Adsorption of Aqueous Selenite [Se(IV)] Species on Synthetic Layered

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Ind. Eng. Chem. Res. 2009, 48, 7893–7898

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Adsorption of Aqueous Selenite [Se(IV)] Species on Synthetic Layered Double Hydroxide Materials Sujata Mandal,* S. Mayadevi, and Bhaskar D. Kulkarni Chemical Engineering and Process DeVelopment DiVision, National Chemical Laboratory, Pune, India

Layered double hydroxide materials (Zn/Al, Mg/Al, Zn/Fe) with varying composition (M2+:M3+ molar ratio, x ) 3, 2, 1, and 0.33) were synthesized and evaluated for their selenium adsorption characteristics in aqueous medium. Zn/Al and Mg/Al LDHs with x ) 3 and 2, exhibited very high selenite adsorption capacity. XRD patterns of the pristine LDHs, LDH after selenite adsorption, and chloride ion leaching studies revealed that adsorption of selenium on the LDHs occurred through both surface adsorption and ion-exchange mechanism. The adsorption data was fitted to Langmuir and Freundlich isotherm models. A pseudo second-order kinetic model was used to describe the adsorption kinetics of selenite on LDH materials. Desorption of selenite ions in water from the LDH-Se matrix was studied up to 5 h. 1. Introduction

2. Materials and Methods

Layered double hydroxides (LDHs), also known as hydrotalcites, have the general formula [Mx2+ My3+ (OH)2(x+y)] Ay/nn–. mH2O, where M2+ is a bivalent metal ion like Zn, Mg, Cu, Co, Ni, etc., M3+ is a trivalent metal ion like Al, Fe, Cr, etc., and An– is the exchangeable anion. Due to the presence of large interlayer spaces and the reasonable number of exchangeable anions, they act as potential ion-exchangers/adsorbents. In the recent decade, LDHs have attracted increasing attention due to their simple synthesis procedure and easy regenerability. They have been used as adsorbents for the removal of various oxyanionic species,1 which includes, pesticides,2 surfactants,3 organic pollutants,4 heavy metals,5,6 fluoride,7,8 phosphate,9 etc. However, a very limited number of publications report selenium adsorption on anionic clays.10-12

2.1. Reagents. Selenite (1000 mg/L) stock solution for adsorption study was prepared using AR grade Sodium Selenite (Na2SeO3) from Merck Chemicals, India. Standard solutions with varying selenite concentrations were prepared by appropriate dilution of the stock solution with distilled water. Anhydrous AlCl3, ZnCl2, FeCl3, and MgCl2. 6H2O used for LDH syntheses were AR grade chemicals from sd Fine Chemicals Limited, India. 2.2. Synthesis of LDHs. Layered double hydroxide materials were synthesized by the coprecipitation of an aqueous solution containing chloride precursor salts of M2+ and M3+ (Mg/Al or Zn/Al or Zn/Fe) using 2 M NaOH solution at 60 °C maintaining a constant pH of 10 ( 0.5. The precipitate formed was aged for 24 h, separated using centrifuge, washed thoroughly with distilled water until the washing was neutral to litmus and subsequently dried at 60 °C in an air oven until constant weight was obtained. A detailed synthesis procedure of the layered double hydroxide materials have been described in our previous article.7 LDHs with varying M2+:M3+ ratios were prepared by varying the concentration of M2+/M3+ in the solution. Four Mg/ Al/Cl LDHs, MACl-3, MACl-2, MACl-1, and MACl-0.33 with M2+:M3+ molar ratios of 3, 2, 1, and 0.33, respectively, were synthesized by this procedure. Zn/Al/Cl LDHs having M2+:M3+ molar ratios 3, 2, 1, and 0.33 (ZACl-3, ZACl-2, ZACl-1, and ZACl-0.33, respectively), and Zn/Fe/Cl LDHs with M2+:M3+ molar ratio 3, 1, and 0.33 (ZFCl-3, ZFCl-1, and ZFCl-0.33, respectively) were also prepared in a similar way. 2.3. Characterization Techniques. The as-synthesized LDHs were characterized for their chemical compositions and physical behavior using different techniques. Composition of Mg, Zn, Al, and Fe in the LDHs were determined using an atomic absorption spectrometer (GBC 908AA). Concentrations of C, H, and N were determined using CHNS analyzer (CARLOERBA, Italy). Chloride content in the LDHs was titrimetrically determined.23 BET surface area of the LDHs was measured by nitrogen adsorptionsdesorption technique using surface area analyzer (Autosorb 1, Quantachrome Instruments, USA). X-ray diffraction patterns of the LDHs were recorded in XPERT-PRO XRD from PANalytical Instruments using Cu KR radiation. Concentrations of selenite in the adsorbate solution before and after adsorption were determined titrimetrically.23 A fixed quantity of 10% KI solution was added to selenite solution

Selenium (Se) is an essential micronutrient for all living organisms, but excess intake of selenium is toxic for both human and animals. Drinking water is one of the primary sources through which selenium can enter the human body. The maximum contaminant level of selenium in drinking water as regulated by US environmental protection agency is 0.05 mg/L.13 Selenium may exist in water as oxy-anionic species, selenite (SeO32-), and selenate (SeO42-). Attempts for the removal of both selenite and selenate from water have been studied by several researchers using different techniques like, adsorption, membrane filtration,14 and chemical reduction.15 Removal of selenium by adsorption on a wide variety of adsorbents has been reported. These include iron and its minerals,16,17 AlPO4,18 microbes,19 biopolymeric materials,20 agricultural wastes,21 natural clays,22 and synthetic clays.10-12 The present work reports synthesis of Mg/Al, Zn/Al, and Zn/ Fe layered double hydroxide (LDHs) materials with different M2+/M3+ molar ratios, their physicochemical characteristics and adsorption behavior for selenite (SeO32-) adsorption. The adsorption behavior of the synthesized clays was evaluated based on isotherm and kinetic studies. The influence of metal ion of the LDH, M2+/M3+ molar ratio, adsorbent dose, initial selenite concentration, and solution pH, on selenite uptake by the LDH materials have also been reported. * To whom correspondence should be addressed. Tel.: +91 20 25902167. Fax: +91 20 25902612. E-mail: [email protected].

10.1021/ie900136s CCC: $40.75  2009 American Chemical Society Published on Web 05/08/2009

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Table 1. Chemical Compositions and Physical Characteristics of the Pristine LDHs LDH

M2+:M3+

Cl (%)

C (%)

H (%)

surf. area (m2/g)

avg. crys. size (nm)

MACl-3 MACl-2 MACl-1 MACl-0.33 ZACl-3 ZACl-2 ZACl-1 ZACl-0.33 ZFCl-3 ZFCl-1 ZFCl-0.33

2.92 2.05 0.98 0.32 3.26 2.12 0.98 0.36 3.10 1.02 0.38

12.25 19.38 5.62 4.34 7.22 4.24 3.76 4.71 0.27 0.16 0.30

0.00 0.00 0.92 0.98 0.38 0.15 0.47 0.61 0.27 0.45 0.15

3.32 3.17 3.28 3.08 2.53 2.69 3.10 3.00 0.60 1.02 0.91

107.7 51.8 64.9 57.2 232.1 67.5 146.7 223.3 52.85 70.00 111.20

27.44 22.23 23.27 15.46 19.52 23.56 29.10 19.43 14.40 12.54 28.71

acidified with concentrated HCl and the liberated iodine was titrated against standard sodium thiosulphate solution using starch as the indicator. The accuracy of the data from volumetric analysis was confirmed by periodic analysis of selected samples using atomic absorption spectrometer (GBC 908AA). 2.4. Batch Adsorption Experiments. Batch adsorption experiments were carried out at 25 °C in a thermostatic shaker (Julabo SW-21C). 100 mL of aqueous Se(IV) solution, of known initial concentrations in a 250 mL stoppered conical flask was contacted with 0.1 g of LDH sample for 4 h. At the end of 4 h, the solution was filtered and analyzed for residual selenite concentration. The adsorption capacity of the LDH for selenium was estimated using the formula: (C0 - Ce) × V (1) w × 1000 LDHs with different M2+/M3+ molar ratio (0.33 to 3) were prepared and used for studying the influence of M2+/M3+ molar ratio on selenite adsorption capacity. Experiments were carried out using 100 mL batches having an initial selenite concentration of 50.34 mg/L, and the typical adsorbent dosage was 1 g/L of solution. The influence of the adsorbent dose on selenite adsorption was studied by contacting 100 mL of selenite solution (53.30 mg/L) with different quantities (0.1 to 4 g/L of solution) of the adsorbent, for 4 h. Isotherm data were collected by carrying out adsorption experiments (100 mL batch, 4 h) using different initial selenite concentrations (0 to 125 mg/L) and a constant adsorbent dose (0.5 g/L). Kinetic data were collected by taking out samples at different time intervals and analyzing the selenite concentration as a function of contact time. The desorption study was carried out using adsorbent saturated with selenium by contacting 0.5 g with 500 mL, 53.30 mg/L selenite solution for 24 h. The adsorbent was separated by centrifuging and the residual selenite concentration was determined. The saturated adsorbent was contacted with 500 mL distilled water and the selenite concentration in the water (due to desorption from the adsorbent) was determined as a function of time. The selenite solution pH was adjusted using dilute HCl/NaOH for studying the influence of initial solution pH on the adsorption capacity of the LDH.

Figure 1. X-ray diffraction patterns of the Mg/Al LDHs, (a) MACl-0.33, (b) MACl-1, (c) MACl-2, and (d) MACl-3 (O represents LDH, | represents NaCl and 0 represents Bayerite phases).

qe(mg/g) )

3. Results and Discussions 3.1. Characterization of the Adsorbent. Table 1 shows the chemical compositions and general physical characteristics of the pristine LDHs. The LDHs showed negligible amounts of carbon indicating less adsorption of carbon dioxide from the ambient atmosphere. Hydrogen in the LDHs was due to the surface as well as interlayer hydroxide ions. Zn/Fe LDHs contained far less hydrogen as compared to the Zn/Al and Mg/ Al LDHs. The residual chlorine present in the Mg/Al LDHs

Figure 2. X-ray diffraction patterns of the Zn/Al LDHs, (a) ZACl-0.33, (b) ZACl-1, (c) ZACl-2, and (d) ZACl-3 (O represents LDH, 0 represents zincite type ZnO and | represents bayerite phases).

was higher than that in Zn/Al and Zn/Fe LDHs. The surface area of the LDHs was between 40 and 232 m2/g. Although no specific relation could be obtained between the relative metal content and surface area in the case of Mg/Al and Zn/Al LDHs, the surface area of Zn/Fe LDHs were found to increase with increase in iron content. Average crystallite size of the LDHs calculated from major XRD phase of LDH using Debye Schurrer formula is also presented in Table 1. The average crystal size for all the Mg/Al and Zn/Al LDHs lie within the narrow range of 20 to 30 nm. The crystal sizes of Zn/Fe LDHs are almost same for ZFCl-3 and ZFCl-1 but doubled on increasing the iron concentration in the case of ZFCl-0.33. This may be due to the phase change, i.e., the crystal size in ZFCl-3 and ZFCl-1 corresponds to the Zincite phase, whereas crystal size in ZFCl0.33 corresponds to the magnetite phase. The X-ray diffraction patterns of the Mg/Al, Zn/Al and Zn/ Fe LDHs with varying M2+:M3+ molar ratios are presented in Figures 1, 2, and 3. The typical LDH phase corresponding to the 003, 006, 012, 015, and 018 diffraction peaks at 2θ positions 11.4, 22.8, 34.5, 38.8, and 46.3, having corresponding d values 0.757, 0.382, 0.259, 0.229, and 0.194 nm, respectively, can be observed in the XRD profiles of Mg- and Zn-/Al LDHs (Figures 1 and 2) with varying M2+:M3+ molar ratios. As the concentration of aluminum in the LDHs increased (MACl-1, MACl-0.33, ZACl-1, and ZACl-0.33) the diffraction peaks due to β-Al(OH)3 (bayerite) phases having d spacings 0.471, 0.435, 0.222, 0.172, and 0.146 nm appeared at the 2θ positions 18.8, 20.3, 40.6, 53.1, and 63.8 respectively. Apart from the LDH and bayerite phases, all of the Mg/Al LDHs (Figure 1) showed sharp diffraction peaks of residual NaCl at 2θ values 27.5, 31.9, 45.6,

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Figure 3. X-ray diffraction patterns of the Zn/Fe LDHs, (a) ZFCl-0.33, (b) ZFCl-1, and (c) ZFCl-3 (| represents zincite type ZnO and O represents Fe2O3 phases).

Figure 4. X-ray diffraction patterns of the LDH ZACl-2 after exchange with 100 mL of (a) 1000 mg/L (b) 50 mg/L of selenite solution and (c) ZACl-2 before exchange (O represents LDH, and | represents new 003 LDH phase).

56.6, 66.4, and 75.4. Zn/Al LDHs (Figure 2) had diffraction peaks corresponding to zincite-type ZnO at 2θ positions 31.8, 34.4, 36.3, 47.6, 56.6, and 62.9 having respective d values 0.281, 0.259, 0.247, 0.190, 0.162, and 0.148 nm, along with LDH and bayerite phases. XRD pattern of Zn/Fe LDHs (Figure 3) did not have peaks corresponding to typical LDH. The low hydrogen content of these samples, obtained from microanalysis (Table 1) also supported this observation. The XRD profiles in Figure 3 revealed that ZFCl-3 (Figure 3c) had zincite-type ZnO as the major phase (100, 002, 101, 102, 110, 103, and 112 diffraction peaks, corresponding to d values 0.281, 0.259, 0.247, 0.190, 0.162, 0.148, and 0.138 nm respectively at 2θ positions 31.8, 34.4, 36.3, 47.6, 56.6, 62.9, and 67.9) with very little magnetite phase. ZFCl-1 and ZFCl-0.33 had magnetite as the major crystalline phase (Fe2O3, diffraction peaks at 2θ positions 29.69, 35.2, 42.5, 52.9, 56.6, and 62.3 with d values 0.298, 0.255, 0.211, 0.173, 0.162, and 0.149 nm, respectively). A comparison of the XRD pattern of fresh LDH ZACl-2 with those after exchange with selenite solution of different concentrations (50 mg/L and 1000 mg/L) is presented in Figure 4a-c. The crystallinity of the LDH decreased with increase in selenite concentration. The intensity of the 003 diffraction peak due to interlayer chloride ions (at 2θ ) 11.4) also decreased on exchange with selenium. This was accompanied by the appearance of two new peaks at 2θ ) 9.5 (d ) 0.93 nm) and 2θ ) 13.4 (d ) 0.67 nm), the intensity of which increased with increase in the initial selenite concentration in the solution. The appearance of the new peak at 2θ ) 9.5 at the expense of the original LDH peak due to interlayer chloride ion (2θ ) 11.4)

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Figure 5. Selenite uptake on Mg/Al, Zn/Al, and Zn/Fe LDHs with varying M2+:M3+ molar ratios (Initial selenite conc. 50.34 mg/L; volume: 100 cm3; adsorbent dose: 0.1 g; contact time: 240 min; temp: 25 °C).

might be due to the exchange of interlayer chloride ions with selenium. An increase in d spacing of 0.16 nm was also observed due to exchange of selenite with interlayer chloride ions. In selenite, SeO32- exists in trigonal planar geometry and for aqueous and adsorbed selenite species, 1.68 to 1.72 Å is the average Se-O bond length.24,25 The value of Se-O bond length matches well with the value of increase in d spacing due to exchange with selenite. You et al.26 reported no shifting of the 003 diffraction peak when selenite was exchanged with Mg/Al LDH, which was contradictory to the result found for selenite exchange on Zn/Al LDH in the present study. The presence of the 110 diffraction peak at the same position before and after selenite uptake indicates there was no change in basic LDH structure due to selenite exchange. 3.2. Influence of M2+:M3+ Molar Ratio of the Adsorbent on Selenite Uptake. Selenite uptake on Mg/Al, Zn/Al, and Zn/ Fe LDHs with different M2+:M3+molar ratios at 25 °C, under similar conditions are compared in Figure 5. Among the different LDHs prepared, Zn/Al LDH with an M2+:M3+ molar ratio of 2 exhibited the highest selenite uptake. The results reveal that the M2+:M3+ molar ratio of the LDH has a significant influence on its selenite uptake. The uptake increased with increase in M2+: M3+ molar ratio up to 2. Subsequent increase in the molar ratio resulted in a decrease in the selenite uptake of the LDH, in the case of Mg/Al and Zn/Al LDHs. Zn/Fe LDHs did not develop the layered structure (Figure 3) which limited the selenite uptake to physical adsorption on the external surface. Hence Zn/Fe LDHs had low selenite uptake compared to Mg/Al and Zn/Al LDHs. Selenium uptake by Mg/Al and Zn/Al LDHs was due to both surface physical adsorption and ion-exchange with the interlayer chloride ions and hence had higher values compared to Zn/Fe LDHs. Detailed adsorption studies were performed on the LDHs, ZACl-2, MACl-2, ZACl-3, and MACl-3, due to their high selenite uptake capacity. 3.3. Influence of Adsorbent Dose on Selenite Uptake. Variation of selenite uptake and percentage adsorption of selenite on MACl and ZACl LDHs at the end of 4 h, as a function of adsorbent dosage is presented in Figure 6. The selenite uptake per gram of the adsorbent is dependent on many parameters, viz. the initial concentration of selenium, the adsorption temperature, contact time, etc. The figure reveals that an adsorbent dose of 0.5 g/L of the solution is sufficient for 71.5-92% selenite adsorption and the increase in % adsorption is marginal above an adsorbent dosage of 2 g/L. For the same adsorbent dosage, the percentage adsorption of the different LDHs followed the order: ZACl-2 > MACl-2 > ZACl-3 > MACl-3. The selenium uptake decreased with an increase in adsorbent dosage, as the total selenium content in the solution

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Figure 6. Selenite uptake per gram of the adsorbent and % selenite adsorption as a function of adsorbent dose (Initial selenite conc. 53.30 mg/ L; volume: 100 cm3; contact time: 240 min; temp: 25 °C; s selenite uptake; ------ % adsorption). Table 2. Comparison of Selenite Adsorption Capacity of Various Adsorbents at 25 °C S.N.

adsorbent

1 2 3 4 5 6 7 8

aluminum oxide coated sand30 tropical soil31 Mg-Al LDH26 Mg-Al-CO3 LDH11 Mg-Al-CO3 LDH11 (calcined) Zn-Al LDH26 Mg/Al/Cl LDHa Zn/Al/Cl LDHa

a

initial selenite adsorption conc. (mg/L) capacity (mg/g) 158 12.9 1612 0.2 0.2 1612 110.5 110.5

1.0 0.25 203 0.6 1.3 160 119.0 129.3

Results from present study.

that got distributed in the increasing weight of the adsorbent was constant. A comparison of the performance of the adsorbents with that available in the literature along with the initial concentration of the selenite solution used is presented in Table 2. The table shows that adsorption capacities above 150 mg/g of LDH can be obtained on Mg-Al and Zn-Al LDHs when they are exposed to highly concentrated selenite solutions.26 The adsorption capacity of adsorbents is known to increase with an increase in the initial concentration of the adsorbate until the equilibrium adsorption capacity at that temperature is reached. The data in Table 2 show that our adsorbents exhibit good adsorption capacities at medium/low initial selenite concentrations. 3.4. Adsorption Isotherm. Influence of initial selenite concentration on adsorption capacity of the LDHs at 25 °C was studied by varying the initial selenite concentration from 0 to 125 mg/L. The equilibrium adsorption capacity obtained for the LDHs MACl-3, MACl-2, ZACl-3, and ZACl-2 was, respectively, 101.86, 119.02, 103.75, and 129.3 mg/g. The equilibrium adsorption data were fitted to the standard Langmuir27 and Freundlich28 isotherm equations presented in eqs 2 and 3, respectively. Langmuir isotherm model: Ce Ce 1 ) + qe Vm bVm

(2)

Freundlich isotherm model: 1 (3) ln qe ) ln kF + ln Ce n The isotherm constants, calculated from slope and intercept of the linearized plot of the respective isotherm equations are presented in Table 3 along with their correlation coefficients

Figure 7. Langmuir and Freundlich adsorption isotherms for selenite uptake on the LDHs at 25 °C. The symbols indicate experimental data. Solid/ dotted lines indicate model predictions.

(R2). The Langmuir isotherm constant, Vm, representing monolayer adsorption capacity matches well with the experimental equilibrium adsorption capacity. The Langmuir equilibrium constant (KL) was obtained from the values of b and Vm (KL ) b · Vm). The values of the Freundlich isotherm constant, n, for all of the LDHs lie within 2 to 8, indicating the feasibility of the adsorption process using these adsorbents. The experimental isotherm data along with the predictions using Langmuir and Freundlich isotherm models are presented in Figure 7. Figure 7 reveals that the Langmuir model gives a better fit for the isotherm data as compared to the Freundlich model. This is also evident from the value of correlation coefficients presented in Table 3. The correlation coefficients for the Langmuir model are unity or very close to unity, whereas the value of the correlation coefficients for the Freundlich model is not close to unity. 3.5. Adsorption-Desorption Kinetics. The change in selenite concentration due to uptake by Mg- and Zn-/Al LDHs, with different M2+:M3+ molar ratios are presented in Figure 8 as a function of time. Figure 8 shows that an adsorption-desorption phenomena occurred between 20 to 60 min for all the LDHs, except MACl-2. The amount of chloride ion leached out due to exchange with selenite ions is presented in Figure 9. It is interesting to note that the chlorine leaching kinetics followed the reverse pattern of the selenite uptake including the adsorption-desorption phenomena around 30 to 60 min contact time. This indicates that the mechanism of selenium removal by the LDH is due to both adsorption and ion-exchange. The total amount of chlorine leached out depended on the total amount of chlorine present in the LDH. Since the Mg/Al LDHs contain maximum chlorine (chemical analyses in Table 1), they showed maximum amount of leached out chlorine during the kinetic study. The amount of chloride ion leached out in water was determined by performing a parallel experiment with the same amount LDHs in distilled water. The values are presented as dotted line in Figure 9. The difference in the values of released chloride ions in selenite solution and in distilled water represents the amount of chloride ions released due to exchange with selenite ions. Table 3. Langmuir and Freundlich Isotherm Parameters at 25 °C LDH MACl-3 MACl-2 ZACl-3 ZACl-2

Langmuir parameters

Freundlich parameters 2

b

Vm

KL

R

n

KF

R2

0.1162 0.3524 0.5529 1.2063

116.28 125.0 106.38 131.58

13.51 44.05 58.82 158.73

0.995 1 1 1

2.76 4.00 4.87 7.28

24.44 46.25 47.40 78.02

0.878 0.859 0.857 0.981

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Figure 8. Change in selenium concentration as a function of time (Initial selenite conc. 53.30 mg/L; volume: 100 cm3; adsorbent dose: 0.05 g; temp: 25 °C).

Figure 9. Rate of chloride ion leaching from the LDHs in water as a function of time (Initial selenite conc. 53.30 mg/L; volume: 100 cm3; adsorbent dose: 0.05 g; temp: 25 °C; s during selenite adsorption, ----- in distilled water). Table 4. Pseudo Second-Order Rate Constants and Correlation Co-Efficients (R2) LDH

rate constant (k), g/(mg.min)

correlation coefficient (R2)

MACl-3 MACl-2 ZACl-3 ZACl-2

16.5 × 10-3 0.94 × 10-3 1.32 × 10-3 0.90 × 10-3

1 0.991 0.995 0.985

The kinetic data were fitted to a pseudo second-order kinetic model (eq 4) proposed by Ho and McKay.29 t 1 t ) + 2 qt q k·qe e

(4)

Values of the rate constants k were calculated from slope and intercept of the linearized plot of the eq 4 and presented in Table 4 along with their correlation coefficients. The R2 values indicate that reasonably good fitting was obtained for all the LDHs except ZACl-2. The metal ion and the M2+:M3+ molar ratio of the LDH were found to influence the rate of adsorption. The rate of selenite adsorption on MACl-3 was almost 12 to 18 times faster than that of the other three LDHs studied. The rate of adsorption was found to follow the order MACl-3 > ZACl-3 > MACl-2 > ZACl-2. Desorption of selenite in water from the selenite adsorbed LDHs was studied with time and the results are presented as Figure 10. A maximum of 12 to 13% desorption of selenite

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Figure 10. Desorption of selenite species from the LDHs to water as a function of time.

Figure 11. Influence of initial pH of the solution on selenite adsorption by ZACl-2 and on the equilibrium pH of the solution (Initial selenite conc. 53.30 mg/L; volume: 100 cm3; adsorbent dose: 0.05 g; contact time: 240 min; temp: 25 °C).

was observed, when kept in water up to 5 h at 25 °C. The low desorption of selenite from Zn/Al LDH may be due to the strong confinement of the selenite ions in the interlayer region of the layered structure. However, desorption of selenite from Mg/Al LDH was relatively higher. The larger surface area of Mg/Al LDHs and the smaller size of the Mg ions (compared to Zn ions) might have limited the adsorption of selenite on Mg/Al LDHs to physical adsorption/ion-exchange on the external surface. 3.6. Influence of Solution pH. Influence of solution pH on equilibrium selenite adsorption by the LDH ZACl-2 in the pH range of 1.9 to 10.6 is presented in Figure 11. The pH of the solution after adsorption was also measured and presented. The percentage of adsorption remained almost constant in the pH range 7.4 to 3.4 but decreased on further lowering the pH toward acidic medium. Also, a sharp decrease in adsorption was observed upon increasing the solution pH toward alkaline medium. The low selenite adsorption in acidic medium may be due to partial dissolution of the LDH material in strong acidic medium (pH < 2). In alkaline medium, the low adsorption was probably due to the competitive adsorption between the increased OH- ions and the selenite ions. It was interesting to observe that the equilibrium pH of the solution after adsorption was not considerably affected (changed from 6.0 to 8.7) upon increasing the initial pH from 1.9 to 10.6, which shows the strong buffering capacity of LDH. Similar results for selenite adsorption on Zn/Al and Mg/Al LDHs were reported by You et al.26

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4. Conclusions Zn- and Mg/Al LDHs with molar ratios 3 and 2 can be effective adsorbents for the adsorption of selenite in aqueous medium. The selenite uptake by Mg- and Zn-/Al LDHs involved both surface adsorption as well as exchange of the interlayer anions. An increase in the interlayer distance of the LDH structure (0.16 nm), due to incorporation of the selenite species (SeO32-) in the interlayer region of the LDHs by exchange with the chloride ions, was observed by XRD study. Selenite uptake by Zn/Fe LDHs was solely due to surface physical adsorption. The Langmuir isotherm model was found to be the most suitable model for describing the adsorption of selenite ions on Zn- and Mg-/Al LDHs. The initial rate of the adsorption was very fast, but slowed down with time as the surface sites were covered by the selenite ions. Selenite adsorption kinetics on the LDHs followed a pseudo-second order kinetic model. A maximum 12-13% desorption of selenite ions was recorded when contacted with water up to 5 h. Adsorption of selenite was maximal and remained unaffected by a change in initial pH, in the solution pH range of 3.4 to 7.4. 5. Nomenclature C0 ) initial selenite concentration (mg/L) Ce ) equilibrium selenite concentration (mg/L) k ) second order rate constant (g/mg · min) t ) time (min) V ) volume of solution (mL) w ) weight of the adsorbent (g) qe ) Amount of adsorbate per unit gram of the adsorbent at equilibrium (mg/g) qt ) Amount of adsorbate per unit gram of the adsorbent at time t (mg/g) Vm ) Langmuir isotherm constant representing monolayer adsorption capacity (mg/g) b ) Langmuir isotherm constant representing adsorption bond energy (L/mg) KL ) Langmuir equilibrium constant (b · Vm) kF ) Freundlich isotherm constant representing adsorption capacity n ) Freundlich isotherm constants representing adsorption intensity Acknowledgment Dr. Sujata Mandal wishes to thank Department of Science and Technology, Government of India, for financial support. Literature Cited (1) Goh, K. H.; Lim, T. T.; Dong, Z. Application of layered double hydroxides for removal of oxyanions: A review. Water Res. 2008, 42, 1343. (2) Inacio, J.; Taviot-Gueho, C.; Forano, C.; Besse, J. P. Adsorption of MCPA pesticide by MgAl-layered double hydroxides. Appl. Clay Sci. 2001, 18, 255. (3) Schouten, N.; van der Ham, L. G. J.; Euverink, G. J. W.; de Haan, A. B. Selection and evaluation of adsorbents for the removal of anionic surfactants from laundry rinsing water. Water Res. 2007, 41, 4233. (4) Chao, Y. F.; Chen, P. C.; Wang, S. L. 2008. Adsorption of 2,4-D on Mg/AlsNO3 layered double hydroxides with varying layer charge density. Appl. Clay Sci. 2008, 40, 193. (5) Prasanna, S. V.; Kamath, P. V. Chromate uptake characteristics of the pristine layered double hydroxides of Mg with Al. Solid Stat. Sci. 2008, 10, 260. (6) Lazaridis, N. K.; Pandi, T. A.; Matis, K. A. Chromium(VI) Removal from Aqueous Solutions by Mg-Al-CO3 Hydrotalcite: SorptionDesorption Kinetic and Equilibrium studies. Ind. Eng. Chem. Res. 2004, 43, 2209.

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ReceiVed for reView January 27, 2009 ReVised manuscript receiVed April 15, 2009 Accepted April 21, 2009 IE900136S