Removal of cd pb hg heavy metal ion from simulation msw incineration flue gas by modified mineral ad

Frontier of Environmental Science September 2014, Volume 3, Issue 3, PP.109-118

Removal of Cd-Pb-Hg Heavy Metal Ion from Simulation MSW Incineration Flue Gas by Modified Mineral Adsorbents Liping Ma #, Yu Mao, Jiantao Chen, Hang Zhang Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China #

Email: maliping22@sina.com

Abstract During MSW incineration process, many hazardous materials such as Pb, Hg and Cd will be volatilized and exhausted into the air.

In this study, natural mineral and molecular sieve impregnated with two kinds of metal oxide were prepared for Cd 0, Pb0, and Hg0 removal from municipal solid waste (MSW) incineration flue gas. Here bentonite, mordenite, 4A molecular sieve and 5Amolecular sieve were used as carriers impregnated with CuO and CeO2 nitrate(or hydrochloride) solution respectively. The adsorption capacity for Hg0, Pb0 and Cd0 were tested at different temperature in the continuous fixed bed equipment. Results show that natural sorbents have lower adsorption capacity. After being modified, the adsorption capacity was increased greatly, SEM, EDS and BET analysis explain the change of adsorption capacity and the characteristic of these adsorbents. The adsorbents impregnated with CeO2 show the best effect on improving the adsorption capacity of these natural mineral adsorbents. The removal percentage was in the order of Cd0>Pb0>Hg0.

Keywords: Removal; Heavy Metal Ion; Incineration Flue Gas; Adsorbent; Modified

1 INTRODUCTION Incineration is one of the useful method to treat municipal solid waste(MSW). However, there are many hazardous material exhaust in the flame gas because of the complex contents of MSW. Among these hazardous materials, heavy metal such as Pb, Hg and Cd are payed more attention for atmosphere pollution. During incineration process, these heavy metals contained in MSW will be volatilized, and then may be condensed and agglomerated on ash with the change of gas temperature, and those which cannot be condensed will exhaust into the atmosphere with tail gas.

The particles (size: 0.01-10Îźm) are hard to catch by ordinary dust removal device, they will be exhausted into the air with tail gas. Emissions of heavy metals during the incineration process of MSW should be controlled[1-2].

According to the pollution protect law of People's Republic of China, the restrict concentration of Pb, Hg and Cd content in tail gas is 1.6 mg/m3,0.2mg/m3 and 0.1 mg/m3 respectively (with dry flue gas containing 11% O2 under standard condition for the reference value conversion). Some pollution control equipment must be used during the process of incineration, such as wet spray, bag filter and other collecting system. However, all of these methods could not collect submicron particles effectively. Many methods have been researched to remove heavy metal contained in gas, such as fly ash solidification technology, chemical stability method and adsorption method[3-8].

Changxing Hu. etc[9] researched on the effect of chemical activation of activated carbon modified with zinc chloride for elemental mercury adsorption, their experiments showed that the zinc chloride (ZnCl2) had significant Hg adsorption capability in the same experimental conditions. Mercedes Diaz-Somoano.etc[10]researched on removal of cadmium from a gasification flue gas using solid sorbents at high-temperature, results showed that kaolin, limestone, alumina and fly ashes all have certain adsorption capacities for cadmium compounds, and the adsorption capacity of fly ash is the best. Han Jun. etc[11] set up a general model of kaolin adsorbing heavy metal vapor, they found that the size of adsorbent particle have great effect on the chemical reaction rate and conversion rate. Ma. etc[12] studied on application of natural zeolite, bentonite and vermiculite and modified sorbents for mercury vapor - 109 http://www.ivypub.org/fes

removal from simulated flue gases. Other researchers had focus on the effect of gas temperature, elementary component and reaction atmosphere on adsorption for heavy metal contained in tail gas[13-17]. The compounds forms transform of these heavy metal contained in ash, organic effect on adsorption had also been researched[1819,3]. Active carbon, active carbon fiber, carbon nanotubes and resins also had been used to remove these metal ion contained in tail gas[8,10,5,16,20-21].

In order to fit the emission limits of air pollution control, most of the MSW incineration plant used active carbon as adsorbent to purify tail gas[22-23]. However, because of the high price and lower effect, developing high efficiency and low price adsorbent become the new purpose.

In this paper, bentonite, mordenite and molecular sieve were used as adsorbents to remove Pb0,Cd0 and Hg0 contained in MSW incineration tail gas, and modified adsorbents with metal oxide were prepared using these mineral as carries for removal of Pb0,Cd0 and Hg0 ion by continuous adsorption process, and the adsorption characteristic of these adsorbents also had been analyzed.

2

EXPERIMENTAL AND EQUIPMENT

2.1 Adsorbent Preparation Two kinds of minerals: bentonite, mordenite and two kinds of molecular sieve were used as adsorbents and carriers impregnated with CeO2, CuO respectively to prepare modified adsorbents. The minerals were pre-treated with 0.5mol/L NH4Ac aqueous solution at 60℃-95℃, then washed with distilled water until PH=7. After dried at 200300℃, these pre-treated mineral were mixed with 0.5mol/L nitric acid(or hydrochloric acid) solution which mixed with transition metal nitrate(or hydrochloride), then stirring for 2-4 hours under constant temperature，standing for 12 hours. After filtering, the solid was dried for more than 8 hours at 90 ℃, and then roasted 4-8 hours in a tube furnace, grinded the size to 60-80 mesh and got the modification adsorbents. the mixing proportion of the transition metal nitrate(or hydrochloride), deionized water, nitric acid (hydrochloric acid) and natural mineral solution is: each gram of calcined mineral added transition metal oxide 0.05-1g, deionized water 2ml, nitric acid (hydrochloric acid) 2ml.

2.2 Adsorption Experiments for Pb,Cd and Hg Adsorption activity measurement were carried out in the continuous fixed bed. The process are shown in Fig.1. The equipment include three parts: heavy metal evaporation section(1,2,4,7,8,9,10,16), heavy metal adsorption section(3,5,6,11,17) and tail gas purifying section(12,13,14). The device provided two gas path, one is the simulation flue gas contained heavy metal ion through the adsorbent bed, another is used for analysis and balance. The adsorbent mixed with a amount of quartz sand was put on the adsorption bed (11), the flow rate of the whole gas was 500mL/min, the flow rate of the balance gas was 200mL/min, and the flow rate of the carrier gas was 300mL/min.The temperature of adsorption experiments was carried out at 100-250 ℃, the amount of adsorbent was 50mg. Excess heavy metal ion contained in tail gas was abstracted with sulfhydryl cotton (12) and analyzed with atom absorption spectrometer[24].

1. N2 cylinder; 2-6.Cut-off valve; 7. Mass FlowmeterⅠ; 8. Mass FlowmeterⅡ; 9. Thermocouple; 10. Railboat; 11. quartz tube fixed bed; 12. Sulfhydryl Cotton; 13.u- tube; 14.Active carbon; 15.Tail gas; 16. Temperature controller; 17. Heating belt

FIG.1. EXPERIMENTAL SET-UP FOR ADSORPTION HEAVY METALS IN SIMULATED FLUE GAS - 110 http://www.ivypub.org/fes

2.3 Adsorbents Characterization The concentration of Hg, Cd and Pb were analyzed with atomic fluorescence analyzer (AF-610D2, Beijing Ruili Company) and Atomic absorption spectrometer (AA240FS,USA). BET analyzer (Autosorb-iQ,USA) was used to analyze the specific surface area and pore volume of adsorbents. Morphology and elements content were determined

by SEM and EDS(S4800SEM, Japan).

3 RESULTS AND DISCUSSION 3.1 Adsorption Heavy Metal by Original Mineral Adsorbents Fig.2 shows the breakthrough of Hg0, Pb0 and Cd0 adsorption by the original mineral adsorbents as a function of time at 200℃. It could be seen that the adsorption rate is very fast initially, about 10%-50% of total gaseous metal ion are adsorbed in 10min by these four kind of adsorbents. The adsorption capacity of bentonite is relatively weak for these three kinds of metal ion, almost 90% of Hg overflow in 10min. While the adsorption capacity of molecular sieve on heavy metals is relatively high. This agree with the results of Ding Feng .etc[25], they pointed out that molecular sieve adsorbent have the active sites for adsorbing heavy metal between layer, these active sites may be the ion and water, hydroxyl groups which filled between the adsorbent layer or the acid sites formed by the characteristics of mineral structure.

(a)

(b)

(c)

(d)

FIG.2. THE ADSORPTION BREAKTHROUGH FOR HG,PB AND CD WITH ORIGINAL MINERAL AT 200 ℃ (A) MORDENITE; (B) BENTONITE; (C) 4AMOLECULAR SIEVE; (C) 5AMOLECULAR SIEVE

3.2 Adsorption Heavy Metal by Modified Adsorbents Fig.3 gives the adsorption breakthrough for three kinds of metal ions by bentonite impregnated with CeO2 and CuO nitrate(or hydrochloride) solution. Comparing with original bentonite, the capacity of adsorption is greatly increased (Fig.2(b)). Fig.4 shows that the modified mordenite by CuO and CeO2 have strong capacity of adsorption on heavy - 111 http://www.ivypub.org/fes

metal ions at same conditions, especially the CeO2/mordenite adsorbent, the adsorption capacity for Cd0 has been more increased than that of original mordenite(Fig.2(a)), the breakthrough point increases almost 40min. Fig.5 shows the adsorption breakthrough for these three kinds of metal by modified molecular sieve. It is also clear that the modified molecular sieve has the better adsorption capacity than that of original molecular sieve,especially for Cd0.

(a) (b) FIG.3.THE BREAKTHROUGH FOR HG,PB AND CD WITH MODIFIED BENTONITE AT 200 ℃ (A) CUO/BENTONITE ADSORBENT; (B) CEO2/BENTONITE ADSORBENT

(a) (b) FIG.4 .THE BREAKTHROUGH CURVE FOR HG,PB AND CD WITH MODIFIED MORDENITE AT 200℃ (A) CUO/MORDENITE ;(B) CEO2/MORDENITE

(a)

(c)

FIG.5. THE BREAKTHROUGH CURVE FOR HG,PB AND CD WITH MODIFIED MOLECULAR SIEVE AT 200℃ (A) CEO2/4AMOLECULAR SIEVE;(B) CEO2/5AMOLECULAR SIEVE - 112 http://www.ivypub.org/fes

During the process of modifying, the water molecules contained in molecules continuously lost with the change of temperature, and the crystal skeleton structure is unchanged, which formed many cavities, can adsorb the small molecules to the internal bore diameter. Therefore the physical capacity of molecular sieve increases after modified.

From Fig.2 to Fig.5, it is clear that modified mineral adsorbents show stronger ability on adsorbing heavy metal than that original adsorbents, the reason may be the chemical adsorption capacity has been activated after modified, and the adsorbents modified by CeO2 show stronger ability than by CuO.

3.3 Temperature Effect on Adsorption The incineration flue gas temperature is an important factor for heavy metal ions removal by adsorption. Fig.6 shows the temperature effect on the adsorption for three kinds of metal ions by unmodified adsorbents. Before loading metal oxide, the original mineral and molecular sieve have the better adsorption capacity for Pb0, Hg0 and Cd0 at 200℃. The removal rate is calculated as follow:

removal rate =

c0  cout c0

Where c0 is the initial concentration of metal ion contained in gas before adsorption, cout is the output concentration of metal ion after adsorption. Fig.7 shows the adsorption capacity for these three kind of metal ion by modified adsorptions. The best adsorption capacity is CeO/5A molecular sieve at 175℃. Table 1 give the best condition and removal rate by different adsorbents.

The results show that the modified adsorbents have the best adsorption capacity than unmodified, and the adsorbents modified by CeO2 has higher adsorption capacity than by CuO, which explain that natural mineral adsorbents for heavy metal ions are not only a single physical process, it may have closely relations with its structure and composition. The removal percentage is in the order of Cr0>Pb0>Hg0 by CeO/5A molecular sieve, and the suitable temperature is similar to the real gas temperature of incineration flue gas.

FIG. 6 THE REMOVAL RATE OF UNMODIFIED ADSORBENTS ON HEAVY METAL AT 100-250℃ (A) BENTONITE; (B) MORDENITE; (C) 4A MOLECULAR SIEVE; (D) 5A MOLECULAR SIEVE

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FIG.7 THE REMOVAL RATE OF MODIFIED ADSORBENTS ON HEAVY METAL AT 100-250℃ CUO/BENTONITE; (B) CEO2/BENTONITE; (C) CUO/MORDENITE ;(D) CEO2/MORDENITE; (E) CEO2/ 4A MOLECULAR SIEVE; (F) CEO2/5A MOLECULAR SIEVE TABLE 1. THE BEST REMOVAL RATE OF UNMODIFIED AND MODIFIED ADSORBENTS ON HEAVY METAL adsorbents bentonite mordenite 4A molecular sieve 5A molecular sieve CeO2/bentonite CuO/bentonite CeO2/mordenite CuO/mordenite CeO2/4Amolecular sieve CeO2/5Amolecular sieve

Best adsorption temperature℃ 200 200 200 200 200 225 175 225 175 175

The best removal rate for Hg,% 11.0 39.3 47.6 49.9 60.3 58.9 79.0 68.8 88.0 90.0

The best removal rate for Pb,% 20.0 27.6 40.8 45.2 55.0 37.0 82.5 66.6 90.0 91.0

The best removal rate for Cd,% 38.0 39.8 49.4 48.7 65.0 62.0 87.5 71.1 98.6 99.5

3.4 Adsorbents Activity Characteristic The BET analysis for the adsorbents prepared above are listed in table 2. After modification, the BET specific surface area of all adsorbents decrease, while the average size of pore volume generally increase. After modification, The - 114 http://www.ivypub.org/fes

aperture of these adsorbents also increase. the more decrease of specific surface area is 5A molecular sieve, from 121 .5133m2路g-1 to 4.7110m2路g-1. At the same time, the average diameter of pore is increased from 2.06436nm to 9.68758nm. According to the size of aperture, all pore structure of the adsorbents are mainly microporous. the specific surface area of modified bentonite decreased, average pore diameter increased, at the same time, after NH4Ac pretreatment, impurity of the adsorbent various pore is clean, therefore the adsorption capacity is increased for heavy metal ions( Fig.3 to Fig.5).

Fig.8 and Fig.10 give the SEM analysis for natural mineral bentonite, 4A molecular sieve and they modified with CeO2 adsorbents. Is could be seen that the surface of bentonite, 4Amolecular sieve are irregular shape. After impregnated with CeO2, it could see a bright block 1(Fig.10b) which may mean the uneven distribution of Ce, and the point 2 means the Ce has been penetrated into the carrier. Further energy spectrum scanning analysis of these four kinds of adsorbents are showed in Fig.9 and Fig.11. It is clear that the Ce have seeped into internal smaller empty structure.

TABLE 2 BET ANALYSIS FOR ADSORBENTS Sample mordenite CeO2/mordenite CuO/mordenite bentonite CeO2/bentonite CuO/bentonite 4Amolecular sieve CeO2/4Amolecular sieve 5Amolecular sieve CeO2/5Amolecular sieve

BET specific surface area (m2路g-1) 9.7268 7.7672 7.9531 86.709 49.1238 50.0021 27.1275 8.6213 121.5133 4.7110

Pore volume (cm3路g-1) 0.01950 0.01871 0.01988 0.14017 0.13076 0.12118 0.10144 0.04373 0.06271 0.01141

Aperture (nm) 8.01812 8.31805 8.21718 6.46604 7.95524 7.75722 14.95788 20.26341 2.06436 9.68758

(a) (b) FIG.8 SEM IMAGE OF BENTONITE (A) AND CEO2/BENTONITE (B)

(a) (b) FIG.9 THE EDS GRAPHS OF POINT 1 AND 2 IN THE CEO2/ BENTONITE SHOWN IN FIG.8 (B) - 115 http://www.ivypub.org/fes

(a) (b) FIG.10 SEM IMAGE OF 4AMOLECULAR SIEVE (A) AND CEO2/ 4AMOLECULAR SIEVE (B)

(a) (b) FIG. 11 THE EDS GRAPHS OF POINT 1 AND 2 IN THE CEO2/ 4AMOLECULAR SIEVE SHOWN IN FIG.10 (B)

4 CONCLUSIONS The adsorption capacity for heavy metal ion by original mineral adsorbents is not strong. Molecular sieve has better adsorption capacity than the other original mineral adsorbents. After modified, the adsorption capacity is increased greatly. The adsorbents modified by CeO2 show stronger ability than by CuO, amongs these modified adsorbents, CeO2/5Amolecular sieve have the best ability on heavy metal ions adsorption at 175℃ than others, the removal percentage is in the order of Cr0>Pb0>Hg0. SEM and EDS analysis further explain the adsorption capacity transform and the characteristic of these adsorbents.

Because the price of natural mineral is cheap and easy to get, it has wide application in the removal of heavy metal ions from flue gas, different natural minerals has different structure and properties, CeO2 shows the best effect on improving the adsorption capacity of these natural mineral adsorbents.

ACKNOWLEDGMENT Financial support of the work by the Natural Science Foundation of China (NO. 21176108) and the National High Technology Research and Development Program of China (863 Program) (NO. 2011AA06A106) is greatly acknowledge. We also would like to thank the key Laboratory of Kunming University of Science and Technology for characterization analysis.The heading of the Acknowledgment section and the References section must not be numbered.

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AUTHORS 1

Liping Ma. Borned in Kunming, 1966,

11.

Ph.D.,

Chemical

Engineering,

Sichuang University, China, Sep. 1999 to Jul. 2002. The main research area is reuse and recycle of solid waste, air pollution

Now her research focus on the solid waste and air pollution control.

Prof. Ma is a Member of Yunnan Environment Association, ACS and AIchE. Yu. Mao. The master of Faculty of Environment Science and

control.

She is a professor of Faculty of Environment Science and Technology, Kunming University of Science and Technology, P. R. China. she is the Director of Department of Resource Recycle Science and Technology.

Technology, Kunming University of Science and Technology, P. R. China. She graduated in May, 2013, and now she works in Xiamen as a research staff in a company Jiantao Chen. The post graduate student of Faculty of Environment Science and Technology, Kunming University of

work experience:

Science and Technology. She research on the removal of Hg and

1992 to 1999: Institute of Chemical Engineering of Yunnan, Kunming, P.R.China;

NOx from fuel gas Hang Zhang. The master of Faculty of Environment Science

2006.2 to 2007.3: University of Michigan, visiting scholar, USA;

and

Technology,

from 2002 to now, Teacher, professor. Her research area is

Technology. He graduated in May, 2013, and now he works in

mainly adsorption and separation, fluidization theory analysis

Fuzhou power design institute as a environmental engineer.

and environmental pollution control technology. In the last 5 years she had published almost 30 research papers as an auther.

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Kunming

University

of

Science

and