Preparation of nanometer magnesia and its properties for fluoride removal

Frontier of Environmental Science September 2014, Volume 3, Issue 3, PP.97-108

Preparation of Nanometer Magnesia and Its Properties for Fluoride Removal Huifang Zhou 1,2, Wen Chen 1,2#, Xianghui Zhang 1,2, Dan Shi 1 1. College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu, Sichuan 610059, China 2. Mineral Chemistry Key Laboratory of Sichuan Higher Education Institution, Chengdu, Sichuan 610059, China #

Email: chenwen2010@foxmail.com

Abstract Nano-MgO powders were prepared by direct chemical precipitation using polyethylene glycol (PEG 400) as dispersing agent. The technological conditions for nano-MgO preparation were ammonia, Mg2+, PEG, reaction at 60 ℃, reaction time in 1.5 h and aging time in 2 h, calcination at 500 ℃ after filtrating and vacuum drying. The optimum conditions for removal of fluoride are as follows: pH range of 6.0~7.0, temperature is 25 ℃, adsorbent dosage is 20 mg/10 ml, initial concentration is 40 µg/mL, and ionic strength is 0.1 mol/L KNO3. It was found that the nano-MgO had the largest adsorption capacity of 74 mg/g. The adsorption data were well described by the Langmuir isotherm model and the two-constant rate equation kinetic model. According to these studies, the nano-MgO has potential application in fluoride ions removal from water.

Keywords: Nano Magnesium; Defluorinate; Adsorption; Kinetics; Thermodynami

1 INTRODUCTION The quality of drinking water is very important for public safety and the living quality. Excess fluoride in drinking water causes dental, skeletal fluorosis, decreases growth and intelligence [1]. For example, skeletal fluorosis is caused by continuous, excessive exposure to fluoride and is characterized by axial osteosclerosis, joint pain, ligamentous ossification and fractures [2]. Fluoride enters the human body mainly through food and water intake.

Studies indicate that soluble fluoride in drinking water is the highest contributor to daily fluoride intake [3], and that drinking water is thus the most significant source of human fluoride ingestion [4]. High fluoride level in groundwater is a worldwide problem; India and China have the highest fluorosis prevalence in humans in Asia [5]. Fluorosis is a serious public health problem throughout most of China. In 2010 there were 41.76 million fluorosis cases in 1325 different counties, out of which 58.2 % were caused by chronic exposure to high levels of fluoride in drinking water[6].

Yuanmou County in Yunnan Province, China, has been identified by the Center for Disease Control and Prevention (CDC) as an area where endemic fluorosis caused by using groundwater high in fluoride content for drinking purposes, prevails [7]. Therefore, it is necessary to treat fluoride-contaminated water and control the fluoride concentration to a permissible limit, which is 1.5 mg/L in the WHO guideline [8]. And The Chinese national guideline value for fluoride in drinking water is 1.0 mg/L. Compared with the common defluoridation methods, adsorption which is simple, high-efficiency, versatile and economical is regarded as the most appropriate method [9]. It is reported that many adsorbents have been used in defluoridation. Synthetic and biomass materials such as different types of aluminas [10], aluminum fluoride complexation [11], Zr(IV)-loaded GP gel [12], stilbite zeolite modified with Fe(III) [13], Mg–Al–Fe [14], hydrous zirconium oxide [15] , granular ceramic adsorption [16], Hydrous bismuth oxides (HBOs) [17], bauxite [18], Zr(IV)– ethylenediamine [19], Zr–Mn composite material [20], and hydroxyapatite [21] .

These adsorbents have shown a certain degree of fluoride adsorption capacity but some of them can only be used in a narrow pH range (5~6) and some of them are too expensive to be considered for full-scale water treatments.

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Recently, considering their large specific area and great mass transfer efficiency, nanosized materials, such as zirconium-loaded garlic peel [22], nano-goethite [23], CeO2-ZrO2 nanocages [24], polypyrrole/Fe3O4 magnetic nanocomposite [25], Fe-Ti oxide nano-adsorbent [26], cellulose/hydroxyapatite nanocomposites [27], Fe-Al-Ce nano-adsorbent [28], Fe3O4/Al2O3 nanoparticles [29] have been developed for effective removal of fluoride. The use of nanoparticles in water treatment, however, is limited by the costly synthetic process , separation process.

Furthermore, some of them need a high temperature; interfering with coexisting ion and resulting in a supplementary difficulty in eliminating residual chemicals in excess bring about secondary pollution.

MgO is avirulence and magnesium resources rich in China, not only exists in sea water, bittern, Well Brine etc liquid resources, but also is very rich with magnesite, dolomite, brucite, serpentine etc natural mineral resources, these provide a solid resource advantage for magnesium oxide adsorbent products. And China’s energy is abundant and low-cost, providing a guarantee for production dedicated magnesia material. Thus, nano-MgO has a high potential to be applied in adsorption systems.

Up to now, A few scholars had done some researches of active magnesium oxide for removal of fluoride. These include conditions of active magnesium oxide for defluorination [30], Preparation of activated magnesium oxide defluorination adsorbents by microwave radiation method [31], fluorine removal of fluorine-containing mineral spring water [32]. However, active magnesium oxide is no match for nano-Mgo which has a larger specific surface area. The study indicated that nano-MgO be able to achieve an adsorption capacity of 20.66 mg /g [33]. Now nanoMgO’s application in thick films of Bi2Sr2CaCu2O8 [34], removal of phosphate and nitrate from aqueous solutions were investigated [35]. There were no literature reported the kinetics and thermodynamics of the F- adsorption on nano-MgO in drinking water. It is an important part of the explanation nano-MgO adsorption mechanism, which is crucial to further research and its application.

In this study, nano-MgO(100 nm) was obtained by high temperature calcination. The optimum removal of fluoride ions on nano-MgO was evaluated. For this, batch and continuous mode sorption experiments were conducted by using nano-MgO as the sorbent. Attempts have also been made to learn the adsorption kinetics and thermodynamics of adsorption.

2 EXPERIMENTAL 2.1 Materials PEG 400, NaOH, HCl, MgCl2 • 6H2O and absolute ethanol used were analytical grade. A stock solution (500.0 µg/mL) was prepared by dissolving 0.5525 g NaF (analytical grade, obtained after forging at 2 h in the temperature range of 120 ℃) in 500 ml of distilled water. All the solutions for fluoride removal experiments and analysis were prepared by an appropriate dilution from the stock solution.

2.2 Synthesis of Nano-Mgo Procedure 10.17 g MgCl2 • 6H2O and 13.7 mL PEG 400 (30 % mass of magnesium chloride) was dissolved in 50 mL of absolute ethanol. 15 ml ammonia solution-ethanol (1:1) solution (with MgCl2 ratio of 2:1) was slowly added into the mixed solution until the solution was heating to 60 ℃. Adding at a rate of one drop per second with continuous magnetic stirring, then white precipitate formed. The solution keeps thermal insulation at 60 ℃ for 1.5 h. Then the solution becomes ageing at room temperature for 2 h. The precipitate formed was in vacuum drying at 60 ℃ at the end of the precipitate was separated by filter. This was followed by repeated washing with distilled water and ethanol.

The nano-MgO was calcined at 500 ℃ for 1.5 h, and then the nano-MgOs were obtained. The synthesis reaction equations may be expressed by: Mg2++2OH-=Mg(OH)2↓

(1)

Mg(OH)2=MgO+H2O

(2)

2.3 Characterization of Nano-Mgo Precursor A NETZSCH STA409PC simultaneous thermal analyzer was used for thermogravimetry (TG) and differential - 98 http://www.ivypub.org/fes

thermal analysis (DTA) with different scanning rates (5, 10, 15, 20 K/min) from 30 to 1100 ℃. An alumina crucible container was used in air. The gas flow rate was 30 ml/min. Sample weight was around 12 mg. The decomposition product was collected at the temperature that was shown on the DTA-TG curves. Prior to experiments, temperature calibration was performed. All experiments were performed twice to show the reproducibility. The thermokinetic calculation was conducted with the help of NETZSCH thermokinetic software, using the detected TG data.

The X-ray diffraction (XRD) patterns of each sample were obtained from random mounts using a Rigaku D/MAXШC powder diffractometer. The X-ray generator is equipped with a Ni filter and generates a beam of CuKa radiation (λ=0.15418 nm). The operational settings for all the XRD scans are voltage: 40 kV, current: 30 mA, range: 5< 2θ <70°, scanning speed: 0.6°/s.

Morphology of the samples was determined by scanning electron microscope (SEM) with S-3000N (Hitachi, Japan).

2.4 Determination of Fluoride Adsorption Capacity 10 ml 40 µg/mL fluoride solutions were added separately into 50 ml plastic pipe, with KNO3 as the background electrolyte in a concentration of 0.1 mol/L, The solid-liquid ratio is 1:500 g/mL. The pH of the test solution was kept at 6.0~7.0 by titrating with 2.0 mol/L HCl or 1.0 mol/L NaOH solution. The test solution was shaken at 25 ℃ for 0.5 h and standing for 24 h during the adsorption test. The blank control should be measured using 10 ml distilled water.

After adsorption, the solutions were filtered and the filtrate was analyzed. The fluoride ion concentration remaining in the filtrate was measured with a JP-303 polarograph. The second order derivative linear sweep voltammetric peak currents (ip〞) were recorded in the potential range from −0.30 V to −1.30 V (vs. SCE) at room temperature, scanning rate and standing time was 500 mvs and 8 s respectively.

The adsorption capacity Q (µg/g); and fluoride removal efficiency P (%) was calculated from the mass balance equation respective presented by: Q=[(C0-C)/m]V

(3)

P=[(C0-C)/C0]100%

(4)

where C0 (µg/mL) is the initial fluoride concentrat ion and C (g/mL) is the final fluoride concentration after adsorption, V is the volume of the solution containing fluoride ions, and m is the adsorbent granules dose of 20 mg/10 mL.

3 RESULTS AND DISCUSSION 3.1 Thermal Decomposition Process of As-Prepared Products

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FIG. 1 XRD PATTERN AND SEM PHOTO OF THE AS-PREPARED PRODUCTS: (a) XRD PATTERN; (b) SEM IMAGE

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The XRD patterns and SEM image of the as-prepared products were given in Fig.1. The XRD pattern shows that all the diffraction peaks in the pattern are in agreement with those of the hexagonal type of Mg(OH)2 with space group Pmp21(18) and lattice constants a= 3.144Å, b= 3.144 Å,c=4.777Å (standard card JCPD No. 44-1482). No diffraction peaks of impurities are observed. Hence the single phase Mg(OH)2 is prepared successfully.

The SEM image (Fig.1b) shows that the Mg(OH)2 particles are uniform, consisting of plane-like grains.

Fig.2 shows the DTA-TG-DTG curves of the as-prepared products at a heating rate of 10 K/min in air below 1000 ℃. The thermal decomposition of the as-prepared products occurs in three steps in concordance with the DTG peaks, and all steps are endothermic reaction.

The first step is from 80 to 220 ℃( Tmax=116.9 ℃ , WL=2.44%) and The second step is from 202 to 400 ℃ (Tmax=373.6 ℃, WL=31.58%), respectively, attributed for the evaporation of adsorbed water and hydroxide of the magnesium and the formation of magnesium oxide. But the whole weight loss is 34.03%, higher than the theoretical value (31.03%). we attempted to explain this due to the reason that the loss of the water of crystallization in the magnesium hydroxide. The corresponding XRD patterns support the reaction process, a pure MgO (standard card JCPD No.JCPD No.65-0476) is observed in Fig.3a. The SEM image (Fig. 3b) shows that the MgO particles are uniform; consisting of sphericity-like grains about 20nm in size. Hence the nano-MgO is prepared successfully.

The third step is from 400 to 1000 ℃ (WL=0%, Tmax=779.4 ℃) without mass loss shows the phase transition process of magnesium oxide.

FIG. 2 DTA-TG-DTG CURVES OF THE Mg(OH)2 AT A HEATING RATE of 10K/min in AIR 1000

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FIG. 3 XRD PATTERN AND SEM IMAGE OF THE AS-PREPARED NANO-MgO (a) XRD PATTERN; (b) SEM IMAGE

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3.2 Fluoride Removal Studies.

1) Effect of pH and Ionic Strength The pH of the medium is one of the important parameters that significantly affect the extent of fluoride adsorption.

Other parameters were equal to the fluoride adsorption capacity determination test. The effect of pH on fluoride removal from aqueous solution by the nano-MgO was studied at various pH values (ranging from 4 to 9). Fig. 4 shows the adsorption capacity of fluorinion onto the nano-MgO in different pH. The nano-MgO exhibited a strong adsorption of fluoride when the pH was 4.0. However, nano-MgO is alkaline in the water.

When the pH was 4.0, MgO has been partially dissolved, it will combine with fluorine ion in the solution of MgF2, reduce fluorine ion concentration in the solution, showing the illusion of a large amount of adsorption. When the pH was between 6.0 and 7.0, the nano-magnesium on fluorine adsorption capacity is high and stable. After pH 9, the fluoride the adsorption adsorption capacity is decreased because the change in surface negative charge of the adsorbent. removal is mainly governed by ion-exchange mechanism. So in the study, we selected the acidity between 6.0 and 7.0. The distilled water we use whose pH is between 6.0 and 7.0, so the experiments without pH adjustment.

To observe the effect of ionic strength on adsorption capacity, 0-0.5 mol/L KNO3 concentration was studied. Ionic strength is very important variable in adsorption. Take the test solution 0.1 mL measured experimental results shown in Fig. 5. When the ion concentration was 0.2 mol/L, the adsorption capacity of fluoride on nano-magnesium was nearly balanced. However, in real life, the ion concentration of the solution reached a highest value of 0.1 mol/L, so we chosen the ion concentration of 0.1 mol/L. 2) Effect of Temperature and Initial Fluoride Concentration When other parameters were unchanged. The samples obtained at temperatures of 25℃, 35℃, and 45℃, respectively. The adsorption rates keep balanced with the temperature changed. The adsorption rates is high when the reaction temperature was 25℃. So the best temperature of this experiment is setting for room temperature, which is 25 ℃.

Equal masses of nano-MgO were put into contact with solutions of different concentrations of fluoride-ion solutions (from 10 to 60 µg/mL). Fig. 6 shows that as fluoride concentration increases, adsorbed amounts of fluoride increase too at first. At 20 µg/mL, the adsorption reached equilibrium and the adsorption rates decreased more than 40 µg/mL. This plateau occurs due to the saturation of active sites on the adsorbent surfaces. At low fluoride concentrations, the ratio of surface active sites to fluoride ions is high. After the The adsorption quantity saturated, the adsorption rate is reduced with the increase of initial fluoride concentration increase(c>40μg/mL). So the optimal initial concentration is 40 µg/mL. 19

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FIG. 4 EFFECT OF pH ON THE ADSORPTION CAPACITY (THE INITIAL F− CONCENTRATION WAS 40 µg/ml; ADSORBENT DOSAGE 20 mg/10 mL; 298 K)

FIG. 5 EFFECT OF IONIC STRENGTH ON THE ADSORPTION CAPACITY (THE pH=6.0~7.0; THE INITIAL F− CONCENTRATION WAS 40 µg/ml; ADSORBENT DOSAGE 20 mg/10 mL; 298 K).

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FIG.6 EFFECT of INITIAL FLUORIDE CONCENTRATION on THE ADSORPTION RATE (ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0; 298 K).

5

FIG..7 EFFECT of SOLID-LIQUID RATIO on THE ADSORPTION RATE (THE INITIAL F− CONCENTRATION WAS 40 µg/ml; pH=6.0~7.0; 298 K).

3) Effect of Adsorbent Dosage Changing adsorbent dosage from 5 mg/10 ml to 40 mg/10 ml in the solution, take the test solution 0.5 mL, measured experimental. Fig. 7 shows the adsorption effect to flouride was increased with the increase of the amounts of nanoMgO. It is evident that the percent of fluoride removal increased with the increase of the adsorbent concentration which is due to the fact that a greater amount of adsorbent provides greater number of available binding sites. When the adsorbent dosage is 20 mg/10 ml, there were reaching a value of steady. Based on the above discussion, the adsorbent dosage of 20 mg/10 ml was used as the optimum value for the fluoride ion adsorption experiments.

By the study of relation between single experimental factor such as solid-liquid ratio, initial concentration, initial pH of solution, adsorption temperature and adsorption effect, the optimum adsorptive removal rate can be obtained: adsorbent dosage = 20 mg/10 ml, pH=6.0 ~ 7.0, C0=40 mg/L, T=25 ℃, ionic strength was 0.1 mol/L. Adsorption capacity of fluoride removal reaches 74 mg/g.

3.3 Effects of Coexisting Ions in Ground Water Contaminated groundwater usually contains several other co-existing ions along with fluoride, which may compete with fluoride for the active adsorption sites. Hence, it is imperative to investigate the possible interference of these ions (HCO3−, Cl−, NO3−, SO42−, PO43−, Ca2+, Mg2+, Fe3+ and Mn2+) on fluoride removal by nano-MgO. The adsorption studies were carried out in the presence of distilled water compare with ground water. This may contain chloride, sulfate, nitrate, bicarbonate, phosphate and calcium. At an initial fluoride concentration of 20 µg/mL, the effects of these co-existing ions on fluoride removal in the ground water did not significantly affect the removal of fluoride. Under the optimum conditions, in the distilled water, the adsorption rates is 96.53 %, and the concentration of fluoride solution is 0.70 µg/mL by defluorinated. Regard to groundwater, the adsorption rates is 96.67 %; the concentration of fluoride solution is 0.67 µg/mL by defluorinated. All these were lower than drinking water standard of China (1.0 µg/mL).

3.4 Adsorption behavior and possible mechanism of fluoride Nano-MgO has smaller particle size than ordinary magnesium oxide, thus shows different surface effect which ordinary magnesium oxide doesn’t have. Nano-MgO’s surface extremely increases when its particle size decreases, and then making the surface adsorption sites increased greatly. The fluoride adsorption may be due to the combined effect of both chemical and electrostatic interaction between the oxide surface and fluoride ion and also availability of active sites on the oxide surfaces. As reported earlier, anion adsorption on metal oxide surface is through - 102 http://www.ivypub.org/fes

columbic forces and/or ligand exchange reactions, where the anions displace OH− or H2O from the surface [37]. In our studies it was observed that the pH of the equilibrated solution slightly increased in acidic pH range, while it slightly decreased when the initial solution was alkaline. Hence, the fluoride adsorption on nano-MgO may be due to anion exchange at acidic pH and by van der Waals forces at alkaline pH ranges (Fig. 4). The most probable mechanism for fluoride adsorption in acidic conditionis mainly as [38]: MOH + H2O + F− ↔ MOH2+ + F− + OH− ↔ MOH2–F + OH−↔ MF + H2O+ OH−

(5)

3.5 Adsorption Isotherm Studies Adsorption capacity is one of the basic parameters required for the design of any batch or fixed-bed adsorption system. The equilibrium adsorption study of fluoride by nano-MgO was carried out at optimum adsorptive condition which obtained above. The initial fluoride concentration was varied over a wide range from 10 µg/mL to 400 µg/mL, in order to attain complete exhaustion of the adsorbent. The saturated adsorption capacity of fluoride on nano-MgO powder was showed (Fig. 8).

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FIG. 8 THE INFLUENCE OF DIFFERENT INITIAL CONCENTRATION ON ADSORPTION CAPACITY (ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0; 298 K)

The data obtained from this study are analyzed with the help of two isotherm models, including the well-known Langmuir and Freundlich. The linearized form of the Langmuir and Freundlich isotherm are given below.

Langmuir model: Ce/qe = Ce/qm + 1/(KL×qm)

(6)

lgqe = lgKF + 1/nlgCe

(7)

Freundlich model:

where qe is the amount of solute adsorbed per unit weight of adsorbent at equilibrium (mg/g); Ce is equilibrium concentration of the adsorbate in the solution (mg/L); qm represent maximum specific up take capacities at equilibrium of Langmuir (mg/g); KL is the Langmuir isotherm constants (L/mg); KF is the Freundlich isotherm constant [(mg/g) (m/g)−1/n].

Fig. 9-10 show the Langmuir isotherm model and Freundlich isotherm model fit for fluoride sorption on the adsorbents.

The Langmuir isotherm in its linear form is represented by: qe =7.913Ce/ (1+0.102Ce).

(8)

The Freundlich isotherm in its linear form is represented by: qe=9.471Ce1/2.107.

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(9)

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y=0.01289x+0.1261 2 R =0.9984

y=0.4746x+0.9764 2 R =0.8493

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FIG. 9 LANGMUIR ISOTHERM FITS FOR ADSORPTION OF FLUORIDE ONTO NANO-MgO (INITIAL FLUORIDE CONCENTRATIONS 40 µg/ml; ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0).

FIG.10 FREUNDLICH ISOTHERM FITS FOR ADSORPTION OF FLUORIDE ONTO NANO-MgO (INITIAL FLUORIDE CONCENTRATIONS 40 µg/ml; ADSORBENT DOSAGE 20 mg/10 mL; pH=6.0~7.0).

The Langmuir isotherm constants calculated are follows: qm=77.58, KL=0.102. The saturated adsorption capacity depends on the quantity of adsorption activity sites, which directly related to the nano-MgO specific surface area of the adsorbent. It is clear that the qmax of Langmuir model fitted reach to 77.58 mg/g. The Freundlich isotherm constants calculated are follows: KF=9.471; n=2.107. It is believed to when n＜0.5, the adsorption is difficult to reaction. now n=2.107, the adsorption is an easy reaction. From Figure 9-10, Langmuir model fitted the isotherm data well, yielding a correlation coefficient of 0.998. The adsorption of defluorination on nano-MgO obeys the Langmuir monolayer adsorption.

The thermodynamic feasibility of the process in the present fluoride-iron oxide system was evaluated from the free energy change following Eq. (10) ΔG= -RT lnKd

(10)

where ΔG is the change in free energy (kJ/mol); T is the absolute temperature (K); R is the universal gas constant (8.314J/K·mol); Kd is the Langmuir isotherm constants (L/mg).

When the sorption equilibrium at studied systems is established, the fluoride adsorbed onto nano-MgO is in equilibrium with the residual fluoride concentration remaining in the liquid phase. The value of the observed equilibrium constants (Kd) of the adsorption process, the constants of F- distribution between the solid and liquid phases at the equilibrium, were calculated with respect to temperature using the method of Khan and Singh,[36] by plotting ln(qe/Ce) versus qe and extrapolating to zero qe. Kd =2.094, ΔG values was estimated at -1.83 kJ/mol. This indicates that the adsorption process is spontaneous. From above indicated that nano-MgO is a wonderful absorbent material for defluorination.

3.6 Adsorption Kinetic Studie In this study, kinetics of adsorption of fluoride at various initial concentrations was carried out by using nano-MgO as adsorbents. We chose three initial concentrations: 30 µg/mL, 100 µg/mL, and 200 µg/mL. Fig. 11 shows the time dependence of fluoride sorption onto nano-MgO at these initial concentrations.

From Fig. 11, In the case of nano-MgO, most of sorption has taken place within 3 h of contact time. After 3 h, the rate of adsorption was negligible and residual fluoride concentration reached almost a constant value. We can also know that the higher initial concentration of absorption liquid, the higher equilibrium adsorption quantity is.

After the equilibrium period, the amount of adsorbed fluoride did not significantly change with time. Theoretically, adsorption kinetics of fluoride onto solid particles is controlled by different mechanisms. These mechanisms involve diffusion or transport of fluoride from bulk solution to exterior surface of the adsorbent particle. The rate of sorption onto a solid surface depends upon a number of parameters such as structural properties of the sorbent, initial concentrations of the solute, and the interaction between the solute and the active sites of the sorbent. Therefore, - 104 http://www.ivypub.org/fes

qt / (mg/g)

kinetics dates analyses for fluoride sorption onto nano-MgO were studied with different models. The pseudo first order dynamic model, pseudo secondary dynamic model, banerm equation model and two-constant rate equation models were tested to describe the kinetic process. The mathematical representations of models are given in Eqs (11), (12), (13) and (14).

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FIG. 11 THE TIME DEPENDENCE OF FLUORIDE SORPTION ONTO NANO-MGO at 30 µg/ml, 100 µg/ml, 200 µg/ml INITIAL CONCENTRATIONS

Pseudo-first-order adsorption kinetic model:

ln (qe−qt) = ln qe−k1t

(11)

t/qt =1/(k2 qe2)+ t / qe

(12)

ln qe/(qe-qt ) = k3 tn

(13)

ln qt = a+ k4 ln t

(14)

Pseudo-second-order adsorption kinetic model:

banerm equation kinetic model:

two-constant rate equation kinetic model:

where qe is the amount of solute adsorbed per unit weight of adsorbent at equilibrium (mg /g); qt is the amount of fluoride sorbed by adsorption at time t (mg/g); k1, k2, k3, k4, a, n are the constants.

The adsorption kinetic model parameters obtained from the above equations are given (Fig. 12). From these results, it is evident that kinetic datas of the fluoride adsorption onto nano-MgO fitted well with two-constant rate equation.

Yielding a correlation coefficient of 0.9 (Fig. 12d), this suggests that the adsorption kinetic of magnesium oxide process is not a simple first order kinetics of reversible reaction, it is a complex reaction process, may be include adsorption diffusion and chemical reaction process.

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FIG. 12 KINETICS OF DIFFERENT CONCENTRATIONS F- ADSORPTION ON THE NANO-MgO (A), (B), (C) AND (D) pH=6.0~7.0, T=25℃, I=0.1 mol/L KNO3, ADSORBENT DOSAGE 20 mg/10 mL. (A) PSEUDO-FIRSTORDER KINETIC; (B) PSEUDO-SECOND-ORDER KINETIC; (C) BANERM EQUATION KINETIC; (D) TWOCONSTANT RATE EQUATION KINETIC.

3.7 The Desorption of Fluoride on Nano-MgO Adsorbent Under the optimum conditions, the adsorptions test was done at an initial fluoride concentration of 30 µg/mL. Separate MgO from solution after oscillating, assaying the fluoride concentration in the solution. The nano-MgO in plastic tubes was washed with distilled water, adjusts the pH of 6, 7, 8, 9, 10, 11, 12 and distilled water respectely, the blank control be measured using distilled water, a total volume of 10 mL. The elution conditions: 3 hour of shaking time and 25 ℃ temperature. The fluoride ions remaining in the test solution were measured with JP-303 Polarograph. The results show that pH has a nonsignificant influence on desorption of fluoride on nano-MgO adsorbent. The hightest desorption rate was 0.73% in the pH of 8.0. It is also do slightly influence in distilled water.

So it can be learn that the fixation of F- by nano-MgO was the very strong. It is suitable for the fluoride adsorption on nano-MgO, Which is hard to desorb of fluoride on nano-MgO adsorbent.

4 CONCLUSIONS 1) The calcination product of nano-MgO can be an efficient adsorbent for fluoride sequestering from water. The optimal parameters for producing high performance adsorbent nano-MgO were at the adsorption temperature of 25 °C, a adsorbent dosage of 20 mg/10 ml, and an ionic strength of 0.1 mol/L KNO3. This nano-MgO had an adsorption capacity for fluoride of 74 mg/g for water with an initial fluoride concentration of 40 µg/ml that was treated at pH 6.0~7.0. That is higher than usual adsorbents.

2) The adsorption process is well described by the Langmuir isotherm model and the two-constant rate equation model, as assessed by the correlation coefficient values. This suggests that the adsorption of defluorination on nanoMgO obeys the Langmuir monolayer adsorption; the adsorption kinetic of magnesium oxide process is not a simple first order kinetics of reversible reaction, it is a complex reaction process, may be include adsorption diffusion and chemical reaction process.

3) The interference of common ions in ground water is also investigated. The experiment showed that it been the difficultly desorbed F- after adsorption. So the nano-MgO has a significant amount of potential by promising environmental materials for fluoride removal from water.

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