Environmental remediation and energy production technologies by C3i IPP

Environmental Remediation and Energy Production Technologies

Ficha Técnica Título

Environmental Remediation and Energy Production Technologies

Edição

Instituto Politécnico de Portalegre C3i – Coordenação Interdisciplinar para a Investigação e Inovação

Autores Paulo Sergio Duque de Brito Anabela Sousa de Oliveira Isabel Luisa Ferreira Machado Luiz Filipe Trepa Torres Rodrigues Instituto Politécnico de Portalegre, Portalegre, Portugal Cesar Sequeira Diogo Santos Instituto Superior Técnico, Portugal Jiri Barek Jiri Zima Charles University in Prague, Czech Republic

Eduardo M. Cuerda Correa Joaquín R. Domínguez Antonio Macías García Awf Al-Kassir Universidade da Extremadura, Spain ISBN 978-989-8806-05-5 Ano 2015

Environmental Remediation and Energy Production Technologies

Ă?ndice 1.

Modern electroanalytical methods for environmental monitoring ............................... 6

1.1. Modern electroanalytical methods for environmental monitoring based on the application of mercury electrodes ................................................................................. 6 1.2. Modern electroanalytical methods for environmental monitoring based on the use of silver amalgam electrodes ......................................................................................... 26 1.3. Carbon paste electrodes for monitoring the environment ....................................... 31 1.3.1. Introduction ................................................................................................................................................ 31 1.3.2. Development, preparation, and characteristics of carbon paste electrodes ................... 31 1.3.3. Practical utilization of carbon paste electrodes........................................................................... 38 1.3.4. Selected examples of CPEs utilization in electroanalysis ........................................................ 41 1.3.5. Practical examples of method development using CPEs .......................................................... 44 1.3.6. Conclusion ................................................................................................................................................... 49 1.4. Modern electroanalytical methods for environmental monitoring based on the use of boron doped diamond film electrode....................................................................... 52

2. Introductory Industrial Electrochemistry ...................................................................... 57

2.1. What is Electrochemistry ..................................................................................... 57 2.2. What can Electrochemistry do? ............................................................................ 59 2.3. Electrochemistry as transfer agent to technology .................................................. 60 2.4. Fundamentals of elementary electrochemistry ...................................................... 65 2.4.1. Conductivities, electrical quantities and properties .................................................................. 65 2.4.2. An electrochemical system is not homogeneous......................................................................... 68 2.4.3. Many things can happen at once ........................................................................................................ 70 2.4.4. Current is an expression of rate ......................................................................................................... 70 2.4.5. Electrode potential and electron energy ........................................................................................ 71 2.4.6. One cannot control both current and potential simultaneously........................................... 73 2.5. Fundamentals of electrochemical engineering ....................................................... 74 2.6. Modified electrodes ............................................................................................ 79 2.7. Photoelectrochemical processes........................................................................... 83

3. Electrochemistry and Environmental Protection ............................................................ 87

3.1. Introduction....................................................................................................... 87 3.2. Electrochemical sensors and monitors .................................................................. 88 3.2.1. Potentiometric sensors .......................................................................................................................... 88 3.2.2. Conductimetric sensors ......................................................................................................................... 91 3.2.3. Voltammetric sensors ............................................................................................................................. 91 3.3. Analysis and removal of gaseous pollutants .......................................................... 93 3.4. Metal ion removal and recovery ........................................................................... 94 3.5. Electrochemistry of ecotoxic metals in water......................................................... 97 3.6. Electrochemistry in drug and food control............................................................. 99 3.7. Electrochemical degradation of organic compounds ............................................. 101 3.8. Electrochemical-assisted photocatalytic degradation ............................................ 103 3.9. Electrochemical cleaning of flue gases................................................................ 104 3.10. Electrochemical carbon dioxide fixation ............................................................ 107

4. Electrochemical energy conversion .............................................................................. 111

4.1. Methodology of electrochemical conversion and storage ...................................... 111 4.1.1. Thermodynamic of galvanic cells ................................................................................................... 112 4.1.2. Kinetics ...................................................................................................................................................... 113 4.1.3. Cell parameters ...................................................................................................................................... 114 4.2. Primary and secondary batteries ........................................................................ 116 4.3. Fuel cells ......................................................................................................... 122 3

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4.3.1. Introduction ............................................................................................................................................. 122 4.3.2. Brief historical perspective ............................................................................................................... 122 4.3.3. Types of fuel cells .................................................................................................................................. 124 4.3.4. Basic components of a fuel cell ........................................................................................................ 132 4.4. Hydrogen as an energy vector ........................................................................... 134 4.4.1. Introduction - Towards a hydrogen economy ........................................................................... 134 4.4.2. Methods for hydrogen production ................................................................................................. 134 4.4.3. Hydrogen storage .................................................................................................................................. 136 5.

Chemical Environmental Carcinogens and Persistent environmental pollutants ..... 138

5.1. Who they are and why is their study relevant ..................................................... 138 5.2. Classes of chemical carcinogens and persistent environmental pollutants .............. 139 5.2.1. Polycyclic aromatic hydrocarbons ................................................................................................. 139 5.2.2. Persistent Organic Polutants (POPs) - Organochlorinated substances [1,3,4,15-30]140 5.2.3. Emergent Pollutants with endocrine disrupting character ................................................. 147 5.2.4. Dyes ............................................................................................................................................................. 150 5.3. Occurrence and Dangers of Chemical Carcinogens and Persistent Environmental Pollutants to Environment and Individuals ................................................................. 151 5.3.1. Polycyclic aromatic hydrocarbons ................................................................................................. 151 5.3.2. Organochlorinated substances ........................................................................................................ 152 5.3.3. Emergent Contaminants ..................................................................................................................... 167 5.4. Monitoring persistent environmental pollutants ................................................... 169 5.4.1. in homogeneous media ....................................................................................................................... 169 5.4.2. in heterogeneous media - Surface Photochemistry Techniques: Diffuse reflectance geometry for the study of reactions on surfaces .................................................................................. 169

6. Advanced Oxidation Processes for Water and Wastewater Treatment ....................... 176

6.1. Introduction..................................................................................................... 176 6.2. AOPs ............................................................................................................... 177 6.2.1. Non-photochemical methods ........................................................................................................... 180 6.2.2. Photochemical methods ..................................................................................................................... 183 6.2.3. AOPs Comparison .................................................................................................................................. 189

7. Industrial Units of Wastewater Treatment by Photocatalysis ..................................... 195

7.1. Basics on Photochemistry.................................................................................. 195 7.1.1. Absorption of UV â&#x20AC;&#x201C; visible light ....................................................................................................... 195 7.1.2. Luminescence Processes .................................................................................................................... 197 7.2. Basics on Solar Photocatalysis ........................................................................... 204 7.3. Treatment of industrial wastewaters containing persistent organic pollutants ........ 206 7.4. Solar Collectors remediation of effluents through Photocatalysis ........................... 208 7.5. Reactors with Solar Collectors ........................................................................... 209 7.5.1. Thin Film Fixed Bed Reactor............................................................................................................. 209 7.5.2. Parabolic Trough Reactor .................................................................................................................. 209 7.5.3. Compound Parabolic Collecting Reactor ..................................................................................... 210 7.5.4. Double Skin Sheet Reactor................................................................................................................. 211 7.5.5. Industrial Units ....................................................................................................................................... 211 7.6. Applications of Photocatalysis on the Treatment of Industrial Effluents ................. 213 7.6.1. Industrial Effluents Containing Dyes............................................................................................. 214 7.6.2. Effluents Containing Pesticides and Pharmaceuticals ........................................................... 214

8. Adsorption for water and wastewater remediation ..................................................... 220

8.1. Fundamentals of Adsorption .............................................................................. 220 8.1.1. Some definitions .................................................................................................................................... 220 8.1.2. Some historical aspects ....................................................................................................................... 223 8.1.3. Characterization of the adsorbents................................................................................................ 227 8.2. Types of Adsorbents ......................................................................................... 234 8.2.1. Activated carbon .................................................................................................................................... 234 4

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8.2.2. Peat .............................................................................................................................................................. 236 8.2.3. Lignite......................................................................................................................................................... 239 8.2.4. Molecular sieves / Zeolites ................................................................................................................ 240 8.2.5. Silica gel ..................................................................................................................................................... 245 8.2.6. Activated alumina.................................................................................................................................. 246 8.2.7. Chitin and chitosan ............................................................................................................................... 246 8.2.8. Other adsorbents ................................................................................................................................... 248 8.3. Adsorbents on environmental remediation .......................................................... 250 8.3.1. General considerations ....................................................................................................................... 250 8.3.2. Adsorbents for heavy metals removal from wastewaters ................................................... 252 8.3.3. Adsorbents for dye removal from wastewaters ....................................................................... 253 9. Photocatalysis and Environmental Cleaning Systems (teaching through Case Studies) .......................................................................................................................................... 256

9.1. Use of spectroscopic methods for environmental monitoring and to follow environmental remediation ...................................................................................... 257 9.2. Photocatalytic remediation of industrial effluents................................................. 265 9.3. Photocatalysis on the remediation of medical wastes and effluents from pharmaceutical industry .......................................................................................... 267

10. Corrosion in Energy Conversion and Environmental Cleaning System ...................... 268

10.1. Introduction ................................................................................................... 268 10.2. Corrosion fundamentals .................................................................................. 270 10.2.1. Corrosion definition and nature ................................................................................................... 270 10.2.2. The corrosion cell and the fight against corrosion ............................................................... 271 10.2.3. Standard electrode potentials and galvanic series ............................................................... 274 10.3. Corrosion classification and mechanisms........................................................... 281 10.3.1. Uniform or generalized corrosion ............................................................................................... 281 10.3.2. Galvanic corrosion.............................................................................................................................. 281 10.3.3. Crevice corrosion ................................................................................................................................ 282 10.3.4. Pitting corrosion ................................................................................................................................. 283 10.3.5. Intergranular corrosion ................................................................................................................... 284 10.3.6. Selective leaching or dealloying.................................................................................................... 285 10.3.7. Erosion-corrosion ............................................................................................................................... 286 10.3.8. Stress-Corrosion Cracking (SCC) .................................................................................................. 288 10.3.9. Corrosion fatigue ................................................................................................................................ 288 10.3.10. Hydrogen Damage ........................................................................................................................... 289 10.3.11. Biocorrosion ....................................................................................................................................... 290 10.3.12. Corrosion of reinforced concrete structures ........................................................................ 292 10.4. Corrosion in energy and environmental cleaning systems ................................... 293

Environmental Remediation and Energy Production Technologies

1. Modern electroanalytical monitoring

methods

for

environmental

Barek Jiri and Zima Jiri

1.1. Modern electroanalytical methods for environmental monitoring based on the application of mercury electrodes Modern electroanalytical methods can play very useful role in the monitoring of trace amounts of electrochemically active organic compounds in the environment. The choice of suitable electrode is crucial for successful determination. Therefore, we shall pay attention to the use of both traditional and no-traditional electrode material for the determination of organic environmental pollutants. In this chapter, our attention will be focused on the application of various types of mercury electrodes since we believe that mercury is the best electrode material available for the determination of submicromolar concentrations of electrochemically reducible organic compounds. Recent developments in the use polarography and voltammetry at mercury electrodes in modern environmental analysis will be described here together with their combination with preliminary separation and preconcentration using liquid or solid phase extraction. Another possibility is the use various forms of environmentally friendly, non-toxic and mechanically more robust electrodes based on various types of solid or paste silver amalgam electrodes. This topic will be discussed later. The sensitivity of polarographic methods of analysis, enabling determination of electrochemically reducible species in concentrations down to about 10â&#x20AC;&#x201C;5 M, was superior to most other contemporary techniques in 1950s and 1960s, DC polarography (DCP) was one of the five most frequently used analytical techniques. Later, with the advance of spectrometric and separation methods for the determination of organic compounds, DCP lost its importance (1). Renaissance of polarography was based on methods effectively eliminating the charging current and thus enabling to reach much lower limit of detection (LOD). Square wave polarography (SWP) and differential pulse polarography (DPP), and their voltammetric variants at a hanging mercury drop electrode (HMDE), namely square wave voltammetry (SWV) and differential pulse voltammetry (DPV), opened new possibilities in trace analysis. These methods can be combined with the stripping analysis, in which prior accumulation of the analyte on the electrode surface leads to the increased sensitivity by about three orders of magnitude (2,3). Even though very successful, HMDE, consisting of a renewable drop of mercury at the end of a fine capillary, has some drawbacks mentioned below. Some of these disadvantages can be successfully eliminated by using a mercury film electrode (MFE), prepared by coating a suitable substrate with a thin film of metallic mercury. The sensitivity of above mentioned methods (LOD form 10â&#x20AC;&#x201C;6 to 10â&#x20AC;&#x201C;11 M) is sufficient for routine environmental electroanalysis, whereas the selectivity is limited by the width of potential window of relevant mercury electrode. However, their selectivity can be increased using preliminary separation with liquid-liquid (LLE) or solid phase (SPE) extraction or column, paper or thin layer chromatography. The combination of modern polarographic and voltammetric techniques with preconcentration and/or separation step enables the use of mercury electrodes for solving of many problems in contemporary environmental analytical chemistry (4-8).

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Therefore, modern polarographic and voltammetric techniques at mercury electrodes can be successfully used for the determination of trace amounts of various genotoxic and ecotoxic organic environmental pollutants. The species to be determined at mercury electrodes must be electroactive (i.e. must undergo electroreduction or electrooxidation within the available potential windows), or must react with Hg ions, or must be catalytically active or be adsorbed on mercury. The list of organic functional groups reducible or oxidisable on mercury electrodes can be found in review (6). The electroactive species must be soluble in a solvent which is conductive and interfering materials must be absent. Mercury enables reaching negative potentials down to –2.5 V and thus following numerous reductions occurring in this potential range. On the other hand the dissolution of mercury at about +0.4 V prevents reaching more positive potentials. Hence mercury electrodes are not suitable for monitoring oxidizable species (9). There are several types of mercury electrodes, DME, HMDE, and MFE, being the most frequently used. Their fundamental technical parameters are well known (6,9). DME and HMDE have been used successfully in countless applications involving reduction of organic and inorganic electroactive compounds. The DME is now less frequently used being replaced by HMDE. Nevertheless, the advantages of DME, such as simplicity, reliability and renewable surface, can be successfully used in many cases of environmental electroanalysis especially in combination with DPP. The main disadvantages of DME (high consumption of mercury and higher charging current) are eliminated by using HMDE as the most frequently used mercury electrode with high reproducibility, low consumption of mercury, and the possibility of adsorptive or electrolytic accumulation of analytes on its surface. Of course, even HMDE has some drawbacks. It is bulky, requires a mercury reservoir and regular maintenance of the capillary and incorporates complicated electronics and mechanics for precise drop generation and disposal. Another serious problem, which is becoming increasingly important, is the very use of metallic mercury. The potential risks of poisoning, contamination and disposal associated with the use of mercury have led some countries to complete ban of mercury. Moreover, HMDE, unlike solid electrodes, is mechanically unstable (i.e. the mercury drops are easily dislodged) so that it is not particularly suitable for on-site and field analysis (e.g. shipboard operations that involve vibrations) or for flow-through applications (where the electrode is subject to high flow rates). Finally, HMDE is not the ideal substrate for permanent modification by chemical reagents or permselective coatings that improve the analytical properties (e.g. selectivity and sensitivity). Even though in principle it is possible to modify the surface of HMDE, such procedures have been used only occasionally due to the electrode sensitivity to mechanical handling and to its very principle of operation (one drop for each measurement). An overview of most widely used polarographic and voltammetric techniques, including their potential programs, can be found in our review (6). The original DCP at DME is nowadays displaced by more sensitive pulse techniques. Nevertheless, there are some situations in which even now the classical DCP can offer some advantages – limiting currents in a given solution for a chosen electrode depend only on the concentration of the electroactive species, number of transferred electrons, and diffusion coefficient of the electroactive species and are not affected by the rate of electrode reaction. The more sensitive pulse variation – DPP at DME – represents modern technique for environmental analysis, especially due to LOD around 10–7 M and renewable surface enabling analysis in matrices causing electrode surface passivation. SWV at HMDE and DPP at HMDE gives LOD around 10–8 M, particularly when the oxidation–reduction process is reversible (9). These methods can be combined with the stripping analysis, in which prior accumulation of the analyte on the electrode surface leads to the increased sensitivity. Anodic stripping voltammetry (ASV) is a well-known method, employed mainly for determination of traces of heavy metal ions. In cathodic stripping voltammetry (CSV), analytes are accumulated at the electrode via reaction with mercury ions formed by previous dissolution of mercury at positive potentials, forming 7

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low soluble compounds. Surface active organic compounds (and complexes of some metals) can be adsorbed at the electrode surface using adsorptive stripping voltammetry (AdSV) (10, 11, 12). In this technique, the analyte is concentrated by adsorption on the electrode surface, and subsequently stripped off in reduction or oxidation scan. AdSV belongs to the most sensitive and frequently used analytical methods; however, its use in environmental analysis is limited because it is less robust and more prone to interferences from surface active substances and other compounds likely to be present in environmental matrices. Thus, it should be used mainly for analysis of relatively clean samples (e.g. of drinking water) or of samples after preliminary clean-up or separation. Another limitation is that not all electroactive compounds are adsorbed at mercury electrodes. AdSV determination of organic genotoxic compounds was reviewed (12). The theory of polarographic and voltammetric techniques is well described in monograph (10). Voltammetric methods used today in analytical laboratories comprise a suite of techniques, the creation of which was made possible by rapid advances in instrumentation, by the computerised processing of analytical data, and particularly by innovative electrochemists. Advances in microelectronics and in particular the early introduction of operational amplifiers and feedback loops have led to major changes in electroanalytical instrumentation. Indeed, many functions can be performed now by small and reliable integrated circuits. Voltammetric analysers consist of two such circuits: a polarising circuit that applies the potential to the cell and a measuring circuit that monitors the cell current. The working electrode is potentiostatically controlled, and this minimises errors from cell resistance. Electroanalytical procedures can be fully programmed and can be driven automatically by means of a personal computer with a user-friendly software. All this results in the possibility of fast â&#x20AC;?time-resolvedâ&#x20AC;? sampling of the current from dropping mercury electrode. The mercury drop emerging from the capillary monitors current which consists of that due to charging of the double layer and the faradaic current produced by reduction or (less frequently) oxidation of the analyte in solution. The contribution of the capacitance current becomes less as the drop increases in size and the rate of increase in area becomes much smaller. Thus if the current is sampled at a long enough time after the drop has started to emerge from the capillary, the capacitance current is discriminated against to the faradaic current: this is utilised in its simplest form in TAST polarography, but it is utilised also when more advanced pulse waveforms are used. Pulse waveforms improve further signal-to-noise ratio for other reasons as well. LOD can be further decreased by a new method to obtain the signal associated with a blank in DPV and stripping voltammetry. In this method, the signal assigned to the blank is obtained by direct integration of the background noise extrapolated values of the base-peak width at different concentrations. All pulse techniques (NPP, DPP, SWP and SCV) are chronoamperometric and are based on a sampled current potential-step experiment. After the potential is stepped, the charging current decreases rapidly (exponentially), while the faradaic current decays more slowly. Another technique that allows the separation of the contributions of the faradaic and charging current is alternating current voltammetry (ACV), which involves the superimposition of a small amplitude AC voltage on a linearly increasing potential, where the charging current is rejected using a phase sensitive lock-in amplifier. For a comprehensive survey of methods mentioned above with their basic parameters see Table 1.1.

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Table 1.1.1 - Basic parameters of modern polarographic and voltammetric techniques

The comparison of polarographic and other analytical techniques is depicted in Figure 1.1. 9

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Figure 1.1.1 - The application range of various analytical techniques and their concentration limits as compared with the requirements in different fields of chemical analysis.

Important features of mercury electrodes in polarography and voltammetry are summarized in Table 1.2.

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Table 1.1.2 - Summary of working mercury electrodes

Working Electrode DME

Characteristics

Advantages

Disadvantages

-mercury freely

-simplicity

-LOD~10-5 M,

dropping

-reliability

-high consumption of

from a capillary,

-renewable surface

mercury -higher current

ď ´ =1- 5 s -valve mechanics SMDE

and a hammer -stopped growth of drop surface for each new drop at a given time

charging

-LOD~10-7 M

-lower reliability

-lower charging current,

-high demands on valve mechanics

-lower consumption mercury

-drop periodically detached by a hammer HMDE

-valve mechanics and a hammer -electrode surface not renewed during one analysis -whole analysis one drop

-LOD~10-7 - 10-10 M -high reproducibility -low consumption of mercury -adsorptive or electrolytic accumulation -possibility of chemical

-demands on stand mechanics -increased danger of passivation -more complex mechanics and electronics

Modifications

MFE

-a thin mercury layer electrolytically plated on a solid electrode

-LOD~10-11 M

-passivation

-possibility of chemical modifications

-time consuming

-stable in flow applications

preparation -irregularities of Hg plating

-no mercury reservoir -no complex mechanics and electronics

The sensitivity of polarographic and voltammetric techniques at mercury electrodes is aboveaverage for the determination of various environmental pollutants (6). However, the selectivity of these techniques is limited by the width of potential window. For the determination in simple matrices (e.g. drinking water, rain water, or snow), the direct determination can be used. The development of polarographic/voltammetric determination is then simplified to finding optimal conditions. In dependence on solubility of the determined 11

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compound, the suitable solvent is chosen (deionized water or mixture of linear low molecular alcohols with water are usually used in the case of most environmental pollutants). Electrochemical behavior of electroactive organic compounds depends on pH so that optimum pH value of supporting electrolyte is important. Optimal conditions applied to direct determination of organic pollutant in drinking, well, rain, or river water can then lead to LODs similar to those in deionized water. In the case of more complicated matrices (e.g. river, sea or waste water, soil, ice, etc.), the preliminary separation step is usually required. This step can be simultaneously used for analyte preconcentration, too. LLE or SPE are most frequently used techniques. Ten-fold to thousand-fold preconcentration is attainable by proper choice of optimal sorbent and eluent (in SPE), or optimal extraction liquid (in LLE), extraction recovery playing important role. High performance liquid chromatography (HPLC) belongs to powerful separation techniques in environmental analysis of liquid samples, although it is much more expensive than LLE or SPE. In combination with electrochemical detection, we obtain very sensitive and selective method for the determination of wide group of electrochemically active organic compounds. Nevertheless, HPLC in combination with polarographic or voltammetric detection using mercury electrodes is not often used. Newly developed mechanically more robust electrode materials are preferred which will be discussed later. Even though electroanalytical methods are sensitive and inexpensive, in the area of environmental pollutants they are not too often used. However, the only prerequisite to voltammetric determination is the presence of reducible or oxidizable moieties – see Table 1.3. Thus, the research in polarographic and voltammetric determination of genotoxic pollutants in environmental samples should further continue. Nitrated polycyclic aromatic hydrocarbons are more mutagenic and/or carcinogenic than their parent PAHs, so that their monitoring is very important (12,13,14). They are easily electrochemically reducible and thus are suitable candidates for the application of modern polarographic and voltammetric methods on mercury electrodes. The same is valid for carbonyl derivatives of PAHs, namely 2-aminoanthraquinone (15) and 9-fluorenone (16). Several examples of methods developed in our UNESCO Laboratory of Environmental Electrochemistry for the determination of these compounds are summarized in Table 1.4. Until recently, there was little interest in heterocyclic compounds in the environment probably because of their low concentrations one or two orders of magnitude lower than those of their homocyclic analogues. However, the first studies focused on biological effects and toxicity of heterocyclic compounds have started discussions on their possible human health and ecological risks. The most studied group of these “new environmental pollutants” is group of polycyclic aromatic nitrogen-containing heterocycles which are mutagenic and/or carcinogenic. Heteroaromatic compounds with a π–electron deficit are polarographically reducible and can be determined using modern polarographic and voltammetric techniques in aqueous and/or aqueous-methanolic solutions. Selected polarographic and voltammetric determinations of environmentally important heterocyclic compounds at mercury electrodes are summarized in Table 1.5.

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Table 1.1.3. - Organic functional groups reducible or oxidizable on mercury electrodes

cathodic waves

aldehydes, ketones,

Ar C O

C C C O

NO2

Ar C C

O O

C N

S S

NHOH

O C C O

ONO

C N

C X

ONO2

C C C C

Ar X

Ar C C

NH NO

N N

Ar C X

heterocycles (O,S,N) with double bonds, alkaloids, vitamines, hormones, steroids, saccharides

anodic waves

groups reacting with Hg : Br

S C

oxidizable substances :

OH Ar OH

NH NH

NH2 Ar NH2

-NH-NH2 NH-CS-NH-

C C HO OH

-CS-NH-R

OH Ar NH2 Ar NHOH

-NH(R)2

-NH-CO-NH-

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Table 1.1.4 - Polarographic and voltammetric determination of selected genotoxic nitrated and carbonyl derivatives of polycyclic aromatic hydrocarbons Analyte

Technique/Electrode

LOD,M

DCTP/DME

6 × 10–7

DPP/DME

DPV/HMDE 2,2’-Dinitrobiphenyl

2,7-Dinitrofluorene

2,7-Dinitro9-fluorenone

1,3-Dinitronaphthalene

1,8-Dinitronaphthalene

1 × 10–7 1 × 10–7 6 × 10–8

DPV/HMDE (direct determination in DW)

2 × 10–7

DPV/HMDE (direct determination in RW)

2 × 10–7

DPV/HMDE (SPE from 100 ml of DW)

2 × 10–8

DPV/HMDE (SPE from 100 ml of DW)

2 × 10–8

DPV/HMDE (SPE from l00 ml of RW)

3 × 10–8

DCTP/DME

2 × 10–6

DPP/DME

2 × 10–7

DPV/HMDE

1 × 10–8

AdSV/HMDE

2 × 10–8

AdSV/HMDE

4 × 10–9

DCTP/DME

1 × 10–6

DPP/DME

1 × 10–7

DPV/HMDE

2 × 10–8

AdSV/HMDE

4 × 10–9

DPV/HMDE (direct determination in DW)

2 × 10–8

DPV/HMDE (SPE from 500 ml of DW)

2 × 10–9

DPV/HMDE (SPE from 500 ml of RW)

4 × 10–9

DCTP/DME

2 × 10–6

DPP/DME

1 × 10–7

DPV/HMDE

3 × 10–7

AdSV/HMDE

1,5-Dinitronaphthalene

2 × 10–7

2 × 10–8 2 × 10–9

DCTP/DME

2 × 10–6

DPP/DME

1 × 10–7

DPV/HMDE

1 × 10–7

AdSV/HMDE

2 × 10–8

DCTP/DME

2 × 10–6

DPP/DME

4 × 10–7

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9-Nitroanthracene

2-Nitrobiphenyl

DPV/HMDE

4 × 10–7

AdSV/HMDE

2 × 10–8

DCTP/DME

1 × 10–6

DPP/DME

2 × 10–7

DPV/HMDE

2 × 10–7

AdSV/HMDE

2 × 10–9

DPV/HMDE (LLE with hexane from 4 l of DW)

5 × 10–10

DPV/HMDE (LLE with hexane from 4 l of RW)

2 × 10–10

DCTP/SMDE

3 × 10–7

DPP/SMDE

2 × 10–7

DPV/HMDE

3 × 10–8

AdSV/HMDE

3-Nitrobiphenyl

4-Nitrobiphenyl

3 × 10–9

DCTP/SMDE

3 × 10–7

DPP/SMDE

3 × 10–8

DPV/HMDE

3 × 10–8

AdSV/HMDE

2 × 10–8 2 × 10–9

DPV/HMDE (direct determination in DW)

2 × 10–8

DPV/HMDE (LE with hexane from 100 ml of DW)

2 × 10–9

DCTP/SMDE

3 × 10–7

DPP/SMDE

3 × 10–8

DPV/HMDE

3 × 10–8

AdSV/HMDE

2 × 10–9

DCTP/DME

3 × 10–6

DPP/DME

1 × 10–7

DPV/HMDE 3-Nitrofluoranthene

2 × 10–8

3 × 10–8 3 × 10–8 2 × 10–8

AdSV/HMDE

2 × 10–8 5 × 10–9

DPV/HMDE (LE with hexane from 100 ml of DW)

4 × 10–9

DPV/HMDE (LE with hexane from 10 ml of RW)

3 × 10–8

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2-Nitrofluorene

DPV/HMDE (SPE from 500 ml of DW)

4 × 10–10

DPV/HMDE (SPE from 500 ml of RW)

2 × 10–9

DCTP/DME

4 × 10–6

DPP/DME

4 × 10–7

DPV/HMDE

4 × 10–8

AdSV/HMDE

2-Nitro-9fluorenone

1 × 10–6

DPP/DME

2 × 10–7

DPV/HMDE

2 × 10–8

AdSV/HMDE

4 × 10–9

DCTP/DME

1 × 10–6

DPP/DME

1 × 10–7

1Nitropyrene

2 × 10–7 3 × 10–8

AdSV/HMDE

2 × 10–9

DPV/HMDE (direct determination in DW)

2 × 10–8

DPV/HMDE (direct determination in RW)

3 × 10–8

DPV/HMDE (SPE from 100 ml of DW)

2 × 10–9

DPV/HMDE (SPE from 1 l of DW)

2 × 10–10

DPV/HMDE (SPE from 100 ml of RW)

2 × 10–9

DCTP/DME

2 × 10–6

DPP/DME

2 × 10–7

DPV/HMDE

2-Nitronaphthalene

3 × 10–9

DCTP/DME

DPV/HMDE 1-Nitronaphthalene

2 × 10–8

3 × 10–7 2 × 10–8

AdSV/HMDE

2 × 10–9

DPV/HMDE (direct determination in DW)

2 × 10–8

DPV/HMDE (direct determination in RW)

4 × 10–8

DPV/HMDE (LE with hexane from 100 ml of DW)

5 × 10–9

DPV/HMDE (LE with hexane from 1 l of DW)

4 × 10–10

DPV/HMDE (SPE from 100 ml of DW)

3 × 10–9

DPV/HMDE (SPE from 1 l of DW)

3 × 10–10

DPV/HMDE (SPE from 100 ml of RW)

3 × 10–9

DCTP/DME

3 × 10–6

DPP/DME

4 × 10–7

Environmental Remediation and Energy Production Technologies

3 × 10–7 DPV/HMDE

2-Aminoanthraquinon e

6 × 10–8

AdSV/HMDE

1 × 10–9

DCTP/DME

5 × 10–6

DPP/DME

4 × 10–6

DCV/HMDE

6 × 10–7

DPV/HMDE

AdS-DCV/HMDE

9-Fluorenone

1 × 10–7

2 × 10–7 4 × 10–7 2 × 10–8 3 × 10–9

AdS-DPV/HMDE

3 × 10–8

DCTP/DME

3 × 10–6

DPP/DME

5 × 10–7

DPV/HMDE

2 × 10–8

DW-drinking water; RW-river water Table 1.1.5 - Polarographic and voltammetric determination of selected genotoxic heterocyclic compounds Analyte

Technique/Electrode DCTP/DME DPP/DME DPV/HMDE

LOD (M) 1 × 10– 6

2 × 10– 7

2 × 10– 8

6-Methyl-

AdSV/HMDE

5-nitroquinoline

2 × 10– 8

DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in RW) AdSV/HMDE (direct determination in DW) DPV/HMDE (LE with hexane from 1 l of DW)

3 × 10– 6

5 × 10– 7

2 × 10– 8

4 × 10–

Environmental Remediation and Energy Production Technologies

DPV/HMDE (LE with hexane from 1 l of drinking water) b

2 × 10–

DPV/HMDE (LE with hexane from 1 l of RW )

2 × 10–

DCTP/DME DPP/DME DPV/HMDE 6-Methyl-5nitrouracil

DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in RW) DPV/HMDE (SPE from 10 ml of DW) DPV/HMDE (SPE from 100 ml of RW)

2 × 10– 6

2 × 10– 7

3 × 10– 7

4 × 10– 8

6 × 10– 8

2 × 10– 6

DCTP/DME

3 × 10– 6

DPP/DME 5-Nitrobenzimidazole

DCV/HMDE DPV/HMDE DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in DW) DCTP/DME DPP/DME

5 × 10– 7

4 × 10– 7

3 × 10– 8

5 × 10– 8

4 × 10– 8

2 × 10– 6

2 × 10– 7

5-Nitroindazole

DPV/HMDE

2 × 10– 7

AdSV/HMDE

1 × 10– 8

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DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in RW) DPV/HMDE (SPE from 100 ml of DW) DPV/HMDE (SPE from 500 ml of DW) DPV/HMDE (SPE from 100 ml of RW) DPV/HMDE (SPE from 500 ml of RW) DCTP/DME DPP/DME DPV/HMDE 5-Nitroquinoline

DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in RW) DPV/HMDE (SPE from 100 ml of DW) DPV/HMDE (SPE from 100 ml of RW) DCTP/DME DPP/DME DPV/HMDE

1 × 10– 7

2 × 10– 7

2 × 10– 8

2 × 10– 9

2 × 10– 8

2 × 10– 9

9 × 10– 7

9 × 10– 8

2 × 10– 8

7 × 10– 9

3 × 10– 9

1 × 10– 9

2 × 10– 6

1 × 10– 7

1 × 10– 8

AdSV/HMDE 8-Nitroquinoline

2 × 10– 8

DPV/HMDE (direct determination in DW) DPV/HMDE (direct determination in RW) DPV/HMDE (SPE from 100 ml of DW) DPV/HMDE (SPE from 200 ml of DW)

9 × 10– 8

1 × 10– 7

1 × 10– 8

2 × 10– 9

Environmental Remediation and Energy Production Technologies

DPV/HMDE (SPE from 100 ml of RW) DCTP / DME DPP / DME Quinazoline DCV / HMDE DPV / HMDE

2 × 10– 8

1 × 10– 6

2 × 10– 7

– substance after evaporation dissolved in 10 ml of supporting electrolyte; a

– substance after evaporation dissolved in 1 ml of supporting electrolyte; DW – drinking water; RW – river water b

Many pesticides contain electroactive groups and thus voltammetry can be used for their determination. However, despite the fact, that many agrochemicals are directly reducible at DME, relatively few polarographic determinations appeared in recent literature. Several examples of the use of mercury electrodes for determination of pesticides or agrochemicals in various environmental matrices are summarized in Table 1.6. Therefore, it can be stated that for some analytes and some types of matrices, polarographic and voltammetric methods at mercury electrodes may be the “best method” and can successfully compete with more widespread separation and spectrometric techniques. Moreover, in many other cases modern polarographic and voltammetric techniques can be among “fit for the purpose” methods. Lower investment and running costs, high speed (an analysis can be routinely carried out in less than 3 minutes, if necessary within fractions of a second), sensitivity, universality, and wide applicability speaks in favor of voltammetric techniques despite their limited selectivity. To increase the use of polarography in modern analytical laboratories it is necessary to improve education in this field and to pay more attention to the validation of newly developed methods.

Environmental Remediation and Energy Production Technologies

Table 1.1.6 - Polarographic and voltammetric determination of selected pesticides Analyte

Ametryne

Chemical class

triazine

Matrix [Recovery (%)]

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

BR buffer pH 10.1 – ethanol (8:2) Amitraz

formamidine

AdSV/HMDE

Atrazine

triazine

DPP/DME

Butralin

thiadiazine

2,6-dinitroaniline

DW and WW [90–97 %]

–––

Cyfluthrin

Cypermethri n

Dialifos

pyridazinone

pyrethroid

organophosphorus

1× 10–7

RW [96 %]

–––

soil [98 %]

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

ACP/SMDE

1 M KCl

–––

AdSV/HMDE

DPP/DME

DPV/HMDE

7× 10–9

soil [90–93 %]

–––

DW [99 %]

–––

WW [97 %]

–––

BR buffer pH 4.0 grains, soils, water samples

Chloridazon

2× 10–9 –––

BR buffer pH 7.1 – ethanol (4:1) Buprofezin

(M)

soil [91–96 %]

BR buffer pH 2.0 Anilazine

LOD

Technique/ Electrode

6× 10–8 –––

citric acid pH 2.3

3× 10–8

BR buffer pH 3.0 – methanol (6:4)

2× 10–8

formulations [99–100 %]

–––

grains [99–100 %]

–––

soils [98–100 %]

–––

DW [93–98 %]

–––

well water [92–95 %]

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

DPP/DME

agricultural formulations, food grains and soil

–––

AdSV/MFE-Au

BR buffer pH 2.0

DPP/DME

2× 10–8

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2,4-Diamino1,3,5-triazine

Dichlorvos

Dimethametr yn

Diquat

2,6Dimethoxy4-chloro1,3,5-triazine

Dinobuton

Dinocap

Ethion

triazine

organophosphorus

triazine

bipyridylium

triazine

dinitrophenol derivative

organophosphorus

soil [92–97 %]

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

0.5 M Na2SO4

1× 10–8

river water

3× 10–8

SWV/HMDE

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

drinking water

3× 10–8

soil

3× 10–8

AdSV/HMDE

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

universal buffer pH 6.0

–––

formulations [99–100 %]

–––

spiked distilled water [94–97 %]

–––

DCTP/DME

BR buffer pH 2.0 – methanol (1:1)

1× 10–6

DPP/DME

BR buffer pH 2.0 – methanol (1:1)

5× 10–7

DPV/HMDE

BR buffer pH 7.0 – methanol (1:1)

2× 10–7

BR buffer pH 8.4

2× 10–8

DPP/DME

LSSV/HMDE

vegetable and fruit samples SWV/HMDE Fenitrothion

organophosphorus

BR buffer pH 10.5 formulations [98–109 %]

AdS-SWV/HMDE

RW [85–99 %]

DCP/DME

buffered aqueous solution BR buffer pH 3.0

Flumethrin

pyrethroid

DPP/DME

–––

2× 10–8 –––

6× 10–9 –––

2× 10–8

formulations [99 %]

–––

grains [98–100 %]

–––

soil [99 %]

–––

DW [98–99 %]

–––

Environmental Remediation and Energy Production Technologies

DPP/DME Imidacloprid

2,6-dinitroaniline

–––

BR buffer pH 8.0

4× 10–8

formulations [99–102 %]

chloro-nicotinyl AdS-SWV/HMDE

Isopropalin

well water [99 %]

DPP/DME

2-Methyl4,6dinitrophenol e

organophosphorus

dinitrophenol derivative

2× 10–8

RW [89–104 %]

–––

BR buffer pH 4.0

2× 10–8

organophosphorus

Mevinphos

organophosphorus

Parathion

organophosphorus

Pendimethali n

2,6-dinitroaniline

Propazine

triazine

Pymetrozine

triazine

–––

DCP/DME

0.1 M buffer (NH4Cl + NH4OH)

DPP/DME

pH 9.0

DPP/DME

vegetables, soil, fruits and soft drinks

DCTP/DME

BR buffer pH 10.0 – methanol (9:1)

1× 10–6

DPP/DME

BR buffer pH 7.0 – methanol (9:1)

1× 10–7

BR buffer pH 6.0 – methanol (9:1)

1× 10–8

DW [94 %]

2× 10–9

RW [73 %]

2× 10–9

BR buffer pH 6.0

1× 10–9

0.01 M CaCl2 – pH 6.8

2× 10–8

soil suspensions

8× 10–8

insecticidal formulations

1× 10–9

grains and soil samples

–––

buffered aqueous solution

–––

DPV/HMDE

AdSV/HMDE

Methylparath ion

BR buffer pH 7.2

grains, soils, water samples

Malathion

–––

DPP/DME

DPP/DME DCP/DME

DPP/DME

buffered water–acetone (3:1) solution

2× 10–8

formulations, grains, soils, water

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

DPP/DME

BR buffer pH 2.0

5× 10–7

Environmental Remediation and Energy Production Technologies

Simazine

triazine

Simetryn

triazine

Terbutylazin e

triazine

Tetrachlorvin phos

organophosphorus

Thiamethoxa m

formulations [97–101 %]

–––

lake water [93–102 %]

–––

orange juice [96–103 %]

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

DCP/DME

buffered aqueous solution

–––

DPP/DME

buffered aqueous solution

–––

ACP/SMDE

1 M KCl

–––

DPP/DME

–––

DPP/DME

insecticidal formulations

1× 10–9

grains and soil samples

–––

BR buffer pH 8 formulations [86–95 %] BR buffer pH 7.0

Thiazopyr

pyridinecarboxylic acid

Thifensulfuro n methyl

sulfonylurea

Triflumizole

Zinc dimethyldithi ocarbamate a

DPP/DME

azole

DPP/DME

dithiocarbamate, inorganic-zinc

– the value of LOD was not investigated;

DPP/DME b

WW – waste water.

1× 10–7 –––

3× 10–7

soil samples [94–99 %]

–––

fruit juice [97–99 %]

–––

BR buffer pH 3.0

4× 10–7

soil [103 %]

3× 10–7

orange juice [102 %]

1× 10–6

BR buffer pH 2.0

7× 10–9

formulations [101–104 %]

–––

soil [101–103 %]

–––

lake water [101–106 %]

–––

acetonitrile [in thye presence of Cu(ClO4)2]

– the recovery value is mentioned only;

c – the value of LOD was not obtainable; DW – drinking water; RW – river water;

ACKNOWLEDGMENT Authors thank the Grant Agency of the Czech Republic (Project p206/12/G151) for financial support. REFERENCES 1. P. Zuman, Current examples of practical and fundamental applications of DC polarography. J. Solid State Electrochem., 10 (2006):841-851. 2. R. Kalvoda, Is polarography still attractive? Chem. Anal. (Warsaw), 52 (2007):869-873. 3. A. Economou, and P. R. Fielden, Mercury film electrodes: developments, trends and potentialities for electroanalysis. Analyst, 128 (2003):205-212. 4. J. Barek, M. Pumera, A. Muck, M. Kaderabkova, and J. Zima, Polarographic and voltammetric determination of selected nitrated polycyclic aromatic hydrocarbons. Anal. Chim. Acta, 393 (1999):141-146. 5. J. Barek, V. Mejstrik, A. Muck, and J. Zima, Polarographic and voltammetric determination of chemical carcinogens. Crit. Rev. Anal. Chem., 30 (2000):37-57. 6. J. Barek, A. G. Fogg, A. Muck, and J. Zima, Polarography and voltammetry at mercury electrodes. Crit. Rev. Anal. Chem., 31 (2001):291-309. 7. V. Vyskocil, J. Barek, I. Jiranek, and J. Zima, Polarographic and voltammetric determination of genotoxic substances in drinking water using mercury electrodes, in Progress on Drinking Water Research, eds. M. H. Lefebvre and M. M. Roux (New York: Nova Science Publishers, 2009), ch. 5, 171-198. 8. K. Rajeshwar, and J. G. Ibanez, Environmental Electrochemistry: Fundamentals and Applications in Pollution Sensors and Abatement (London: Academic Press, 1997). 9. P. Zuman, Role of mercury electrodes in contemporary analytical chemistry. Electroanalysis, 12 (2000):1187-1194. 10. J. Wang, Analytical Electrochemistry, 3rd ed. (Hoboken: John Wiley & Sons, 2006). 11. R. Kalvoda, Adsorptive stripping voltammetry and/or chronopotentiometry. Comparison and assessment. Electroanalysis, 12 (2000):1207-1210. 12. J. Barek, K. Peckova, and V. Vyskocil, Adsorptive stripping voltammetry of environmental carcinogens. Curr. Anal. Chem., 4 (2008):242-249. 13. V. Vyskocil, I. Jiranek, J. Barek, K. Peckova, and J. Zima, Are mercury electrodes still useful sensors? Polarographic and voltammetric determination of polarographically reducible environmental carcinogens, in Sensing in Electroanalysis, vol. 2, eds. K. Vytras and K. Kalcher (Pardubice: University of Pardubice, 2007), 105-119. 14. J. C. Moreira, and J. Barek, Analysis of carcinogenic nitrated polycyclic aromatic hydrocarbons - a review. Quim. Nova, 18 (1995):362-367. 15. K. Peckova, H. F. Ayyildiz, M. Topkafa, H. Kara, M. Ersoz, and J. Barek, Polarographic and voltammetric determination of trace amounts of 2-aminoanthraquinone. Chem. Anal. (Warsaw), 52 (2007):989-1001. 16. V. Vyskocil, P. Bologa, K. Peckova, and J. Barek, Polarographic and voltammetric determination of selected genotoxic fluorene derivatives using traditional mercury electrodes. Abstract of the Modern Analytical Chemistry 2008 Conference, Prague, Czech Republic (2008):108-112.

1.2. Modern electroanalytical methods for environmental monitoring based on the use of silver amalgam electrodes Even though we believe that mercury is the best electrode material available for the determination of submicromolar concentrations of electrochemically reducible organic compounds, it has some disadvantages, namely mechanical instability. Moreover, there are increasing problems connected with somewhat unreasonable fears of mercury toxicity. Therefore, in this chapter we pay attention to the use of various forms of environmentally friendly, non-toxic and mechanically more robust electrodes based on various types of solid or paste silver amalgam electrodes. These electrode materials represent a transition between solid metal surfaces and liquid mercury saturated by dissolved silver. Amalgam electrodes could substitute mercury electrodes thanks to their relatively high hydrogen overvoltage and simple surface regeneration [1-3]. Mikkelsen and Schroder introduced electrodes based on dental amalgam [4,5] and Yosypchuk and Novotny independently developed the electrodes based on soft metal amalgamated powders [6]. The final amalgam form is dependent on a metal/mercury ratio. Thus if the content of mercury decreases, the amalgam changes from liquid through paste consistency to a solid form. Working electrodes based on liquid, paste and/or solid metal amalgam were described and summarized in reviews [1,3,7].This chapter describes methods of voltammetric and amperometric detection applied for determination of various organic compounds using amalgam electrodes. Attention is paid to nitrated polycyclic aromatic hydrocarbons, herbicides, and anti-cancer drugs presenting serious environmental problems. The amalgam electrodes may be based on liquid, paste or solid amalgam in dependence on metal/mercury ratio. The requirement to decrease amounts of metallic mercury used complicates the use of liquid mercury and liquid amalgams. Therefore, the paste and solid amalgams are preferable electrode materials for construction of working electrodes in agreement with so called “green analytical chemistry” requirements. That is why a major attention is being paid to the solid and paste types of electrodes. The metal/mercury ratio suitable for different types of amalgam depends on solubility of the metal in mercury. In the case of silver, the amalgam containing less than 10 % Ag (w/w) is liquid, 10 – 12 % Ag is a paste (advisable for paste electrodes) and more than 15 % Ag becomes quickly to be solid. The silver solid amalgam electrodes (AgSAE) are the most frequently used. Their surfaces can be mechanically polished or modified by mercury meniscus, electrochemically covered by mercury film or chemically modified by organic compounds (e.g. DNA, proteins, phospholipids, thiols). Silver solid amalgam was also used for preparation of silver solid amalgam paste electrodes (AgSA-PE) consisted from the solid amalgam fine powder and a suitable organic pasting liquid commonly used for preparation of carbon pastes. The paraffin oil mixed with the silver amalgam powder (1:20 (w/w)) was found as the most convenient [8]. A silver amalgam paste electrode without pasting liquid (AgA-PE) is another simple working electrode eliminating problems with passivation of the electrode surface by products of electrochemical reaction. AgA-PE is based on the paste amalgams (e.g. 10 – 12 % Ag (w/w)) which can be packed into disposable plastic pipette tips (2 µl) thus forming a disposable electrode for voltammetric methods. Recently prepared single crystals of silver amalgam were used for the construction of single crystal silver amalgam electrode (SCAgAE), where the crystal sticks out from the electrode body and makes a “cylindrical” electrode [9]. The most frequently used shape of working electrode is so called “pen shape/type/like” electrode. It consists of cylindrical polymer body made from polytetrafluoethylene (PTFE), polyetheretherketone (PEEK) or more advisable from a glass tube filled by an amalgam making a disc at the end of electrode. It is easy to prepare electrodes with different diameters. Large surface electrodes (diameter > 3 mm) are convenient for coulometric studies and the electrodes with diameter < 1 mm are commonly used in voltammetry. Preparation, pretreatment and

characterization of the common amalgam electrodes are summarized in review [7] including their classification, usable potential ranges in selected supporting electrolytes and their use for electrochemical studies and determination of various inorganic and organic substances. Direct current voltammetry (DCV), differential pulse voltammetry (DPV), square wave voltammetry (SWV) pertain to the most frequent voltammetric methods offering limits of detection (LoD) usually from 10-6 to 10-7 mol l-1. Nanomolar and even subnanomolar concentrations is possible to reach by cathodic stripping voltammetry (CSV), anodic stripping voltammetry (ASV), adsorptive stripping voltammetry (AdSV) or for chemically modified electrodes preferably using adsorptive transfer stripping voltammetry (AdTSV) or constant current chronopotentiometric stripping analysis (CPSA). Cyclic voltammetry (CV), elimination voltammetry with linear scan (EVLS) and potentiostatic coulometry provide information about electrode processes and reaction mechanisms at the electrode surfaces. The main advantages of the amalgam electrodes are their wide range of working potentials, simple regeneration of the electrode surface, rapid pretreatment procedure (5 â&#x20AC;&#x201C; 10 min), long-lasting activity without significant changes (years) in their sensitivity, mechanical stability, non-toxicity of the solid amalgams enabling the use in mobile laboratories, simple preparation of the electrodes with different size and shapes, simple design and mechanical stability making them compatible with measurements in flowing systems (HPLC, FIA etc.). Flowing systems with electrochemical detection present suitable sensitive and selective instrument for effective analysis. The polished silver solid amalgam electrodes (p-AgSAE) were firstly used in thin layer (Figure 1.2.1A) and wall jet (Figure 1.2.1B) arrangements of amperometric detectors for determination of nitrophenols mixture after their separation by HPLC [10]. Due to higher signal stability and lower signal noise it is more advisable to use thin layer detector, however the wall jet arrangement does not need special instrumentations and can be easily constructed in any lab. It is possible to use common three electrode system and an overflow vessel. This arrangement was already used for determination of 5-nitroquinoline by FIA-ED with LoD 4 Âľmol l-1 [11]. Promising results obtained at platinum microcylindrical and platinum tubular amperometric detectors inspired the construction of microcylindrical single crystal silver amalgam detector (Figure 1C) and silver solid amalgam tubular detector (Figure 1D) for amperometric detection of nitrophenol mixture and mixture of herbicides (acifluorphen, nitrophen and oxyfluorphen) after HPLC separation, respectively. Both detectors offer low background current, low noise, good signal stability and sufficient sensitivity for amperometric determination of electrochemically reducible organic substances. Thanks to the similar electrochemical behavior of amalgam electrodes (mainly p-AgSAE and mAgSAE) and mercury electrodes, electrochemically reducible substances at mercury electrodes are in many cases possible to determine at amalgam electrodes using the same methods. The cathodic signals are provided by reducible function groups containing polar or unsaturated bond in their structures (e.g. nitroso, nitro, azo, carbonyl, imine group. Well-arranged tables containing techniques, types of amalgam electrodes, supporting electrolytes and limits of quantifications of the determined organic substances are in the review [7]. Nitro, nitroso, azo, and oxo derivatives of aromatic or heterocyclic compounds and their derivatives are easily reducible and intensively studied in our UNESCO Laboratory of Environmental Electrochemistry. As stated above, DCV and DPV are the most common voltammetric methods for determination of reducible organic compounds with limits of detection (LoD) down to 10-6 and 10-7 mol l-1, respectively. Even lower LoD down to 10-8 mol l-1 can be reached by AdSV and/or using some preliminary preconcentration procedure (e.g. liquid-liquid extraction, solid phase extraction). Two cathodic waves or peaks of nitro compounds at mercury electrodes using DCV or DPV are observed at AgSAE as well in acidic and neutral aqueous medium. In alkaline medium, the nitro

Figure 1.2.1 - Detector flow cells with amalgam electrodes. THIN LAYER(A): (1) outlet, (2) hole for RE, (3) inlet, (4) steel block – CE, (5) PTFE gasket, (6) WE, (7) contact – Pt wire, (8) PTFE body of WE; WALL JET (B): (1) inlet, (2) overflow vessel, (3) outlet, (4) Pt wire CE, (5) WE, (6) RE; CYLINDRICAL (C): (1) WE, (2) PTFE tube; TUBULAR (D): (1) inlet, (2) contact – Pt wire, (3) WE, (4) sleeve – PTFE tube, (5) PTFE tube, (6) outlet.

group is irreversibly reduced to a nitro anion radical first (earlier also observed at silver and other solid electrodes) and than the nitro anion radical is reduced to hydroxylamine with the exchange of three electrons [12]. AgSAE was found to be suitable for the determination of submicromolar concentrations of a number of azodyes [13]. Voltammetric and amperometric methods using AgSAE for detection of selected nitro and/or heterocyclic herbicides were developed with LoD in the range of 10-6 - 10-8 mol l-1. Different types of AgSAE were utilized for the determination of 1,3-dinitrobenzene and 2,4-dinitrotoluene, 2- and 4-nitrophenol, 2-methoxy-5-nitrophenol, and 2,4-dinitrophenol. A mixture of selected nitrophenols was also determined by means of an amperometric detector utilizing p-AgSAE in a wall jet and thin layer arrangement after HPLC separation. 2-Methyl-4,6-dinitrophenol, Pendimethalin, Bifenox [14,15], Nitrophen, acifluorphen and oxyfluorphen [16] were also determined in mixture by HPLCED using wall jet and tubular AgSAE. Bipyrylidium herbicide Diquat was determined using dental amalgam electrode and the developed method was used for determination of Diquat in water samples and potatoes. [17]. Examples of easily electrochemically reducible antineoplastic drugs are derivatives of nitrosourea. Carmustine and Lomustine were determined using voltammetry at m-AgSAE with LoD 10-7 mol l-1 [18]. FIA and HPLC systems with amperometric detection in wall jet arrangements were successfully used for fast and automated analysis of high number of Carmustine samples with LoD around 10-6 mol l-1 [19]. Structurally similar Streptozotocin was also determined at SAE. The

nitroso group is irreversibly reduced with the exchange of two electrons to hydroxylamino group in the whole pH range. Flutamide containing electrochemically reducible nitro group was determined by DPV at m-AgSAE with LoD at micromolar level [20]. Flutamide and its metabolite 4-nitro-3trifluoromethylaniline were also determined at HMDE in model samples of urine. Preliminary separation and preconcentration using solid phase extraction enabled to reach even lower LoD around 10-7 mol l-1 [21]. Voltammetry at m-AgSAE was successfully used for determination of Lomustine, Flutamide and Azidothymidine in commercial drugs CeeNU (Lomustine 40 mg, ApoFlutamide 250 mg and Zidovudina 100 mg). Anticancer drugs Daunomycin and Doxorubicine were also electrochemically investigated at SAE. Daunomycin was determined by DPV at AgSAE and AuSAE. Adsorption of Doxorubicine at m-AgSAE was used for its determination by AdSV with LoD around 10-8 mol l-1. Doxorubicine was also determined by DPV in concentration range 110-6 – 510-5 mol l-1 in urine after its extraction to chloroform [22]. It can be concluded that the solid amalgam electrodes represent suitable alternatives to the mercury electrodes or they are in some cases even more preferable. Wide range of applications (determination of selected derivatives of aromatic hydrocarbons, azo dyes, herbicides, pharmaceuticals) in batch measurements as well as in flow systems were described. Last but not least, the future of the solid amalgam electrodes is evidently connected with the development of selective and sensitive sensors for flow systems and/or proteomic analysis. Acknowledgment Authors thank the Grant Agency of the Czech Republic (Project P206/12/G151) for financial support. REFERENCES 1.

Barek, J.; Fischer, J.; Navratil, T.; Peckova, K.; Yosypchuk, B.; Zima, J. Nontraditional electrode materials in environmental analysis of biologically active organic compounds. Electroanalysis, 2007, 19(19-20), 2003-2014.

Yosypchuk, B.; Novotny, L. Nontoxic electrodes of solid amalgams. Critical Reviews in Analytical Chemistry, 2002, 32(2), 141-151.

Yosypchuk, B.; Novotny, L. Electrodes of nontoxic solid amalgams for electrochemical measurements. Electroanalysis, 2002, 14(24), 1733-1738.

Mikkelsen, O.; Schroder, K. Dental amalgam in voltammetry some preliminary results. Analytical Letters, 2000, 33(15), 32533269.

Mikkelsen, O.; Schroder, K. H. Alloy electrodes with high hydrogen overvoltage for analytical use in voltammetry. Some preliminary results. Analyst, 2000, 125(12), 2163-2165.

Novotny, L.; Yosypchuk, B. Solid silver amalgam electrodes. Chemicke Listy, 2000, 94(12), 1118-1120.

Yosypchuk, B.; Barek, J. Analytical applications of solid and paste amalgam electrodes. Critical Reviews in Analytical Chemistry, 2009, 39(3), 189-203.

Danhel, A.; Yosypchuk, B.; Vyskocil, V.; Barek, J. In: New silver solid amalgam paste electrode for environmental application, 6th Spring Meeting of the International Society of Electrochemistry, Foz do Iguaçu, Brazil, 2008; Foz do Iguaçu, Brazil, 2008; p 131.

Danhel, A., Mansfeldova,V., Janda,P., Vyskocil,V., Barek, J. Crystalic silver amalgam – a novel electrode material. Analyst, 2011, 136, 3656–3662.

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Barek, J.; Dodova, E.; Navratil, T.; Yosypchuk, B.; Novotny, L.; Zima, J. Voltammetric determination of N,N-dimethyl-4-aminecarboxyazobenzene at a silver solid amalgam electrode. Electroanalysis, 2003, 15(22), 1778-1781.

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1.3. Carbon paste electrodes for monitoring the environment 1.3.1. Introduction It is already more than 55 years when carbon paste electrodes were introduced into the electrochemistry and electroanalysis [1. Since then, many thousands of papers appeared in the literature covering different fields of electrochemistry which are well described in numerous reviews, books and book chapters, or papers in journals, the nice example of it being paper by Švancara et al. [2. The success of carbon pastes is connected not only with the possibility to study oxidation reactions (as a complement to polarography to some extent) but primarily with the ease of carbon paste preparation in each laboratory from carbon (graphite) powder and a suitable liquid binder [2-4. Moreover, the availability of carbon powders of various sizes, shapes and surface properties together with a vast choice of binding pasting liquids which enable the preparation of the electrode with controlled properties, present so many advantages and special electrode or sensor properties that make carbon paste electrodes one of the most successful materials in electroanalysis with thousands of applications reflected in thousands of papers in the field of electrochemistry and electroanalysis. The success of carbon paste electrodes is connected with the possibility to chemically or biologically modify its composition and to tailor their properties what brings another attractive feature for their practical utilization. At least some references are abstracted, commented, documented in tables, and described in numerous reviews, from these reviews at least some review articles could be cited [5-9. In this chapter, the emphasis on preparation, properties and utilization of bare carbon paste electrodes and chemically and biologically modified electrodes will be laid, both in the field of determination of organic and inorganic analytes using batch methods like voltammetry, and as sensors in flowing methods like high performance liquid chromatography and capillary zone electrophoresis. The intention is to provide basic overview in the field of carbon pastes utilization in electroanalysis enabling the reader to get information about the advantages and disadvantages of carbon pastes [5-13. 1.3.2. Development, preparation, and characteristics of carbon paste electrodes From the very first report on the carbon paste electrode [1] for several years, only bare (unmodified or native) carbon pastes were studied from the physical chemistry points of view of their basic characterisation and utilization in analytical determinations of both organic and inorganic analytes. In the half of the sixties of 20th century, the first papers on chemical modifications of carbon pastes and thus alterations of their physico-chemical and electrochemical properties appeared, e.g. [14], later reviewed in [10]. Then, relatively slow expansion of bare carbon paste electrodes and chemically modified carbon paste electrodes (CM CPE) in electrochemical laboratories followed which was among other things supported also by the Adams book on electrochemistry at solid electrodes [15]. This expansion was supported and enhanced by the development of pulsed electrochemical techniques enabling marked decrease in limits of analyte detection. In 1988, carbon paste electrodes modified with enzymes as a new type of biosensors applicable in analysis of organic substances and biologically important compounds, as well as some inorganic ions and molecules appeared, with the first application in flow injection analysis [16]. These biologically modified carbon paste electrodes (BM CPE) became intensively studied as the enzyme incorporation into the paste brought the selectivity for the analyte determination and accounted further impulse for the field development. Another contribution was connected with the introduction solid heterogeneous carbon composites and screen-printed

electrodes (SPE) [17], later used for inorganic ions [18]. The end of 20th century and first decade of this century is characterised by the utilization of carbon pastes in new methodologies in instrumental analysis as in environmentally friendly procedures, tasting electronic tongues, with the use of new carbon forms such as fullerenes, nanotubes, doped diamonds), hybrid inorganic/organic films, ionic liquids, nanomaterials, etc. [19-23]. The properties of carbon pastes reflect properties of carbon powder, of pasting liquid binder and of the modifier or modifiers, which create the base for their favourable mechanical, physico-chemical, and electrochemical properties. The most simple carbon paste electrodes are considered as a special type of solid carbon electrodes which are ranked among heterogeneous carbon electrodes. Classical CPEs which are made from spectrographic graphites and liquid organic binders are characterised by their chemical and electrochemical inactivity. The simplicity of their preparation makes them up to now the most frequently CPEs used in analytical application and first choice for studying the oxidation properties of studied analytes. Other type of CPE utilizes as a pasting liquid an electroactive binder. These CPEs with electroactive binders (CPEEs) form the group of usually strong inorganic electrolytes and they have some interesting properties which are of use especially in physico-chemical applications but their higher baseline currents complicate their efficient use for analytes determination which could be seen in much lower number of papers dealing with them. CPE electrodes could be prepared with various densities (consistencies) which correspond mainly to the carbon-to-pasting liquid ratio used but also to the density or viscosity of pasting liquid. Dry pastes are more difficult to protrude from the electrode bodies which could be risky for the body when pushing the paste by a piston out from the body. Wet pastes are more difficult to deal with when preparing the electrode but have advantages when using them in mixed aqueous-organic solvent media which is normally detrimental for classical pastes. Special types of SPEs are solid pastes which could be prepared from easily melted binders or very viscous compounds as rubbers. These CPEs more resemble solid glassy carbon electrodes rather than traditional soft CPEs, and their cleaning too. Modified CPEs (chemically or biologically) are except from carbon powder and liquid binder comprised from an additional component(s). This modifier imposes a new character of CPE, thus bringing higher selectivity, higher sensitivity, higher repeatability or other use characteristic not experienced by bare CPE. The modifications are done by numerous ways. The simplest modification is mechanical admixing of modifiers in a liquid or a solid state into the bare carbon paste during or after its preparation, such a method being probably the most frequently used. Other possibility is the preparation of carbon pastes made of chemically active binder. As typical cases pastes containing liquid ion-exchangers or esters of organic acids could be mentioned. The binders then actively take part in ion-pairing processes leading to accumulation of the analytes from the solutions and thus decreasing the limits of detections. Also admixing of lipophilic compounds to the composition of the paste is frequently used with the intention to help the accumulation of hydrophobic compounds from the solutions on the surface of the electrode. Modifying reagents could be inserted into the paste by impregnating the carbon particles with a chosen modifying liquid, evaporating it, and then preparing the paste from the modified particles. When working with BM CPE, very frequent method of CPE modification (apart from admixing the enzyme or whole cells containing the enzyme directly into the paste) is the immobilization of the enzymes in additional membranes. Nevertheless, these membranes can also serve as protecting layers keeping the electroactive surface of CPE free from passivating components from the media. As the modification of carbon paste in situ is considered the situation when the reagent of choice is present in the solution during the measurement when the formation of a reaction product with the analyte takes place, with much favourable final analytical properties when determining this product. As modifications of CPEs or better surface alterations of CPEs are considered cases where the CPE is chemically pre-treated which could lead to effectively â&#x20AC;&#x153;newâ&#x20AC;? electrode material with generated

active groups on the electrode surface and tailored properties for given task. Finally, the electrolytic pre-treatment of CPE by keeping it at high anodic or cathodic potentials or cycling it between high anodic or cathodic potentials is another way of CPE modification. During this pretreatment the surface generation of gases removes either part of the binder or passivating products of former electrode reactions, alterating or re-generating the electroactive surface, making it ready for new measurements For preparation of bare carbon pastes, common spectral graphite powders were used at first. These powders fulfilled the best the requirements for mechanical and electrochemical properties of the electrode. They are produced with particle size in micrometres, and of quite uniform size distribution; they are pure enough a have low adsorption properties. Later also other carbon materials have been tested as the electroactive part of the paste, e.g. glassy carbon powders or spherical microparticles of glassy carbon, acetylene black, pulverised boron doped diamonds, single-walled or multi-walled carbon nanotubes, nanoparticles of graphite or at present also graphenes. As binding liquids for carbon pastes, frequently organic liquids are used with the main task to form the connective medium for carbon particles, whose main function is connecting mechanically the individual carbon particles. The binders should be chemically and better also electrochemically inert with proper viscosity (usually higher) and low volatility. Quite important is also their low solubility in aqueous or mixed aqueous-organic solvent solutions, and limited miscibility with pure organic solvents. Frequently used are mineral (paraffin) oils, and silicon oils. Some other binders have been tested and successfully used in the determination of various analytes but much less frequently. Among them organic esters, halogenated hydrocarbons, or wax-like polymers or resins or ionic liquids could be named. As was unexpectedly found, carbon paste based on spherical microparticles of glassy carbon are much more resistant to organic solvents than classical pastes based of spectral graphites with irregular particle shapes. The reason probably lies in the more stable structure of carbon paste formed by spherical microparticles sticking more densely to each other while allowing the replenishment of pasting liquid from the depth of the paste, which was removed by organic solvent and thus lost in the medium. In Figure 1, there is the SEM photograph of the two pastes based on either spectral graphite, or spherical microparticles. Although there exist major differences in shapes, sizes and densities of carbonaceous materials and major differences in densities and viscosities of binding liquids, the optimum carbonď&#x20AC;toď&#x20AC;binding liquid ratio is usually within the range of about 1.0 g : 0.5 ml. Though the carbon pastes are commercially available, in the vast majority of laboratories working with carbon pastes hand mixing and manual preparation of the CPE is common. The paste preparation consists of thorough and intimate hand mixing of the components in an porcelain or agate mortar using pestle, repeating the mixing and collecting the paste from mortar walls for several times, thoroughly homogenizing the paste several times. When the paste is homogenized enough the electrode body is carefully filled by the prepared paste. Moreover, it is recommended to leave this freshly prepared paste in the electrode holder to finish the homogenisation spontaneously and to start using it for measurement the following day. When in excess, the prepared paste could be stored in small plastic bags till the final use in several weeks period from the paste preparation. It is recommended to keep the CPE immersed in small amount of distilled water when not working with it overnight, just to prevent the losses of pasting liquid. When working daily with the CPE, it is not necessary to always immerse it into the distilled water, especially when working with viscose binders. Actually, the immersing of CPE into the distilled water could lead to water getting along the walls of the electrode body up to the piston which could complicate work with CPE when only small column of paste remains in the holder. The electrode bodies or holders are usually made

Figure 1.3.1 - SEM photographs of carbon paste based on spherical microparticles of glassy carbon (upper part) and carbon paste based on particles of spectral graphite (lower part)

Figure 1.3.2 - A set of necessary tools or requisites for preparation of carbon paste electrode. The purpose of each item is evident or described in the text

from inert plastic tubes or glass tubes, much more frequently the plastic ones, often made from Teflon. In Figure 1.3.2, a simple set for preparation of carbon paste, its loading, filling, or cleaning the electrode, is depicted. The usually stainless steel pistons enable fast carbon paste renewal by carefully and slowly screwing the piston and pushing the paste out of the electrode body, and then wiping it off by wet filter or weighing paper. Other electrode types are also available, so that carbon paste miniaturized electrodes, microelectrodes and arrays of ultramicroelectrodes, or carbon paste flow-cell detectors in either wall-jet or thin-layer configuration are successfully used in the electroanalysis of both organic and inorganic analytes. Especially the linkage of highly sensitive electrochemical principals of detection with high separation power of HPLC or capillary electrophoresis seems to be very promising and competitive technique. Another advantage of carbon paste detectors used as

detectors in flowing liquids (HPLC, capillary electrophoresis, flow injection analysis, sequential injection analysis) is the fact that there is practically no passivation of the electrode surface because the surface is most of the time washed by flowing mobile phase or running buffer with no electrode reaction going on so that the products of former electrode reactions are efficiently washed away and are not decreasing the peaks. When working in mobile phases with very high content of organic modifier as acetonitrile or methanol (around 80 % of organic part) the opposite problems appears. The removal of pasting liquid by the organic phase is not compensated enough by bringing the pasting liquid from the depths of the paste to the surface so that larger area of carbon is accessible for the analyte and higher peaks are measured (up to 15 % when comparing the peak height at the beginning of the measurement and peak height following the continuous 3 hr measurement without renewing the electrode surface). Figure 1.3.3 shows CPEs designed for work in volumes of 5 to 10 ml, 1 ml, and 100 Âľl. The principal of using CPE in flow methods and a photograph of a real configuration is depicted in Figure 1.3.4.

Figure 1.3.3 - CPE bodies used for batch voltammetric methods in volumes of 1 ml and 5-10 ml (left), minielectrode immersed in a volume of 1 ml (center), and pipette tip electrode designed for volumes around 100 Âľl (right)

Figure 1.3.4 - Scheme of CPE sensor in wall-jet configuration for flow methods and a photograph of the CPE in an overflow vessel, a carbon paste, b reference electrode, c auxiliary electrode, d mobile phase in overflow vessel, e inlet capillary

Carbon paste electrodes exhibit properties typical for carbonaceous materials, so that they have the several advantages, and of course, also some disadvantages. The advantage of CPEs is their relative chemical inertness which is in fact one of the requirements for successful use so they are resistant to chemical transformations when not working under extreme conditions involving either very high or low pH values or at very positive or negative potentials. The potential window of CPEs or CN CPEs during faradic measurements involving electron transfer and current flow is in the range of approx. 1.1 V to +1.1 V vs. SCE in near neutral media, certainly depending also on pH of the media as in alkaline medium the reduction potential could be even more negative and in acidic medium the oxidation potential more positive, in comparison with the values indicated. Proper choice of particular carbon paste constituents or special modifiers could also extend the potential range in both anodic and cathodic direction. Further advantage is the possibility to easily modify the paste composition which could result in increased adsorption of the analyte on the electrode surface utilizing either adsorption phenomena, extraction of the analyte into the paste and ion-pair formation, which all contributes to decreasing of limit of determinations. The heterogeneous structure of common carbon pastes can be described as a solid dispersion of carbonaceous particles in a liquid binder. Some papers confirmed that carbon paste surface forms a kind of array of carbon microparticles connected together by pasting liquid. The molecules of pasting liquid on one hand form a very thin film on carbon particles decreasing the electroactive area of the electrode, on the other hand they keep the paste together ensuring the contact between carbonaceous particles. One of the most pronounced advantages of classical CPEs is their very low ohmic resistance (usually around 5 to 15 ) for carbon paste “columns” of about 1 to 2 cm in diameter and 5 cm long). This outstanding conductivity of carbon pastes contributes to low baseline currents which is another favourable characteristics of carbon pastes as it allows measurements of quite low current signals on reasonably low and stable background current. The typical feature of carbon paste electrodes is due to the properties of mostly utilized binding liquids. Hydrophobicity of electrode material promotes adsorption of hydrophobic analytes from the analysed solutions on the electrode surface, thus increasing the sensitivity of the analyte determination. On the contrary, the hydrophobic electrode surface leads to irreversible character of redox reactions at these electrodes which may result in lower and broader current peaks, thus decreasing the sensitivity of the analyte determination. Probably the most serious disadvantage of carbon pastes is their already mentioned instability in organic and mixed aqueous-organic solvent media with high content of organic solvents, especially the most frequently used methanol or acetonitrile. This is especially true for pastes based on classical spectral carbon of irregular particle sizes. These pastes in the presence of ca 30-40 % methanol or acetonitrile decompose very quickly which is almost immediately manifested in spikes and disturbances of current line and final complete disintegration of the paste which falls out of the electrode body, the electrical contact with the piston is lost and the measurement is disabled. Partial solution of this problem is offered by pastes based on spherical microparticles of glassy carbon and one of the ordinary binding liquids. The problem of slow washing off the liquid binder to some extent persists but the paste keeps together, with suspected re-supply of pasting liquid from the depths of the paste. Most importantly, no spikes or disturbances on current lines are recorded up to ca 80 % of methanol or acetonitrile in mobile phase, running buffer or electrolyte. Partial problem connected with the use of carbon pastes is also their ageing. Why partial? Because the constructions of electrode bodies usually enable the measurement of ca 100 to 200 curves and then new paste must be filled into the body. CPEs and CM CPEs (especially when inorganic modifiers are used) could be stored under proper conditions (in plastic bags, in refrigerator…) for weeks, sometimes for months. The other question is the stability of BM CPEs, especially when enzymes are used for the modification. Also here some authors claim reasonable stability of these BM CPEs or biosensors being at least one month, with probable one digit decrease in sensitivity.

Nevertheless, the laboratory preparation of all kind of carbon pastes is so easy, fast and not complicated that possible paste ageing presents no obstacle for their use. Carbon paste electrodes are utilized in electroanalytical chemistry mainly for the electrochemical processes of both faradic and no-faradic character. It is possible to state that oxidation reactions are much more frequently utilized for analyte determinations than reduction reactions. Although the potential window available is almost of the same with for cathodic and anodic reaction, the measurement at cathodic potentials is complicated by oxygen content in the pastes. Oxygen is adsorbed on particles of carbonaceous material and thus passes with it into the paste and is then reduced giving broad signals which often coincide with analyte signals. Attempts have been made to get rid of the adsorbed oxygen from carbonaceous particles e.g. by thermal treatment or by preliminary electrochemical reduction but with no marked success. The reason is either in a way of paste preparation with the access of oxygen which adsorbs readily on the particles again, or in a latter case in substitution of the reduced oxygen on the electroactive surface by oxygen migrating to the surface from the depths of the paste. At carbon paste electrodes, electrolytic processes of both inorganic and organic compounds could be studied. These processes involve transfer of one or more electrons, which are followed by various voltammetric, amperometric, potentiometric or coulometric techniques. In electroanalysis of organic compounds, CPEs have been used for the determination of numerous classes of compounds including amino derivatives of aromatic hydrocarbons, hydroxy derivatives of aromatic hydrocarbons, thiols, nitro derivatives of aromatic hydrocarbons, carboxylic acids, amino acids, peptides, sugars etc., which will be discussed and documented below. These analytes very often belong to important classes of biologically active organic compounds like pharmaceuticals, environmental pollutants, pesticides, herbicides, carcinogens, etc. Inorganic analysis with CPEs is mainly focused on the determination of heavy metal ions, often using anodic stripping voltammetry, as heavy metals are also a priority group of analytes from the point of view of the importance of their monitoring in the environment. The reaction kinetics at the carbon paste electrodes could be tailored by paste incorporation of modifiers or electron transfer mediators into the paste bulk. At bare CPEs, the typical behaviour of organic analytes is irreversible or in some cases quasi-reversible, similar behaviour is found also for inorganic analytes. Nevertheless, incorporation of some mediators and electrolytic hydrophilisation of the electrode surface could result in potential shifts +/ď&#x20AC;500 mV, depending on the type of reaction. This is especially true when working with CPE bio sensors with electron transfer mediators. The analyte signal is moved from the high potential region (with already high background currents) to lower potential, thus bringing at the same time the increase in selectivity, and usually in sensitivity, too. Non-electrolytic processes at CPEs are used also in the applications of potentiometry, chronopotentiometry, conductometric measurement or electrochemically evoked chemiluminescence measurements. In efforts for decreasing the limits of detection the major role is played by the accumulation of the analytes from the solution on the electrode surface. The combination of adsorption properties of carbonaceous materials (remember the absorption and adsorption properties of charcoal) and of hydrophobic binders forms one base for success of CPEs in analysis of organic compounds. In analysis of inorganic compounds, adsorption plays minor role with more pronounced importance of electrostatic effects, ion pair formation or analyte extraction. Still, when complexes of inorganic metals with voluminous ligands are to be determined then adsorption could also play the decisive role in increasing the sensitivity of analytical method developed. Ion pair or ion associates formation utilizes electrostatic interactions between charged analytes and charged reagent counter ions which are present either in the paste as the modifiers or dissolved in the solution. The typical application of ion pairing or ion exchange process is the incorporation of an ion exchanger into the paste and then determining oppositely charged analyte ions following their electrostatic immobilization on the electrode surface, their pre-concentration as a result, and then the

determination. This principle is utilizable for both inorganic analytes as e.g. halides or complexes, and for organic compounds with ionisable groups. As ion-exchanger pasting liquid could also serve. Suitable pre-concentration mechanism is also extraction of the analyte into the paste. Extraction into the carbon paste is a very efficient tool for the analyte accumulation on the CPE but in some cases it is also the source of problems if the analyte penetrates too deep. As a result, very sensitive analytical methods could be developed but too deep penetration leads to very high paste consumptions as for each new measurement it is necessary to remove quite long piece of paste (up to more than 1 mm). This was the case of determining doxorubicin in our laboratory [24]. The extent of the penetration of an analyte into the carbon paste depends mainly on the nature of liquid binder and is in a good agreement with the rule similia similibus solvuntur. As the important feature of extraction it is possible to name non charged character of the analyte. This is easy to ensure by proper choice of media pH depending on possible moieties present in the analyte structure. As a special type of accumulation, the intercalation of the analyte into the wisely selected modifier could be considered. Intercalating modifiers are of both inorganic and organic origin, where the most important characteristics of the modifier are similar dimensions of the cavities and of the analyte which must be also compatible with the groups present around and inside the cavity. The boom of nanomaterials of carbonaceous origin and other origins is reflected also in new applications dealing with the electrocatalytic phenomena. These materials either catalyse the electrode reactions or have very large active areas in comparison with usually micrometer sized particles so that much higher signal could be recorded and as a result much lower limits of detection reached. It is not extraordinary to determine nanogram analyte quantities, or differently written sub-nanomolar analyte concentrations. 1.3.3. Practical utilization of carbon paste electrodes 1.3.3.1. Inorganic analytes In electroanalysis of inorganic compounds, the ions, complex species and molecules are being determined using CPEs. As Svancara et al. state [4], more than 70 elements of the periodic table were already determined using CPEs. Various pre-concentration methods are often the integral part of the analytical procedure, just preceding the respective final voltammetric stripping method of determination. Also medium exchange is quite often applied in analysis of inorganic compounds as it enables to choose different accumulation conditions from the ones for the electroanalytical determination. Probably the most frequently studied group of inorganics is the class of heavy metals, among them the most frequently determined species being lead, copper, mercury, zinc, and cadmium, usually by direct voltammetry or anodic stripping voltammetry. Direct determination is sometimes complicated by the relatively high reduction potential of the respective cation as a consequence of high overpotential or irreversible reaction. The possible solution is to use metal plated CPEs as mercury, bismuth and to some extent also antimony films where the reactions proceed well, quickly, and reproducibly with good sensitivity of the respective method. From the above analytes, probably the most frequently studied metal is lead, which is connected not only with ease of lead determination on CPE but with quite broad occurrence of lead in the environment resulting from former extensive use of leaded petrol, and industrial use of lead, too. Electroanalytical methods of determination of noble metals using CPEs including gold, silver, platinum, iridium, palladium and other species were worked out and described. Practically all methods include an accumulation step with limit of determination in micromolar range, in some cases even in nanomolar concentrations or below.

Rather demanding are direct electrochemical determination methods for metals from the groups of iron, manganese and vanadium. The existence of several oxidation states and relative ease of transition among them further complicates the method development. Usually the use of various complex forming ligands is involved there. From these metals relatively high attention was paid to the determination of chromium (both Cr(VI) and Cr(III) which is, apart from reasonable sensitive anodic and cathodic stripping analysis of these species, connected also with the carcinogenic properties of solid Cr(VI) compounds and attempts to find a substitution for polarographic determination utilizing as the electrode material “questionable” mercury. Rare earths metals and metals of the third and fourth group were successfully determined using CPEs, too. Aluminium, titanium, zirconium, and gallium oxidation states form stable complexes with many electroactive ligands so that indirect methods of their determination are numerous. Although indirect methods for the determination of rare earth metals exist, too, they could probably never compete with spectral methods which are the first methods of choice for them. Practically the same is valid for metals from the group of alkaline metals and metals of alkaline earths. From the non-metallic species, ions, molecules or complexes could be determined. CPEs when used for analytes like halides (chlorides, bromides, etc.) halo-oxygen species (chlorates, bromates, iodates, etc.), pseudohalides (cyanides, rhodanides), sulphur containing species (sulphides, sulphites, sulphates, etc.), or nitrogen anion species (nitrites and nitrates) face not only severe competition from either already well-established traditional methods and newer separation methods like ion-exchange chromatography or capillary zone electrophoresis but the electroanalytical methods with CPEs are not too extraordinary sensitive. 1.3.3.2 Organic analytes CPEs are used in analysis of organic compounds for the determination of several classes of compounds, and especially those which contain amino, hydroxyl, and sulfhydryl groups on an aromatic or heteroaromatic rings. There are clearly reasons to use CPEs for this: a) the relative ease of oxidation of these compound at CPEs or CM CPEs, and b) the occurrence of these compounds in many areas of human beings activities as intermediates in industry, disinfectants, pharmaceuticals, pesticides etc., resulting in the abundance of these compounds in our environment. Among organic compounds, aromatic amines play important role. Anodic oxidation of primary amines like aniline and other derivatives of this kind proceeds with the exchange of one electron under the formation of radical cations, as in equation 1.3.1. C6H5NH2  C6H5NH2+ + e

(1.3.1)

Cation radical is usually not the last reaction product, as intramolecular rearrangements, dimerisations, polymerizations, alternative coupling and further oxidations leading to more than 1 electron exchanges could occur, and they really often occur. The electron-deficient monocation radical is thus either stable enough during the time interval of particular electrochemical reaction in particular medium or the above mentioned reaction proceeds. The relative stability of monocation radical could be ensured by high degree of charge delocalization in radical, by blocking the reactive sites in the radical or by stabilization of the radical by specific functional moieties. When not stable enough, then the non-stability of the monocation radical usually results in chemical follow-up reactions by well-known EC, ECE processes (mechanisms), or other processes

involving another direct electron transfer without chemical reaction. The final products of the reaction pathway of aniline oxidation by ECE mechanism is p-aminodiphenylamine and benzidine. In [25], the determination of 2-, 3-, 4-aminobiphenyls using DPV at CPE with limits of determination in sub micromolar range is described. The selectivity of the method was improved by preliminary separation using liquid-liquid and solid phase extraction. Cyclodextrin modified carbon paste based electrodes as sensors enabled the determination of carcinogenic polycyclic aromatic amines, namely 1- and 2-aminonaphthalene and 2-aminobiphenyl [26] with nanomolar limits of determination. Beta-cyclodextrin was found as the most successful modifier of CPE composition which enabled the most efficient accumulation of the analytes on the surface of the paste, in comparison to alfa- and gamma-cyclodextrin. Direct oxidation of mother aromatic hydrocarbons usually proceeds at potentials pretty above 1 V (vs. SCE or Ag/AgCl reference electrodes, and depending on medium pH used) but much easily oxidizable amino derivatives of aromatic hydrocarbons or heterocyclic aromatic compounds are more often the target analytes when utilizing CPE in electroanalysis. Quite a lot of attention was paid to the determination of detrimental substances in drinking water (and surface waters) in [27], especially to environmental pollutants and toxic substances of biological importance, and among them to amino derivatives of aromatic hydrocarbons. Some amino derivatives of aromatic hydrocarbons belong to the group of proven carcinogens [28], therefore, they are largely monitored in the environment utilizing mainly efficient separation methods like GC, HPLC or electrophoresis with MS or other types of detection. High sensitivity of electrochemical methods could be utilized usually only in connection with preliminary analyte separation from the matrices using either solid phase extraction or liquid-liquid extraction, or in analysing quite simple matrices like various pharmaceutical forms where the interfering substances are not present. The oxidation of phenolic compounds with two hydroxyl groups present on the aromatic ring proceeds irreversibly with the exchange of two electrons under the formation of hydroquinones, (see equation 2) more easily when the hydroxyl groups are in either p-position or o-position. The presence of an electrophilic group on the ring is favourable for the reaction with facilitating the course of the reaction. Still, the reaction mechanism of substituted divalent phenols proceeds in the same way. Similarly, as in the case of aromatic amines also some hydroxyl derivatives are oxidized to unstable products of quinoid compounds which could either undergo hydrolytic reactions or form oligomer products. The slow kinetics of phenolics reactions gives broad peaks, although the oxidation potentials of phenolic products are relatively low and often below 1 V vs. SCE. The ease of their oxidation mirrors the use of phenolic compounds as antioxidants which could protect the organisms from attacks of free radicals. When compounds with both amino and hydroxy group on an aromatic ring are determined on CPEs then their oxidation proceeds quite easily at potentials below 1 V vs. SCE. In some cases the oxidation signals could be seen the first one being the oxidation of the amino group, the second one being slower oxidation of hydroxyl group. The products of the reactions are quinoimines which could undergo further chemical reactions. It is possible to say that during the oxidation of these aminophenols quite complicated reaction mechanisms take place and the reaction products could not be easily identified. Quite frequently analysed compounds using CPEs are also catecholamines. This is because of the fact that they could be determined on CPEs relatively easily, catecholamines are popular analytes because of their neurotransmittant properties and CPEs could be miniaturized and used for determination of catecholamines even in complicated matrices where mechanical cleaning is necessary following each measurement; just this possibility CPEs are offering. The oxidation of catecholamines proceeds with the exchange of either one or two electrons with the formation of unstable aldehydes which could undergo further anodic oxidation to acidic products or could be reduced to alcoholic compounds.

It is necessary to mention that many phenolic compounds could be determined using enzyme reactions utilizing enzymes like polyphenol oxidase, laccase, tyrosinase etc. These applications will be mentioned in the part of chosen examples on the use of CPEs in analysis of biologically active organic compounds. CPEs were used also for the determination of compounds with sulfhydryl moieties. Especially cysteine was the frequent target the two electron oxidation of which results in cystine. Thiols usually severely passivate the electrode surfaces of the electrode used. The possibility of quick and repeatable renewal of the CPE working area is advantageous here. Very large part of the applications for carbon paste electrodes is connected with the area of biologically modified carbon paste electrodes sometimes called biosensors. These biosensors utilize either pure enzymes or enzyme producing whole cells contained in the pastes. Several generations and types of biosensors exist according to the mechanism of signal production, enzyme entrapment, type of enzyme used, presence of electron transfer mediator or use of two enzymes. These biosensors are used for voltammetric or amperometric determinations of sugars, alcohols, carboxylic acids, etc. At present, DNA sensors based on CPEs are becoming increasingly popular. DNA is used in these sensors as the recognition part of the sensor which is following proper accumulation of the analyte reacting, intercalating, or interfering with it which is used as a measure for evaluation of the DNA destruction which could lead to DNA breaks, destruction or other destructive effects leading to wrong DNA function with known consequences. The last to be mentioned here are immunosensors utilizing CPEs. The ease of modification of carbon paste is responsible for the development in this field where antigen or antibodies are entrapped in the paste or on the electrode or sensor surface giving the signal following the reaction with their immunochemical partner. Biosensors, DNA sensors and immunosensors with all their complexity are out of the range of this chapter and will not be discussed in detail here. The other reason is that except phenolics or some herbicides or pesticides, these analytes (sugars, carbohydrates, NADH, DNA and its bases, immunoglobulins) are not targets for environmental analyses. 1.3.4. Selected examples of CPEs utilization in electroanalysis In this part, some examples of the utilization of carbon paste electrodes in electroanalysis will be given with the intention to cover both the field of inorganic and organic compounds to choose either recent papers or papers dealing with the determination of quite popular (equals frequently studied and from various reasons determined) analytes in environmental matrices. It is just the illustrative choice. Table 1.3.1 lists the selected examples in alphabetical order with inorganic and organic analytes forming separate parts of the table. The majority of references is based on chemically modified electrodes, where the modifier is selectivity or sensitivity assuring component of the electrode system. The unifying feature is also use of the method developed for determination of the analytes in environmental samples. Table 1.3.1 - Some examples of CPEs utilization in electroanalysis Analyte

Working electrode

Medium

Method

Matrix

LOD

Ref.

Ag+

CPE

0.01M amonnia buffer,

LSV

wastewaters, ores

0.1 nM

As(III), As(V)

CM CPE

acidic

ASV

drinking water

ppb

medium

III, 2 ppb As V

AuIII

CPE

NaCl + HCl

DP CSV

tap and mineral water

0.8 M

Bi3+

CM CPE

0.05K KNO3 + 0.01 M HNO3

CV, DPV

alloy, waters

0.05 M

Cd2+

CM CPE

0.01M ammonia buffer, and 0,1M HCl

DPV

river water

5 ng ml-1

Cd2+, Pb2+, Tl+, Zn2+

CM CPE

0.01M HCl + Kcl/KBr

PSA

model samples

10 ppb Cd

Ce3+

CM CPE

AcB+ PhB pH 4-5

ASV

waste waters

0.8 nM 35

Cr(III)

CM CPE

pH 4.5-7.7, EDTA

POT

river water, content in drug

2 M

Cu2+

CPE

0.1M KNO3

ASV

waste waters

0.08 M

Fe3+

CPE

0.01M KCl + 0.01mM EDTA

CV, DPV

river water, tap water

2 M

Hg2+

CNT CPE

0.1M AcB pH 4

CV, SW ASV

water samples

0.4 M

Ni2+

CM CPE

0.005M HCL pH 3

AdSV

tap water, mineral water

1 g l-1

NO3-

CM CPE

HCl, pH 2-3

DP ASV

drinking water

CM CPE

0.01 M HCl

CV, DP ASV

water samples

6 nM

Sn2+ (SnIV)

CPE

0.1M AcB

DP ASV

waste waters, canned food

0.1 M

SO32-

ILM CPE

0,1M PB pH 7.0

CV, DPV

mineral water, juices, beers

4 M

1- and 2aminonaphthalenes, 2-aminobiphenyl

CM CPE

BRB pH 7.0 or 9.0

CV, DPV, AdS DPV

model solutions

below 1 nM

Amitrol

CNT CPE

0.05M PhB

AdSV

tap water

40 g l-1

anthracene, fluorene, naphthalene

CPE

0.1 M H2SO4, MeOH or AcN

environmental samples

0.2 nM

aromatic hydrazines

CoPc CPE

10 mM phosphate run buffer pH 6.5

CZE, Am 0.5 V (Ag/AgCl)

environm. water samples

0.5 M

catechol, phenol

Ir PPO CPE

0.05 M PB pH 7.4

Amp, -0.1 V

intended for environmental samples

to 0.1 M

chlorpyrifos

Sep CPE

BRB pH 2.0

AdS DPV

tap water, soil samples, juices

0.08 ppb

clofibric acid, diclofenac, ofloxacin, propranolol

CNT CPE

0.2 M NaClO4

CV, DPV, NPV

spiked river water, SPE enrichment

to 0.5 nM

cyromazine

CNT CPE

0.1M H2SO4

SW ASV

tap water

0.12 g mL-1

dinoseb, dinoterb

CPE, Sep CPE

BRB pH 4.0

AdS DPV

tap water, soil samples, juices

0.1 M, 0.5 nM

Dipyrone

CoPC CPE

0.1 M phosphate buffer pH 7

FIA, 0.3 V

pharm., environm. samples

ca 5 M

Estradiol

CNT CPE

0.1 M PB pH 7.0

CVA

environmental samples, rabbit blood serum

5 nM

Estrogens

CPE

60% ACN + 40% v/v Tris (5 mmol/L, pH 8.0

CEC-Amp

chicken eggs, milk samples

2 to 50 ng mL-1

fluorene deriv.

CPE

0.1 M H2SO4 or BRB pH 7

SWV, DPV

water samples

to 1 nM

Linuron

TCP CPE

BR buffer pH 2

DPV

river water

ca 5 M

Phenolics

laccase CPE

weakly acidic

Amp

pharm., environm. samples

2.4 M

phenol (Salmonella t.)

tyrosinase CPE

pH 7.0

FIA (ICEM)

carcass wash water

5x105 CFU/ ml

phenylhydrazine

Feroccene+ CNT CPE

0.1 M PB pH 7.0

DPV

water, urine samples

0.6 and 14 M

OP pesticides

whole cells modified CPE

50 mM citratephosphate buffer pH 7.5, 50 M CoCl2

AmB

waters

0.3 ppb

2,4,6trichlorophenol

polymer CPE

0.1 M AcB pH 5.0, 0.001 M H2O2

DPV, CV, oxidative dehalogenation

environm. wastes

25 M

1.3.5. Practical examples of method development using CPEs In this part, several examples of method development using CPEs and coming from the laboratory of the authors will be described and briefly documented by figures and results achieved with the emphasis on the practical utilization of the developed methods on samples of environmental origin. In [63], voltammetric behaviour of 3-aminofluoranthene (3-AF) in a mixed water-methanol (1:1, v/v) medium has been investigated at two types of electrodes - glassy carbon electrode (GCE) and a carbon paste electrode using differential pulse voltammetry and adsorptive stripping voltammetry. The anodic oxidation of 3-aminofluoranthene was studied in Britton-Robinson buffers pH 2 to 12 (before mixing with methanol), Figure 1.3.5. A

Figure 1.3.5 - Selected DP voltammograms of 3-AF (c = 1 × 10-4 M) at GCE (A) and at CPE (B) in BrittonRobinson buffer-methanol (1:1, v/v) medium of pH: 2.8 (1), 4.8 (2), 6.8 (3), 8.9 (4), 10.9 (5), and 12.2 (6); 1st (p1), 2nd (p2, major peak), 3rd (p3), and 4th peak (p4)

The reaction mechanism of the analyte oxidation is quite complex, nevertheless, one peak is always the major one so that either first or second peak could be successfully used for the analytical purposes. Under the optimum conditions, i.e. Britton-Robinson buffer pH 4 in a mixture with methanol (1:1), the calibration dependences of 3-aminofluorathnene were measured in the concentration range 2 × 10-8 to 1 × 10-4 mol l-1. The recorded curves together with the calibration line are depicted in Figure 6. The achieved limits of quantitation for DPV at GCE were about one order of magnitude higher than that obtained for DPV at CPE. With accumulation time tacc = 120 s the limit of quantitation was 2 × 10-8 mol l-1. In comparison to GCE the advantage of CPE was the possibility of a noticeable improvement of sensitivity due to the presence of hydrophobic Nujol as pasting liquid, and thus the possibility of adsorptive pre-concentration of 3-AF at the CPE and easier renewal of this surface in comparison to GCE. It was also possible to decrease the content of methanol for the lowest concentration range to only 1 % methanol, see Figure 1.3.6.

Figure 1.3.6 - AdS DP voltammograms of 3-AF in methanol-BR buffer (pH 4) mixture (1:99) measured at CPE with Eacc = -150 mV and tacc = 120 s, c (3-AF): 0 (1), 2 × 10.8 (2), 4 × 10.8 (3), 6 × 10.8 (4), 8 × 10.8 (5), 10 × 10.8 (6) M. The corresponding calibration line is depicted in the inset

Quite a lot of attention was paid to the electroanalytical determination of amino derivatives of aminoquinolines at CPEs [64]. In this paper, optimum conditions have been found for voltammetric determination of 3-aminoquinoline 5-aminoquinoline, 6-aminoquinoline by differential pulse voltammetry and adsorptive stripping differential pulse voltammetry. The possibility to determine mixtures of 8-aminoquinoline (studied previously) with 3-aminoquinoline or 5-aminoquinoline or 6aminoquinoline, and mixtures of 5-aminoquinoline with 3-aminoquinoline or 6-aminoquinoline by differential pulse voltammetry was verified. Binary mixtures of 8-aminoquinoline with 3aminoquinoline or 6-aminoquinoline, and of 3-aminoquinoline with 5-aminoquinoline could be successfully analysed. It was impossible to analyse the other mixtures because of the similarity of the respective analyte peak potentials. With all the three analytes the influence of pH on DP voltammetric behaviour of them was investigated in Britton–Robinson buffers (BR buffers) in the pH range from 2 to 12 with the concentration of the analyte 1x10-4 M. In Figure 1.3.7, the measured curves are depicted for 5-aminoquinoline.

Figure 1.3.7 - DP voltammograms of 1x10-4 M 5-aminoquinoline on CPE in Britton – Robinson buffer pH 2 (1), 3(2), 4 (3), 5 (4), 6(5), 7 (6), 8 (7), 9 (8), 10 (9), 11 (10), and 12 (11).

The peak potentials of all studied analytes are close to each other, nevertheless in cases of reasonable potential differences between the respective peak potentials the attempts were made to determine binary mixtures. Only partial success was achieved, see Figure 8, thus differential pulse voltammetry could be used for analysis of binary mixtures of 8-AQ with 3-AQ, 8-AQ with 6AQ, and 3-AQ with 5-AQ (in limited ratios only). In the case of all other remaining aminoquinolines binary mixtures, the quantitative analysis using differential pulse voltammetry on CPE was not possible. For analysis of complex mixtures preliminary separation will be necessary or HPLC with electrochemical detection on CPE should be used. Nevertheless, the achieved limits of determination using adsorptive accumulation on the working electrode surface and then measurement using DPV were reasonably low. The lowest limits of determination were found for adsorptive stripping differential pulse voltammetry in 0.1M H3PO4 (5x10-7 M, 1x10-7 M, and 1x10-7 M for 5-aminoquinoline, 6-aminoquinoline and 3-aminoquinoline, respectively). As already mentioned above, one of the disadvantages of carbon paste electrodes is their limited stability in media containing high amounts of organic solvents, as is the case of mobile phases in HPLC. But the interconnection of highly sensitive electrochemical methods of detection of electrochemically active compounds with a high separation power of HPLC is frequently demanded. The mentioned spherical microparticles of glassy carbon in a mixture with appropriate pasting liquid are able to resist the paste decomposition so that HPLC-ED utilizing up to about 80 % of organic modifier could be successfully used even with carbon pastes. We have utilized HPLC with electrochemical detection using carbon paste electrode based on glassy carbon spherical microparticles with an organic pasting liquid for the determination of trace amounts of carcinogenic 1-naphthylamine (1-AN), 2-naphthylamine (2-AN), 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonaphthalene (1,8-DAN) [65]. In HPLC-ED following finding the optimum separation conditions, basically with UV detector at appropriate wavelength, the measurement of hydrodynamic voltammograms at pH range compatible with the used column follows is necessary. The influence of methanol content in the mobile phase consisting of phosphate buffer pH 3 on retention characteristics of the tested analytes was studied in the range from 50 to 70 % of methanol (v/v). In Figure 1.3.9, the hydrodynamic voltammograms of studied analytes are depicted.

Figure 1.3.8 - DP voltammetric determination of 8-AQ on CPE in the presence of 2x10-5 M 3-AQ in 0.1 M H3PO4. C (8-AQ): 6 (1), 8 (2), 10 (3), 20 (4), 40 (5) and 60 (6) mM.

Figure 1.3.9 - Hydrodynamic voltammograms of 1-AN, 2-AN, 1,5-DAN and 1,8-DAN (Kromasil 100-7 m, 250 x 4,6 mm column, 0.01 mol.L-1 phosphate buffer pH 3 : methanol (40:60, v/v) mobile phase, 1 ml.min -1 flow rate, injected 20 l solution of an analyte mixture in mobile phase with c = 1.10-4 mol.L-1 each)

It follows from the hydrodynamic voltammograms in Figure 1.3.9 that the optimum potential of +1.0 V could be chosen, at which the electrochemical sensor was further operated. The lowest limit of detection found was 5.410-9 M for 1,5-diaminonaphthalene. The limits of detection for all studied analytes using HPLC with electrochemical detection at carbon paste electrode were in all cases lower than corresponding limits of detection for spectrophotometric detection. The developed method of determination was verified on model samples of drinking and river water. Quite encouraging results were obtained during the study of the voltammetric properties and possibilities to determine carcinogenic amino derivatives of polycyclic aromatic hydrocarbons, namely 1- and 2-aminonaphthalene and 2-aminobiphenyl, on carbon paste electrodes modified with monomeric -, - or -cyclodextrin [26]. The modified CPE was prepared from 200 µl of the respective cyclodextrin stock solution in deionized water (5 mg ml-1) which were diluted by 1.0 ml of Britton-Robinson buffer pH 7.0 and added to 500 mg of the carbon powder in a mortar. The suspension was thoroughly homogenized by a pestle for 5 min. Then, the aqueous phase was allowed to evaporate at about 60 oC, white pharmaceutical vaseline was added as a binder (the modified carbon powder to vaseline mass ratio was 9 : 2) and the paste was made by thorough hand-mixing. When bare carbon pastes are used for the determination of amino derivatives of aromatic hydrocarbons then usually micromolar limits of determination are reached, when adsorptive accumulation is utilized, then the limits decrease by one or one and a half concentration order. From tested cyclodextrins the best accumulation characteristics were found for cyclodextrin. It was found, that the pre-concentration effect of monomeric -cyclodextrin is higher than that of - and -cyclodextrin which was reflected in lower limits of determination. The lowest calculated limit of determination using AdS DPV was 1x10-9 M for 2-aminobiphenyl, the highest was 9x10-9 M for 2-aminonaphthalene which is still quite low value. Similar approach to the modification of CPE was applied when developing the determination of aminobiphenyls. As modifiers either polyedric montmorillonite or octaedric sepiolite were used. Although the cavities sizes correspond to the analyte dimensions only relatively small increase of analyte signals was recorded with stirred adsorptive accumulation, thus indicating existing barrier for extraction or adsorption of the analyte by the modifiers used.

Carbon paste electrodes are used mainly with solution volumes from 5 to 10 ml. Nevertheless, miniaturized electrodes were also constructed (Figure 1.3.11).

Figure 1.3.10 - Accumulation time dependence of the DPV peak current for 2x10-7 M 2-aminobiphenyl (1), 1-aminonaphthalene (2) and 2-aminonaphthalene (3) at ď ˘-CD/CPE under optimized conditions in BrittonRobinson buffer of pH 7.0 (1) and 9.0 (2, 3).

Figure 1.3.11 - Photographs of miniaturized CPEs based on Teflon capillaries (left) and pipette plastic tips (right), in comparison to 1 Eurocent coin.

These CPEs in pipette tips or glass or Teflon capillaries could be easily and successfully used for measurements in volumes of about 100 Âľl. The electrode active surface renewal is done simply by pushing the paste from the tip and wiping it by wet filter or weighing paper or by cutting the capillary by a knife thus getting new electrode surface. This is in accordance with principals of

green chemistry where small volumes of possibly dangerous chemicals are used while keeping high sensitivity or sensitivity. 1.3.6. Conclusion Carbon paste electrodes clearly present the independent alternative to more frequently used separation methods, or as electrochemical detectors for separation methods like HPLC or capillary zone electrophoresis where they offer high sensitivity, selectivity and utilize the separation power of the above mentioned methods. Their advantage is inexpensive apparatus needed, the ease of renewal of electroactive surface. There already exist thousands of applications for the determination of both inorganic and organic species making solid base for their practical use in environmental, pharmaceutical or food analyses. The methods are suited for analysis of drinking, mineral, natural surface and waste waters, various types of food after clearly the proper type of analyte extraction from the matrix. Analysis of pharmaceuticals is also becoming an important issue with natural waters being burdened by extensively used antibiotics, compounds with endocrine disrupting properties etc. CPEs are suited for analysis of metals, inorganic compounds, biologically active organic compounds from the group of environmental pollutants, carcinogens, pesticides, herbicides, which increasingly worth of attention among analytical chemists or general public. REFERENCES 1

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Safavi A., Maleki N., Momeni S., Tajabadi F., Highly improved electro-catalytic behaviour of sulphite at carbon ionic liquid electrode: Application to the analysis of some real samples. Anal. Chim. Acta, 2008, 625, 8-12.

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1.4. Modern electroanalytical methods for environmental monitoring based on the use of boron doped diamond film electrode Nowadays it was well established that conductive diamond thin films are in many ways ideal as electrode materials. Boron-doped diamond (BDD) thin films introduced in 1992 by Fujishima (1) are most frequently used. In this chapter we pay attention to applications of BDD-based sensors in electroanalysis of organic compounds of environmental importance since the first proposal in 1993 (2). The applications of BDD electrodes in flowing systems started in 1997 for flow injection analysis with amperometric detection (FIA-AD) in thin layer cell (3). In 1999, the same detection cell was coupled with ion chromatography (4). In 1998, the first BDD microelectrodes (BDDÂľE) exhibited steady state cyclic voltammograms (CVs) (5) and five years later were used in capillary zone electrophoresis (CZE) (6,7), chip-based devices (8), or under in vitro/in vivo conditions (911). Aarrays of BDDÂľE (12) and a random array of BDD nano-disc electrodes (13) were described latter. To extend selectivity of BDDE, intensive research on surface oxidation (14) and other modifications was done. The easy electrochemical oxidation and the surprising inertness of such O-terminated BDD (OBDD) surface towards adsorption was shown (15) on the example of serotonin electrooxidation. Together with earlier reports on electrochemical properties of Oterminated surfaces (16) this drew attention to their use especially for electroanalysis of charged organic species. Further biofunctionalization of bare and oxidized diamond surfaces was enabled by introduction of carboxylic (21) and amino groups (22). Reviews on general electrochemical properties and electroanalytical applications (27) and surface modifications (28,29) appeared. The common BDD films used in electroanalysis usually grow on Si supports from dilute mixtures of a hydrocarbon gas (typically methane) in hydrogen using one of several energy-assisted chemical vapor deposition (CVD) methods, the most popular being hot-filament (HFCVD) and microwave plasma assisted CVD (MPCVD). BDD materials produced in research laboratories are gradually substituted by commercially available materials. The analytical techniques routinely used to characterize the morphological, optical, chemical and electronic properties of diamond thin films, include Raman, Auger electron and X-ray photoelectron spectroscopies, scanning electron micrography, scanning tunneling and force microscopies, powder X-ray diffraction analysis, and secondary ion mass spectrometry. BDD thin films possess several excellent electrochemical properties: Low and stable background current over a wide potential range, corrosion resistance, high thermal conductivity and high current densities. They offer superb micro structural stability at extreme cathodic and anodic potentials and resistance to fouling because of weak adsorption of polar species on the H- and Oterminated surface, which results in good responsiveness for many redox analytes without pretreatment. Besides other electrochemical applications of BDDE described in monograph (17), great attention is paid to their in electroanalysis as simple electrochemical sensors employed in voltammetric methods or coupled to liquid flow methods (HPLC, FIA, CZE) for detection of organic and inorganic species, or specialized selective applications of BDD-based bioelectrochemical sensors. The analytical applications of BDDE were reviewed (18-20). There is obvious prevalence of oxidizable analytes. The only determinations based on reduction were suggested for some nitrophenols and nitro-group containing pesticides and drugs and for cytochrome c. The popularity of BDDE for oxidizable substances is given by the wide potential window in anodic region. This enabled direct determination of aliphatic amines, polyaromatic hydrocarbons and sulfur containing analytes (e.g., aminothiols, disulfides) which are rarely detectable at conventional bare electrodes. The other advantage is the fouling resistance or easy removal of adsorbed reaction by-products and products by rinsing BDDE with appropriate solvent or treatment at high anodic or cathodic potential. Methods for problematic surface passivators (chlorophenols, nitrophenols and amino

group containing aromatics) were reported with signal repeatability typically better than 5 %. BDDE represents usually no exception on fouling problems when using batch voltammetric methods. Nevertheless, in contrary to other solid surfaces where the activation approaches rely either on in situ repetitive electrochemical treatment in the presence of various deactivating compounds (21,22), or on mechanical removal by polishing with diamond or alumina powder, simple regeneration of BDDE as described above is sufficient. Nowadays, there are at least six commercial suppliers of BDD materials and equipment, but many research groups still use BDD from their own sources. HFCVD and MPCVD reactors are also commercially available. The placement as the bottom of electrochemical cell requires foolproof sealing and has the disadvantage in the need of manipulation with the whole cell during measurements. In this case, the electrode area is given by the opening in the gasket and the ohmic contact made by placing the backside of the Si substrate on a conductive metal (brass, copper) plate. Similar principle is used in the pen-type holders, where the reusable Si/BDD disc is pressed against the gasket in the bottom part of the holder. These robust electrodes are easier to manipulate, nevertheless, they may also incline to leaking, especially in mixed aqueous-organic and nonaqueous media. As the BDD disc is dipped into the bottom part of the holder exposed to the solution, problems with bubbles sticking in the cavern may complicate the handling. The other approach relies on simple electrodes prepared by gluing of the Si/BDD disc onto a conductive plate (usually using an Ag paste) and insulating of all other parts by a suitable insulator. The amperometry coupled to FIA or HPLC is most frequently realized in home-made or commercial thin layer cells. The wall-jet arrangement with pen-type electrodes has been also tested (23). Voltammetric methods are used to investigate electrochemical processes at the electrode surface and as analytical tool for quantitation of analytes. In the former case, CV is most frequently used. Therefore, brief results on linearity of concentration dependences in a limited range without investigation of the lowest and high concentrations using CV or linear scan voltammetry appear in many studies devoted to other topics, e.g. electrochemical combustion, comparison of performance of BDD and other carbon electrodes or determinations using amperometric methods. In these cases, very often the LOD is not given or it is relatively high, in 10–5 to 10–6 mol L–1 range (24). The specialized electroanalytical studies use most frequently differential pulse and square wave voltammetry possessing the advantage of good discrimination against background current. The enhancement of analytical sensitivity by using an adsorptive step to preconcentrate the analyte into, or onto the working electrode, which is very popular at mercury and carbon electrodes (25) is in principle difficultly achievable due to the well-known adsorption resistivity of the BDD surface because of lack of adsorption sites. Slower kinetic in comparison to GC was demonstrated e.g. on the example of dopamine oxidation, which is catalyzed by hydrogen bonding of surface carbonyl to adsorbed molecules; these bondings are rarely present on H-terminated surface of BDD (HBDD). In contrary, adsorption on HBDD prepared by annealing of OBDD in hydrogen flame was proved for glucose, readily adsorbed on almost all electrode materials. The few examples of adsorptive stripping voltammetry (AdSV) for organic analytes using bare BDD surfaces rely in fact on determination of oxidation products of the analyte of interest. In the case of aniline these are dimeric species (p-aminodiphenylamine and benzidine) formed by its anodic oxidation during the accumulation period. These studies document that quantitative analysis using AdSV at bare BDD surfaces provides interesting results in infrequent specialized cases contrary to common applications of stripping methods for inorganic analytes. The other general strategy to increase the sensitivity – employment of the ultrasound – has also the advantage of overcoming potential electrode fouling problems. Both issues were appreciated in the sono-voltammetric determination of commonly surface passivating 4-chlorophenol and 4nitrophenol. Nevertheless, the possibility of BDD reactivation in situ using high anodic potential in the region of water decomposition favorits classical voltammetric measurements in simple

detection cells and wide-spread use of sono methods is not probable despite the fact that BDD shows usually no signs of mechanical damage under sonication. More frequently, chronoamperometric determinations in stirred solutions under potentiostatic conditions may be expected as suggested in several studies of Fujishima. When considering batch voltammetric methods, their selectivity is a big issue in complex matrices. In comparison to classical electrode materials with relatively narrow potential window, the wider potential window of BDD is not that big advantage, as the structurally relative group of organic compounds, which are often found together in an environmental or biological matrix, possess usually near oxidation / reduction potentials. Nevertheless, several reports appeared analyzing two to three component mixtures. Insufficient selectivity can be solved also by preliminary off-line separation of analytes using common extraction techniques, which complicates the analysis. Therefore, AD of mixtures of organic analytes in flowing liquids is preferred to batch voltammetric analysis because of lower problems with passivation (reaction products and intermediates creating the passivation films are removed from the electrode) and because of possible separation of complex mixtures using HPLC or CZE. BDDE offer several advantages compared to other solid electrodes used in flowing systems. Usually no mechanical or electrochemical pretreatment of BDDE is needed. The creation of passivation films is less probable due to decreased adsorptivity of reaction by-products and products at their relatively hydrophobic surface. The low electrostatic capacity of BDD surface minimizes the time to stabilize the background current prior and the current drift during AD. Thus, the background current stabilizes within seconds to few minutes after detector turn-on in contrast to solid, especially other carbon based electrodes, where it takes frequently about one hour to reach a constant current value. Less common is the CZE-AD coupling, as this requires the technically exacting miniaturization of BDDE and adaptation of the appropriate electrophoretic system. The surface termination contributes greatly to the physical and chemical properties of BDD and thus is of big importance for electroanalysis. Usually, the as-grown BDD electrodes produced commercially or in research laboratories are initially H-terminated as they are deposited in a hydrogen plasma CVD chamber. The HBDD surface was first believed to be responsible for the adsorptive inertness as shown by Swain on the example of polar 2,6-anthraquinonedisulfonate on intentionally hydrogenated glassy carbon and BDD surfaces. Surprisingly, the results of Fujishima indicated that the OBDD surface behaves differently from a polished GC electrode with oxygen surface groups and is also inert with respect to adsorption. Since that time the intensive research on oxidative functionalization of BDD surfaces resulted in interesting results for electrochemists and several comparative studies appeared on HBDD and OBDD. BDD surface oxygenation may be achieved by several methods including vapor phase oxidation in O2, oxygen plasma treatment, boiling in strong acid, oxidizing agent or radical oxidation, long-term exposure to air and electrochemical oxidation. The last methods is very convenient for electroanalysis, as no specific instrumentation is needed, the oxidation is simply accomplished either by anodic treatment of the BDD surface at high positive potentials or repetitive cycling in positive potential range. Under these conditions, the powerful oxidants OH radicals are produced from water at the BDD surface, which precedes the oxygen evolution having high anodic overpotential at BDD. The re-hydrogenation of an OBDD surface is achievable only by hydrogenflame annealing or hydrogen-plasma treatment. The structure of OBDD surface depends on the oxygenation technique and on the type of Sisupport. Based on the diamond structure, it is expected that the sp3 Câ&#x20AC;&#x201C;H bonds on the (111) facets are terminated with hydroxyl groups, while the CH2 bonds on the (100) facets are transformed to carbonyl and ether functional groups. By surface oxygenation the unique BDD properties are not affected, the OBDD surfaces are hydrophilic, have lower conductivity and relatively negative surface charge while the HBDD are hydrophobic and have high conductivity.

The advantages of the OBDD electrodes include somewhat wider potential window, higher surface stability to fouling and the possibility of on-line reactivation by applying a highly anodic potential, which enables the oxidative destruction of the adsorbed species. The preference of HBDD or OBDD surface for electroanalysis of some analytes was announced, while for the others negligible differences were reported. Compounds with positive charge may be more easily oxidized at OBDD than at HBDD due to the electrostatic attraction between these compounds and negatively charged OBDD. A typical example is the shift of response of oxidized aminothiols. Also the redox species with negative charge are sensitive to the surface oxygenation, exhibiting slower electron transfer. Anodic peaks for such species were more clearly observed at a HBDD than at an OBDD electrode due to existence of the electrostatic repulsion between the analyte and the negative charge on the electrode surface. Decreased adsorbability of oxidation products on OBDD in comparison with HBDD may favorite the former surface as reported for di- and trichlorophenols with negligible fouling of OBDD in contrary to fast passivation of HBDD. Surprisingly, no significant electrode fouling of HBDD even without any reactivation was reported for phenol and monochlorophenols in aqueous media. Nevertheless, the authors admitted that in this case the H-termination is questionable due to experiments performed at relatively high anodic potentials. This problem arises also in other studies reported for HBDD surfaces. The merits of cathodic pretreatment prior to detection of chlorophenols The use of OBDD electrodes is also advantageous for all analytes passivating the electrode surface by oxidation products, because in these cases its regeneration by anodic oxidation is compatible with O-termination. The cathodic pretreatment of BDD surfaces was also reported in some electroanalytical studies because it may improve the voltammetric response. It should be performed just before measurement because the loss of superficial hydrogen due to the oxidation by air oxygen was reported. Cathodic reduction may be also used for the regeneration of passivated electrode surface as shown for bovine serum albumin. It is believed that hydrogen generation by reduction treatment plays an important role in the process. Negligible effect of the surface termination on the peak potential was noted for several purines and pyrimidines and procaine. It is obvious that the anodic or cathodic pretreatment of BDD surface performed easily in situ can change the response of the analyte of interest. This is on one side undoubtedly a substantial advantage, on the other it represents a potential risk of unwanted surface change. Therefore, the compliance of pretreatment and cleaning of BDDE with defined standard operation procedures must be strictly enforced when considering their applications in practice. Electroanalytical methods developed for OBDD will be presumably preferred due to the long-term stability of such surface and possibility of its regeneration using high anodic potentials. Both HBDD and OBDD outperform usually classical carbon and metal electrode materials thanks to chemical inertness and fouling resistivity. Therefore, the efforts on its modifications must be driven by a concrete purpose, i.e. impart of catalytic activity or increase of selectivity toward the analyte of interest, which includes also the surface biofuctionalization for biosensing. ACKNOWLEDGMENT Authors thank the Grant Agency of the Czech Republic (Project P 206/12/G151) for financial support.

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Proc. Electrochem. Soc. 2000-19 (Microfabricated Systems and MEMS V

2. Introductory Industrial Electrochemistry César A.C. Sequeira

2.1. What is Electrochemistry The science of electrochemistry was born at the end of the eighteenth century following intense interest in the production of electricity by animals such as the torpedo ray and the electric eel. The first half of the nineteenth century was dominated by the work of Davy and Faraday. Davy used electrolysis to discover the alkali metals but also applied his electrochemical knowledge to suggest cathodic protection to the Royal Navy as a way of preventing the corrosion of their copper-sheathed wooden ships. Faraday showed that all known forms of electricity were identical and then discovered the quantitative relation between the electrical charge passed through a cell and the amount of chemical change which occurs. He also laid the basis for the understanding of passivation which plays such an important role in the protection of materials. Not long after Faraday’s electrochemical work, in 1842 Sir George Grove showed that Nicholson and Carlisle’s experiment of the decomposition of water could be reversed, that is, hydrogen and oxygen could be combined to form water, in an electrochemical cell, with the production of electricity. This led the way to a fuel cell in which a fuel, hydrogen, could be continuously oxidized electrochemically. Grove’s fuel cell was the forerunner of the hydrogen-oxygen fuel cells used in the Gemini and Apollo space capsules. Developments of Volta’s pile which are known as primary batteries, were made during the nineteenth century. The most enduring is the cell invented by Leclanché in 1867 in which zinc dissolves at the anode and manganese dioxide is reduced at the cathode. These cells are essentially irreversible. Rechargeable cells or secondary batteries are more economical and the best known of these is the lead accumulator invented by Planté in 1860. Humphry Davy’s use of electrolysis using molten electrolytes to produce active metals culminated near the end of the century in the industrial production of aluminium on the basis of independent discoveries by Hall and Héroult in 1886 that alumina could be electrolysed in a molten criolite bath. The intense activity in electrochemistry in the nineteenth century did not increase its understanding, except after the discovery of the electron by J.J. Thomson in 1897 and the identification of the species called by Faraday as ions. When there is a change in the mechanism of electrical conduction, the production of electrons from the ions or the consumption of electrons by the ions is inevitably accompanied by reduction or oxidation of the ions concerned, that is, a change in their chemical nature. This is the beginning of a clear understanding of the nature of the electrochemical process. Towards the end of the nineteenth century, Walther Nernst had used the powerful system of energy relations known as thermodynamics to show how one could calculate the maximum amount of electrical energy that could be obtained from a chemical reaction occurring in an electrochemical cell. Although the work of Tafel in 1905 brought some light on the interfacial nature of the electrode reaction that done by Georges Gouy was the first showing clearly the structure of the electrode interface and that it is frequently only a few molecular diameters thick. Thus, reliable results can be obtained only if a scrupulous control of the purity of the system is maintained. The first step

towards a satisfactorily controlled environment for the study of electrode reactions were made in Frumkin’s laboratory in Moscow in the 1930’s but an important contribution was also made by Heyrovsky who introduced the renewable surface of the dropping mercury electrode; of course, mercury has an advantage other than that of providing a clean surface easily, i.e. its surface is uniform and atomically smooth. During the twenties and thirties of last century the idea emerged clearly that electrode reactions were heterogeneous chemical reactions which obeyed the same sort of kinetic laws as ordinary chemical reactions; that is, the rate was proportional to a product of concentrations at the reacting species: Rate = k A B

(2.1.1)

where the square brackets indicate concentration of the species A, B, etc. This arises because the reactants must collide to react. The proportionality constant k is known as the rate constant of the reaction. The unique feature of electrochemical reactions is that this “constant” can be adjusted by changing the electrode potential. In other words, there is a simple electrical way of controlling the rate of this type of chemical transformation. Since the relation between the rate constant and the electrode potential is exponential, this control can be exercised over a very wide range; in the extreme example it is possible to change the rate by a factor of 1010 in this way. It should be emphasized that there is a limit to which an electrochemical reaction can be accelerated. This is because it is an interfacial reaction and the reacting species must be brought to the interface to react. As the interfacial reaction is accelerated a point is always reached when it is this transport of matter to the electrode which ultimately determines the rate of the whole process. Such behaviour is of great practical importance because the rate of the transport process is directly proportional to the concentration of the reacting species in the solution. Hence, this is the basis for a whole series of powerful methods of quantitative analysis. Of the many new developments in electrochemistry, one of the most interesting is that in which electrochemical processes are combined with photo-excitation, i.e. the use of light energy. The basic principle is that the light energy is used to form a positive charge and an electron in a region where an electric field can separate these charges and lead them to produce useful work. In the semiconductor p-n junction, the region of electric field is produced by a very slight change in the composition of the semiconductor which is brought about by very carefully controlled diffusion of a dopant using very sophisticated furnaces. A similar type of field can be produced at the surface of a semiconductor simply by dipping it into an electrolyte. This semiconductor electrode also has the advantage that the electron driven towards the electrolyte can be used either to produce electricity or to carry out a useful chemical reaction, whereas the p-n junction can only do the former. This holds out the promise of a solar energy based chemical production or of the possibility of storing solar energy in chemical form. It should be remarked, in conclusion, that the natural sources of energy on which mankind has so far depended are the result of a process which is essentially photo-electrochemical: photosynthesis. Light is absorbed by an array of chlorophyll molecules resulting in the production of a pair of charges which are separated in the natural analogue of the field described previously and a series of oxidation-reduction reactions ensure resulting ultimately in the production of carbohydrates from carbon dioxide and water. This and other biological processes such as nerve conduction involve the transfer of charged ions across membranes and the ideas of electrochemical reactions are proving of great help in their understanding.

2.2. What can Electrochemistry do? The use of electrochemistry seems to be older than electrochemistry itself. In the valley of Mesopotamia, close to the Tigris, archeologists have found some 2200 years old remains of an iron rod and a copper sheet bent into a cylinder around it, both placed in a ceramic jar filled possibly with grape juice; in other words, they found a battery or, technically, a chemical power source of the primary cell type. However, the development of electrochemical practices had to await the rediscovery of sources of continuous flow of electricity within the last two hundred years. These practices, with applications in many aspects of modern life, include: (i) Electrosynthesis of Inorganic chemicals (Cl2/NaOH, H2, F2, , …) Organic chemicals (NCCH2CH2CH2CH2CN, …) Metals and alloys (Al, Zn, Sn, Cu, Pb, Au, …) (Figure 1) Semiconductors (SiGe, …) Conductive polymers (poly(3,4-ethylenedioxythiophene:poly(styrenesulphonate) (Figure 2) Composites (ceramic films, nanocomposites, ...) (ii) Materials stability and processing Electrochemical machining, grinding, electroforming Etching of metals Etching of semiconductors Electrophoretic painting Electroplating and anodising (Zn, Sn, Cu, Ni, Fe, Cr, Au, …) Corrosion and protection of metals (anodic and cathodic protection, sacrificial anodes) (iii) Electrical power conversion Fuel cells Storage batteries (Figure 3) Redox batteries Solar cells Cold fusion (iv) Environmental protection techniques Sensors, monitors and control (Figure 4) Metal ion removal Removal of inorganic and organic compounds Water purification Recycling of redox and other reagents “Clean” electrosyntheses Salt splitting (v) Biomedical application In vivo sensors and monitors In vitro diagnostics Electrotherapy and transdermal drug delivery Electrochemotherapy and electrogenetherapy (Figure 5) These examples show that electrochemistry has a wide range of applications, which illustrate its many-sided interactions with other branches of science and technology, as shown in Figure 6.

2.3. Electrochemistry as transfer agent to technology Starting with the brief referenced work on the history of electrochemistry (Galvani, Sir Humphry Davy, Faraday, …) and its applications, as described in sections 1. and 2., it is clear the tendency of electrochemistry in quickly turning its science to use. The ubiquitous dry cell in its long-standing form used zinc, ammonium chloride, manganese dioxide and carbon. More exotic systems of this kind involve very high power density, short life packages for special military purposes. Similarly, the widely used rechargeable systems using lead or nickel are also examples of the art of electrochemistry in everyday use. Another important contribution of electrochemistry is the conversion of natural inorganic materials to useful metals (Al, Mg, Cu, Na, Li, …) and the application of metals to solid surfaces by electroplating. The production of several vital, inorganic chemicals (chlorine, sodium hydroxide, sodium hypochlorite, …) has long been carried out via electrochemical processes. Organic chemicals (silicilaldehyde, adiponitrile, …), molten salts, room temperature ionic liquids, etc., have been produced commercially and at laboratory scale, also by electrochemical means.

Figure 2.3.1 - Aluminium electrolysis cell with prebaked anode.

Figure 2.3.2 - Conductive polymer-based MEMS devices. Conductive polymer of PEDOT:PSS transfers holes for conduction. The PEDOT:PSS is coated with water-dispersion and the resulting film is transparent. The devices consist of bio-chemical sensors, pixel displays, touch screens, and large area touch sensors.

Figure 2.3.3 - Discharge reactions in a lead-acid battery.

Figure 2.3.4 - Meter-scale fabric touch sensors. Touch sensors consist of conductive polymer-coated fibers. Capacitance between fiber and human finger is measured for detecting human touch. The keyboard input is demonstrated.

Figure 2.3.5 - Electrochemotherapy of liver metastases. A: Overlay of the computational geometry and patientâ&#x20AC;&#x2122;s anatomy. The red lines represent the direction of the electrodes, while the blue line represents a cross-section. B: Photograph of the surgical setup with electrodes penetrating into the tumor is clearly seen (cables not connected).

Figure 2.3.6 - The applications of electrochemistry and its interactions with other branches of science and technology.

Electroanalytical procedures have long been in use but recent instrument developments make for new procedure plus very helpful automation.This provides for effective and beneficial use of these methods both for programming and for rapid, accurate quality control. Another example is the electrochemical machining which was a key to turbine bucket manufacturing for aircraft engines, here the science elicited in the study of corrosion processes in conducting media paid off handsomely. Recently, applications in the medical and health fields with ion transport through membranes have also been very fruitful (Figure 7). The implication being made here is that electrochemical science transfers readily into technology, i.e. is an effective agent for this often difficult process. Very good recent examples are concerned with very high speed integrated circuits, genetic engineering, biotechnology and molecular medicine. So, the field of electrochemistry has been and continues to be a fertile area to work and have impact both on human affairs and on science.

Figure 2.3.7 - A membrane before the pulse with outside concentrations of not easy permeable cations, drugs, DNA, etc. and after the pulse showing schematically the enlargement of pores, the disturbance and the breakdown of bilayer for electroporation.

2.4. Fundamentals of elementary electrochemistry 2.4.1. Conductivities, electrical quantities and properties Electrochemical reactions are heterogeneous chemical reactions which occur via the transfer of charge across the interface between an electrode and an electrolyte. The minimum components required for an electrochemical cell (and hence a cell reaction) are an anode, a cathode, ionic contact between the electrodes (via an electrolyte), and electronic contact between them, i.e. an external electrical circuit. A critical question arises: if the electrodes are externally connected, will a cell reaction take place spontaneously, or will it be necessary to connect a power supply between the electrodes ? Actually, spontaneous or galvanic cells are self-driving when an external electrical connection is made. Indeed, if a load is connected between the electrodes, electrical energy may be used to provide useful work in the form of heat, light, magnetism or power for an electronic device. The discharging lead-acid battery shown in Figure 3 is an example of a galvanic cell. In contrast to spontaneous cells, electrolytic or “driven” cells require an input of electrical energy (in the form of a dc power supply or a battery) to produce chemical transformations. The aluminium electrolysis cell shown in Figure 1 is an example of an electrolytic cell. So, electrochemistry deals essentially with electrodes and cells, involving basic concepts and manipulative skills as shown in Tables 1 and 2. A qualitative description of electrical conductivity in metals, semiconductors, and insulators is an appropriate way to initiate the presentation of electrochemistry. The discussion should include the concept of valence and conduction bands and the idea of a delocalized sea of electrons in the Fermi levels of metals. These concepts, which are germane to several aspects of energy conversion and solid state electronics, can be related at the molecular level to things chemical. The important distinction between electronic and protonic charge on metal atoms in conductors should be restated. Thus, a feeling for the nature of electronic charge in metals will help to understand electrochemical cells. The distinction between the flow of electrons (or positive holes) in electronic conductors and ionic conductivity in solution should be drawn carefully. One way this can be done is by discussion of the simple diagram shown in Figure 8 that clearly shows that current is not carried through solution by electrons and that overall electroneutrality is maintained in solution during electrolysis. To appreciate the production of electricity from an electrochemical cell, certain notions concerning electrical circuits have to be introduced. Essentially what is needed is a brief description of electrical quantities and properties. These should include the flow of current in a battery/resistor circuit and Ohm’s law. The suggested level of the treatment is such that the battery discharge experiment shown in Figure 9 can be understood. For this simple series circuit, the battery acts as an electron pump with an associated cell potential (Ecell) and an internal resistance (Rcell). Presentation of these concepts in an introductory discussion of electrochemistry has several attractive features. Once the idea of an electrochemical cell has been given, this experiment shows the direct relation between the “downhill” cell reaction and the generation of electricity. Without the background understanding of electrical quantities and properties, this experiment lacks full meaning for most of the students. Furthermore, students can relate to the process of discharging a battery, and the experiment is readily performed with relatively modest equipment. All that is required is a resistor, an inexpensive digital voltmeter, and a supply of batteries. For example, the Zn-Cu Daniell cell which appears in almost all general chemistry textbooks can be studied with instructive results. The open circuit voltage (Ecell) can be obtained with the switch (S) open, and the current measured under

Table 2.4.1 - Basic electrochemical concepts 

Conductivity in metals, semiconductors and insulators



Ionic conductivity



Electrolysis



Electrode reactions

Anode/Cathode terminology

Electroneutrality principle

Electrical quantities and properties o

Charge, current, electrical potential energy

Current flow in battery/resistor circuits

Battery discharge curves



Faraday’s law



Galvanic cells



Electrode potentials and the activity series

Eºcell, Gº, Keq relationship

Nernst equation

Membrane potentials

Applications o

Membrane potentials

Batteries

Photovoltaic cells

Fuel Cells

Corrosion

Table 2.4.2 - Manipulative skills for presentation of introductory electrochemistry 

Balance oxidation/reduction reactions



Calculate charge and mass using Faraday’s law



Calculate Ecell from table of electrode potentials



Predict relative reducing and oxidising tendencies



Identify positive and negative electrodes in galvanic cells



Calculate Gº and Keq from electrode potentials



Calculate concentration dependence of Ecell using Nernst equation



Estimate concentrations from potential measurements with ion selective electrodes (advanced topic)

Figure 2.4.1 - Schematic ionic conductivity in a NaCl solution. The electrode reactions are AgCl + e  Ag + Cl- at the left-hand electrode and Ag + Cl-  AgCl + e- at the right-hand electrode. Note that by migration of the cations and anions, electroneutrality is maintained in the solution.

Figure 2.4.2 - Battery discharge experiment for discussion of electrical circuits.

load from the potential drop across the resistor (R). For a typical Daniell cell with gravitational separation of electrolytes a load resistor of 20  gives a voltage drop of approximately 0.7 V. Since Ecell = IR + IRcell, this corresponds to an internal cell resistance of 12 . This experiment also constitutes an excellent and significant introduction to Faraday’s law. The representation of battery discharge shown in Figure 10 permits calculation of the energy capacity from the product of current x time, i.e. Q = Iav.t1. Once the concept of the cell reaction has been introduced, this quantity can be related quantitatively to the consumption of reactants in the usual fashion. Following the brief approach to introductory electrochemistry, it seems to be of paramount importance to synthesize an understanding of the heterogeneity of an electrochemical system and a chemical sense for the important electrochemical variables of potential, current, and charge. Fuller treatments are given in the textbooks of Bard, Faulkner, Lingane and many other excellent electrochemists. The points can be made here in the following five subsections.

Figure 2.4.3 - Schematic battery discharge for the calculation of the energy capacity.

2.4.2. An electrochemical system is not homogeneous Newcomers to electrochemistry have had much more experience with homogeneous systems than with heterogeneous ones. Heterogeneities almost always exist in electrochemical systems. One cannot understand electrochemistry without grasping the locations at which important events occur. There is a good deal of structure to a typical system; Figure 11(a) offers a representation. Electrode reactions, such as the evolution of hydrogen, 2H+ + 2e-  H2

(2.4.1)

CU2+ + 2e-  Cu

(2.4.2)

or the plating of copper,

or the oxidation of ferrocene to ferricenium, FeCp2  FeCp2+ + e-

(2.4.3)

can only take place at the interface between the electrode and the solution of electrolyte. The important point for the moment is that the electrode can only affect or sense the part of the solution in immediate contact with itself. No ferrocene can be oxidized if there is none at the interface, even if it does exist somewhere else in the cell. Electrode reactions, in fact, tend to make the composition in the nearby solution different from that further away. This effect is shown in Figure 11 (b) for an electrode immersed in 1 mM ferrocene in acetonitrile. If reaction (4) takes place at the interface, the concentration of ferrocene will be drawn down at the surface, as shown in Figure 11 (b). Since the normal Brownian motion of molecules tends to homogenize the solution, there is a net diffusion of new ferrocene molecules into the depleted zone from more remote regions. Some molecules diffuse all the way to the surface where they undergo reaction (4). Thus, there is a tendency for the system to transport

reactants to the electrode from remote points, and the depleted zone widens as the reaction proceeds. If there is stirring then the zone widens to a limit, because the stirring keeps the solution homogenized beyond that limit. Just as the electro-reactant is depleted near the interface, the product accumulates nearby, as shown in Figure 11 (c). The product gradually diffuses outward from the surface, so its profile also broadens with time. The part of the solution near the electrode, where the composition is affected by an electrode reaction, is called the diffusion layer. It is large on a molecular scale, perhaps 104â&#x20AC;&#x201C;107 Ă&#x2026;, but quite small on the usual dimensions of cells, only 10-4â&#x20AC;&#x201C;10-1 cm. The bulk is the part of the solution, far from the electrode (but still perhaps not more than 0.1 mm away) where the composition is uniform. Of even smaller dimensions is the structure in the immediate vicinity of the interface. It is intuitive that the part of the solution very near the electrode, within a few molecular layers, must differ in structure from the characteristics of more remote zones, simply because this part of the system is forced to interact with the electrode, which obviously has a very different character from the bulk solvent. Likewise, the distribution of mobile electrons in the part of the electrode in contact with solution must differ from the distribution found in the interior of the metal. These interfacial zones on the two sides of the interface have dimensions of a few tens of angstroms, because that is the scale on which interatomic and intermolecular forces operate. The whole structure is called the double layer. At this point, a very important idea must be grasped. One is taught early to rely on the electroneutrality of chemical phases, i.e. on the idea that positive and negative charges exist in equal numbers within a phase. This is not true of the separate phases which constitute the interface. Usually there are differences, so that phases retain net electrical charges. The differences are small compared to the total number of positive and negative charges present hence electroneutrality is a very good approximation for most stoichiometric thinking. However, the excess charge has a big effect on the electrical (and electrochemical) properties of a phase.

Figure 2.4.4 - (a) Spatial structure of an electrochemical system. (b) Concentration profiles of ferrocene undergoing oxidation of the electrode. Distance is measured from the electrode toward the bulk. Times are elapsed periods from start of electrolysis. (c) Concentration profiles of ferricenium produced by oxidation of ferrocene.

All conducting phases, such as metals and electrolytes, tend to push the excess charges to their outer boundaries. Thus, the double layer between an electrode and a solution will contain such excess of charge as shown in Figure 11 (a). If the metal is positively charged, then the solution side has an equal excess of negative ions in the interfacial region. One can show that the whole double layer is electrically neutral, even if both sides are charged. The excess charge on the metal can be changed at will with a power supply, which is just a pump for electrons. The supply forces electrons to enter or leave the interface until the repulsion of charges remaining there will not allow the supply to remove or add more. The metal can carry positive, zero or negative charges of continuously tunable magnitude. This feature is the basis for control of potential (see below). 2.4.3. Many things can happen at once An electrode reaction can be rather complicated. Invariably, it takes place in a mechanism of several steps, and these steps in electrode processes are often very different in kind. There is at least one heterogeneous electron-transfer step, like (2), (3), or (4) above. However, there can also be coupled heterogeneous processes of other types, such as (a) adsorption or desorption precursors, intermediate e.g. H atoms in (2) or products, (b) migration of atoms across a surface during electrocrystallization of metals e.g. Cu in (3), or (c) surface-mediated recombination of atoms or radicals. There can also be coupled homogeneous reactions in the solution. Acid-base reactions, metal-ligand chemistry, homogeneous redox reactions, and radical-radical recombination processes are quite common. These reactions can precede or follow the heterogeneous electron transfer. All of these chemical steps are kinetic events, and they all proceed at rates described by rate constant. The overall rate of the electrode reaction is determined by all of the individual rates together, much as for a mechanism of purely homogeneous steps. The chief differences for electrode reactions are that the heterogeneous steps (a) have rate constants that depend on the state of charge of the interface, i.e. the potential of the electrode, and (b) have rates that depend on concentration of reactants at the surface. Since the electrode process can happen only at the electrode, another factor in the overall reaction rate is the rate at which reactants can be brought to the electrode or the rate at which products can be dispersed. Mass transport is usefully regarded as part of the overall mechanism, even though it is not a chemical process. Mass transport occurs by diffusion (microscopic, random motion), by convection (bulk flow, stirring movement of segments of solution), or by migration (movement of ions along an electrical field). As elsewhere in science, one obtains chemical information from a complex electrochemical mechanism by trying to arrange conditions so that a single step, such as mass transfer or heterogeneous electron-transfer, controls the overall rate. It is very often possible to obtain such selective control. The main message here is that an electrochemical system has a dynamic nature of unusual variety. It is essential for a newcomer to obtain a feeling for this aspect as early as possible. 2.4.4. Current is an expression of rate Figure 12 is a schematic representation of the chemistry in a cell. Usually we focus attention on a single electrode, called the working electrode. Suppose ferrocene is being oxidized there according

Figure 2.4.5 - Structure of an electrochemical cell. The voltmeter has a high impedance, so current does not flow in the circuit between the working and reference electrodes.

to step (4). Each molecule that is oxidized gives up one electron to the working electrode. These electrons will not be allowed to be separated permanently from the solution on any stoichiometric scale, because huge potential differences would be produced. Instead, the electron will pass through the external circuit and back into the solution at a second electrode called the counter electrode. Since the power supply will force the electrons onto species in the solution at the counter electrode, reduction takes place there to an extent equal to the oxidation of ferrocene at the working electrode. Electrons at either electrode can flow into the electrode or out of the electrode by way of the external circuit. That is, the current can be reducing or oxidizing, i.e. cathodic or anodic. However, a current at the working electrode is compensated exactly at the counter electrode. Anodic current at the working electrode implies cathodic current at the counter electrode, and vice versa. Since electrons are used or supplied on an integral, stoichiometric basis, in electrode reactions, the flow of electrons at an electrode is directly proportional to the rate of reaction there. 2.4.5. Electrode potential and electron energy The best starting point is the definition from physics that potential is the work required to bring a unit test charge from infinite distance to a point of interest, such as inside a working electrode. It is obvious that the potential of an electrode depends on any excess charge that exists on the electrode. It is also clear that the energy required to add or subtract an electron from the electrode can also be expressed in terms of this potential. The linkage between potential, electron energy, and excess charge on the electrode is the key to an understanding of potential as a chemical variable. An external power supply, like that in Figure 12, can force extra electrons into or out of the metal side of the double layer until its driving force is counterbalanced by the repulsions between the excess charges. With a large excess of negative charges, the potential is very negative and the energy of electrons on the electrode is high. Likewise, a large excess of positive charge implies a

Figure 2.4.6 - Illustration of potential as an expression of electron energy. At potential (a) ferricenium at the surface would be reduced to ferrocene. At potential (b) ferrocene at the surface is oxidized to ferricenium. The potential limits correspond to values for which the solvent, supporting electrolyte, or electrode material are oxidized or reduced.

very positive potential and low electron energy. By changing the voltage on the power supply, the excess charge, the potential, and the electron energy can be tuned continuously over a wide range. With this idea in mind, it is easy to understand oxidation and reduction at the electrode. Consider a Pt electrode in contact with a solution of ferrocene in acetonitrile. Ferrocene has the property of being fairly easy to oxidize, i.e. it gives up an electron readily. The energy of the highest electron in the ferrocene molecule is, of course, determined by the molecular structure of ferrocene, and it has the same value for all ferrocene molecules. This highest electron can be lost to an electrode only if the electrode offers a lower energy to the electron, that is, only if the potential is sufficiently positive. The system can be usefully pictured as in Figure 13, where potential and energy of electrons on the electrode run vertically. There is a well-defined energy (potential) where oxidation becomes possible. The standard potential for the ferrocene/ferricenium couple is essentially that critical energy. At more negative potentials than Eº, ferricenium would be reduced at the electrode by relatively high energy electrons present there. Any electrode process is characterized by its own standard potential Eº. On the positive side of Eº, the oxidized form of the couple is stable at the electrode, and the reduced form tends to undergo oxidation if it reaches the electrode. Likewise, the zone of potential more negative than Eº is a region where the reduced form is stable and the oxidized form tends to be reduced. Actually, in a zone about 100 mV wide centered on Eº, statistical equilibrium of electrons on the electrode and on species in the solution permits mixtures of oxidized and reduced forms with appreciable amounts of both species. These ideas can tell us what may be possible at a given energy, but the kinetics of the reactions determine whether an energetically allowed reaction actually does occur. Some reactions, such as (2) on Hg, are very sluggish and do not occur at appreciable rates unless a large excess potential beyond Eº is applied. Others, like (4), are quite fast.

Experimentally, one can measure only differences in potentials between two electrodes, so the word potential in electrochemical usage means the voltage difference (Figure 12) between a working electrode and an independent reference electrode. The reference electrode is usually arranged, so that it does not pass current and is at equilibrium. It is constructed so that it contains both forms of a redox couple (e.g. H+/H2, Ag/AgCl) at fixed composition. Electron energies in the metallic part of the electrode are, therefore, fixed at a value near that corresponding to Eยบ for the couple. This arrangement provides an invariant reference point against which working electrodes can be measured. To understand the manner in which an electrode at open circuit senses the composition of a solution by providing a characteristic potential given by the Nernst equation, careful discussions are available in the open literature for the interested readers. 2.4.6. One cannot control both current and potential simultaneously Once the energy available to a reaction has been determined in any chemical system the reaction proceeds at a rate corresponding to that energy. In homogeneous kinetics, one can dictate the temperature and accept the rate, or one can dictate the rate and accept the corresponding temperature. Likewise in electrochemical systems one can set the potential of an electrode and accept the corresponding current flow, or vice versa. Choosing one variable for control precludes any separate influence over the other. The five major concepts discussed above give only a foundation for an understanding of electrochemistry. However, experience has shown that they do provide a sound basis for the development of a contemporary working knowledge of the subject.

2.5. Fundamentals of electrochemical engineering The availability of new materials for electrodes, membranes and other cell components has exerted an enormous influence on industrial electrolysis during the recent past. Additional influences of feedstock and energy availability, and of wastewater treatment procedures, have also led to changes in industrial practice. Many processes have thus been re-optimized to new constraints, while other entirely new technologies have emerged to fill needs. Activities such as these require effective engineering methodologies for accurate design, scale-up and optimization. Electrochemical processes are complex because they involve many different phenomena simultaneously. Included among these, for example, would be ohmic resistance effects, mass transport limitations on reactants and products, and charge transfer rate processes. The relative importance of such processes depends upon geometry and current density. Because the reaction rate along a surface is generally not uniform, the relative importance of such processes can therefore vary strongly with position inside a cell. As a consequence, it is usually difficult to predict behaviour of electrolysis cells by intuition alone. Effective engineering methodologies are needed for transforming understanding of fundamental principles into properly designed hardware. Electrochemical engineering consists on applying scientific principles to understand and improve technological devices. The designs illustrated in Figure 14 include some electrochemical reactors favourite in industrial practice. Fundamental principles upon which the field draws heavily include  Thermodynamics, which describes the equilibrium state of the electrode/electrolyte interface;  Kinetics, which relates the rate of passage of current through the interface to the driving forces across the interface;  Transport phenomena, which determines the rate at which species and energy can become available to the interface region;  Current and potential field distribution, which determine the flow of current between electrodes, and the variation of potential along the electrode surfaces. The potential enters into every one of the four areas listed above. The extension of chemical understanding requires mainly an appreciation of the role of the potential. The voltage of any electrochemical cell consists on several components. At equilibrium the cell voltage would exhibit a thermodynamic potential. In order to pass current, it is necessary that driving forces in excess of the thermodynamic potential be supplied. The overpotential caused by irreversible processes arises from several regions in the cell. Ohmic resistance, for example, arises throughout the entire region, both in the electrolyte between electrodes, and in the electrodes and connecting wires. In the diffusion layers near the electrodes, concentration differences arise which beget concentration overpotentials. Within a few angstroms of the electrode-electrolyte interface, additional overpotentials arise in order to drive charge-transfer reactions at the surface. These various overpotentials depend upon the level of current which passes through the cell following, as is nature’s way, the path of least resistance. The simple act of separating the overall cell voltage into several components (thermodynamic potential and irreversible overpotentials) provides an important conceptual basis for the study of complex electrochemical systems.

Figure 2.5.1 - Schematic diagrams of some electrochemical reactors.

These component areas are being emphasized in many symposia on fuel cells and batteries, aluminium manufacture, electroplating, chlor-alkali technologies, corrosion, etc.; key among these component areas are the distribution of current and potential within a cell, the evaluation of tradeoffs between the influences of different phenomena, the use of dimensionless numbers to assist in scale-over to new operating conditions, and economics. The central point to be recognized is that the rate of an electrochemical reaction varies from point to point along an electrode surface. In hardly any case is the reaction rate uniform, usually because the shape of the electrode does not lend itself to that result, or else the process economics do not permit it. For example, a platinum flag electrode attached to a wire and immersed in a beaker of solution and facing another similar flag electrode may be considered. In such a cell, electrochemical reactions will occur at a high rate on the surfaces which face each other in comparison with the surface on the â&#x20AC;&#x153;back sideâ&#x20AC;? of the electrodes. For the same reason, the reaction rate will be higher near the edge of the flag than in the central region of the surface because, from potential field considerations, the current finds an easier path to the edges than to the central region. The very corners of the flag have the highest reaction rates since they are the most exposed to the volume of electrolyte through which current flows. Thus, simple platinum flag electrodes are not very simple at all and are rarely used in modern laboratories. In addition to the effect of cell geometry, the current and potential distribution is influenced by mass transfer and by charge transfer. Engineering tradeoffs are therefore required in order to balance the need for cell operation against economic considerations. For example, a uniform distribution of current can be achieved over a surface, but only by operating at very small currents. Also, mass transfer complications can be avoided by stirring the solution vigorously. While these procedures are easy to accomplish in a beaker, they may not be profitable in an industrial process. Operating at low current means that the manufacturing rate is low; stirring on a

large scale requires pumps and special cells designed to guide the fluid over the electrode surface effectively. Electrochemical engineering consists in large part of evaluating different possible courses of action on the basis of their effect on cell operation and upon the process economics. A key step in this process, at least when the design of the cell is involved, is evaluation of the potential distribution to determine what factors influence it to the greatest extent. At this stage it should be noted that it is always important to quantify the performance of an electrochemical reactor as a key unit process in many overall schemes. It is, therefore, useful to define and calculate various figures of merit which act as performance indicators for electrochemical reactors. The indicators which are commonly used or are of developing interest are: 1. Yield, ď ąp This quantity is the amount of desired product obtained from a unit amount of reactant, taking the reaction stoichiometry into account

(2.5.1)

2. Overall selectivity, Sp This quantity describes the relative formation of a desired product with respect to the others also produced

(2.5.2) 3. Current efficiency or charge yield, ď Ś This is a product yield based upon the electrical charge passed during an electrochemical reaction, i.e. it is a yield based upon the electron as a reactant (2.5.3) 4. Cell voltage, Ecell In any electrochemical cell operating away from equilibrium, in addition to the difference between the cathode and the anode potentials (measured with respect to the same reference electrode), it is necessary to consider ohmic potential drops within the cell, IRcell, and in the electrical circuit, IRcirc, such that we have (2.5.4) 5. Electrolytic energy consumption, Es The molar energy consumption is given by (2.5.5) The specific energy consumption is given by

(2.5.6) The volumetric energy consumption is given by

(2.5.7) where Vm is the molar volume and  is the current efficiency. 6. The active electrode area per unit volume, As This is given by (2.5.8) where A is the active electrode area and VR is the reactor volume. 7. Mass transport coefficient, Km This is an important figure of merit for processes under mass transport control via convective-diffusion of species, (2.5.9) and may be considered as a limiting current density, jL, which is normalised with respect to the bulk concentration of electroactive species, C. 8. Space-time, st and space-time yield, st The space-time is defined as the ratio of reactor volume to volumetric flow rate, (2.5.11) If VR is taken to be the effective volume of electrolyte within the reactor, then st is equivalent to the mean residence time, . The space-time is then effectively a fill-up time, i.e. the time in which the volumetric flow rate at the inlet would charge an empty reactor. In S.I. units, the use of m3 for VR and m3 s-1 for QV will produce st in units of seconds. The space-time yield is the mass of product per unit time (dw/dt) which can be obtained in a unit reactor volume VR: (2.5.12) 9. Space-velocity, s, and normalised space-velocity, sn The space-velocity is defined as the ratio of volumetric flow rate to the effective reactor volume, and is reciprocal of space-time

(2.5.13) The space-velocity describes the investment costs per unit volume of electrolyte. The figure of merit is important in cases where the electrolyte has a high value (as in the case of concentrated solutions of precious metals) or when the reactor is used for waste-water treatment. To facilitate comparisons between reactors, it is convenient to define a normalised space velocity as (2.5.14) where is the residence time in the reactor which is needed for 90% fractional conversion of the reactant, Vn is the volume of electrolyte which has experienced this 90% conversion of the reactant, and is the mean residence time in the reactor (= VR/QV). 10. Normalised volumetric energy and power consumptions The normalised volumetric energy consumption may be defined as (cf. eq. 11) (2.5.15) where Ecell is the cell voltage, Q is the overall electrical charge and Vn is the normalised volume of electrolyte, i.e. the volume in which the concentration of reactant has been reduced by 90%. is the electrolyte energy consumed in treating a unit volume of electrolyte such that 90% of the reactant is depleted in unit time in a unit reactor volume. If Ecell is in volts, Q is in coulombs and Vn is in m3, and will take units of Jm-3. The electrolyte power consumption may be stated as the rate of energy utilization with respect to time. The normalised volumetric power consumption for electrolysis is given by (2.5.16) This is the electrolyte power required to treat a unit volume of electrolyte such that 90% of the reactant is depleted (in Wm-3). It is common in science courses, to speak of “energy balances” or “mass balances”. By application of these balances, one can evaluate what goes into a system, and in what form it comes out. In the same spirit, one can talk of a “bulk balance” where euros are used to buy and operate equipment, from which product, unwanted waste, and old equipment emerge. Economic balances are used to identify costly elements in an overall process, so that improvements can be made which will have an economic impact. Economic balance equations can also be made dimensionless in order to evaluate how profit, return on investment, or other economic indicators scale with changes in process operation. Applied research and development activities are always carried out in the presence of economic awareness.

2.6. Modified electrodes When an electrode, such as a piece of platinum or carbon, is dipped into a solution, its surface becomes covered with a layer of water molecules. Sometimes species present in the solution that have been purposely added or are present as impurities will also attach to the electrode surface. The presence of such adsorbed species will often modify the electrochemical behaviour of the electrode. For example, they may cause the current observed for a given electrochemical process to be much smaller, because they block access to the electrode surface. Such effects, with electrode surfaces modified by adsorption of species from solution, often accidentally, have been studied extensively by electrochemists for many years. In recent times electrochemists have become interested in purposely modifying an electrode by adsorbing, coating, or attaching specific molecules to the surface. This deliberate and controlled modification of the electrode surface can produce electrodes with new and interesting properties that may form the basis of new applications of electrochemistry and novel devices. Fundamental studies of such modified electrodes have also provided a better insight into the nature of charge transfer and charge transport processes in thin films. Modified electrodes can be prepared by several different techniques and are, therefore, often referred to by such names as derivatized, polymer-coated, functionalized, and electrostaticallybound electrodes. Most frequently the layer or coating on the electrode surface is electroactive, i.e. it can exchange electrons with the underlying substrate material and be oxidized or reduced, although some applications of non-electroactive films have also been discussed.

Strong chemisorption. Some species find the surface of the electrode much more hospitable than the bulk solution and so attach spontaneously to the surface. For example organic species, such as those containing double bonds, are often hydrophobic and strongly adsorb from aqueous solutions on carbon or platinum surfaces. In these cases the amount adsorbed on the electrode surface, usually written as I’, is at most a monolayer, i.e. about 1 x 10-10 mol cm-2, or 6 x 1013 molecules cm-2. Covalent attachment. Chemical reactions can be carried out to form bonds between the

substrate and a molecule of interest. For example, a metal or carbon can be oxidized so that the surface can be considered as consisting of hydroxyl groups. Such a surface can be “silanized” by reaction with an organosilane and then reacted with another molecule of interest. In this case the silane is a kind of glue for fixing the molecule to the surface. One would expect that this technique would also only form a monolayer on the electrode. Frequently, however, the silanization reaction causes polymerization to take place and thicker layers, equivalent to several monolayers, form.

Polymer layers. By dipping the electrode into a solution containing a dissolved polymer, e.g. poly(vinylferrocene), and allowing the solvent to evaporate, one form a thin (0.1-10 m) film on the surface. A somewhat better method than “dip-coating” for producing more uniform films is “spin-coating” (which is also widely used in the production of semiconductor chips). Polymer layers can also be produced by electrodeposition or by inducing the polymerization of monomers at the electrode surface by electrochemical or other (rf plasma) means. Several types of polymer electrodes have been studied. In some, the polymer itself is electroactive and can undergo redox reactions. In others the polymer is a polyelectrolyte, that is, a material which contains ionic groups, which can extract charged ions from the solution and hold them by electrostatic binding (e.g. poly(vinylpyridine) or Nafion). After the surface of an electrode has been subjected to one of the above treatments, one must prove that the surface has indeed been modified and find out about the properties and nature of the layer. Since one is dealing with a very small amount of material on the surface, rather sensitive analytical techniques are required. Generally one wants to learn about the amount

of material on the surface layer, how easy it is to oxidize or reduce this material, what is its composition and structure, and what are its other chemical and physical properties.

Electrochemical methods. Even monolayer amounts of material can be analyzed electrochemically, since small currents can be measured quite readily and, by Faraday’s law, 10-10 moles of material is equivalent to about 10 microcoulombs (n = 1). Thus, the cyclic voltammetry of a monolayer of material (Figure 15) will show a peak with an integrated area equivalent to that amount of material on the electrode surface. The location of the peak on the potential axis is a direct measure of the redox potential of the couple on the surface. Frequently this potential is very near that found for the same or a closely related couple in solution. For thicker layers, the electrochemical response will show larger integrated areas, representing greater amounts of material on the electrode. A detailed inspection of the nature of the response and the shape of the curves can provide information about chemical interactions in the films and the rate and mechanism of charge transport through them.

Spectroscopic techniques. Electron spectroscopy (such as X-ray photoelectron and Auger

electron spectroscopy) can be used for an analysis of the elements present in the surface layers. For example, a nitrobenzene group attached to a tin oxide electrode via a silane bridge will show peaks for Sn, Si, C, and N. Direct optical spectroscopic measurements in the visible, IR and UV regions are also possible, either by absorption (when the films are on a transparent electrode substrate), or by reflectance (e.g. with metal substrates).

Other methods. Direct observation of the surface of optical or scanning electron microscopy is often useful for multilayer films to provide information about the texture and porosity of the layers. An important measurement that is surprisingly difficult to make is the determination of the thickness of the coating. Techniques for measuring hard, dry, films with thicknesses greater than 100 Å have been developed for applications in semiconductor technology. There are problems in applying such techniques to the softer films on electrode surfaces. Moreover, such films change dimension when they are immersed in the electrolyte solution, so that measurements of dry thickness can only be considered as rough estimates for the electrodes as used under electrochemical conditions. Many of the studies of modified electrodes were undertaken simply because electrochemists were curious about how species attached to the electrode surfaces behave compared to these species in solution. However, such modified electrodes have possible applications in all kinds of devices; several will be described briefly below.

Electrocatalysis. Many reactions that one would like to carry out in electrochemical cells, such as the reduction of oxygen to water or the oxidation of natural gas components to CO 2 (for use in fuel cells), do not occur readily at inexpensive electrode materials (e.g. carbon). Thus, it is necessary to catalyse these reactions by introducing suitable, stable, layers to the electrode surface. It is hoped that by applying known chemical principles about structure and reactivity, useful surface layers can be designed for particular reactions of interest. Although practical catalysts of this type have not yet emerged, this remains an important and promising area of research. Perhaps the most successful example of this work is the modification of a graphite surface by irreversible adsorption of a cofacial dicobalt porphyrin dimer which allowed the reduction of oxygen under conditions where reduction was not possible on the substrate itself. More importantly, the path of the reaction was shown to be very sensitive to details of the structure of the attached molecule; these results could be rationalized by how the oxygen molecules “fit” into the structure of the catalyst.

Figure 2.6.1 - (A) Ideal cyclic voltammetric behaviour for a surface layer on an electrode. The amount of material on the electrode, Iâ&#x20AC;&#x2122;, can be obtained from the area under the wave, Q (in coulombs) (n is the number of electrons in the electrode reaction, F is the Faraday, and A is the electrode area). Such ideal behaviour is only approached for slow scan rates for films that show no intermolecular interactions and rapid electron transfers. In most cases, the cyclic voltammetric behaviour is less ideal, as shown in B. (B) Experimental cyclic voltammograms for a poly(vinylferrocene) film on a platinum electrode in acetonitrile solution at scan rates of (a) 2 mV s-1; (b) 10 mV s-1; (c) 0.2 V s-1; (d) 10 V s-1.

Display devices. Electrodes in electrochemical cells that change colour or emit light when excited electrically are of interest in the production of displays for electronic devices. For example, electrodes modified with polymer layers can change colour when they are oxidized or reduced (i.e. show electrochromic behaviour). Specially designed polymer electrodes will emit light when electrochemically reduced or oxidized (electrogenerated chemiluminescence). Analytical applications. Molecules may be preferentially extracted into a surface layer and thus be concentrated from a bulk solution. For example, Ru(II) (EDTA) will extract into a poly(4vinylpyridine) layer from a solution as dilute as 5 x 10-8 M and become detectable by electrochemical techniques (Figure 16). Nonelectroactive polymer layers may also be used to produce a selective electrode that will show only certain molecules from solution to reach the electrode surface. For example, thick membranes of polyethylene placed on surfaces of gold electrodes while allowing oxygen to get through. These devices are used to monitor the oxygen concentration in the blood. Similarly designed modified electrodes with even better discriminative properties could find numerous analytical uses.

Figure 2.6.2 - (A) Cyclic voltammogram for a poly(4-vinylpyridine)-coated electrode after immersion in a 5 x 10-8 M solution of Ru(II) (EDTA) at scan rate of 200 mV s-1. The dashed curve is the background current obtained in the absence of Ru(II). The area under the small wave represents 6 x 10 -11 mol cm-2 of Ru-complex extracted from the solution. (B) Schematic picture of the extraction of the Ru-complex by the polymer film.

Photoelectrochemical applications. Techniques for surface modification are useful in preventing photocorrosion in semiconductor electrodes used in photoelectrochemical cells for solar energy conversion. For example, a silicon electrode in an aqueous solution usually rapidly forms an insulating oxide layer upon immersion that prevents useful operation of the cell. When a layer of the electronically conductive polymer polypyrrole is attached to the electrode surface, the silicon electrode shows much more stable behaviour. Modification of semiconductors can also improve the efficiency of operation (in terms of solar energy converted to electrical energy) and can be employed to incorporate catalysts on the electrode to promote desired reactions. Modification techniques also appear useful in attaching dyes to electrode surfaces; these can absorb light and sensitize electron transfer reactions at the electrode. The study of modified electrodes remains a field of high activity. Many new types of surface structures are being prepared, and electrochemical studies are leading to better insights into the way charge is transported through surface layers and how charge is exchanged between surface species and molecules in solution.

2.7. Photoelectrochemical processes In 1839 the French physicist, Edmond Becquerel (not to be confused with his son, the discoverer of natural radioactivity, Henry Becquerel), noticed that a voltage and a current were produced when a silver electrode immersed in a chlorine electrolyte was illuminated. This was the first observation of the photovoltaic effect. The next major advances in photoelectrochemistry occurred some 125 years later at Bell Laboratories, shortly after the invention of the p-n junction silicon solar cell, when Brattain and Garrett studied the physics of illuminated semiconductor/electrolyte interfaces. Subsequent work at Bell Laboratories by Dewald, Boddy and Turner, in Germany by Gerischer and Memming, and in Russia by Myamlin and Pleskov resulted in the beginning of a fundamental understanding of the behaviour of semiconductor electrodes. The conceptual leap which brought photoelectrochemistry from the fundamental to the practical occurred in Japan in 1972 when Fujishima and Honda studied the photooxidation of water to oxygen at illuminated semiconducting TiO2 electrodes. They suggested that such a system would be applicable to the problem of using sunlight to photoelectrolyse water into H2 and O2, a process which results in the conversion of sunlight to stored chemical energy. The large band gap, but very stable, metal oxide semiconductors (i.e. TiO2, SrTiO3, WO3) have the disadvantage that they are capable of converting only a small fraction of the solar spectrum into electrical or chemical energy. Since 1974 the main research efforts, which have been pursued by an increasing number of researchers in laboratories all over the world, have been to study semiconductors which have smaller band gap materials (e.g. 1.1 to 1.5 eV), which are capable of converting sunlight to usable energy at high efficiency, tend to be unstable when illuminated in electrolyte solutions. Photoelectrochemical systems have been very useful for carrying out chemical reactions. Those of particular interest for practical use are as follows: (i) Fuel production such as the splitting of water to produce H2 or the reduction of CO2 to produce formaldehyde and methanol; (ii) Water purification; (iii) Photo fertilization via N2 reduction; (iv) Generation of Cl2 from seawater; (v) New photosynthetic and photocatalytic reactions, etc. In the area of photoelectrochemical cells which have only electrical energy as an output, we have the photovoltaic cells and the electrochemical photovoltaic cells or semiconductor liquid ignition cells. Photogalvanic cells use metal electrodes and produce electricity as a result of light interacting with photosensitive chemicals in the solution; these are more academic than of practical interest. The electrochemical photovoltaic cell makes use of a semiconductor electrode for both light absorption and separation of the photogenerated electron-hole pair. This type of cell is the most easily constructed of all solar cells because all that needs to be done is to immerse a semiconductor electrode and another inert electrode into an appropriate redox electrolyte and connect them through an appropriate load. Electricity is produced when the electron hole pairs created by illumination of the semiconductor are separated in the space charge layer near the semiconductorâ&#x20AC;&#x201C; electrolyte interface with the majority carrier being driven into the bulk of the semiconductor while the minority carrier is driven to the semiconductor-electrolyte interface. The minority carrier (holes in an n-type material or electrons in a p-type material) oxidizes or reduces the redox species in the electrolyte while the majority carrier travels around the circuit through a load and accomplishes the opposite reaction at the dark electrode. (The dark electrode is often called the counter

electrode and can be constructed of any inert conducting material, i.e. platinum or carbon). The difficulty, for this type of cell, arises because the minority carriers are usually highly reactive and so the semi-conductor itself may undergo oxidation or reduction reactions with the photogenerated minority carrier. This process is known as photocorrosion and has been the major obstacle in the path for the development of photoelectrochemical solar cells. Despite the fact that all small band gap semiconductor electrodes are thermodynamically unstable toward photocorrosion, a high degree of kinetic stability has been achieved in several systems. Perhaps the major advantage of a semiconductor liquid junction, over solid state junctions, is that a high percentage of the single crystal efficiency can be retained in a polycrystralline material. This fact is due to the ability of the junction-forming liquid to conform to the uneven surface of the small crystalline grains which make up a polycrystalline material. This is important because the cost of producing thin films of polycrystalline semiconductors is considerably less than that for large single crystals of any given semiconductor. Ultimately, it is the cost per watt of electrical power produced that determines the practical utility of any alternative energy source. Table 3 shows sunlight conversion efficiencies, and the preparation method for the most efficient polycrystalline semiconductor liquid junction solar cells, which have been reported in the literature. One feature to notice from Table 3 is that many different preparation techniques, even for the same material, can result in reasonably efficient solar cells. Table 2.7.1 - Sunlight conversion efficiencies and preparation method for efficient polycrystalline semiconductor liquid junction solar cells Semiconductor

Redox electrolyte

Sunlight efficiency

Preparation technique

n-CdSe0.65Te0.35

1M Na2S2, 1M KOH

7.9%

Painting slurry

1M K2Se, 0.1M K2Se2, 1M KOH

7.8%

CVDa

n-GaAs p-InP

0.3M V+3, 0.05M V+2, 5M HCl

7.0%

CVDa

n-CdSe

1M Na2S2, 1M NaOH

6.5%

Vacuum coevaporation

n-CdSe

1M Na2S2, 1M NaOH

6.3%

CBDc

n-CdSe

1M Na2S2, 1M NaOH

5.5

Electrodepositon

n-CdSe

1M Na2S2, 1M NaOH

5.3

Hot pressed

Chemical vapour deposition

Transient efficiency after treatment with Ag+

Chemical bath deposition

Photoelectrosynthetic cells are devices in which a chemical change is brought about at an

electrode-electrolyte interface when the semiconductor electrode is illuminated with light of energy equal to or exceeding the band gap energy of the semiconductor. If the photoinduced chemical reactions at the cell electrodes result in energy being stored (a positive change in free energy, G) the cells are called photoelectrolysis cells. If the opposite is true, the cells are called photocatalytic cells because the light energy is used to overcome an energy barrier of a normally “downhill” reaction. The fact that photocatalytic cells do not result in the net conversion of solar energy to stored energy does not imply that such devices are of no use. Many of the reactions which are carried out on an industrial scale in Europe require a large energy input to promote faster rates for thermodynamically “downhill” reactions (e.g. N2 to NH3 and sulphuric acid production).

If a photoelectrosynthetic cell requires an external electrical energy input as well as a light energy input to accomplish the desired electrochemical reactions, the cell would be called a photoassisted electrolysis cell or a photoassisted electrocatalytic cell. If both electrodes are semiconductors and one is n-type and the other p-type, it would be called a photoelectrochemical diode or a p-n photoelectrosynthetic cell. The advantage of using two semiconductors is that the photovoltages of the two electrodes add such that both the oxidation reaction (at the n-type electrode) and the reduction reaction (at the p-type electrode) are photodriven. This is analogous to photosynthesis in green plants where both the water oxidation and carbon dioxide reduction reactions are light driven. Much of the fundamental research being conducted in photoelectrochemistry is still directed toward solving the ever-present problem of stabilizing smaller band gap semiconductors. Techniques which have been tried include the use of non-aqueous solvents, applying polymer layers to the surfaces of semiconductors, testing new materials with more intrinsic stability, and attaching catalysts to the semiconductor surface to promote the desirable electrochemical reactions over photocorrosion. Recent studies have advanced our understanding of the factors which control the energy positions of the semiconductor band edges at the semiconductor-electrolyte interface.

Surface states, i.e. states associated with the atoms at the interface, play an important role in the

energetic and chemistry of the semiconductor, and recent studies have addressed their measurement and chemical nature.

Hot carrier processes at the semiconductor-electrolyte interface have been predicted and observed, with several semiconductors. A hot electron or a hot hole is a carrier which is not in thermal equilibrium with the band edges.

Theoretical models being developed for the current voltage behaviour of photoelectrochemical cells have been increasing not only in their sophistication but also in their relevance and applicability to real systems. An evaluation of the physical configurations of electrochemical photovoltaic cells has recently appeared. Many other significant contributions to this emerging field have been made but are too numerous to be discussed in this chapter. However, the reader can find more detailed discussions in the many excellent reviews of various aspects of photoelectrochemistry found in text books and scientific journals. It is clear that the worldâ&#x20AC;&#x2122;s long range energy future must contain a substantial contribution from renewable energy sources. Electrochemical cells and novel reactions for the production of electricity, new chemicals and fuels seem to be the next challenge for the field of industrial electrochemistry.

REFERENCES 1.

M.M. Baizer, H. Lund, eds., Organic Electrochemistry, Marcel Dekker, New York, 1991.

A.J. Bard, L.F. Faulkner, Electrochemical Methods, 2nd ed., Wiley, New York, 2001.

G. Dryhurst, K.M. Kadish, F. Scheller, R. Renneberg, Biological Electrochemistry, Vol. I, Academic Press, New York, 1982.

T.Z. Fahidy, Principles of Electrochemical Reactor Analysis, Elsevier, Amsterdam, 1985.

C.H. Hamann, A. Hammett, W. Vielstich, Electrochemistry, Wiley-VCH, Weinheim, 1998

F. Hine, Electrode Processes and Electrochemical Engineering, Plenum Press, New York, 1985.

A.E. Kaifer, M. Gomez-Kaifer, Supramolecular Electrochemistry, Wiley-VCH, Weinheim, 1999.

J. Koryta, Ions, Electrodes and Membranes, 2nd ed., Wiley, Chichester, 1991.

A. Memming, Semiconductor Electrochemistry, Wiley-VCH, Weinheim, 2001.

10.

D. Pletcher, F.C. Walsh, Industrial Electrochemistry, Chapman and Hall, London, 1990.

11.

Roušar, K. Micka, A. Kimla, Electrochemical Engineering, Elsevier, Amsterdam, 1986.

12.

K. Scott, Electrochemical Reaction Engineering, Academic Press, London, 1992.

13.

C.A.C. Sequeira, ed., Environmentally Oriented Electrochemistry, Elsevier, Amsterdam, 1994.

14.

W. Wallace, A. Nozik, S. Deb, eds., Photoelectrochemistry: Fundamental Processes and Measurement Techniques, Electrochemical Society, Princeton, 1982.

3. Electrochemistry and Environmental Protection CĂŠsar A.C. Sequeira and Luiz Rodrigues

3.1. Introduction One of the most pressing tasks for mankind during the recent times is the protection of the environment and creation of conditions for modification of the environment in an ecologically optimal way for the further development of our life in this planet. From the historical point of view, it can be mentioned that the problem of environmental protection is not new: these questions were discussed already by Hippocrates (460-377 B.C.) in his book on Air, Water and Environment. A great variety of branches of science participate actively in the solution of environmental problems, including medicine, biology, physics, chemistry and electrochemistry. The latter is important in several ways. In addition to the analytical aspects, electrochemistry is used in the removal of toxic substances or other products and wastes polluting the environment as a result of industrial activity. Electrochemistry can also help in decreasing or eliminating the occurrence of some substances in several industrial fields. Moreover, electrochemistry can, and most probably will, affect energy production and all kinds of transport, which are among the major polluters of the environment. This chapter summarizes some potentialities offered by electrochemistry in the area of environmental protection.

3.2. Electrochemical sensors and monitors Electroanalytical methods are often seen as an effective tool to study chemical and biological systems. This section gives an overview of the most common electrochemical sensing techniques, their basic working principles, paying a brief look to some recent applications and developments. The various kinds of electrochemical sensors can be split into the following main classes:

I. II.

Potentiometric, implying a pure voltage measurement; Amperometric, where current in a close loop involving two electrodes is the measured

III. IV.

Conductimetric, where the conductance of an electrochemical cell is determined; Voltammetric, where information about the analyte is derived from the measurement of

variable;

current-voltage or current-time curves, in a three-electrode cell configuration.

3.2.1. Potentiometric sensors Electrodes for potentiometry are based on the interface between two electrolyte phases, which are put into contact. A non-polarized interface (selective or not) exhibits an half-cell potential, that is related to the activities of the species involved into the equilibrium of the electrochemical potentials. The half-cell potential (or formal potential) is defined as the electrical potential measured with respect to a standard hydrogen reference electrode. The electrochemical potential, in a location P, can be defined as (3.2.1) where zi is the charge of the species i, F is the Faraday’s constant,  is the electrical potential at the location P for phase , is the standard chemical potential of the species i in phase , and is the activity of the species i in phase . Non-polarized interfaces reach equilibrium conditions that are governed by activities of the ions; the difference in electrical potential that makes the electrochemical potential constant across the interface, depends on said activities as explained by the Nernst equation put into the following form:

(3.2.2)

where  is the interfacial potential difference, 0 is the standard interfacial potential difference, measurable with equal activities in the phase , , is the activity of the species in phase , and is the activity of the species in phase . The two phases  and  may represent the sample phase and the electrode phase, according to the described approach. Thus,  is the voltage measurable across a semi-permeable membrane

between the sample and an unpolarized reference. It is now clear how selective membranes, that respond in a Nernstian fashion to specific electroactive species, can be the fundamental building block of Ion-Selective Electrodes (ISE). In this context, ion activity can be measured with a twoelectrode configuration, where the WE (often called indicator) is actually a reference electrode interfaced to the sample via a specialized membrane and a reference solution in between, having the analyte at a fixed activity. 3.2.1.1. Ion-selective electrodes Ion-selective electrodes are based on an ion selective membrane that interfaces an internal reference to the external environment (that is, the sample). Glass membranes are probably the most commonly used; in fact, they have been developed and used since the beginning of the 20th century, especially for the measurement of pH. Other ion-selective membranes, both solid-state and liquid, have been developed and are commercially available; the use of some particular kinds of membranes, such as the lipid/polymer ones and chalcogenide glasses, is still matter of research, especially for the development of arrays and sensors. When the ion selective membrane uses a glass/electrolyte interface, the whole cell is configured in the following way: Ag/AgCl/HCl (c)/Glass membrane/Sample//Reference

(3.2.3)

where / denotes the interfaces, // denotes the junctions, and c is the concentration of the reference solution. The internal reference in this example is of the Ag/AgCl kind; the external reference electrode can be of the same kind, but not necessarily. In the case of pH electrodes, the membrane essentially responds to H+ ions, and is formed by a dry bulk and two thinner dry layers that come into contact with the liquid phase. In the bulk structure only mobile cations present in the glass, such as Na+ or Li+, are substantially responsible for electrical conduction. 3.2.1.2. Semiconductor field-effect sensors A typical issue in the use of glass electrodes lies in their high output impedance, that is, the need for the voltmeter device, placed between the ISE and REF electrode, to drain the smallest possible current from the measurement loop. In fact, typical impedance values for commercially combined pH electrodes (embedding their REF) are in the order of the hundreds of MOhms. The front-end electronic component typically used in the electrometers for potentiometry is a kind of FET (Field-Effect Transistor) that may be carried out in different ways, according to the particular technology employed. In general, the active building-blocks of the amplifier and buffer stage will be one or more FETs, with insulated gates in bipolar or MOS (Metal Oxide Semiconductor) technology; such stage has to process the weak signal coming from the electrodes and adapt its impedance level to the input of data acquisition and display devices, connected to the output of this buffer/ amplifier stage.

The idea to embed an ion-sensitive capability into a FET device comes from the early 70s, and has generated the first integrated chemical sensor. CHEMFETs (CHEMically sensitive FETs) can be mainly split into two categories:

I. II.

ISFETs, based on an ion-sensitive membrane on behalf of the metalized gate of standard FETs ; ENFETs, based on a chemically selective enzyme layer.

To avoid confusion, it must be clarified that some authors consider ENFETs a mere kind of ISFETs, and do not use the term CHEMFET. The structure and a simplified schematic diagram for potentiometric measurements performed by using an ISFET device are shown in Figure 1. The VREF generator provides the right Gate voltage to polarize the FET for a correct modulation of the channel into the desired working region. Channel conductance is a function of the ion activity because of the electric field generated by the charges accumulated on the membrane; thus, the observable is the current i, which is also a function of the Drain.

Figure 3.2.1 - Structure of a CHEMFET/ISFET device. Electrical connections are shown.

3.2.1.3. Potentiometric measurements Among the many sensor arrays developed recently, we can mention food and beverage quality tests, pollution monitoring of superficial and ground water, drinking water quality tests and classification, including water distribution networks. This class of devices is referred to as electronic tongue devices. This is the wet counterpart of the electronic nose concept, which has been very much explored in the scientific literature, especially in the early 90s. Arrays of screenprinted electrodes made of carbon paste on a polymeric substrate can be found in the last ten years. Good discrimination capability has been shown in the classification of fruit juice, wine and water. Data fusion between potentiometry, gas chromatography and spectrophotometry has been presented for the characterization of wines, while a general approach for the combination of liquidphase information (electronic tongue) and vapor-phase information (electronic nose) has been proposed by Winquist and collaborators, in 1999.

3.2.2. Conductimetric sensors Conductance and impedance measurement techniques play a significant role in the arena of electrochemical methods. The current flowing in a solution under the influence of an electric field is due to the migration of ions; individual ionic currents are proportional to the strength of such field, and are determined by the balance between the frictional drag and said electrical field, which bounds the terminal velocity of each ionic species, thus its associated current density. Conductance, which is the reciprocal of resistance, is generally defined for a given volume, delimited by parallel current flow lines, as: (3.2.4)

where  is the electrical conductivity of the medium, S is the cross-sectional area of two finite surfaces normal to the flow lines, and L is the path length between the two said surfaces. G is expressed in -1 (or Siemens) while , which is an intrinsic property of the solution, is expressed in -1 cm-1. The contribution of each ionic species in the solution to the total conductivity is called transference number, and is defined as follows:

(3.2.5)

where i, j are the subscripts denoting the generic i, j ion species, i is the mobility of the generic ith ion species, Ci is the concentration of the i-th ion species, and is the normalized charge for the generic i-th ion species. In most of the applications, the conductivity of a solution is measured in a two-electrode cell configuration, with two identical metal electrodes; conductance is usually measured between two terminals, either by measuring the current flowing at a known imposed voltage, or by measuring the potential drop at a known injected current, finally, various versions of bridge configurations have been seen in the literature. 3.2.3. Voltammetric sensors There is a wide range of possible implementations of measurement methods based on voltammetry, according to the type of the working electrodes (i.e. solid metal microelectrodes, dropping mercury, planar rotating disks, carbon electrodes, etc.), and according to the particular electrochemical experiment, which depends on the program imposed by the generator. There is a huge quantity of useful readings to get a deep knowledge about polarographic methods and more recent applications of voltammetry. Under the theoretical point of view, the difference of electrochemical potential between two points (A,B) of a solution can arise because of:

I. II.

A difference in the concentration of electroactive species over the distance A to B ; An electric field with a non-zero component along the direction A-B.

These two phenomena contribute to the mass transport by diffusion and migration, respectively; the flux of charged species transported can be expressed in terms of current density. For non-convective mass transfer, in the proximity of the electrode, the electroactive substance is transported by both the processes previously described (migration and diffusion). In this case, the general flux equation is given by:

(3.2.6)

where Dj is the diffusion coefficient of species j, and Cj is the local concentration of species j. Stirred solutions, which are mostly used in practice, involve forced convection. In this case, a model has been proposed where it is assumed that convection maintains the concentrations of all the species uniform and equal to the bulk values, till a certain distance ď ¤ from the electrode. Within this layer having thickness ď ¤, it is assumed that there is no solution movement, thus mass transfer is thought to be due by diffusion only. The use of pulse voltammetry, with a series of rectangular pulses modulated by a ramp, has been presented in different contexts as an efficient sensor technique for automatic classification/ characterization purposes. In fact, some recent developments are concerning signal processing aspects, in the framework of pattern recognition techniques for automatic monitoring systems. In many of these works, focus has been given to drinkable water distribution surveillance and to food processing applications. Methods based on polarography and chronoamperometry are also in deep advancement; for example, biosensors are being researched which combine impedance spectroscopy with chronoamperometry in order to determine the charge-transfer resistance in the measurement of the faradaic impedance. Biosensor may be defined as a self-contained integrated device that is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor), which is retained in direct spatial contact with a transduction element. Or, easier, biosensor is an analytical device composed of a biological sensing element (enzyme, receptor antibody or DNA) in intimate contact with a physical transducer (optical, mass or electrochemical) which together relate the concentration of an analyte to a measurable electrical signal. The main advantages offered by biosensors over conventional analytical techniques, in particular for environmental applications, are the possibility of portability, of miniaturisation and working on-site, and the ability to measure pollutants in complex matrices with minimal sample preparation. Despite these advantages, the application of biosensors in the environmental field is still limited in comparison to medical or pharmaceutical applications, where most research and development has converged. The development of electrochemical biosensors is probably one of the most promising ways to solve some problems concerning sensitive, fast and cheap analytical techniques for environmental applications. In fact, different combinations of electrochemical transduction with enzymatic, bacterial, cellular and DNA biochemical recognition elements has been used for determination of pollutants such as phenols, linear alkyl benzene sulphonate (LAS), toxicity, and various metals and anions in river waters or wastewaters and some commercial biosensors for environmental applications using electrochemical sensing element are available nowadays.

3.3. Analysis and removal of gaseous pollutants The rapid detection of ammonia or oxygen plays a vital role in pollution control. Gas sensing electrodes are highly selective devices for monitoring these (and other) gases. Such sensors commonly incorporate a conventional ion selective electrode, surrounded by an electrolyte solution and enclosed by a gas permeable membrane. The target gas diffuses through the membrane and reacts with the internal electrolyte, thus forming or consuming a detectable ionic species. The ammonia selective probe uses an internal pH glass electrode in connection with an ammonium chloride electrolyte. The glass electrode detects the decreased activity of protons. While most gas sensors rely on potentiometric detection, the important oxygen probe is based on covering an amperometric platinum cathode with a Teflon or silicon rubber membrane. Handheld and remote oxygen probes are available commercially. Potentiometric sensors for other gases (SO2, NO2, HF, etc.) have been designed by using different membranes and equilibrium processes. Among recent developments, we should refer several types of semiconductor sensors that serve for detection and determination of hydrocarbons, alcohols, ethers, ketones, esters, nitrated compounds, ammonia, carbon monoxide, hydrogen, methane, etc., with a detection limit which is often lower than 0.1 ppm.

3.4. Metal ion removal and recovery The need to protect the environment from further contamination by transition and heavy metal ions is well established and universally reinforced by legislation which sets impositions to chemical plants, factories and other facilities employing solutions of such metals. A situation of particular concern, that we will address here, is the metal ion removal and recovery from aqueous effluents. Several issues must be considered before deciding on an appropriate approach:

I. II. III. IV. V.

What is the real objective of the effluent treatment process: merely to meet the legal limit on discharge or also to recover and recycle the metal? Can the metal ion be reduced to metal at a cathode? What forms of the metal are acceptable products from the process? What are the concentrations of metal ions in the effluent? What is the overall composition of the solution to be treated?

Several technologies are based on the precipitation of the metal ions as the hydroxide. The hydroxide ion results from the cathodic reduction of water: 2H2O + 2e- ď&#x201A;Ž H2 + 2OH-

(3.4.1)

which then reacts with metal ions in the medium. In the Ionsep process, the contaminated solution is fed to the anode compartment of a cell separated by a cation exchange membrane. During water electrolysis within the cell, the metal ions migrate through the cation permeable membrane into the catholyte where they are precipitated by the hydroxide formed by reaction (7) at the cathode. The process has been used for the removal of ions such as (Cu)II, Cd(II), Fe(II), Ni(II), Al(III) and Cr(III). The Andco process employs an undivided cell with an iron anode. During operation, the cell produces Fe(II) at the anode and hydroxide at the cathode; these combine to precipitate iron hydroxide and it is found that other ions in solution co-precipitate with the iron hydroxide. The co-precipitation occurs by a combination of mechanisms which include simple precipitation of the metal hydroxide/oxide as well as surface complexation, adsorption and electrostatic interaction with the surface of the iron hydroxide. The addition of polymers assists the co-precipitation and filtration and large amounts of ion removal are possible for relatively low charges through the cell. For example, the arsenic content of an effluent from a GaAs semiconductor plant could be reduced from 5.2 ppm to 45 ppb with 50 ppm of anodically generated iron hydroxide. In another application, groundwater from a superfund site is treated and the Hg, Cd, Pb, As, Se and Cu can all be removed to below their consent levels. It is claimed that the process can be used to remove Be, F, Mg, Al, P, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Mo, Po, Ag, Cd, In, Sn, Sb, Te, Ba, W, Pt, Au, Hg, Ti, Pb and Bi and the precipitating metal hydroxides/oxides can further adsorb toxic organics from the effluent. The cathodic deposition of metals has the attraction that the metals can be recovered from the cell in a valuable form, either high purity metal or a concentrated and pure metal salt solution. In all these technologies, the effluent will be fed to the cathode in a divided or undivided cell where the reaction Mn+ + ne- ď&#x201A;Ž M

(3.4.2)

is carried out. Clearly, the method is limited to those ions which reduce to the metal at potentials less negative than water reduction (equation (7)), which is now an unwanted, competing reaction leading to loss of current efficiency. Even so, many heavy and transition metals including Ag, Au, Pt, Pd, Ir, Rh, Cu, Ni, Hg, Cd, Pb, Bi, Zn, Ni, Co, As, Sb and Te can be removed. No electrode reaction can occur more rapidly than the rate at which the reactant reaches the electrode surface. To obtain the highest rate of removal, the reduction of Mn+ should be mass transport controlled and under these conditions, the rate of removal of metal ion from an effluent by cathodic reduction may be written - V dc/dt = - IL/nF = kmAc

(3.4.3)

where V is the volume of effluent to be treated, c the concentration of metal ion (Mn+), t is the time, IL the mass transport limited current, F the Faraday constant, Km the mass transfer coefficient and A the electrode area. Integration of equation (9) with respect to time gives an expression for the fraction of metal ion removed, i.e. c(t)/c(0) = exp â&#x20AC;&#x201C; (km A/V) t

(3.4.4)

where c(0) is the initial concentration of metal ion and c(t) the concentration after electrolysis for time t. Hence, it can be seen that the rate of removal depends on two factors: (i) the electrode area and (ii) the mass transport regime determined by the flow conditions (electrolyte flow rate or electrode movement) and the presence of turbulence promoters. Note that although the current density (I/A) is proportional to the concentration of Mn+ and a higher current density leads to more rapid removal, the time taken to achieve a defined fractional removal is not dependent on the concentration. For the technology of metal ion removal by cathodic reduction, the conclusions are clear. Cells must be designed to give very high mass transport coefficients and/or to have a very high cathode surface area. The former is the basis of rotating cylinder electrode cells; particularly when plated under mass transfer control, the metal is deposited as a rough layer and rotation of this rough layer leads to greatly enhanced values of km. Increases in both km and A enhance the performance of cells with three dimensional electrodes. In such cells, A = Ae V e

(3.4.5)

where Ve is the volume of the three dimensional cathode and Ae is the electrochemically active surface area per unit volume of the three dimensional material. In employing three dimensional electrodes, there is always a concern about the potential distribution through the electrode and hence whether all the electrode is active for deposition. Fortunately, the treatment of effluents fits well with three dimensional electrodes because of the low metal ion concentration, hence low current density and the minimum possibility of unwanted voltage drops. Even so, there are limitations on the dimensions of three dimensional electrodes which can be used with advantage. The Chemelec and RETEC systems operate well as intermediate concentrations of metal ion and have commonly been used to maintain plating bath wash-waters at metal levels of 50-100 ppm and allow their continuous recycle; in this type of application, the loaded cathodes may be recycled directly as anodes dissolving in the plating bath. The enViro-Cell, Martineau and Porocell are intended more as effluent treatment systems where the recovery of metal is secondary. The metal is more difficult to recover (perhaps involving incineration of carbon based cathodes) and it

is more common to recycle the metal as a concentrated salt solution. The metal is dissolved off the cathode into a small volume of solution either chemically or anodically. Although beyond the scope of this section, two other approaches to metal ion concentration should be mentioned. These are electrodialysis, which has been applied to the concentration of several transition metals, and electrochemical ion exchange. The latter involves electrodes coated with ion exchange resin and the electrode is essentially used to control the flux of ions into/out of the resin. This technology has been successfully applied to the control of radioactive ions in the nuclear industry and also as components of systems for the remediation of soils, sludges and groundwater.

3.5. Electrochemistry of ecotoxic metals in water The most sensitive electroanalytical technique, stripping analysis, is highly suitable for the task of field monitoring of toxic metals. The remarkable sensitivity of stripping analysis is attributed to its pre-concentration step, in which trace metals are accumulated onto the working electrode. This step is followed by the stripping (measurement) step, in which the metals are â&#x20AC;&#x153;strippedâ&#x20AC;? away from the electrode during an appropriate potential scan. About 30 metals can thus be determined by using electrolytic (reductive) deposition or adsorptive accumulation of a suitable complex onto the electrode surface (Figure 2). Stripping electrodes thus represent a unique type of chemical sensors, where the recognition (accumulation) and transduction (stripping) processes are temporarily resolved. Short accumulation times (of 3-5 min.) are thus sufficient for convenient quantitation down to the sub-ppb level, with shorter periods (1-2 min) allowing measurements of ppb and sub-ppb concentrations. The time consuming deaeration step has been eliminated by using modern stripping modes (e.g. potentiometric or square-wave stripping), that are not prone to oxygen interferences. Stripping analysis can provide useful information on the total metal content, as well as characterization of its chemical form (e.g. oxidation state, labile fraction, etc.). Field measurements of chromium (VI) represent one such example. Overlapping peaks, formation of intermetallic compounds and surfactant adsorption represent the most common problems in stripping analysis. Various advances in stripping analysis should accelerate the performance of on-site environmental testing of toxic metals. New sensor technology has thus replaced the traditional laboratory-based mercury electrodes and associated cumbersome operation (oxygen removal, solution stirring, cell cleaning, etc.). Of particular significance are new stripping-based tools such as automated flow systems for continuous on-line monitoring, disposable screen-printed stripping electrodes for single-use applications or remote/submersible devices for down-hole well monitoring or unattended operations. Portable and compact (hand-held) battery-operated stripping analysers are currently being commercialized for controlling these field-deployable devices. In addition to providing on-site real-time information, such in situ devices should minimise errors (due to contamination or loss) inherent to trace metal measurement in a centralized laboratory. Stripping analysis has been extensively used by marine chemists on board ships for numerous oceanographic surveys. Relevant examples of environmental applications of stripping analysis are given in Table I. In addition to trace metal pollutants, it is possible to employ new adsorptive stripping procedures for measuring low levels of organic contaminants that display surface-active properties (e.g. detergents, oil components). However, due to competitive adsorption such schemes usually require a prior separation step. Another version of stripping analysis, cathodic stripping voltammetry, can be used for measuring environmentally-relevant anions (e.g. S-2, I-, Br-) or sulphur- or chlorine-containing pollutants (e.g. pesticides) following their oxidative deposition onto the working electrode. Additional background information on stripping analysis and its environmental opportu-nities can be found in various books or reviews.

Figure 3.5.1 - Steps in anodic (A) and adsorptive (B) stripping voltammetry of trace metals based on electrolytic and adsorptive accumulation, respectively, of target metal analytes.

Table 3.5.1 - Typical environmental applications of stripping analysis Trace metal

Matrix

Electrode

Stripping mode

Natural waters

Gold

Differential pulse

Lakes and oceans

Mercury film

Differential pulse

Seawater sediments

Mercury drop

Adsorptive

Tap water

Mercury film

Potentiometric

Seawater

Gold

Differential pulse

Natural waters

Mercury drop

Potentiometric

Seawater

Mercury drop

Adsorptive

Lakes and oceans

Mercury film

Differential pulse

Sediments

Mercury film

Potentiometric

River water

Gold

Potentiometric

Natural waters

Mercury film

Differential pulse

Groundwater sediments

Mercury drop

Adsorptive

3.6. Electrochemistry in drug and food control A radical change in the applicability of voltammetric methods in food and drug control took place in the 1970â&#x20AC;&#x2122;s and is closely connected with the introduction of pulse-polarographic metods, in particular with that of DPP (differential pulse polarography), and with the appearance of pulse polarographs on the market. The DPP increased the sensitivity by several orders of magnitude from 10-4 to 10-6 â&#x20AC;&#x201C; 10-8 mol.L-1, or perhaps 10-9 mol.L-1, in practice this allows a determination of a substance with M = 300 g and n = 2 at a concentration in the range of 1-10 mg.mL-1. Very low concentrations may be measured with the help of the stripping method based on preceding adsorption without an electrochemical process at the electrode. However, there is another recently introduced improvement in polarographic techniques, namely static mercury drop electrode (SMDE). This electrode represents a non-electronic approach to obtaining a constant electrode surface. It follows from recent literature on analytical applications of polarography and from the comparison of pertinent equations for the polarographic current in DC and DP polarography (under assumption of linear diffusion): id = nFAc D/tD

(3.6.1)

where A is the electrode area, id is the diffusion-controlled current, tD is the drop time, c is the concentration of the electroactive species, D is the diffusion coefficient, and F is the Faraday constant for the diffusion-controlled current, and (id)p = nFAc D/tm

(3.6.2)

for the current in the pulse polarographic method (in normal pulse polarography). Here, t m is the time at which the current is measured after application of the pulse. The equations demonstrate that a longer decay is associated with a DC measurement; this leads to a smaller faradaic current in DC polarography. It further implies that the faradaic current/charging current ratio is more favourable. With a small noise, the DC method can be superior. The following experimental results in Table 2 show that DC polarography at the SMDE rivals the DPP techniques because of no difference in the detection limit and much simpler instrumentation. Hence, a renaissance of the use of DC polarography was followed because the use of SMDE became widespread.

Table 3.6.1 - DC, normal pulse and differential pulse polarography at the DME and SMDE tD/s

Detection limit c/mol.L-1

Reversible: Cd(II) + 2e-

DME

(0.1M NaNO3)

SMDE

Irreversible: Ni(II) + 2e-

DME

(1M KCl)

SMDE

Pulse amplitude E = -50 mV * Electrode growth period 50 ms. ** Electrode growth period 200 ms.

DC 5.0

2.10-6

NP 2.0

4.10-7

DPP 2.0

8.10-8

DC 0.5*

1.10-7

NP 0.5*

2.10-7

DPP 0.5*

1.10-7

DC 2.0

5.10-6

NP 2.0

3.10-6

DPP 2.0

7.10-7

DC 0.2*

3.10-7

NP 0.5**

5.10-7

DPP 0.5*

5.10-7

3.7. Electrochemical degradation of organic compounds Besides to the application in environmental sensors and monitoring systems, electrochemical methods are increasingly being used for environmental remediation, in particular, in the degradation and destruction of organic compounds contained in wastewaters and contaminated soils [1], in CO2 fixation, in the electrochemistry assisted photocatalytic degradation of recalcitrant compounds [2,3], and in the electrochemical clean-up of flue gases. In fact, electrochemical techniques may offer an attractive alternative and environmental friendly process for treating aqueous effluents containing organic compounds, since it does not require the addition of any chemicals, electrons being the only reactants added to the system to stimulate the reaction. Electrochemical techniques used in wastewater treatment generally include electrooxidation and electrocoagulation. Electrooxidation is a mediated reaction and occurs via oxygen atom’s transfer from water in the solvent phase to the oxidation product [1], as represented in equation 14: R + x H2O  ROx + 2 x H+ + 2 x e-

(3.7.1)

where R is an organic compound that may mineralize in a “combustion” reaction [1] expressed through reaction scheme (15): R + x H2O  x/2 CO2 + 2 x H+ + 2 x e-

(3.7.2)

Direct electrochemical oxidation has however an important handicap: the anodic oxidation of water to give O2 cannot to be suppressed, and consequently only low currents yields can be achieved. On the other hand, the low miscibility of most organic compounds with water, the mass transfer from the bulk solution to the anode is hindered, and, as a result, the space-time yields are also low [1]. Besides, electrolysis consumes a lot of energy, in particular in dilute wastewater treatment, therefore indirect electrooxidation of contaminants, involving electrogeneration of strong oxidants is now preferred. In this process the toxic organic waste is oxidized in the bulk solution by a mediator, often a transition metal in a higher oxidation state, which after that is reoxidized (recycled) at the anode, allowing the oxidation of further organic molecules. To avoid the mineralization of the organic compound, which is more likely to happen on higher oxygen-evolution overpotential surfaces (containing PbO2, SnO2 or Sb2O5), the pollutants can be treated with electrogenerated H2O2 in a two-compartment cell, in which H2O2 is continuously supplied to the cathodic compartment from the reduction of O2 (equation 16) sparged on graphite or reticulated carbon electrodes [1,5,6]. O2 +2 H+ + 2 e-  H2O2

(3.7.3)

If the wastewater is treated in an undivided electrolytic cell, consisting of a Pt anode and a carbonPTFE cathode, where the H2O2 is generated, the pollutants are mainly destroyed by the combined action of the •HO electrogenerated at the anode, and, therefore, this type of processes are also known as advanced electrochemical oxidation processes (AEOP’s) [1].

The oxidizing power may be enhanced by addition of Fe2+ (acidic media) due to the increase of • OH radical concentration produced by the electrochemical Fenton process given in equation (17): Fe2+ + H2O2  Fe(OH)2+ + •OH

(3.7.4)

This catalytic effect is based on the regeneration of Fe2+ through the reduction of Fe3+ in the reaction with H2O2 and at the cathode. Under UV radiation of wavelengths between 320 and 480 nm, mineralization of the pollutants may be accelerated by the photolysis of Fe3+ complexes and the reinforcement of Fe2+ regeneration due to additional photoreduction of ferric species through a photo-Fenton reaction. One consequence of this is that the electrogenerated H2O2 can also be oxidized to O2 at the anode [1]. In addition to the free radicals (•OH), other oxidizing species such as percarbonates, persulfates or chlorine, bromine and iodine can be simultaneously generated during the electrolysis of water on electrodes of Boron Doped Diamond (BDD), that may the degradation of organic pollutants [4].

3.8. Electrochemical-assisted photocatalytic degradation Electrochemical-assisted photocatalytic degradation, also called photochemically-assisted electrochemical, or simply photoelectrochemical degradation of organic contaminants in wastewater, in particular in washwater of the textile industry [7-11], and other effluents [12] have been extensively studied and applied. TiO2, in different presentation forms, was the most used photocatalyst [2,3,7,8], alone or combined with other materials, in particular PbO2, which display favorable electrochemical properties [9,10,13,14] or SnO2 [15]. Combination of electrochemical, microbial and photochemical methods for the degradation of Procion blue contained in industrial effluent was also tried with interesting success [11]. In the electrocoagulation, instead of achieving neutralization and agglomeration of charged particles of the colloidal suspensions by neutral collision with counter ions present in a added coagulant, for instance, alum, i.e., Al2(SO4)3â&#x2C6;&#x2122;18H2O, the coagulant is generated in situ by electrolytic oxidation of an appropriate sacrificial anode, mainly iron and aluminum. In this process, charged particles are removed from the effluent by allowing it to react either with ions of opposite charge , or with floc of metallic hydroxides generated within the effluent. Electrocoagulation is an alternative to the use of metal salts or polymers and polyelectrolyte additions for breaking stable emulsions and suspensions, removing metals, colloidal and solid particles, and soluble inorganic pollutants from aqueous media by introducing highly charged polymeric metal hydroxide species, which neutralize the charge on suspended solids and oil droplets to facilitate coagulation. The mechanism of electrocoagulation is highly dependent on the chemistry of aqueous medium, especially conductivity, but also on pH, particle size, and chemical constituent concentrations. Electrochemical oxidation in aqueous medium has been studied to be applied in the treatment of industrial effluents containing phenols [1,16], nitrophenols [17], chlorophenols [18,19], carboxylic, cynuric, coumaric acids [1], pesticides [1] and herbicides [4,20], 5-amino-6- methyl-2benzimidazolone (AMBI), benzoquinone [18,5], dyes and pigments [21, 25] and surfactants [21], chlorinated organic compounds [2]. Electrocoagulation has been also used for treatment of industrial wastewater containing metals and from pulp and paper, mining and metal processing industries [1]. Electrocoagulation techniques were also applied in the removal nitrate ion, and to treat wastewater containing chemical and mechanical polishing residues, organic matter from landfill leachates, and deflourination of water [1]. Another electrochemical techniques that are being increasingly used in the remediation of soils contaminated both with metals and organic compounds is the so called â&#x20AC;&#x153;Electrokinecticsâ&#x20AC;? [1]. In this technique an electric current is applied between electrodes inserted into the soil, generating hydrogen ion at the anode and hydroxyl ion at the cathode. The pH gradient formed in the soil facilitates the movement of metals to the cathode where they are removed and collected, while anionic metal complexes, inorganic anions, and negatively charged organic species migrate towards the cathode. On the other hand, dipolar interactions between water molecules and the soil surface originate an electroosmotic flow of water molecules towards the cathode, enabling the transportation of uncharged organics [1]. Electrokinetics was applied to the remediation of soils contaminated by coal tars constituents such as PAHs, and combined with biodegradation to treat soils contaminated with 2,4-diclophenoxyacetic acid [1].

3.9. Electrochemical cleaning of flue gases Electrochemical methods have been used to clean flue gases, in particular for the desulfurization of flue gases resulting from burning of coal and other sulfur containing fuels in thermal power plants, but also resulting from other chemical or metallurgical industries. Sulfur dioxide (SO2) is certainly one of the most important air pollutants in micro and macro scale because of its effect on human and nature. SO2 is a quite irritant, non flammable, colorless, non explosive, and toxic gas with a suffocating smell. Further, SO2 is catalytically or photochemically affected by other atmospheric components to produce SO3, H2SO4 mists and H2SO4 salts, which deposit as acid deposit, as rain drops and snowflakes, on plants, soil, living organisms, buildings, lakes, rivers and seas. These acid precipitations directly and/or indirectly affect the health of humans, plants and animals. Different methods have been used to remove SO2from industrial flue gases: (i) wet gas washing with lime (calcium carbonate) to convert to calcium sulfate; (ii) aqueous absorption and desorption (Wellman-Lord Process), concentration followed by catalytic oxidation of concentrated SO2 to sulfuric acid; (iii) high temperature reduction with natural gas and production of elemental sulfur; (iv) oxidation to sulfuric acid using using ammonia, hydrogen peroxide or manganese as redox couples [26,27]. All these processes serves mainly to transform S(IV) to S(VI) and frequently to immobilize the SO2 waste in the form of a solid. Of the referred processes, the wet limestone flue gas desulfurization (FGD) process represents over 90 % of installed desulfurization capacities in the world [27] and may expressed by the equation: SO2 + CaCO3 + 2 H2O + 1/2 O2  CaSO4∙2H2O + CO2

(3.9.1)

But all the mentioned methods show some handicaps, requiring a lot of chemicals, regeneration of reagents or oxidants, reheating of stack gases before emitting to the atmosphere. Although catalytic oxidation is attractive, it has low removal rate. Electrochemical removal of SO2 seems to present important advantages in relation to these methods. Electrochemical gas purification methods imply basically two steps: i) gases to be removed are absorbed in the aqueous electrolyte; ii) dissolved gases are converted to harmless components by electrochemical oxidation or reduction. In particular, in the electrochemical SO2 removal, SO2, dissolved in acidic solution, is converted to SO42- at the anode, while H+ is converted to H2 at the cathode of an electrolytic cell [28,29,30]: Anode: SO2 + 2 H2O  SO42- + 4 H+ + 2 e-

(3.9.2)

Cathode: 2 H+ + 2 e-  H2

(3.9.3)

In some cases the electrochemical step aim the regeneration of a mediator, after absorption and redox conversion of the flue gas component [31]. For instance, SO2 and NOx were removed simultaneously from a flue-gas mixture, after absorption in a HNO3 solution and oxidation with Ag(II), which was recovered in a subsequent electrochemical cell [31,32].

Townley and Winnik [33] presented an electrochemical concentration cell for FGD (Figure 1) based following redox reactions (15,23,25), that remove SO2 from coal-burning power plant stack gases and concentrate it another gas stream: “Driven” mode: Cathode: SO2 + O2 + 2 e-  SO42-

(3.9.4)

Anode: SO42-  SO3 + ½ O2 + 2 e-

(3.9.5)

Cathode: SO2 + O2 + 2 e-  SO42-

(3.9.6)

Anode: SO42- + 5 H2  4 H2O + H2S + 2 e-

(3.9.7)

“Reducing-gas” mode:

The driving force for the cell comes either from the application of a small potential, less than 1.0 V (“driven” mode), or from the oxidation of a reducing gas (“reducing-gas” mode). In the first case the product stream contains sulfur oxides at high concentration and in the second case a mixture of H2S, H2O, and elemental S.

Figure 3.9.1 - Cell configuration for the electrochemical SO2 concentrator [33]

Finally, electrochemical desulfurization may be also performed in liquid or slurry state [32,34] in the so called oxidative desulfurization (ODS). For example, a bituminous coal in acid slurry was electroliticaly desulfurized with H2 production in cathode and desulfurization efficiency up to 80 % [32], while electrochemical ODS of crude oil showed enhanced desulfurization effect relative to simple ODS, in absence of applied electric field [35].

Electrochemical techniques were also used to separate CO2 from flue gas produced by pulverized coal combustion for power generation, using one or more electrochemical cells or cell stacks [36].

3.10. Electrochemical carbon dioxide fixation Carbon dioxide fixation is an important issue in view of the global warming and the subsequent climate change problems significantly caused by the rapid increase of the CO2 concentration in the earthâ&#x20AC;&#x2122;s atmosphere in the last 150-200 years. CO2 utilization is being increasingly recognized as a method by which global CO2 emissions can be reduced in an economical manner. Chemical fixation of CO2 is an attractive technique for utilization of carbon resources, as well as for the reduction of the atmospheric concentration of CO2. Nevertheless, CO2 is the stablest among carbon based substances under the environmental conditions [40]. Carbon dioxide can be electrochemically reduced to useful products under mild conditions. However, the energy conversion efficiency, defined as the ratio of the free energy of the products obtained in electrochemical CO2 reduction and that consumed in the reduction, would be roughly 30 to 40% [38]. Such a low efficiency may discourage practical application of CO2 reduction in the very near future, but for the moment, electrochemical reduction of CO2 is an interesting as well as an important topic in chemistry. Some advantages of the electrochemical reduction of CO2 include [22]: i) extensive research during the last several decades has yielded high selectivity, low cost, heterogeneous catalysts for CO2 electrochemical reduction to various useful products for aqueous reaction systems; ii) electrochemical conversion can be performed at room temperature and ambient pressure; iii) If the supporting electrolytes are fully recycled and the anode reactions can be performed using wastewater, then the overall chemical consumption can be minimized to just water or wastewater; iv) a renewable source of electricity can be used to drive the process, including solar, wind, hydroelectric, geothermal, tidal, and thermoelectric sources. Therefore this method can also be used as a renewable electricity storage mechanism; converting the electrical energy to chemical energy by producing fuels from CO2, such as methanol and formic acid. The stored energy can be released later for endues by oxidization of the fuels through fuel cells or the normal fuel-burning engines; v) electrochemical conversion can be augmented using light energy or solar thermal energy; vi) the electrochemical reaction system is modular and thus scale-up is relatively simple and; vii) in general, the electrochemical systems have a compact design. Using metal or alloy electrodes/catalysts, various products can be produced by electrochemical reduction of CO2, including carbon monoxide (CO), formic acid (HCOOH), oxalates (C2O42-), hydrocarbons (e.g., ethylene C2H4), and alcohols (e.g., methanol, CH3OH) [39]. Tokuda [39] presented the results on an efficient electrochemical fixation of an atmospheric pressure of carbon dioxide and its application to a synthesis of various useful carboxylic acids, including the precursor of anti-inflammatory agents. The method has several advantages: i) use of a simple one-compartment cell (platinum cathode and a magnesium anode); ii) simple electrolysis at a constant-current; iii) high yields in a synthesis of useful carboxylic acids, and iv) easy application to a synthesis of the precursor of non-steroidal anti-inflammatory agents. The use of supercritical CO2 in the electrochemical fixation of carbon dioxide would be useful in the future since it is a good solvent for organic compounds, and may act as a reagent in this fixation technique [40]. Electrochemical reduction of carbon dioxide on indium (In) electrode to synthesis dimethyl carbonate was studied in ionic liquid, 1-butyl-3-methylimidazoliumtetrafluoborate (BMIMBF4)[41]. The experiments, carried out by constant-potential electrolysis in an undivided cell under mild conditions without any toxic solvents, catalysts and supporting electrolytes, afforded a yield of 76% in dimethyl carbonate, at a potential of -1.7 V (vs Ag/AgCl). The results showed that the yields were affected by temperature, working potential and charge passed.

Figure 3.10.1 - Current density versus time for A. ferrooxidans in the presence of Fe2+ ions at 0.4 V/SHE. The inset presents the variation of the total organic carbon (TOC) and the dissolved organic carbon (DOC) with the incubation time in the electrochemical cultivation [41].

“Electrochemical cultivation” of bacteria, bacterial cultivation under potential control, is being used to fixe atmospheric CO2. For instance, Ishii et al. [41] investigated the ability of the bacterium Acidithiobacillus ferrooxidans to convert CO2 into organic compounds in batch culture and during electrochemical cultivation and that pyruvic acid was one of the organic acids present in both cultures. The authors concluded that that Acidithiobacillus ferrooxidans, a chemoautotrophic extremophile that derives energy for growth from the oxidation of Fe2+ to Fe3+ under strongly acidic conditions. is a promising bioelectrocatalyst for the conversion of CO2 into extracellular organic compounds. Figure 2 shows the current density versus time for A. ferrooxidans in the presence of Fe2+ ions at 0.4 V/SHE- The inset in the same figure presents the variation of the total organic carbon concentration (TOC) and the dissolved organic carbon concentration (DOC) with the incubation time in the electrochemical cultivation. As can be seen, in contrast to growth in batch culture, electrochemical cultivation of A. ferrooxidans showed continuous increases in TOC and DOC, even after prolonged cultivation of 50 days. REFERENCES 1.

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4. Electrochemical energy conversion Paulo Brito e Diogo Santos

4.1. Methodology of electrochemical conversion and storage Batteries and fuel cells are electrochemical systems that produce electrical energy by direct conversion of chemical energy. These systems are called galvanic cells in honour of Luigi Galvani (1737-1798), an Italian physician, researcher, physicist and philosopher who made a major contribution in the area of electrochemistry in the development of the first batteries. These types of energy-producing devices are getting increasing impact in our society as a result of, on one hand, a larger use of portable equipment that require electrical energy systems to work and, on the other hand, a need to seek sources and alternative energy production processes to fossil fuels. As an example, the current personal worldwide consumption of batteries in developing countries is about 5 per year and in industrialised countries it is about 15 per year [1-4]. This type of chemical system is based on spontaneous reduction-oxidation (redox) reactions, that is, reactions where the chemical species involved either lose electrons in oxidation (fuel) or gain electrons in reduction (oxidant). For the device to work the semi reactions of oxidation and reduction must occur separately in special vessels, so that the electrons may flow and generate an electron current (electricity). The basic elements of a galvanic cell are shown in Figure 4.1.1, for a cell of Cu and Zn, where the spontaneous reaction is as follows: Cu2+(aq) + Zn(s) ď&#x201A;Ž Cu(s) + Zn2+(aq)

(4.1.1)

Generically, a cell is composed of two solid electrodes made from materials that conduct electrons (metals, graphite, etc.), an electrolyte solution in each compartment, kept apart by a salt bridge separator that allows the transfer of ions. When the external circuit is electrically closed between the two electrodes, the conditions are created to start the redox process, thus occurring a semireaction of oxidation in the anode and a semi-reaction of reduction in the cathode (Fig. 4.1.1). A galvanic cell is normally represented by the chemical characterisation of each of the phases involved in the cell starting with the anode and separated by vertical lines: Anode|Anolyte||Catholyte|Cathode

(4.1.2)

In the case of the cell shown in Figure 4.1.1 it would be as follows: Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)

(4.1.3)

V A e-

Anions

Cations

Cathode (+)

Anode (-) Cu Zn

Cu Zn Cu2+

Zn2+ Anolyte

Zn(s) 

Zn2+(aq)

Catholyte

Cu2+(aq) + 2e-  Cu(s)

2e-

Figure 4.1.1 – Schematic representation of a galvanic cell.

4.1.1. Thermodynamic of galvanic cells The polarisation of the cell occurs when the oxidation reaction on discharging the electrons in the anode creates an excess negative charge; whereas in the cathode the reduction reaction removes electrons, creating an excess positive charge. This differential of electrical potential promotes the dislocation of charges and produces continuous electric current. These differences of potential may be thermodynamically related to the maximum useful energy, in this case electrical energy that the chemical system may supply. This useful energy, or Gibbs energy (G), may be related to the electric potential of the cell based on the following formula: G = -n F 

(4.1.4)

Where G is the variation of Gibbs energy for each mole of reagent or product formed (J mol-1), n is the number of moles of electrons exchanged for each mole of reagent or product formed, F is the Faraday constant (96500 C mol-1), and  is the electric potential of the cell (V, J/C). We can consider the cell potential as the difference of the standard electrode potentials of each electrode process, based on the equation of Nernst, for conditions outside the standard,  = cathode-anode = ocathode-oanode – (RT/nF) ln(Q)

(4.1.5)

where R is the constant of ideal gases (8.314 J K-1 mol-1), T is the absolute temperature and Q is the reaction quotient of the redox processes occurring in the cell.

4.1.2. Kinetics As in any chemical system, besides the thermodynamic approach that enables the evaluation of the energetic balance of the system and its spontaneity, it is necessary to take into account the related kinetic aspects, connected with the resistance throughout the processes inside the cell. In fact, for the cell to work it is necessary to have a dynamic of transport of electrical charges, ions and electrons throughout the device. This transport is motivated essentially by diffusion phenomena as a result of the gradients of concentration that are generated amongst the electrodes due to the consumption of chemical reagents and the formation of reaction products. The easier these various processes of transport may occur the quicker and less resistive the global process will be, and less energy will be spent by the cell. This loss of energy, or reduction of available potential is known as the electrodes transport overpotential. Other phenomenon that implies resistance to cell functioning is the charge transfer (electron transfer) that is processed at the surface of the electrodes among the chemical species that oxidise or reduce and the anode or the cathode, respectively. This type of resistance is associated with the activation energy of the process of charge transfer, named as the activation overpotential. Figure 4.1.2 shows a schematic representation of a polarisation curve of an electrode process. A polarisation curve can determine the overpotentials associated to the electrode processes. The overpotential in the electrode is the difference in potential between the balanced potential generated in the electrode without charge being passed, for example, M+/M for a balance between a metallic electrode and its ions in a solution, M(s)  M+(aq) + e-

M+/M

(4.1.6)

and the potential that the electrode acquires when a charge i is passed (i):  = M+/M - i

(4.1.7)

The overpotential may be anodic, if the process being promoted is the oxidation at the electrode, or cathodic, in the case of a reduction process. The overpotential ends up being a loss of tension necessary to overcome the resistance of the processes that occur in the cell. When the currents are low the resistance associated to the charge transfer (electrons), predominate in the electrode, when the current increases it starts to transport the chemical species, particularly by diffusion processes that demand greater resistance. For very high currents, the process is limited in terms of currents (current limit). In addition to the resistance associated to the electrode processes, we must also consider the resistance associated to the transport of charges in the electrolyte. To close the circuit in the cell the charge must be transported by ions. Its resistance will depend on the mobility of the ions present and their quantity.

electrode (versus reference electrode)

Zones where activation resistance prevails

M  M+ + e(anodic branch)

’

 anodic M+/M

{

cathodic ’

M + e-  M (cathodic branch)

Anodic limiting current

log ireduction log ioxidation log i Figure 4.1.2 – Polarisation curve of an electrode Clearly, in a system that produces energy it is of interest that all processes have low resistance, i.e. low overpotentials. Therefore in order to achieve low overpotentials the following situations are normally introduced and studied: a) Introduction of catalytic material in the electrodes in order to accelerate the reactions of charge transfer in the electrodes; b) Use of large surface area electrodes, most of the times porous in nature, to accelerate the reactions of charge transfer in the electrodes; c) Use of electrolyte additives to increase the ionic conductivity of the solution. 4.1.3. Cell parameters Galvanic cells are divided in primary, secondary and fuel cells in accordance to their modus operandi. Primary cells are systems that only allow one discharge, secondary cells allow several cycles of charge and discharge and fuel cells are those that externally store the reagents (fuel and oxidant) which are then continuously added to the device. In any cell whose objective is to obtain electrical energy three theoretical parameters are defined to characterise the same: the electromotive force (e.m.f.), the capacity (C) and the specific energy (E). The e.m.f. is the property of any device that mainly generators have, to produce electrical current in a circuit. It is a scaled measure (whose unit is the volt) that designates the existing tension in the terminals of a battery or electric generator before passing any charge. We can estimate the e.m.f. of a cell by calculating the potential reduction difference of the cathode and anode: e.m.f. = cathode - anode

(4.1.8)

The capacity of a cell measures the quantity of electrical charge (electrons) that the cell may supply for a given mass. The theoretical capacity is related with the total stoichiometric mass (M) of the reagents and may be calculated according to the following expression: C (Ah/g) = [n (mol) × F (C/mol)] / [3600 (C/Ah) × M (g)]

(4.1.9)

where n is the number of electron moles exchanged and F is the Faraday constant (F = 96500 C mol-1). The theoretical specific energy of the cell is the product of the e.m.f. with its capacity and can be determined as: E (Wh/g) = C (Ah/g) × e.m.f. (V)

(4.1.10)

These parameters are the maximum that can be supplied by the cell. When it starts working and generating current the previously referred resistances impede the maintenance of these parameters. In Figure 4.1.3 are shown the generic curves of cell discharge. Ideally, the cell would maintain a constant potential close to the value of e.m.f. up to the point when all the reagents are consumed and the value would fall to zero. In a non ideal situation the cell has an initial abrupt fall as a result essentially of the resistance of the electrode processes, followed by a gradual loss throughout the discharge process. This loss is due to the cumulative growth of the reaction products that limit the process.

Figure 4.1.3 – Discharge curves.

4.2. Primary and secondary batteries The different types of galvanic cells vary in size, physical characteristics and mainly in the electrochemical reactions involved. By associating a series of galvanic cells, a galvanic battery is obtained. As previously referred, the cells and the batteries are divided into three groups: i. Primary These systems only allow one discharge, in other words, when the finite quantity of reagents is consumed, the cells are deactivated either by reasons of system irreversibility or by reasons of an economic nature. ii. Secondary These are also known as rechargeable or cumulative. The systems can be recharged after being discharged, in other words, an external source of energy can be admitted, thus regenerating the reagents. iii. Fuel cells The fuel and the oxidant are continuously supplied to the device and the reaction products are continuously removed. There is still another type of system to be considered between secondary batteries and fuel cells, which may be called semi fuel cell; this is because only one of the reagents is continuously admitted, the other being periodically replaced or regenerated inside the cell. The voltaic cell built by the Italian physicist Alessandro Volta in 1800, was the first galvanic cell to be commercialised. The battery was formed by a positive copper (or silver) electrode and a negative zinc electrode, separated by a cloth soaked in water. Batteries with bipolar electrodes were formed as shown in Figure 4.2.1.

Figure 4.2.1 â&#x20AC;&#x201C; Representation of a voltaic cell.

As a result of using a weak oxidant, water, the open circuit voltage was very small, about 0.4 V. The anodic and cathodic reactions were that described by Eqs. 4.2.2 and 4.2.3, respectively. Battery

(-) Zn | KOH | Cu (+)

(4.2.1)

Anodic

Zn  Zn2  2 e 

(4.2.2)

2 H 2 O  2 e   H 2  2 OH

(4.2.3)

Cathodic

Since then, a significant number of cells have appeared. Table 4.2.1 compiles the main galvanic cells that have been developed and commercialised in our days. The table divides the cells according to the previously cited classification, indicating for each of the systems its voltage in open circuit (Vi=0), its theoretical capacity (C), and its theoretical and practical specific energy (E). Within the primary cells, the cells of known as Leclanché cells – named 1865 –, or with alkaline electrolytes, their low cost, with over 12 million 4.2.2).

zinc - manganese dioxide (Zn-MnO2) using saline electrolytes, after the French engineer G. Leclanché that developed it in are without a doubt the most popular due in large measure to cells sold per year in different shapes and sizes (see Figure

Figure 4.2.2 – Zn-MnO2 cells. a) Zinc-carbon (Leclanché); b) Alkaline

Table 4.2.1 – Main types of primary and secondary cells. Vi=0

Etheoretical

Epractice

(V)

(Ah kg-1)

(Wh kg-1)

(-) Zn | NH4Cl, ZnCl2 | MnO2 (+)

1.6

224

358

Alkaline

(-) Zn | KOH | MnO2 (+)

1.5

224

336

145

Mercury

(-) Zn | KOH | HgO (+)

1.3

190

247

100

Silver

(-) Zn | KOH | Ag2O (+)

1.6

180

288

135

Li-SO2

(-) Li | LiBr (SO2, AN, PC) | SO2, C (+)

3.6

379

1364

260

(-) Li | LiBr (SOCl2, AN, PC) | SO2, C (+)

3.1

403

1249

590

(-) Li | LiBF4, butyrolacton | CFx, C (+)

3.1

706

2189

250

Lead-acid

(-) Pb | H2SO4 | PbO2 (+)

2.0

120

240

Nickel-cadmium

(-) Cd | KOH | NiOOH (+)

1.2

181

217

(-) LixC6 | LiBr | Li(1-x)CoO2 (+)

1.4

100

140

150

Silver oxide-zinc

(-) Zn | KOH | Ag2O2 (+)

1.8

283

509

105

Silver oxide-cadmium

(-) Cd | KOH | Ag2O2 (+)

1.4

227

318

Cell

System

Primary Cells Leclanché

Li-SOCl2 Li-CFx

Secondary Cells

Lithium-ion

The high energy loss due to the formation of agglomerates of reaction products, oxides and oxyhydroxides and the considerable decline of pH at the cathode during the discharge are the main problems of cells that use saline electrolytes. When the discharge is interrupted a gain in open circuit voltage is observed as a result of the standardisation of pH and the concentration of oxyhydroxides. This centenary cell has been the target of innumerate innovations. Of these, we highlight the use of dusted zinc anodes with a greater efficiency in discharge; the use of cathodic additives with the purpose of increasing electronic conductivity; and the use of its own system as a secondary rechargeable cell. Lithium is a metal with excellent characteristics for galvanic cell anodes, due to its high electropositive characteristics and low density. These properties have long been recognised in lithium, but its ability to react with air, water and other solvents delayed its application until the beginning of the 1980’s. In effect, only in the last decade, and following the discovery of the stability of lithium in solvents such as cyclic esters, butyrolacton, hydrofuran, propylenecarbonate, etc., solvents that at the same time allow the dissolution of lithium salts, such as LiBr, LiBF4, LiClO4, etc., have created the conditions for the development and commercialisation of this type of cells (see Figure 4.2.3).

Figure 4.2.3 – Structure of a lithium-ion battery.

However, despite being a very recent technology, it has a large implementation in the market, mainly in the use of all types of electronic devices. Lithium cells are normally divided in two groups: cells with liquid cathodes (e.g., cells of Li-SO2 and Li-SOCl2), where the cathodic reagents, SO2 and SOCl2, are dissolved in the electrolyte and are reduced in inert substrates of graphite, and cells with solid cathodes (e.g., Li-CFx and Li-(PEO)xLiY,V2O5), where the active substance is the actual cathode. Considering secondary cells, about 75% of cumulative are produced for the automobile industry – over 100 million startup batteries of lead-acid per year – the other 25% being for applications as diverse as forklift truck traction, emergency electric power stations, portable devices, computers, telephones and military and aerospace applications. Currently, the biggest priority is the development of even more reliable systems that are more flexible, efficient and compact, in order to satisfy the needs of a series of new applications, namely electric vehicles, solar cells, miniaturisation of electronic devices, etc.

The wide deployment of lead-acid accumulators is due essentially to its low cost and good electrical characteristics – a 2.0 V tension per cell that only varies slightly with the temperature and discharge current. The first lead-acid cell, developed by the Frenchman Gaston Planté in 1859, was made of two lead sheets separated by a cloth soaked in sulphuric acid. The cell was activated, as is the case today, by passing current in the system to produce a layer of lead oxide in the positive electrode. Since then, technological advances such as the use of antimony lead alloys and expanders of oxide of negative electrodes, as well as the construction of electrodes with higher specific areas has led to a progressive increase of the number of cycles of charge-discharge of these accumulators (see Figure 4.2.4). In a lead-acid battery, the negative electrode is made of spongy lead, the positive electrode of lead dioxide (PbO2) and the electrolyte is a solution of sulphuric acid. The reactions involved in the process of charge and discharge are shown as follows.

Cathode

Discharge PbO2  3H  HSO-4  2e-   PbSO4  2H2O

(4.2.4)

Anode

Discharge Pb + HSO-4   PbSO4 + H+ + 2e-

(4.2.5)

Global

PbO2  Pb  2H2SO4  2PbSO4  2H2O

(4.2.6)

Figure 4.2.4 – Lead-acid battery.

The open circuit voltage is practically equal to the electromotive force of the cell and varies in accordance to expression 4.2.7: E  2.047 

2.3RT a log H2SO4 F aH2O

(4.2.7)

The three most relevant applications of lead-acid accumulators are: for startup batteries of internal combustion engines (accumulators with capacities between 2 and 200 Ah) that are subject to discharges at high currents and for short time intervals; for traction batteries (capacities between 40 and 1200 Ah) that are subject to intense discharges but at medium currents; and for stationary batteries (capacities between 40 and 5000 Ah) that are subject to discharges at high currents and for long periods of time. Although the first patent of a nickel-cadmium cell was registered at the end of the 19th century, in 1899, by the Swedish engineer W. Junger, it was only after the Second World War, in 1950, that it was mass-produced, it being actually the second most produced secondary cell in the world. This cell, which is also classified as an alkaline accumulator, has the advantages of having good behaviour at low temperatures and of suffering low energy losses. The high cost is probably its weakest point. The cell is composed of a positive electrode of trivalent nickel hydroxide, a negative electrode of cadmium and the electrolyte is normally a solution of 20-22 wt.% of KOH with additions of LiOH. In a simplified manner, the reactions involved are as follows:

Cathode

Discharge 2NiOOH  2H2O  2e   2Ni  OH2  2OH

(4.2.8)

Anode

Discharge Cd  2OH   Cd  OH2  2e

(4.2.9)

Global

Cd  2NiOOH  2H2O  Cd  OH2  2Ni  OH2

(4.2.10)

When a cell enters in overload or over discharge process, a very common process when different cells are connected in series, the formation of oxygen and hydrogen may occur as a result of the oxidation and reduction reactions of water, respectively. This phenomenon can be particularly problematic in sealed cells where the mixture of these two gases at higher temperatures may lead to explosions. In order to prevent the formation of such mixes, cadmium hydroxide is added to the positive nickel electrode to electrochemically reduce the oxygen formed by larger discharges. To avoid bigger problems, sealed cells have an escape valve that is triggered when the gas pressure exceeds a critical maximum value.

4.3. Fuel cells 4.3.1. Introduction Over the last twenty years, the problem of limited oil and gas supply together with the new demands of developing countries has created an immediate need for new technologies to relieve global dependence on a hydrocarbon-based energy policy. This requires significant changes in the global energy policy and the rapid introduction of an array of new technologies that produce and use energy in a more efficient and clean way. The basis of a new hydrogen-based economy should be supported by fuel cells. Considering that fuel cells and hydrogen will play a significant role in the global energy economy, governments are currently committing funds for research, development, and demonstration of hydrogen and fuel cells, as they struggle to create programs for viable infrastructure to support their use. Fuel cells generate energy from controlled, spontaneous redox (reduction-oxidation) reactions. A fuel cell is a multi-component device containing two electrodes separated by an ion-exchange membrane. As with battery systems, there are several kinds of fuel cells. In the most common fuel cells, hydrogen (H2) oxidation occurs in the negative electrode, the anode, and in the positive electrode, the cathode, occurs the oxygen (O2) reduction reaction. Electrocatalysts are used to lower the activation energy necessary for the reactions to occur. The electrons extracted at the anode flow through the external circuit (generating electricity) in the direction of the cathode, where they are typically used for the O2 reduction. At the same time, to keep the charge balance, protons migrate in the electrolyte, and through the membrane, to react at the cathode electrocatalyst surface with O2 (and electrons) and generate water and heat. As long as fuel and oxidant are supplied, the fuel cell will generate electricity. A single fuel cell generates a small amount of electricity so, in practice, similarly to what is done with loading multiple batteries to operate an electronic device, fuel cells are usually assembled into a stack of multiple cells to meet the specific power and energy requirements of a particular application. In summary, within a fuel cell (i) energy is generated by redox reactions, with only water/vapour emissions; (ii) there are no moving parts making fuel cells quiet and reliable; (iii) electricity is created electrochemically, so thermodynamic laws that limit internal combustion engines (ICEs) are not applicable, making fuel cells more efficient in extracting energy; and (iv) the fuel, oxidant, and products are renewable and green, making fuel cells essential to a sustainable energy program. Next sections will give an overview of the existing types of fuel cells. 4.3.2. Brief historical perspective In the early 1800s it was already known that water could be split into H2 and O2 using electricity, but it wasnâ&#x20AC;&#x2122;t until 1839 that Sir William Grove was able to demonstrate in practice that the reverse of the electrolysis process, i.e., generating electricity from the reaction of gaseous O2 with H2, should also be possible [5]. Experimentally, Grove enclosed two platinum strips in separate sealed bottles, one containing H2 and the other containing O2, and immersed them in dilute acid solution. The current that began to flow between the two electrodes was used to illuminate himself with a carbon-filament light bulb. Grove is also considered the inventor of the fuel cell stack (Figure 4.3.1) because he was able to increase the voltage produced by linking several of these devices in series, referring to it as the â&#x20AC;&#x153;gas battery.â&#x20AC;?

Figure 4.3.1 â&#x20AC;&#x201C; Groveâ&#x20AC;&#x2122;s original scheme of the first fuel cell stack (adapted from [6]).

However, it often happens that certain developments in science are forgotten, not because they lack importance, but rather due to circumstances. At the end of the 19th century, the ICE and the widespread exploitation of fossil fuels sent the fuel cell to oblivion, and it was labelled a mere curiosity. Although some minor progresses were obtained in the middle, it was not until 1932 that Dr. Francis Thomas Bacon resurrected the fuel cell last version from 1889, by the implementation of several modifications to the original design. Still, it took another 27 years until Bacon could produce a truly workable fuel cell stack when, in 1959, he demonstrated a machine capable of producing 5 kW of power. Meanwhile, in the late 1950s and early 1960s NASA was looking for a way to power a series of upcoming manned space flights and decided that fuel cells were the only reasonable choice. These developments led the first proton exchange membrane fuel cell (PEMFC). In parallel, the alkaline fuel cell (AFC) also started to be developed. The goal was to reduce the weight, an important consideration in space flight, and develop a longer-lasting fuel cell. The net result of this was the NASA use of AFCs in the Apollo spacecraft. Upgrades of these alkaline cells have been used on most subsequent manned US space missions, including those of the Space Shuttle and the International Space Station. Since necessity is the mother of invention, fuel cell research was reinvigorated in the USA and Europe during the oil embargos of the 1970s. In the following two decades, research aimed at the development of the required materials, selecting the best fuel source, and decreasing the cost of this technology. Technical breakthroughs during the 1980s and early 1990s led to the development of the first marketable fuel cell vehicle (FCV) in 1993, by the Canadian company Ballard. The next major breakthroughs should deal with the problem of H2 storage and the development of inexpensive non-noble metal electrpcatalysts, and it will be then that the full implementation of fuel cells will be more feasible.

4.3.3. Types of fuel cells As shown in Table 1, fuel cells can be classified in different categories, depending on the type of fuel, its operation temperature, and type of used electrolyte. These include (i) proton-exchange membrane fuel cells (PEMFCs), (ii) alkaline fuel cells (AFCs), (iii) phosphoric acid fuel cells (PAFCs), (iv) molten-carbonate fuel cells (MCFCs), (v) solid-oxide fuel cells (SOFCs), (vi) direct methanol fuel cells (DMFCs) or (vii) direct borohydride fuel cells (DBFCs), among other new types still having less relevance, such as biological or microbial fuel cells (BioFCs/MFCs), direct ethanol fuel cells (DEFCs), or regenerative fuel cells (RFCs). For example, regenerative fuel cells produce electricity from H2 and O2 and generate heat and water as byproducts, just like other typical fuel cells. However, regenerative fuel cell systems can also use electricity from solar power or some other source to carry out electrolysis of the excess water and generate more O2 and H2 fuel, which can again be used in the fuel cell mode; this is a comparatively young fuel cell technology being developed by NASA. Fuel cells may also receive an alternative classification according to their application. The different categories include (i) automotive fuel cells, (ii) stationary fuel cells, (iii) residential fuel cells, (iv) backup power fuel cells, and (v) portable-power fuel cells: -

Automotive fuel cells produce ca. 80–130 kW of peak power and are intended for use in cars and buses. Recently, forklifts and golf carts use similar fuel cell power plants, but require much lower power, around 20–30 kW. Fuel cells for buses and larger vehicles are designed to generate about 200 kW.

Stationary fuel cells are a category of larger fuel cell power plants that may produce 100–250 kW or more of power using reformate derived from various fossil fuels. Several stationary fuel cell stacks are located at universities, hospitals and schools across USA and Canada.

Residential fuel cells are intended to power a home or small store, produce power in the range 2–10 kW, and are fuelled by reformed methane or propane.

An intermediate class of fuel cells generating power in the range 1–5 kW has application as back-up power.

Portable-power fuel cells are devices having power ratings of <100 W for use in small electronic appliances such cell phones and laptop computers, with the most promising ones being DMFCs, which typically use methanol stored in a removable cartridge. DBFCs are also competing for this type of applications.

In the next subchapters, the several fuel cell types will be quickly described. 4.3.3.1. Proton-exchange membrane fuel cell (PEMFC) PEMFCs typically operate at temperatures around 80 ºC. The temperature is limited to not more than 100 ºC mainly by the lack of a proton-exchange membrane (PEM) that can durably withstand higher temperatures. Typically, H2, methanol, or a reformed fossil fuel containing very small quantities of carbon monoxide (CO, less than 10 ppm) and carbon dioxide (CO2, a few percent) are the choices for the fuel. The PEMFCs employ a perfluorosulphonic acid (PFSA) or hydrocarbon material in the form of a membrane that functions as the proton-conducting electrolyte/separator and avoids system complications involved when dealing with a liquid acid or alkaline electrolyte.

Table 4.3.1 â&#x20AC;&#x201C; Main types of fuel cells (adapted from [7]). Type

Fuel

Electrolyte

Operating temperature (ÂşC)

Power level (kW)

Typical applications

Proton exchange membrane fuel cell (PEMFC)

hydrogen

perfluorosulphonic acid membrane

50-100

0.01-100

transportation, backup power, distributed generation

Alkaline fuel cell (AFC)

hydrogen

potassium hydroxide

80-150

10-100

space and military power generation

Phosphoric acid fuel cell (PAFC)

hydrogen

phosphoric acid

150-210

100-400

combined heat and power (CHP), distributed power generation

Molten-carbonate fuel cell (MCFC)

hydrogen, natural gas

molten lithium carbonate

600-700

300-300,000

large stationary power, distributed power generation, CHP

Solid-oxide fuel cell (SOFC)

hydrogen, methane, carbon monoxide

ceramic, solid oxide, yttria stabilised zirconia

800-1000

100-100,000

auxiliary power, distributed generation, transportation, CHP

Direct methanol fuel cell (DMFC)

methanol

sulphuric acid, sulphonic acid membrane

50-110

0.001-100

portable power, consumer electronics

Direct borohydride fuel cell (DBFC)

sodium borohydride

sodium hydroxide, ionexchange membrane

25-70

0.001-100

portable power, consumer electronics

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Figure 4.3.2 – Basic scheme of the processes occurring in a PEMFC.

A generic PEMFC, specifically the typical H2/O2 fuel cell system is shown in Figure 4.3.2. The basic operation of all PEMFCs is the same, with slight changes in the used fuel and whether it operates in acid or in alkaline medium, with the ion moving across the electrolyte changing from the proton (H+) to the hydroxyl (OH-) ion, respectively. As mentioned before, the reaction on the anode of hydrogen-fuelled PEMFCs is the oxidation of H2 to form protons and electrons (Eq. 4.3.1) H2 → 2H+ + 2e-

(4.3.1)

The reaction on the cathode is a slow 4e- reduction of O2, where it reacts with protons that were generated on the anode side (and diffused to the cathode) and electrons to form water (Eq. 4.3.2), O2 + 4H+ + 4e- → 2H2O

(4.3.2)

Therefore, Eq. 4.3.3 describes the global cell reaction valid for most H2-based fuel cells, with only electricity, water and heat being produced in the process. O2 + 2H2 → 2H2O

(4.3.3)

Today PEMFCs are rapidly approaching the initial stages of commercialisation in automotive and residential applications; therefore, many efforts are being put in research concerning the technological challenges involved in lowering their cost and increasing the durability of their components.

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Furthermore, a significant barrier to using PEMFCs in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure H2 must store the H2 on-board as a compressed gas in pressurised tanks. Due to the low-energy density of H2 it is difficult to store enough H2 on-board to allow vehicles to travel the same distance as gasoline-powered vehicles before refuelling. Liquid fuels with higher energy density (e.g., methanol, ethanol, natural gas, liquefied petroleum gas (LPG), gasoline) can be used in the PEMFC, but the vehicles need an on-board fuel processor to reform it to H2. This requirement increases costs and maintenance. Moreover, the reformer also releases CO2, although less than that emitted from current gasoline-powered engines. 4.3.3.2. Alkaline fuel cell (AFC) AFCs typically operate on H2 and O2 with 35%–50% potassium hydroxide (KOH) as the electrolyte (in a porous matrix) and gas-diffusion electrodes (GDEs) as anodes and cathodes. The gases have to be extremely pure and free of CO2 in order to avoid the formation of solid carbonates in the highly alkaline electrolyte (Eq. 4.3.4), which lowers its ionic conductivity and block the electrode pores, limiting fuel cell operation. 2KOH + CO2 → K2CO3 + H2O

(4.3.4)

Alkaline fuel cells present advantages that include lower kinetic losses at the cathode for O2 reduction (than for acid fuel cells) and the possibility of using non-precious metals as catalysts. While the global fuel cell reaction is the same (Eq. 4.3.3), the partial reactions for the AFC are now different from those previously shown for the PEMFC in acid media. In the alkaline media of the AFC, Eq. 4.3.5 describes the H2 oxidation at the anode, while Eq. 4.3.6 gives the cathodic oxygen reduction reaction (ORR). 2H2 + 4OH- → 4H2O + 4e-

(4.3.5)

O2 + 2H2O + 4e- → 4OH-

(4.3.6)

AFCs can have efficiencies as high as 60 to 70%. They are classified into flowing-electrolyte cells and static-electrolyte cells; the type used in space applications is the static type. AFCs generally use anion-exchange membranes, being the OH- anion that works as the charge carrier. Nonprecious-metal catalysts are coated onto the membrane in a way similar to what is done for PEMFCs. AFCs are expected to have an impact on the 2020 and 2030 future-generation devices. 4.3.3.3. Phosphoric acid fuel cell (PAFC) PAFCs are intermediate-temperature fuel cells as they typically operate at temperatures ranging from 150 to 210 ºC. The PACF is one of the first fuel cell technologies to be commercialised for use as stationary power plants. With H2 as the fuel, the electrode reactions are similar to those in PEMFCs. The electrolyte is concentrated phosphoric acid soaked into a SiC paste, and the anode and cathode are Teflon (PTFE)-bonded GDEs employing Pt or Pt alloys as catalysts and graphitised carbon black as the catalyst support. H2 is extracted from various hydrocarbon fuels that are reformed externally. An advantage of the higher temperature is that the PAFC anode can tolerate about 1% of CO in the reformate simply using a Pt/C catalyst of moderate loading. The efficiency

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of PAFCs falls within the range of 40–50% and increases to 80% if the waste heat is used in a cogeneration scheme. Commercially available PAFC power plants can produce 400 kW of electricity and heat and are used for combined cooling, heating, and power applications in a variety of facilities such as supermarkets, hospitals, hotels, schools, and data centres. Although PAFCs are typically used for stationary power generation, some have already been used to power large vehicles such as city buses. 4.3.3.4. Molten carbonate fuel cell (MCFC) MCFCs operate in the range 600–700 ºC to obtain reasonable conductivity of the carbonate electrolyte, enabling the use of non-precious-metal catalysts such as Ni-Cr and Ni-Al alloys for anodes and NiO for cathodes. The electrolyte is composed of a mixture of lithium and either potassium or sodium carbonate salt that is suspended in a ceramic matrix of LiAlO2. The MCFC anodic reaction is given by Eq. 4.3.7, H2 + CO32- → H2O + CO2 + 2e-

(4.3.7)

with the CO2 produced at the anode being sent back to the cathodic side for the O2 reduction described by Eq. 4.3.8, forming more carbonate ions, with the global reaction being again that described by Eq. 4.3.3. O2 + 2CO2 + 4e- → 2CO32-

(4.3.8)

A basic scheme of the processes occurring in the MCFC is illustrated in Figure 4.3.3. MCFCs have efficiencies as high as 60%, approaching 85% when the waste heat is collected and used in a cogeneration cycle for combined heat and power (CHP). MCFCs do not need a reformer since the fuel is converted into H2 within the fuel cell itself (internal reforming).

Figure 4.3.3 – Basic scheme of the processes occurring in a MCFC (adapted from [8]).

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The high temperature protects the anode from CO2 poisoning, and therefore, such systems are being developed to work with fuels such as natural gas and coal gas. However, the high temperatures also bring some drawbacks; the contact with the highly corrosive molten-carbonate electrolyte limits the choice of materials that can be used, especially in the electrodes, which need to have a controlled electrode structure, namely controlled pore size and distribution. The high operating temperature accelerates the corrosion and degradation of cathode and cell hardware and lowers the life of the stack. 4.3.3.5. Solid oxide fuel cell (SOFC) By opposition to other fuel cell types, SOFCs employ a solid electrolyte, which avoids the complexity involved with liquid electrolyte management. In a SOFC, the electrochemical oxidation of the fuel (H2, CH4, or even CO) with oxygen ions, to generate water, takes place at the anode side (Eq. 4.3.9), with the oxygen reduction ocurring at the cathode (Eq. 4.3.10). H2 + O2- → H2O + 2e-

(4.3.9)

O2 + 4e- → 2O2-

(4.3.10)

However, the high operating temperature of the SOFC allows the use of a variety of fossil fuels without penalty; since water is generated at the anode, reforming of hydrocarbon fuels may be carried out within the fuel cell. SOFCs employ an electrolyte consisting of a non-porous metal oxide or ceramic such as Y2O3-stabilised Zr2O3 (YSZ), Sc2O3-stabilised Zr2O3 (SCZ), or gadoliniadoped ceria (GDC) and operate at temperatures ranging from 700 to 1000 ºC, at which the ceramic material is a good conductor of oxide anions. Figure 4.3.4 schematises the typical processes occurring in a SOFC.

Figure 4.3.4 – Typical processes occurring in a SOFC [9].

Ohmic losses are mostly related with electrolyte conductivity, and can be decreased by lowering the thickness, doping the material, or raising the temperature. The most typical cathode material is lanthanum strontium manganite (LSM), while the anode material is generally a cermet (i.e., ceramic-metal composite) constituted of Ni combined with the electrolyte material, for example, Ni-ZrO2.

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SOFCs have high efficiencies (60%) and fuel flexibility (almost any hydrocarbon can be used) but, although the problems of liquid-electrolyte management are eliminated, cheap materials and construction of stacks that can withstand the elevated temperatures and longer startup times are technical challenges still to be met. The applications of SOFCs range from stationary power generation (up to 2 MW) to their use for auxiliary power in automobiles. The extremely high operating temperature allows the SOFC to be used in combined heat and power systems, and results in further increased efficiencies up to 85%. 4.3.3.6. Direct methanol fuel cell (DMFC) Up to this point we have discussed fuel cells that are either directly powered by H2 or by hydrogen-rich fuels (e.g., hydrocarbons) that are then reformed. However, there are also lowtemperature fuel cells that can be powered directly by a fuel, without prior conversion to hydrogen. The most known case is the DMFC, in which methanol is directly oxidised in the anode according to Eq. 4.3.11, with O2 being reduced in the cathode (Eq. 4.3.2), giving the net cell reaction described by Eq. 4.3.12. CH3OH + H2O → CO2 + 6H+ + 6e-

(4.3.11)

CH3OH + 3/2O2 → CO2 + 2H2O

(4.3.12)

DMFCs do not have the problems concerning fuel storage typically associated with PEM fuel cells, since methanol has a higher energy volume density than H2. Moreover, since methanol is a liquid, it will also be easier to transport and supply to the public using the current infrastructure used for gasoline. DMFC technology is relatively new compared with that of fuel cells powered by pure H2, but it appears to be a possible battery replacement for portable applications. 4.3.3.7. Direct borohydride fuel cell (DBFC) This recent low-temperature fuel cell competes for the same applications as the DMFC. It is based on the use of sodium borohydride (NaBH4) alkaline solutions as an anodic fuel. There are two types of fuel cell systems using NaBH4 aqueous solution as the fuel: one is a regular PEMFC that uses the H2 generated in situ in a NaBH4 hydrolysis reactor connected to the cell; the other is the DBFC system, which is fed directly by a NaBH4 solution. While the fuel for both these two systems is the same NaBH4 aqueous solution, they have one major difference: In the PEMFC it is necessary to maximise the H2 generation from the borohydride (BH4−) hydrolysis (Eq. 4.3.13), whereas in the DBFC system, the production of H2 must be suppressed as much as possible for adequate cell performance and to maximise the number of electrons exchanged in the BH4− oxidation process (Eq. 4.3.14). BH4− + 2H2O → BO2− + 4H2 BH4− + 8OH− → BO2− + 6H2O + 8e−

(E0 = -1.24 V vs. SHE)

(4.3.13) (4.3.14)

Accordingly, NaBH4, which was previously known mostly for being a specialty reducing agent in the manufacture of pharmaceuticals and a bleaching agent in the manufacture of paper, is now

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acquiring increasing importance as an energy/hydrogen carrier [10]. It is an energy carrier when it directly powers a DBFC. It works as a hydrogen carrier when it stores and releases H2 that can then be used to power a regular PEMFC. This means that NaBH4 can directly or indirectly power a PEMFC. Therefore, in a DBFC there is the BH4− oxidation at the anode (Eq. 4.3.14), whereas humidified oxygen (or air) is electrochemically reduced at the cathode, with the eight electrons being consumed according to Eq. 4.3.15. Coupling of these two equations leads to the overall cell reaction defined by Eq. 4.3.16. 2O2 + 4H2O + 8e− → 8OH−

(E0 = 0.40 V vs. SHE)

BH4− + 2O2 → BO2− + 2H2O (E0 = 1.64 V)

(4.3.15) (4.3.16)

There is a recent interest in DBFCs that use hydrogen peroxide (H2O2) as the oxidant. The focus of research on the direct borohydride/peroxide fuel cell (DBPFC) is in developing a high energy density power source for space applications, underwater vehicles, and specific terrestrial applications. Figure 4.3.5 shows the basic electrochemical processes in a DBPFC.

Figure 4.3.5 – Electrochemical processes in a direct borohydride/peroxide fuel cell (reprinted from [11] with permission from Elsevier).

One of the major advantages of the DBPFC is the use of reactants (both fuel and oxidant) that are liquid at ambient temperature. The use of liquid reactants simplifies their storage, thermal management and internal processing. The general electrochemical reduction of H2O2 is given by Eq. 4.3.17. 4H2O2 + 8e− → 8OH− (E0 = 0.87 V vs. SHE)

(4.3.17)

However, when the pH of H2O2 oxidant solution is low (pH<1), H2O2 reduction is described by Eq. 4.3.18, which leads to the global fuel cell reaction given by Eq. 4.3.19.

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4H2O2 + 8H+ + 8e− → 8H2O

(E0 = 1.77 V vs. SHE)

(4.3.18)

BH4− + 4H2O2 → BO2− + 6H2O (E0 = 3.01 V vs. SHE)

(4.3.19)

Most of the present research in DBFCs focus the development of lower-cost anode electrocatalysts that can achieve the complete 8-electron oxidation of BH4- with high kinetics. 4.3.4. Basic components of a fuel cell The heart of the fuel cell is the membrane electrode assembly (MEA). It is defined as the five-layer assembly of (i) a membrane, (ii) the anode, (iii) the cathode, (iv) the anode diffusion medium (DM) or gas-diffusion layer (GDL), and (v) the cathode DM or GDL. The MEA is sandwiched by two bipolar plates that contain channels or flow fields for reactant-gas flow. A set of repeating units of MEAs and bipolar plates held together by a set of end plates is referred to as a fuel cell stack. Figure MEAs are generally prepared by coating the catalyst onto the membrane to form a three-layer catalyst-coated membrane. The catalyst is typically a powder of carbon-supported platinum (Pt/C) that is made into a slurry or ink (using alcohol, water, glycerol, etc.) and then adding a binder (generally a Nafion solution) in a consistency suitable for the coating method employed. The most used electrocatalysts, both for anode and for cathode, are based in Pt nanoparticles. The kinetics of H2 oxidation is extremely fast and thus requires low Pt loadings (ca. 0.05 mg cm-2). On the other hand, the reaction on the cathode is generally a slower 4e- reduction of O2. O2 reacts with protons that were generated on the anode side and electrons to form water. Most of the potential losses (~400 mV) in the fuel cell occur due to the sluggish kinetics of the ORR, and much effort has been put on understanding the ORR mechanism and finding cheaper materials that exhibit lower overpotentials. The loading on cathodes that employ Pt/C is typically 0.35 mg cm-2. Most of the research work on catalysts is focused on improving the activity of the catalyst in an attempt to lower the Pt loadings. As an example, the amount of Pt used in a stack is targeted to be 0.1 g kW-1 or 10 g in a 100-kW stack to ensure acceptable costs as well as adequate availability of Pt. Today’s PEMFC systems are limited to maximum temperatures of about 80-90 ºC. The constraint is imposed by the low proton conductivity of membrane materials at lower relative humidity (below 50%). Membranes for PEMFCs are required to have (i) high protonic conductivity, (ii) high electronic resistance, (iii) low H2 and O2 crossover, (iv) low cost, (v) high mechanical durability, and (vi) high chemical durability. Recently, thinner membranes have been tested to lower the resistance and enhance cell performance, but typically a backbone or support is required for mechanical strength, which in turn increases the resistance. The most commonly used membranes are PFSA-based, such as Nafion from Dupont, Gore, Asahi Glass, and 3M. Finally, the bipolar plates are the external components of each individual cell. They incorporate flow fields or channels for distributing the reactants to the entire fuel cell area, and play the role of current collectors. The materials used for bipolar plates include graphite foils, porous graphite plates, and metal sheets. Figure 4.3.6 shows the several basic components of a typical fuel cell.

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Figure 4.3.6 â&#x20AC;&#x201C; Basic components of a typical PEM fuel cell (adapted from [12]).

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4.4. Hydrogen as an energy vector 4.4.1. Introduction - Towards a hydrogen economy For the world to move towards a hydrogen (H2) economy it is necessary to build a H2 infrastructure that includes a combination of distributed and centralised H2 production. There are already several H2 pipeline networks to provide H2 to the refining and food-processing industry, although the transport by trucks is still prevalent. In addition, nowadays there are several hundred H2-fuelling stations, many kilometres of H2 pipelines, with millions of tonnes of H2 being produced every year [13]. H2 storage is often categorised as physical or chemical storage: on-board physical storage methods include compressed gas, liquid H2, and cryoadsorbed H2; chemical storage includes metal hydrides, liquid organic carriers, and complex hydrides (e.g., NaBH4). Compressed H2 (35–70 MPa) in one or two tanks (~4-8 kg H2 depending on the target range) is stored on board in today’s fuel cell vehicles (FCVs). Current FCVs already meet the driving range of conventional internal combustion engine (ICE) vehicles and have a lifespan of 70% of the target value. The H2 fuel tank has been demonstrated to be completely safe – FCVs successfully pass the front, rear, and side impact tests that are typically applied to ICE vehicles. But for a H2 economy to become a reality, first the fuel cell stack must be commercially viable and widely available, which will then be naturally followed by a H2 infrastructure for distribution. 4.4.2. Methods for hydrogen production Several methods can be used to generate H2 from different sources. Though H2 can be produced in a clean and renewable way from water electrolysis, nowadays, unfortunately, fossil fuels are still the main source for industrial mass scale H2 production, owing to their low cost and to the existence of a system that is already built for fossil fuel usage. However, this fact is absolutely in contradiction with the present policies towards a green and sustainable energy society. 4.4.2.1. Hydrogen production from fossil fuels Fossil fuels have large and heavy hydrocarbon based molecular structure. Extracting H2 by breaking the bonds between hydrogen and carbon atoms is one of the most popular methods of H2 production. H2 can be extracted from biomass, coal, gasoline, oil (heavy and light), methanol and methane. Steam-methane reforming is presently the most economical method and has the largest share in global H2 production (almost 48%). This highly endothermic process is described by the reaction given in Eq. 4.4.1. H2O + CH4 → CO + 3H2

(4.4.1)

Coal and oil have the second and third place in this ranking with a share of 30% and 18%, respectively. H2 production by water electrolysis has the smallest share (only 4%) among the

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available methods of large scale H2 production, where other existing methods are not currently being used in industrial scale. In many cases, H2 can also be obtained as a byproduct of oil refinement. The main advantages of fossil fuel-based H2 production are its reasonable price and possibility of mass production. However, this approach for H2 production suffers from problems concerning high pollution ratings and limited resources. These fossil fuel-based processes do not fit in a new H2 economy that aims at following “green” strategies and build itself on renewable and sustainable development. H2 production using these methods definitely cannot be considered “green”, since they usually emit CO, CO2 and other greenhouse gases, and their resources are not renewable. 4.4.2.2. Hydrogen production from water electrolysis The process of splitting water molecules by means of electrolysis is known for a long time. When an electric current passes through water, its molecules are forced to decompose into its basic constituents – O2 and H2. The half reactions at each of the electrodes are given by Eqs. 4.4.2 and 4.4.3, with the overall chemical reaction of water electrolysis being given by Eq. 4.4.4. These equations describe the exact opposite processes that were presented earlier for the H2/O2 fuel cells. Cathode:

2H+ + 2e- → H2

(4.4.2)

Anode:

2OH → ½O2 + H2O + 2e

(4.4.3)

Global:

H2O → H2 + ½O2

(4.4.4)

Unfortunately, this method does not have a large share in global H2 production. High production costs due to low conversion efficiency and electrical power expenses are the main drawbacks of electrochemical H2 production. Hence, water electrolysis is not currently a method of choice for large-scale H2 production. As a result, electrolytic H2 has not found yet a place as a competitive alternative to traditional fuels. Water electrolysis requires a minimum energy of 39.4 kWh kg-1 of H2 generation at full conversion efficiency. However, typical electrolysers consume about 50 kWh to generate 1 kg of H2. Many efforts are being made to increase the efficiency of the electrolysis process. Higher efficiencies have been obtained in extreme pressure and temperature conditions. Therefore, increased investment has been required to build more complex and sophisticated electrolysers that are able to perform efficiently under such harsh conditions. In these cases, higher production efficiency comes with dramatically increased corrosion, higher operation and maintenance costs, and reduced lifespan. For these reasons, most of the available electrolysers work at temperatures lower than the boiling point of water and do not exceed the pressure barrier of 50 bar. Despite the cost disadvantages, water electrolysis has some unique qualities. Electrolysis can be used for in situ H2 production in any place. The only requirements of this production are electricity and water, and the production capacity can be tuned for a given demand in any place. With regard to the characteristics of water electrolysis, this method is able to produce absolutely sustainable and clean H2. The goal can be achieved when the required electricity is obtained from an emissionfree method such as wind, solar, geothermal, tidal, or other renewable and green energy source. Each renewable energy harvesting system has its own capital cost. Utilising one, all, or a combination of few of the new energy production systems is inevitable for future energy production demands. However, current concern is to analyse the possibilities of H2 production based on the available social, industrial and political infrastructures. It has been estimated that a

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H2 production plant of nominal power of about 1000 $ per kWh would require an investment of 50,000 $ for each 1 kg h-1 capacity of electrolytic H2. On the other hand, remarkable reduction of expenses as the production capacity increases have already been demonstrated. 4.4.3. Hydrogen storage A major barrier to the wide use of H2 as an energy carrier, and consequently, to the implementation of a H2 economy, are the problems concerning the H2 storage, as uncompressed gas state H2 has a very low density and low energy content. There are several H2 storage techniques, of which the most conventional is to store it as a compressed gas in pressurised tanks. However, the necessary high pressure levels lead to construction constraints, high cost of production and maintenance, and some problems of operational safety of the tanks and compressors. Alternatively, H2 may be liquefied, as it has much higher energy content in liquid form than in compressed gas state. The main advantage of liquefied H2 storage is its high density in low pressure. These features enable compact and lightweight storage and efficient delivery options. However, H2 only liquefies at temperatures below -250 ÂşC and that is why the liquefying process of H2 adds an excess 30% to the production power demand. Furthermore, the use of gas liquefiers make the production system more complex. As a result, liquid H2 costs 4-5 times more than the compressed gas state product. Storing H2 in high pressure containers is currently the preferred method for most of the vehicle manufacturers due to the efficiency, design, cost, and environmental advantages. It is considered a simple and efficient method that is able to provide an easy to use source for consumers. The main disadvantage is the low storage density and it is anticipated by many experts that H2 storage in high pressure cylinders is very unlikely to be a popular method in future. H2 can also be stored in materials with high storage capacity, such as metal hydrides, Mg-based alloys, some carbon-based materials, and chemical and complex hydrides, including boron compounds. Some metals are able to build a chemical bond with hydrogen as per their interatomic lattice; the hydrogen keeps bonded to the metal in reduced temperatures and is released when heated. An advantage of this method is the ability of bonding at normal or low pressures and releasing at high pressure conditions. A variation of this method is called the â&#x20AC;&#x153;chemical hydride slurry approachâ&#x20AC;?, in which there is a reaction between H2 and a chemical hydride/organic slurry. Then, the high purity H2 can be extracted in situ from the media by reacting the slurry with water. These slurries usually have a fluid-like nature that brings unique opportunities of storage, transportation and pumping. This method brings important advantages in the energy transmission of H2, in the stabilisation of the stored fuel at normal temperature and pressure, in the high volumetric energy content, and in the very low harmful emissions. Calcium, lithium, magnesium and sodium are the most used metals for this type of approach. As an example, alkali-stabilised sodium borohydride (NaBH4) solutions are a good way to store large quantities of H2 in a simple and compact form. When required, H2 can be obtained on demand by simply taking advantage of the NaBH4 catalytic hydrolysis reaction (Eq. 4.3.13). All the methods described still require further improvements. Certainly, we will not have a single answer for the H2 storage problems in the upcoming H2 economy but really a panoply of different solutions for each specific application.

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REFERENCES 1.

Dominik Rutz e Rainer Janssen, “Biofuel Technology Handbook”, WIP Renewable Energies, 2008

Hoogers, Gregor, “Fuel Cell Technology Handbook”, http://lib.myilibrary.com/Browse/open.asp?ID=51859 )

Sorrell, C.C.; Nowotny, Janusz; Sugihara, Shiro, “Materials for Energy Conversion Devices”, Woodhead Publishing, 2005 (available in MyiLibrary with proxy on: http://lib.myilibrary.com/Browse/open.asp?ID=54450)

Messenger, Roger A., “Photovoltaic Systems Engineering”, Taylor & Francis, 2003 (available in MyiLibrary with proxy on: http://lib.myilibrary.com/Browse/open.asp?ID=16913)

"Note sur une pile voltaïque d'une grande énergie, construite par M. Grove; communication de M. Becquerel", Comptes Rendus 8, 497 (1839).

Grove, W.R., “On the Gas Voltaic Battery. Experiments Made with a View of Ascertaining the Rationale of Its Action and Its Application to Eudiometry”, Philos. Trans. Roy. Soc. 133, 91-112 (1843).

Ginley, D.S.; Cahen, D. (Eds.), “Fundamentals of Materials for Energy and Environmental Sustainability”, Cambridge University Press, Cambridge, UK, 2011.

U.S. Department of Energy, “Types of Fuel Cells”, http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html

De Guire, E.J., “Solid Oxide Fuel Cells”, ProQuest, April 2003, http://www.csa.com/discoveryguides/fuecel/overview.php

CRC

Press,

2003

(available

MyiLibrary

with

proxy

on:

10. Santos, D.M.F; Sequeira, C.A.C., “Sodium borohydride as a fuel for the future”, Renew. Sustain. Energy Rev. 15, 3980-4001 (2011). 11. Santos, D.M.F.; Saturnino, P.G.; Lobo, R.F.M.; Sequeira, C.A.C., “Direct borohydride/peroxide fuel cells using Prussian Blue cathodes”, J. Power Sources 208, 131-137 (2012). 12. Britannica Online for Kids, “Fuel cell: proton http://kids.britannica.com/comptons/art-106689

exchange

membrane

13. Penner, S.S., “Steps toward the hydrogen economy”, Energy 31, 33-43 (2006).

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fuel

cell”,

accessed

February

12,

2014,

Environmental Remediation and Energy Production Technologies

5. Chemical Environmental environmental pollutants

Carcinogens

and

Persistent

Anabela S. Oliveira

5.1. Who they are and why is their study relevant Chemical Environmental Carcinogens [1,2] are a group of chemical substances capable of inducing the formation of cancers in men and / or animals. Although some environmental carcinogens have natural origin, most of them are anthropogenic, being produced intentionally or as non-desirable by-product on several industrial / urban processes. In fact, environmental carcinogens of anthropogenic origin are a direct consequence of the recent industrial development, which is responsible for the great increase on pollution levels of different environmental compartment and their consequent negative impacts on all terrestrial ecosystems. Some chemical environmental carcinogens also possess high residence time on the environment and are highly biorecalcitrant being therefore named persistent environmental pollutants [1, 3 , 4]. The high environmental persistence coupled to the chemical characteristics of these substances, especially large lipid solubility and low water solubility, favors the process of bioaccumulation and biomagnification along food chains by environmental carcinogens that because of those processes constitute a high risk to ecosystems and public health. A pollutant of synthetic origin, ie not natural, it is even more dangerous when there are no specific natural and efficient mechanisms for their elimination. Theirs dangers are magnified when although present in undetectable concentrations in air, water and soil they have the ability to concentrate in organisms that directly feed from them (bioconcentration) and then have the ability to reach extremely high concentrations in individues at the top of food chains (biomagnification). The behavior and fate of chemical pollutants in the environment is determined by its physicalchemical and toxicological characteristics and those of the environment itself. If a compound has low toxicity, it is easy to degrade (so has low persistence) and has low environmental mobility, the risk that they can cause adverse effects to the environment (death, illness and birth defects in humans and other animals) is generally neglectless. On the other hand if the compound is highly toxic, has high persistence and has a high environmental mobility, that compound it is certainly associated to a high risk for public and environmental health, posing serious risks to the biodiversity of the ecosystems they interact with. Among the environmental pollutants witch are carcinogens and/or mutagens, two classes of substances attracted special attention until the end of last century: polycyclic aromatic hydrocarbons (PAHs) and persistent organic pollutants (POPs) (ie, organochlorinated compounds) are groups that have received great attention from the scientific community. Although originally (nearly up to the end of last century) most POPs substances were organochlorinated compounds, more recently (last decade of XX century and first decade of XXI) attention was directed to new classes of persistent organic compounds: substances as endocrine disruptors, emergent pollutants and pharmaceuticals residues like are now groups of substances of environmental concern [1]. Most of them have similar chemical character to the original organoclorinated POP compounds and are nowadays frequently subject of similar concern and study and will be also addressed on this chapter.

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5.2. Classes of chemical carcinogens and persistent environmental pollutants 5.2.1. Polycyclic aromatic hydrocarbons Polycyclic aromatic hydrocarbons (PAHs) [1, 3, 5] are a family of more than one hundred organic compounds, which contains two or more aromatic rings, (connected in a way that each aromatic ring shares two carbon atoms). PAHs may also present on their structure five carbon non-aromatic rings, fused with 6-carbon aromatic rings. In Figure 5.1 we can see examples of their characteristics structures. Some definitions of PAHs are summarized in Table 5.1 The 16 PAHs shown in Figure 5.1 were classified as priority pollutants by EPA 5 (Environmental Protection Agency) of the United States of America. Of these, seven are classified by EPA as probably carcinogenic to humans. In Figure 5.1 their Toxic Equivalency Factors relatively to benzo [a] pyrene are indicated in brackets [5].

Figure 5.1 â&#x20AC;&#x201C; PAHs identified by Environmental Protection Agency (EPA) as prioritary organic pollutants for environmental monitoring. Toxic equivalent factors relatively to Benzo[ a]pyrene are displayed in brackets for the PAHs which are probably carcinogenic to humans.[5, 6]

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Table 5.1 – Definitions on PAHs (adapted from [7]) Polynuclear Aromatic Hydrocarbons (PAHs)/ Polycyclic Aromatic Hydrocarbons (PAHs) “PAHs often are byproducts of petroleum processing or combustion. Many of these compounds are highly carcinogenic at relatively low levels. Although they are relatively insoluble in water, their highly hazardous nature merits their positioning in potable waters and wastewaters."[8] “PAHs are a group of organic contaminants that form from the incomplete combustion of hydrocarbons, such as coal and gasoline. PAHs are an environmental concern because they are toxic to aquatic life and because several are suspected human carcinogens." [9] "A compound built from two or more benzene rings. Sources of PAHs include fossil fuels and incomplete combustion of organic matter (in auto engines, incinerators, and even forest fires)." [10] "PAH compounds are a generally hazardous class of organic compounds found in petroleum and emissions from fossil fuel utilization and conversion processes. PAHs are neutral, nonpolar organic molecules that comprise two or more benzene rings arranged in various configurations ... Members of this class of compounds have been identified as exhibiting toxic and hazardous properties, and for this reason the EPA [U.S. Environmental Protection Agency] has included 16 PAHs on its list of priority pollutants to monitored in water and wastes." [11] “PAHs, in general, are ubiquitous environmental pollutants and are formed from both natural and anthropogenic sources. The latter are by far the major contributors. Natural sources include forest fires [12], volcanic eruptions [13], and degradation of biological materials, which has led to the formation of these compounds in various sediments and fossil fuels. Major anthropogenic sources include the burning of coal refuse banks, coke production, automobiles, commercial incinerators, and wood gasifers."[14]

5.2.2. Persistent Organic Polutants (POPs) - Organochlorinated substances [1,3,4,1530] Internationally, the most systematic attempt to regulate persistent organic pollutants and organohalogenated compounds is being carried out by UNEP (United Nations Environmental Program, Stockholm Convention page http://chm.pops.int/Home/tabid/2121/Default.aspx) that since 1997 has mandate to coordinate the establishment and negotiation of a treaty to control, reduce or eliminate the use, production, discharges, emissions and losses of POPs. Since that date, the UNEP has coordinated the international action to protect public health and the environment through measures that reduce or eliminate emissions and / or discharges of POPs. They promoted the collection and sharing of information and the preparation of specific legislation. A group of 12 substances and / or families of POPs substances have been identified as priorities. The toxicological significance of these 12 POPs is so large that the first negotiations involved more than 120 countries and resulted in the approval of an international resolution for their removal which prohibits their use, production and sale (Johannesburg conversations, December 10th, 2000). This agreement, the Convention Priority Organic Pollutants (POPs), was endorsed by the governments of many countries still during 2001, May 23, at a diplomatic conference in Stockholm. In 2009 the list was extended to include 9 new chemicals [30], and some more were already propose to include the list. Nowadays the Stockholm convention is endorsed already by more than 197 countries. It should be noted that this POPs priority list is not a list of the most toxic substances, but a priority list based on a number of parameters such as toxicity, persistence, bioaccumulation potential, potential for atmospheric long range transport, volume of production and, last but not the least, significant economic aspects.

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The physico-chemical properties and, in particular, the stability of any substance results from its molecular structure and in the case of persistent organic pollutants (POPs) are associated with their high degree of molecular halogenation [1]. The group of halogenated hydrocarbons includes some organochlorinated compounds that have proven to be the most resistant to degradation and the most persistent among the compounds ever produced by man [ 1,4 and references quoted there]. Included in this class are various pesticides, dibenzo polychlorinated dioxins (PCDDs), dibenzo polychlorinated furans (PCDFs) and polychlorinated biphenyls (PCBs) (see structures in Figure 5.2) [1]. The C-Cl bond is very stable to hydrolysis and the greater the number of chlorine atoms and / or other functional groups present in the structure of these compounds the higher its resistance to degradation. Chlorine bound to an aromatic ring is more stable than an aliphatic structure. Thus, POPs are typically chlorinated halogenated aromatic ring structures and / or other halogenated chains. The high degree of halogenation strongly decreases their solubility in water and increases the solubility in lipids, thus facilitating the passage through cellular membranes and their accumulation in body fat. For example, it is known that the more chlorinated biphenyls are more likely to accumulate in a greater extent than the less chlorinated ones. Likewise also the metabolism and excretion are faster for the less chlorinated PCBs. 5.2.2.1. The dirty dozen â&#x20AC;&#x201C; The 12 initial POPs A group of 12 substances and / or families of POPs substances have so been at first identified as priorities, namely: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, dioxins (PCDDs), furans (PCDFs) and polychlorinated biphenyls (PCBs). Figure 5.2 shows the structures of these 12 POPs. This list is known as the dirty dozen. The compounds in the list are all organochlorinated and the list comprises pesticides (first nine compounds) used in agriculture and / or the control of epidemic diseases; industrial chemicals (PCBs and hexachlorobenzene) and the rest are byproducts industrial processes (PCDDs, PCDFs and PCBs and hexachlorobenzene). Note that two POPs, namely PCBs and hexachlorobenzene, are comprised in more than one category of use or origin. It should be noted that this POPs priority list is not a list of the most toxic substances, but a priority list based on a number of parameters such as toxicity, persistence, bioaccumulation potential, potential for atmospheric long range transport, volume of production and, last but not the least, significant economic aspects. 5.2.2.1.1. Pesticides The Organochlorinated pesticides included in the original list of the Stockholm Convention of Priority Organic Pollutants are listed in Figure 5.2. They are [1, 24]: Aldrin and Dieldrin are insecticides with similar chemical structures. They are frequently addressed together because, either in the body or in the environment, aldrin quickly breaks in to dieldrin. Pure aldrin and dieldrin are white powders with a mild chemical odor. The less pure commercial powders have a tan color. Endrin is a solid, white, almost odorless substance that was used as a pesticide to control insects, rodents, and birds. Little is known about the properties of endrin aldehyde (an impurity and breakdown product of endrin) or endrin ketone (a product of endrin when it is exposed to light).

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Chlordane is a manufactured chemical that was used as a pesticide in the United States from 1948 to 1988. Technical chlordane is not a single chemical, but is actually a mixture of pure chlordane mixed with many related chemicals, of which about 10 are major components. Some of the major components are trans-chlordane, cis-chlordane, beta-chlordene, heptachlor, and transnonachlor. Chlordane is a thick liquid whose color ranges from colorless to amber. Chlordane has a mild, irritating smell. Some of its trade names were Octachlor and Velsicol 1068. Chlordane does not dissolve in water. Therefore, before it can be used as a spray, it must be placed in water with emulsifiers (soap-like substances), which results in a milky-looking mixture.

Figure 5.2. - Structures of the 12 POPs covered by the Stockholm Convention of Priority Organic Pollutants [1].

Until 1983, chlordane was used as a pesticide on crops like corn and citrus and on home lawns and gardens. From 1983 until 1988, chlordane's only approved use was to control termites in homes. The pesticide was applied underground around the foundation of homes. When chlordane is used

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in the soil around a house, it kills termites that come into contact with it. Before 1978, chlordane was also used as a pesticide on agricultural crops, lawns, and gardens and as a fumigating agent. Because of concerns over cancer risk, evidence of human exposure and build up in body fat, persistence in the environment, and danger to wildlife, the EPA canceled the use of chlordane on food crops and phased out other above-ground uses over the next 5 years. In 1988, when the EPA canceled chlordane's use for controlling termites, all approved use of chlordane in the United States stopped. Manufacture for export continued. Heptachlor is a manufactured chemical and does not occur naturally. Pure heptachlor is a white powder that smells like camphor (mothballs). The less pure grade is tan. Trade names include Heptagran®, Basaklor®, Drinox®, Soleptax®, Termide®, and Velsicol 104®. Heptachlor was used extensively in the past for killing insects in homes, buildings, and on food crops, especially corn. These uses stopped in 1988. Currently it can only be used for fire ant control in power transformers. Heptachlor epoxide is also a white powder. Bacteria and animals break down heptachlor to form heptachlor epoxide. The epoxide is more likely to be found in the environment than heptachlor. Hexachlorobenzene was widely used as a pesticide to protect the seeds of onions and sorghum, wheat, and other grains against fungus until 1965. It was also used to make fireworks, ammunition, and synthetic rubber. Currently, there are no commercial uses of hexachlorobenzene in the United States. Hexachlorobenzene is a white crystalline solid that is not very soluble in water. It does not occur naturally in the environment. It is formed as a by-product while making other chemicals, in the waste streams of chloralkali and wood-preserving plants, and when burning municipal waste. Mirex and Chlordecone are two separate, but chemically similar, manufactured insecticides that do not occur naturally in the environment. Mirex is a white crystalline solid, and chlordecone is a tan-white crystalline solid. Both chemicals are odorless. Mirex was used to control fire ants, and as a flame retardant in plastics, rubber, paint, paper, and electrical goods from 1959 to 1972. Chlordecone was used as an insecticide on tobacco, ornamental shrubs, bananas, and citrus trees, and in ant and roach traps. Mirex was sold as a flame retardant under the trade name Dechlorane, and chlordecone was also known as Kepone. Toxaphene was one of the most heavily used pesticides in the United States in the 1970s and early 1980s. It was used primarily to control insect pests on cotton and other crops in the southern United States. Other uses included controlling insect pests on livestock and killing unwanted fish in lakes. Toxaphene is made by reacting chlorine gas with a substance called camphene. The resulting product (toxaphene) is a mixture of hundreds of different chlorinated camphenes and related chemicals. DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) is a pesticide that was once widely used to control insects on agricultural crops and insects that carry diseases like malaria and typhus, but is now used in only a few countries to control malaria. Technical-grade DDT is a mixture of three forms, p,p'-DDT (85%), o,p'-DDT (15%), and o,o'-DDT (trace amounts). All of these are white, crystalline, tasteless, and almost odorless solids. Technical grade DDT may also contain DDE (1,1dichloro-2,2-bis(p-chlorophenyl)ethylene) and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane) as contaminants. DDD was also used to kill pests, but to a far lesser extent than DDT. One form of DDD (o,p'-DDD) has been used medically to treat cancer of the adrenal gland. Both DDE and DDD are breakdown products of DDT.

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5.2.2.1.2. Dioxines and furanes Polychlorinated dibenzodioxins (PCDDs) are a group of polyhalogenated organic compounds that are environmental pollutants. They are commonly referred just as dioxins because each PCDD molecule contains as central ring a dioxin. This gives the molecule a dibenzo-p-dioxin ring system. The word "dioxins" may also refer to other similarly acting chlorinated compounds (Dioxins and dioxin-like compounds) [1,4, 24]. Some definitions of Dioxines are given in Table 5.2. The structure of dibenzo-p-dioxin comprises two benzene rings joined by two oxygen bridges. This makes the compound an aromatic diether (see Figure 5.2). The name dioxin formally refers to the central dioxygenated ring, which is stabilized by the two flanking benzene rings. In PCDDs, chlorine atoms are attached to this structure at any of 8 different places on the molecule, at positions 1–4 and 6–9. There are 75 different PCDD congeners [1, 4, 24]. Polychlorinated dibenzofurans (PCDFs) are a family of chemicals that contain one to eight chlorine atoms attached to the carbon atoms of the parent chemical, dibenzofuran. There are 135 different types of PCDFs with varying harmful health and environmental effects [1, 4, 24]. Not all of the different types have been found in large enough quantities to study the physical properties. However, of those that have been studied, they do not dissolve in water easily and appear to be in the form of colorless solids. There is no known use for these chemicals. Other than for research purposes, they are not deliberately produced by industry. As PCDDs most PCDFs are produced in small amounts as undesirable by-products of certain processes, such as manufacturing other chemicals or bleaching at paper and pulp mills. PCDFs can also be released from incinerators. PCDFs are often found in association with dibenzo-pdioxins (PCDDs), which cause similar toxic effects [3, 4, 24, 25]. Table 5.2 – Definitions on Dioxins adapted from [25] Dioxins "Dioxins are a class of chemical contaminants that are formed during combustion processes such as waste incineration, forest fires, and backyard trash burning, as well as during some industrial processes such as paper pulp bleaching and herbicide manufacturing. The most toxic chemical in the class is 2,3,7,8tetrachlorodibenzo-para-dioxin (TCDD). The highest environmental concentrations of dioxin are usually found in soil and sediment, with much lower levels found in air and water. Humans are primarily exposed to dioxins by eating food contaminated by these chemicals." [26] "The term 'dioxin' is commonly used to refer to a family of toxic chemicals that share a similar chemical structure and induce harm through a similar mechanism. Dioxins have been characterized by EPA as likely human carcinogens and are anticipated to increase the risk of cancer at background levels of exposure. Examples of dioxin include polychlorinated biphenyls (PCBs), polychlorinated dibenzo dioxins (PCDDs), and polychlorinated dibenzo furans (PCDFs)." [27] "The main pathway for exposure to humans is via airborne emissions of dioxin that settle on plants and are passed on and accumulated through the food chain. While dioxin is produced in very small quantities in comparison with other pollutants (equivalent to around 30 pounds of the most toxic member of the class annually), its high toxicity and properties of bioaccumulation and persistence in the environment have led EPA to treat dioxin as a major health threat." [28] "CDDs (chlorinated dibenzo-p-dioxins) are a family of 75 chemically related compounds commonly known as chlorinated dioxins. In the pure form, CDDs are crystals or colorless solids. CDDs enter the environment as mixtures containing a number of individual components." [29]

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5.2.2.1.3. Polychlorinated biphenyls (PCBs) Polychlorinated biphenyls (see structure at Figure 5.2) are mixtures of up to 209 individual chlorinated compounds (known as congeners). PCBs are either oily liquids or solids that are colorless to light yellow. Some PCBs can exist as a vapor in air. PCBs have no known smell or taste. Many commercial PCB mixtures are known in the U.S. by the trade name Aroclor. 5.2.2.2. The new POPs [30] As said above the original POPsâ&#x20AC;&#x2122;s list was extended to include 9 new substances. Those are again pesticides (Chlordecone, alpha and beta hexaclorocyclohexane, lindane and pentachlorobenzene), industrial chemicals (hexabromobiphenyl, , penta, hexa and hepta bromodiphenyl ethers, perfluorooctane sulfonyl fluoride and sulfonic acid and salts of the later, and pentachlorobenzene) and industrial by-products (alpha and beta hexaclorocyclohexane and pentachlorobenzene). They structures are presented in Figure 5.3.

Figure 5.3 â&#x20AC;&#x201C; The 9 new POPs [30]

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Chlordecone is chemically related to Mirex, a pesticide listed already among the initial 12. Chlordecone is a synthetic chlorinated organic compound, which was mainly used as an agricultural pesticide. It was first produced in 1951 and commercially introduced in 1958. Currently, no use or production of the chemical is reported, as many countries have already banned its sale and use. Hexabromobiphenyl belongs to the group of polybrominated biphenyls, which are brominated hydrocarbons formed by substituting hydrogen with bromine in biphenyl. Hexabromobiphenyl is an industrial chemical that has been used as a flame retardant, mainly in the 1970s. According to available information, hexabromobiphenyl is no longer produced or used in most countries due to restrictions under national and international regulations. Alpha hexachlorocyclohexane and beta hexachlorocyclohexane came from the technical mixture of hexachlorocyclohexane (HCH) which contains mainly five forms of isomers, namely alpha-, beta-, gamma-, delta- and epsilon-HCH. Lindane is the common name for the gamma isomer of HCH. Use of alpha- and beta-HCH as insecticides was phased out years ago, but these chemicals have been produced as byproducts of lindane. For each ton of lindane produced, around 6-10 tons of alpha- and beta-HCH are also produced. Therefore there are large stockpiles leading to site contamination. Lindane has been used as a broad-spectrum insecticide for seed and soil treatment, foliar applications, tree and wood treatment and against ectoparasites in both veterinary and human applications. The production of lindane has decreased rapidly in the last few years, due to regulations in several countries. However, a few countries are still known to produce it and it has a specific exemption for use as a human health pharmaceutical for control of head lice and scabies as second line treatment. Pentachlorobenzene (PeCB) belongs to a group of chlorobenzenes that are characterized by a benzene ring in which the hydrogen atoms are substituted by one or more chlorines. Previously, PeCB was used in PCB products, in dyestuff carriers, as a fungicide and a flame retardant. It might still be used as a chemical intermediate and it is also produced unintentionally during combustion, thermal and industrial processes, and present under the form of impurities, in products such as solvents or pesticides. PeCB production ceased several decades ago in the main producing countries, as efficient and cost-effective alternatives became available. In order to significantly reduce the unintentional production of PeCB, strong efforts on process control need to be applied. Tetrabromodiphenyl ether and pentabromodiphenyl ether are the main components of commercial pentabromodiphenyl ether. They belong to a group of chemicals known as â&#x20AC;&#x153;polybromodiphenyl ethersâ&#x20AC;? (PBDEs). Polybromodiphenyl ethers including tetra-, penta-, hexa-, and heptaBDEs. They inhibit or suppress combustion in organic materials and therefore are used as additive flame retardants. The production of tetra- and pentaBDEs has ceased in certain regions of the world, while no production of hexa- and heptaBDEs is reported. Hexabromodiphenyl ether and heptabromodiphenyl ether are the main components of commercial octabromodiphenyl ether. The only route to decompose polybromodiphenyl ethers is to subject them to debromination, i.e. the replacement of bromine on the aromatic ring with hydrogen. This mean that higher bromodiphenyl ether congeners may be converted to lower congeners that can be even more toxics. The higher congeners might therefore be precursors to the tetraBDE, pentaBDE, hexaBDE or heptaBDE. Identification and handling of equipment and wastes containing brominated diphenyl ethers is demanding and is the only approved exception for their use. Perfluorooctane sulfonic acid (PFOS), its salts and perfluorooctane sulfonyl fluoride (PFOS-F). PFOS is a fully fluorinated anion, which is commonly used as a salt or incorporated into larger polymers. PFOS and its closely related compounds, which may contain PFOS impurities or substances that can result in PFOS, are members of the large family of perfluoroalkyl sulfonate substances.

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5.2.2.3. Substances proposed to include the Stockholm convention list At the moment several substances were proposed to include the list of the Stockholm convention [30]. Substances like endosulfan and hexabromocyclodecane were recently reviewed by the Persistent Organic Polutants Review Comite (POPRC). The substances under review are at the moment short-chained chlorinated paraffins, chlorinated naphthalenes, hexachlorobutadienes and pentachlorophenol: Endosulfan, a synthetic organochlorine compound, is widely used as an agricultural insecticide. It was introduced into the market already back in the mid 1950s but plant production products containing endosulfan are still used in a number of countries worldwide. Hexabromocyclododecane (HBCDD) is a white solid substance which is used as an additive flame retardant on its own, or in combination with other flame retardants. HBCDD is used mainly in expanded and extruded polystyrene. Most of this HBCDD-treated polystyrene is used for insulation boards in, e.g., buildings and vehicles. Other applications include its use in textile coatings and in high impact polystyrene for electrical and electronic equipment. HBCDD is the third most used brominated flame retardant and the global market demand in 2001 was 16 700 tonnes. Technical HBCDD is a mix of mainly three diastereomers (compounds that are identical except for the spatial disposition of the atoms), alpha-, beta-, and gamma-HBCDD, and the final distribution of these diastereomers in technical HBCDD varies with a range of about 70-95 % γ-HBCDD and 530 % α- and β-HBCDD. Hexachlorobutadiene (HCBD) is primarily produced in chlorinolysis plants as a by-product in the production of carbon tetrachloride and tetrachloroethene. Chlorinolysis is a radical chain reaction that occurs when hydrocarbons are exposed to chlorine gas under pyrolytic conditions. The hydrocarbon is chlorinated and the resulting chlorocarbons are broken down. This process is analogous to combustion, but with chlorine instead of oxygen. Short-chained chlorinated paraffins (SCCPs) are a group of synthetic compounds mainly used in metal working fluids, sealants, as flame retardants in rubbers and textiles, in leather processing and in paints and coatings. Chlorinated naphthalenes (CNs) are a group of theoretically 75 possible chlorinated naphthalenes, containing one to eight chlorine atoms. They are structurally similar to the PCBs. Hexachlorobutadiene (HCBD) is a halogenated aliphatic compound, mainly created as a byproduct in the manufacture of chlorinated hydrocarbons like tri- and tetrachloroethene and tetrachloromethae. Pentachlorophenol (PCP) is an aromatic hydrocarbon of the chlorophenol family. PCP was first introduced for use as wood preservative in the 1930’s. Since its introduction PCP has had a variety of other applications (biocide, pesticide, disinfectant, defoliant, anti-sapstain agent, anti-microbial agent, wood preservative and on the production of pentachlorophenyl laurate). The salt sodium pentachlorophenate (Na-PCP) was used for similar purposes as PCP and readily degrades to PCP. The ester pentachlorophenyl laurate (PCPL) is used in textiles. In addition, formation of dioxins and furans during incineration of wastes is known in the presence of corresponding chlorinated precursors like PCP compounds. 5.2.3. Emergent Pollutants with endocrine disrupting character More recently the scientific community has also been devoting great attention to a wide group of substances called emerging contaminants. These are a potential risk to public health since their list includes substances widely used by persons in their day-to-day life, being present in the composition of food, medicines, hygiene products and personal use products, pans coatings’ and plastic containers for food and beverages, included in electronic equipment of common use , etc [31-38].

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The endocrine disruptor character of both persistent pollutants as emerging contaminants makes these compounds targets for environmental monitoring and remediation [31, 33-38]. If the study of persistent pollutants is already routine or its dangers are extensively reported [33], it is still very scarce and recent the study of emerging contaminants in the world, especially those who have action as endocrine disruptors, both in aquatic environments and in other environmental matrices (wastewater treatment plants, sediment, etc..) and that can affect their quality and biodiversity. No body nows what are their effects after their ingestion, external application or disposal. 5.2.3.1. Pharmaceutical and Personal Care Products (PPCPs) Certain pharmaceutically active compounds (caffeine, nicotine or aspirin, among others) were known for over 20 years to enter the environment via wastewaters. However just recently became evident that large amount of drugs and personal care products (PPCPs) from a wide spectrum of therapeutic and consumer-use classes are released unattendly to the environment, every day over their production. Just a limited number of their classes have been monitored in the environment. These compounds do not have long residence times in the environment but their continuous introduction on it through continuous discharge gives them the persistent character. The full extent of the presence of PPCPs in the aquatic environment is largely unknown. The toxicological significance of PPCPs in the environment with regard to either humans or terrestrial wildlife is poorly known for most PPCPs [42, 43]. Among the substances used as medicines we can identify in first place antipyretics, antiinflammatories, antibiotic and oral anticontraceptives. In those are families of compounds with broad pharmaceutical use. For antipyretic drugs, we can mention compounds like aspirin, salicylic acid and paracetamol, for anti-inflammatory drugs, compounds like diclofenac sodium and ibuprofen, for antibiotics, amoxicillin, clavulanic acid, etc. As hormones and residues of oral anticontraceptive pils estradiol and progestin are the most used [31-38, 42-47]. Some of the hygiene products and other daily personal used products as shampoos (and other surfactant containing products as detergents and soaps), perfumes and fragances or sunscreens for solar protection frequently contain endocrine disrupting substances [42, 43].. For example triclosan and triclocarban are two bactericides frequently present in the composition of various personal products like shampoos, soaps, creams, tooth pastes. Theirs wide and varied consumption made frequent the detection of these compounds both in the sewer as surface waters [45]. Some of the products already identified as potentially harmful and being screened are listed in Table 5.3. Most elements of classes of pharmaceutics have never been monitored as anorexiants (diet drugs, ex: xenical), antiarrhythmics, anticoagulants, antidepressants, antidiabetic agents, antipsychotics, diuretics, expectorants, gastrointestinal agents (ulcer drugs), HIV drugs, hormonally active agents, muscle relaxants, osteoporosis agents, street drugs (illicit, illegal, recreational), etc, etc.

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Table 5.3 - Representative classes and members of Pharmaceutics and Personal Care Products reported in wastewater and environmental samples (adapted from [42,43]). Therapeutic class

example generic name

analgesics/anti-inflammatories

acetaminophen diclofenac ibuprofen ketoprofen naproxen

antimicrobials

sulfonamides, fluoroquinolones

antiepileptics

carbamazepine

Antihypertensives, betablockers

bisoprolol metoprolol

antiseptics

triclosan

contraceptives

-estradiol 17-ethinyl estradiol

anti-anxiety/hypnotic agents

diazepam

sun screen agents

methybenzylidene camphor avobenzene octyl methoxycinnamate

X-ray contrast agents

diatrizoate

5.2.3.2. Coatings and plastic containers for food and beverages Bisphenol A (BPA) is an industrial chemical widely used in common plastic products, such as baby bottles, childrenâ&#x20AC;&#x2122;s toys, and the linings of most food and beverage cans. Many scientific studies have found links between BPA and serious health problems, from heart disease, diabetes and liver abnormalities in adults to developmental problems in the brains and hormonal systems of children. BPA exhibits hormone-like properties at high dosage levels that raise concern about its suitability in consumer products and food containers where exposure is orders of magnitude lower. In 2010 the EPA reported that over one million pounds of BPA are released into the environment annually [48]. The antisticking coating of cooking instruments is constituted of perfluorinated compounds (PFCs). They are a family of fluorine-containing chemicals with excellent ability to make materials they coat stain and stick resistant. These chemicals have been used since the 1950s in innumerous familiar products of frequent use that require antisticking properties as nonstick cookware, stainresistant carpets and fabrics, coatings on some food packaging (especially microwave popcorn bags and fast food wrappers), as components of fire-combat foam and many other industrial applications. More recently, scientific community became aware of their persistent, long range

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transport and bioacomulative character that determine their ubiquity. Due to the strength of the CF bond, PFCs are extremely resistant to degradation. Several of their elements are already under control (see section 5.2.2.2), but other deserve attention. 5.2.3.3. Flame retardants As we have seen above a lot of flame retardants are chlorinated organic compounds that attracted already attention due to their characteristics of persistence, accumulation in environment and living organisms and toxicity and/or carcinogenicity. Among them we stress the importance of: Polybrominated biphenyls (PBBs) are manufactured chemicals. They are added to the plastics used to make products like computer monitors, televisions, textiles, plastic foams, etc. to make them difficult to burn, so they can be called of flame-retardants. PBBs can leave these plastics and find their way into the environment, where they easily build up (bioaccumulation and biomagnification). PBBs are usually colorless to off-white solids. PBBs are mixtures of brominated biphenyl compounds known as congeners. Although in the United States manufacturing of PBBs was stopped in 1976, they are still around in the environment because they do not degrade easily or quickly. Polybrominated diphenyl ethers (PBDEs) are flame-retardant chemicals that are added to plastics and foam products to make them difficult to burn. There are different kinds of PBDEs; some have only a few bromine atoms attached, while some others have as many as ten bromines attached to the central molecule. PBDEs exist as mixtures of similar chemicals called congeners. Because they are mixed into plastics and foams rather than bound to them, PBDEs can easily leave the products that contain them and enter the environment. 5.2.4. Dyes Although being much less harmful to individues and environment than all above mentioned chemical, dyes and pigments are also frequently cause of concern. Organic dyes are one of the largest groups of wastewaters pollutants due to their use in color industrie. In general they are released into the environment by textile and some other industries [31, 49]. They are extensively used in dying industry, being lost during dying procedures and going in their wastewaters. Wastewaters containing 5-15% of untreated dye can be released into the environment and are considered to pose serious problems [31, 50, 51]. Even if present in small amounts they impart to waters strong color, quickly reducing water transparency and creating problems on the water bodies they reach. They have in common with persistent pollutants the fact that their structures are strongly resistant [31, 49-54]. In fact, dyes and pigment must resist to light in order their colors not to fade. These dyes pose an environmental concern due to both their high visibility and their recalcitrant character. Moreover, many dyes and their degradation products have been associated with toxicity and/or mutagenicity [31,52].

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5.3. Occurrence and Dangers of Chemical Carcinogens and Persistent Environmental Pollutants to Environment and Individuals 5.3.1. Polycyclic aromatic hydrocarbons PAHs occur naturally on fossil fuels or as suspended particulate matter on urban atmosphere, originating from organic matter pyrolysis. As a matter of fact, combustion processes using fossil fuels on vehicles motor or for heating and energy production are the primary sources of PAHs, nevertheless other industrial processes, usually associated with the production of synthetic fuels or the production of secondary products of the petroleum industry, also contribute for they production. Although in much smaller amounts PAHs are formed also on the course of some food preparation processes as grilling, frying or roasting. The use of naked fires for heating or cooking proposes can be also a domestic source of PAHs, also contributing for the increase of human exposition to this class of pollutants. PAHs are considered to be toxic and some of them are already know as being carcinogenic to humans. This evidence was furnished by occupational studies on workers after inhalation or dermal exposure [1-7, 9, 12, 21-23]. Benzo[a]pyrene is one of most known and studied PAHs since it was the first member of this family to be identified as being a strong carcinogenic agent [3, 30, 33]. Usually when PAHsâ&#x20AC;&#x2122;s molecular weight increases, melting and boiling point increase also, their water solubility decreases and increases their solubility on fat tissues. The danger of these compounds is therefore directly related with the increase of their molecular weight. Because, on environment PAHs are usually semi-volatile and posse low water solubility, they have high affinity for particulate matter. Aqueous solubility decreases approximately one order of magnitude for each additional ring. From a same source, PAHs are produced together but on variable concentrations and properties and effect of those mixtures change according to the PAHs present and to their relative concentrations. Although they have variable individual toxicological effects, 16 PAHs (naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]flouranthene, benzo[a]pyrene, dibenz(ah)anthracene, benzo[ghi]perylene, and indeno(1,2,3-cd)pyrene, see structures on Figure 5.1) were identified first by Environmental Protection Agency (EPA) from United States of America as prioritary for environmental monitoring [33]. Of these, seven are classified by EPA as probably carcinogenic to humans. In Figure 5.1 their Toxic Equivalency Factors relatively to benzo [a] pyrene are indicated in brackets [5]. Nowadays the list has already 32 PAHs but the original list of 16 EPA priority PAHs is still often the targeted one for measurement in environmental samples. PAHs integrate also the list of the most dangerous substances from Agency for Toxic Substances and Disease Registry (ATSDR) / EPA, being PAHs as a family, benzo[a]pyrene, benzo[b]fluoranthene and dibenzo[a,h]anthracene between the 20 first substances of the list from 2005 [40]. In Table 5.4 we resume fate of PAHs in environment according to ATSDR in their toxicological profile [40].

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Table 5.4 - What happens to polycyclic aromatic hydrocarbons (PAHs) when they enter the environment? [40]  PAHs enter the air mostly as releases from volcanoes, forest fires, burning coal, and automobile exhaust. 

PAHs can occur in air attached to dust particles.



Some PAH particles can readily evaporate into the air from soil or surface waters.

 PAHs can break down by reacting with sunlight and other chemicals in the air, over a period of days to weeks. 

PAHs enter water through discharges from industrial and wastewater treatment plants.

 Most PAHs do not dissolve easily in water. They stick to solid particles and settle to the bottoms of lakes or rivers. 

Microorganisms can break down PAHs in soil or water after a period of weeks to months.

 In soils, PAHs are most likely to stick tightly to particles; certain PAHs move through soil to contaminate underground water.  PAH contents of plants and animals may be much higher than PAH contents of soil or water in which they live.

5.3.2. Organochlorinated substances Persistent Organic Pollutants (POPs) are compounds of high environmental persistence, which combined with a strong lipophilic and hydrophilic character determines that such individual compounds undergo bioaccumulation and biomagnification in food chains. Many of these substances are mutagenic and carcinogenic character, many of whom are also endocrine disruptors [1, 30, 31]. POPs and several other carcinogenic substances are semi-volatile; this characteristic confers them a degree of atmospheric mobility that is sufficient to allow that significant amounts are transported over long distances [1, 22]. Its moderate volatility prevents them to remain permanently in the atmosphere (which would pose a relatively small risk to living organisms), and facilitates their transition to the vapor phase and its subsequent deposition or adsorption on airborne particulate matter. This atmospheric pathway facilitates their transport over long distances, usually from warm regions of the globe, where they pass to the vapor phase to the cold regions, where they are deposited [1, 21, 23]. From the moment this mechanism was understood, the international community began to be aware of the ubiquity of these substances and of the need to take urgent global action to reduce and eliminate them. They are produced intentionally by man for agricultural or industrial use (pesticides, PCBs), unintentionally as by-products of industrial activity (PCDDs and PCDFs) or human (PCDDs processes burning at temperatures below 800 º C -; PAHs - burning fossil fuels). The hazardous properties mentioned above, the diversity of its sources, the fact that they are semi-volatile, have high environmental mobility and ubiquitous character makes that they represent an high risk to human health and the environment, strongly threatening biodiversity, especially relatively to individuals on the top of the food chain where they reach exceedingly high concentrations (present in trace amounts in water, bioconcentrate on aquatic organisms and food chains featuring biomagnification with concentration factors of several orders of magnitude). [1, 3, 4, 24, 30 -33, 39].

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POPs’s toxicological properties lead to a growing concern about their toxic effects on species living in places near or far from their sources, particularly those who are at the top of the food pyramids, even when in their natural environment they are exposed to low concentrations of these compounds. For example, polychlorinated biphenyls have been found in all regions of the globe including oceans, deserts and Polar Regions, where there are no local sources. The only plausible explanation for their presence is their long-distance transport mechanism originating in other regions of the globe. Humans are exposed to POPs through their diet from eating foods that already contain significant amounts of these contaminants, in their work environment when handling or moving the sites containing these substances, the environment in general and even in their own environment household. Exposure to POPs can occur chronic or sporadically. An example of a point exposition is an industrial plant accident resulting in the inadverted release of one or more substances of this kind. As an example of chronic exposure we can mention the workers of an industrial facility where persistent pollutants are produce, either voluntarily (voluntary production of a specific compound for which a practical application exists and that subsequently was found to be a persistent pollutants) or involuntarily (producing a persistent pollutant involuntarily due to his occurrence as a byproduct of the manufacturing process). Currently it is known that POPs can cause a variety of adverse effects on living organisms, as: 

endocrine disruption



immune dysfunctions



neurobehavioral disorders



Induction and promotion of tumors / cancers.

The risk posed by the exposure to these types of compounds may seem negligible, without being noticed any signs or symptoms for years or even generations after exposure. A good example of the risk posed by the exposure to POPs is the production and widespread use of organochlorine pesticides after the second world war, when they had an important role in the control of pests and consequent production of larger quantities of food, contributing to the quick pos world war reduction of severe food shortages in some regions the globe. However, nowadays the use of many of them has already been banned or is tightly controlled because of their enormous stability and toxicity and negative effects that they were reported to cause in enviroment being a strong risk to the biodiversity of the contaminated ecosystems. Scientists are now discovering discovered that many POPs, even at levels considered safe by experts less than a decade ago, can cause irreversible damage in people and animals, and have their use banned or controlled now. In fact, often the environmental risks are only understood in its true dimension after the damages are already hopelessly consummated. This results from chronic toxicity studies being not simple and being expensive and time consuming, which usually does not matter to producers in the moment the products started to be commercialized. Thus, as there is still no scientific evidence of their danger, substances currently sold as safe may well prove in a near future to be harmful, after some years of use, when their side effects start to be perceived. Nowadays more and more attention is being given to regulation of dangerous substances either nationally, or internationally. As said above, at international level, the most systematic attempt to regulate persistent organic pollutants and organohalogenated compounds is being carried out by UNEP [39] (United Nations Environmental Program) that since 1997 has mandate to coordinate the establishment and

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negotiation of a treaty to control, reduce or eliminate the use, production, discharges, emissions and losses of POPs. Since that date, the UNEP has coordinated the international action to protect public health and the environment through measures that reduce or eliminate emissions and / or discarges of POPs. They promoted the collection and sharing of information and the preparation of specific legislation for the initial group of 12 substances (aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, PCDDs, PCDFs and PCBs (see Figure 5.2) The compounds in the list are all organochlorinated and the list comprises pesticides (first nine compounds) used in agriculture and / or the control of epidemic diseases; industrial chemicals (PCBs and hexachlorobenzene) and the rest are byproducts industrial processes (PCDDs, PCDFs and PCBs and hexachlorobenzene). Note that two POPs, namely PCBs and hexachlorobenzene, are comprised in more than one category of use or origin. The toxicological significance of these 12 POPs is so large that the first negotiations involved more than 120 countries and resulted in the approval of an international resolution for their removal which prohibits their use, production and sale (Johannesburg conversations, December 10th, 2000). The text of the Convention of Priority Organic Pollutants (POPs), was endorsed by the governments of many countries still during 2001, May 23, at a diplomatic conference in Stockholm. In 2009 the list was extended to include 9 new chemicals (Chlordecone, alpha and beta hexaclorocyclohexane, lindane, pentachlorobenzene, hexabromobiphenyl, penta, hexa and hepta bromodiphenyl ethers, perfluorooctane sulfonyl fluoride and sulfonic acid and salts of the later) and more substances were recently reviewed or are under review (see section 5.2.2.2 and Figure 5.3 for details). They all are organoclorinated compounds. It should be noted that the POPs priority list is not a list of the most toxic substances, but a priority list based on a number of parameters such as toxicity, persistence, bioaccumulation potential, potential for atmospheric long range transport, volume of production and, last but not the least, significant economic aspects. Similarly, the 16 PAHs shown in Figure 5.1 were classified as priority pollutants by EPA 5 (Environmental Protection Agency) of the United States of America. Of these, seven are classified by EPA as probably carcinogenic to humans. In Figure 5.1 their Toxic Equivalency Factors relatively to benzo [a] pyrene are indicated in brackets [5]. These 16 PAHs and 12 POPs are also part of the list of the 275 most hazardous substances published by ATSDR [40] (Agency for Toxic Substances and Disease Registry) / EPA United States, with PCBs, benzo [a] pyrene, PAHs, benzo [b] fluoranthene, DDT, dibenzo [a, h] anthracene and dieldrin in the top 20 substances on this list. ATSDR list (see Figure 5.5) is actualized every two years and include the compounds which are determined to pose the most significant potential threat to human health due to their known or suspected toxicity and potential for human exposure to the substances found on the National Priorities List (NPL). Thus, it is possible for substances with low toxicity but high NPL frequency of occurrence and exposure to be on this priority list. The objective of this priority list is to rank substances across all NPL hazardous waste sites to provide guidance in selecting which substances will be the subject of toxicological profiles prepared by ATSDR. Table 5.5. -The ATSDR 2011 Substance Priority List (adapted [40]) 8

SUBSTANCE NAME

ARSENIC

LEAD

MERCURY

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SUBSTANCE NAME

VINYL CHLORIDE

POLYCHLORINATED BIPHENYLS

BENZENE

CADMIUM

BENZO(A)PYRENE

POLYCYCLIC AROMATIC HYDROCARBONS

BENZO(B)FLUORANTHENE

CHLOROFORM

AROCLOR 1260

DDT, P,P'-

AROCLOR 1254

DIBENZO(A,H)ANTHRACENE

TRICHLOROETHYLENE

CHROMIUM, HEXAVALENT

DIELDRIN

PHOSPHORUS, WHITE

HEXACHLOROBUTADIENE

DDE, P,P'-

CHLORDANE

COAL TAR CREOSOTE

AROCLOR 1242

ALDRIN

DDD, P,P'-

AROCLOR 1248

HEPTACHLOR

AROCLOR

BENZIDINE

ACROLEIN

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SUBSTANCE NAME

TOXAPHENE

TETRACHLOROETHYLENE

HEXACHLOROCYCLOHEXANE, GAMMA-

CYANIDE

HEXACHLOROCYCLOHEXANE, BETA-

BENZO(A)ANTHRACENE

DISULFOTON

1,2-DIBROMOETHANE

ENDRIN

DIAZINON

HEXACHLOROCYCLOHEXANE, DELTA-

BERYLLIUM

ENDOSULFAN

AROCLOR 1221

1,2-DIBROMO-3-CHLOROPROPANE

HEPTACHLOR EPOXIDE

ENDOSULFAN, ALPHA

CIS-CHLORDANE

CARBON TETRACHLORIDE

AROCLOR 1016

COBALT

PENTACHLOROPHENOL

DDT, O,P'-

METHOXYCHLOR

ENDOSULFAN SULFATE

NICKEL

DI-N-BUTYL PHTHALATE

ENDRIN KETONE

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SUBSTANCE NAME

DIBROMOCHLOROPROPANE

BENZO(K)FLUORANTHENE

XYLENES, TOTAL

5.3.2.1. Pesticides Neither aldrin nor dieldrin occurs naturally in the environment. From the 1950s until 1970, aldrin and dieldrin were widely used pesticides for crops like corn and cotton. Because of concerns about damage to the environment and potentially to human health, EPA banned all uses of aldrin and dieldrin in 1974, except to control termites. In 1987, EPA banned their all uses. Endrin has not been produced or sold for general use in the United States since 1986. Chlordane does not occur naturally in the environment. Because of concern about damage to the environment and harm to human health, the Environmental Protection Agency (EPA) banned all uses of chlordane in 1983 except to control termites. In 1988, EPA banned all uses. Mirex and chlordecone have not been manufactured or used in the United States since 1978. Toxaphene was banned for all registered uses by 1990. DDT use in the U.S. was banned in 1972 because of damage to wildlife, but is still used in some countries in malaria control. DDE has no commercial use. DDD was also used to kill pests, but its use has also been banned. One form of DDD has been used medically to treat cancer of the adrenal gland. According to ATSDR [40], the environmental path of the above mentioned organochlorinated pesticides is the one identified bellows in Table 5.6. Theirs routes of human exposition are displayed in Table 5.7, still according to ATSDR [40]. Table 5.6 - What happens to aldrin, dieldrin, endrin, chlordane, mirex, toxaphene and DDT when they enter the environment? (Adapted from [40]

Aldrin and Dieldrin Aldrin and dieldrin can enter the environment from accidental spills or leaks from storage containers at waste sites. In the past, aldrin and dieldrin entered the environment when farmers used these compounds to kill pests on crops and when exterminators used them to kill termites. Aldrin and dieldrin are still present in the environment from these past uses. Sunlight and bacteria in the environment can change aldrin to dieldrin. Therefore, you can find dieldrin in places where aldrin was originally released. Dieldrin in soil or water breaks down (degrades) very slowly. Dieldrin sticks to soil and may stay there unchanged for many years. Water does not easily wash dieldrin off soil. Dieldrin does not dissolve in water very well and is therefore not found in water at high concentrations. Most dieldrin in the environment attaches to soil and to sediments at the bottoms of lakes, ponds, and streams. Dieldrin can travel large distances by attaching to dust particles, which can then be transported great distances by the wind. Dieldrin can evaporate slowly from surface water or soil. In the air, dieldrin changes to photodieldrin within a few days. Plants can take up dieldrin from the soil and store it in their leaves and roots. Fish or animals that eat dieldrin-contaminated materials store a large amount of the dieldrin in their fat. Animals or fish that eat other animals have levels of dieldrin in their fat many times higher than animals or fish that eat plants.

Endrin Endrin does not dissolve very well in water. It has been found in ground water and surface water, but only at very low levels. It is more likely to cling to the bottom sediments of rivers, lakes, and other bodies of water. Endrin is generally not found in the air except when it was applied to fields during agricultural applications. The persistence of endrin in the environment depends highly on local conditions. Some estimates indicate that endrin can stay in soil for over 10 years. Endrin may also be broken down by exposure to high temperatures (230째C) or light to form primarily endrin ketone and endrin aldehyde. It is not known what happens to endrin aldehyde or endrin ketone once they are released to the environment; however, the amount of endrin broken down to endrin aldehyde or endrin ketone is very small (less than 5%).

Chlordane When used as a pesticide on crops, on lawns and gardens, and to control termites in houses, chlordane enters the environment. Although it is no longer used in the United States, it may be used in other countries. In soil, it attaches strongly to particles in the upper layers of soil and is unlikely to enter into groundwater. It is not known whether chlordane breaks down in most soils. If breakdown occurs, it is very slow. Chlordane is known to remain in some soils for over 20 years. Persistence is greater in heavy, clayey or organic soil than in sandy soil. Most chlordane is lost from soil by evaporation. Evaporation is more rapid from light, sandy soils than from heavy

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Environmental Remediation and Energy Production Technologies soils. Half of the chlordane applied to the soil surface may evaporate in 2 to 3 days. Evaporation is much slower after chlordane penetrates into the soil. In water, some chlordane attaches strongly to sediment and particles in the water column and some is lost by evaporation. It is not known whether much breakdown of chlordane occurs in water or in sediment. Chlordane breaks down in the atmosphere by reacting with light and with some chemicals in the atmosphere. However, it is sufficiently long lived that it may travel long distances and be deposited on land or in water far from its source. Chlordane or the chemicals that chlordane changes into accumulate in fish, birds, and mammals. Chlordane stays in the environment for many years and is still found in food, air, water, and soil. Chlordane is still commonly found in some form in the fat of fish, birds, mammals, and almost all humans.

mirex and cholordecone Mirex and chlordecone contaminated water and soil while they were being manufactured and used in the 1960s and 1970s. These substances can still enter surface water through runoff of contaminated soil at facilities that once manufactured these chemicals or by seeping from waste disposal sites. Mirex and chlordecone do not evaporate to any great extent into the air. They also do not dissolve easily in water. Most of the mirex and chlordecone in water attaches to soil particles suspended in the water or to sediment. When they bind to soil particles in water, they can travel long distances. Both compounds bind strongly to soil. Because they are not likely to move through the soil, very little will get into underground water. Mirex and chlordecone can stay in soil, water, and sediment for years. Both compounds are slowly broken down in soil, water, and sediment. Mirex is broken down more quickly than chlordecone. Mirex is broken down to photomirex, which can also cause harmful health effects. Photomirex is even more poisonous than mirex. It is produced when sunlight reacts with mirex in water or in the air. Fish or animals that live in waters that contain mirex or chlordecone, or that eat other animals contaminated with mirex or chlordecone, can build up these substances in their bodies. The amounts of mirex and chlordecone in their bodies may be several times greater than the amount in their prey or in the surrounding water.

Toxaphene When toxaphene is released to the environment, it can enter the air (by evaporation), the soil (by sticking to soil particles), and the water (from runoff after rains). Toxaphene does not dissolve well and evaporates easly. Toxaphene is more likely to be found in air, soil, or the sediment at the bottom of lakes and streams. Once toxaphene is in the environment, it can last for many years because it breaks down very slowly. Toxaphene has been found in water, soil, sediment, air, and animals in places far from where it has been used. This shows that toxaphene can be carried long distances by the air. Toxaphene levels may be high in some predatory fish and mammals because toxaphene accumulates in fatty tissues. Even when levels are low or confined to a certain area, they could be high in individual animals.

DDT, DDE, and DDD Before 1973 when it was banned, DDT entered the air, water, and soil during its production and use as an insecticide. DDT is present at many waste sites, including NPL sites; releases from these sites might continue to contaminate the environment. Most DDT in the environment is a result of past use; DDD was also used as a pesticide to a limited extent in the past. DDT still enters the environment because of its current use in other areas of the world. DDE is only found in the environment as a result of contamination or breakdown of DDT. DDD also enters the environment during the breakdown of DDT. Large amounts of DDT were released into the air and on soil or water when it was sprayed on crops and forests to control insects. DDT was also sprayed in the environment to control mosquitos. Although the use of DDT is no longer permitted in the United States, DDT may be released into the atmosphere in other countries where it is still manufactured and used, including Mexico. DDT, DDE and DDD may also enter the air when they evaporate from contaminated water and soil. DDT, DDE, and DDD in the air will then be deposited on land or surface water. This cycle of evaporation and deposition may be repeated many times. As a result, DDT, DDE, and DDD can be carried long distances in the atmosphere. These chemicals have been found in bogs, snow, and animals in the Arctic and Antarctic regions, far from where they were ever used. Some DDT may have entered the soil from waste sites. DDT, DDE, and DDD may occur in the atmosphere as a vapor or be attached to solids in air. Vapor phase DDT, DDE, and DDD may break down in the atmosphere due to reactions caused by the sun. The half-life of these chemicals in the atmosphere as vapors (the time it takes for one-half of the chemical to turn into something else) has been calculated to be approximately 1.5-3 days. However, in reality, this half-life estimate is too short to account for the ability of DDT, DDE, and DDD to be carried long distances in the atmosphere. DDT, DDE, and DDD last in the soil for a very long time, potentially for hundreds of years. Most DDT breaks down slowly into DDE and DDD, generally by the action of microorganisms. These chemicals may also evaporate into the air and be deposited in other places. They stick strongly to soil, and therefore generally remain in the surface layers of soil. Some soil particles with attached DDT, DDE, or DDD may get into rivers and lakes in runoff. Only a very small amount, if any, will seep into the ground and get into groundwater. The length of time that DDT will last in soil depends on many factors including temperature, type of soil, and whether the soil is wet. DDT lasts for a much shorter time in the tropics where the chemical evaporates faster and where microorganisms degrade it faster. DDT disappears faster when the soil is flooded or wet than when it is dry. DDT disappears faster when it initially enters the soil. Later on, evaporation slows down and some DDT moves into spaces in the soil that are so small that microorganisms cannot reach the DDT to break it down efficiently. In tropical areas, Ă&#x201C;DDT may disappear in much less than a year. In temperate areas, half of the Ă&#x201C;DDT initially present usually disappears in about 5 years. However, in some cases, half of the Ă&#x201C;DDT initially present will remain for 20, 30, or more years. In surface water, DDT will bind to particles in the water, settle, and be deposited in the sediment. DDT is taken up by small organisms and fish in the water. It accumulates to high levels in fish and marine mammals (such as seals and whales), reaching levels many thousands of times higher than in water. In these animals, the highest levels of DDT are found in their adipose tissue. DDT in soil can also be absorbed by some plants and by the animals or people who eat those crops.

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Table 5.7 - How might I be exposed to aldrin, dieldrin, endrin, chlordane, mirex, toxaphene and DDT? (Adapted from [40]) Aldrin and Dieldrin For most people, exposure to aldrin and dieldrin occurs when they eat foods contaminated with either chemical. Contaminated foods might include fish or shellfish from contaminated lakes or streams, root crops, dairy products, and meats. Exposure to aldrin and dieldrin also occurs when you drink water, breathe air, or come into contact with contaminated soil at hazardous waste sites. Skin contact and breathing of aldrin and dieldrin by workers who used these chemicals to kill insects were at one time common. However, aldrin and dieldrin are no longer produced and no longer used. People with the greatest potential for exposure include those who live in homes that were once treated for termites using aldrin or dieldrin. Studies indicate that people can be exposed to aldrin and dieldrin years after they were applied in a home. Exposure to aldrin is generally limited because aldrin is changed quickly to dieldrin in the environment. Dieldrin remains in the environment for a long time and is usually detected in soil, sediment, and animal fat. Levels of both aldrin and dieldrin have decreased over the years since they are no longer produced or used. The levels of aldrin and dieldrin in air and water are typically very low.

Endrin Since endrin is no longer produced or used in the United States, you can probably be exposed to it only in areas where it is concentrated, such as a hazardous waste site. You may be exposed to endrin in air, water, or soil if you live near a hazardous waste site. Endrin has been detected at 120 (8.4%) such sites. Children living near hazardous waste sites could be exposed to endrin in contaminated soils, if they eat dirt. Detection of endrin in ground water or drinking water is rare. In the U.S. EPA 1989 National Pesticide in Groundwater Study, in which ground water was collected from areas with significant agricultural land uses as well as urban areas, only two wells were found with detectable levels of endrin. In wells drilled to access ground water near hazardous waste sites, 1.3% of 156 Resource Conservation and Recovery Act (RCRA) sites and 0.9% of 178 Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) or Superfund sites had detectable levels of endrin in the early 1980s. No information about the presence of endrin aldehyde or endrin ketone in the environment was found. You may also be exposed to endrin by eating foods that contain endrin. Before cancellation of endrin use, reported concentrations of endrin in domestic and imported food samples ranged from 0.05 to 0.50 parts per million (ppm; where 1 ppm = 1 microgram per gram (ĂŹg/g) of food). However, no endrin was detected in food samples from a Texas survey and only 0.084% of over 13,000 food samples were found to contain endrin in 1989 after cancellation of endrin use. Endrin was found in less than 1% of all food sampled by the U.S. Food and Drug Administration (FDA) in 1991. Because endrin is no longer used in the United States, residues on imported foods are the main source of potential human exposure in food. The levels of endrin aldehyde or endrin ketone in foods are not known. Endrin levels can build up (bioaccumulate) in the tissues of organisms that live in water. In the 1986 EPA National Study of Chemical Residues in Fish, concentrations of endrin were found in fish at 11% of 362 sites surveyed (average 1.69 parts per billion [ppb; where 1 ppb = 1 nanogram per gram (ng/g) of food]; maximum 162 ppb). Endrin was also detected in 21 of 31 samples of 2 commercial shrimp species from a Gulf Coast estuary receiving both industrial discharges, and urban and agricultural runoff. The average concentration was 1,070 and the maximum concentration was 9,470 ppb. Levels of endrin have probably declined, even in such polluted areas, since using endrin was banned. Endrin has been detected in human breast milk (0.02-6.24 milligrams endrin in each kilogram milk fat [mg/kg]); this may be a route of exposure for nursing infants. However, no studies of endrin in breast milk in United States or Canadian populations have been conducted.

Chlordane Everyone in the United States has been exposed to low levels of chlordane. A more relevant question is whether or not you may have been exposed to high levels of chlordane. Before its ban in 1988, you might have been exposed to high levels of chlordane if you worked in the manufacture, formulation, or application of chlordane. Therefore, farmers and lawn-care workers may have been exposed to chlordane before 1978, and pest control workers may have been exposed to chlordane before 1988 by skin contact and breathing dust and vapor. A national survey conducted from 1980 to 1983 estimated that 3,732 workers were potentially exposed to chlordane in the United States. This number of potentially exposed workers should have decreased after chlordane's use was banned in the United States. However, the ban on chlordane did not eliminate it from your environment, and some of your opportunities for exposure to chlordane continue. Today, people receive the highest exposure to chlordane from living in homes that were treated with chlordane for termites. Chlordane may be found in the air in these homes for many years after treatment. Houses in the deep south and southwest were most commonly treated. However, chlordane use extended from the lower New England States south and west to California. Houses built since 1988 have not been treated with chlordane for termite control. You can determine if your home was treated with chlordane by examining your records or contacting your termite treatment service. Over 50 million persons have lived in chlordanetreated homes. Indoor air in the living spaces of treated homes have been found to contain average levels of between 0.00003 and 0.002 milligram (mg) of chlordane in a cubic meter of air (mg/mÂł). However, levels as high as 0.06 mg/mÂł have been measured in the living areas of these homes. Even higher levels are found in basements and crawl spaces. The most common source of chlordane exposure is from ingesting chlordane- contaminated food. Chlordane remains in the food supply because much of the farmland was treated with chlordane in the 1960s and 1970s, and it remains in some soil for over 20 years. However, since chlordane has been banned, the levels in soils would be expected to decrease with the passage of time. Chlordane may also be found in fish and shellfish caught in chlordane-contaminated waters. If you are in doubt about whether a lake or river is contaminated, call your local Game and Fish or Health departments. Chlordane is almost never detected in drinking water. A survey conducted by the Food and Drug Administration (FDA) determined daily intake of chlordane from food to be 0.0013 microgram per

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Environmental Remediation and Energy Production Technologies kilogram of body weight (ĂŹg/kg) for infants and 0.0005-0.0015 ĂŹg/kg for teenagers and adults (a microgram is one thousandth of a milligram). The average adult would, therefore, consume about 0.11 ĂŹg of chlordane. You may come into contact with chlordane while digging in soil around the foundation of homes where it was applied to protect the homes against termites. Soil may also be contaminated with chlordane around certain NPL hazardous waste sites. Chlordane has been found at 176 of 1,350 hazardous waste sites on the NPL in the United States. The highest level of chlordane found in soil near an NPL site was 344 ppm. People may be exposed to chlordane at these sites by breathing low levels of chlordane volatilizing from the soil or from touching the soil. Levels of chlordane found in groundwater near NPL sites containing chlordane ranged from 0.02 to 830 parts of chlordane per billion parts of water (ppb). Finally, some chlordane may be left over from preban days. Old containers of material thought to contain chlordane should be disposed of carefully and contact with the skin and breathing vapors should be avoided.

mirex and cholordecone Most people are exposed to very low levels of mirex and chlordecone. The most likely way for people in the general population to be exposed to mirex or chlordecone is by eating food, particularly fish, taken from contaminated areas. Currently, three states (Ohio, New York, and Pennsylvania) have issued a warning to the public that fish may contain mirex. This warning applies mostly to fish caught in Lake Ontario. The state of Virginia has also issued a warning to the public about possible chlordecone contamination in fish and shellfish caught in the lower 113 miles of the James River. This contamination was caused when chlordecone was manufactured in one factory in Hopewell, Virginia, polluting the James River. People who live in areas where these compounds were used or made have higher levels in their tissues. Mirex was found in the milk of women who live in these areas, so nursing infants could be exposed. People who live near hazardous waste sites may be exposed to mirex or chlordecone by touching or eating contaminated soil that is on unwashed hands, food containers, or food itself, since these compounds bind to soil particles. Because mirex and chlordecone do not dissolve easily in water or evaporate easily in air, people are not likely to be exposed to them by drinking water or by inhaling air. Since mirex and chlordecone are no longer produced, the only people likely to be exposed through their work are those involved in the cleanup and removal of contaminated soils and sediments. Mirex and chlordecone do not occur naturally in the environment. Although mirex is not usually found in the air, it was detected at very low levels of up to 10 parts of mirex per quadrillion (1,000,000,000,000,000) parts of air in air samples from southern Ontario, Canada. Surface water concentrations of mirex ranged from 0.06 to 2.6 parts mirex per one trillion (1,000,000,000,000) parts of water in the Niagara River between 1981 and 1983. More recent monitoring data from 1987 show that mirex concentrations are decreasing in the surface waters of the Great Lakes to about 0.022 parts per trillion (ppt). In the mid-1980s, mirex was found in sediments of Lake Ontario at levels ranging from 6.4 parts per billion (ppb) to 38 ppb. Nationwide, the average level of mirex in fish was less than 4 ppb in 1986. However, fish from Lake Ontario had levels as high as 225 ppb. Chlordecone was found in surface water samples from the James River estuary at levels less than 10 ppt in 1977. More recent data were not available. In 1978, chlordecone was detected in sediments from the James River below its production site at concentrations of less than one part chlordecone in one million parts of sediment. In 1981, chlordecone was found in clams from the James River at levels ranging from 60 to 140 ppb.

Toxaphene People living near a location with heavy toxaphene contamination, such as a hazardous waste site, may be exposed to higher levels through breathing contaminated air or through direct skin contact with contaminated soil or water. Infants and toddlers, who are likely to put things in their mouth, may be exposed to toxaphene by eating contaminated soil. People who eat large quantities of fish, shellfish, or wild game animals from areas contaminated by toxaphene may have higher exposure to this substance since these animals tend to concentrate toxaphene in their fatty tissues. Individuals may be exposed to toxaphene through drinking water contaminated with toxaphene runoff from contaminated soils.

DDT, DDE, and DDD People in the United States are exposed to DDT, DDE, and DDD mainly by eating foods containing small amounts of these compounds. Although not common today, exposure to DDT could also occur through inhalation or absorption through the skin during the handling or application of DDT. Even though DDT has not been used in this country since 1972, soil may still contain some DDT that may be taken up by plants and eaten by animals and people. DDT from contaminated water and sediment may be taken up by fish. The amount of DDT in food has greatly decreased since DDT was banned and should continue to decline. In the years 1986 to 1991, the average adult in the United States consumed an average of 0.8 micrograms (a microgram is a millionth of a gram) of DDT a day. Adults consumed slightly different amounts based on their age and sex. The largest fraction of DDT in a person's diet comes from meat, poultry, dairy products, and fish, including the consumption of sport fish. Leafy vegetables generally contain more DDT than other vegetables, possibly because DDT in the air is deposited on the leaves. Infants may be exposed by drinking breast milk. DDT or its breakdown products are still present in some air, water, and soil samples. However, levels in most air and water samples are presently so low that exposure is of little concern. DDT levels in air have declined to such low levels that it often cannot be detected. In cases where DDT has been detected in air, it is associated with air masses coming from regions where DDT is still used or from the evaporated DDT from contaminated water or soil. p,p'-DDT and p,p'-DDE concentrations measured in air in the Great Lakes region in 1990 reached maximum levels of 0.035 and 0.119 nanograms (a nanogram is a billionth of a gram) of chemical per cubic meter of air (ng/m3), respectively. Levels were generally much lower, especially during the winter months. In 1995-1996, soils in the corn belt, where DDT was heavily used in the past, contained on the average about 10 nanograms of DDT in a gram of soil. In recent years, most surface water has not contained detectable amounts of DDT. People who work or live around NPL sites or work with contaminated soil or sediment would most likely be exposed by accidentally swallowing soil, having skin contact with the soil, inhaling DDT vapor, or breathing in DDT in dust.

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5.3.2.2. Dioxines and furanes Dioxins occur as by-products in the manufacture of organochlorinated compounds, in the incineration of chlorine-containing substances such as PVC (polyvinyl chloride), in bleaching of paper with chlorine, and from natural sources such as volcanoes and forest fires. PCDDs bioaccumulate in humans and wildlife because of their hydrophobic and lipophilic properties, and may cause developmental disturbances and cancer. PCDF have similar behavior. Theirs environmental path and routes of human exposition are displayed in Table 5.8 and 5.9 [40]. Table 5.8. - What happens to PCDDs, PCDFs and PCBs when it enters the environment? (adapted from [40])

PCDDs CDDs are released into the air in emissions from municipal solid waste and industrial incinerators. Exhaust from vehicles powered with leaded and unleaded gasoline and diesel fuel also release CDDs to the air. Other sources of CDDs in air include: emissions from oilor coal-fired power plants, burning of chlorinated compounds such as PCBs, and cigarette smoke. CDDs formed during combustion processes are associated with small particles in the air, such as ash. The larger particles will be deposited close to the emission source, while very small particles may be transported longer distances. Some of the lower chlorinated CDDs (DCDD, TrCDD, and some of the TCDDs) may vaporize from the particles (and soil or water surfaces) and be transported long distances in the atmosphere, even around the globe. It has been estimated that 20 to 60% of 2,3,7,8-TCDD in the air is in the vapor phase. Sunlight and atmospheric chemicals will break down a very small portion of the CDDs, but most CDDs will be deposited on land or water. CDDs occur as a contaminant in the manufacture of various chlorinated pesticides and herbicides, and releases to the environment have occurred during the use of these chemicals. Because CDDs remain in the environment for a long time, contamination from past pesticide and herbicide use may still be of concern. In addition, improper storage or disposal of these pesticides and waste generated during their production can lead to CDD contamination of soil and water. CDDs are released in waste waters from pulp and paper mills that use chlorine or chlorine-containing chemicals in the bleaching process. Some of the CDDs deposited on or near the water surface will be broken down by sunlight. A very small portion of the total CDDs in water will evaporate to air. Because CDDs do not dissolve easily in water, most of the CDDs in water will attach strongly to small particles of soil or organic matter and eventually settle to the bottom. CDDs may also attach to microscopic plants and animals (plankton) which are eaten by larger animals, that are in turn eaten by even larger animals. This is called a food chain. Concentrations of chemicals such as the most toxic, 2,3,7,8-chlorine substituted CDDs, which are difficult for the animals to break down, usually increase at each step in the food chain. This process, called biomagnification, is the reason why undetectable levels of CDDs in water can result in measurable concentrations in aquatic animals. The food chain is the main route by which CDD concentrations build up in larger fish, although some fish may accumulate CDDs by eating particles containing CDDs directly off the bottom. CDDs deposited on land from combustion sources or from herbicide or pesticide applications bind strongly to the soil, and therefore are not likely to contaminate groundwater by moving deeper into the soil. However, the presence of other chemical pollutants in contaminated soils, such as those found at hazardous waste sites or associated with chemical spills (for example, oil spills), may dissolve CDDs, making it easier for CDDs to move through the soil. The movement of chemical waste containing CDDs through soil has resulted in contamination of groundwater. Soil erosion and surface runoff can also transport CDDs into surface waters. A very small amount of CDDs at the soil surface will evaporate into air. Certain types of soil bacteria and fungus can break CDDs down, but the process is very slow. In fact, CDDs can exist in soil for many years. Plants take up only very small amounts of CDDs by their roots. Most of the CDDs found on the parts of plants above the ground probably come from air and dust and/or previous use of CDD-containing pesticides or herbicides. Animals (such as cattle) feeding on the plants may accumulate CDDs in their body tissues (meat) and milk.

PCDFs Small amounts of CDFs can enter the environment from a number of sources. Accidental fires or breakdowns involving capacitors, transformers, and other electrical equipment (e.g., fluorescent light fixtures) that contain polychlorinated biphenyls (PCBs) are known to release high levels of CDFs formed by thermal degradation. A fire involving a transformer containing PCBs contaminated the State Office Building in Binghamton, New York, with CDFs. Accidents of a different kind involving heated PCBs occurred in Japan (Yusho incident) and Taiwan (Yu-Cheng incident). These incidents involved exposure to CDFs-contaminated PCBs that were used as a heat exchanger fluid for processing rice oil and which accidentally leaked into the oil. CDFs are also produced as unwanted compounds during the manufacture of several chlorinated chemicals and consumer products, such as wood treatment chemicals, some metals, and paper products. When the waste water, sludge, or solids from these processes are released into waterways or soil in dumpsites, they become contaminated with CDFs. CDFs also enter into the environment from burning municipal and industrial waste in incinerators. The exhaust from cars that use leaded gasoline, which contains chlorine, releases small amounts of CDFs in the environment. Small amounts of CDFs may also enter into the environment from burning of coal, wood, or oil for home heating and production of electricity. Many of these chemicals or processes that produce CDFs in the environment are either being slowly phased out or strictly controlled. CDFs in air are present mostly as solid particles and to a much lesser extent as vapor. Some of the CDFs present in air return to the land and water by settling, snow, and rainwater. An amount of CDFs in the vapor phase is destroyed by reacting with certain chemical agents (called hydroxyl radicals) naturally present in the atmosphere. CDFs may remain in air for an average of more than 10 days

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Environmental Remediation and Energy Production Technologies depending on the CDF compound. Once in the air, CDFs can be carried long distances. They have been found in air and waters and at the bottom of lakes and rivers in areas far away from where they were released into the environment. CDFs tend to stick to suspended particles and settled particles in lakes and rivers and can remain at the bottom of lakes and rivers for several years. Sediment acts as a medium where CDFs that are present in air or water eventually settle. CDFs can build up in fish, and the amount of CDFs in fish can be tens of thousands times higher than the levels in water. The CDFs in water can get into birds or other animals and humans that eat fish containing CDFs. CDFs bind strongly to soil and are not likely to move from the surface soil into groundwater. In some instances, CDFs from some waste landfills may reach underground water. CDFs are more likely to move from soil to water or other soils by soil erosion and flooding. The breakdown or loss of CDFs in soil occurs over years, so CDFs remain in soil for years. Most CDFs found in plants are probably deposited by air. Cattle that eat plants on which CDFs have been deposited will build up some of the CDFs in their bodies. Some of the CDFs will enter the milk and meat of cattle.

PCBs Before 1977, PCBs entered the air, water, and soil during their manufacture and use in the United States. Wastes that contained PCBs were generated at that time, and these wastes were often placed in landfills. PCBs also entered the environment from accidental spills and leaks during the transport of the chemicals, or from leaks or fires in transformers, capacitors, or other products containing PCBs. Today, PCBs can still be released into the environment from poorly maintained hazardous waste sites that contain PCBs; illegal or improper dumping of PCB wastes, such as old transformer fluids; leaks or releases from electrical transformers containing PCBs; and disposal of PCB-containing consumer products into municipal or other landfills not designed to handle hazardous waste. PCBs may be released into the environment by the burning of some wastes in municipal and industrial incinerators. Once in the environment, PCBs do not readily break down and therefore may remain for very long periods of time. They can easily cycle between air, water, and soil. For example, PCBs can enter the air by evaporation from both soil and water. In air, PCBs can be carried long distances and have been found in snow and sea water in areas far away from where they were released into the environment, such as in the arctic. As a consequence, PCBs are found all over the world. In general, the lighter the type of PCBs, the further they may be transported from the source of contamination. PCBs are present as solid particles or as a vapor in the atmosphere. They will eventually return to land and water by settling as dust or in rain and snow. In water, PCBs may be transported by currents, attach to bottom sediment or particles in the water, and evaporate into air. Heavy kinds of PCBs are more likely to settle into sediments while lighter PCBs are more likely to evaporate to air. Sediments that contain PCBs can also release the PCBs into the surrounding water. PCBs stick strongly to soil and will not usually be carried deep into the soil with rainwater. They do not readily break down in soil and may stay in the soil for months or years; generally, the more chlorine atoms that the PCBs contain, the more slowly they break down. Evaporation appears to be an important way by which the lighter PCBs leave soil. As a gas, PCBs can accumulate in the leaves and above-ground parts of plants and food crops. PCBs are taken up into the bodies of small organisms and fish in water. They are also taken up by other animals that eat these aquatic animals as food. PCBs especially accumulate in fish and marine mammals (such as seals and whales) reaching levels that may be many thousands of times higher than in water. PCB levels are highest in animals high up in the food chain

Table 5.9. - How might I be exposed to PCDDs, PCDFs and PCBs when it enters the environment? (adapted from [40])

PCDDs CDDs are found at very low levels in the environment. These levels are measured in nanograms and picograms. One nanogram (ng) is one billionth of a gram, and one picogram (pg) is one trillionth of a gram. In some contaminated soils, concentrations of CDDs are reported as parts per billion. One part per billion (ppb) is one part CDD per billion parts of soil. The concentration of CDDs is often reported as parts per trillion, in samples of air, water, or soil. One part per trillion (ppt) is one part CDD per trillion parts of air, water, or soil. In some rural areas where CDD concentrations are very low in air or water, measurements are given in parts per quadrillion (ppq), which means one part CDD per quadrillion parts of air or water. CDDs are found everywhere in the environment, and most people are exposed to very small background levels of CDDs when they breath air, consume food or milk, or have skin contact with materials contaminated with CDDs. For the general population, more than 90% of the daily intake of CDDs, CDFs, and other dioxin-like compounds comes from food, primarily meat, dairy products, and fish. CDDs may be present at much lower levels in fruits and vegetables. The actual intake of CDDs from food for any one person will depend on the amount and type of food consumed and the level of contamination. Higher levels may be found in foods from areas contaminated with chemicals, such as pesticides or herbicides, containing CDDs as impurities. CDDs have been measured in human milk, cow's milk, and infant formula, so infants are known to be exposed to CDDs. Most surface water in the United States typically does not contain 2,3,7,8-TCDD and other CDDs at levels that are high enough to be measured (1 ppq or more). Municipal drinking water does not usually contain CDDs because the CDDs do not dissolve in water and primarily stick to particles, which are usually filtered out of treated drinking water. This means that using tap water to wash clothes or to bathe or shower, or swimming in pools or in uncontaminated lakes, rivers, or at ocean beaches will not expose people to significant levels of CDDs. Although CDDs are not usually found in filtered, treated drinking water, they have, on occasion, been detected in unfiltered groundwater from areas with known CDD contamination. Exposure to CDDs can also occur through skin contact with chlorinated pesticides and herbicides, contaminated soils, or other materials such as PCP-treated wood and PCB transformer fluids. Background levels of CDDs in soil are higher than background levels in both air and water. Background levels of CDDs detected in uncontaminated soils in the United States are generally very low or not detectable. 2,3,7,8-TCDD is not usually found in rural soil, but is typically found in soil in industrialized areas at levels ranging from 0.001 to 0.01 ppb. However, higher levels of 2,3,7,8-TCDD may be found in areas where CDDs have contaminated the soil. For example, contaminated soil at Times Beach, Missouri, had levels of 2,3,7,8-TCDD ranging from 4.4â&#x20AC;&#x201C;317 ppb. If CDDs are present at all in outdoor air in rural areas, they are generally present at very low levels or at concentrations near the

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Environmental Remediation and Energy Production Technologies detection limits for testing equipment. In winter, because of the burning of wood and other fuels for home heating, CDD levels may be slightly higher than during other seasons. In general, the background air levels of CDDs in urban areas are higher than in rural areas. Typical levels of CDDs in outdoor air in urban areas and industrial areas averaged 2.3 picograms per meter cubed (pg/m3). 2,3,7,8TCDD is not usually found in rural or urban air, but it is found in air near urban waste incinerators and high-traffic areas. The air around people who are smoking cigarettes may also have CDDs at levels above background levels. Although breathing contaminated air is a minor route of exposure for most people, exposure may be greater in areas near these CDD sources. CDDs have been found in all samples of adipose tissue and blood (serum lipids) from individuals with no known previous exposure. This indicates that all people are exposed to small amounts of CDDs. Levels of 2,3,7,8-TCDD in serum from the general population typically range from 3 to 7 ppt (on a lipid basis), and rarely exceed 10 ppt. Typically, lower levels of CDDs are found in less industrialized countries and in younger people. The production, use, and disposal of pesticides and phenoxy herbicides, disposal of production waste containing 2,3,7,8-TCDD, industrial accidents involving 2,4,5-trichlorophenol (2,3,5-TCP), and the consumption of CDD-contaminated food, have all led to increased potential for excess exposure of some groups of people. 2,3,7,8-TCDD has been detected at 91 of the 126 hazardous waste sites on the NPL that have been reported to contain CDDs. People living around these sites may be exposed to above-background levels of 2,3,7,8-TCDD and other CDDs. Elevated levels of CDDs have been reported in fish, shellfish, birds, and mammals collected in areas surrounding various chemical production facilities, various hazardous waste sites, and pulp and paper mills using the chlorine bleaching process. Sometimes these findings have resulted in closure of these areas for the purpose of fishing. People who eat contaminated food from these contaminated areas are at risk of increased exposure to CDDs. Occupational exposure to CDDs generally occurs through breathing contaminated air, or through skin contact with materials containing CDDs. Workers with the potential to be exposed to above-average levels of CDDs include those involved in the production or handling of certain chlorinated phenols (such as 2,4,5-TCP, PCP) or chlorinated pesticides or herbicides (such as 2,4,5-T, 2,4-D, hexachlorophene, Silvex速), and those involved in application of chlorinated pesticides containing CDDs as impurities. Workers whose jobs involve pressure treatment of wood with PCP and the handling of PCP-treated wood products, chlorination processes at pulp and paper mills, or operation of municipal solid waste or hazardous waste incinerators may have increased exposure to CDDs. Finally, workers involved in hazardous waste clean-up or clean-up of PCB transformer and/or capacitor fires including emergency service personnel like fire fighters and police who respond to such fires are also at additional risk of exposure to CDDs. Most of these occupational exposures have been significantly reduced in recent years. In general, workers involved in the manufacture of 2,4,5-TCP and subsequent products were exposed to far greater levels of 2,3,7,8TCDD than those involved in the handling and application of chlorinated pesticides containing CDDs. Current serum lipid levels of 2,3,7,8-TCDD in a small number of U.S. Air Force veterans who were directly involved in the aerial spraying of herbicides (Agent Orange contaminated with 2,3,7,8-TCDD) in Vietnam as part of Operation Ranch Hand, are up to 3 times higher than the general population. However, while studies on blood or fatty tissue 2,3,7,8-TCDD levels in U.S. Army ground combat Vietnam veterans also found some individuals with 2,3,7,8-TCDD levels higher than those of the general population, overall, most Vietnam veterans and Vietnamese living in Vietnam studied to dated have blood and fatty tissue 2,3,7,8-TCDD levels comparable to members of the general U.S. population.

PCDFs CDFs are found at very low levels in the environment of industrial countries and at even lower levels in nonindustrial countries. People are exposed to very small levels of CDFs by breathing air, drinking water, and eating food, but most human exposure comes from food containing CDFs. The levels of CDFs in air are usually higher in city and suburb areas than in rural areas. The concentration of CDFs in city and suburb areas ranges from less than one femtogram (fg) (one quadrillionth of a gram, that is 1/1000,000,000,000,000th of a gram) to a few picograms (pg) in a cubic meter (m続) of air. The levels in rural air are usually so low that measurements are not possible. The levels of CDFs in most drinking waters are also below the level that can be measured. CDFs were found in drinking water of one of the 20 water supplies in New York State at a concentration of 3.4 parts of CDF in a quadrillion part of water. CDFs are not found in soils that have not been polluted. CDFs have been detected in the stack emissions and ash from certain industries and processes that are sources of these compounds in air at levels that are thousands of times higher than the levels in the air that we usually breathe. Once emitted in the air from stacks, CDFs are dispersed by the cleaner air and the level of CDFs drops substantially. Similarly, the levels of CDFs in waste waters from certain industries and in soil at dumpsites can be thousands to millions times higher than the levels found in clean water and soil. Some products you use, such as paper towels, coffee filters, tampons, and milk cartons, can contain extremely low levels of CDFs. The intake of CDFs from these sources is very low. Since CDFs tend to concentrate in the fat, and milk contains fat, mother's milk can be a source of CDFs for babies. But considering the small amounts of CDFs in milk and the other beneficial effects of human milk to a baby and the length of time a baby uses mother's milk, scientists believe that mother's milk, on balance, is still beneficial to babies. Cow's milk and formula usually contain lower amounts of CDFs than human milk. Children playing in dumpsites may come in contact with CDFs through their skin and by eating dirt. It has been estimated that over 90% of the total daily intake of CDFs (on the order of a few pg per day) for the general adult population occurs from eating food containing them. The rest comes from air, consumer products, and drinking water. Meat and meat products, fish and fish products, and milk and milk products contribute equally to intake of CDFs from food, while intake from vegetable products contributes much less. Eating large amounts of fatty fish from water containing CDFs may increase your daily intake of CDFs from food. People in certain occupations may be exposed to higher levels of CDFs than the general population. Exposure in the workplace occurs mostly by breathing air and touching substances that contain CDFs. Workers involved with cleaning up after transformer fires, workers in the pulp and papermill industry, workers in municipal incinerators, and workers in sawmills may be exposed to higher levels of CDFs than the general population. Contact with CDFs at hazardous waste sites can happen when workers breathe air or touch soil containing CDFs.

PCBs Although PCBs are no longer produced, people can still be exposed to them. Many older transformers and capacitors may still contain PCBs, and this equipment can be used for 30 years or more. Old fluorescent lighting fixtures and old electrical devices and appliances,

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Environmental Remediation and Energy Production Technologies such as television sets and refrigerators, therefore may contain PCBs if they were made before PCB use was stopped. When these electric devices get hot during operation, small amounts of PCBs may get into the air and raise the level of PCBs in indoor air. Because devices that contain PCBs can leak with age, they could also be a source of skin exposure to PCBs. Small amounts of PCBs can be found in almost all outdoor and indoor air, soil, sediments, surface water, and animals. However, PCB levels have generally decreased since PCB production stopped in 1977. People are exposed to PCBs primarily from contaminated food and breathing contaminated air. The major dietary sources of PCBs are fish (especially sportfish that were caught in contaminated lakes or rivers), meat, and dairy products. Between 1978 and 1991, the estimated daily intake of PCBs in adults from dietary sources declined from about 1.9 nanograms (a nanogram is a billionth part of a gram) to less than 0.7 nanograms. PCB levels in sportfish are still high enough so that eating PCB-contaminated fish may be an important source of exposure for some people. Recent studies on fish indicate maximum concentrations of PCBs are a few parts of PCBs in a million parts (ppm) of fish, with higher levels found in bottom-feeders such as carp. Meat and dairy products are other important sources of PCBs in food, with PCB levels in meat and dairy products usually ranging from less than 1 part in a billion parts (ppb) of food to a few ppb. Concentrations of PCBs in subsurface soil at a Superfund site have been as high as 750 ppm. People who live near hazardous waste sites may be exposed to PCBs by consuming PCB-contaminated sportfish and game animals, by breathing PCBs in air, or by drinking PCB-contaminated well water. Adults and children may come into contact with PCBs when swimming in contaminated water and by accidentally swallowing water during swimming. However, both of these exposures are far less serious than exposures from ingesting PCB-contaminated food (particularly sportfish and wildlife) or from breathing PCB-contaminated air. Workplace exposure to PCBs can occur during repair and maintenance of PCB transformers; accidents, fires, or spills involving PCB transformers and older computers and instruments; and disposal of PCB materials. In addition to older electrical instruments and fluorescent lights that contain PCB-filled capacitors, caulking materials, elastic sealants, and heat insulation have also been known to contain PCBs. Contact with PCBs at hazardous waste sites can happen when workers breathe air and touch soil containing PCBs. Exposure in the contaminated workplace occurs mostly by breathing air containing PCBs and by touching substances that contain PCBs

The toxicity of PCDDs depends on the number and positions of the chlorine atoms. Congeners that have chlorines in the 2, 3, 7, and 8 positions have been found to be significantly toxic. In fact, 7 congeners have chlorine atoms in the relevant positions which were considered toxic by the World Health Organization toxic equivalent (WHO-TEQ) scheme [4, 24] as presented is Table 5.3. The compounds that contain chlorine atoms at the 2,3,7,8-positions of the dibenzofuran molecule are known to be especially harmful [1, 4, 24]; see their toxic equivalent factor in Table 5.3. TCDD Equivalents for PCDD, PCDFs and PCBs can be calculated as the sum of the concentrations of chlorinated dibenzodioxins and chlorinated dibenzofurans multiplied by their respective toxicity factors [4, 30], as shown in the Table 5.10 and 5.11 (that also contain data for PCBs). Table 5.10 - Toxicity Equivalence Factor of dibenzodioxins and chlorinated dibenzofurans [4, 30]

chlorinated

Isomer Group

Toxicity equivalent Factor

2,3,7,8-tetra CDD

1.0

2,3,7,8-penta CDD

0.5

2,3,7,8-hexa CDDs

0.1

2,3,7,8-hepta CDD

0.01

octa CDD

0.001

2,3,7,8 tetra CDF

0.1

1,2,3,7,8 penta CDF

0.005

2,3,4,7,8 penta CDF

0.5

2,3,7,8 hexa CDFs

0.1

2,3,7,8 hepta CDFs

0.01

octa CDF

0.001

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Table 5.11 - WHO Toxic Equivalence Factors (WHO-TEF) for the dioxin-like congeners of concern [40] Polychlorinated dioxins

WHO-TEF

2,3,7,8-TCDD

1,2,3,7,8-PeCDD

1,2,3,4,7,8-HxCDD

0.1

1,2,3,6,7,8-HxCDD

0.1

1,2,3,7,8,9-HxCDD

0.1

1,2,3,4,6,7,8-HpCDD

0.01

OCDD

0.0003

Polychlorinated dibenzofurans 2,3,7,8-TCDF

0.1

1,2,3,7,8-PeCDF

0.03

2,3,4,7,8-PeCDF

0.3

1,2,3,4,7,8-HxCDF

0.1

1,2,3,6,7,8-HxCDF

0.1

1,2,3,7,8,9-HxCDF

0.1

2,3,4,6,7,8-HxCDF

0.1

1,2,3,4,6,7,8-HpCDF

0.01

1,2,3,4,7,8,9-HpCDF

0.01

OCDF

0.0003

Non-ortho-substituted PCBs 3,3',4,4'-TCB (PCB77)

0.0001

3,4,4',5-TCB (PCB81)

0.0003

3,3',4,4',5-PeCB (PCB126)

0.1

3,3',4,4',5,5'-HxCB (PCB169)

0.03

Mono-ortho-substituted PCBs 2,3,3',4,4'-PeCB (PCB105)

0.00003

2,3,4,4',5-PeCB (PCB114)

0.00003

2,3',4,4',5-PeCB (PCB118)

0.00003

2',3,4,4',5-PeCB (PCB123)

0.00003

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2,3,3',4,4',5-HxCB (PCB156)

0.00003

2,3,3',4,4',5'-HxCB (PCB157)

0.00003

2,3',4,4',5,5'-HxCB (PCB167)

0.00003

2,3,3',4,4',5,5'-HpCB (PCB189)

0.00003

(T = tetra, Pe = penta, Hx = hexa, Hp = hepta, O = octa)

5.3.2.3. PCBs PCBs are a group of synthetic organic chemicals that can cause a number of different harmful effects. There are no known natural sources of PCBs in the environment. PCBs are either oily liquids or solids and are colorless to light yellow. There are no known natural sources of PCBs. PCBs have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment because they don't burn easily and are good insulators. The manufacture of PCBs was stopped in the U.S. in 1977 because of evidence they build up in the environment and can cause harmful health effects. Products made before 1977 that may contain PCBs include old fluorescent lighting fixtures and electrical devices containing PCB capacitors, and old microscope and hydraulic oils [3, 4, 24, 30]. Some PCBs are volatile and may exist as a vapor in air. They have no known smell or taste. PCBs enter the environment as mixtures containing a variety of individual chlorinated biphenyl components, known as congeners, as well as impurities. Because the health effects of environmental mixtures of PCBs are difficult to evaluate, most of the information in this toxicological profile is about seven types of PCB mixtures that were commercially produced. These seven kinds of PCB mixtures include 35% of all the PCBs commercially produced and 98% of PCBs sold in the United States since 1970. Some commercial PCB mixtures are known in the United States by their industrial trade name, Aroclor. For example, the name Aroclor 1254 means that the mixture contains approximately 54% chlorine by weight, as indicated by the second two digits in the name. Because they don't burn easily and are good insulating materials, PCBs were used widely as coolants and lubricants in transformers, capacitors, and other electrical equipment. The manufacture of PCBs stopped in the United States in August 1977 because there was evidence that PCBs build up in the environment and may cause harmful effects. Consumer products that may contain PCBs include old fluorescent lighting fixtures, electrical devices or appliances containing PCB capacitors made before PCB use was stopped, old microscope oil, and old hydraulic oil. 5.3.2.4. New POPs and substances under review All the above mentioned new POP compounds are persistent in the environment, suffer long range transport and are highly bioaccomulative in food chains through a similar mechanism to any of the dirty dozen POPs. They all have toxic and or mutagenic/ carcinogenic effects reported. PFOS is extremely persistent and has substantial bioaccumulations and biomagnifying properties, although it does not follow the classic pattern of other POPs by partitioning into fatty tissues, but instead binds to proteins in the blood and the liver. However as they have also the capacity to undergo long-range transport and also fulfills the toxicity criteria of the Stockholm Convention, they includes the list. Some applications like photo imaging, use for semi-conductors or aviation hydraulic fluids are considered as acceptable purposes, because for these, technically feasible alternatives to PFOS are not available to date.

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5.3.3. Emergent Contaminants These compounds enter the aquatic environment continually coming from the effluent of sewage treatment plants as they go without any type of control or treatment. Even though present in small quantities, are capable of interfering with the endocrine system, causing cellular alterations and changes in the reproductive system. This class of compounds has gained wide visibility though the scientific community has for its disposal today analytical techniques that allow the determination of these compounds in increasingly lower concentrations (ng L-1) and can already be aware of their harmfulness to human health and environmental. [31, 34-88 and references cited therein] The endocrine disruptors are compounds that can interfere / disrupt the hormone system of mammals, being responsible for the appearance of tumors and cancers, born defects and other developmental disorders ranging from learning disabilities and attention deficit to changes in sexual development related respectively with feminization / masculinization of males / females. The critical period for most individues to be affected by this type of substance is during the embryonic stage. At this stage of development, hormonal and protein balances alter quickly and they can be greatly affected if the fetus is exposed to compounds with endocrine disruptor character; this happens without the mother suffering any disturbance. [31, 32 and references therein] Emerging pollutants generally have characteristics very similar to persistent pollutants, makes that both classes have the same kind of effects on the environment, particularly with regard to its ability to act as endocrine disruptors. Both classes represent significant danger to human health and the environment may actually turn out to reduce the biodiversity of the ecosystems in which they are present, including the aquatic ecosystems [31]. If persistent pollutants have ubiquitous character, contaminating all environmental segments, emerging contaminants, since they are products commonly used by man, enter the environment mainly be via domestic sewage, providing a constant source of pollution of watercourses. Many of them are not even considered as pollutants (for some of them, present in trace amounts, only the most modern, sophisticated and expensive analytical methods are able to detect them), are not subject to any restriction or control legislation, going through it by sewage treatment plants without any control or treatment, eventually contaminating the receiving freely, continuing to increase their concentrations in the media, with the consequent risk to public and environmental health. Thus, the treatment of water in order to eliminate persistent and emerging becomes extremely important from the standpoint of environmental quality, since the conventional treatment stations have not efficient enough to handle all the new compounds present in water [31]. 5.3.3.1. PBDEs Polybrominated biphenyls (PBBs) are chemicals that were added to plastics used in a variety of consumer products, such as computer monitors, televisions, textiles, and plastic foams, to make them difficult to burn. Because PBBs were mixed into plastics rather than bound to them, they were able to leave the plastic and find their way into the environment. Commercial production of PBBs began in the 1970s. Manufacture of PBBs was discontinued in the United States in 1976. Concern regarding PBBs is mainly related to exposures resulting from an agriculture contamination episode that occurred in Michigan over a 10-month period during 1973-1974. PBDEs are flameretardant chemicals that are added to a variety of consumer products to make them difficult to burn. Because PBDEs are added rather than reacted to the product, they could leave the product under ideal conditions and enter the environment, but this rarely happens. The first commercial productions of PBDEs began in the 1970s in Germany. Production of PBDEs has continued until the present. There are three commercial PBDE products (i.e., penta-, octa-, and decabromodiphenyl

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ethers). Deca- and octa-brominated types of PBDEs are also produced outside of the United States (in China and Israel). Decabromodiphenyl ether (decaBDE) makes up 82% of these products manufactured globally. Its main use is for electronic enclosures, such as television cabinets. Octabromodiphenyl ether (octaBDE) product is used in plastics for business equipment. Pentabromodiphenyl ether (pentaBDE) product is used in foam for cushioning in upholstery. PBDEs have not been associated with actual health-related effects. Concerns have increased, however, because some of these chemicals (particularly the pentaBDEs) have been found in the environment at varying concentrations. Environmental concentrations of lower brominated PBDEs, which may be leveling off in Europe, appear to be increasing in certain areas of Canada and the United States [40]. There are no known natural sources of PBBs in the environment. PBBs are solids and are colorless to off-white. PBBs enter the environment as mixtures containing a variety of individual brominated biphenyl (for PBBs) components, known as congeners. Some commercial PBB mixtures are known in the United States under the industrial trade name, FireMaster速. However, other flame retardant chemicals also may be identified by this name. PBBs are no longer used in North America because the agriculture contamination episode that occurred in Michigan in 1973-1974 led to the cessation of its production. PBDEs are a group of synthetic organic chemicals with no known natural sources in the environment, except for a few marine organisms that produce forms of PBDEs that contain higher levels of oxygen. Commercial decaBDE and octaBDE products are colorless to off-white solids, whereas commercial pentaBDE product is a thick liquid. PBDEs are not expected to evaporate into the air. PBDEs in the air are mostly found with dust rather than as a vapor. PBDEs enter the environment as mixtures containing a variety of individual brominated diphenyl ether (for PBDEs) components, known as congeners. Congeners are distinct members of a class of chemical substances. Some commercial mixtures of PBDEs may be known by their industrial trade names, (i.e., DE-60F Special, DE-61, DE-62, DE-71, DE-79, DE 83R, Saytex速 102E). PBDEs are still produced and widely used in the United States, although the sole manufacturer of penta- and octaPBDE commercial products in the United States is expected to quit making these chemicals by the end of 2004. This family consists of 209 congeners of which the most abundant are penta, octa and decaPBDEs. PBDEs are lipophilic, have low vapor pressure and octanol / water in the range between 5.9 and 10.0. Are persistent, have low water solubility and high affinity to bind to the particulate material [55]. Their physicochemical characteristics favor the biomagnification along the food chain. In fact, bioconcentration factors of approximately 1.4 million have been found in mussels as well as biomagnification that RTEM been reported in fish [56, 57]. Due to its wide dispersion characteristics and environmental, this family of substances has been identified in mammals and even humans. The provision of electrical and electronic equipment is one of the major sources of these toxic agents to the environment. To get an idea of the magnitude of this contamination, it is estimated that every person born in Europe in 2003 will produce about 8 tons of waste electrical and electronic for your life that are currently being produced about 9 million tonnes of waste. An estimate made in the United States shows that there is a potential of 747 million items electronics representing about 13 million metric tons [58]. Currently these substances are found in all terrestrial environments as a result of migration resulting from the disposal of products containing them and their leaching and dispersion. Because of their toxicological effects, the production and use of formulations containing PBDEs has been banned in Europe and the penta and octa formulations containing, in the United States. However even with these bans these substances continue to contaminate the environment through its release of products in use and those willing in landfills [59].

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5.4. Monitoring persistent environmental pollutants 5.4.1. in homogeneous media PAHs possess very characteristic UV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors. Detection of PAHs is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods. Organochlorinated compounds are usually determined by gas chromatography in its different variations. Analysis of the environmental samples containing POPs frequently need extraction, concentration and clean up procedures to put the elements to analyze conveniently in the liquid state. This makes frequently the methods difficult and time consuming to develop and operate as can easily e found in literature. EPA site [33] is a good source of methods for this type of substances. 5.4.2. in heterogeneous media - Surface Photochemistry Techniques: Diffuse reflectance geometry for the study of reactions on surfaces An easier way to analyze environmental samples would be not to need to have samples in liquid or gas and do not have to extract and concentrate them from solids. For that spectroscopic techniques that can operate for the analysis of non-transparent media are a good option as we will show in this chapter [32, 60-62 and references quoted there]. The growing interest on the photophysical and photochemical properties of heterogeneous systems, most of them opaque highly scattering solids, determined the strong interest that on last two decades was devoted to the implementation of new experimental techniques for their study. Conventional techniques for absorption and emission studies on transparent medium, either in stationary or transient modes, always operate on the validity limit of Beer Law. When an incident radiation beam is transmitted through an absorbing transparent sample the beam decreases exponentially with the sample optical path whilst it is partially absorbed by the absorbing species present therein. The transmitted radiation, emerge from the sample with 180Âş angle with incident radiation. The difference of intensity between the two beams, (incident and transmitted) is the amount of radiation absorbed by the sample. After sample irradiation, if the absorbing species are also emissive them luminescence is emitted in all directions. When the initial excitation pulse comes from a source with high intensity and short duration, higher excited states are significatively populated and their transient absorptions can be measured following the variations on a monitoring light that crosses the sample. This is the basis of laser â&#x20AC;&#x153;flash-photolysis", a powerful technique developed in the 50â&#x20AC;&#x2122;s by G. Porter (Chemistry Nobel Prize, 1967 [63]). When light radiation reaches the surface of a powdered solid sample, the incident radiation it is strongly attenuated due to simultaneous strong absorption and strong dispersion, as a consequence of the highly scattering nature of the sample. All the incident radiation is absorbed or dispersed before to completely cross the sample, there is no radiation transmitted through the sample and only scattered radiation is observed. During the pathway of incident radiation through the sample, part of the incident radiation is absorbed by the absorbing species present in there, reason why once dispersed radiation emerges from the sample, it can be used to analyze and

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quantify the absorbing species present on the sample in the same way that transmitted light is used for the analysis of transparent media. Like that, it is possible to measure the ground state absorption of highly scattering solid samples, by measuring the reflectance of the sample as a function of the wavelength [60-62, 64]. The reflectance of the sample is measured with the help of an integrating sphere internally coated with a high diffusing material (R ≅ 1), that collects efficiently all the scattered radiation that comes out in all direction from the sample surface. (Figure 5.4a)). Because ideal scatters and ideal absorbers does not exist it is necessary to calibrate the light measuring system. The calibration of the integrating sphere is usually performed with the best white and black standards available which, from 200 to 1000 nm, typically display reflectance values of about 97 - 98 % and 1 - 2 %, (Figure 5.4b)), respectively. Using reflection geometry (sample excitation and signal collection at the same sample side) and an intense excitation source, which is simultaneously short and monochromatic (typically a pulsed laser) it is also possible to observe luminescence originating from opaque solid samples. On our laboratory in Lisbon, the laser induced luminescence set-up uses an intensified charged coupled device with a minimum time gate of 2.2 ns, coupled to a time-delay unit, that enable to obtain immediately after a single laser shot a luminescence spectra (fluorescence or phosphorescence depending on the luminescence lifetime of the species being analyzed). On the beginning of the 80’s, F. Wilkinson and co-workers showed that laser flash-photolysis principles for the analysis of transparent media could be extended to the analysis of solid highly scattering opaque media, provided that the changes on the monitoring light level due to the absorption of transients generated following laser pulse excitation are measured in diffuse reflectance mode [65 – 66]. So, the geometry of the excitation beam, that of the analysing light and of detection system, relatively to the opaque sample has to be built in a way that any scattered light originated from laser or analysing light could reach the analysing monochromateur and that the maximum of scattered light originated from the sample reach the entrance slit of the monochromateur of analysis. As is evidenced at Figure 5.5, the reflection geometry (very much different from the one used on the conventional transmission techniques) is the essential characteristic of both laser induced luminescence and laser flash photolysis in diffuse reflectance mode. In fact, it is possible to transform a laser induced luminescence system on a laser flash photolysis system, only by adding to the laser excitation-detection system on reflectance geometry an appropriated monitoring light, arranged also on reflection geometry. from a white standard, a black standard and for a microcrystalline cellulose sample.[60 – 62]. The use of diffuse reflectance and laser induced luminescence enabled already the study of numerous probes deposited or adsorbed on different opaque subtract, namely microcrystalline cellulose [67 – 73], cyclodextrins [74 – 76], calix[n]arenes [77-80], silicas [81], silicalite [74 – 76] and other zeolite like materials [82 – 84]. Recently we tested the use of these techniques to the study of chemical contaminant environmentally relevant on different solid supports [75-80, 85 – 89] namely PAHs adsorbed on model and real solid samples [85 - 88]. Laboratorial methods currently used on the determination of chemical environmental pollutants are expensive and time consuming, because on environment pollutants are usually present as complex mixtures or included on complex solid matrixes [23, 90]. The more usual methods of analysis use particulate matter extraction with organic solvents, column or thin layer chromatography to eliminate interfering substances followed by detection after chromatographic separation (e.g. gas chromatography coupled to mass spectrometry (GC/MS) or high performance liquid chromatography (HPLC)) [23, 90]. Therefore it is important to find alternative simpler methods for detection and quantification of those substances, namely real samples direct analysis methods (preferentially field methods) that do not require the extraction of the substance to be analyzed from their natural matrix and that do not suffer with their interference [85].

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b) Figure 5.4 â&#x20AC;&#x201C; Ground state diffuse reflectance. a) Radiation optical path within the integration sphere of a diffuse reflectance spectrophotometer; b) Ground state diffuse reflectance spectra [1, 62]

Room temperature luminescence is strongly dependent on probe-matrix interactions and can be strongly favoured by the rigid environment frequently enabled by deposition or inclusion on a solid support of the probe to be study [1, 67-89]. This fact can be successfully used on the determination of extremely low levels of luminescent probes, as PAHs, adsorbed on surfaces [1, 85-88]. When diffuse reflectance and luminescence from solids are combined with the use of powerful lasers as excitation source and intensified charged coupled devices cameras (ICCDs) as detectors, they constitute extremely powerful and sensitive methods of analysis, and they can be use as methods for solid substrates analysis with excellent detection limits [1, 85].

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b) Figure 5.5. â&#x20AC;&#x201C; Reflection geometry for the analysis of opaque powdered samples by a) Laser induced luminescence and b) Diffuse reflectance laser flash-photolysis [1, 62]

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6. Advanced Oxidation Processes for Water and Wastewater Treatment JoaquĂn R. DomĂnguez, Eduardo M. Cuerda-Correa and Anabela Oliveira

6.1. Introduction The last 30 years have witnessed a growing awareness of the fragile state of the planetsâ&#x20AC;&#x2122; drinking water resources. In order to cope with the growing pollution of our planet, two strategies of water treatment begin to be applied: (a) chemical treatment of polluted drinking water and surfacewater or groundwater; (2) chemical treatment of wastewaters containing toxic and/or nonbiodegradable components. Pollutant removal in drinking water may only involve techniques adopted in governmental regulations, such as coagulation, flocculation, sedimentation, filtration, chlorination, to which have been added chemical treatment techniques involving a limited number of chemicals, mostly stable precursors for hydroxyl radical production. Chemical treatment of contaminated wastewaters containing toxic/non-biodegradable components is part of a long-term strategy to improve the quality of our drinking water resources by eliminating toxic materials before releasing the used waters into the natural cycles. Used waters of normal anthropogenic origin can be efficiently treated in conventional biological wastewater treatment plants (WWTPs). These stations are elementary for the safe guard of the sanitary quality of a more and more urbanized environment. Therefore, it has been frequently observed [1,2] non-biodegradable pollutants characterised by high chemical stability and/or by strong difficulty to be completely mineralized. In these cases, it is necessary to adopt reactive systems much more effective than those adopted in conventional purification processes. Chemical oxidation aims at the mineralization of the contaminants to carbon dioxide, water and inorganics or, at least, at their transformation into harmless products. Recent developments in the domain of chemical water treatment have led to an improvement in oxidative degradation procedures for organic compounds in aqueous solution, applying catalytic and photochemical methods. They are generally named as advanced oxidation processes (AOPs) which usually operate at or near ambient temperature and pressure [3-5]. A rather fast evolution of the research devoted to environment remediation using AOPs has been recorded as the consequence of the special attention paid to the environment by legislative international authorities leading in some cases to the delivery of very severe regulations [6-8].

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6.2. AOPs Glaze et al. [4] defined AOPs as “near ambient temperature and pressure water treatment

processes which involve the generation of hydroxyl radicals in sufficient quantity to effect water purification”. The hydroxyl radical (·OH) is a powerful, non-selective chemical oxidant (see Table

1), which acts very rapidly with most organic compounds. ·OH radicals are extraordinarily reactive species, they attack the most part of organic molecules with rate constants usually in the order of 106–109 M−1 s−1 [7,9,10] (see Table 6.2). They are also characterised by a little selectivity of attack which is a useful attribute for an oxidant used in wastewater treatment and for solving pollution problems. The versatility of AOPs is also enhanced by the fact that they offer different possible ways for ·OH radicals production thus allowing a better compliance with the specific treatment requirements. Depending upon the nature of the organic species, two types of attack are possible: the hydroxyl radical can abstract a hydrogen atom from water, alkanes or alcohols (Eqs. (1-5) and (6)), or it can add itself to the contaminant (in the case of olefins and aromatics) [7]. Table 6.1. Relative oxidation power of some oxidizing species [7].

Oxidizing species

Oxidation power

Chlorine

1.00

Permanganate

1.24

Hydrogen peroxide

1.31

Ozone

1.52

Atomic oxygen

1.78

Hydroxyl radical

2.05

Hole on TiO2+

2.35

Table 6.2. Reaction rate constants (k, M–1 s–1) of ozone vs. hydroxyl radical [7].

Compound

·OH

Chlorinated alkenes

103-104

109-1011

Phenols

103

109-1010

N-containing organics

10-102

108-1010

Alcohols

10-2-1

108-109

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In the case of methanol, the reaction proceeds as follows:

(6) OH radical, in the presence of oxygen, initiates a complex cascade of oxidative reactions leading to mineralization of the organic compound. The exact routes of these reactions are still not quite clear. For example, chlorinated organic compounds are oxidized first to intermediates, such as aldehydes and carboxylic acids, and finally to CO2 and H2O. The rate of destruction of a contaminant is approximately proportional to the rate constant for the contaminant with 路OH radical. From Table 2 we can see that chlorinated alkenes are treated most efficiently because the double bond is very susceptible to hydroxyl attack. Alkanes react at a much slower rate and, therefore, are more difficult to oxidize. AOPs can often achieve oxidative destruction of compounds refractory to conventional ozonation. AOPs are suited for destroying dissolved organic contaminants such as halogenated hydrocarbons (trichloroethane, trichloroethylene), aromatic compounds (benzene, toluene, ethylbenzene, xylene), pentachlorophenol, nitrophenols, detergents, pesticides, etc [6,7]. A suitable application of AOPs to waste water treatments must consider that they make use of expensive reactants as H2O2, and/or O3 and therefore it is obvious that their application should not replace, whenever possible, the more economic treatments as the biological degradation. The potentialities offered by AOPs can be exploited integrating biological treatments by an oxidative degradation of toxic or bio-refractory substances entering or leaving the biological stage. A list of the different possibilities offered by AOP is given in the Figure 6.1.

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Figure 6.1. Advanced oxidation processes.

Other important aspect concerning the opportunity of AOP application is that referring to the polluting load of wastes. Only wastes with relatively small COD contents (≤5.000 mg O2/l) can be suitably treated by means of these techniques since higher COD contents would require the consumption of too large amounts of expensive reactants. Wastes with more massive pollutants contents can be more conveniently treated by means of wet oxidation or incineration [11] (see Figure 6.2). Wet oxidation makes use of oxygen or air to achieve pollutant oxidation at high temperatures (130–300 ºC) and pressure (0.5–20 Mpa). Since oxidation is an exothermic process, simple thermal balance shows that wastes with COD contents higher than approximately 20 g/l undergo autothermic wet oxidation, whereas fuel consumption should be taken into account to achieve combustion temperatures for leaner wastewaters. In general, AOPs when applied in a right place, give a good opportunity to reduce the contaminants’ concentration from several hundred ppm to less than 5 ppb. That is why they are called the “water treatment processes of the 21st Century”. The practical applications of AOPs have been made largely by equipment manufacturers, who have not carried out systematic studies of AOPs with the view of understanding their advantages and disadvantages.

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Figure 6.2. Suitability water treatment technologies according to COD contents.

6.2.1. Non-photochemical methods There are five well-known methods for generating hydroxyl radicals without using light energy. Two of the methods involve the reaction of ozone while one uses iron ions as the catalyst. These methods are ozonation at elevated values of pH (>8), combining ozone with hydrogen peroxide, ozone with catalyst, and the Fenton or Fenton like system. 6.2.1.1. Fenton system (H2O2/Fe2+) The oxidation with Fenton’s reagent is based on the hydrogen peroxide decomposition in acidic medium, catalyzed by ferrous ion. This process involves several parallel and consecutive reactions. The Fenton process was reported by Fenton [16] already over a hundred years ago for maleic acid oxidation. Production of OH radicals by Fenton reagent [17] occurs by means of addition of H2O2 to Fe2+ salts: Fe2+ + H2O2 → Fe3+ + OH− + OH•

(7)

Fe3+ + H2O2 → H+ + FeOOH2+

(8)

FeOOH2+ → HO2 • + Fe2+

(9)

The rate constant for the reaction of ferrous ion with hydrogen peroxide is high and Fe (II) oxidizes to Fe(III) in a few seconds to minutes in the presence of excess amounts of hydrogen peroxide. Hydrogen peroxide decomposes catalytically by Fe (III) and generates again Fe2+ and hydroxi-peroxide radicals according to the reactions (7-9). The use of Fe(II)/H2O2 as an oxidant for wastewater treatment is attractive due to the facts that: (1) iron is a highly abundant and non-toxic element, and (2) hydrogen peroxide is easy to handle and environmentally benign. Thus, the Fenton process is very effective for · O H

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Environmental Remediation and Energy Production Technologies 2+

radicals generation; however, it involves consumption of one molecule of Fe radical produced, demanding a high concentration of Fe(II).

for each · OH

6.2.1.2. Fenton like system (H2O2/Fe3+) Hydrogen peroxide decomposes catalytically by Fe(III) and generates again hydroxyl radicals according to the reactions (8-9) regenerating also ferrous ions to activate reaction (7). The reaction of hydrogen peroxide with ferric ions is referred to as “Fenton-like reaction”. For this reason, it is believed that most waste destruction catalyzed by Fenton’s reagent is simply a Fe(III)/H2O2 system, and Fenton’s reagent with an excess of hydrogen peroxide is essentially a Fe(III)/H2O2 process (known as a Fenton-like reagent). Thus, the ferrous ion in Fenton’s reagent can be replaced with the ferric ion [18]. 6.2.1.3. Non-photoassisted ozone systems: O3/high pH As the pH rises, the decomposition rate of ozone in water increases. For example, at pH 9,5, the half-life of ozone in water can be less than 1 min. Oxidation of organic species may occur due to a combination of reactions with molecular ozone and reactions with ·OH radicals [7]. Hoigné [19] showed that the ozone decomposition in aqueous solution develops through the formation of ·OH radicals. In the reaction mechanism OH− ion has the role of initiator: The reaction between hydroxide ions and ozone leads to the formation of superoxide anion radical O2·and hydroperoxyl radical HO2·. Reaction between ozone and the superoxide anion radical results the ozonide anion radical O3-·, which decomposes immediately (Eqs. (10,11)) giving ·OH radical. O3·- + H+ →HO3·

(10)

HO3·

(11)

→ ·OH + O2

Summarizing, three ozone molecules produce two ·OH radicals [5]: 3O3 + OH– + H+ → 2·OH + 4O2

(12)

The rate of the attack by ·OH radicals is typically 106 to 109 times faster than the corresponding reaction rate for molecular ozone. The major operating cost for the ozone oxidation process is the cost of electricity for ozone generation. The energy requirement for ozone synthesis using air as a feed gas ranges 22-33 kWh/kg O3, including air handling and ozone contacting with water [20]. 6.2.1.4. Non-photoassisted ozone systems: O3/H2O2 Addition of hydrogen peroxide to ozone can initiate the decomposition cycle of ozone, resulting in the formation of ·OH radicals (Eqs. 13,14). The ozonide anion radical O3- decomposes immediately giving ·OH radical (Eqs. 10,11).

H2O2 → HO2– + H+

[ 181 ]

(13)

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HO2– + O3 → HO2· + O3·-

(14)

The combination of different reaction steps shows that two ozone molecules produce two ·OH radicals: 2O3 + H2O2 → 2·OH + 3O2 (15) Paillard et al. [21] showed better degradation with ozone–hydrogen peroxide combination as compared to ozone alone. The optimum H2O2/O3 mass ratio was 0.4. The implementation of a radical system makes oxidation of refractory molecules possible: it allows getting full advantage of selective molecular ozone reactions before converting the process to non-selective free radical attack. On the other hand, hydrogen peroxide is a relatively inexpensive, readily available chemical oxidant. The electrolytic process consumes approximately 7.7 kWh per 1 kg of H2O2 produced [20]. 6.2.1.5. Non-photoassisted ozone systems: O3/Catalyst Another method to accelerate ozonation reactions is to use heterogeneous (as Fe2O3, Al2O3, TiO2, Me, MnO2, Ru/CeO2, TiO2–Me) or homogeneous catalysts (as Fe2+, Fe3+, Mn2+). Mn2+ catalysed ozonation of oxalic acid has been shown to develop according to a radical mechanism at pH >4.0 at which Mn(III)-dioxalate Mn(III)-trioxalate are formed. In these conditions, the oxidation process proceeds presumably through the formation of ·OH radicals as a result of a reaction between manganese complexes and ozone [22]. The system has been demonstrated to be effective for the abatement of refractory pollutants such as pyrazine and pyridine [23]. Mn(III)/(AO2−)n + O3 + H+ → Mn(II) + (n-1)(AO2-) + 2CO2 + O2 + ·OH (16) Legube et al. [24] studied the catalytic ozonation process using Al2O3, TiO2 in its anatase form, and clay as the support for metal catalysts. In contrast to unassisted ozonation, TOC measurements showed complete removal of organics in catalytic ozonation. Paillard et al. [25] compared the efficiency of catalytic ozonation O3/TiO2 with plain ozonation and a combination of O3/H2O2. Oxalic acid was chosen as a model compound. As a result the O3/TiO2 system was preferable in terms of process efficiency. Ozone–Granulated Activated Carbon systems (O3/GAC) make a special case of catalytic ozonation. Quite well-known is the combined system O3/GAC for biorefractory compounds (for example, pesticides) destruction where the GACs bed life is prolonged due to the ozonated water [26]. Jans and Hoigne declare [27] that a few milligrams of activated carbon or carbon black per litre in ozone-containing water initiate a radical-type chain reaction that then proceeds in the aqueous phase and forms ·OH radicals.

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6.2.2. Photochemical methods Conventional ozone or hydrogen peroxide oxidation of organic compounds does not completely oxidize organics to CO2 and H2O [20]. In some reactions, the intermediate oxidation products remaining in the solution may be as toxic as or even more toxic than the initial compound [7]. Completion of oxidation reactions, as well as oxidative destruction of compounds can be achieved by supplementing there action with UV radiation. UV lamps must have a maximum radiation output at 254 nm for an efficient ozone photolysis. Many organic contaminants absorb UV energy in the range of 200–300 nm and decompose due to direct photolysis or become excited and more reactive with chemical oxidants. However, commercially available high-power UV lamps have an energy efficiency of only 15-30% [7]. Photochemical methods electronic excitation of the organic substrate imply in most cases an electron transfer from the excited-state (C*, Eq. (17)) to ground-state molecular oxygen (Eq. (18)), with subsequent recombination of the radical ions or hydrolysis of the radical cation, or homolysis (Eq. (19)) to form radicals which then react with oxygen (Eq. (20)) [8]. C + hʋ →C*

(17)

C* + O2 →C·+ + O2·- (18) R-X + hʋ →R· + X·

(19)

R· + O2 →RO2

(20)

6.2.2.1. Photochemical methods. H2O2/UV system The use of hydrogen peroxide as an oxidant brings a number of advantages in comparison to other methods of chemical water treatment: commercial availability of the oxidant, thermal stability and storage on-site, infinite solubility in water, no mass transfer problems associated, two hydroxyl radicals are formed for each molecule of H2O2 photolyzed, peroxyl radicals are generated after ·OH attack on most organic substrates, minimal capital investment, very cost-effective source of hydroxyl radicals, and simple operation procedure [6-8]. There are, however, also obstacles encountered with the H2O2/UV process. In fact, the rate of chemical oxidation of the contaminant is limited by the rate of formation of hydroxyl radicals, and the rather small absorption cross section of H2O2 at 254 nm is a real disadvantage, in particular, in the cases where organic substrates will act as inner filters. Higher rates of ·OH radical formation may, nevertheless, be realized by the use of Xe-doped Hg arcs exhibiting a strong emission in the spectral region of 210-240 nm, where H2O2 has a higher molar absorption coefficient. The mechanism most commonly accepted for the photolysis of H2O2 is the cleavage of the molecule into hydroxyl radicals with a quantum yield of two ·OH radicals formed per quantum of radiation absorbed:

H2O2 + hʋ → 2 ·OH

(21)

The rate of photolysis of aqueous H2O2 has been found to be pH dependent and increases when more alkaline conditions are used. This might be primarily due to the higher molar absorption

[ 183 ]

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coefficient of the peroxide anion at 254 nm. Hydrogen peroxide is known to decompose by a dismutation reaction (Eq. (22)). H2O2 → HO2- + H+

(22)

HO2– + hʋ → ·OH + O·– (23) Reactions of hydroxyl radicals generated in presence of an organic substrate may be differentiated by their mechanisms into three different classes [8]: -

Hydrogen abstraction: ·OH + RH → R· + H2O

(24) Electrophilic addition

·OH + PhX → HOPhX· -

(25) Electron transfer

·OH + RX → RX·+ + HO- (26) 6.2.2.2. Photochemical methods. O3/UV system The O3/UV process seems at present to be the most frequently applied AOP for a wide range of compounds. This is mainly due to the fact that ozonization is a well-known procedure in water technology and that ozonizers are therefore in most cases readily available in drinking water treatment stations. From the photochemistry point of view, the absorption spectrum of ozone provides a much higher absorption cross section at 254 nm than H2O2, and inner filter effects by e.g. aromatics are less problematic [8]. There remain, however, many questions related to mechanisms of free radicals production and subsequent oxidation of organic substrates. In fact, the literature contains many conflicting reports on the efficiency of this oxidation method which may be linked to mechanistic problems as well as to the difficult asks of dissolving and photolyzing ozone with high efficiency. Ozone readily absorbs UV radiation at 254 nm wavelength (the extinction coefficient ε254= 3300 M–1 cm–1) producing H2O2 as an intermediate, which then decomposes to ·OH [6,7]: O3 + hʋ → O2 + O(1D)

(27)

O (1D) + H2O → H2O2 + hʋ → 2 ·OH

(28)

Common low pressure mercury lamps generate over 80% of their UV energy at this wavelength. Photolysis of ozone therefore appears only to be an expensive way to make hydrogen peroxide that is subsequently photolyzed to ·OH radicals. Although photochemical cleavage of H2O2 is conceptionally the simplest method for the production of hydroxyl radicals, the exceptionally low molecular absorptivity of H2O2 at 254 nm (ε254 = 18.6 M–1 cm–1) limits the ·OH yield in the solution. Table 6.3 shows that photolysis of ozone yields more radicals than the UV/H2O2 process.

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Table 6.3. Formation of ·OH from photolysis of ozone and H2O2 [20]

Oxidant

ε254nm (M-1 cm-1)

·OH production per oxidant molecule

·OH production per incident photon

H2O2

0.09

3300

2/3

2.00

The O3/UV process is an advanced water treatment method for the effective oxidation and destruction of toxic and refractory organics, bacteria, and viruses in water. The efficiency of the process has been proven on a pilot and technical scale with the destruction of toxic or refractory organic pollutants from the ppm or ppb range to acceptable or non-detectable limits without generation of hazardous waste. Like other ·OH radical generating degradation processes, O3/UV oxidizes a wide range of organic compounds. Using the O3/UV system complete mineralization of organic compounds with a short molecular chain (glyoxal, glyoxylic acid, oxalic acid, formic acid) can be achieved according to Gurol and Vatistas [25] and Takahashi [26]. Peyton et al. [27] demonstrated the efficiency of O3/UV system for C2Cl4 elimination from water compared to ozonation and photolysis only. 6.2.2.3. Photochemical methods. O3/H2O2/UV system The addition of H2O2 to the O3/UV process accelerates the decomposition of ozone, which results in an increased rate of ·OH generation [7,20]. Reaction pathways leading to the generation of ·OH radicals are summarized in Eqs. (29-32). H2O2 + H2O → H3O+ + HO2-

(29)

O3 + H2O2 → O2 + HO· + HO2· (slow)

(30)

O3 + HO2- → HO· + O3·-+ O2

(31)

O3·- + H2O → HO· + HO- + O2

(32)

Again ·OH radicals are considered to be the most important intermediate, initiating oxidative degradation of organic compounds. Corresponding rate constants are usually in the order of 1081010 M-1. Compared to the rates of oxidative degradation observed in reactions of ozone with organic pollutants, addition of hydrogen peroxide results in a net enhancement due to the dominant production of HO· radicals. This process is further enhanced by the photochemical generation of HO· radicals [7]. In processes involving pollutants that are weak absorbers of UV radiation, it is more cost effective to add hydrogen peroxide externally at a reduced UV flux. If direct photolysis of pollutants is not a major factor, O3/H2O2 should be considered as an alternative to photooxidation processes. The capital and operating costs for the UV/O3 and UV/H2O2 systems vary widely depending on the wastewater flow rate, types and concentrations of contaminants present, and the degree of removal required. Table 4 presents a comparison of the operating costs of various AOPs [7].

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Table 6.4. Comparative operating costs of some AOPs [20]

System

Oxidant costs

UV costs

O3/UV

High

Medium

O3/H2O2

High

No cost

H2O2/UV

Medium

High

Photocatalytic oxidation

Very low

Medium/High

6.2.2.4. Photochemical methods. TiO2/UV system The degradation of organic pollutants present in wastewaters using irradiated dispersions of titanium dioxide is a fast growing field in basic and applied research. The development of this process in order to achieve complete mineralization of organic pollutants has been widely tested for a large variety of chemicals. The basis of photocatalysis is the photo-excitation of a semiconductor that is solid as a result of the absorption of electromagnetic radiation, often, but not exclusively, in the near UV spectrum [7]. Photoexcitation with light of an energy greater than the TiO2 band gap promotes an electron from the valence band to the conduction band, and leaves an electronic vacancy or hole (h+) in the valence band. Thus the act of photoexcitation generates an electron-hole pair (see Figure 3) [28]: TiO2 + hʋ → TiO2 (h+ +e−)

(33)

In order to achieve chemically productive photocatalysis, electron-hole pair recombination must be suppressed. This can be achieved by “trapping” these species with the surface adsorbates. The photo-excited electrons are trapped by molecular oxygen: e− +O2(ads.) → O2−(ads.)

(34)

The principal hole traps are adsorbed water molecules and OH− ions [28] producing ·OH radicals h+ +H2O(ads.) → ·OH(ads.)+H+ (35) h+ +OH− → ·OH(ads.)

[ 186 ]

(36)

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Figure 6.3. Mechanism of electron-hole pair formation in a TiO2 particle in the presence of pollutant in water [29].

Heterogeneous photoprocess has been found to be pH dependent [8] the properties of the solid liquid interface being modified as the pH of the solution is varied. Consequently, the efficiency of the adsorption-desorption processes and, hence, the separation of the electron-hole pairs are also significantly affected. Numerous observations can indeed be explained by the intermediacy of ·OH. However, due to the short life-time and high reactivity of this radical no experimental evidence for the formation of hydroxyl radicals has been given so far. Authors [30–32] found that there is no need to bubble the air through the reaction mixture as the performance does not depend on aeration. The absorption of oxygen by the surface of solution is sufficient for photocatalytic oxidation. This means that the absorption of oxygen by the liquid phase is not the stage limiting the process rate. On the other hand, titanium dioxide, both in the forms of anatase and rutile, is one of the most widely used metal oxides in industry. Its high refractive index in the visible range permits preparation of thin films, and thus its use as a pigment material, and its use as a catalyst support or as a catalyst and photocatalyst itself is well known [7]. Titanium dioxide acts not only as a catalyst support, but also interacts with the supported phase as a promoter [33]. Titanium dioxide (anatase) has an energy bandgap of 3.2 eV and can be activated by UV illumination with a wavelength up to 387.5 nm. At the ground level, solar irradiation starts at a wavelength of about 300 nm. Therefore only 4–5% of the solar energy reaching the surface of the earth could in principle be utilized as direct and diffused components when TiO2 is used as a photocatalyst [34]. The TiO2/UV process is known to have many important advantages, in particular: a large number of organic compounds dissolved or dispersed in water can be completely mineralized; the rate of reaction is relatively high if large surface areas of the photocatalyst can be used; TiO2 is available at a relative modest price and would be recycled on a technical scale; UV lamps emitting in the spectral region required to initiate the photocatalytic oxidation are well known and are produced in various sizes; improvements to increase the absorption cross section and to widen the spectral domain of absorption are sought by surface modifications and transition metal ion doping [8]. The quantum yield of the TiO2/UV process is relatively low (ɸ≤ 0.05) this method of water treatment

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has, however, the advantage of being operational in the UV-A domain with a potential use of solar radiation. The development of a practical treatment system based on heterogeneous photocatalysis has not yet been successfully achieved, because there are many operating parameters which must be considered, in particular, type and geometry of photoreactor, the photocatalyst, the optimal use of energy, and wavelengths of the radiation [8]. The central issue in the design of commercial units is the irradiation field in a scattering and absorbing heterogeneous medium. In addition, rates of reaction seem rather slow, but combinations of catalysts (e.g. TiO2/Pt) show some potential, as the overall rate of oxidative degradation seems to be governed by the electron-transfer reaction from the conduction band of the semiconductor. 6.2.2.5. Photochemical methods: Photo-Fenton systems The rate of degradation of organic pollutants with Fenton–Fenton like reagents is strongly accelerated by irradiation with UV-VIS light [35]. This is an extension of Fenton process which takes advantage from UV-VIS light irradiation at wavelength values higher than 300 nm. In these conditions, the photolysis of Fe3+ complexes allows Fe2+ regeneration: Fe(OH)2+ + hʋ → Fe2+ + ·OH

(37)

and the occurrence of Fenton reactions due to the presence of H2O2 (Eqn. (7)). Despite of the great deal of work devoted by researchers to these processes scanty indications have been found about their industrial applications. This is not surprising since Fenton processes application requires strict pH control and sludges can be formed with related disposal problems [6]. When Fe3+ ions are added to the H2O2/UV process, the process is commonly named photo-Fenton-type. It is apparent that the photo-Fenton-type reaction relies heavily on the UV irradiation to initiate the generation of ·OH. If desired, organic pollutants can be mineralized completely with UV/VIS irradiation. For example, Sun and Pignatello [33] showed that a number of herbicides and pesticides can be totally mineralized by the UV-VIS/Fe(III)/H2O2 process. The increased efficiency of Fenton/Fenton-like reagents with UV/VIS irradiation is attributed to [7]: – Photo-reduction of ferric ion: irradiation of ferric ion produces ferrous ion according to reaction (37). The ferrous ion produced reacts with hydrogen peroxide generating a second hydroxyl radical and ferric ion, and the cycle continues; – Efficient use of light quanta: the absorption spectrum of hydrogen peroxide does not extend beyond 300 nm and has a low extinction coefficient beyond 250 nm. On the other hand, the absorption spectrum of hydroxyl ferric ions extends to the near-UV/visible region and has a relatively large extinction coefficient, thus enabling photo-oxidation and mineralization even by VIS-light [7]. As a photo-active catalyst ferrioxalate can be used [7,37]. Irradiation of ferrioxalate in an acidic solution generates Fe(II) and carbon dioxide: [Fe(C2O4)3]3– + hʋ → [Fe(C2O4)2]2– + C2O4·– C2O4·

2–

+ [Fe(C2O4)3] → [Fe(C2O4)2] + C2O4 3–

2–

[ 188 ]

2–

(38) + 2CO2 (39)

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C2O4·– + O2→ O2·- + 2CO2

(40)

The quantum yield of Fe(II) formation is about 1.0–1.2, independent of irradiation wavelength in the range of 250–450 nm (UV/VIS) and decreases with further increase in the irradiation wavelength. The photolysis of ferrioxalate produces ferrous (free or complexed with oxalate), which in combination with hydrogen peroxide provides a continuous source of Fenton’s reagent and hydroxyl radicals [7,38]. 6.2.3. AOPs Comparison Beltran de Heredia et al. [39] have made a comparative study of 12 methods of chemical oxidation applied to degrading p-hydroxybenzoic acid in aqueous solution. In order to study further the improvements provided by the different AOPs, the different oxidation systems are divided into three groups, corresponding to the three basic oxidation processes from which they derive: UV irradiation, ozonation, and Fenton's reagent. The overall reaction process is taken to consist of three contributions: direct oxidation by UV irradiation, (photolysis), direct oxidation by ozone (ozonation), and oxidation by free radicals (mainly ·OH). It will also be assumed that the first two reactions (UV irradiation and ozonation) will take place whenever they form part of the combined oxidation system. This assumption is based on two facts: first, that in combined oxidation systems (binaries, ternaries, etc.) in which UV radiation is involved, only a minimal part of the emitted radiation is consumed in generating free radicals, most being absorbed by the compound; second, in combined systems involving ozone, the amount of ozone decomposed in generating free radicals is negligible with respect to the total amount of ozone. In accord with this reaction scheme, the overall process rate can be written as the sum of three processes: direct photolysis (rP), direct reaction with molecular ozone (rO3), and free radical reactions (rR) [39,40]. 6.2.3.1. Oxidation processes deriving from the application of UV radiation Taking as reference the level of degradation attained at 5 and 10 min of reaction time, the different oxidation processes were ranked in terms of efficacy as follows [39]: UV < UV/TiO2 < UV/O3 < UV/H2O2< UV/H2O2/O3 Figure 6.4 shows the p-hydroxybenzoic acid removals attained at 10 min for this group of experiments. Table 5 lists the values of the pseudo-first-order kinetic constants (kT) obtained for each system. This constant was also split up into its three components: photolysis, kP, ozonation, kO3, and free radical, kR.

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Removal, X10 min (%) 95 82

70 49 34

UV/TiO2

UV/O3

UV/H2O2

UV/O3/H2O2

Figure 6.4. p-Hydroxybenzoic removal at 10 min of reaction time. Oxidation processes deriving from the application of UV radiation [39].

It can be seen that there is a synergic effect when the different oxidation systems are combined, due specifically to the increase in the free radical component. Table 6.5. Split-up of kinetic rate constants for processes deriving from the application of UV radiation [39].

Oxidation process

kT (min-1)

kP (min-1)

0.032

UV/TiO2

0.063

0.032

UV/O3

0.151

0.032

UV/H2O2

0.176

0.032

UV/H2O2/O3

0.358

0.032

kO3 (min-1)

kR (min-1)

0.031 0.100

0.019 0.144

0.100

0.226

One observes in Table 6.5 that a binary UV/O3 process is not the sum of two individual processes O3 and UV. This is because of the generation, albeit at a very low concentration, of free radicals. This synergic effect can also be observed in the ternary process UV/H2O2/O3, which is not equal to the sum of the binary UV/H2O2 process plus the simple process O3. Neither is it equal to the sum of the three individual processes, so that there is a clear synergy involved. This synergy is the reason behind the recent trend towards using combinations of different oxidants [39]. 6.2.3.2. Oxidation processes deriving from the application of ozone The order of efficacy in the oxidation systems deriving from O3 was as follows: O3 < O3/Fe2+ = O3/H2O2 < O3/UV < O3/H2O2/Fe2+ < O3/UV/H2O2 < O3/UV/H2O2/Fe2+

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Removal, X10 min (%) 95

100

82 62

Figure 6.5. p-Hydroxybenzoic removal at 10 min of reaction time. Oxidation processes based on the application of ozone [39].

Figure 6.5 shows the p-hydroxybenzoic acid removals attained at 10 min of reaction time for this group of experiments. Table 6 lists the total pseudo-first-order kinetic constants and their three components for each oxidation process. Once it is found that the addition of Fe2+ or H2O2 by themselves exerts no oxidizing activity against p-hydroxybenzoic acid, it can be deduced that the addition of these substances has a slight synergic or catalytic effect on the oxidant activity of ozone, presumably due to free radical generation. Free-radical-mediated synergic effects were observed in the O3/UV combination as well as in the above two combinations [39]. Table 6.6. Split up of kinetic rate constants for processes based on the application of ozone [39].

Oxidation process

kT (min-1)

0.100

O3/Fe2+

0.135

0.100

0.035

O3/H2O2

0.122

0.100

0.022

O3/UV

0.151

0.100

0.019

O3/H2O2/Fe2+

0.128

0.100

0.028

O3/UV/H2O2

0.358

0.032

0.100

0.226

O3/UV/H2O2/Fe2+

0.869

0.032

0.100

0.737

kP (min-1)

0.032

kO3 (min-1)

kR (min-1)

With respect to the last two oxidation systems of Table 6, one observes more marked synergic effects together with greater importance of the free radical pathway in the oxidation process. The O3/UV/H2O2 system is not the simple sum of the systems UV/H2O2+O3, nor it is the sum of the three individual systems UV + H2O2 + O3. The case is similar, although greater in degree, for O3/UV/H2O2/Fe2+ system, which is far more effective than would be expected from the sum of the

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oxidants. As can be seen, more oxidizing agents making up the oxidation system, the greater the synergic effect with the consequent increase in the free radical oxidation pathway [39]. 6.2.3.3. Oxidation processes deriving from the application of Fentonâ&#x20AC;&#x2122;s reagent. Considering the degree of conversion of p-hydroxybenzoic acid at 5 and 10 min reaction time, the order of reactivity of the oxidation systems is as follows [39]: Fe2+/H2O2/O3 < Fe2+/H2O2 < Fe2+/H2O2/UV < Fe2+/H2O2/UV/O3 Figure 6.6 shows the removals at 10 min of reaction time for the experiments belonging to this group. Table 7 lists the pseudo-first-order rate constants and their corresponding three components.

Removal, X10 min (%)

100

90 82

H2O2/Fe2+/O3

H2O2/Fe2+

H2O2/Fe2+/UV

H2O2/Fe2+/UV/O3

Figure 6.6. p-Hydroxybenzoic removal at 10 min of reaction time. Oxidation systems based on Fenton's reagent [39].

Table 6.7. Split up of kinetic rate constants for processes based on Fenton's reagent [39].

Oxidation process

kT (min-1)

Fe2+/H2O2/O3

0.128

Fe2+/H2O2

0.227

Fe2+/H2O2/UV

0.263

0.032

Fe2+/H2O2/UV/O3

0.869

0.032

kP (min-1)

kO3 (min-1)

kR (min-1)

0.100

0.028 0.227

[ 192 ]

0.231 0.100

0.737

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The last two oxidation systems of Table 6.7 have a very high synergic effect due to the addition of ozone to the photo-Fenton's reagent system (with a notable increasing in the free radical pathway). The synergic effect observed for the combined photo-Fenton's reagent system with respect to the sum of the two systems (Fenton's reagent+UV radiation) has recently been demonstrated.

References [1] P.M. Fedorak, S.E. Hrudey, The effects of phenol and some alkyl phenolics on batch anaerobic methanogenesis, Water Res. 18 (1984) 361. [2] T. Reemtsma, M. Jekel, Dissolved organics in tannery wastewaters and their alteration by a combined anaerobic and aerobic treatment, Water Res. 31 (1997) 1035. [3] W.H. Glaze, J.W. Kang, E.M. Aieta, Ozone–Hydrogen peroxide systems for control of organics in municipal water supplies, in: Proc. 2nd Int. Conf. on the Role of Ozone in Water and Wastewater Treatment. Tek Tran Intern., Ltd., Edmonton, Alberta, April 28– 29, 1987, p. 233. [4] W.H. Glaze, J.W. Kang, D.H. Chapin, The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation, Ozone Sci. Eng. 9 (1987) 335. [5] E.M. Aieta, K.M. Regan, J.S. Lang, L. McReynolds, J.W. Kang, W.H. Glaze, Advanced oxidation processes for treating groundwater contaminated with TCE and PCE: pilot scale evaluations, J. AWWA 80 (1988) 64. [6] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Advanced oxidation processes (AOP) for water purification and recovery, Catalysis Today 53 (1999) 51–59. [7] R. Munter, Advanced oxidation processes – current status and prospects, Proc. Estonian Acad. Sci. Chem., 50 (2001) 59. [8] O. Legrini, E. Oliveros, A.M. Braun, Photochemical processes for water treatment, Chem. Rev. 93 (1993) 671. [9] Farhataziz, A.B Ross, Selective specific rates of reactions of transients in water and aqueous solutions. Part III. Hydroxyl radical and perhydroxyl radical and their radical ions, Natl. Stand. Ref. Data Ser., (USA Natl. Bur. Stand.), 1977, 59. [10] J. Hoigné, H. Bader, Rate constants of reaction of ozone with organic and inorganic compounds in water. Part II. Dissociating organic compounds, Water Res. 17 (1983) 185. [11] V.S. Mishra, V.V. Mahajani, J.B. Joshi, Wet air oxidation, Ind. Eng. Chem. Res. 34 (1995) 2. [16] H.J. Fenton, Oxidative properties of the H2O2/Fe2+ system and its application, J. Chem. Soc. 65 (1884) 889. [17] F. Haber, J. Weiss, The catalytic decomposition of hydrogen peroxide by iron salts, Proc. R. Soc. Series A 147 (1934) 332. [18] C. Gottschalk, J.A. Libra, A. Saupe, Ozonation of Water and Waste Water. Wiley-VCH, 2000. [19] J. Hoigné, Chemistry of aqueous ozone and transformation of pollutants by ozone and advanced oxidation processes, in: J. Hrubec (Ed.), The Handbook of Environmental Chemistry, vol. 5, part C, Quality and Treatment of Drinking Water, Part II, Springer, Berlin Heidelberg, 1998. [20] TECHCOMMENTARY: Advanced Oxidation Processes for Treatment of Industrial Wastewater. An EPRI Community Environmental Center Publ. No. 1, 1996. [21] H. Paillard, R. Brunet, M. Dore, Optimal conditions for applying an ozone/hydrogen peroxide oxidizing system. Water Res. 22 (1988) 91. [22] R. Andreozzi, V. Caprio, A. Insola, M.G. D’Amore, The kinetic of Mn(II) catalysed ozonation of oxalic acid in aqueous solution, Water Res. 26 (1992) 917. [23] R. Andreozzi, V. Caprio, M.G. D’Amore, A. Insola, Manganese catalysis in water pollutants abatement by ozone, Environ. Technol. 16 (1995) 885. [24] B. Legube, B. Delouane, N. Karpel Vel Leitner, F. Luck, Catalytic ozonation of salicylic acid in aqueous solution: Efficiency and mechanisms. In Proc. Reg. Conf. Ozone, UV-light, AOPs Water Treatm., September 24–26, 1996, Amsterdam, Netherlands, 509. [25] H. Paillard, M. Dore, M. Bourbigot, Prospect concerning applications of catalytic ozonation in drinking water treatment. In Proc. 10th Ozone World Congr., March, 1991, Monaco, 1, 313. [26] B. Pinker, W.D. Henderson, The effect of ozonation on the performance of GAC. In Proc. Reg. Conf. Ozone, UV-light, AOPs Water Treatm., September 24–26, 1996, Amsterdam, Netherlands, 307. [27] U. Jans, J. Hoigne, Activated carbon and carbon black catalyzed transformation of aqueous ozone into ·OH radicals. Ozone: Sci. Eng. 20 (1998) 67. [25] M.D. Gurol, R. Vatistas, Oxidation of phenolic compounds by ozone and ozone/UV radiation: A comparative study. Water Res. 21 (1987) 895.

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Environmental Remediation and Energy Production Technologies [26] N. Takahashi, Ozonation of several organic compounds having low molecular weight under UV irradiation. Ozone Sci. Eng. 12 (1990) 1. [27] G.R. Peyton, F.Y. Huang, J.L. Burleson, W.H. Glaze, Destruction of pollutants in water with ozone in combination with UV radiation. Environ. Sci. Technol. 16 (1982) 448. [28] J. Beltran de Heredia, J. Torregrosa, J.R. Dominguez, J.A. Peres, Oxidation of p-hydroxybenzoic acid by UV radiation and by TiO2/UV radiation: Comparison and modelling of reaction kinetic, Journal of Hazard. Mater. 83 (2001) 255. [29] M. Umar, H. Abdul, Photocatalytic Degradation of Organic Pollutants in Water in: Organic Pollutants - Monitoring, Risk and Treatment, N. Rashed Ed., DOI: 10.5772/55953. [30] J.F. Klausner, D.Y. Goswami, Solar detoxification of wastewater using nonconcentrating reactors. AlChE Symp. Ser., Heat Transfer, Atlanta, 89 (1995) 445. [31] P. Wyness, J.F. Klausner, D.Y. Goswami, K.S. Schanze, Performance of nonconcentrating solar photocatalytic oxidation reactors, Part 1: Flat plate configuration, J. Sol.Energy Eng. 116 (1994) 2. [32] J. Bedford, J.F. Klausner, D.Y. Goswami, K.S. Schanze, Performance of nonconcentrating solar photocatalytic oxidation reactors, Part 2: Shallow pond configuration, J. Sol. Energy Eng. 116 (1994) 8. [33] C. Martin, I. Martin, V. Rives, Effect of sulfate removal on the surface texture and acid-base properties of TiO2 (anatase), J. Math. Sci. 30 (1995) 3847. [34] D. Bahnemann, D. Bockelmann, R. Goslich, Mechanistic studies of water detoxification in illuminated TiO2 suspensions, Sol. Energy Mater. 24 (1991) 564. [35] C. Pulgarin, J. Kiwi, Overview on photocatalytic and electrocatalytic pretreatment of industrial non-biodegradable pollutants and pesticides, Chimia 50 (1996) 50. [36] Y. Sun, J.J. Pignatello, Photochemical reactions involved in the total mineralization of 2,4-D by Fe3+/H2O2/UV, Environ. Sci. Technol. 27 (1993) 304. [37] C. Gottschalk, J. Libra, A. Saupe, Ozonation of Water and Waste Water. Wiley-VCH, 2000. [38] A. Safarzadeh-Amiri, J. Bolton, S. Cater, Ferrioxalate-mediated photodegradation of organic pollutants in contaminated water, Water Res. 31 (1997) 787. [39] J. Beltrán de Heredia, J. Torregrosa, J.R. Dominguez, J.A. Peres, Comparison of the degradation of p-hydroxybenzoic acid in aqueous solution by several oxidation processes, Chemosphere 42 (2001) 351. [40] J.R. Dominguez, J. Beltrán de Heredia, O. Rodríguez, Vis and UV photocatalytic detoxification methods (using TiO 2, TiO2/H2O2, TiO2/O3, TiO2/S2O82_, O3, H2O2, S2O82_, Fe3+/H2O2 and Fe3+/H2O2/C2O42_) for dyes treatment, Catal. Today 101 (2005) 389.

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7. Industrial Units of Wastewater Treatment by Photocatalysis Anabela S. Oliveira,, Isabel Ferreira Machado,, Eduardo Cuerda, Joaquin Dominguez

7.1. Basics on Photochemistry The subject of Photochemistry is the interaction of light with matter. The absorption of electromagnetic radiation results in the excitation of an electron from a lower to a higher energy level. The electronically excited molecule is obviously energetically unstable with respect to the ground state. If the molecule does not rearrange or fragment (chemistry), it will find some way of losing its excitation energy to return to the ground state (physics). In a photophysical process energy is put into a molecule by the absorption of radiation and although the electronically excited molecule may experience a variety of physical changes as a consequence, it nonetheless retains its chemical integrity, i.e., no new chemical are produced (Gilbert and Baggott, 1991). In this lecture we are dealing with interactions of light resulting in radiative and nonradiative changes, i.e., photophysical processes. Chemical changes arising from interactions of light and molecules are beyond the scope of this session. 7.1.1. Absorption of UV – visible light 7.1.1.1 Types of electronic transitions in polyatomic molecules. The Nature of Electronic States An electronic transition consists of the promotion of an electron from an orbital of a molecule in the ground state to an unoccupied orbital by absorption of a photon. The molecule is then said to be in an excited state. There are various types of molecular orbitals. A  orbital can be formed either from two s atomic orbitals, or from one s and one p atomic orbital, or from two p atomic orbitals having a collinear axis of symmetry. The bond formed in this way is called a  bond. A  orbital is formed from two p atomic orbitals overlapping laterally. The resulting bond is called a  bond. For example in ethylene (CH2= CH2), the two carbon atoms are linked by one  and one bond. Absorption of a photon of appropriate energy can promote one of the p electrons to an antibonding orbital denoted by  *.The transition is then called    *. The promotion of a  electron requires a much higher energy (Valeur, 2002). A molecule may also possess non-bonding electrons located on heteroatoms such as oxygen or nitrogen. The corresponding molecular orbitals are called n orbitals (Valeur, 2002). Promotion of a non-bonding electron to an antibonding orbital is possible and the associated transition is denoted by n  *, depicted in Figure 1 for formaldehyde case.

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.. O ..

C H

Energy





HOMO  LUMO n  * Figure 7.1 – Energy levels of molecular orbitals. Two important types of orbitals must be considered: the Highest Occupied Molecular Orbitals (HOMO) and the Lowest Unoccupied Molecular Orbitals (LUMO). An electronic transition consists on promoting an electron from an orbital of a molecule in the ground state to an unoccupied orbital by absorption of a photon, usually from HOMO to LUMO. In formaldehyde case, the HOMO is the n orbital and the LUMO is the * orbital.

The energy of these electronic transitions is generally in the following order (Valeur, 2002): n  * <    * < n  * <    * <    * The total spin angular momentum possessed by a many-electron atom or molecule is represented by the total spin quantum number S (which may be calculated as S =  si, with si = + ½ or - ½). Two electrons maybe present with their spins parallel or opposed. If spins are opposed the total spin quantum number S is zero; if the electron spins are parallel the total quantum number S is 1. Thus, a molecule with all electrons spin- paired possesses S = 0 and spin multiplicity M (M = 2S + 1) of 1. Such an electronic state is referred to as a singlet state (Gilbert and Baggott, 1991). When one of the two electrons of opposite spins (belonging to a molecular orbital of a molecule in the ground state) is promoted to a molecular orbital of higher energy, its spin is in principle unchanged so that the total spin quantum number S remains equal to zero. Because the multiplicities of both the ground and excited states (M = 2S + 1) is equal to 1, both are called singlet state (usually denoted S0 for the ground state, and S1; S2; . . for the excited states), portrayed in Figure 7.2. If a molecule in a singlet excited state undergo conversion so that the spin of the excited electron becomes aligned parallel to the one “left behind”, an excited state will be generated with a total spin quantum number S = 1 and therefore a spin multiplicity M of 3. Such a state is called a triplet state, abbreviated by symbol T (T1 for the first excited triplet state). According to Hund’s Rule, the triplet state has a lower energy than that of the singlet state of the same configuration (Gilbert and Baggott, 1991; Valeur, 2002).

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Figure 7.2 – Electronic states are labeled using their spin multiplicity with singlet state having all electron spins paired and triplet state having two unpaired. Below: Generalized state diagram: each state is indicated by the (relative) energy of the lowest vibrational level (Gilbert and Baggott, 1991; Valeur, 2002)

7.1.1.2. The Franck–Condon principle According to the Born–Oppenheimer approximation, the motions of electrons are much more rapid than those of the nuclei. Promotion of an electron to an antibonding molecular orbital upon excitation takes about 10-15 s, which is very quick compared to the characteristic time for molecular vibrations (10-10- 10-12 s). This observation is the basis of the Franck–Condon principle: an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment, thus, the nuclei remain essentially frozen at the equilibrium configuration of the ground state molecule during the transition (Gilbert and Baggott, 1991; Valeur, 2002). 7.1.2. Luminescence Processes What happens to an electronically excited molecule that does not undergo some kind of chemical reaction? Electron de-excitation must somehow occur, the excess energy being released as thermal or radiation energy. Transitions involving the de-excitation of electronically excited states that do not involve the emission of radiation are called nonradiative transitions.

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Transitions that give rise to the emission of radiation are called radiative transitions a (Gilbert and Baggott, 1991). 7.1.2.1. The Jablonsky diagram The Jablonsky diagram represents all different types of transition that may occur between the different energy levels, Figure 7.3. The singlet electronic states are denoted S0 (ground state), S1; S2; … and the triplet states, T1; T2; … One should note that each of the electronic states (ground or excited) has a number of vibrational levels superimposed on it. The vibrational levels arise because a molecule in a given electronic state may absorb small increments of energy corresponding to changes in vibrational modes, although retaining the same electronic configuration and so, vibrational levels are associated with each electronic state (sometimes called the “manifold”) (Gilbert and Baggott, 1991; Valeur, 2002). Radiative transitions are usually indicated by vertical arrows (and nonradiative transitions as wavy arrows). Thus, absorption of a photon occurs from the lowest vibrational energy level of S0 because the majority of molecules are in this level at room temperature b, and can bring a molecule to one of the vibrational levels of S1; S2… (Valeur, 2002).

Vibrational relaxation (VR) through collisions with solvent molecules collapses the vibrational

population into the lowest energy level of S1, from which all subsequent processes occur. As we mentioned before, the absorption is very fast (~ 10-15 s) with respect to all other processes (so that with no changes in the positions of the nuclei, according to the Franck–Condon principle). These processes may include fluorescence, internal conversion (IC) and intersystem crossing (ISC). The last two are nonradiative processes and are therefore indicated by horizontal wavy arrows. Once formed, the T1 state may emit phosphorescence, but it can also undergo reverse ISC to give rise to E-type delayed fluorescence or further absorption of radiation to produce the T2 state (Gilbert and Baggott, 1991), processes that later will be referred to. Characteristic Times (Valeur, 2002) Absorption: 10-15 s Vibrational relaxation: 10-12 - 10

-10

Fluorescence: 10-10- 10-7 s Intersystem Crossing: 10-10- 10-8 s Internal Conversion: 10-11- 10-9 s Phosphorescence: 10-6 s – 1

a The radiative transitions may be viewed as “vertical” in the sense that they involve electron motion which is more rapid than nuclear motion – the transitions can be understood in terms of the Frank-Condon principle. They involve a change in the total energy of the molecule due to the absorption and emission of a photon. If radiative transitions are imagined to be “vertical” transitions, nonradiative transitions are “horizontal”. The implication of a horizontal transition is that it occurs between quantum states of the same energy (Gilbert and Baggott, 1991).

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Environmental Remediation and Energy Production Technologies b The ratio of the numbers of molecules N1 and N0 in the 1 and 0 vibrational levels of energy E1 and E0, respectively, is given by the Boltzmann Law:

N1 / N0 = exp [- (E1-E0) / kT], where k is the Boltzmann constant (k = 1.3807 x 10-23 J K-1) and T is the absolute temperature (Valeur, 2002)

Figure 7.3 â&#x20AC;&#x201C; Jablonsky diagram showing absorption, the emission processes of fluorescence and phosphorescence and the nonradiative transitions. Schematic representation of the relative positions of absorption, fluorescence and phosphorescence spectra (based on Valeur, 2002)

7.1.2.2. Radiative and non-radiative transitions between electronic states Let us now refer with some detail the possible de-excitation processes. With very few exceptions, organic photochemistry is concerned with the properties and reactivities of the S1 and T1 states of organic molecules. In the cases where the S2 (or higher) state is formed directly, rapid internal conversion to the S1 state usually follows and the subsequent photochemistry is that of the S1 state. Notable exceptions to this rule are the thiocarbonyl compounds which react from their S2 states (Gilbert and Baggott, 1991; Maciejewski and Steer, 1993; Ferreira Machado, 2007).

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7.1.2.2.1. Fluorescence Once a molecule arrives at the lowest vibrational level of the first electronic state, S1, it can do a number of things, one of which is to return to the ground state by photon emission. This process is called fluorescence (Gilbert and Baggott, 1991). Fluorescence is therefore the emission of photons accompanying the S1 → S0 transition, i.e., is a radiative transition between states of the same multiplicity. The fluorescence is strongly allowed with the result that is occurs on relatively fast timescales. Typical timescales lie in the picosecond (10-12 s) to microsecond (10-6 s) range. It should be emphasized that, apart from a few exceptions (namely azulene and thiocarbonyl compounds) fluorescence emission occurs from S1 and therefore its characteristics do not depend on the excitation wavelength c (Gilbert and Baggott, 1991; Valeur, 2002; Wayne and Wayne, 1996). Owing to the energy loss to the surroundings (due to of vibrational relaxation and internal conversion) the fluorescence spectrum is located at higher wavelengths than the absorption spectrum, that is, spectrally red-shifted (the so-called Stokes shift). Additionally, since the energy spacing between the vibrational levels in S0 or S1 is similar, the applicability of the Franck–Condon principle in both absorption and emission, along with Kasha’s rule, leads to the mirror image symmetry between the emission spectrum and the S0 → S1 absorption spectrum. The shift between the maximum of the first absorption band and the maximum of fluorescence is called the Stokes shift (Gilbert and Baggott, 1991; Valeur, 2002).

7.1.2.2.2. Internal conversion (IC) Internal conversion is a nonradiative transition between two isoenergetic vibrational levels belonging to electronic states of the same spin multiplicity. The excitation of a molecule to an energy level higher than the lowest vibrational level of the first electronic state, S1, might be followed not only by vibrational relaxation but also by internal conversion (if the singlet excited state is higher than S1) leading the excited molecule towards the lowest vibrational level of the S1 singlet state with a time-scale of 10-13–10-11 s (Valeur, 2002). From the lowest vibrational level of S1, internal conversion to S0 is possible, competing with emission of photons (fluorescence), as well as intersystem crossing to triplet state. Since internal conversion occurs to an isoenergetic level of S0 (the transition is horizontal) it is easy to see that S0 state thus formed will possess a considerable amount of vibrational excitation. In solution, this process is followed by a vibrational relaxation towards the lowest vibrational level of the final electronic state. The excess vibrational energy can be indeed transferred to the solvent during collisions of the excited molecule with the surrounding solvent molecules. Molecules formed in their ground electronic states with high vibrational excitation are frequently referred to as “hot” molecules (Gilbert and Baggott, 1991). d Intersystem crossing and subsequent processes As we just refer, another possible de-excitation process from S1 is intersystem crossing towards the T1 triplet state followed by other processes, according to Scheme of Figure 7.4.

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7.1.2.2.3. Intersystem crossing (ISC) Intersystem crossing is a non-radiative transition between two isoenergetic vibrational levels belonging to electronic states of different multiplicities. Therefore, ISC involves transfer of population between electronic states of the different spin multiplicity. For example, an excited molecule in the lowest vibrational level of the S1 state can move to the isoenergetic vibrational level of the T1 triplet state; then vibrational relaxation brings it into the lowest vibrational level of T1. Intersystem crossing may be fast enough (10-7–10-9 s) to compete with other pathways of deexcitation from S1 (fluorescence and internal conversion S1 → S0) (Valeur, 2002). c Kasha’s rule: Emission always occurs from the lowest excited electronic state, independent of the energy of the electronic state excited initially (Gilbert and Baggott, 1991).

The energy differences appropriate for internal conversions among higher singlet states (S2→ S1, S3→ S2, etc) are smaller than that for the S1→ S0 transition, and smaller the energy gap between the initial and final electronic states the larger the efficiency of internal conversion. This gives a relatively straightforward interpretation of Kasha´s rule. The higher rates of internal conversions among the higher energy S2 and S3 states allow these processes to compete effectively with radiative processes – the latter compete only when the molecule has relaxed to the lowest vibrational level of the S1 state (Kasha’s rule) (Gilbert and Baggott, 1991).

Figure 7.4 – Illustration of intersystem crossing and subsequent processes (based on (Valeur, 2002))

This process is referred to as intersystem crossing, and is a spin-dependent internal conversion process.

7.1.2.2.4. Phosphorescence Once ISC has occurred, we have a route to the production of the T1 state and phosphorescence from this state may be observed. Phosphorescence is a radiative transition between states of different spin multiplicity. However, in solution at room temperature, nonradiative de-excitation

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from the triplet state T1, is predominant over radiative de-excitation, i.e., over phosphorescence. There are two factors which tend to enhance a nonradiative transition between the lowest triplet state and the ground state. First the energy difference between the triplet state and the ground state is smaller than the difference between the lowest singlet state and the ground state. This tends to enhance vibrational coupling between these two states, and therefore to enhance internal conversion. Second, and more important, the lifetime of a triplet state is much longer than that of an excited singlet state and therefore loss of excitation energy by numerous collisions with solvent molecules is generally enhanced. Nonetheless, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state, under these conditions, may be long enough to observe phosphorescence and typical timescales for phosphorescence lie in the microsecond (10-6 s) to second range (Valeur, 2002). The phosphorescence spectrum is located at wavelengths higher than the fluorescence spectrum because the energy of the lowest vibrational level of the triplet state T1 is lower than that of the singlet state S1 (Gilbert and Baggott, 1991; Valeur, 2002; Wayne and Wayne, 1996).

7.1.2.2.4.1. Delayed Fluorescence In some circumstances a molecule may exhibit two kinds of fluorescence, termed prompt and delayed. Prompt fluorescence is the type we have already described. Delayed fluorescence is an emission that decays rather more slowly (it appears delayed with respect to the prompt fluorescence) and occurs by two principal mechanisms (Gilbert and Baggott, 1991).

Thermally activated delayed fluorescence If the difference between the lowest vibrational levels of S1 and T1 is small, a significant Boltzmann population of vibrational levels in T1 which are isoenergetic with the lowest vibrational levels in S1 might be achieved by thermal activation. Under these circumstances, reverse intersystem crossing from T1 → S1 is possible and results in emission with the same spectral distribution as normal fluorescence but with a much longer decay time constant. This fluorescence emission is thermally activated; consequently, its efficiency increases with increasing temperature. It is also called Etype delayed fluorescence from S1 because it was observed for the first time with Eosin. The requirement of a small singlet-triplet energy difference, EST, is met by some organic dye molecules and aromatic thiocarbonyls (Gilbert and Baggott, 1991; Valeur, 2002).

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the T1 state can provide enough energy to allow one of them to return to the S1 state. Such triplet–triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with Pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. M ** + M ** →

M* + M

→ M + M + h

where M ** is a triplet and M * is a singlet excited molecule (Gilbert and Baggott, 1991).

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7.1.2.2.4.2. Triplet-triplet transitions According with scheme shown on Figure 7.4, once a molecule is excited and reaches triplet state T1, it can absorb another photon at a different wavelength because tripletâ&#x20AC;&#x201C;triplet transitions are spin allowed. These transitions can be observed provided that the population of molecules in the triplet state is large enough, which can be achieved by illumination with an intense pulse of light (Valeur, 2002).

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7.2. Basics on Solar Photocatalysis Photocatalysis is the combination of photochemistry and catalysis, a process where light and catalysis are simultaneously used to promote or accelerate a chemical reaction. So, photocatalysis can be defined as â&#x20AC;&#x153;catalysis driven acceleration of a light induced reactionâ&#x20AC;?.Direct light absorption is one of photocatalysis bigger advantages compared to thermally activate catalytic processes. Nowadays, photocatalysis appears as an excellent tool for final treatments of samples containing persistent organic pollutants (POPs) when compared to classical treatments (Doll & Frimmel, 2005; HincapiĂŠ, 2005). In a near future they can turn in one of the most used technologies for POPs remediation. Advanced oxidation processes (AOPs) is the common name of several chemical oxidation methods used to remediate substances that are highly resistant the biological degradation. Although oxidation can be total, frequently a partial oxidation is sufficient to decrease the toxicity of the biorecalcitrant compound enabling their final treatment by conventionalbiological treatment. The complete oxidation leads to mineralization and yields CO2, H2O and inorganic ions. and inorganic ions. The partial oxidation can be enough to decrease toxicity enabling biological degradation, but is essential to verify if the intermediary products formed are not more toxic than the parent compound under treatment. In the last 30 years several books (Bahnemann, 1999; Halmann, 1995; Pelizzetti & Serpone, 1989; Schiavello, 1988) and reviews (Byrne, et al., 2011; Dusek, 2010; Gogate & Pandit; 2004a, 2004b; Legrini, et al., 1993; Linsebigler, et al, 1995) were published on the subject. Blake, 2001 contains more than 1200 references on the subject. AOPs can remediate all different types of organic pollutants in liquid, gaseous or solid media, reason why they are used on the remediation of contaminated waters, liquid or gaseous effluents and also on the treatment of different hazardous wastes namely on contaminated soils. Some of the above mentioned reviews present comprehensive compilations of the substances and residues already mineralized using different advanced oxidation processes (Blake, 2001, Legrini, et al., 1993). There are two advanced oxidation processes that enable the use of sunlight as energy source: heterogeneous photocatalysis using semiconductors and homogeneous photocatalysis using photoFenton processes (Fujishima et al., 2000; Pirkanniemi & Sillanpaa, 2002).The degradation of persistent organic pollutant using advanced oxidation processes with sunlight as energy source have as great advantage their lower costs. We can compare the solar emission spectra (starting at 300 nm) with the absorption spectra of titanium dioxide and of Fenton reactant (Malato et al., 2002). Heterogeneous photocatalysis activated by sunlight uses near ultraviolet solar spectrum (wavelength under 380 nm) and homogeneous photocatalysis by photo-Fenton uses a larger portion of solar spectrum (wavelength up to 580 nm). Both processes are efficient in the photodegradation of persistent organic pollutants, they are an innovative way of using a renewable energy and they are very promising technologies in what regards environmental remediation (Gogate &Pandit, 2004a, 2004b).

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Figure 7.5 â&#x20AC;&#x201C; Comparation of Solar emission spectra with TiO 2 and Fenton reactant absorption (Malato, 2002).

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7.3. Treatment of industrial wastewaters containing persistent organic pollutants Generally the term wastewater refers to any residual fluid released into the environment and that contains polluting potential. The equivalent term effluent, which means to spill, derives from the latin effluente. In the last decades the growing environmental awareness led to the implementation of national, international and communitary legislation (Simonsen, 2007) which prohibits or severely restricts the discharge into the environment of untreated industrial effluents containing various classes of substances (a list of controlled or restricted organic pollutants is found in Metcalf & Eddy, 2003, chapter 2, pages 99 – 104). Therefore, particular attention has been devoted to the development of methodologies for industrial wastewater treatment able to destroy or reduce the concentration of restricted chemicals within the allowed legal limits (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Tchobanoglous at al., 1986; Nevers, 2000). Effluents discharges into the environment may be liquid and gaseous. Domestic sewage and several different industrial effluents are the main sources of liquid effluents. After treatment, these effluents are generally released into water bodies (rivers or sea), being usually designated as wastewaters (industrial or domestic). Deficient or incomplete wastewater treatment can lead to surface and groundwater contamination. The extensive use of chemicals such as pesticide, fertilizers, pharmaceuticals, detergents, etc. and soil deposition of urban solid residues are the most important causes of water contamination (Tchobanoglous, et al., 1993). Because of the risk posed to public health by consumption of contaminated water, special care must be taken in water source preservation and water treatment. Water and wastewater treatment sequences consists of several different mechanical, physical, chemical and biological treatments that frequently include harrowing, filtration, flocculation, sedimentation, sterilization and chemistry oxidation of organic pollutants, among others. After physical treatment (filtration and sedimentation) the water still has considerable amounts of organic matter (including organic contaminants), which, in general, can be efficiently degraded under biological treatment (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003). The gaseous effluents are often released to the air through tall chimneys and their main treatments include masking, adsorption on active carbon, contact liquid method, combustion and biological treatment (Davis & Cornwell, 1998; Kiely, 1998; Nevers, 2000). The methods for the treatment of water and wastewater (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003) and gases (Davis & Cornwell, 1998; Eckenfelder, 2000; Nevers, 2000) are deeply revised in the literature. However, the treatment of wastewater containing some organic substances cannot be achieved by traditional processes, because they resist to biological degradation (biorecalcitrant or persistent organic pollutants - POPs) or they are not completely removedby traditional treatment. Nowadays, the persistent organic pollutants (POPs) are a matter of great importance, because they cannot be eliminated by the ordinary water or wastewater treatments (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Nevers, 2000). POPs are xenobiotic chemicals of natural or anthropogenic origin witch accumulated in the environment and biota, due to theirs highly refractory chemical structures and physicalchemical properties. Structurally they are polycyclic conjugated compounds (polycyclic aromatic hydrocarbons) or they have a high number of halogen atoms, especially chlorine or bromine (pesticides, polychlorinated dibenzodioxins – PCDDs -, polychlorinated dibenzofurans – PCDFs -, polychlorinated biphenyls – PCBs -, brominated flame retardants, etc). Because most POPs are semi-volatile they suffer long range transport and can be found anywhere, even in distant regions where they have never been produced or released. POPs have a lipophylic and hydrophobic characters and so they consequently bioaccumulate in fatty tissues of organisms and are capable of bioaccumulating or

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biomagnificating into food chains, reaching extremely high concentrations (in comparison with their environmental concentrations) on the top species (Baird, 1999). Many of these compounds are biologically actives possessing mutagenic and/or carcinogenic or even endocrine disruption properties. Although several of them have natural sources, the fast industrial development since the late nineteenth century lead to an enormous increase either on the quantity and on the diversity of the persistent organic pollutants from anthropogenic origin present in the environment. Conjugation of their above mentioned characteristics determines that these compounds represent a high risk to public and environmental health. Several of those substances have already been classified as prioritary substances for environmental monitoring (see Baird, 1999, chapter 7, pages 293 to 379). Dibenzodioxins, dibenzofurans, polychlorinated biphenyls and organochlorinated pesticides join the list of priority organic pollutants of World Health Organization (WHO), United Nations Environmental Program (UNEP) and other Environmental Protection Agencies (Kiely, 1998; Metcalf & Eddy, 2003). The Stockholm Convention regulates this matter worldwide. This Convention presents a list of POPs (originally 12 substances: aldrin, dieldrin, endrin, chlordane, PCDDs, PCDFs, BHC, DDT, heptachlor, mirex, PCBs, toxaphene). Nowadays there are other under consideration: HCH, chlordecone, hexabromobiphenyl, hexa and heptabromobiphenyl ether, pentachlorobenzene tetra and pentabromodiphenyl ether e perfluorooctanosulfonic acid and its salts) which production, use and trading are banned or severely restricted (United Nations Environment Programme, 2005; Stockholm Convention on Persistent Organic Pollutants, 2005; Oliveira, et al., 2004, 2008, 2011). There are many other synthetic substances that have been identified as priority pollutants for environmental monitoring by the United States Environmental Protection Agency (USEPA) based on theirs probable or confirmed carcinogenic, mutagenic, teratogenic or acute toxicity characteristics. Among them we can mention volatile organic compounds, agricultural fertilizers and chlorinated residues resultant from disinfection processes at water public supply systems. Many of those substances can either be found in the air (as is the case of the volatile organic compounds) or in surface and groundwater and they reach the reception media through domestic or industrial wastewater systems or due to drain-off from agriculture (as appends with pesticides and fertilizers). There are also several substances (i.e. dyes, pharmaceuticals, etc) some of them specially synthesized to be resistant to degradation and conventional wastewater treatment processes are not able to remove them efficiently (Eckenfelder, 2000). Although these substances are not classified as prioritary pollutants, their negative impact in aquatic life and the changes of physical-chemical characteristics of the water bodies even when present in low concentrations make the control of their concentration very important. Once the use and discharge of bioactive organic substances in the different environmental segments is not easy to eliminate and appears extremely difficult to control its essential to develop new powerful, clean and safe environmental remediation technologies for their treatment especially for the biorecalcitrant organic pollutants. One of the new most promising technologies available uses hydroxyl radical, a highly reactive chemical species that can attack and destroy organic molecules and is denominated advanced oxidation processes (Eckenfelder, 2000; Metcalf & Eddy, 2003).

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7.4. Solar Collectors remediation of effluents through Photocatalysis The use of light activated advanced oxidation processes requires the development of dedicated photochemical solar technology that include the design of efficient solar photons collection technologies and the direction of those photons to the appropriate reactor in order to promote the photodegradation of the persistent organic pollutants to be remediated. For solar photochemical processes it is more interesting the collection of photons with high-energy and low wavelength, since typically the majority of the photocatalysis processes use solar radiation in the ultraviolet (300-400 nm). The exception is photo-Fenton process, which uses all sunlight below 580 nm. Usually radiation with wavelengths higher than 600 nm does not have any utility for photocatalysis processes. Solar flux at ground level is about 20 to 30 W per square meter, so sun approximately provides 0.2 to 0.3 moles of photons per square meter per hour (Bahnemamm, 1999; Malato et al., 2002). This equipment represents the largest source of operating costs of a photocatalysis unit for treatment of effluents. The equipment that makes the efficient collection of photons is the solar collector. Solar collectors can be classified into three types according to the level of solar concentration achieved, which is usually directly related to the temperature reached by the system. So we have solar collectors which are non-concentrators, moderately concentrators or highly concentrators. They can also be called concentrators of low (<150 째 C), medium (150 - 400 째 C) and high (> 400 째 C) temperature (Hernandez, 2010). The non-concentrating collectors or low temperature collectors are static and usually consist of flat plates directed towards the sun with a certain inclination, depending on the geographic location. Its main advantage is their great simplicity and low cost. The moderately concentrating collectors or medium temperature collectors concentrate the sun 5 to 50 times; to achieve this concentration factor the equipment must be able to continuously follow the sun. The parabolic and holographic are such type of collectors. Parabolic collectors have a parabolic reflecting surface which concentrates the radiation in a tubular collector located at the focus of the parabola and can have uni or biaxial movement. Holographic collectors consist of reflective surfaces like convex lenses that deflect radiation and concentrate it in a focus. The highly concentrating collectors or high temperature collectors have a punctual focus rather than a linear one and they are based on a paraboloid with solar tracking. These reactors ensure solar concentrations from 100 to 10,000 times, reason why they require high precision optical elements. As the temperature plays no role in photochemical reactions, their technological applications typically only use low and medium temperature collectors, which have a much more economical construction. An important difference between these two reactors is that the first type of concentrators uses both direct and diffuse radiation while the concentrators collectors only use direct radiation. In terms of collector and reactor design itself, the systems have much in common with that of conventional thermal collectors; however, as the effluent to be cleaned must be directly exposed to sunlight, the absorber must be transparent to the photons. As temperature is not important, the systems are not thermally isolated. Most photocatalytic remediation systems involve wastewaters (Duran et al., 2010, 2011; Malato et al., 2007a ,2007b; Marugan et al., 2007; Miranda-Garcia et al., 2011; Navarro et al., 2011; Oyama et al., 2011; Vilar et al., 2009; Zayani et al., 2009), but the appropriated technology for gas phase photocatalytic processes is also possible (Lim et al., 2009).

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7.5. Reactors with Solar Collectors Several types of solar reactors for effluents decontamination have been tested. We describe below the four most commonly used.

7.5.1. Thin Film Fixed Bed Reactor This reactor, whose simplified diagram is presented in Figure 5, is one of the first nonconcentrators solar reactors, so it can use the total solar radiation (direct and diffuse) for the photocatalytic process. Quantities of direct and diffuse radiation reaching earth are nearly identical, so concentrator reactors, by not using the diffuse radiation, profit from only half of the maximum radiation available. The most important part of the reactor is a tilted fixed dish (0.6 m x 1.2 m) coated with a thin film of photocatalyst, typically Degussa P25 TiO2, which is continuously washed with a film of about 100ď&#x20AC; ď m from the wastewater to be treated at a rate of 1 to 6.5 liters per hour (Bahnemamm, 1999; Malato et al., 2002).

7.5.2. Parabolic Trough Reactor This reactor directly concentrates sunlight by a factor from 5 to 50. Tracking of solar radiation is done by a single or dual motors system that allow the continued alignment of the solar concentrator with the sun and various reactors can be connected in series or in parallel. In the parabolic trough reactor, the reflector has a parabolic profile and the tube where the photocatalytic reaction takes place is in its focus, in this way, only the light that enters parallel in the reflector can be focused on the reaction tube (Figure 6). This type of reactor is being used in solar decontamination circuits installed in the United States (Albuquerque, Sandia National Laboratories, and California, Lawrence Livermoore Laboratories) and Spain (Plataforma Solar de Almeria), Figure 7 (Navntoft et al., 2009). The concentrated radiation is focused into a tube containing an aqueous suspension of TiO2 and the effluent to be treated. In fact, only about 60% of the radiation collected is effectively used, the rest being lost by various causes. At Almeria reactors, the total volume of effluent are about 400 liters, but the illuminated tube of about 180 meters contains only about half of that volume of effluent, which moves at speeds between 250 to 3500 liters per hour (Bahnemamm, 1999; Malato et al., 2002).

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Figure 7.6. - Simplified diagram of a thin film fixed bed reactor

Figure 7.7. - Simplified diagram of a plant using parabolic trough reactors

7.5.3. Compound Parabolic Collecting Reactor This reactor, whose simplified diagram is presented in Figure 7, is an open reactor without solar concentration. Basically this reactor differs from the conventional open parabolic reactor in the form of the reflectors. These reactors are static collectors with a reflective surface that surrounds a circular reactor, as shown in Figure 7. They had shown to provide better efficiency in the treatment of low pollutant concentration effluents. This reactor is an effective combination of the two reactors types described above (Bahnemamm, 1999; Malato et al., 2002).

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Figure 7.8. - Simplified diagram of a compound parabolic collecting reactor

7.5.4. Double Skin Sheet Reactor This type of reactor without concentration consists of a transparent box with an internal structure similar to that shown in Figure 8, through which is pumped the suspension containing the pollutant and the photocatalyst. It has the advantage of using the total radiation and be very simple to operate (Bahnemamm, 1999; Malato et al., 2002).

Figure 7.9. - Simplified diagram of a double sheet reactor

7.5.5. Industrial Units Figure 7.9 shows a diagram of a photocatalytic installation that can be alternatively used for heterogeneous TiO2 photocatalysis or for homogeneous photo-Fenton photocatalysis (or any other

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of the treatments previously described). In both cases the catalyst (TiO2 or iron) must be separated at end of treatment to be recycled and reused. The surface area of the solar collector depends essentially on the effluent to be treated, mandatorily of the type and concentration of the contaminant, and on solar irradiation conditions and the location where treatment plant will be installed. The lifetime of the catalysts depends on the type of effluent to be treated and of the desired treatment final quality. In the end, the toxicity of the treated effluent must always be evaluated. The project of an industrial effluent decontamination plant by photocatalysis requires a careful selection of the type of reactor to use, the arrangement of the reactor at the installation (series or parallel), the operation mode of the photocatalyst (fixed or suspended), the system for recycling catalysts and flow velocity, among others. The concentration of photocatalyst is also a key parameter and must be adjusted according to the following basic principles: for suspensions of TiO2, the speed of reaction is maximum for concentrations among 1 to 2 grams of TiO2 per liter of effluent to be treated, when the optical path is small (1-2 cm maximum). When the optical path is substantially higher, the appropriate concentration of photocatalyst is several hundred milligrams per liter. Anyway, when TiO2 concentrations is too high there is an internal filter effect and the rate of photodegradation decreases due to excessive opacity of the solution, which itself inhibits the illumination of the photocatalyst.

Figure 7.10. â&#x20AC;&#x201C; Simplified diagram of a photocatalytic effluent treatment plant

If the treatment plant is intended for treatment of a specific effluent, it does not need to be versatile and will be very similar to that shown in Figure 10. On the other hand, if an installation needs to treat various types of wastes, it must have the versatility to adapt to the optimal photodegradation conditions of the various types of effluents, e.g., having different types of solar collectors and reactors (as is the case of Almeria solar platform) and the project will be much more complicated.

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7.6. Applications of Photocatalysis on the Treatment of Industrial Effluents Solar driven AOPs proved to be an excellent environmental remediation method to destroy persistent organic compounds not treatable by biological processes. In many cases, they allow the degradation of several persistent organic toxic pollutants decreasing the toxicity of the effluents released into the environment. These methods are particularly suitable for treating recalcitrant substances including those requiring special attention (hazardous or controlled ones). Several organochlorinated substances (dioxins, PCBs, etc) are persistent and sufficient toxic to disturb the environmental health and must be degraded prior to their environmental release. Without being exhaustive on the list of applications and systems to be treated, we will refer some examples that we consider most significant.

Figure 7.11. - Simplified diagram of a unit for dyes degradation with thin film fixed reactors

Water is essential for life and therefore a key resource for humanity. Although it may seem that the water is very handy on our planet that is not true. Of the entire planet's water, 97.5% of the water is salty, among the remaining 2.5%, 70% is frozen and the rest is largely inaccessible in underground aquifers or as soil moisture. In fact, less than 1% of world potable water is available for immediate human consumption and even that is not uniformly distributed around the globe. For this reason, methodologies such as advanced oxidation processes that allow the maintenance of water quality are essential (Andreozzi et al., 1999; Chong et al., 2010; Comninellis et al., 2008; Matilainen & Sillanpaa, 2010). The problematic of water treatment and industrial wastewater treatment are inseparable issues since these industrial effluents constitute a major source of water contamination and are usually discharged into the environment in aqueous media. The classical treatment processes of drinking water include treatment with ozone and filtration through granular activated carbon beds. Photocatalysis emerged as a promising tool for the treatment of water (and for the degradation of persistent substances even when they are present in low concentrations or complex matrices). So the advanced oxidation processes have been widely reported as an appropriate remediation methodology of all kinds of biorecalcitrants

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pollutants in water and industrial wastewater and their application to large-scale treatment facility is already being implemented, as discussed in section 4. Typical examples of water pollutants that were efficiently mineralized by photocatalysis are effluents from industries containing dyes (Guillard et al., 2003), pesticides (Burrows et al., 2002; Marinas et al., 2001) and the effluents from the paper industry (Peiro et al., 2001). Various applications are also known for the decontamination of waste gases (Fu et al., 1996; Hay & Obee, 1999) including those involving self-cleaning surfaces (Hashimoto & Watanabe, 1999).

7.6.1. Industrial Effluents Containing Dyes The dyes are common industrial residues present in wastewaters of different industries, ordinarily in textile dyeing process, inks, and photographic industries, among others. The environmental aspects of the use of dyes, including their degradation mechanisms in various environment compartments, have been a target of increasingly interest. It is estimated that nearly 15% of world production of dyes is lost during synthesis and dyeing process. Concomitantly, the major problem related with dyes is the removal of their colour from effluents. The non treated effluents frequently are highly colored and then particularity susceptible to public objection when disposed in water bodies. The dye concentration in residual waters can be smaller than others contaminants, but because of its high molar absorption coefficients they are visible even in very low concentrations. So, methodologies of effluents discoloration became very relevant. The oxidation processes are very much used in treatment of dye containing effluents (Khataee & Kasiri, 2010; Oliveira et al., 2008, 2011; Rauf & Ashraf, 2009; Saggioro et al., 2011; Soon & Hameed, 2011). Figure 10 presents a pilot unit commonly used on the degradation of dyes with thin film fixed bed (Guillard et al., 2003).

7.6.2. Effluents Containing Pesticides and Pharmaceuticals The photodegradation and mineralization of pesticides and pharmaceuticals has been widely studied because of the danger they represent to the environment and also due to the highly recalcitrant nature of some of these compounds. For a comprehensive review of pesticide degradation see Blake, 1999 or some of the reviews listed here (Atheba et al., 2009; Bae & Choi, 2003; Felsot et al., 2003). Either titanium photocatalysis or Fenton parent methodologies usually promote rapid destruction of persistent pesticides. Municipal water recycling for industrial, agricultural, and non-potable municipal may contain several different pharmaceuticals including antibiotics, hormones and other endocrine disruptors, sulphonamides, antipyretics, etc. Those are present in municipal sewage, largely as a result of human use and/or excretion. Much of the concern regarding the presence of these substances in wastewater and their persistence through wastewater treatment processes is because they may contribute to directly or indirectly affect the environmental and human health (Exall, 2004; Vigneswaran & Sundaravadivel, 2004). In spite of the variable removal of antibiotics during conventional waste water treatment processes, many of these chemicals are often observed in secondary treated effluents. Conventional water and wastewater treatment are inefficient for substantially removing many of these compounds. While there appears to be no standard treatment for removal of all residual pharmaceuticals under conventional treatment processes, there is a strong opinion that advanced oxidation processes can be used for the effective removal of these compounds (Auriol et al., 2006;

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8. Adsorption for water and wastewater remediation Eduardo M. Cuerda Correa, Joaquín R. Domínguez, Antonio Macías García, Anabela S. Oliveira, Isabel Ferreira Machado

8.1. Fundamentals of Adsorption 8.1.1. Some definitions The term “sorption” is used to describe every type of capture of a substance from the external surface of solids, liquids, or mesomorphs as well as from the internal surface of porous solids or liquids (Skoulikides, 1989). Depending on the type of bonding involved, sorption can be classified as follows. (a) Physical sorption. In physical sorption (or physisorption), no exchange of electrons is observed; rather, intermolecular attractions between favorable energy sites take place and are therefore independent of the electronic properties of the molecules involved. Physisorption is characterized by interaction energies comparable to heats of vaporization (condensation). The adsorbate is held to the surface by relatively weak van der Waals forces and multiple layers may be formed with approximately the same heat of adsorption. The heat of adsorption for physisorption is at most a few kcal/mole and therefore this type of adsorption is stable only at temperatures below 150 °C. (b) Chemical sorption. Chemical sorption (or chemisorption) involves an exchange of electrons between specific surface sites and solute molecules, and as a result a chemical bond is formed. Chemisorption is characterized by interaction energies between the surface and adsorbate comparable to the strength of chemical bonds (tens of kcal/mol), and is consequently much stronger and more stable at high temperatures than physisorption. Generally, only a single molecular layer can be adsorbed. (c) Electrostatic sorption (ion exchange). This is a term reserved for Coulomb attractive forces between ions and charged functional groups and is commonly classified as ion exchange. The most important characteristics of physical and chemical sorption are presented in Table 8.1. The term “adsorption” includes the uptake of gaseous or liquid components of mixtures from the external and/or internal surface of porous solids. In chemical engineering, adsorption is called the separation process during which specific components of one phase of a fluid are transferred onto the surface of a solid adsorbent (McCabe et al., 1993). When the species of the adsorbate travel between the atoms, ions, or the molecules of the adsorbent, the phenomenon of “absorption” takes place and this discriminates absorption from the main phenomenon of adsorption that takes place on the interface. The adsorption of various substances from solids is due to the increased free surface energy of the solids due to their extensive surface. According to the second law of thermodynamics, this energy has to be reduced. This is achieved by reducing the surface tension via the capture of extrinsic substances.

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Table 8.1.- Physical versus chemical sorption

Chemisorption

Physisorption

Virtually unlimited; however, a given molecule may be effectively adsorbed only over a small range

Near or below the condensation point of the gas (e.g. CO2 < 200 K)

Adsorption enthalpy

Wide range, related to the chemical bond strength typically 40–800 kJ/mol

Related to factors like molecular mass and polarity but typically 5–40 kJ/mol (i.e. ≈ heat of liquefaction)

Nature of adsorption

Often dissociative and may be irreversible

Nondissociative and reversible

Saturation uptake

Limited to one monolayer

Multilayer uptake is possible

Kinetics of adsorption

Very variable; often is an activated process

Temperature range over which adsorption occurs

Fast, because it is a nonactivated process

Consider a molecule above a surface with the distance from the surface being normal to the surface. There are two competitive types of influence occurring: (a) repulsion between the cloud of electrons in the atoms that form the surface and those of the molecule and (b) van der Waals nuclear attraction force. The nuclear attraction has a much shorter radius of influence and as a result of the balance of these two forces, there is a “well” in the potential energy curve at a short distance from the surface, as shown in Figure 8.1. Molecules or atoms that reach this “well” are trapped or “adsorbed” by this potential energy “well” and cannot escape, unless they obtain enough kinetic energy to be desorbed.

Figure 8.1.- The potential energy versus distance.

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The surface can be characterized either as external when it involves bulges or cavities with width greater than the depth, or as internal when it involves pores and cavities that have depth greater than the width (Gregg and Sing, 1967). All surfaces are not really smooth and they exhibit valleys and peaks at a microscopic level. These areas are sensitive to force fields. In these areas, the atoms of the solid can attract atoms or molecules from a fluid nearby. The most important property of adsorbent materials, the property that is decisive for the adsorbent’s usage, is the pore structure. The total number of pores, their shape, and size determine the adsorption capacity and even the dynamic adsorption rate of the material. Generally, pores are divided into macro-, meso- and micropores. According to IUPAC, pores are classified as shown in Table 8.2. Table 8.2 – Pore size

Pore diameter d (nm)

Type Macropores

d>50

Mesopores

2<d<50

Micropores

d<2

Supermicropores

0.7 < d < 2

Ultramicropores

d < 0.7

d is the pore width for slit-type pores or the pore diameter for cylindrical pores.

Porosity is a property of solids that is attributed to their structure and is evident by the presence of pores between internal supermolecular structures (Tager, 1978). It is not considered to be an intrinsic property of the solids, but depends on the treatment of the materials. The porosity can be developed by the aggregation of particles as well as by the detachment of a part of the mass of the solid. The pores shaped during the second process are comparable in shape and size with the particles detached. Adsorptive molecules transport through macropores to the mesopores and finally enter the micropores. The micropores usually constitute the largest portion of the internal surface and contribute the most to the total pore volume. The attractive forces are stronger and the pores are filled at low relative pressures in the microporosity, and therefore, most of the adsorption of gaseous adsorptives occurs within that region. Thus, the total pore volume and the pore size distribution determine the adsorption capacity. A phenomenon closely related to adsorption is ion exchange. Ion exchangers are solid materials that are able to take up charged ions from a solution and release an equivalent amount of other ions into the solution. The ability to exchange ions is due to the properties of the structure of the materials. The exchanger consists of a so-called matrix, with positive or negative excess charge. This excess charge is localized in specific locations in the solid structure or in functional groups. The charge of the matrix is compensated by the so-called counterions, which can move within the free space of the matrix and can be replaced by other ions of equal charge sign (Helfferich, 1995). The pores sometimes contain not only counterions but also solvent. When the exchanger is in contact with the liquid phase, the solvent can travel through the exchanger and cause “swelling” to an extent that depends on the kind of counterions. Some electrolytes can also penetrate into the exchanger along with the solvent. As a result, there are additional counterions, the so-called coions, which have the same charge sign as the fixed ions. Normally, an exchanger has many open areas of variable size and shape that are altogether called “pores.” Only a few inorganic exchangers contain pores of uniform cross section. So, the exchangers exhibit a three-dimensional network of channels with irregular size.

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Although ion exchange is similar to sorption since a substance is captured by a solid in both processes, there is a characteristic difference between them: ion exchange is a stoichiometric process in contrast to sorption (Helfferich, 1995). It means that in the ion-exchange process, for every ion that is removed, another ion of the same sign is released into the solution. In contrast, in sorption, no replacement of the solute takes place. Ion exchange can be seen as a reversible reaction involving chemically equivalent quantities (Treybal, 1980; Perry and Green, 1999). The water-softening reaction constitutes a characteristic example of cation exchange. However, the characterization of an ion exchange as a “chemical process” is rather misleading. Ion exchange is in principle a redistribution of ions between two phases by diffusion, and chemical factors are less significant or even absent. The absence of any actual chemical reaction explains why the heat evolved in the course of an ion exchange is usually very small to negligible, often less than 2 kcal/mol (Helfferich, 1995). Only when an ion exchange is accompanied or followed by a reaction such as neutralization can the whole phenomenon be characterized as “chemical.” A characteristic example is in chelating resins where the ion exchange is followed by a chemical reaction and bond formation between the incoming ion and the solid matrix. Ion removal by solids could involve more phenomena, as for example in inorganic natural materials where ion uptake is attributed to ion exchange and adsorption processes or even to internal precipitation mechanisms (Inglezakis et al., 2004). 8.1.2. Some historical aspects The first known use of adsorption was made in 3750 B.C. by Egyptians and Sumerians who used charcoal for the reduction of copper, zinc, and tin ores for the manufacture of bronze. Around 1550 B.C., Egyptians applied charcoal for medicinal purposes, whereas around 460 B.C., Hippocrates and Pliny introduced the use of charcoal to treat a wide range of infections. Around the same age, Phoenicians used charcoal filters to treat drinking water. So, this must have been the first use of adsorption for environmental purposes. In 157 B.C., Claudius Galen introduced the use of carbons of vegetable and animal origin to treat a wide range of complaints. These early applications of adsorption were based on intuition and not on a systematic study. It was in 1773 that Scheele made the first quantitative observations in connection with adsorption, whereas F. Fontana in 1777 reported his experiments on the uptake of gases from charcoal and clays. However, the modern application of adsorption is attributed to Lowitz. Lowitz used charcoal for the decolorization of tartaric acid solutions in 1788. The next systematic studies were published by Saussure in 1814. He concluded that all types of gases can be taken up by a number of porous substances and this process is accompanied by the evolution of heat (Dabrowski, 2001). The term “adsorption” was first used by H. Kayser in 1881. J. W. McBain introduced a similar term in 1909, i.e. “absorption”, to determine an uptake of hydrogen by carbon much slower than adsorption. He proposed the term “sorption” for adsorption and absorption (Dabrowski, 2001). In 1903, Tswett was the first to study selective adsorption. He investigated the separation of chlorophyll and other plant pigments using silica materials. This technique proposed by Tswett has been called “column solid–liquid adsorption chromatography.” However, there was no sound theory that enabled the interpretation of adsorption isotherm data until 1914. Despite the fact that the Freundlich equation was used, there was no theoretical justification for it. It was an empirical equation, proposed actually by van Bemmelen in 1888. However, it is today known as the Freundlich equation because Freundlich assigned great importance to it and popularized its use. Langmuir was the first to have introduced a clear concept of the monomolecular adsorption on energetically homogeneous surfaces in 1918 and derived the homonymous equation based on kinetic studies (Dabrowski, 2001). The first practical applications of adsorption were based on the selective removal of individual components from their mixtures using other substances. The first filters for water treatment were

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installed in Europe and the United States in 1929 and 1930, respectively. Activated carbon was recognized as an efficient purification and separation material for the synthetic chemical industry in the 1940s. By the late 1960s and early 1970s, activated carbon was used in many applications for removing a broad spectrum of synthetic chemicals from water and gases. In Table 8.3, the history of adsorption is presented briefly. Table 8.3.-Brief history of adsorption (Dabrowski, 2001)

Scientist(s) Name(s) C. W. Fontana

Scheele,

Breakthrough Experiments on the uptake of gases by charcoal and clays

1773–1777

T. Lowitz

Decolorization of tartaric acid utilizing charcoal

1776–1778

D. M. Kehl

Application of carbons of animal origin for the removal of colors from sugar. The English sugar industry used charcoal as a decolorization agent in 1794.

1793

T. de Saussure

Systematic studies on adsorption. He discovered the exothermic character of adsorption

1814

H. Kayser Van Bemmelen, H. Freundlich

Year

Introduced the term “adsorption” first and

1888

R. Von Ostreyko

Set the basis for the commercial development of activated carbons

1901

M. S. Tswett

Discovered selective adsorption. He used the term and technology of “column solid–liquid adsorption chromatography”

1903

J. Dewar

Found selective adsorption of oxygen from a mixture with nitrogen, during the uptake of air by charcoal

1904

W. A. Zelinsky

Applied the use of active carbons as an adsorption medium in a gas mask for the needs of World War I

1915

I. Langmuir

Derived the concept of monolayer adsorption, formed on energetically homogeneous solid surfaces. Was awarded the Nobel Prize in chemistry in 1932

1918

The milestone in the development of adsorption science was the multilayer isotherm equation, known as BET

1938

S. Brunauer, P. H. Emmet, E. Teller

The Freundlich equation was proposed by van Bemmelen popularized by Freundlich

1881

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Scientist(s) Name(s) A. J. P. Martin B. L. M. Synge

R. M. Barrer D. W. Breck

Breakthrough

Year

Introduced to laboratory practice the solid–liquid partition chromatography, both in column and planar form

1941

Invented the method of zeolite synthesis. In the same year, the NorthAmerican Linde Company started the production of synthetic zeolites on a commercial scale

1956

On the other hand, the first citation of an application of ion exchange can be found in Aristotle’s Problematica, where it is mentioned that sand filters were used for the purification of sea and impure drinking waters. That is also the first environmental application. In the same book, Aristotle suggested that desalination resulted from density effects. It seems that practical applications of ion exchange were well recognized before the 19th century. However, the underlying physical phenomenon was not known. Credit for the identification of the ion-exchange phenomenon is attributed to two agriculture chemists, Thomson and Way. In 1848, Thomson reported to Way that he had found that urine was decolorized and deodorized during the filtration of liquid manure through a bed of an ordinary loamy soil. It was Way, who illustrated the basic characteristics of ion exchange after conducting several experiments (Lucy, 2003). After soil and clays, natural and synthetic aluminum silicates and synthetic zeolites were tested as ion-exchange materials by other scientists. However, the first practical applications of ion exchange took place in the early 20th century. The first synthetic organic resins were synthesized in 1935. This spectacular evolution began with the finding of two English chemists, Adams and Holmes, who found that crushed phonograph records exhibited ion-exchange properties (Helfferich, 1962). Much progress was made during World War II in the field of ion exchange, but the results obtained were not published for some years due to reasons of confidentiality. Afterwards, there was a rapid development of ionexchange materials and methods (Lucy, 2003). A brief history of ion exchange is presented in Table 8.4. Table 8.4.- Brief history of ion exchange (Lucy, 2003; Helfferich, 1962) Scientist(s) name(s) H. S. Thompson

Breakthrough

Year

Thompson passed a solution of manure through a filter made of ordinary garden soil and found that the ammonia was removed from solution.

1845

H. S. Thompson, J. Recognition of the phenomenon of ion exchange and a 1848 T. Way, J. Spence description of its basic characteristics. The ion exchange –1852 property of soils was found to be based on their containing small amounts of zeolites. H. Eichorn

Proved that the adsorption of ions by clays and zeolites constitutes a reversible reaction

1858

J. Lemberg

Zeolites recognized as carriers of base exchange in soils; equivalence of exchange of bases proved.

1876

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Scientist(s) name(s)

Breakthrough

Year

F. Harm, A. Artificial zeolites used for removal of potassium from sugar 1901 Rumler, S. Mayert, juices. First synthetic industrial ion exchanger. Manufacture of –1902 K. Halse sulfonated coals and suggestion for the removal of potassium from sugar juices. R. Gans

Discovered that zeolites could be used to soften hard water. He also invented processes for synthesizing zeolite and designed the equipment—the zeolite water softener— used for the recovery of gold from sea water. S

1905

O. Folin, R. Bell

The first analytical application of ion exchange.

1917

J. Whitehorn

The first use of ion exchange in column chromatography.

1923

A. Bahrdt

The first use of an ion-exchange column for anion analysis.

1927

O. Liebknecht

Entirely new types of cation exchangers were developed.

P. Smit

B. A. Adams,E. L. Holmes G. F. D’Alelio

1934 –1939

Not only could they be used in the sodium cycle when regenerated with salt, but also in the hydrogen cycle when regenerated with an acid. One group of these cation exchangers was the carbonaceous type, which was made by the sulfonation of coal. Synthesis of the first organic ion exchanger.

1934 –1935

Invention of sulfonated polystyrene polymerization cation exchangers.

1942

G. E. Boyd, J. Demonstration of the applicability of ion exchange for Schubert, A.W. adsorption of fission products in trace amounts (lanthanides). Adamson

1942

C. H. McBurney A. Skogseid

Invention of aminated polystyrene polymerization anion exchangers. Preparation of a potassium-specific polystyrene

J. A. Marinsky, L. The discovery of promethium (element 61), an element not E. Glendenin, C. D. found in nature, is attributed to ion exchange Coryell

1947 1947 1947

D. K. Hale, D. Development of carboxylic addition polymers as weak acid 1949 Reichenberg, N. E. cation exchangers. –1956 Topp, C. G. Thomas R. M. Barrer, D. W. Breck

New zeolites as molecular sieves with ion-exchange properties.

1951 –1956

H. P. Gregor, K. W. Pepper, L. R. Morris

Invention and development of chelating polymers.

1952 –1971

M. A. Peterson, H. Development of cellulose ion exchangers. Preparation and A. Sober studies of nonsiliceous inorganic ion exchangers—insoluble salts, heteropolyacids

[ 226 ]

1956

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Scientist(s) name(s)

Breakthrough

F. Helfferich T. R. Kressmann, J. Millar

E. R.

J. Weiss

Year

Foundations laid for the new theoretical treatment of ion exchange

1959

Invention and development of isoporous ion-exchange resins.

1960

Thermally regenerable ion-exchange desalination based on them.

resins

and

water

1964

Ion exchange is similar to adsorption, since mass transfer from a fluid to a solid phase is common in both processes, i.e. they are basically diffusion processes. Ion exchange is also a sorption process, but ions are the sorbed species in contrast to adsorption, where electrically neutral species are sorbed (Noble and Terry, 2004; Perry and Green, 1999). It is generally accepted that adsorption and ion exchange can be grouped together as sorption for a unified treatment in practical applications. Most of the mathematical theories and approaches have been developed originally for sorption rather than ion exchange. However, they are sufficiently general to be applicable with minor, if any, modifications to a number of similar phenomena such as ion exclusion and ligand exchange. According to Helfferich (1995), the applicability of a simplified theory depends more on the mode of operation than on the particular mechanism of solute uptake. A significant feature of physical adsorption is that the rate of the phenomenon is generally too high and consequently, the overall rate is controlled by mass (or heat transfer) resistance, rather than by the intrinsic sorption kinetics (Ruthven, 1984). Thus, sorption is viewed and termed in this book as a â&#x20AC;&#x153;diffusion-controlledâ&#x20AC;? process. The same holds for ion exchange. 8.1.3. Characterization of the adsorbents The treatment of aqueous effluents is an increasing problem, with tighter government legislation being imposed, particularly as environmental issues have become political ones. One of the major methods for the removal of pollutants from wastewater is using porous solid adsorbents. The properties of porous solids that render them useful for water treatment include high porosity and surface area as well as the physical and chemical nature of the internal adsorptive surfaces. In order to identify whether a solid has potential application in water purification by adsorption, numerous tests (standard) are performed that are used in its characterization. This section describes the principal methods of characterizing porous solids and quotes figures for some commonly used adsorbents. The properties quoted are helpful in specifying adsorbents in terms of their general performance or their ability to perform specific tasks. 8.1.3.1. Surface area This is perhaps one of the most informative tests performed to identify the usefulness of an adsorbent in wastewater treatment. The surface area is expressed in square meters per gram and is usually measured by the B.E.T. method (Brunnauer et al., 1938). When a gas is physically adsorbed (condensed) on the surface of a solid, the quantity adsorbed varies with pressure, as shown in Figure 8.2. The B.E.T. equation was developed to permit the measurement of Vm the volume of gas necessary to form a monolayer of liquid across a solid surface. The equation is

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x 1 (C  1) x   V (1  x) Vm Vm where x is the relative pressure, P /Pº, at which a gas of volume V (m3 at S.T.P.) is adsorbed, P is the pressure of the gas, and Pº is the saturated vapor pressure at the temperature of the vessel holding the adsorbent. Vm is the volume of gas required to form a monolayer on the adsorbent at the system temperature, and C is a constant. Therefore, a plot of [x/V(1-x)] against x should be linear, with slope [(C - 1)/Vm] and intercept (Vm)-1. Determination of these values from the plot gives two simultaneous equations, enabling the calculation of Vm. Linear plots of [x/V(1-x)] against x are usually observed over the relative pressure range 0.05 P/Pº to 0.30 P/Pº, although exceptions to this are known. Equation 8.1 is a generalization of the Langmuir adsorption isotherm applied to multilayer adsorption. The theory is based on the establishment of an equilibrium between gas and adsorbed material involving the dynamic transfer of molecules between the gas phase and the surface. The B.E.T. method yields a value for the volume of a monolayer of adsorbed gas on the solid surface. The surface area of one molecule of adsorbed gas (adsorbate) may be calculated from its density and molecular weight. Using this datum the total surface area of the solid may be calculated. In general, good adsorbents will have surface areas in the range 100 to 1000 m2g-1. However, this method, although very useful, is not wholly definitive, and misleading values are occasionally gained when the sizes of the smallest micropores are not great enough to allow admittance of a nitrogen molecule with cross-sectional area 16 Å2. This disadvantage is at least partially solved by using other gases such as carbon dioxide, which is able to access narrower pores than nitrogen. The total surface areas of solid adsorbents are most frequently characterized by carrying out the B.E.T. gas adsorption isotherm. Nitrogen gas is normally used since it is well characterized, but –as indicated above- other gases may be used. The volume, Vm, of nitrogen gas absorbed is determined and then the total surface area, A, of adsorbent can be calculated, since A = NA·Nm·Vm where NA is Avogadro's number and Nm is the number of moles of nitrogen adsorbed. 8.1.3.2. Porosity The particle porosity of a degassed adsorbent may be determined using a mercury porosimeter. This technique also provides information on the internal pore size distribution, but is limited to pore sizes with diameters greater than that of the mercury molecule. Mercury porosimetry is a powerful technique utilized for the evaluation of porosity, pore size distribution, and pore volume (among others) to characterize a wide variety of solid and powder materials. The instrument, known as a porosimeter, employs a pressurized chamber to force mercury to intrude into the voids in a porous substrate. As pressure is applied, mercury fills the larger pores first. As pressure increases, the filling proceeds to smaller and smaller pores. Both the inter-particle pores (between the individual particles) and the intra-particle pores (within the particle itself) can be characterized using this technique. The determination of pore size following the technique of mercury intrusion is based on the behavior of “nonwetting” liquids in capillary. A liquid coming in contact with a solid porous material and behaving as a non wetting agent (namely if the contact angle of the liquid with that solid

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material exceeds 90°) cannot be spontaneously absorbed by the pores of the solid itself, because of surface tension. However, this resistance to penetration can be won by applying an external pressure. Required pressure depends on the pore size. The relation between the pore size and the applied pressure, assuming the pore is cylindrical, is expressed as:

p

2 cos  r

This relation is commonly known as Washburn Equation. The Washburn Equation relates the applied pressure (p) to pore radius (r) using physical properties of the non wetting liquid (mercury in this case). The physical properties include the contact angle between the mercury and the ranging from approximately 1 psi up to 60,000 psi which correlates to measurement of pores from about 250 microns to 0.003 microns (3 nanometers). The contact angle of the mercury on the material under test is an important consideration for optimal results. The contact angle and the surface tension are usually assumed to be 140º and 0.48 N/m, respectively. The volume of mercury intruded into the sample is monitored by a capacitance change in a metal clad capillary analytical cell called a penetrometer. The sample is held in a section of the penetrometer cell, which is available in a variety of volumes to accommodate powder or intact solid pieces. Sample size is limited to dimensions of approximately 2.5 cm long by 1.5 cm wide. 8.1.3.3. Density When dealing with density, three different concepts are applicable. Firstly, the apparent density of an adsorbent is defined as the mass of a unit volume, including pores and spaces between particles. Secondly, packed density is normally determined by ascertaining the volume of a known mass of sample in a graduated cylinder after tamping shows no further shrinkage in volume. The data may be expressed in grams per cubic centimeter or kilograms per cubic meter. The packed density of a specified particle size range of adsorbent of specified moisture content is then calculated using the tamped volume occupied by a certain mass of adsorbent. Finally, the absolute densities of most common adsorbents are well documented in literature. The absolute density of an adsorbent may be determined using a calibrated specific gravity (relative density) bottle and solvent that is capable of penetrating the pores of the adsorbent and whose density itself is accurately known. The method is based on solvent displacement by an accurately weighed mass of adsorbent. The measurement of sample density by pycnometry is usually undertaken through two different analytical techniques, each providing fundamentally different information regarding sample density. Mercury pycnometry is used to measure the bulk density and skeletal density of a sample (also termed the envelope density or mercury displacement density) whereas helium pycnometry is applied to the measurement of the absolute density of a sample (also termed the true density or helium displacement density). These techniques may be combined for the measurement of the total pore volume of a sample. Mercury picnometry is performed using a mercury porosimeter. This may be performed alone or in conjunction with our mercury porosimetry analysis. The technique allows for the selection of the optimum mercury pressure for the calculation of sample density is selected, ensuring the sample is

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fully enveloped by mercury without intrusion to the sample pores. This ensures that the most accurate bulk density value is generated for any given sample. Two density values are reported for this test: Bulk density of the sample (also termed envelope density or mercury density) - includes the volume of all pores within the sample. Skeletal density of the sample (when in conjunction with full mercury porosimetry). Helium pycnometry is used to measure the absolute density of a material, and is performed stereopycnometer, yielding highly accurate and reliable density measurements. A variety of sample cells of differing volume are available to accommodate a wide range of sample types and sizes. The true density / absolute density, of a sample excludes the volume of the pores and voids within the sample. It is also possible to combine mercury pycnometry and helium pycnometry to calculate the total volume of pores within a sample. Hence, the total pore volume can be determined according to the following equation:

VT 

Where VT respectively.

1 1   Hg  He

are the total pore volume and the mercury and helium densities,

8.1.3.4. Electron microscopy The texture of an adsorbent may be examined using scanning electron microscopy (SEM). Such studies give an appreciation of the porosity of an adsorbent and hence a qualitative appreciation of its ability to adsorb materials in solution. The specimen is then rendered conducting by evaporation of a Au/Pd alloy, the vapor of which condenses on the surface of the sample. The adsorbent may then be placed in the evacuated microscope chamber.1 8.1.3.5. Physicochemical characterization For the physicochemical characterization of the adsorbents some routine techniques are used. In this section, some of these techniques will be briefly described. 8.1.3.5.1. Dye Adsorption-Methylene Blue Test To qualify for use, a particular adsorbent should be placed in contact with a synthetic test solution that reflects the application under consideration. Dyes are frequently used in this respect as they often constitute an undesirable part of wastewater effluent. One dye often used to characterize

Sorne adsorbents, e.g., silicates, may be susceptible to charging during scanning, causing the sample to "jump" or escape from the stage. In such cases a low accelerating voltage (about 10 kv) should be used to prevent darnage and deterioration of the sample.

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the capacity of an adsorbent is methylene blue. The test procedure, the methylene blue test, is as outlined (CITA). A sample, 0.10 dm3 of methylene blue is introduced into a 0.2 dm3 Erlenmeyer flask. The adsorbent to be tested, 0.15 g, is added and the flask shaken for 1 h at 25Â°C. This procedure is repeated for several other adsorbent dosages up to 0.59 g. The adsorbent-dye mixture is filtered through a 15-cm No. 5 Whatman filter paper under gravity, allowing all the solution to filter. To ensure homogeneity of the filtrates, the contents of the flask are stirred and the concentration of dye is determined. This concentration may be determined either by preparing standards and comparing the test solution visually or, as is now more practicable, by using a visible spectrophotometer. The results so obtained provide the distribution of dye at particular initial ratios of adsorbent mass to dye concentration and volume. In doing so, one may construct an adsorption isotherm. This procedure gives several pieces of information, including the amount of adsorbent required to remove effluent of a particular concentration and the total capacity of the adsorbent. 8.1.3.5.2. Iodine Sorption Value For the test with iodine, a stock solution is made up with 2.7 g of iodine and 4.1 g of potassium iodide in 1 dm3 distilled water. The iodine concentration should be determined before use, and if it falls below 2.65 gÂˇL-1 a fresh solution should be prepared. The method is outlined. The adsorbent, 0.5 g, being tested and 10 mL 5% HCl are placed in an Erlenmeyer flask, which is shaken until the sample is wetted. The iodine solution, 100 mL, is then added to the flask, which is agitated for 5 min. The mixture is filtered, under gravity, through a Whatrnan No. 5 filter paper. The filtrate is stirred to ensure homogeneity and a sample of the solution titrated with 0.1 N sodium thiosulfate solution using starch indicator. The above procedure is repeated using adsorbent dosages in the range 0.2 to 0.4 g per 100 mL iodine solution. A related yet distinct test often used as an alternative to the previous technique is described below. 8.1.3.5.3. lodine Number The iodine number may be defined as the number of milligrams of iodine adsorbed by 1 g adsorbent, when the iodine concentration of the residual filtrate is 0.02 N. A representative sample of adsorbent is crushed until 90%, or more, will pass through a 325-mesh sieve (by wet screen analysis). The sorbent sample is dried for a minimum of 3 h in an electric drying oven at 150Â°C, and then 1.000 g of dried pulverized adsorbent is weighed out. (See Note A). The sample is transferred into a dry, glass-stoppered, 250-mL Erlenmeyer flask. A 10-mL volume of 5% w /w HCl is added to the flask and stirred until the adsorbent is wetted. The contents are brought to the boil on a hot plate and maintained at boiling point for 30 s. After allowing the flask to cool to room temperature, 100 mL of standardized 0.1 N iodine solution is added to the flask. The flask is immediately stoppered and agitated vigorously for 30 s. The mixture is filtered by gravity immediately after the 30s shaking period through a Whatman No. 2 filter paper. The first 20 to 30 mL filtrate is discarded and the rest is collected in a clean beaker. The residue on the filter paper must not be rinsed. The filtrate is stirred in a beaker with a rod, and 50 mL of the filtrate is pipetted into a 250 mL Erlenmeyer flask. The 50 mL sample is titrated

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with standardized 0.1N sodium thiosulfate solution until the yellow color has almost disappeared. Approximately 2 mL of starch solution is added and the titration continued until the blue indicator color just disappears. The volume of sodium thiosulfate used is recorded. The iodine number is calculated as follows:

A  (2.2 B·N 1 mL of thiosulfat e solution used ) X  M weight of sample C

N 2 mL of thiosulfat e solution 50

Iodine Number 

XD M

where X/M mg iodine adsorbed per gram adsorbent N1

Normality of iodine solution

Normality of sodium thiosulfate solution

N1 X 12693.0

N2 X 126.93

Residual filtrate normality

A correction factor

Notes for iodine number calculation: A. The capacity of an adsorbent for any adsorbate is very dependent on the adsorbent concentration In the contactíng medium, and the adsorbate concentration must be specified or known, so that the appropriate factors may be applied to correct the concentration to agree with the definition. B. The quantity of sample used in the analysis is governed by the activity of the adsorbent. If the residual filtrate normality (C) is not within the range 0.008 to 0.035 N, as shown on an iodine correction curve, the analyses should be repeated using a different size sample. C. The accuracy of the test is dependent on the KI to I2 weight ratio being 1.5 to 1 in the standard iodine solution. Then the iodine correction factor (D) vs. residual filtrate normality (C) is plotted to obtain the correction factor.

8.1.3.5.4. Elemental Analysis i. X-Ray Elemental Analysis: The quantitative elemental analyses of adsorbents may be ascertained using an electron microprobe X-ray technique which involves comparing the intensity (number of X-ray photons emitted in unit time) of characteristic X-ray emission from the element of interest in the specimen with that of a standard of known composition. An iron reference material is normally used. The sample is mounted on an aluminum probe and coated with carbon before being placed in the evacuated microscope chamber. This

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method combined with S.E.M. or electron backscattering microscopy facilitates quantitative identification of elements as well as giving an indication of their distribution throughout the adsorbent. The technique is limited during normal use to the measurement of elements of atomic number 11 and above. ii. C H N Elemental Analysis: For the chemical characterization of carbonaceous adsorbents, a CHN analysis is used. The carbon, hydrogen, and nitrogen contents of the sample are determined from the quantities of CO2, H2O, and NO2 produced by the combustion of the dried solid adsorbent in oxygen using an automatic elemental analyzer interfaced with a computer. iii. Surface Group IR Analysis: Surface groups may also be characterized by IR spectroscopy (CITA). This method has been used this method to characterize carbonaceous materials, but it has also been applied to more homogeneous adsorbents such as silicates (CITA). Infrared spectra may be measured on potassium bromide disks as follows: 2.0 mg of each sample, previously dried in a vacuum desiccator containing phosphorus pentoxide, is ground with 0.7 g potassium bromide and is again dried in the vacuum desiccator at 60°C for 3 days. The mixture is then pressed at 5 ton/cm2 under vacuum for 20 min. The disk thus obtained is dried again in a vacuum desiccator at 60°C for 3 days. The dried disks are examined on a double-beam IR spectrophotometer. Functional groups on the surface of adsorbents often play an important role in the adsorbent-adsorbate interaction. Polar compounds may be accommodated by carbonyl or hydroxyl functionalities; metallic dyes may be bonded by amine groups and nonpolar adsorbates bound by surface aliphatic or other nonpolar groups in nature. 8.1.3.5.5. End Group Titration (Cation Exchange Capacity) Many adsorbents are used to treat wastewater containing metals. These adsorbents often work as a result of their good cation exchange capacities. This is a measure of the total amount of exchangeable cations that can be held by the adsorbent, expressed as milliequivalents per 100 grams air-dried material sorbent (CITA). This procedure is outlined. A sample, 2.00 g, of the air-dried ground sorbent is thoroughly mixed and placed in a 300-mL Erlenmeyer flask. Approximately 100 mL 0.5 N HCl is added and the flask is stoppered and shaken using a mechanical stirrer for 30 min. The mixture is filtered through a fluted, rapid filter paper in a large powder funnel. The residue is washed with 100 mL distilled water until a 10-mL wash shows no precipitate with around 3 mL 1% w/w silver nitrate solution. The filtrate is discarded. The moist adsorbent is immediately transferred to a 300 mL Erlenmeyer flask using about 100 ml 0.5 N Ba(OAc)2 to wash the filter paper. The flask is stoppered and shaken mechanically for 15 min. The mixture is filtered and washed with three 100-mL portions of distilled water. The residue is discarded and the washings titrated with 0.1 N NaOH, using 5 drops phthalein, to first pink color. The exchange capacity is calculated as follows:

mequiv  (mL · normality NaOH · 100 / g sample) 100 g air  dried adsorbent

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8.2. Types of Adsorbents Although the concept of applying the adsorption process to treat contaminated liquids and gases has been realized for many years, adsorption remains one of the more novel chemical engineering processes. Part of the reason for this relates to the increasing understanding of the role of surface chemistry and the physical properties of the adsorbent materials in the sorption processes. Adsorption and ion exchange from aqueous solutions are important processes in water purification, mineral beneficiation, soil conservation, and many other areas, as reflected by the ever increasing range and types of adsorbents that are employed. Much attention has now been given to the fundamental characterization of adsorbents, especially to the porosity, surface area, and surface charges. Such information in turn leads to the development of new and higher qualities of adsorbents and insight on how such qualities will be affected by the processes and conditions of manufacture of the adsorbents. This relation between the manufacturing processes and the eventual quality of the adsorbent will lead to further development and application of adsorption processes in effluent treatment. Solid surfaces are a field of growing interest in applied areas as different as environmental photochemistry, drugs photochemistry, biocompatibility, composite materials, corrosion, adsorption, dyeing and lightfastness of dyes on fabrics, heterogeneous catalysis, among many other applications. Microporous solids play a special role due to their large area/volume ratio providing the opportunity for adsorption of large amounts of guest molecules on very small amounts of substrate, due to the very high specific area of the adsorbent. In particular, powdered solids with controlled pore size solids may have specific uses such as molecular sieves, microreactors of controlled microsize, or catalysts. The surface-probe interaction may simply be of electrostatic nature, of hydrogen bonding, or in terms of an acid or basic behavior (BrĂ¸nsted or Lewis). Substrates may also interact with the adsorbed probe as electronically active supports and have an important role in redox processes (as in advanced oxidative processes for persistent pollutant destruction) [1-6]. It is possible to develop and manufacture adsorbents for specific adsorption and ion exchange dependent upon the particular application. Flow hydraulics, pressure drop, and fluidization and elutriation of the bed must be considered. Some of the main adsorbents in commercial and laboratory use include activated carbons, decolorizing carbons, bone char, alumina, silica, bauxite, bentonite, Fuller's earth, molecular sieves, peat, lignite, chitin, chitosan, and ion exchange resins. In this section some examples of the adsorptive properties and uses of these adsorbents will be outlined and discussed.

8.2.1. Activated carbon Activated carbons, and carbons in general, are one of the oldest and most widely used of adsorbents. Indeed, one of the earliest known uses of carbons for purification purposes dates back to the early Egyptians. However, the real modern history and role of carbon and activated carbons as adsorbents did not begin until the 18th century, when physicists and chemists began to take an interest in the subjects of gas adsorption and the decolorization of liquids. There is an increasing market trend for the use of activated carbons. This is perhaps reflected by the current widespread interest in the manufacture and use of activated carbons made from coals and a range of other raw materials such as peat, lignite, coconut shell, hardwoods and other carbon-containing materials.

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Granular activated carbons (GAC) have been widely applied in the treatment of wastewater. Most carbonaceous materials are porous and have an internal surface area of around 10m2路g-1. The activation process greatly expands the internal surface structure and consequently the carbons will acquire an internal surface area of some 1000 m2路g-1. The total world production capacity of activated carbon is presented in Table 8.5. Table 8.5.- World Production of Activated Carbon

Countries

Tonnes

U.S.

150,000

Holland

35,000

Germany

20,000

U.K.

16,000

France

12,000

Belgium

10,000

Philippines

15,000

Japan

30,000

India

5,000

Italy

4,000

Sri Lanka

3,000

Taiwan

1,000

The activation process consists of two distinct phases. The first phase, or carbonization phase, involves heating of the carbon source up to temperatures of 600-900掳C in the absence of air. This carbonized char is then activated by different physical and/or chemical agents at a temperature below that used in the previous carbonization stage. During the activation processes the properties of the new adsorbent carbons can be developed and controlled to produce carbons with special affinities. The activation process increases the surface area and develops porosity. Activated carbon is perhaps one of the most widely used adsorbents in industry. Its use can be broken down into discrete areas such as effluent treatment, potable water treatment, solvent recovery, air treatment, decolorizing, metal ores processing, and many more general domestic applications. The EC directive on drinking water standards puts forward tight restrictions on levels of contaminants in water. Many water authorities are using activated carbon as part of the chemical treatment processes. Powdered activated carbon and granular carbons are used to improve taste, odor, and color. Once again, European directives and local authorities are imposing tighter restrictions on emission by industry and commerce to the environment. Active carbon adsorption systems are in the forefront of the battle to reduce emissions to the atmosphere and to the watercourses. Many organic compounds can be adsorbed onto activated carbons. The presence of small concentrations of organics such as phenols can manifest themselves in many ways. Although carbon adsorption cannot remove all organics, it has emerged as an efficient process for removing undesirable constituents. The removal of phenols, chlorophenols, and nitrophenols is well established (CITA).

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The adsorption of phenolic compounds by granular carbon is extremely rapid; approximately 60 to 80% of the ultimate adsorption occurs within the first hour of contact. The adsorption of benzene onto activated carbon has been reported (CITA) attaining of equilibrium after only 3 h of contact. Using benzene water systems desorption does occur but with a reduced driving force. The adsorption of basic dyes onto granular activated carbon has also been reported (CITA) showing the competitive nature of the adsorbent carbon for the three different basic dyes in single- and multicomponent mixtures. Active carbons are used to decolorize products such as sugar and alcohols to give aesthetic consideration to the product. CFCs and organic solvents can be adsorbed onto active carbons. Many industries use hydrocarbon solvents that can find their way into the atmosphere. Active carbons are effective at adsorbing most of the industrial solvents. On the domestic front, offensive odors and smells can be removed using active carbon filters.

8.2.2. Peat The application of peat to pollution control in water treatment systems has received increasing attention over the past years and currently offers a very attractive method of wastewater treatment. Besides being plentiful, inexpensive, and easily accessible, peat possesses several of the characteristics that make it effective for adsorption or ion exchange operations. Peatlands comprise a significant portion of the land surface in many regions of the world (Table 8.6), with many minable resources totaling over 1 thousand billion metric tonnes. The largest peat deposits are found in the Northern hemisphere, particularly in the Commonwealth of Independent States (CIS), but significant reserves have been discovered in Brazil, Indonesia, and other subtropical regions (CITA). Peat is a young coal in the making. Peat falls in rank as one of the lowest grades of solid carbonaceous fuels (Table 8.7). Peat formation is the first stage of coalification and will take up to 10,000 years to complete. The process commences with the decomposition of trees and various plant species in the waterlogged environment of marshes, bogs, and swamps. Preserved by the water, the vegetation is slowly oxidized by microorganisms to form peat. This is the biochemical stage of metamorphosis which normally occurs in the first few meters of the earth, and hence peat is found with significant moisture content at depths of between 2 and 5 m. The precise nature and composition of the peat that is formed depends on such factors as the nature of the vegetation, the regional climate, the acidity of the water, and the degree of metamorphosis. Peat is a rather complex material containing lignin, cellulose, and humic acids as its major constituents. These constituents bear polar functional groups such as alcohols, aldehydes, carboxylic acids, ketones, and phenolic hydroxides, which can all be involved in chemical bonding with the absorbed pollutants. Peat, under microscopic examination, presents a cellular structure. Because of the very polar character of peat, the specific adsorption potential for dissolved solids such as metals and polar organic molecules is quite high. These properties have consequently led to the examination of the potential for peat to act as an agent for the purification of contaminated wastewaters. Peat is known to have excellent ion exchange properties similar to natural zeolites (CITA). The drainage of mining leachate through a white cedar bog has resulted in the complete adsorption of some trace metals and a significant reduction in the levels of other metals.

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In addition to studying the natural constituents of peat bogs, considerable attention has been focused on the potential of peat as a commercial adsorbent for the removal of toxic metals in contaminated wastewaters. As far back as 1939 the potential of peat as an ion exchange medium for metals such as copper, zinc, lead, and mercury was realized. Table 8.6.- World Resources of Peat

Table 8.7.- Technological Properties of some carbon fuels

Fuel

% Moisture % Carbon (daf) Calorific Value, kJ kg-1

Peat

>75

50-60

7,000

Lignite

30-70

60-70

17,000

Bituminous coal

36,000

Anthracite

37,000

Peat is capable of removing calcium and magnesium ions from wastewaters, and cation removal is due to a weak ion exchange mechanism. In batch and column studies peat has been found to be an efficient means of removing mercury from water (CITA). Peat would also be an effective

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material for removing other metals such as cadmium, zinc, lead, copper, iron, nickel, chromium, and silver. Peat can compete with other adsorbents to remove metals, provided that low flow rates are maintained and that the quantities of water to be treated are small. Furthermore, experimental studies have revealed a much lower sorption capacity for metals than for dyestuffs and a much more rapid attainment of equilibrium. The nature of metal binding in soil organic matter, including peat and lignite, has been extensively investigated, and a common viewpoint on the exact mechanism of adsorption is yet to be reached. Additionally the comparison of results is very difficult because pretreatment methods vary among authors, and the type of peat (and to lesser. extent, lignite) investigated will have a significant effect on the ion exchange capacity (CITA). There is a good correlation between the content of unesterified polyuronic acids in the cell wall of sphagnum peat and the cation exchange capacity. The adsorption of copper and zinc ions occurs by the formation of complexes with the carbonyl and nitrile groups in peat. The presence of humic acids in peat is primarily responsible for the ability of peat to adsorb metals. The reaction of metal ions, such as Cu and Fe, with humic acids is one of chelate ring formation involving adjacent aromatic carboxilate -COOH and phenolic -OH groups or, less predominantly, two adjacent -COOH groups, which participate in ion exchange reactions by binding metal ions with the release of H+ ions. However, the adsorption processes could not be solely explained by the formation of humic acid complexes. Humic acids in their soluble form are responsible for the fixation of metals, but in the solid form have quite different properties and can play only a very minor part in the adsorption process. Authors differ in their theories concerning the mechanism of adsorption. They do, however, all appear to be in agreement on one issue: the rate of adsorption is rapid. Moreover, all authors discussing the subject agree that the natural capacity of peat to retain cations is related to the pH of the solution. In fact pH is critical to the adsorption process (CITA). Other authors (CITA) concluded that the percentage of metal extraction from solution onto peat varies from 0% to almost 100% within 4 to 5 pH units. The pH of the system should be lower than 8 for adsorption to be significant, but a pH greater than 3.0 is required to prevent the metal ions from being exchanged by hydrogen ions resulting in leaching or stripping of metals from the peat (CITA). In the pH range 3.0 to 3.5, the removal of most metal ions from solution ceases. Optimum adsorption occurs within the range pH 3.5 to 6.5, and there is little difference in the amount of sorption of copper ions onto peat moss within this pH range (CITA). Adsorption increases as the surface area of the organic substance increases (CITA). This explains the increase in adsorption capacity after humic acids were extracted from the peat and lignite. Increasing metamorphism in natural carbonaceous deposits, i.e., peat > lignite > coal, results in compaction, which produces a decrease in the surface area and a corresponding decrease in adsorption capacity. Peats contain extensive porosity and, consequently, a large internal surface area, which facilitates access of the cations to carboxylic and phenolic functional groups. 8.2.2.1. Modified Forms of Peat and Lignite Some studies have pursued the development of peat and lignite to their maximum potential as adsorbents of metals. This has been achieved by physical and chemical pretreatment of the peat and lignite prior to contact with metal-bearing wastes. A number of authors have observed that peat and lignite exhibit enhanced cation exchange capacities after treatment with phosphoric and/ or sulfuric acid (CITA).

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The development of activated carbons from peat and lignite would seem a natural progression following the success of coal-based activated carbons. Indeed lignite-derived activated carbons are commercially available, whereas peat-based carbons are a relatively new idea. 8.2.2.2. Recovery In the areas of recovery of metals from peat and lignite and regeneration of spent adsorbents, the literature survey produced very little information. Many authors have observed the effect of low pH (<3) in stripping metals from peat, but have failed to connect it with possible regeneration. This may be because recovery and regeneration were not as economically or politically attractive in the past as they are now. After several cycles of adsorption, elution, and washing with distilled water, it was found that the ion exchange capacity of the peat remained within narrow limits during repeated Cu2+ adsorptions; thus the cations adsorbed can be released with a small volume of acid and the peat adsorbent repeatedly used.

8.2.3. Lignite Lignite is a member of the solid fuel family and is often referred to as brown coal. Table 5.3 shows that lignite fits between peat and bituminous coals. It is drier than peat and has a higher carbon content. Lignite and coal are formed from decaying trees and plants. The transformation occurs in two stages, biochemical and geochemical. The decaying vegetation falls to the ground and, if it is preserved by water, is slowly oxidized by microorganisms and peat is formed. The microorganisms principally consume the cellulose that is present. This is the biochemical stage of the transformation (geologists use the term diagenesis or, more specifically, coalification instead of transformation). Peat becomes lignite and coal when it becomes buried below the earth's surface. This is the geochemical stage of the transformation. As one proceeds toward the center of the earth, the temperature increases by about 3째C per 100 meters unless, of course, special features such as volcanoes are present, when the rise in temperature is more rapid. It is this heat which gradually dries out the peat and causes the chemical changes that result first in lignite and then ultimately in coal. Coal formation requires the vegetation to reach temperatures of about 150째C, so the peat must become buried to a depth of around 5 km. Lignite formation needs a lower temperature, and this is the reason why lignite is usually found fairly close to the surface and can be recovered by opencast methods, whereas coal normally is buried deeply and has to be mined underground. During the transformation into lignite, the vegetation has lost hydrogen and oxygen atoms, and the lignite now contains 65 to 70% carbon, about 5% hydrogen, and 25 to 30% oxygen (by weight). Much of the oxygen is present in acidic groups. Lignites contain a little paraffinic material-and generally some waxes and resins-but mainly they are composed of aromatic structures based on benzene and naphthalene rings. Many different structures are present. Lignites have a rich chemistry; they appear to consist of a rigid, complicated lattice in which molecules are trapped. Lignite possesses adsorptive properties. The surface area, porosity, functional groups, and calcium and magnesium ions all contribute to these properties. Lignite has a strong affinity for basic dyes.

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The lignite-cationic dye sorption process is facilitated by the presence of the humic acid groups in the lignite. The adsorption process is a function of the dye structure and the lignite structure. Different ions will experience different physical and electrical attractive and repulsive forces according to their structure, molecular size, and functional groups. The process is a combination of ionic attraction/ repulsion, hydrogen bonding, ion-dipole forces, covalent bonds, and van der Waals forces. The adsorptive powers of lignite have been shown to reduce the COD of an effluent. Lignite treated with 50% sulfuric acid at 100°C is partly activated. This activated lignite is claimed to reduce the COD of slaughterhouse wastes by up to 60%. Lignite and lignite-derived activated carbons also exhibit adsorption of organics such as benzoic acid and chlorobenzoic acids. Lignites treated with calcium phosphate show enhanced adsorption of the organics. The retention of metallic cations such as copper, cadmium, zinc, strontium, lead, uranium, and thorium by lignites is well established. The humic acid can exert a strong concentration effect upon heavy metals and lignite and therefore may have a future in waste disposal problems associated with the nuclear industry.

8.2.4. Molecular sieves / Zeolites One of the most widely used of the synthetic adsorbents is the range of zeolite metal aluminosilicates known as molecular sieves. These are synthesized adsorbents that have exact pore sizes caused by the driving off of water molecules by heating. The zeolites consist of porous aluminosilicate frameworks of SiO4 and AlO4 forming polyhedra. The gaps in the framework lattice permit the passage of molecules through to the body of the framework. The sieves can separate according to molecular size and by molecular polarity. Molecular sieves can therefore be tailormade for particular applications. They are used for dehydration of gases and liquids and the separation of gaseous and liquid hydrocarbon mixtures. Molecular sieves can be regenerated by heating or by elution. Due to the catalytic potentialities and hosts’ unique characteristics, zeolites are among the most versatile powdered solids. They host various molecules including organics in their cavities and channels, and such inclusions have often been shown to modify the normal solution photochemistry of a given species. The confinement of molecules within the constrained space and the catalytic activity of the surface adsorption sites were considered decisive for product distributions unique to zeolites. Zeolite science covers a broad spectrum, from synthesis to structural characterization, adsorption and transport phenomena, and applications such as catalysts and other classical uses (adsorbents or desiccants, water softeners, soil improvers, and so forth) [1]. 8.2.4.1. Structure of zeolites Zeolites (from the Greek words zeo, "to boil" and lithos "stone") are synthetic or natural minerals that often expel water so violently when heated that they appear to boil [7]. Zeolites are microporous crystalline aluminosilicate materials, possessing a tri-dimensonal framework of [SiO4]4− and [AlO4]5− tetrahedra, linked to each other at the corners by sharing their oxygens [1].

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M O _ Al O

Hydrophilic

Si O

M _ Al

Hydrophobic

Hydrophilic

Figure 8.1. - Constitution of an aluminosilicate zeolite framework [7].

Each AlO4- unit bears a net negative charge which must be compensated by an appropriate number of cations, usually mono- or dipositive ones, occupying various sites within the framework. These cations are generally mobile, because they are ionically and not covalently bound to the framework structure. To maintain electrical neutrality there must exist a 1:1 relationship between the Al content of the framework composition and the number of positive charges provided by the mobile cations [7]. The presence of these charge compensating cations determine the hydrophilic character of the zeolite framework, i.e., the greater the number of Al atoms, the greater the number of cations and sites capable of adsorbing water. Therefore, the Al content of an aluminosilicate and the Si/Al ratio significantly influences the zeolite properties. The Si/Al ratio may vary from Si/Al = 1 (zeolite X) to Si/Al = â&#x2C6;&#x17E; (silicalite). Typically, zeolites with small Si/Al ratios are strongly hydrophilic; zeolites with large Si/Al ratios or dealuminated are hydrophobic [8]. In contrast to amorphous silica or silica-alumina, which lack long-range ordering, zeolites are crystalline; their tetrahedra are spatially arranged in strictly regular fashion. The building blocks are repeated in the three principal directions of the unit cell. The zeolite frameworks contain pores, channels, cages and interconnected voids. The combination of the topology of the internal void space and the chemical characteristics of the internal framework structure provide chemists with "designer microscopic reactors" in which chemical reactions can be performed on molecules adsorbed on zeolites. The ability of zeolites to selectivity adsorb molecules based on size/shape selectivity rules has led to their designation as "molecular sieves"[7]. Several types of zeolites are frequently employed and their common names are X, Y, A, mordenite, L, ZSM-5, silicalite, MCM-41, to name a few [1]. Important families of zeolites are usually considered. Falling near the extremes of the hydrophilic and hydrophobic classes we have the faujasites and the pentasils, respectively. Both families enjoy an enormous and wide usage in the chemical industry as catalysts [7, 8]. Among the most famous and useful members of the faujasite and pentasil families of zeolites are the classes termed X and ZSM, respectively. In X zeolites the Si and Al tetrahedra are linked together to form a cubooctahedron - sodalite which is the building block for the faujasite zeolites. The sodalite cages are connected via the double 6-membered ring (D6R) frame as portrayed in Figure 2. Besides X zeolites with a Si/Al ratios: 1.0 < Si/Al < 1.5, another important faujasite zeolites are Y zeolites have different framework Si/Al ratios, 1.5 < Si/Al < 3 for zeolite Y [1].

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Sodalite cage

D6R

Supercage

Figure 8.2. – Faujasite zeolites NaX, NaY. Faujasite supercavities constructed of sodalite cages and hexagonal prism subunits; the charge compensating cations occupy different crystallographic positions, designated by Roman numerals [8].

For example, NaX refers an X zeolite which contains Na+ cations; therefore NaX corresponds to the faujasite type with Si/Al of ca. 1.5, i.e., NaX is a strongly hydrophilic zeolite. The important topological characteristics (size/shape characteristics of the internal void space) of NaX are the relatively large cavities (roughly spherical "supercages" of ca. l3Å diameter) which are connected by pores (roughly circular "windows" of ca. 8Å diameter). These "supercages" and “windows" constitute the internal surface of NaX zeolites and may be represented topologically as a sequence of periodic "supercages" and "windows" extending in three dimensions [7]. In ZSM class the Si/Al ratio is generally high (≥ 20) [7]. Pentasil units (5-membered ring) are building blocks for the channel- type zeolite ZSM-5. ZSM-5 and its dealuminated analogue silicalite possess a structure consisting of two intersecting channels systems, with the vertical channel being elliptical (5.75 Å × 5.15 Å), whereas the horizontal channels have nearly circular (5.40 Å × 5.60 Å) openings [1, 7], represented in Figure 3.

Elliptical Channels 5.75x5.15Ǻ

Circular Channels 5.40 x5.60Ǻ Figure 8.3. – Schematic representation of ZSM class of zeolites (ZSM-5 or silicalite) [7, 9].

On the other hand, another channel type zeolite, zeolite L has a unidimensional channel system as in the mordenite, formed by 6- and 4-membered rings, depicted in Figure 4.

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Figure 8.4. - Zeolite L: a) Unidimensional channel system formed by 6- and 4-membered rings; b) Schematic representation of zeolite L [1].

A three-letter framework type codes were assigned by the Structure Commission of the International Zeolite Association (http://www.iza-structure.org/) to classify the zeolites. According, X and Y belong to FAU (faujasite), L to LTL and ZSM-5 and silicalite to MFI [1]. Zeolites can be classified as small-, medium-, and large pore, depending on the number of O atoms (8, 10, or 12) defining the pore apertures [9]. Many of the limitations to incorporating large organic species inside a solid host of a narrow pore size distribution have now been overcome since novel mesoporous (extra-large-pore) channel-type aluminosilicates, the M41S family of mesoporous molecular sieves have been reported [10]. Among them, the most widely used is MCM-41, whose internal voids are formed by parallel hexagonal channels, with uniform pore distribution, high surface area and long range ordering which are potentially important in catalysis involving large molecules [9]. Besides the pore dimensions, the topology of the internal voids is very important characteristic of zeolites (Figure 5). In this regard, zeolites can be divided into tri-, bi- and monodirectional materials. Faujasites X and Y are typical tridirectional zeolites; their internal space consists of almost spherical cages (“supercavities”), 13 Å in diameter, interconnected tetrahedrally through four smaller round openings of 7.4 Å diameter. ZSM-5 is a bidirectional zeolite; its structure is formed by two systems of oval channels, one straight (5.2 x 5.7 Å), the other sinusoidal (5.3 x 5.6 Å), crossing each other at right angles. Mordenite can be considered a monodirectional zeolite; its channels (12-oxygen opening) are parallel and arranged like a honeycomb [8, 11].

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Pentasil (silicalite and ZSM 5): bidirectional; medium pore (5.4x5.6 Å 2); 10-oxygen opening

Faujasite (zeolites X and Y): tridirectional; large pore (13 Å); 12-oxygen opening

MCM41: unidirectional; mesoporous (20 Å)

Mordenite: unidirectional; large pore (7.4 Å); 12-oxygen opening

Figure 8.5. – Topologies of the internal voids of zeolites (based on reference [9])

Active sites in zeolites Zeolites contain Brønsted as well as Lewis acid sites. The bridging Si ― OH · · · Al group is usually referred to as a Brønsted acid site in zeolites [1]. The three-coordinated aluminum sites on the framework and non-framework Al sites are normally considered to be Lewis sites [1, 8].

Bronsted Acid Site O

_ Al

Lewis Acid Site

H+

Si O

_ Al

Al O

Figure 8.6. - Schematic representation of Brønsted and Lewis acid sites in zeolites [1]

Additionally, charge-compensating cations act as Lewis acids, while the framework oxygens represent a base. In particular, the oxygens adjacent to Al (Si–O–Al oxygens) are more basic because of a larger negative charge on oxygen. The Lewis acidity is usually connected to an electron-accepting property, and the basicity to an electron-donating property. Several studies performed in NaZSM-5 zeolites with different Si/Al ratios showed an increase in the electron-

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accepting ability with the increase of Al content [12]. Thus, dealuminated zeolites are not able to produce radical cations [8]. Therefore, the zeolites behave both as electron donors and as acceptors of moderate strength to the guest species, depending on the adsorption site. Remarks The reason for the zeolites’ success in catalysis is related to the following specific features of these materials: 

They have very high surface area and adsorption capacity;

 The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials;  Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application;  The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 Å);  Their intricate channel structure allows the zeolites to present different types of shape selectivity, i.e., product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions [2]. Adsorption from liquids into molecular sieve zeolites is finding increased applications in separation and purification processes. The separation by adsorption of liquid hydrocarbon mixtures and fructose/ glucose mixtures by simulated moving bed applications has been reported. Silicalite is a microporous crystalline silica that has received attention in both gaseous and liquid applications. The hydrophobic nature of silicalite has led to its use in the separation of ethanol from dilute aqueous solutions. The adsorption of methanol, ethanol, acetone, toluene, and cyclohexane in silicalite crystals has also been investigated. The use of pressure sieving adsorption for air separation has grown in popularity. Compressed air is fed to a combination adsorption/ desorption cycle system. During the adsorption phase, nitrogen is preferentially adsorbed from the air resulting in a stream enriched in oxygen and argon. Molecular sieves are giving good performance over wider temperature ranges (CITA). 8.2.5. Silica gel A very common adsorbent is silica gel. It is used mainly in granular form. It is made by heating an acidified sodium silicate gel to about 350°C. Silica gel is a hard, white, glassy, highly porous, granular solid. Silica is used chiefly for dehydration of air and other gases, but does find application in decolorizing operations. When used as a drying agent, the silica gel can be regenerated by evaporation of the adsorbed water. Silica can adsorb organics such as toluene and xylene from heptane in single- component and binary systems (CITA). Sorbsil silica gel is reported to adsorb basic dyes (CITA).

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8.2.6. Activated alumina This adsorbent is manufactured by heating a precipitated mixture of alumina mono- and trihydrates up to 400°C. The heating process drives off the moisture leaving an open porous structure with a high internal surface area. Alumina is widely used as a desiccant and can be regenerated by evaporation of the adsorbed species. Alcoa S-100, a commercial transition alumina, is treated with NaOH solution and dried overnight at l10°C. This activated alumina can adsorb hydrogen chloride gas from a nitrogen carrier. Ethylene dichloride is widely used in the petrochemical industry as a refrigerant in cooling systems and as a feed material for the production of polyvinyl chloride. During typical processing of the ethylene dichloride, it can become mechanically and thermally degraded. Low levels of hydrochloric acid, water, and iron chloride (ferric) can contaminate the ethylene dichloride. An alumina can be modified chemically to adsorb these contaminants. One such alumina is SelexsorbHCl. At 40°C, removal efficiencies for the three contaminants were 99.4% for HCI, 17.9% for H2O, and 98.2% for Fe (III). The alumina can be regenerated by methanol rinse or by thermal regeneration. 8.2.7. Chitin and chitosan Chitin was described for the first time by Braconnot in 1811, when he isolated a substance from fungi which he named "fungine". The first scientific reference to chitin was given from the Greek word "chiton", meaning a "coat of mail", for the material obtained from the elytra of May beetles. Chitin is the second most widely occurring natural carbohydrate polymer next to cellulose. Chitin is a long, unbranched polysaccharide and can be regarded as a naturally occurring derivative of cellulose, where the C2 hydroxyl group has been replaced by the acetyl amino group -NHCOCH3. The primary unit in the chain polymer is 2-deoxy-2-(acetyl-amino) glucose. These units are linked by 4) glycosidic bonds forming a long-chain linear polymer having degrees of polymerization around 2000 to 4000. It is insoluble in almost all solvents except strong mineral acids, notably nitric and hydrochloric and certain other solvents, due to the presence of the carbonyl functionality in the acetyl group. The carbonyl group is responsible for hydrogen bonds, resulting in a rigid structure, less permeable to water and other reagents, as in the case with cellulose. Chitin is widely distributed throughout nature, being the main structural polysaccharide that forms the characteristic exoskeleton of most of the invertebrates. High concentrations of up to 85% are found in Arthropoda (especially the edible crab), which are particularly able to synthesize chitin (CITA). Chitin does not naturally occur as a free molecule as described, but is covalently cross-linked to other molecules which are of great importance to the final form and function of chitin. They include proteins, pigments, and minerals. The major derivative of chitin, collectively named "chitosan" by Hoppe-Seiler (1894), was discovered by Rouget in 1859. However, a good deal of fundamental research on chitin and chitosan occurred in the next century, with most of the information available today arising since 1950. Chitin is found in conjunction with other components such as calcium carbonate and proteins. Chitosan, however, can occur free in nature in the cell walls of certain fungi. However, these sources are not yet utilized as a potential source. Chitosan is obtained predominantly by the alkaline deacetylation of chitin obtained from crustacean shells.

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Chitin is generally accepted as the linear polymer composed of 2-acetamide-2- deoxy-D-glucose (N-acetylglucosamine) units. The term "chitosan" refers to a family of polymers derived from chitin by deacetylation. The process is called N-deacetylation, which removes the acetyl radical (-COCH3) from the amino group (-NH-) of most of the glucose rings to which it is attached. Chitosan is an aminopolysaccharide and consists of [(1ď&#x20AC;˘4)-2-amino-2-deoxy- -D-glucan] monomer units. Like chitin it is nontoxic and biodegradable and has a molecular weight (about 120,000) depending on the source of chitin and the severity of N-deacetylation reaction. It is commercially produced by the chemical modification of chitin by alkaline hydrolysis. This cleaves the acetyl group leaving the free amine, and, depending on controllable process parameters, varying degrees of deacetylation can be achieved. There are numerous methods for deacetylating chitin, and a number were outlined by Muzzarelli (1976). The systematic repetition of the alkali deacetylation procedure produces chitosan with degrees of deacetylation up to 90%, but also leads to hydrolysis of the glycosidic linkages and a much reduced molecular weight. Adsorption of metal cations (e.g., lead , chromium (III) and (VI), nickel, copper, zinc, and cadmium) is one of the most attractive functions of chitin and chitosan. The mechanism of complex formation of metals with chitosan is manifold and is probably dominated by different processes such as adsorption, ion exchange, and chelation under different conditions. Adsorption studies indicated that boundary layer resistance as well as intraparticle diffusion could be a rate-controlling step. In such studies it is of utmost importance to work with well characterized chitosan, so that one knows the degree of deacetylation and the distribution of acetyl groups along the molecule and throughout the chitosan particle. Spectroscopic studies have shown that different complexes exist depending on pH. Mechanisms for the binding of metal ions to chitosan are based on the acid-base equilibria and related properties. The chelation of transition metal ions by chitosan has been demonstrated to be due mainly to macroscopic consequences such as the separation of metal ions by chelation chromatography (CITA). The chelating property is due to the presence of the amine groups in the polymer chains. These donor groups are capable of combining with a metal ion by donating a lone pair of electrons, thus forming dative coordination bonds. Chelation with chitosan is accompanied by a visible color change, i.e., green for nickel, blue for copper, pink for chromium, and yellow /brown for cadmium. Some color change may occur in chitin, but this is not as intense as that in chitosan due to the greater amount of amine groups present involving nucleation and growth of modules on the polymer surface. In solution, chelating anions are proton acceptors, and protons compete with metal ions for the anions. Hence, if the test solution contains a sulfate anion it will enhance metal ion collection, while chlorides depress the collection. If HL represents the un-ionized form of chelator, the overall equilibrium with a divalent metal ion may be represented as: M2+ + 2HL ď&#x20AC;˘ [ML2] + 2H+ Control of solution pH solution is an important factor. At higher pH's this equilibrium is shifted to the right so that the metal uptake is increased significantly. Similarly at low pH the chelation of metals by chitosan is poor. This pH effect was also found to apply to the desorption of copper from copper-chitosan complexes, which is substantial in fresh water, but greatly reduced in the high pH conditions of sea water. The performance of Chitoplex, which is the cross-linked form of N-carboxymethyl chitosan has been investigated (CITA). Chitoplex is a chitin-based polysaccharide in the form of a rigid granular gel. It enhanced the chelation capacity, while depressing swelling and chain association. Derivatives are, however, often more expensive to use and from a cost/benefit viewpoint less

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interesting for bulk applications. Another aspect of derivatization is the fear of residual amounts of unreacted compounds, in case these could create undesired effects. Other authors (CITA) have reviewed the complex binding of metals onto membranes. This new technique was developed for coating expanded areas of silica with a large surface area of chitosan. Preliminary results indicate very high complex binding capacities when simulating removal of metals from drinking water by filtration purification. Higher demand in reduction of harmful metals from wastewater to protect the environment and increased skepticism toward the use of synthetic flocculants make chitosan interesting in removal of heavy metals and radioactive isotopes and in the recovery of valuable metals. The use of chitosan as a metal sorbent has been well studied. Favorable sorption may be achieved by buffering the system. This is required because chitosan has the ability to sorb protons. It was found that, when chitosan was contacted with an unbuffered aqueous metal solution, a shift to a higher pH was observed. In the case of the lead system, the initial pH of the chitosan-metal solution shifted from pH 4.9 to pH 6.32 within the first 30 min of contact. A fine, white precipitate was formed because a pH greater than pH 6.0 leads to the formation of lead hydroxide. Also at the higher pH levels the metal ions will essentially act as Lewis acids, thereby increasing the initial concentration of the metal solution, hence the metal hydroxide is more easily formed. The final metal ions used in this study were Cu2+,Zn2+,Cd2+, and Cr3+. The pH of the metal solutions after contact was kept below the pH at which the metal hydroxide was formed. Thus the removal of metal ions was due to the chitosan and not due to precipitation. Where possible, the metal sulfate salt was used, as chitosan has an enhanced capacity for transition metal ions in a sulfate environment. This may be explained as increased crystallinity in the polymer due to the formation of chitosan sulfate. Chitosan has almost three times the capacity for copper than that of cadmium or chromium. These effects have proved complex to interpret, but are a function of a number of parameters: ionic radii; ionic charge; electron structure and, possibly, some hydration capacity for the metal ions; solution pH and nature; and availability of sites for chitosan. The collection rates of metal ions onto chitosan have been reported (CITA) to follow the trend Cu2+ > Zn2+ > Cd2+ as the same as the Irving and Williams series, and there was a relation between the order and the second ionization potentials. Chitosan can also adsorb pesticides (CITA). Significant uptake was observed for acidic pesticides on chitosan. An initial rapid uptake was followed by a decrease until equilibrium was reached after 24 h. 8.2.8. Other adsorbents 8.2.8.1. Bagasse pith Bagasse pith is a waste product from the sugar refining industry. It is the name given to the residual can pulp remaining after the sugar has been extracted. Bagasse pith is composed largely of cellulose, pentosan, and lignin. Pith does exhibit adsorptive properties. 8.2.8.2. Decolorizing Carbons Decolorizing carbons are made by carbonizing an organic material such as vegetable matter, sawdust, lignite, and other coals. The organic material is charred in the absence of air and

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sometimes in the presence of an inorganic material such as calcium chloride or pumice. They are used for a variety of purposes, including decolorizing solutions, cleaning of solvents, refining of oils, and metal recovery systems. 8.2.8.3. Fuller's Earths These are natural clays that contain magnesium aluminium silicates. The clay is given an open, porous structure by heating and drying processes. The adsorbent can be regenerated by washing and burning the organic contaminants that adhere to the clay surface. Fuller's earths find use in decolorizing and drying of oils.

8.2.8.4. Bentonite Bentonite is a clay that requires activation by acid washing before it exhibits adsorptive properties. It is normally used as a fine powder for decolorizing and clarifying liquids and is discarded after use. 8.2.8.5. Bauxite Bauxite is a naturally occurring alurnina that requires thermal activation to facilitate its use as an adsorbent. It can be reactivated by heating. 8.2.8.6. Bone Char This is obtained by the thermal destruction of bones at temperatures up to 900째C.

8.2.8.7. Synthetic Polymer Adsorbents Polymer adsorbents can be made from monomers such as styrene, divinylbenzene, and acrylic esters. The former two monomers result in adsorbents that have an affinity for nonpolar organics in aqueous solution, whereas the acrylic esters result in adsorbents with an affinity for polar salutes. The basic polystyrene cage can be made into a cation exchange resin by sulfonation. Treatment with monochloroacetone results in an anion exchange resin.

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8.3. Adsorbents on environmental remediation 8.3.1. General considerations There are many environmental applications of adsorption in practice and many others are being developed (Noble and Terry, 2004). Activated carbons and clays are frequently used for the removal of organic contaminants, such as phenol and aniline, both of which are prevalent in industry wastewaters and are known to have a significant negative impact on marine life and human health (IRIS, 1998; Dabrowski et al., 2005). Moreover, the adsorption on inexpensive and efficient solid supports has been considered a simple and economical viable method for the removal of dyes from water and wastewater (Forgacsa et al., 2004). Activated carbon, clays, coal, vermiculite, and other adsorbents have been used for this purpose. Specifically, adsorption can be employed in (Noble and Terry, 2004; Dabrowski, 2001): • the removal of water from organic solvents • the removal of organics from water • taste and odor regulation in wastewater treatment • the removal of radon, hydrogen sulfide, and other sulfur compounds from gas streams • mercury removal from chlor-alkali-cell gas effluent • heavy-metal removal in clay barriers • nitrogen and phosphorus removal from wastewater, i.e. removal and recovery of nutrients • solvent recovery and solvent vapor fractionation • volatile organic compounds recovery from gas streams and groundwater • water removal from gas streams containing acid gases Other important applications of adsorption are the control of “greenhouse” gases (CO, CH4, N2O), the utilization of CH4, the flue gas treatment (SOx, NOx, Hg removal), and the recovery of the ozone-depleting CFCs (Dabrowski, 2001). Activated carbons and hydrophobic zeolites are used for the adsorption of HCFCs (Tsai, 2002). As any process, adsorption has both some advantages and disadvantages:

Advantages • high removal efficiency • enables removal of refractory and/or toxic organic compounds • possibility of compounds recovery (preferably with zeolites) • simple installation and maintenance • capability of systems for fully automatic operation • a large variety of adsorbents available

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Disadvantages • adsorbents deteriorate in capacity gradually • particulates in the feed can cause problems • high content of macromolecular compounds decreases efficiency and may cause irreversible blockage of active sites • risk of bed fires in the VOC abatement • spent adsorbent has to be regenerated (high energy consumption) or disposed (causing waste) •

relatively high capital cost

Special applications: The environmental control and life support system on a spacecraft

maintains a safe and comfortable environment, in which the crew can live and work, by supplying oxygen and water and by removing carbon dioxide, water vapor, and trace contaminants from cabin air. It is apparent that the processes aimed at the recycling of air and water are vital for supporting life in the cabin. These recycling processes include separation and reduction of carbon dioxide, removal of trace gas-phase contaminants, recovery and purification of humidity condensate, purification and polishing of wastewater streams, and are performed totally or in part by adsorption equipment (Dabrowski, 2001). Another special application of adsorption in space is presented by Grover et al. (1998). The University of Washington has designed an in situ resource utilization system to provide water to the life-support system in the laboratory module of the NASA Mars Reference Mission, a piloted mission to Mars. In this system, the Water Vapor Adsorption Reactor (WAVAR) extracts water vapor from the Martian atmosphere by adsorption in a bed of type 3A zeolite molecular sieve. Using ambient winds and fan power to move atmosphere, the WAVAR adsorbs the water vapor until the zeolite 3A bed is nearly saturated, and then heats the bed within a sealed chamber by microwave radiation to drive off water for collection. The water vapor flows to a condenser where it freezes and is later liquefied for use in the life-support system. On the other hand, although there are some applications in gas emissions reduction, for example, hydrogen sulfide and ammonia removal by utilizing carboxylic acid resins and ammonium anionexchange resins, ion exchange is mainly used in wastewater treatment. Some characteristic environmental applications are the following (Noble and Terry, 2004): • treatment of mine drainage water: removal of metal cations and anions using silicotitanates and layered titanates • removal of nitrates and ammonia from groundwater • treatment of nuclear waste solutions: (1) strontium removal by clinoptilolite and heulandite (Chernjatskaja, 1988), (2) cesium removal using hexacyanoferrate exchanger and phenolic resins (Harjula et al., 1994; Samanta et al., 1992), (3) treatment of liquid nuclear wastes using titanate ion exchangers (Dosch et al., 1993), and (4) thorium ions removal using zeolites (Sinha et al., 1994) • plating industry: (1) treatment of raw water to produce high-quality rinse water, (2) chemical recovery from rinse water, (3) treatment of plating baths to remove contaminants, and (4) as a primary end-of-pipe treatment process

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Some of the cons and pros of ion exchange are

Advantages • in principle, all ions or ionizable species can be removed from aqueous liquids • recovery of valuable species is possible • high efficiency • a large variety of specific resins is available

Disadvantages • prefiltration is required (suspended particles in the feed should be less than about 50 mg/L to prevent plugging) • interference of competing cations in the wastewater • low-temperature resistance of organic (resin) ion exchangers

Special application: Ion exchange was in the foreground in World War II during the Manhattan

Project. The need for separation of reactor fusion products for analysis purposes led Boyd and coworkers to suggest the use of resins for the uptake of several fusion products. This study paved the way for the development of several ion-exchange methods. However, the results of the Manhattan Project in connection with ion exchange were not published until 1947 on grounds of confidentiality.

8.3.2. Adsorbents for heavy metals removal from wastewaters Brown et al. (1992) used 17 carbon-based adsorbents to compare the performance and cost of carbon adsorption treatments for the removal of dissolved metal pollutants from wastewaters. Cadmium, copper, and lead were the cations investigated. Twelve of the adsorbents studied were peats supplied by Bordna Mona, Ireland. These samples represented different peat types, geographic origin, and processing history. The remainder comprised lignite, lignite char, bone char, a Chemviron activated carbon, and a Noritt activated carbon. The results show a wide range in the performance of the sorbents; however, the capacities follow a similar trend for the metals for any specific adsorbent sample. The most effective sorbent tested was bone char, with a capacity of over 100 g cadmium uptake per kilogram bone char, followed by Bordna Mona peat type 5, a milled peat ex. Cloncreen 1 bog, with a capacity of 60 g/kg. Other good performers were activated carbon (Noritt), Bordna Mona (type 10), and lignite (ex. Crurnlin), all with capacities around 40 g/kg adsorbent sample. The poorest performers were the peat char, 1 g/kg; and lignite char, 5 g/kg. This low result could be anticipated if one considers that the process of charring will burn off the vast majority of the functional groups associated with the binding of metal ions onto peat and lignite. The surface areas of the peat and lignite chars are two orders of magnitude lower than those of the activated carbons. This partially explains the difference in capacities between the chars and the activated carbons. The difference in capacities between the chars and the raw peats and lignite can be attributed largely to the presence of chemical species in these materials. This suggests an exchange sorption mechanism is predominant for these materials. The high capacity and

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irreversibility of the metal sorption onto bone chars may be attributed to the phosphorus-based hydroxyapatite/metal association. The capacity data for cadmium was then applied to a costing analysis to determine the metal removed per pound sterling material cost. It can be seen that, on the basis of adsorbent material cost per unit metal ion uptake, peat and lignite are the most economically attractive adsorbents for heavy metals. This analysis is a simple approach since it takes into account neither the additional process costs associated with regeneration nor the economic benefits associated with metal recovery; however, it does provide a relative cornparison. The regeneration analysis is complicated because it involves a consideration of adsorbent cost; adsorption capacity (this will affect the frequency of regeneration); the ease of regeneration (cornbustion temperature, acid required for leaching);sorbent activity after regeneration; plus process equipment costs and operating costs. 8.3.3. Adsorbents for dye removal from wastewaters Many factors influence the rate and extent of dye uptake on adsorbents and include pH, temperature, particle size, initial dye concentration, and adsorbent mass. Consequently all the experiments have been undertaken using similar conditions, and using two basic dyes, namely, basic red 22 (maxillon red) and basic blue 3 (astrazone blue). A uniform range of adsorbent particle size, between 150 adsorbent were agitated with a constant volume and constant concentration of dye solution. The isotherms for the various dyes on different adsorbents were determined and plotted as q against e, where q is milligrams dye adsorbed per gram adsorbent and e is the concentration of dye at equilibrium in solution. Experimental conditions were selected so that the isotherms reached a plateau in order that the saturation value of the adsorptive capacity, q, could be determined. The values were used to assess the quantity of adsorbent required to remove 1 kg of dye. These quantities have been used as a basis for costing the adsorption process. In this simplified approach, no account has been taken of contact time data, and it was assumed that the adsorbent was saturated with dye. In addition, regeneration costs have been neglected, since very few figures are available. The relative costs of the adsorbents for the two basic dye systems are shown in Tables 8.8 and 8.9, together with the adsorption costs in removing 1 kg of dye. Carbon was taken as a standard, having a comparative cost of unity per kilogram, and the relative costs of the other adsorbents are shown in the third column of Tables 8.8 and 8.9. The mass (kg) of adsorbent required to remove 1 kg of dye is obtained from the dye isotherms and, consequently, using columns 3 and 5, a comparative cost of different adsorbents to remove 1 kg of dye may be obtained; this is shown in column 4. The results given in Table 8.8 indicate that the activated carbons and Fuller's earth adsorb the largest quantities of dye, that is, 560, 500, and 450 mg dye per gram adsorbent. Lignite and peat show sorption capacities in the region of 380 mg dye per gram adsorbent. The final group of four materials, namely, pith, lignite char, silica, and alumina have sorption capacities in the range 50 to 120 mg dye per gram adsorbent. In economic terms the order changes somewhat, peat being the most economical adsorbent, closely followed by lignite and pith. In the case of the second basic dye, namely, basic red 22, the maximum sorption capacities are again shown by Fuller's earth and the activated carbons; these are of the order of 500 mg dye per gram sorbent. The lignite and peat capacities are 410 and 310 mg dye per gram sorbent. The pith, lignite char, silica, and alumina are in the range of 40 to 100 mg dye per gram sorbent. The results indicate a very similar trend to those of basic blue 3 dye, and the costs are reviewed in Table 8.9.

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Table 8.8.- Adsorption Costs of C.l. Basic Blue 3

Comparative Cost

Adsorbent Material

Mass of adsorbent required to Per kg of To remove remove 1 kg Adsorbent 1 kg of BB3 of Dye (kg)

Activated carbon

448

1.00

1.000

2.232

Peat

375

0.04

0.048

2.667

Silica

87.5 1.50

7.682

11.430

Pith

0.04

0.286

16.000

Lignite activated carbon 560

0.85

0.674

1.770

Char

125

0.25

0.896

8.000

Lignite

392

0.15

0.1714

2.551

Fuller's earth

500

0.66

0.60

2.000

Table 8.9.- Adsorption Costs of C.l. Basic Red 22

Comparative Cost

Adsorbent Material

Mass of adsorbent required to Per kg of To remove remove 1 kg Adsorbent 1 kg of BR22 of Dye (kg)

Activated carbon

500

1.00

1.0000

2.000

Peat

314

0.04

0.0637

3.185

Pith

0.04

0.2999

15.000

Lignite activated carbon 520

0.85

0.8173

1.923

Char

0.25

2.9762

23.809

Lignite

400

0.15

0.1875

2.550

Silica

24.5 1.50

30.6120

40:816

Fuller's earth

460

0.72

2.500

0.66

The results presented in Table 8.10 indicate that wood is the most economical adsorbent, followed by peat, carbon, pith, and Fuller's earth. The results show that activated carbons have the highest adsorption capacity for dyes in most of the systems studied, but that natural materials such as peat and lignite also have high capacities. Furthermore, economic analysis indicates that peat, wood, lignite, and pith are the cheapest, with silica, alumina, and Fuller's earth being more expensive. Although having a lower overall sorption capacity, on an economic basis pith is an attractive adsorbent. Its "market" value is assessed on its calorific value relative to fuel oil. However, it can still be used with maximum effect, as once the pith has been used for dye adsorption, its calorific

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value has actually been enhanced by about 5%. Therefore the pith is participating in an interstage operation for effluent treatment prior to its combustion. Table 8.10.- Adsorption Costs of C.l. Acid Blue 25

Comparative Cost

Adsorbent Material

X/M

Mass of adsorbent required to Per kg of To remove remove 1 kg Adsorbent 1 kg of AB25 of Dye (kg)

Decolorizing carbon

82.75

1.00

1.0

12.085

Fuller's earth

0.66

1.517

27.777

Silica

1.50

5.397

43.48

Wood

53.5

0.01

0.0155

18.692

Peat

99.0

0.04

0.334

10.101

Alumina

96.0

1.82

1.5687

10.417

Pith

22.0

0.04

1.503

45.45

References [1] Hashimoto S.: Zeolite Photochemistry: impact of zeolites on photochemistry and feedback from photochemistry to zeolite science , J. Photochem. Photobiol. C: Reviews 4, 19-49 and references quoted there, (2003). [2] Corma A.: From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis, Chem. Rev. 97, 2373-2419, (1997). [3] Botelho do Rego A.M., Vieira Ferreira L.F.: Photonic and electronic spectroscopies for the characterization of organic surfaces and organic molecules adsorbed on surfaces. In: Handbook of Surfaces and Interfaces of Materials, Nalwa H.S. (Ed.), Academic Press, New York, Vol.2, Ch.7, 275-313, (2001).

[4] a) Anpo M. (Ed.): Surface Photochemistry, Wiley, Baffins Lane, (1996). b) Anpo M., Matsuara T. (Eds.): Photochemistry on Solid Surfaces, Elsevier, Amsterdam, (1989). [5] a) Johnston L.J.: Phototransformations of Organic Molecules Adsorbed on Silica and Alumina. In: Photochemistry in Microheterogeneous Systems, Kalyanasundaram K. (Ed.), Academic Press, Orlando, Ch. 8, p. 359, (1987). b) Kamat P. V.: Photochemistry on nonreactive and reactive (semiconductor) surfaces. Chem. Rev. 93, 267-273, (1993). [6] Cosa G., Scaiano J.C.: Laser techniques in the study of drug photochemistry, Photochem. Photobiol. Review 80, 159-174, (2004). [7] Turro N.J.: Photochemistry of organic molecules in microscopic reactors, Pure & Appl. Chem. 58, 1219-1228, (1986). [8] Garcia H, Roth H.D.: Generation and Reactions of Organic Radical Cations in Zeolites, Chem. Rev. 102, 3947-4007, (2002). [9] Scaiano J.C., Garcia H.: Intrazeolite Photochemistry: Toward Supramolecular Control of Molecular Photochemistry, Acc Chem. Res. 32, 783-793, (1999). [10] Beck J. S., Vartuli J.C., Roth W. J., Leonowicz M. E., Kresge C. T., Schmitt K. D., Chu C. T.-W., Olson D. H., Sheppard E. W., McCullen S. B., Higgins J. B., Schlenker J. L.: A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates, J. Am. Chem. Soc. 114, 10834-10843, (1992). [11] Meier, W. M.; Olson, D. H.; Baerlocher, C. Zeolites 1996, 17, 1. [12] Ramamurthy V., Caspar J.V., Corbin D.R.:

ď ˇ-Diphenyl Polyenes within the Channels of Pentasil Zeolites, J. Am. Chem. Soc. 113, 594-600, (1991).

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9. Photocatalysis and Environmental Cleaning Systems (teaching through Case Studies) Anabela S. Oliveira Advanced oxidation processes offer a consistent path to the treatment of recalcitrant substances that cannot be treated by conventional effluents treatments. Either TiO2 mediated photocatalysis or Fenton related methodologies offer feasible alternatives for the treatment of dyes, organochlorinated substances (pesticides, dioxines, furanes, PCBs, etc.) and pharmaceutical products, enabling the decomposition of such substances. Those methods, which are very attractive from the point of view of sustainable and green chemistry because they can use solar light as energy source, are being increasingly tested in several treatment plants (some of them pilot plans) with the help of solar collecting technology. Diffuse reflectance techniques enables the direct study of photophysical processes on opaque heterogeneous systems (either real or model systems) avoiding the recurrent use of classic liquid model systems (transparent homogeneous systems). Ground state diffuse reflectance, laser induced luminescence and diffuse reflectance laser flash photolysis proved to be complementary techniques extremely useful on the study of probes adsorbed on solid supports and on the elucidation of their photodegradation mechanisms, namely on the study of polycyclic aromatic hydrocarbons deposited on different solid supports, either model or real. Surface photochemistry techniques have shown to be valid alternatives to reduce environmental monitoring cost with the advantage of being quick and enabling a large number of determinations. Furthermore, there is a varied set of compact commercial components (ex: lasers, CCDs e diffuse reflectance spectrophotometers) that can be easily adapted to field analysis with excellent detection limits. Finally laser radiation and/or TiO2 can be used on the photodegradation of persistent environmental pollutants.

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9.1. Use of spectroscopic methods for environmental monitoring and to follow environmental remediation Case Study 1 - Environmental carcinogens analyzed by photochemical techniques surfaces (Ground State Diffuse Reflectance and Laser induced Time Resolved Fluorescence) on model solid samples [1,2,3] From A. S. Oliveira, M. B. Fernandes, M. M. Higarashi, J. C.Moreira, M. E. A. Vieira Ferreira and L. F. Vieira Ferreira, Scientia, Rev. QuĂm. Ind., 2004, 9, 15-23.

Figure 9.1.a) shows the diffuse reflectance spectra of samples in the ground state with increasing concentrations of pyrene deposited between cellulose chains. The spectra were obtained by placing the sample on the surface of an opaque integrating sphere capable of measuring light scattered by a illuminated sample. The integrating sphere is coated internally with a perfect reflector and is calibrated in advance with a white (R = 100%) and black (R = 0%). Other details of the method are given in reference 1 and 2. The calculation of the refemission function for each of the opaque solid samples in Figure 9.1.b), allows us to obtain comparable spectra of the absorption spectra of solutions for ground-state pyrene. This result shows that it is possible to determine the absorption spectrum of the ground state pyrene deposited on a solid matrix and quantify their levels in different supports.

Figure 9.1. - a) Percentage Reflectance b) remission function micromolg-1 of pyrene on microcrystalline cellulose.

Figure 9.2 shows the remission functions of fluorene, anthracene, dibenzo [a, h] anthracene, pyrene and benzo [a] pyrene on plates chromatographic sĂlica. The reference function is characteristic of each compound and identifies the various PAHs deposited on the support.

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Figure 9.2 â&#x20AC;&#x201C; Remission function of fluorene, anthracene, dibenz [a, h] anthracene, pyrene and benzo [a] pyrene adsorbed onto silica chromatographic plates. Concentration of approximately 10 micromolg-1 of PAH silica.

These results show that the diffuse reflectance technique in the ground state can be selective and quantitative analysis of PAHs on various solid supports not transparentes. Figure 9.3 presents fluorescence spectra of laser-induced dibenzo [a, h] anthracene, anthracene, benzo [a] pyrene deposited individually in sĂlica 10 chromatographic plates. The results show that the PAHs deposited in a solid opaque matrix have a characteristic fluorescence spectra different and which in many cases may allow its identification, in particular by measuring its wavelength of maximum emission. As the absorption spectra opaque samples, also the fluorescence spectra of opaque samples are obtained in reflection geometry. The technique is described in detail in reference 1 and 2.

Figure 9.3 - laser-induced luminescence of N2 (337 nm, 1.6 mJ / pulse) dibenzo [a, h] anthracene, anthracene, benzo [a] pyrene adsorbed onto silica chromatographic plate. Concentration of approximately -1 PAH silica.

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Figures 9.4 and 9.5 shows spectra of laser induced luminescence with time resolution for samples of different concentrations of pyrene and benzo [a] pyrene deposited microcrystalline cellulose and silica. For a given set of decay curves of fluorescence emission differ in the time range indicated in nanoseconds on each of figures. By the analysis of the figures it is observed that: The pyrene and benzo [a] pyrene deposited in a low concentration (0.5 micromolg-1) microcrystalline cellulose present themselves exclusively as monomers emission at 395 nm and 405 nm and emission lifetimes of about 200-250 nm and 15-20 nm, respectively. When deposited in high concentration (25 micromolg-1), both the PAHs have a small formation of excimer identifiable by the growth of a band at wavelengths greater than the emission of the monomer. The lifetime of the excimer for the two PAH is much lower than the lifetime of the respective monomer. When pyrene and benzo [a] pyrene are deposited on silica, there is the coexistence of monomers and excimer from low concentrations (0.5 micromolg-1). In this case the lifetimes are much lower than those observed in microcrystalline cellulose, not exceeding respectively 40 ns and 2.5 ns. When deposited in high concentration (25 micromolg-1) so as pyrene, benzo [a] pyrene show an almost exclusive presence excimer identifiable by bands at wavelengths greater than the emission monomer respectively at 450 nm and 525 nm. This set of results illustrates the potential of coupling temporal resolution to luminescence techniques. In fact, the luminescence time-resolved spectral information adds to the conventional luminescence temporal resolution and is reflected in the addition of a third coordinate to our experimental data. For a given concentration, each PAH emits light at a certain wavelength, intensity and lifetime resultant of the interaction of a specific PAH himself to the matrix in which it is deposited.These results show once more that in the photophysics of PAHs in surface is heavily dependent on specific interactions of the probe with the substrate.

Figure 9.4 - laser-induced luminescence of N2 (337 nm, 1.6 mJ / pulse) a) 25.0 b) 0.5 pyrene on microcrystalline cellulose and silica.

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Figure 9.5 - laser-induced luminescence of N2 (337 nm, 1.6 mJ / pulse) a) 25.0 b) 0.5 micromolg-1 of benz [a] pyrene on microcrystalline cellulose and silica.

Case Study 2 - Determination of total PAHs level on atmospheric particulate matter (Real environmental samples) From A. S. Oliveira, M. B. Fernandes, J. C Moreira, L. F. Vieira Ferreira, J. Bras. Chem. Soc., 2002, 13, 245â&#x20AC;&#x201C; 250.

Air pollution on urban areas is associated to the incomplete combustion of fossil fuels and to the consequent emission of carcinogenic compounds, like PAHs, which are considered target pollutants for environmental monitoring. Methods used on their detection are expensive and time consuming and use sophisticated methods of extraction followed by chromatographic detection. We tested the use of ground state diffuse reflectance techniques and of time resolved fluorescence with intensified charged coupled device detection on the evaluation of PAHs from atmospheric particulate matter samples, collected at four collection points at Rio de Janeiro, Brazil. As shows Figure 9.6, the total PAHs content on atmospheric particulate measured by ground state diffuse reflectance and laser induced luminescence is in good agreement with chromatographic results published before for the same samples and for a standard reference material (SRM 1648). Ground state diffuse reflectance proved to be a good technique to the evaluation of the total PAHs content in urban atmospheric particulate matter while laser induced luminescence appeared as a less efficient method due to the strong quenching effect promoted by the high black carbon content of the samples. As a matter of fact, for samples with high black carbon content it was not possible to fully correct the observed fluorescence extinction effect. This fact leaded us to the determination of lower PAHs levels than expected for the most contaminated samples and for the standard reference material. However, fluorescence still proved to be valid as analysing method on a local scale, provided the black carbon content of the samples is similar.

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P A H s , ng/ m g

D ec em b er January M arc h

60 40 20 0

A lto

Ilha

C enter

Bras il

A lto

Ilha

C enter

SRM 1648

F (R)

15 10 5 0

If, arb. uni ts

100000

408922

Bras il

SRM 1648

252351

80000 60000 40000 20000 0 A lto

Ilha

C enter

Bras il

SRM 1648

Figura 9.6 â&#x20AC;&#x201C; (a) Content of total PAHs measured using chromatographic techniques (b) Remission Function, (c) Fluorescence intensity of urban air particulate samples from the city of Rio de Janeiro and SRM 1648. Collection points in the city of Rio de Janeiro were the Alto da Boa Vista, the Governor's Island, the Central Avenue and Brazil.

Our techniques proved to be a valid alternative to reduce the cost of air monitoring, with the advantage of being rapid and simple, enabling a large number of measurements in a short time. Case Study 3 - Photodegradation studies of pyrene on microcrystalline cellulose and silica (Model Samples) From A. S. Oliveira, L. F. Vieira Ferreira, J. P. Silva, J. C. Moreira, Internat. J. Photoenergy, 2004, 6, 205213.

The study of powdered solid samples of pyrene adsorbed on microcrystalline cellulose and silica by surface photochemistry techniques enabled the acquisition of experimental evidence of a distinct photochemical behaviour of this PAH on the two substrates. The information obtained was extremely useful on the elucidation on the pyrene photodegradation mechanism on those substrates. Ground state absorption studies showed that at low concentrations pyrene is on the monomer form on both substrates. For high concentrations of pyrene on microcrystalline cellulose and silica different aggregated species where detected: for pyrene on microcrystalline cellulose ground state absorption spectra becomes broader and its maxima shifts to higher wavelengths. Pyrene adsorbed on silica presents a new band, shifted to the red relatively to the location of monomer absorption. On silica, the appearance of such bands is described on literature as originating from the formation of ground state dimers. Laser induced luminescence studies showed that pyrene on microcrystalline cellulose presents only fluorescence from monomers, for the all range of concentrations investigated. For pyrene adsorbed on silica, we observe fluorescence arising from excimers from very low concentrations.

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For the two substrates, diffuse reflectance laser flash-photolysis and photodegradation studies were performed for concentration where mainly monomers exist; we feared that dimers could trap all the energy furnished to the sample preventing its photodegradation. Diffuse reflectance laser flash-photolysis studies showed that pyrene presents a distinct photochemistry on both substrates: While on silica only transient absorption from pyrene radical cation was observed, on microcrystalline cellulose we detected the simultaneous transient absorption of pyrene radical anion, pyrene radical cation and pyrene triplet. Figure 5 presents the transient observed for both substrates. On silica, in the presence of O2, pyrene radical cation is formed by photoionization or from pyrene

triplet, being O2 the molecular species that receives the electron released on the reaction. On microcrystalline cellulose and in the absence of O2, radical cation formation occurs in presence of -

another pyrene molecule which receives the e this turn, originating the simultaneous formation of pyrene radical anion. Photodegradation studies followed from chromatographic analysis confirm the two distinct phtodegradation pathways on the two substrates: For pyrene included on microcrystalline cellulose polymer chains (i.e. in the absence of oxygen) the main observed product is hydroxypyrene; for pyrene on silica photodegradation showed to be very much efficient and it was almost impossible to detect hydroxypyrene. Either on microcrystalline cellulose or on silica, the intermediate species that leads to photoproduct formation is pyrene radical cation. Hydroxypyrene can be easily formed from the radical cation, on both substrates. For pyrene deposited into microcrystalline cellulose polymer chains, and in oxygen absence, the oxidation of the formed hydroxypyrene is extremely slow. Nevertheless for pyrene on sĂlica and in the presence of oxygen, almost all the hydroxypyrene previously formed is quickly oxidized origination other photodegradation studies. The use of surface photochemistry techniques for the detection and identification of intermediates and photoproducts, allied to chromatography, for product identification, shoed to be complementary techniques on the study of the photodegradarion of pyrene on cellulose and silica. Case Study 4 - Use of laser induced fluorescence for monitoring the laser photodegradation of nitropyrene on microcrystalline cellulose (Model Samples) From A. Muck, P. Kybat, A. S. Oliveira, L. F. Vieira Ferreira, J. Cvaka, S. Civis, Z. Zelinger, J. Barek, J. Zima, J. Hazard. Mat., 2002, 3891, 1-10.

We further investigate the photodegradation of nitropyrene adsorbed on microcrystalline cellulose when irradiated with a 355 nm (100mJ/pulso) pulse from a de Nd:Yag laser. Figure 9.7 presents the changes on fluorescence emission of a sample of nitropyrene deposited on cellulose with the number of laser shots. For the non-irradiated sample the simultaneous presence of monomers and excimers can be observed. After Nd:Yag laser irradiation the intensity of the excimer band decreases as the same time that that from the monomer increases up to a maxima from what it also start to decreases. Figure 9.7b) shows excimer disappearance with the first 200 laser shots while figure 9.7c) puts in evidence the changes on monomer concentration with the increase of sample irradiation. The results show that is possible to use laser radiation for the efficiently destruction of chemical environmental carcinogens.

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Figure 9.7 – Time resolved transient absorption for a) Pyrene on microcrystalline cellulose (air equilibrated sample; 10 μmolg-1). Curve 1 and 2 were registered 20 μs and 1ms after laser pulse. b) Pyrene on silica (argon purged sample; 0.1 μmolg-1). Curves 1, 2 and 3 were registered 5 μs, 100 μs and 20 ms after laser pulse.

Figure 9.8 – a) Changes on laser induced fluorescence (N2 laser, 337 nm 1,6 mJ/pulse) of a 6 μmolg-1 sample of nitropyrene onto microcrystalline cellulose with the number of shots from a Nd:Yag laser (355 nm, 80mJ/pulse); Variation on the fluorescence intensity of the b) excímer and of the c) monomer.

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The efficiency of surface photochemistry techniques to monitor PAHs photodegradation in the presence or absence of TiO2 on the above mentioned solid supports and the use of laser radiation

for the photodegradation of PAHs and other chemical compounds of environmentally relevant have shown to be very useful for environmental studies and very much promising for the development of methods able to destroy and/or monitor the destruction of that substances on different environmental compartments. Case Study 5 – Monitoring PAHs in Fishs Bile in Guanabara Bay using Sincronous Fluorescence From Moreira Freire, Marina, Master Thesis, Oswaldo Cruz Fondation, 2008

Analysis of PAHs on sediments and surface waters and on costal and estuarial waters has been done year around providing important information on PAH levels and sources on environment. Meanwhile, these data did not provide any information on the effects these substances cause in living species. Environmental monitoring based exclusively on chemical analysis of sediments and waters it is not appropriated for evaluation and prediction of the effects caused by PAH contamination. In this way, to evaluate the impact that pollutants are inducing in the quality of the ecosystems it is pertinent to evaluate the exposition to stressor substances of the living organisms from these same environments. An appropriated group of biological responses can be useful to evaluate the level of impact of stressors or pollutants on biota’s health. Biomarkers can detect exposition to, or the toxic effects of those compounds and their metabolites. These compound and their metabolites in some cases are quickly metabolized and eliminated by organisms, as is the case of PAHs, which are mainly metabolized in liver and are stored in gall blader up to excretion. The presence of PAH metabolites in bile revealed to be a risk factor for species exposed to this class of compounds due to their toxic effects. Taking this in account, the analysis of PAHs metabolites in bile constitutes an excellent exposition biomarker of living organisms to aquatic PAH contamination and can be largely used in environmental monitoring programs. PAH’s metabolites analysis in fish bile is being used to monitor exposition to these compound in several studies. Metabolites analysis is commonly based on determination of PAHs fluorescence, once PAHs are strong fluorophores. Case study 6 – Phodegradation of polycyclic aromatic hydrocarbons over silica gel chromatographic plates impregnated with TiO2 From L. F. W. Xavier, I. M. N. S. Moreira, M. M. Higarashi, J. C. Moreira, L. F. Vieira Ferreira, A. S. Oliveira, Quim. Nova, 2005, 28, 409-413.

solution of naphthalene, chrysene and benzo[k]fluoranthene (PAH) was directly deposited over silica gel chromatographic plates impregnated or not with TiO2 and also over glass plates holding only TiO2. The silica gel plates holding the substances under study were exposed to solar radiation and, after irradiation, were developed with hexane and photographed under ultra-violet light. The results show that, in the plates holding only silica, the PAH degradation occurs after a long period of solar exposition, but with the silica plates impregnated with TiO2, there is a rapid degradation. The analysis of the glass plates impregnated only with TiO2 showed also very rapid PAH degradation.

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9.2. Photocatalytic remediation of industrial effluents Case study 7 – Photodegradation of Indigo Dyes on Surfaces by TiO2 From Oliveira, A.S ; E.M. Saggioro ; N.R Barbosa ; A. Mazzei ; Ferreira, L.F.V. Ferreira ; C.J. Moreira. Surface Photocatalysis: A Study of the Thickness of TiO2 Layers on the Photocatalytic Decomposition of Soluble Indigo Blue Dye., v. 62, p. 462-468, 2011 The effect of the thickness of a TiO2 layer coated on glass slides, obtained by the use of a simple multiple soaking method, was tested evaluating its effect on the efficiency of photodegradation of adsorbed soluble indigo blue dye on it. Photodegradation was undertaken on a laboratorial bench top photoreactor, equipped with a mercury lamp covered by a glass bulb, and photodegradation results were monitored directly on the photocatalytic surface by ground state diffuse reflectance. Case study 8 – Photodegradation of Azo Dyes in Model Textile Effluents by TiO2, Fenton and photo-Fenton Saggioro, Enrico Mendes; Oliveira, Anabela Sousa ; Pavesi, Thelma ; Maia, Cátia Gil ; Vieira Ferreira, Luis Filipe; Moreira, Josino Costa . Use of Titanium Dioxide Photocatalysis on the Remediation of Model Textile Wastewaters Containing Azo Dyes. Molecules (Basel. Online), v. 16, p. 10370-10386, 2011. Large quantities of dyes are extensively used in fundamental processing steps of textile industries. Hence, wastewaters resulting from these industries are highly contaminated when discharged in rivers or public sewage treatment plants. The aim of this work was optimize the fotocatalytic process of the degradation of three textile azo dyes in model effluents, respectively Remazol Yellow Gold RGB, Remazol Blue RGB and Remazol Carbon RGB. To assess the operational parameters on the photcatalytic degradation was evaluated: concentrations of TiO2, concentration of the dyes, effect of H2O2, effect of pH, recycling of TiO2 were investigated. The effect of the simultaneous photodegradation of the three azo dyes was also investigated and we observed that the degradation rates achieved in mono and three-component system were identical. Degradation was analyzed using UV-Vis spectroscopy. Results show that the use of an efficient photocatalyst and the adequate selection of optimal operational parameters may easily lead to a complete decolorization of the aqueous solutions of both azo dyes. For the same dyes Fenton and Photo-Fenton are also being investigated with very promising results. Case Study 9 – Photodegradation of Azo Dyes on model textile dyes effluents in CPC reactor with solar irradiation Recent results obtained in CPC reactor will be presented Case Study 10 – Photodegradation of Indigo Dyes on model textile dyes Effluents by TiO2 Photocatalysis in laboratory and CPC The aim of this study was to analyze the photocatalytic degradation of indigo carmine dye under artificial and solar irradiation using TiO2 (Titanium Dioxide). To determine the efficacy of three artificial photoreactors, we evaluated the ideal amount of TiO2, the necessary period of UV-light irradiation, the use of activated carbon in the dark and/or under irradiation, and the effect of activated carbon/TiO2/UV. Degradation was analyzed using UV-Vis spectroscopy. Among the tested

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reactors, the one equipped with the mercury vapor lamp (reactor 1) was the most efficient. Solar photocatalysis was also very efficient, regardless of season (100% and 70% in 30 min irradiation, during summer and winter, respectively). Ecotoxicological evaluation with Daphnia similis and Pseudokirchneriella subcapitata showed that by-products of photodegradation were more toxic than untreated indigo carmine, and that nanoTiO2 residues affected species and may have potentialized toxic effects of the photoproducts. These results show the importance of toxicity evaluation of photoproducts for water management. Also, we stress the need for treatment processes that guarantees the complete removal of TiO2. For this dye solar photocatalysis onC PC reactor was recently undertaken.

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9.3. Photocatalysis on the remediation of medical wastes and effluents from pharmaceutical industry Case Study 11 – Photodegradation of Bisphenol A and several medicines on CPC reactor Bisphenol A (BPA) is used as an intermediate in the manufacture of polycarbonate, epoxy and polysulfonate resins. It is released into the aquatic environment from industrial discharges, landfill leachate. The experiments were carried out in batch magnetically stirred reactor and solar photocatalytic degradation was performed in a CPC pilot plant at Plataforma Solar de Almería. The analytical measurements were made by HPLC, TOC and ions chromatography. The batch reactor photodegradation of BPA was performed with 1gL-1 of TiO2 and TiO2 P25 demonstrated more efficiency than mineral forms of TiO2. Solar CPC reactor degraded and mineralized BPA with 0.1gL1 of TiO2. Carboxylic acids like Glycolate, Oxalate, Acetate and Formate were formed on photocatalytic degradation of BPA References [1] Vieira Ferreira, L.F., Química, 72 (1999) 28 e referências aí citadas; [2] Botelho do Rego, A.M., Vieira Ferreira, L.F, in Handbook of Surfaces and Interfaces of Materials, Ed. H.S. Nalwa, Academic Press, Vol.2, Cap.2, 2001, pág. 275-313, e referências aí citadas. [3] Oliveira, AS, Ferreira, LFV and Moreira, JC, surface photochemistry techniques apllied to the study of environmental carcinogens , Rev. Roum.Chim., 53 (10) 893-902, 2008 [4] A.S. Oliveira, E.M. Saggioro, T. Pavesi, J.C. Moreira, L.F. Vieira Ferreira, 2012, Solar Photochemistry for Environmental Remediation - advanced oxidation processes for industrial wastewater treatment, Molecular Photochemistry- Various aspects, ISBN 979-953-307364-3 Edited by Dr. Satyen Saha.

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10. Corrosion in Energy Conversion and Environmental Cleaning System Paulo Brito & Luiz Rodrigues

10.1. Introduction Due the nature of the environments and working conditions in which energy conversion and environmental cleaning systems operate, they are subject to different kinds of corrosion phenomena. Some of these systems must operate in relatively harsh conditions from the point of view of materials chemical stability and mechanical integrity, effecting largely the efficacy and efficiency of equipments and devices. In this chapter, some examples of corrosion failures in energy conversion and environmental cleaning systems are presented and discussed in terms causes, factors, mechanisms and possible solutions. Corrosion may be defined as the chemical interaction of materials with the environment in which it operates. Chemical interactions always leads to changing in the nature, and therefore in the properties and functionalities of materials and equipment in which they are applied, with obvious economical, safety and environmental consequences. In this chapter some cases of the corrosion failure of metallic materials employed in the construction of devices and equipments installed in energy conversion and in environmental cleaning systems are presented and discussed. Energy conversion systems and also environmental cleaning devices frequently operate at temperature, pressure, mechanical, and chemical conditions that are relatively aggressive to the materials applied in their constructions. For example, metallic furnaces and tubes in a heat power stations are subjected to the aggressivity of oxygen and exhaust gases and water vapor at high temperatures, and also to wearing effects due to the presence of solid and liquid fuel particles particles at high speed impinging the walls of containers and tubes. Also electrochemical and photoelectrochemical systems, such electrolyzers and fuels cells, involve very aggressive environment and operations conditions for common electrodes and container materials, affecting their performances [1]. Concerning environmental cleaning systems, for example, oxyfuel or O2/CO2 recycle combustion seems to be a highly interesting option for lignite-based power generation with CO2 capture, due to the possibility to use advanced steam technology, reduce the boiler size and cost and to design a zero-emission power plant[2]. In particular, one interesting question in this process would be whether it is possible to co-capture SO2 with CO2, resulting from the combustion of lignite, and if the resulting stream has a composition that is acceptable for transport and storage, and is compliant with legal demands. If the answer is yes, the expensive desulphurisation system could be omitted. The main obstacles for the co-capture of SO2 with CO2 will be related to corrosion problems in connection to transport and storage, the concerns of safety, environmental regulation and legal related issues. Dehydration to remove the water still remaining in the flue gas after the flue gas condenser may also be necessary to avoid corrosion and hydrate formation, in particular if the SO2 is not removed from the CO2-rich stream [2]. CO2 capture from flue gases emitted from flue gases of existing energy infrastructure may be performed with significant capital and operating cost savings using an oxygen tolerant amine absorption process, for example, compared with oxyfuel combustion [3]. But also here corrosion is an important issue, since amine degradation byproducts, particularly at higher water content of

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the amine solutions lead to corrosion problems and cause significant deterioration in the overall capture performance [3]. Corrosion is an obvious and daily concern for designers and operators of geothermal or tidal energy conversion systems as well as of industrial or domestic waste water treatment plants (WWTP). Due to the consequences that corrosion have to the performances, economics, and ecology of those systems, the rationale of corrosion must be understood by the engineers and technicians that design and run the systems, before they can contribute to reduce or avoid corrosion problems in the systems they design or operate. In the next section the basic of corrosion science will be introduced to allow a better understanding of the case studies presented in the following sections.

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10.2. Corrosion fundamentals 10.2.1. Corrosion definition and nature Corrosion may be defined in different ways, but the majority of the authors and corrosionists will agree, as stated above, that corrosion is the degradation of materials due to its chemical interaction with species present in the environment with which they contact. The word chemical was underlined to distinguish corrosion from other types of material degradation as a consequence of interaction with the environment. For example, metallic or plastic materials may erode due to friction (mechanical or physical interaction) with other surface in the environment or may degrade because bacteria and other microorganisms may eat them (biological interaction). Nothing is said about the physical state of the materials in the above definition, but it must be inferred that the definition applies only to solid material of any nature (metallic, polymeric, ceramic or composite). Due their overwhelming significance as construction material metals will be treated here in more detail, but corrosion of polymeric elements, and specially, of reinforced concrete is also important in the context of energy conversion and environmental cleaning systems, and therefore, will be discussed in this chapter. Corrosion of polymeric and ceramic materials is essentially chemical in the nature of the materialenvironment interaction, involving mainly dissolution, solvatation, and acid-base processes. On the other hand, corrosion of pure metals and alloys, particularly that which occurs in aqueous, soil and atmospheric environments, and inside concrete, the so called “wet corrosion”, is often of electrochemical nature. Corrosion of metallic materials at high temperature, the so called hot corrosion or dry corrosion, is also based in electrochemical processes, but involves solid state electrolytes. Due to the importance of wet corrosion its general mechanism will explained in some detail. As stated wet corrosion or corrosion in aqueous solutions, including condensate over metallic surface, is an electrochemical, i.e., a redox (reduction-oxidation) process, in which the metal loses one or more electrons to a species in the environment, moving to a more stable and higher oxidation state (eq. 1): M(s) Mn+(aq) + n e-

(10.1)

In neutral and alkaline environments, quite oftenly the species that receives the electron(s) given off by the metal is oxygen, O2, dissolved in the environment, which is reduced from the oxidation state 0 to -2, in aqueous solutions, incorporated in hydroxide ion, HO-, (eq. 2): O2(g) + 2 H2O(l) + 4 e-  4 HO-(aq)

(10.2)

In acidic media, the reduction reaction that conjugates with the oxidation of the metal is the reduction of H+ ions to molecular hydrogen (H2), the hydrogen evolution, since in acidic solutions the concentration of H+ ion is relatively high:

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2 H+(aq) + 2 e- ď&#x192;ł H2(g)

(10.3)

In some situations, a metallic ion with higher reduction tendency, i.e., an ion of a more noble metal, present in the media, may be reduced to a lower oxidation state causing the oxidative dissolution of the metal in the structure with the solution contacts. This may be the case of complex piping and container systems, with accessories of different materials or with complex electrolytic solutions (eg.: industrial, thermal power stations, primary solar circuits, WWTP piping systems). 10.2.2. The corrosion cell and the fight against corrosion As stated above, corrosion in aqueous media, wet corrosion, is an electrochemical or oxidationreduction (redox) process. Each redox reaction is composed by two half reactions: the reduction semi-reaction and the oxidation semi-reaction. In the particular case of corrosion reactions the semi-reactions is always that of the oxidation of the construction metal (eq. 1). The reduction semi-reaction is that of the reduction of a species in the surrounding environment, for instance, O2 reduction, in neutral or alkaline media (eq. 2), hydrogen evolution in acidic media (eq.3), etc. One distinctive characteristic of redox reactions is the transference of electrons from one species (the oxidized species or reductant) to the other (the reduced species or oxidant). Another special particularly of redox reactions among the different chemical reactions is that the reagents (species that interact) do not need to be in direct contact for reaction to take place! It is only needed that there is a way for electrons to flow from the reductant to the oxidant. The place where each of the semi-reactions happens is called electrode, normally, located at the surface of conducting metallic bodies. The place where oxidation semi-reaction takes place is called anode and the place where reduction occurs is the cathode. Therefore, frequently, oxidation semi-reaction is said the anodic process and reduction semi-reaction is named the cathodic process. Corrosion as any electrochemical or redox process may be described with the help of an electrochemical cell, having an anode, a cathode, an electronic conductor through which electrons flow from anode to cathode, and an electrolytic conductor, or simply, electrolyte, that allow the migration of positive and negative ions to the electrode with opposite electrical charge. Figure 1 shows a representation of the 4 essential elements (anode, cathode, electronic conductor, and electrolyte) of a corrosion cell.

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(Electrolytic conductor)

(Anode)

(Electronic conductor) (Cathode)

Figure 10.1 - Corrosion cell with its 4 essential elements: anode, cathode, electronic conductor and electrolyte. (Adapted from http://www.sperchemical.com/html/corrosion_inhibition.html, consulted at 01 August 2013)

Similarly to the batteries used in different gadgets and equipments, corrosion cells are spontaneous or galvanic electrochemical cells. That means the global reaction takes place spontaneously and give off energy, particularly, electric energy. The different to batteries is that, in batteries cathode and anode were purposely separated from each other allow electrons to flow through a external circuit and an energy consuming device and take advantage of the electric energy produced. In a corrosion cell anode and cathode may be placed in different (near or relatively far) places of the surface of the corroding metal which cannot be separated and the metallic surface works itself as electronic conductor and carries electrons from anode to the cathode. The role of electrolyte is played the aqueous media (process solutions, seawater, river water, waste waters, condensate films, etc.) with which the metallic surface keeps contact. In the corrosion cell, the anode appears where the oxidation half reaction of the metal (eq. 1) takes places, while the cathode is located where the reduction semi-reaction is most probable, for instance where the concentration of the oxidant or the chemical potential is high. And how is it possible that cathodes and anodes appear spontaneously on the surface a metallic body? Any small difference in the composition of the solution or of the alloy, or even crystalline defects in the metal, may lead to electrical potential differences that will drive electrons from anode to cathode. For example, differential aeration cells appear whenever significant differences in oxygen concentration near the metallic surfaces builds up. The cathodic reaction (2) occurs always in the region where a relatively oxygen concentration exists, and the anodic reaction (1) corresponding to the dissolution of the metal happens where oxygen deficient. The referred 4 elements are essential for the occurrence of the corrosion process. That means that each of these 4 elements (anode, cathode, metallic or electronic conductor and electrolyte) are necessary for the process to happen. If one of them is absent the process cannot exist! This observation is quite important in the fight against corrosion. Indeed, 3 features of the corrosion reaction must be pointed out due its importance in the context of the fighting against corrosion: 1.

If corrosion exists, there is an corresponding corrosion cell with their 4 fundamental elements working properly;

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Due to the principle of electrical neutrality, electrons are consumed at the cathode at same rate at which they are produced at the anode;

Corrosion is a spontaneous process.

Feature 1 indicates that if one of the 4 elements of the corrosion cell is absent of it there is no possibility for corrosion to exist or to continue. That means that if of the elements is moved away corrosion will stop. For instance, if the one could avoid the formation of condensate films over a metallic surface corrosion would also be kept out, since the respective corrosion cell would not have the necessary electrolytic conductor. This is what is done, for example to avoid corrosion of metallic museum pieces (e.g.: coins) by keeping them inside closed boxes and controlling the humidity in the room atmosphere. This action eliminates the possibility of appearance of cathodes, since the oxidant (oxygen) is kept out of the metallic surfaces and, on the other hand, the probability of emergence of electrolytic films is reduced. Theoretically, corrosion of metallic bodies immersed in seawater could be ceased if all the salts (electrolytes) would be removed from it, even keeping oxygen or other dissolved oxidants, due to the reduced or no ionic conductivity of the surrounding “electrolyte”. Can you guess why is recommended to dry the surface of tools, for example scissors, pliers, knifes, etc., after use and before storage, even those made of stainless steel? Feature 2 teaches that corrosion rates may be minimized either by reduction the rate of the anodic process, the semi-reaction that truly effects the degradation of the metallic body, or by lowering the corrosion at which the oxidant is reduced at the cathode. The reduction of anodic or cathodic reactions may reached, for examples, through the use of anodic or cathodic inhibitors, a kind of negative catalyst, that instead of to speed up the reaction, hinders it. On the hand, since the rate of consumption of electrons at the cathode is the same as of their production at the anode, the ratio cathodic area to anodic area is quite important for the rate of the degradation of the metallic piece. A relatively large cathode will ask from the anode electrons at a relatively higher rate (of corrosion) than a smaller cathode! This knowledge will be very important later in this chapter to understand the effect of area ratios in some types of corrosion, in the so called localized corrosion. Finally, the 3rd. feature allows the prediction about the possibility of a certain corrosion process to take place, or not. This knowledge is very important, particularly in the design of new equipments or processes, since, as “corrosionists” know, the best solutions for corrosion problems are those implemented at the design stage. Since corrosion is a spontaneous phenomenon, presumed corrosion process that show to be spontaneous through thermodynamic calculation in the operational conditions, will probably give corrosion problems, therefore they must be avoided. Those that thermodynamic calculation predict to be non-spontaneous will not occur, and this is already good news for someone the wants to make changes in the process conditions, or, for any reason, wants to substitute any material by another. For examples in a WWTP conditions may change due changes in the nature of wastes treated. Knowledge of the corrosion consequences of these changes for materials some critical equipments of the plant would be of great importance for maintenance guys.

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10.2.3. Standard electrode potentials and galvanic series How can be predicted if a presumed corrosion will, or will not, happen? Corrosion of a metal will occur whenever the oxidant species shows a greater tendency to reduce than the cation (positive ion) of the considered metal, in the considered media and conditions (concentrations of species, temperature, pressure, etc.). Reduction tendency may be roughly guessed with the aid of “standard potentials” of redox pairs (electrodes) that can be found in almost every basic chemistry book or handbook. A sample of some standard potential of interesting redox pairs may be found in table I. Table I: Standard redox potentials at 298,15 K (25 ºC). Extracted from reference (4)

Reaction

E0298 K (V)

Reaction

E0298 K (V)

H2O2 + 2 H+ + 2 e-  2 H2O

+1.776

2 H+ + 2 e-  H2

0.00000

HClO2 + 3 H+ + 4 e-  Cl- + 2 H2O

+1.570

Pb2+ + 2 e-  Pb

-0.1262

Au3+ + 3 e-  Au

+1.498

Sn2+ + 2 e-  Sn

-0.1375

HClO + H+ + 2e-  Cl- + H2O

+ 1.482

Ni2+ + 2 e-  Ni

-0.257

Cl2 + 2 e-  2 Cl-

+ 1.35827

Co2+ + 2 e-  Co

-0.277*

N2 + 2 H2O + 6 H++ 6 e-  2 NH4OH

+0.092

Cd2+ + 2 e-  Cd

-0.4030

Pt2+ + 2 e-  Pt

+1.18

Fe2+ + 2 e-  Fe

-0.447

Pd2+ + 2 e-  Pd

+0.951

Cr3+ + 3 e-  Cr

-0.744

NO3- + 3 H+ + 2 e-  HNO2 + H2 O

+0.934

Zn2+ + 2 e-  Zn

-0.7618

NO3- + 4 H+ + 3 e-  NO + 2 H2O

+0.957

2 H2O + 2 e-  H2 + 2 HO-

-0.8297

2 NO3- + 4 H+ + 2 e- N2O4 + 2 H2 O

+0.803

Al3+ + 3 e-  Al

-1.662

Ag+ + e-  Ag

+0.7996

Mg2+ + 2 e-  Mg

-2.372

Cu2+ + 2 e-  Cu

+0.3419

Na+ + e-  Na

-2.714*

Hg22+ + 2 e-  2 Hg

+0.0977

K+ + e-  K

-2.931

*From [6]. Prediction of occurrence, or not, of corrosion problems is quite ease with aid of standard electrodes potentials. Consider for example the question: would a pure zinc plate corrode if immersed in 1.0 mol/dm3 chloride acid, HCl, at standard conditions?

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To answer this question, let begin by considering all the species present in the system: besides pure solid Zn(s), there liquid H2O(l), hydrated hydrogen ion, H+(aq), and chloride ion, Cl-(aq), coming from the ionic dissociation of hydrochloric acid, and hydroxyl ion, HO-(aq), resulting from the water self ionic dissociation, but its quantity is quite tiny to be considered here. From the standard reduction potential table in reference [4], following values of electrode potential may be extracted, considering the species present in the system (in bold in the respective chemical equations) and the clearly acidic environment we are dealing with: Redox reaction involving the species in the system

E0298 K (V)

Description of the possible reaction

2 H2O + 2 e-  H2 + 2 HO-

-0.8297 Water reduction with H2 evolution. (direct reaction)

H2O2 + 2 H+ + 2 e-  2 H2O

+1.776 Water oxidation to give oxygenated water. (reverse reaction)

Zn2+ + 2 e-  Zn

-0.7618 Zinc oxidation to give the respective cation. (reverse reaction)

2 H+ + 2 e-  H2

0.00000 Hydrogen ion reduction with hydrogen gas evolution. (direct reaction)

Cl2 + 2 e-  2 Cl-

+ 1.35827 Chloride ion oxidation with chlorine gas evolution. (reverse reaction)

HClO + H+ + 2e-  Cl- + H2O

+ 1.482 Chloride ion oxidation to give hypochlorous acid. (reverse reaction)

HClO2 + 3 H+ + 4 e-  Cl- + 2 H2O

+1.570 Chloride ion oxidation to give chlorous acid. (reverse reaction)

It must be noticed here that standard potential tables present always the semi-reactions as reduction processes, for example, as Zn2+ + 2 e-  Zn or, simply as Zn2+/Zn. Standard potential of the reverse processes (respective oxidations) have the same absolute value, but with opposite signal. The relationship between the standard electrode potential and the degree of spontaneity is given through the equation: ΔG0 = -n∙F∙E0

(10.4)

where ΔG0 is the standard Gibbs free energy (ΔG0 < 0 – spontaneous process; ΔG0 > 0 – forced process; ΔG0 = 0 – equilibrium), n is the number of electrons involved in the process, F = 96487

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C∙mol-1 is the Faraday constant (charge of 1 mol of electrons) and E0 is the cell potential corresponding to the electrochemical process). It is clear that the more positive the standard potential is, the higher is the probability of the spontaneous occurrence of the process. The more negative the standard potential, the lower is the probability for the direct process to run spontaneously, the higher is the probability for the reverse process to be spontaneous. For example, the reduction Zn2+ + 2 e- → Zn has a E0 = -0.7618 V, and, therefore, has a ΔG0 > 0, meaning the process may not be spontaneous. On the other hand, the oxidation Zn → Zn2+ + 2e- will be spontaneous, because its E0 = +0.7618 V, corresponding to a ΔG0 < 0! Note that, from the 7 processes listed in the table above, all reactions except 2, the oxidation of metallic Zn to Zn2+ and the reduction of H+ to H2, have E0 > 0, and, therefore, have ΔG0 < 0, i.e., are spontaneous. Since oxidation of Zn corresponds to the corrosion of Zn plate, the answer to the placed question is: “yes a zinc plate immersed in a hydrochloric solution will corrode”. This methodology seems be tiresome, because it was decided to show all the possible reactions involving the species present in the system. But it was certainly clear that the majority of those reactions were not spontaneous. For instance, the reactions involving water (oxidation and reduction) have sufficiently large negative E0 values to be almost always disregarded. Another formal way to determine the possibility of occurrence a given corrosion phenomena may be completed through the calculation of the standard potential of the presumed corrosion cell: ΔE0cell = E0catod – E0anod

(10.5)

where the E0catod is the standard potential of supposed cathodic redox pair and E0anod is the standard potential of the anodic redox pair. In the case of corrosion process of a metallic body the anodic redox pair must be always the pair Mn+/M, i.e. the redox pair formed by the metal and respective cation. As mentioned above, depending from the media where the metal is immersed in, cathodic redox pair, most commonly may be H+/H2 (acidic environments), O2/HO- (neutral and alkaline media) or in some worst cases combinations of both (aerated acidic environments). Based on equation (10.4) following conclusion may be taken through the calculation of ΔE0cell: ΔE0cell

ΔG0process

Spontaneity of process

Possibility for corrosion occurrence

Yes

Corrosion

No corrosion

Lets apply this new methodology to answer to the following question: “Would a cooper plate corrode when immersed in a 1,0 mol/dm3 HCl solution at standard conditions”? If the cooper plate would corrode the anodic reaction would be Cu(s)  Cu2+(aq) + 2 e-

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E0298 K = +0.3419 V

(10.6)

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The standard potentials table, once again, would allow identify the most probable cathodic reaction as being the hydrogen ion discharge: 2 H+(aq) + 2 e-  H2(g)

E0298 K = 0,00000 V

(10.7)

Therefore, following equation 10.8, ΔE0cell = E0catod – E0anod = E0 (H+/H2) – E0(Cu2+/Cu) = = 0,00000 V – 0,3419 V = - 0,3419 V

ΔE0cell < 0

(10.8)

The process is not spontaneous, therefore a plate of Cu immersed in a 1.0 M HCl would not corrode. Let’s apply the methodology learned to answers the following questions: “Is it possible to store a 1.0 mol∙dm-3 HCl solution in a cooper container? Can this container serve to store a 1.0 M HNO3 solution?” Since nothing is said about the conditions, it will be assumed standard conditions at 298 K (pure Cu, and 1.0 mol/dm3 Cu2+). Notice that would advise against the use of a cooper container in these situations would be the possibility of corrosion in these acid solutions. This implies the oxidation of Cu to Cu2+ ion by any species in the solutions. It was already shown that in a HCl solution such a species do not exist, therefore, cooper will not corrode in the referred HCl solution. What about 1.0 M HNO3? Is that possible that it corrodes cooper? In HNO3 solution a different species exists: the nitrate ion, NO3-, that, as can be seen in the standard potentials tables supplied is involved in the following reduction half reactions:

Reduction reaction

E0298 K (V)

NO3- + 3 H+ + 2 e-  HNO2 + H2O

+0.934

NO3- + 4 H+ + 3 e-  NO + 2 H2O

+0.957

2 NO3- + 4 H+ + 2 e- N2O4 + 2 H2O

+0.803

Clearly, the most probable semi-reaction is that that comprises the reduction of NO3- to NO. Therefore this would be, certainly, the cathodic reaction of the supposed corrosion cell, whose standard potential difference will be: ΔE0cell = E0catod – E0anod = E0 (NO3-/NO) – E0(Cu2+/Cu) = 0,957 V – 0,3419 V = - 0,615 V ΔE0cell > 0

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(10.9)

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That means that Cu would be corroded by the HNO3 solution and, soon or later, a leak would appear. Prediction of corrosion phenomena based on standard potentials is useful, but in many situations, it could constitute a quite erroneous forecast. In fact, real working conditions of many metals inserted in a certain equipment or device, may be very far from standard conditions. For example, for many metals it is almost impossible to reach the required 1 molar concentration of its cation in aqueous solutions due to the formation of insoluble oxides and hydroxides, especially in the presence of oxygen (aerated solutions, seawater, etc.). It is true that corrosion cell potentials may be used to predict the occurrence, or not, of corrosion phenomena out of standard conditions, namely through the application of the Nernst equation (10.10): (10.10)

where E(Ox/Red) is the potential of the redox pair Ox/Red at any condition different from standard condition, E0(Ox/ Red) is the respective potential at standard conditions, R is gas constant (8,314 J∙K-1∙mol-1), T is the absolute temperature, n the number of involved in the semi-reduction process Ox + n e- → Red, and F is the Faraday constant. Based on Nernst equation and the acid/based and dissolution/precipitation behavior of the oxides and hydroxides formed due to interaction of the solutions components, such as HO- ions, with metallic ions formed by oxidation of the metal, Pourbaix, constructed electrode potential versus pH value diagram, E X pH. Pourbaix diagrams are a type of “phase diagrams” that show the regions on the E-pH domain where the solid metal (immune region) or the oxides/hydroxides resulting from the corrosion process and protect the metal from further corrosion (passivation region) are thermodynamically stable, or where corrosion will proceed actively (corrosion region). Figure 2, shows the Pourbaix diagram for aluminium [8]. Form this diagram, it can be seen that in the region where water is stable, between the dashed lines a) and b), Al is passivated in environments with pH between 4 and ±8.3, where alumina, Al2O3 is a stable compact solid, that avoids the contact of the metal with O2 dissolved in the solution. At sufficiently low potentials, for example at -2 V versus SHE (standard Hydrogen Electrode), Al is immune to corrosion, even in very acid solutions, with pH = -2, for instance! At high (“anodic”) potential Al corrodes actively for pH < 4 or pH > 8.3. Standard electrode potentials and Nernst equation do not consider alloys, such as steels or brass that are quite important as building material, but only pure metals. Therefore in some activity branches, such as in the shipbuilding industry, instead of standard potentials tables, special “galvanic series” are used. Galvanic series is a qualitative or quantitative sequence of materials (metal and alloys) following its the corrosion resistance in the considered environment. This sequence is determined experimentally and may assume difference presentations formats. Figure 2 shows an example of a galvanic series in seawater.

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Figure 10.2: The Pourbaix diagram for aluminium at 1 atm and 25 ºC in aqueous solutions.

Figure 10.3: Example of a galvanic series in seawater [4].

Galvanic series are particularly important in the prediction of “galvanic corrosion”, a type of corrosion cause by the direct electric contact between 2 different metals in an corrosive environment, where the less noble (more “active”) of the metals will be the anode, and therefore

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will corrode, while the more noble will be the cathode, where an semi-reduction reaction, for example (2) or (3) will take place. For example, if a mild steel plate is connected with a stainless steel type AISI 316, (SS 316) bolt in seawater, a potential difference of about 700 mV exists, and the mild steel plate will corrode, while the SS 316 bolt will be the stage for the oxygen reduction semi-reaction.

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10.3. Corrosion classification and mechanisms There are different ways in which materials can interact with the environment they contact with, and also there are quite diverse consequences of those interactions. Since corrosion is an ubiquitous phenomenon corrosion was classified, throughout the history, from diverse point of views, depending from the sector of activity or interest on the metals. For instance, corrosion may be classified as wet or dry, depending of the involvement of water in the process. Corrosion may also be classified as a function of the environment where it occurs, for examples, as marine corrosion, atmospheric corrosion, or corrosion in reinforced concrete structures, etc., Corrosion may be yet classified as a function of the place where it happens, for instance, “under deposit corrosion”, “under bolt corrosion”, “under film corrosion”, etc. Corrosion types may further more differentiated based on the appearance of the corrosion products over the metallic surface. For instance, corrosion may be filliform, plug corrosion, etc. Modern corrosion science and engineering allow a clear and easier classification of corrosion in 8 types, widely accepted by authors and practitioners [6]. This classification system, a mixture of a classification based on corrosion appearance and on corrosion causes or mechanisms, is succinctly +presented following paragraphs. 10.3.1. Uniform or generalized corrosion Metallic bodies subjected to uniform or generalized corrosion, present their surface covered almost uniformly with the corrosion products. This is the most common form of corrosion and represents greatest destruction of metal on ton basis [6]. The main cause for this type of corrosion is the thermodynamic incompatibility of the metal with the environment where it operates. Evidently, corrosion rates increases with oxidative power of the environment, which depends mainly from nature and concentration of oxidants present. A large numbers of micro corrosion cells is formed simultaneously at the surface, with anodic and cathodic regions changing constantly. Substituting the metal by other with higher corrosion resistant in the considered environment is the most reasonable advise. When this is impossible, the metal surface may be painted with anticorrosive coatings, cladded or modified by a more resistant metal. 10.3.2. Galvanic corrosion As referred above, galvanic corrosion is a kind of corrosion in which a macro corrosion cell is built due to the contact of a two different metals in a corrosive environment. Degradation appears on the more active (less noble) of the two metals, which functions as the anode of the corrosion cell, suffering oxidation to the respective cation (eq. 1). On the other hand, the nobler of the 2 metals plays the role of cathode, serving as stage for the occurrence of the reduction semi-reaction, for example, O2 or H+ reductions (equations 2 or 3). So, in opposition to uniform corrosion, galvanic corrosion, is quite localized, with anode and cathode perfectly distinguishable. Even when both metals corrode in the considered environment in isolated condition, when connected electrically, the corrosion rate of the more active metal will be incresead, while that of the less active will be decreased. This is indeed the fundament of the “cathodic protection” of metals by “sacrificial

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anodes”: to avoid corrosion of metal, a more active me metal is connected electrically with the first to turn it the cathode of the corrosion cell and the second, the anode, corrodes to save the first. It is important to stress that galvanic series and standard electrode potentials are not universal. Sometimes the order of two metals or alloys in the galvanic series for a given media, may be reversed when the environment is changed, for instance, due to passivation of one them, turning it more resistant to corrosion, “more noble” than the cathodic metal in the first environment. Therefore, prediction of galvanic corrosion must be made on the basis of galvanic series for the considered working environment, and preferably at the operational conditions. Aggressiveness of the environment, including nature and concentration of oxidants and temperature and flow rate of the environment, and the cathodic to anodic areas ratio are the most important factors to consider. Many methods exist to avoid or minimize galvanic corrosion [6]: 1. Choose combinations of metals (or alloys) as close together as possible in the galvanic series in the operating environment and conditions; 2. Avoid unfavorable effect of small anode to large cathode; 3. whenever practicable, insulate dissimilar metal, and, completely if possible; 4. apply coatings with caution and when applied keep it in good repair conditions, particularly the ones on the anodic surface; 5. use inhibitors or oxygen scavengers to reduce aggressiveness of the environment, and 6. avoid threaded joints for materials far apart in the galvanic series (threading consume part of the available wall or flange thickness and helps to collect spilled liquids and condensed moisture and vapours. 10.3.3. Crevice corrosion Another art of localized corrosion, i.e., working through a macro corrosion cell, that frequently occurs within crevices and other occlusion areas of the metallic surfaces exposed to corrosive environments. Crevice corrosion is the general names of corrosion forms named as “under deposit” or “under gasket corrosion”. Actually, these types of corrosion are usually associated with small volumes of stagnant solutions caused by holes, gasket joints, surface deposits, and in crevices under bolts, rivets heads, among others. To function as a corrosion site (anode!), a crevice must be wide enough to permit liquid entrance but sufficiently narrow to maintain a stagnant zone. Stainless steels and other metals and alloys whose corrosion resistance depends on the formation of a passive film are particularly to crevice attack [6]. Crevice corrosion macro cell may be seen as a “concentration cell” that is formed after initial uniform attack inside and outside of the crevice. At this initial stage both semi-reactions, oxidation (eq. 1) and reduction (eq. 2), occurs inside and outside of the crevice. However, as the uniform attack continues the region inside the crevice become depleted in oxygen, and the semi-reduction reaction (eq. 3) stops to occurs at this place, but pursue at the larger surface outside the crevice, turning the crevice a specialized anode and the rest of surface a very large cathode. As seen above in relation to the galvanic corrosion, this unfavorable cathode/anode ratio leads to an intense (deep) degradation inside the crevice. In addition to that, the building up of the concentration of metal cation Mn+ inside the crevice, has two harmful consequences: 1.

The excess of this positive charge is compensated by the migration of anions, such as chloride ions, Cl-, into the crevice, that has noxious effect to the stability of passivation films;

2. Cations do normally hydrolise (react with) and produces acid solutions inside the crevice (eq. 7), that, once again, generally conducts the system to active corrosion regions of the Pourbaix diagram.

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Mn+ + H2O  M(HO)m(n-m)+ + m H+

(10.11)

It was shown that even starting from a neutral solution (seawater, for example) the pH may reach values such as 2 to 3, and the Cl- concentration may increase 3 to 10 times inside the crevice [6]. Some methods and procedures for avoid or minimize corrosion crevice corrosion may be listed as: 1. use welded butt joints instead of riveted or bolted joints in new equipments; 2. Close crevices in existing lap joints by continuous welding caulking, or soldering; 3. Design vessels for complet drainage; 4. Inspect equipment and remove deposits frequently; if possible, remove solid in suspense early in the process; 6. Remove wet packing materials during long shutdowns; 7. If possible, provide uniform environments; 8. Wherever possible, use “solid”, nonabsorbent gaskets (eg.: Teflon). A special case of crevice corrosion is “filiform corrosion”, a quite common type of corrosion, which manifests as a red-brown filaments that develops under protective films (therefore is also known as “underfilm corrosion”), affecting the appearance of the metallic surface. Examples is the attack of enammeled or lacquered surfaces of food and beverage cans exposed to the atmosphere [6]. 10.3.4. Pitting corrosion Is another form of extremely localized attack that results in the formation of holes (pits) throughout the surface of the metallic body. The pits may appear as isolated holes, small or large in diameter, but sometimes they are close together that the surface looks rough. Pit may be described as a cavity or hole with a surface diameter about the same as or less than its depth. Pitting is one of the most destructive and insidious forms of corrosion, causing failure due to perforation with only a small percent weight loss of the entire structure. This makes pitting particularly dangerous due to its localized and intense nature, leading to extremely sudden failures. Pits usually grow in direction of gravity, growing downward from horizontal surfaces. Lesser numbers start on vertical surfaces, and only rarely do pits grow from the bottom of horizontal surfaces [6]. Pits usually require and an extend initiation period, months to years, depending on both metal and corrosive environment, before they become visible. Once started, however, pits penetrate the metal at an ever-increasing rate. Normally, subsurface damage is usually much more severe than the surface appearance indicate. Similarly to crevice corrosion, pitting corrosion is an autocatalytic process, i.e., the corrosion within a pit produce conditions which are both stimulating and necessary for the continuing activity of the pit. Pitting corrosion may be seen as a “self-initiating crevice corrosion”: it does not require a crevice, it creates its own. For example, random solution composition, differences in chloride concentration in the environment, a surface scratch, an emerging dislocation or other defect on the metal surface may locally favor a rapid dissolution of the metal, creating the conditions for the emergence of a corrosion cell and for the autocatalytic stimulation and survival of pits. However, new pits are unstable and many of them become inactive after a few minutes of growth. Important to notice is that “all systems that show pitting attack are particularly susceptible to crevice corrosion” [6], however, the reverse is not always true.

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Some important factors for the development and rate of pitting attack include solution composition (chloride and chlorine-containing ions and oxidizing metal ions such as cupric and ferric ions are quite dangerous! On the other hand, presence of anions such as hydroxide, chromate and silicate above a certain threshold concentration may prevent pitting corrosion), velocity of the environment (pitting is usually associated with stagnant conditions!) and metallurgical variables such as alloy composition (SS are more susceptible than other classes of metals or alloys, and mild steel is better than SS!), cold working, and surface finishing (polished surfaces are more resistant). The methods suggested for combating crevice corrosion generally apply also for fight pitting corrosion. Important rule is that: never use to build a plant or equipment, materials that show pitting, or tendencies to pit, during corrosion tests. The best procedure is to use materials that are known not to pit in the environment under consideration [6]. The following list of metals and alloys may be used as a qualitative guide of suitable materials to resist to pitting corrosion [6]. Increasing pitting resistance

Type 304 Stainless Steel (18 % Cr, 8 % Ni) Type 304 Stainless Steel (18 % Cr, 8 % Ni, 2 % Mo) Hastelloy F (Ni alloy plus 22 % Cr, 6.5 % Mo, 2.10 % Nb, 1.25 % Co); Nionel, or Durimet 20 (Fe alloy plus 30 % Ni, 20 % Cr, 3.5 % Cu, 2.5 % Mo) Hastelloy C (Ni alloy plus 40 % W, 17 % Mo, 16 % Cr, 5.75 % Fe, 1.25 % Co) or Chlorimet 3 (Ni alloy plus 12.5-17.5 % Mo, 15.5-22.5 % Cr, 2.0-7.5 % Fe, 2.5-5.25 % W) Titanium

10.3.5. Intergranular corrosion Intergranular corrosion is a localized attack at and adjacent to crystal grain boundaries, with relatively little attack of the grains. The metal tends to the disintegration and loses its strength because the grains fall out. This attack may be caused by impurities at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements in the grain-boundary areas. For example, impurities of iron in aluminum, may be segregated to the grain boundaries, due to the low solubility of Fe in Al, and cause the emergence of a corrosion cell in this region. Another example is the depletion of Cr in the grain-boundary regions that results in intergranular corrosion of austenitic stainless steels subjected to thermal treatment in certain temperature interval. Indeed, when austenitic SS are heated in approximately the temperature range between 500 and 800 ยบC they become sensitized, i.e., susceptible to intergranular corrosion. This sensitization is explained, in the case of SS due to the fact that, generally more than 10 % Cr be needed to make steel stainless. When heated in the referred temperatures interval, and when the C content is higher than 0.02 %, the compound Cr23C6, virtually insoluble in steel, precipitates out of the solid

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solution, decreasing the Cr content to values lower than the referred threshold and conferring to the grain boundary the poor corrosion resistance of ordinary steel. The chromium carbide (Cr23C6) in the boundary is not attacked, but the Cr-depleted zone near the grain boundary is corroded. Common 18-8 SS (AISI 304) possesses 0.06 to 0.08 % C, therefore, an excess C is available to combine with Cr to form Cr23C6 that can precipitate in the referred temperatures range, for example, during gas welding processes, leading to SS sensitization. This explains degradation types such as “weld decay” and “knife line attack”, quite common with 18-8 SSs. In this type of attack the metal must have been heated in the sensitizing temperature range, particularly when gas welding is used, keeping a wide zone (“heat affected zone”) of the metal in the sensitizing range for relatively long time, implying great carbide precipitation. Electric arc welding is used more than gas welding for SS exactly because the former produces higher and more intense heating in shorter time. Methods to control or minimize intergranular corrosion of austenitic SSs include: 1.high temperature solution treatment (quenching-annealing or solution-quenching: heating the alloy to 1050-1120 ºC to allow Cr23C6 redissolution, followed of rapid water quenching, i.e. cooling); 2. Addition of strong carbide-former elements, i.e., “stabilizers” (Nb, Nb+Ta, or Ti that much greater affinity to C than Cr); and lowering the C-content to below 0.03 % (AISI 304L and 316L SS have a Low C content below 0.03 %!).

Carbon pickup (surface carburization) during casting of austenitic SSs, even of the low C SSs, into molds containing carbonaceous materials (organic binders and washes or baked oil sand) may cause premature failure due to intergranular corrosion, pitting corrosion and stress corrosion cracking. Metal cast in ceramic molds shows practically no C pickup [6]. The difference between weld decay and knife line attack (KLA) is that: 1. KLA occurs in a narrow band in parent metal immediately adjacent to the weld (hence its name!); 2. KLA occurs in stabilized steels, for example with type 347 SS (18-8 + Nb) and 3. The mechanism is different (the problem happens due to quenching annealing treatment!). Other alloys other than SSs may suffer from intergranular corrosion. For examples, Duraluminum Al-Cu alloys, similarly to other high-strength Al alloys, are strong because of precipitation of the CuAl2 compound. The substantial potential differences between Cu-depleted areas and adjacent material drives a corrosion cell that explains its susceptibility to intergranular corrosion. 10.3.6. Selective leaching or dealloying Selective leaching or dealloying is the removal of one element from a solid alloy by corrosion processes. Examples of this type of corrosion are dezincification (selective leaching of Zn from brass alloys), dealuminumification (removal of Al from Al bronzes), decobaltification (removal of Co from Co-W-Cr alloys), or graphitization (selective removal of Fe from gray cast iron), etc. Dezinfication of common yellow brass (~30 % Zn and ~70 % Cu) may be readly observed with naked eye because the alloy assumes a red (cooper) color. Uniform, or layer-type, dezincification favor the high brasses (high Zn content) and acid environments. A localized, or plug-type, dezincification occur more often in the low brasses (low Zn content) and neutral, alkaline, or slightly acidic conditions. Failures of red brasses (15 % Zn) because of dezincification rarely were observed. Stagnant conditions and presence of O2 increases the rate of attack. Graphitization or “graphitic corrosion” is the selective leaching of the Fe from iron or steel matrix, particularly in relatively mild environments, leaving the graphite network. The name comes from the fact that the cast iron body seems to be “graphitized” in that the surface layer acquires the appearance of graphite and can be easily cut with a knife. A corrosion cell is established since

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graphite (C) is cathodic to Fe that is dissolved, leaving a porous mass consisting of graphite, without the mechanical strength and metallic properties of the original alloy. White cast iron has essentially no free C and therefore is not subject to graphitization. Selective oxidation of an element in a alloy may occur in high temperature conditions. For examples, stainless steels subjected to high temperatures may show selective oxidation of chromium when exposed to low-oxygen atmosphere due to the relatively higher affinity of that metal for oxygen. 10.3.7. Erosion-corrosion Till now all types of corrosion presented had as main process the chemical interaction of the material with the environment where it operates. Erosion-corrosion is one of the classes of corrosion that involves not only a chemical process, but also mechanical interactions. Erosion corrosion implies an increase in the degradation rate of deterioration of a metal due to relative movement of the corrosive media to the metal surface, generally involving mechanical wear effects or abrasion. It is characterized by the appearance of grooves, gullies, waves, rounded holes, and valleys, usually exhibiting a directional pattern and, in many cases, failure occur in a relatively short time. Most metals and alloys, such as aluminum, lead, and SSs, whose corrosion resistance depends on the development of a protective surface film (passivation) are susceptible to erosion-corrosion. Erosion-corrosion results when these protective surfaces are damaged or worn and the metal and alloy are attacked at a elevated rate. Metals that are soft and readily damaged or worn mechanically, such as Cu and Pb are specially susceptible to erosion corrosion. Many types of corrosion medium could cause erosion corrosion, including gases, aqueous solutions, organic systems, and liquid metals. Solids in suspension in liquids (slurries) are particularly destructive. Since corrosion is involved in erosion-corrosion process all the factors that affect corrosion may also influence this type of degradation. The nature and properties of the protective films are very important from the standpoint of resistance to erosion corrosion. Hard, dense, adherent, and continuous films provide better protection that those that are easily removed by mechanical means or worn off. Velocity of the environment plays an important role in erosion corrosion, influencing strongly the mechanisms of the corrosion reactions. It could have mechanical wear effects at high values and particularly when the solution contains solids in suspension. Increased velocity may increase or reduce attack, depending on its effect on the corrosion mechanism involved. It may increase attack by increasing the supply of O2, CO2, or H2S, or by the scouring effect of solids in suspension on the protective layers. But it may also decrease attack in other situations by increasing the rate of supply of inhibitors, by preventing the deposition of silt or dirt (crevice corrosion), by sweeping corrosive agents from the surface (e.g.: SSs are used successfully in seawater and other chloride solutions without suffering from pitting or crevice corrosion). Turbulence or turbulent flow may contribute to erosion corrosion (e.g.: â&#x20AC;&#x153;inlet-tube corrosion in shell-and-tube heat exchangers, and corrosion in impellers and propellers) by promoting agitation of liquid and intimate contact between environment and metal surface. Impingement corrosion is typical failure in piping elbows and tees, steam-turbine blades, particularly in exhaust or wet-steam ends, cyclones, etc., where fluids are forced to turn its direction of flow. Solids and sometimes bubbles of gas, in particular air bubbles, in the liquid increase the impingement effect.

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Other important factors that influence significantly erosion corrosion rates include the presence of galvanic effects (destruction of passive films under combination action of galvanic corrosion and erosion), nature of the metal or alloy, particularly, the chemical composition, corrosion resistance, hardness, and metallurgical history. In general the addition of a 3rd. element to an alloy improves the resistance to erosion corrosion (e.g. addition of Fe to cupronickel alloy, Mo to 18-8 SS). Soft metals are more susceptible to erosion corrosion because they are subject to mechanical wear. Solid solution hardening is one sure method for producing good erosion corrosion resistance [6]. Prevention or minimizing erosion corrosion may be achieved by following actions (in order of importance or extent of use): 1. Better materials; 2. Better design (change in shape or geometry to attain low velocity or laminar flow, reduction of impingement, use of replaceable impingement plates or baffles, extended inlet tubes, etc.); 3. Alteration of the environment (e.g.: deaeration, addition of inhibitors, settling and filtration to remove solids, reduction of temperature) ; 4. Application of coatings (not always feasible, use of hard facings or welded overlays are sometimes helpful) ; 5. Cathodic protection. A special form of erosion corrosion, typically observed in hydraulic turbines, ship propellers pump impellers, and other surfaces where high velocity liquid flow and pressure changes are encountered is cavitation damage, caused by the formation and collapse of vapor bubbles in a liquid near a metal surface. Collapsing vapor bubbles may produces shock waves that reach pressures higher than 400 MPa, able to inducing plastic deformation in many metals [6], and, certainly, many protective surface films. The damage is explained by the repetitive sequence of corrosion with protective layer formation and destruction of the film by a collapsing vapor bubble in the same spot. Once the surface has been roughened at a point, this will serve as nucleus for the new cavitation bubbles, in a quasi-autocatalitic way. Techniques used to combat erosion carrion may be used to prevent cavitation damage, particularly through changing the design to minimize hydrodynamic pressure differences in flow streams, substitution by more corrosion resistant materials, smoothing the surface finishes of pump impellers and propellers (no sites for bubble nucleation!), coating the metallic surface with resilient anticorrosive coatings (rubber, plastic), and applying cathodic protection (formation of H2 bubbles may cushion the shock waves produced). Another special case of erosion corrosion is fretting corrosion, also known as friction corrosion, wear oxidation, chafing, and false brinelling. Fretting corrosion happens at the contact areas between surfaces of metallic bodies under load, subjected to vibration and slip (relative motion), in the atmosphere rather than in aqueous media, for instance, in engine components, automotive parts, bolted parts, and other machinery. This type of corrosion is very detrimental due to the destruction of metallic components and the formation of oxide debris, and consequent sizing, galling, loss of tolerance and loosening of mating parts. Fretting corrosion is the combined action of corrosion and wear and, as such, the minimizing or preventing measures include both chemical and mechanical actions: 1. Lubricate with low-viscosity and high tenacity oils and greases (reduction of friction and exclusion of O2); 2. Increase the hardness of one or both of the contacting surfaces (material selection, shot-penning, or cold working); 3. Increase friction between mating parts by roughening the surfaces (covering with lead); 4. Use gasket to absorb vibration and exclude O2 at bearing surfaces; 5. Increase load to reduce slip between mating surfaces; 6. Decrease load at bearing surfaces (not always recommended); 7. Increase the relative motion between parts to reduce attack.

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10.3.8. Stress-Corrosion Cracking (SCC) SCC refers to cracking caused by the combination of the presence of tensile stress and the chemical action of a corrosive environment. The metallic or alloy body is virtually unattacked over most of its surface, while fine cracks progress through it. This cracking phenomenon has serious consequences since it can occur at stresses within the range of typical design stress. There are essentially 2 types of SCC: the special “season cracking” of brass (cartridge cases at the point where the case is crimped to the bullet, during periods of heavy rainfall in the tropics, related to NH3 resulting from the decomposition of organic matter) and “caustic embrittlement” of steel. Stress corrosion cracks show an appearance of a brittle mechanical fracture, but it results, in fact, from local corrosion processes. Both, intergranular (proceeding along grain boundary) as well as transgranular (advancing without apparent preference for grain boundaries) SCC may be observed, even in the same alloy, depending on the environment or the metal crystalline structure. Cracking generally proceed perpendicular to the applied stress. In some cases cracks are virtually without branching, and in other cases they present a “river delta” pattern. Stress alone may provoke creep, fatigue, and tensile failure, and corrosion alone causes characteristic dissolution reactions. When combined disastrous results may occur! The stress that causes SCC must be tensile in nature of sufficient magnitude (the minimum depends on temperature, and environment and metal compositions) of various sources: applied, residual, thermal, from welding, and even from the wedging action of corrosion products in constricted regions. SSs crack in chloride-containing environments, but not in environments having NH3. The contrary applies to brasses. Further, the number of environments in which a given alloy will crack is generally small (e.g.: SSs crack in Cl--containing and caustic environments, but not H2SO4, HNO3, CH3COOH solutions or pure water). Important variables affecting SCC are temperature, solution composition, metal composition, physical state of the environment, and metal crystalline structure. The presence of oxidizers often has a pronounced influence on cracking tendencies: the presence of O2 or other oxidizing species is critical to cracking of austenitic SSs in chloride solutions, and if the O2 is removed, cracking will not occur. Characteristic of cracking producing environment is that the alloy is negligibly attacked in the non-stressed condition. SCC is accelerated by increasing temperature, and also by exposition to alternate wetting and drying conditions. Additionally, the susceptibility to SCC is affected by the average chemical composition, preferential orientation of grains, composition and distribution of precipitates, dislocation interactions, and progress of the phase transformation (degree of metastability). SCC may be reduced or prevented by application of one or more of the following methods: 1. Lowering the stress below the threshold value if one exists; 2. Eliminating the the critical environmental species; 3. Changing the alloy; 4. Applying cathodic protection; 5. Adding inhibitors; 6. Applying coatings, and 7. Shot-peening (shot-blasting) to produce residual compressive stresses in the surface of the metal, instead of tensile stresses. 10.3.9. Corrosion fatigue Fatigue is the tendency of a metal to fracture under repeated cycling stressing, usually at stress levels below the yield point and after many cycles of stress application. Characteristically the

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fatigue failures show a large smooth area (“hammering” effect) and a smaller area which has a roughened appearance (brittle fracture). In general, steels and ferrous materials show a fatigue limit, a stress value below which the metal will endure an infinite number of cycles without fracture. Non ferrous metals such as Al and Mg do not fatigue limit. On the other hand, corrosion fatigue, is the reduction of fatigue resistance due to the presence of a corrosive medium. In this case, the fatigue section shows a large are covered with corrosion products and a smaller roughened area resulting the final brittle fracture (purely mechanical, does not require presence of a corrosive). Corrosion fatigue is probably a special case of SCC, with different mode of fracture and preventative measures [6]. Environmental factors strongly influence corrosion fatigue behavior. In opposition to the ordinary fatigue resistance, corrosion fatigue resistance is markedly affected by the stress-cycle frequency, fatigue being more pronounced at low stress frequencies due to the increase in the contact time between metal and corrosive. Oxygen content, temperature, pH value, and solution composition influence corrosion fatigue behavior. E.g.: Austenic SSs and Al bronzes retain only 70-80 % of their normal fatigue resistance when moved from water to seawater! Corrosion fatigue seems to be most prevalent in mediums that produce pitting attack (pits act as stress raisers and initiate cracks), being usually transgranular, without branching characteristic of many SCC. Concerning to prevention methods, it must be noted that increasing the tensile strength of a metal or alloy that improves ordinary fatigue resistance is detrimental corrosion fatigue, since once a crack starts in a high-tensile strength material, it usually progresses more rapidly than in a material with lower strength. But, during corrosion fatigue a crack is readily initiated by the corrosive action. Corrosion fatigue may be prevented or reduced by lowering the stress on the component, for instance by changing the design (avoidance of structural vibration), by stress-relieving heat treatments, or by shot-peening the surface to induce comprehensive stresses. Use of corrosion inhibitors, coatings such as electrodeposited Zn, Cr, Ni, Cu, or nitride coatings, may also help improve corrosion-fatigue resistance. 10.3.10. Hydrogen Damage Hydrogen damage is a general term which refers to mechanical damage of metal caused by the presence of, or interaction with, H2. It may assume 4 forms: 1. Hydrogen blistering; 2. Hydrogen embrittlement; 3. Decarburization, and 4. Hydrogen attack. Hydrogen blistering is a phenomena that results from the penetration of hydrogen into the metal crystalline structure, and manifests as a local deformation, for instance of vessel wall. Hydrogen embrittlement results also from penetration of hydrogen into the metal, but with chemical interaction with it, resulting in products (hydrides) with lower ductility and tensile strength. Decarburization is the removal of carbon from steels, at high temperatures, which results in the reduction of its tensile strength. Hydrogen attack refers to the interaction between hydrogen and a component of an alloy at high temperatures.(disintegration of oxygen-containing cooper in presence of hydrogen. Hydrogen blistering and embrittlement may be explained by the diffusion of atomic hydrogen, H, through the crystalline structure of metals and alloys. Molecular hydrogen, H2, is too large to penetrate these structures! Typical sources of “nascent” or atomic H are high-temperature moist

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atmospheres, corrosion processes, cathodic protection, and electrolysis (electroplating). The presence of species such as sulfide ions, phosphorous, and arsenic compounds may increase the rate of these type of attack because they contribute to reduce the H+ reduction rate, increasing the amount of H that can diffuse into crystalline structure. During hydrogen blistering atomic H produced by H+-reduction at the metal surface penetrate into voids (a common defect in rimmed steel, for example) and forms H2 molecules that accumulate at these sites since it cannot diffuse out through the crystalline structure, and its concentration and pressure increases to values to rupture of any known engineering material. On the other hand, in the case of hydrogen embrittlement, after penetration into the metallic structure, the atomic H reacts with the metal itself, in particular with strong hydride-forming metals (e.g.: Ti), to form brittle hydride compounds. A few ppm of absorbed hydrogen can cause cracking [6]. Sulfide stress cracking, i.e., hydrogen embrittlement in presence of water and H2S is of great importance in the petroleum industry from production through refining. Apparently the iron sulfide formed on the metal surface has a catalytic effect in increasing the amount of H that enters the metal. H-blistering may be stopped by the use of one or more of the following preventive measures: 1. Using â&#x20AC;&#x153;cleanâ&#x20AC;? steel (use killed steel, instead of rimmed steel); 2. Using metallic, inorganic, and organic coatings or liners, which must be impervious to H penetration and resistant to degradation by the medium that contact with (e.g.: SS or Ni cladding of mild steel); 3. Using inhibitors which decreases corrosion rate or H-reduction rate; 4. Removing poisons, such as sulfide, arsenic compounds, cyanides, and phosphorous-containing ions; 5. Replacing alloys (e.g.: Ni-containing steels and Ni-based alloys have very low hydrogen diffusion rates. In turn, H-embrittlement may be limited by application of one or more of the following measures: 1. Reducing corrosion rate by careful addition of inhibitors (e.g.: pickling processes); 2. Altering plating conditions by proper choice of plating baths, and careful control of plating current intensity; 3. Baking at relatively low temperatures (90 to 150 ÂşC) to remove hydrogen; 4. Substituting alloys (e.g.: very-high-strength steels are more susceptible. Alloying with Ni or Mo reduces susceptibility); 5. Practicing proper welding by using low-H welding rods, maintaining dry conditions during welding (water and vapor are major sources of H). 10.3.11. Biocorrosion Biocorrosion, biological corrosion, microbiologically assisted corrosion (MAC), microbiologically induced corrosion (MIC), or microbial corrosion are all names for the deleterious combination of the chemical action of corrosion with biological effects. It may be described as the deterioration of a metallic body by corrosion processes that occur directly or indirectly as a result of the activity of living organisms. Biological activity may influence corrosion in a variety of environments, including soil, natural water, waste waters, and seawater, natural petroleum products, and oil emulsioncutting fluids. Living organisms ingest food, that may be a reactant in the corrosion system, and eliminate, metabolites that may be a corrosive. So biological activity can affect corrosion in the following ways: 1. By directly influencing anodic and/or cathodic reactions (e.g.: depolarization); 2. By influencing the stability of protective surface films (e.g.: depassivation); 3. By creating corrosive conditions (e.g.: corrosive metabolite); 4. By producing deposits (crevice corrosion). Probably the most important anaerobic bacteria that influence the corrosion behavior of buried steel structures are the sulfate reducing types (D. desulfuricans). These sulfate reducing bacteria (SRB) reduce sulfate to sulfide, according to

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SO42- + 4 H2 â&#x2020;&#x2019; S2- + 4 H2O

(10.12)

Source of the required H2 can be that evolved during the corrosion reaction or that derived from cellulose, sugars, or other organic compounds present in the soil. SRB are most prevalent under anaerobic conditions as in wet clay, boggy soils, and marshes. The S2- tends to retard the cathodic (particularly hydrogen evolution) reaction, and accelerates the metallic anodic dissolution. Under most conditions, acceleration of metal dissolution predominates over retardation of cathodic reaction. Therefore, in most situations, corrosion is increased. Aerobic sulphur oxidizing bacteria (SOB), such as thiobaccillus tiooxidans are capable of oxidizing elemental S or sulfur-bearing compounds to ulfuric acid according to 2 S + 3 O2 + 2 H2O â&#x2020;&#x2019; 2 H2SO4

(10.13)

SOB thrive best in environments with low pH value and can produce localized H2SO4 concentrations up to 5 %(w/w), i.e., SOB are capable of creating extremely corrosive conditions. SRB and SOB can operate in cycle fashion when soil conditions change, i.e., SRB grow rapidly during rainy seasons when the soil is wet and air is excluded, and SOB grow rapidly during dry seasons, when air permeates the soil. In certain areas, this cyclic effect causes extensive corrosion damage of buried steel pipelines. From equation (9) it is evident that the presence of microorganisms can accentuate conditions of differential aeration in soils. Microorganisms, such as bacteria, may affect corrosion processes indirectly, for example, by using as food hydrocarbons from the protective coatings (e.g.: asphaltic pipe coatings), or producing metabolites that induces different types of corrosion (e.g.: iron bacteria assimilates ferrous iron from the solution and precipitate it as Fe(HO)2 or Fe(HO)3 in sheets surrounding their cell walls. Therefore the growth of iron bacteria frequently results in tubercles on steel surfaces and to produce crevice attack. There bacteria that are capable of oxidizing ammonia to nitric acid, that may corrode iron and most other metals; some other bacteria produce CO2, which can contribute to the formation of carbonic acid and increase corrosivity of the environment.) There are several general techniques for preventing microbialogical corrosion: 1. Coating the buried structure with asphalt, enamel, plastic tape, or concrete; 2. Applying cathodic protection (especially effective when used with coatings); 3. Altering the environment (e.g.: removal of S and S-containing compounds by aeration of sewage); 4. Using corrosion inhibitors in recirculation systems. Macroorganisms such as fungus and mold assimilate organic matter and produce considerable quantities of organic acids, including oxalic, lactic, acetic, and citric acids. The most familiar attack of fungi is the mildewing of leather and other fabrics, but they can also attack rubber and bare and coated metal surfaces, in many instances without causing severe mechanical damage, but affecting undesirably the appearance of the product. In addition to the corrosion caused by the acids produced, fungi can also initiate crevice attack of metal surfaces. Mold growth on coated and uncoated metal surfaces can be prevented or reduced by periodic cleaning, by reducing relative humidity during storage and by using toxic organic agents. Fresh and salt water organisms such as barnacles, mussels, algae, and others attach themselves to solid surfaces during their growth cycle, and its accumulation causes crevice corrosion and, more importantly, fouling of structures (e.g.: barnacles can colonize rapidly ship bottoms and, in severe situations, this may increase its power requirements in as much as 30 %. Similarly,

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accumulation of macroorganisms in heat exchangers and other such devices severely limits heat transfer and fluid flow and may result in complete obstruction. Accumulation of aqueous macroorganisms, such as barnacles and mussels, is more severe in relatively shallow water (e.g.: harbor) and in warm waters (long breeding seasons and rapid multiplication). Relative motion between an d object and water inhibits attachment of organisms (e.g.: rapidly moving vessels accumulate only small quantities of organisms; High flow rate of seawater used as coolant in heat exchanges suppresses fouling). The nature of the surface strongly influences the accumulation of macroorganisms: smooth, hard surfaces offer an excellent point for adhesion, whereas rough, flaking surfaces tend to inhibit colonization. The best way to attack fouling is the use of antifouling paints, that contain toxic substances, usually Cu compounds. They function by slowly releasing Cu ion into the aqueous environment, inhibiting the growth of barnacles and others organisms. In closed systems various toxic agents and algaecides are used, specially chlorine and chlorine-containg compounds. 10.3.12. Corrosion of reinforced concrete structures Corrosion of reinforced concrete structures is mainly to the corrosion of steel reinforcement bars (rebars) in aggressive environment, it is not another form of corrosion, but, as in the case of biological corrosion, it may involve one or of the corrosion forms described above. In fact, the concrete covering layer does work as physical against the ingress of aggressive agents, but the porous nature of concrete cannot avoid that oxidants, such as O2, and harmful species reach the rebar surface. Fresh sound concrete possesses in its porosity an alkaline solution (pH > 12) made essentially of Ca(HO)2 that contributes to the stability of the passive film that protects steel from corrosion by O2. The penetration and accumulation of CO2 into concrete contributes to the acidification of the pore solution “concrete carbonation”), leading the dissolution of the passive films and exposure of the surface to the uniform attack by oxidants. Ingress and accumulation of chloride ion (“concrete chlorination”) do not alter the pH value of pore solution, but also contributes to instability of passive film, to the development of pitting corrosion that may evolve to a generalized corrosion. Crevice, SCC, and other forms of corrosion may happen inside the concrete layer, and corrosion products create expansion forces much greater than the tensile strength of the concrete around the reinforcement bars. These forces crack, fracture, and disaggregate the concrete layer, allowing more water and harmful agents to reach the steel surface, and accelerating corrosion and structure degradation, in an “autocatalytic” fashion.

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10.4. Corrosion in energy and environmental cleaning systems Corrosion can be found in any energy system and affect decisively its performances. Although galvanic corrosion constitutes the basis of the working capability of energy producing devices, such as galvanic cells and fuels cells, any spurious corrosion in one of the electrodes may considerably influence their efficiency. Avoidance and resolution of corrosion problems does certainly influence the capital and maintenance costs of facilities such as thermal and nuclear power plants. Safety questions related to the deposition of radioactive wastes oblige the considerations of corrosion of containers materials for this end. On the other hand, the efficiency of a heat engine or power plant is increased by operating at higher temperatures, which brings more corrosion problems for the majority of construction materials. Even modern alternative energy sources, such as solar, thermal and photovoltaic, geothermal, and tidal energy production systems or the application of biomasses for the production of energy involve some important corrosion problems that must be faced seriously before they can be truly competitive with the traditional energy sources. On the other hand environmental cleaning installations, such as incinerators for solid trash, scrubbers for flue gas desulfurization (FGD) of power plants and metallurgical plants and industrial and municipal sewage and waste water treatment plants, undergo different types of corrosion, according to the compositional complexity of effluents and of the processes they must be subjected. In fact, various wet and dry environments â&#x20AC;&#x201C; gases, liquids and solids â&#x20AC;&#x201C; are involved, and temperatures vary from low to very high. Metals from steel to platinum and nonmetals, including linings for metals are used as materials of construction, making of these facilities real chemical plants. To illustrate the importance of corrosion in energy and environmental cleaning system two text will be presented to the student as case studies for discussion in the class based on a proposed questionnaire.

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Case study 1: Metallic corrosion caused by biodiesel Read the text of Munoz et al.[7] on production, characterization and metallic corrosion caused by biodiesel, in particular the § 5 (“Metallic corrosion in biodiesel”) and answer to following questions: 1. In what sense corrosivity of biodiesel may affect its own storage stability and performance in engines? What metals and alloys and engine parts may be affected the most by this corrosivity? 2. Which characteristics of biodiesel make more corrosive to engine parts than normal diesel? Which types of corrosion may be expected to happen in engine parts and in biodiesel storage tanks? 3. What roles does water play in the corrosion aggressiveness of biodiesel? 4. How is the corrosivity of biodiesel tested following, for instance the EN and ASTM standards? What are the fundaments for these tests? 5. Why are the engine injection systems more prone to corrosion by biodiesel? 6. Which metals are more affected by the presence of residual alcohol in biodiesel? How can this susceptibility be explained? 7. How can be explained the development of microbial corrosion in presence of biodiesel? 8. How can the presence of catalyst residues, namely of Na+, K+, Ca2+, and Mg2+ ions, and trace metals influence biodiesel oil stability index (OSI) and metallic corrosion? How do the biodiesel influence its corrositivity towards metallic parts? 9. What is the equivalent mass loss of an aluminum alloy, in mg, that in an immersion test during 300 days in jatropha curcas biodiesel presented a corrosion rate of 11.7∙10-3 mpy? 10. Based on the experiments cited in the text, propose a short galvanic series for the following metals and alloys in palm oil diesel at room temperature: cooper, brass, aluminum, and cast iron. Compare the proposed galvanic series with the corrosion susceptibility of these metals in seawater. 11. How is it explained in the text that aluminum and stainless steels presents higher corrosion resistance than cooper and carbon steel in rapeseed oil biodiesel? 12. What the roles of oxygen, water and fatty acids in the corrosivity of biodiesel? 13. Discuss the following assertion that appears in the text “… oxygen concentration increases with increasing temperatures, which may explain the high corrosion rates at higher temperatures”, referring to the increasing biodiesel corrosivity at relatively higher temperature. 14. How does the movement of biodiesel inside of an engine affect the corrosion rate of metallic parts? How could the influence be explained? 15. Discuss on how the following methods may reduce the corrosiveness of biodiesels: 1. blending with normal diesel oil; 2. Using inhibitors (how they work? What is their influence in biodiesel performance?); 3. Using anti-oxidants (how they work? What is their influence in biodiesel performance?). 16. Based on the information achieved in the text, propose the corrosion cell of a galvanic contact between cooper and aluminum in a standard biodiesel. Specify all the elements of this corrosion cell.

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17. In the text it is referred that “metal ions cause the formation of deposits of insoluble soaps, as well catalyze polymerization reactions of biodiesel degradation.” Discuss the consequences of these processes to the corrosion of engine parts. Case study 2: Corrosion in Waste Water Treatment Plants at Pulp and Paper Mills Read the text of reference 8 and answer to following question, based in the knowledge acquired: 1. List the different forms of corrosion present in WWTP at pulp and paper mills cited by the authors an try a quick definition of each. 2. Based on the iron Pourbaix-diagram, predict the behavior of that metal if it would be used to collect the different process effluents streams that arrive to the WWTP of a paper mill. 3. What is the main difference between types 304 and 316 AISI stainless steels? Which 4. types of corrosion may both undergo when chloride ion is present in acid effluents of a paper mill? Why the SCC of austenitic SSs is rare in paper mill effluent treatment plants? 5. How can be explained that effluent waters with pH > 10 are benign to carbon steel? 6. How does chloride ion presence in waste waters influence the degradation of reinforced concrete structures? Explain the mechanism of chloride actuation. 7. According to the authors sulfate ion has no harmful effect on corrosion of carbon steel. How does SO42- contribute to the degradation of reinforced concrete structures? What relationship does sulfate ion maintain with microbial corrosion? 8. What are the corrosive consequences of the presence of sulfate reducing bacteria (SRB) in the return activated sludge (RAS) systems of pulp mill WWTP? Which type of corrosion does sulfide oxidizing bacteria (SOB) origins in carbon steel and stainless steel in some municipal sewers? 9. What is the main consequence of the presence of carbon dioxide in some equipment in a pulp mill WWTP? 10. Where is it located the anode of a differential aeration corrosion cell: in occluded surfaces, or on surfaces more accessible to aerated water? 11. Why do some corrosion problems occur around air spargers and blowers? 12. Based on the affirmation that “carbon steel corrodes by uniform acid corrosion when pH is below 4.5”, propose the corrosion cell, pointing out, in particular, the cathodic and the anodic reactions. 13. In addition, authors claim that “carbon steel corrosion is mostly mitigated with protective coatings, sometimes combined with cathodic protection”. What is cathodic protection? 14. What is the method that the authors point to prove that MIC of mild steel is caused by sulphate-reducing bacteria? 15. Propose an explanation for the fact pointed out by the authors of the austenitic stainless steel AISI 304 and 316 suffer from SCC in doluções containing ion Cl-above 55 º C and tensile loads sufficiently high, while ferritic stainless steels resist SCC in effluents treated (neutralized) and untreated (acidic). Propose an explanation for the fact pointed out by the authors of the austenitic stainless steel AISI 304 and 316 suffer from SCC in media containing ion Cl-, above 55 º C and with tensile loads sufficiently high, while ferritic stainless steels resist SCC in treated (neutralized) and untreated (acidic) effluents.

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16. What is the main source of carbon dioxide which attacks the concrete structures in wastewater treatment plants in pulp and paper mills? 17. Indicate some ways to prevent carbonation of the concrete. 18. Indicate some ways to prevent the attack of concrete by chloride ions. 19. Indicate some ways to prevent the attack of concrete by sulfate ions. 20. Explain the mechanism of the concrete and reinforced concrete structures attack by sulfate ions. 1. References 1. D.H. Grantham, “Hydrogen gas generation utilizing a bromide electrolyte, an amorphous silicon semiconductor, and radiant energy”, US Patent 4236984, 1980 2. K. Jordal, M. Anheden, J. Yan, L. Strömberg, “OXYFUEL COMBUSTION FOR COAL-FIRED POWER GENERATION WITH CO2 CAPTURE – OPPORTUNITIES AND CHALLENGES”, consulted at http://www.vattenfall.com/en/ccs/file/OXYFUEL-COMBUSTION-FOR-COAL-F_8470503.pdf, at 2013, July, the 5th. 3. S. Chakravarti, A. Gupta, B. Hunek, “Advanced Technology for the Capture of Carbon Dioxide from Flue Gases”, First National Conference on Carbon Sequestration, Washington, DC, May 15-17, 2001, consulted at http://netl.doe.gov/publications/proceedings/01/carbon_seq/p9.pdf, at 2013, July, the 5th. 4. P. Vanýsek, “Electrochemical Series”, CRC Press LLC, 2000 5. Consulted at http://www.copper.org/applications/cuni/txt_materials_selection.html, at 2013, July, the 10th 6. Corrosion Engineering, M.G. Fontana, 3rd. ed., McGraw-Hill, 1987, Singapore 7. R.A.A. Munoz, D.M. Fernandes, D.Q.Santos, T.G.G. Barbosa, R.M.F. Sousa, “Biodiesel: Production, Characterization, Metallic Corrosion and Analytical Methods for Contaminants”, InTech, http://dx.doi.org/10.5772/53655, pag. 129-176 8. R.A. Nixon, D.C. Bennett, “Corrosion in Waste Water Treatment Plants at Pulp and Paper Mill”, Corrosion Probe, Inc.,

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