Nanostructure Iron(III)−Zirconium(IV) Binary Mixed Oxide: Synthesis

Ind. Eng. Chem. Res. 2008, 47, 9903–9912

9903

Nanostructure Iron(III)-Zirconium(IV) Binary Mixed Oxide: Synthesis, Characterization, and Physicochemical Aspects of Arsenic(III) Sorption from the Aqueous Solution Kaushik Gupta, Krishna Biswas, and Uday Chand Ghosh* Department of Chemistry, Presidency College, 86/1 College Street, Kolkata 700 073, India

Characterization of synthetic Fe(III)-Zr(IV) mixed oxide (NHIZO) by the X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses confirmed the material as agglomerated nanocrystallite particles (16-21 nm) which was used for As(III) sorption from water. The optimum pH and equilibrium time (As(III) concentrations (mgL-1), 5.0 and 10.0; NHIZO dose, 2 g · L-1; temperature, 303 K) were 7.0 ( 0.2 and 2.0 h, respectively. The kinetic and equilibrium data described, respectively, the pseudo-second-order equation and the Langmuir as well as the Redlich-Peterson isotherm models very well. The Langmuir capacity was 65.5 ( 1.0 mg · g-1 at 303 K, which increased with increasing temperature. The positive enthalpy (∆H°) and negative free energy (∆G°) changes indicated the endothermic and spontaneous nature of the reaction, respectively. The sorption energy (4.64-5.20 kJ · mol-1) and Fourier transform infrared (FTIR) analyses suggested physissorption of As(III) by NHIZO. The sorbed arsenic could be desorbed (∼80%) by 2.0 M alkali. The toxicity leaching characteristic procedure test marked As(III)-NHIZO as nonhazardous waste. 1. Introduction Nanomaterials show various unique enhanced properties that are not shown by the bulk materials and find their application in multivariate technological fields1,2 Materials with nanostructure have gained special attention very recently in the field of solute sorption from the liquid phase due to the small particle size, large surface area, and high in situ reactivity. The use of such materials for scavenging water pollutants should generate low sludge volume and reduce disposal problems. Zhang had demonstrated the use of nanoscale iron particles for environmental remediation.3 On the other hand, occurrences of arsenic in groundwater much exceeding4 the tolerance limit (0.01 mg L-1) are a global problem and pose an ever-increasing degree of health hazard. The Bengal delta basin in India and Bangladesh has become infested with this menace, and in some pockets of this region it has assumed a life-threatening proportion, causing deaths of a good number of inhabitants. The cause of accumulation of arsenic in groundwater in this delta region is an anoxic environment around its alluvial deposits of geogenic arsenic pyrites and iron oxyhydroxide with adsorbed arsenic undergoing microbial reduction.5,6 The aquifers thus become rich in this reduced As(III) along with Fe(II). The ratios of As(III)/Astotal at a depth of 30-40 m reported in these aquifers are in the range of 0.6-0.9,6 which is a matter of great concern since As(III), being more toxic than As(V), has much greater combining affinity with the thiol (-SH) part of the protein due to soft-soft acid-base reaction. Remembering the adverse health impact of arsenic toxicity for long-term drinking of high arsenic contaminated groundwater, researchers had been forced to undertake work for developing methods on reducing arsenic levels below or equal to the permissible value (>0.01 mg · L-1) in the past 2 decades. Consequently, several methods such as oxidation-precipitation, coagulation/electrocoagulation/precipitation, membrane filtra* To whom correspondence should be addressed. Tel.: +91-33-22413893. E-mail: [email protected].

tion, and surface sorption and ion exchange, etc., have been reported.4,7,8 However, the surface sorption method has been found to be an alternative option for the treatment of high arsenic groundwater and well-accepted by the rural people of underdeveloped countries such as India and Bangladesh for simple operation and low recurring cost. Numerous sorbent materials,4,7,8 viz., activated carbon, agricultural products and byproduct, biomasses, and metal oxides or metal ion loaded biomaterials, had been tested for the treatment of high arsenic contaminated ground-/wastewater and industrial effluents. The solid inorganic materials used in bulk phase for the arsenic sorption from the aqueous solution are mostly the different mineralogical forms of iron(III)/aluminum(III) oxide and hydroxide.9-22 Some polyvalent metal oxides had been synthesized and tested for the arsenic sorption/removal in our laboratory.23-28 The recent use of nanoscale zerovalent iron29,30 and nanocrystalline titanium oxide31,32 for the arsenic sorption from aqueous solution has encouraged us in undertaking the present work with the synthetic nanostructured Fe(III)-based hydrous mixed metal oxide materials. Thus, the present paper reports herein the (i) synthesis and characterization of nanocrystalline-hydrated Fe(III)-Zr(IV) bimetal mixed oxide (NHIZO) and (ii) As(III) sorption on NHIZO from aqueous solution for the kinetics and thermodynamics. 2. Materials and Methods 2.1. Chemicals. All reagents used were of guaranteed reagent (G.R.) grade (E. Merck, India) except arsenic(III) oxide (99.9% Aldrich, USA), ferric chloride (laboratory reagent, Merck, India), sodium hydroxide (laboratory reagent, SD Fine Chemicals), and zirconium oxychloride (ordinary grade, Loba Chemie, India). 2.2. Preparation of NHIZO. The ferric chloride (0.18 M) and the zirconium oxychloride (0.02 M) solutions prepared separately in hydrochloric acid (0.1 M) were mixed together in 1:1 volume ratios. The mixed solutions were stirred for 0.5 h and heated to ∼60 °C. To the hot well-stirred mixed solutions, sodium hydroxide (1.0 M) was added slowly until the pH reached ∼6.0, and stirring was continuedfor 0.5 h more. The brown precipitates that appeared

10.1021/ie8002107 CCC: $40.75  2008 American Chemical Society Published on Web 11/13/2008

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were aged with the liquid for 48 h, filtered, and washed five times with deionized water. The filtered mass obtained was dried at ∼80 °C into an air oven. The dried hot mass when treated with cold water was broken to the fine agglomerates of grain size ranged in 140-290 µm. These grains were analyzed for nanostructure and used for the As(III)-sorption studies. The method described herein for the material synthesis is environmentally friendly because the dissolved material present in the filtered solution was the recoverable sodium chloride, and that can be disposed of safely. 2.3. Arsenic Solutions. A standard stock As(III) solution (1000 mg · L-1) was prepared by dissolving 0.1320 g of arsenic(III) oxide in l0 mL of 4% (w/v) sodium hydroxide, acidified with 2.0 mL of concentrated hydrochloric acid, and diluted to 100 mL with arsenic free deionized water. The working solutions of required As(III) concentrations were made by diluting the stock with 0.2% (v/v) hydrochloric acid. The stock solution was prepared freshly after every 3 days and frozen to prevent oxidation. 2.4. Analytical Methods. 2.4.1. Arsenic Analysis. Arsenic in samples was analyzed by UV-vis spectrophotometer (Hitachi model 3210) using the procedure described in Standard Methods for Examination of Water and Wastewater.33 Here, the dissolved inorganic arsenic in samples was determined by adding hydrochloric acid (32%, v/v), potassium iodide (10%, w/v), and sodium borohydride (3%, w/v). The arsine (AsH3) gas generated was absorbed in silver diethyl dithiocarbamate (SDDC) solution in chloroform solvent, and the absorbance was measured at 535 nm against a blank and compared with the standard curve for the value. The detection limit of the used method is 10 µg · L-1 with precision of (5%. 2.4.2. Adsorbent Characterization. The X-ray diffraction (XRD) analysis of the synthetic oxide was conducted by the powder method with Philips diffractometer (Analytical PW1710) using the radiation source Cu KR, a current of 30 mA, and voltage of 40 kV. Transmission electron micrography (TEM) for the particle size of the oxide was recorded on a H800 transmission electron micrograph (Hitachi). Scanning electron microscopy (SEM) for the material was taken by using a Cambridge-360 scanning electron microscope. The pHzpc (pH for zero surface charge) was determined according to the procedure described by Babic et al.34 The Fo¨urier transform infrared (FTIR) spectra of the mixed oxide and associated two pure oxides were recorded by the Perkin-Elmer system 2000 spectrophotometer with a resolution of 2 cm-1. Thermal analyses (thermogravimetric analysis (TGA) and differential thermal analysis (DTA)) of the sample were conducted using a Simudza-made thermal analyzer in argon atmosphere at a heating rate of 20 °C · min-1 over a temperature range of 30-700 °C. 2.5. Batch Experimental Program. 2.5.1. pH Effect. To determine the influence of pH on the sorption capacity, the As(III) solutions [concentrations (mg · L-1), 5.0 and 10.0] were prepared and adjusted to pH 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, and 11.0, respectively, with 0.1 M hydrochloric acid and/ or 0.1 M sodium hydroxide, as required. A 50.0 mL aliquot of each solution was taken into the nine separate 200 mL polythene bottles with 0.1 g of NHIZO, and then shaken (speed, 280 ( 5 rpm) for 4 h. The residual arsenic in the filtered solutions was analyzed. The arsenic sorption capacity was calculated using the relation [(Ci - Cf)V]/w, where Ci and Cf are the initial and the filtered solution arsenic concentrations (mg · L-1), V is the solution volume (L), and w is the mass (g) of the sorbent added.

2.5.2. Sorption Kinetics. The As(III) sorption kinetics on NHIZO at 303 ( 1.6 K and at pH 7.0 ( 0.2 was determined using the batch procedure. Here, 500.0 mL of As(III) solution [concentrations (mg · L-1), 5.0 and 10.0] put into a liter glass vessel was placed in a thermostat bath to attain the desired temperature. To that thermostatic solution, 1 g of NHIZO was added and agitated (280 ( 5 rpm). The agitated solution pH was frequently checked by immersing a pH meter (ELICO, LI127) electrode. Any change of pH, if noted, was adjusted by adding a small amount of 0.1 M hydrochloric acid and/or 0.1 M sodium hydroxide, as required. A 2.0 mL aliquot at the initial successive three stages and 5.0 mL in the later stages were sampled at an interval of 0.25 h. The sample solutions were filtered using a membrane (0.45 µm) filter and, the filtrates were analyzed for arsenic. The time-dependent sorption capacities (qt) were calculated using the relation [(Ci - Ct)V]/w, where Ct is the arsenic concentration (mg · L-1) at any time, t (min), and the significance of the other terms mentioned earlier. 2.5.3. Isotherm Experiment. The isotherm experiments were conducted at three separate temperatures ((1.6 K) such as 288, 303, and 318, and at pH 7.0 ( 0.2 by batch sorption procedure. Here, the concentrations (mg · L-1) of As(III) solution used ranged between 5.0 and 350.0 and, the dose of NHIZO added was 2.0 g/L of the solute solution. The agitation (speed, 280 ( 5 rpm) time used was 2.0 h. The solution pH was adjusted twice during the experimental run (at 1.0 and 2 h of agitation from zero time) using 0.1 M hydrochloric acid and/or 0.1 M sodium hydroxide, as required. Residual arsenic in the filtered samples was analyzed by the standard method,33 and the equilibrium sorption capacity (qe) was calculated using the relation as shown in the subsection 2.4.1. 2.6. Error Analysis. All samples were analyzed in 2 days and analyzed in triplicate, indicating a precision better than (5%. The computer software analyzed the error function of the data collected from the experiments. The kinetic data were analyzed by least-squares regression method using an Excel software spread sheet. The closer the regression coefficient value to unity, the greater is the data fit with a model. The isotherm data were analyzed by the nonlinear χ2 test in an Origin software spread sheet. The χ2 test statistic is basically the sum of the squares of the differences between the experimental data and data obtained by calculating from models, with each squared difference divided by the corresponding data obtained by calculating from models. If data from the model are similar to the experimental data, χ2 will be a small number; if they are different, χ2 will be a large number. 2.7. Toxicity Characteristic Leaching Procedure.35 The toxicity characteristic leaching procedure (TCLP) test of arsenic sorbed solid material (32.8 (mg of As) · g-1) was carried out according to the procedure described by U.S. EPA method 1311. Here, the arsenic sorbed dry solid material was mixed with the TCLP fluid (sodium acetate plus acetic acid buffer of pH 4.93 ( 0.05) by the 1:20 ratio and agitated (speed, 280 ( 5 rpm) for 18 h at 30 °C using a mechanical shaker. Leached arsenic in the filtered TCLP fluid was analyzed.33 3. Results and Discussion 3.1. Characterization of the Mixed Oxide Used. The powder X-ray diffraction (XRD) pattern (Figure 1) obtained for the used deep brown mixed oxide (NHIZO, Fe:Zr ) 9:1, mol/mol) between the start and end angles (2θ, deg), 10 and 80°, has shown seven peaks which indicate the crystalline nature of the material. The approximate size of particles has been calculated from the XRD peak data using Scherer’s formula (a standard

Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9905

Figure 1. Powder X-ray diffraction (XRD) pattern of NHIZO.

Figure 2. Scanning electron microscopic (SEM) image of NHIZO.

Figure 3. Transmission electron microscopic (TEM) image of NHIZO.

equation of solid-state physics) and was found to be in the range of 10.2-34.6 nm. Figure 2 shows the SEM image of NHIZO, which has indicated the agglomerated surface morphology with irregular shape, the porous nature, and the high surface area. The TEM image (Figure 3) has shown the exact size of the particles, which ranged between 16 and 21 nm, present in the agglomerated grains of size 140-290 µm. The FTIR spectrum (figure omitted) obtained for the studied oxide has shown five

absorption bands at wave numbers (cm-1) 3400.8, 2368.8, 1628.0, 670.4, and 448.7. The bands at 3400.8 and 1628.0 cm-1 are due to the stretching and bending modes of the O-H bond. The sharp band at 2368.8 cm-1 is for the CO2, which was presumably present in alkali used for synthesis of the mixed oxide. The band at 448.7 cm-1 is for the Fe-O-Zr, and that at 670.4 cm-1 is for the Fe-O. However, the band intensity has reduced compared to that of the Fe-O bond in the pure ferric oxide. The two steps’ weight loss found in TG analysis (spectrum omitted) respectively were ∼16.7 and ∼11.4% at 100 and 200-600 °C correspond to the loss of water molecules being adsorbed physically and dehydroxylation from -OH groups and, these correspond to the total water loss by 28.10%. The wet chemical analyses have shown that the mixed oxide contains Fe2O3 (65.06%) and ZrO2 (6.84%). Thus, the agglomerated particles (140-290 µm) used for the study are found to be nanocrystalline and hydrous Fe(III)-Zr(IV) mixed oxide (NHIZO). The pHzpc value estimated34 for NHIZO was 6.8 ( 0.2, which is closed to the neutral pH, and that indicates the surface of the synthetic solid should be neutral at pH ∼ 7.0. Thus, NHIZO can be used for scavenging the (HO)3AsIII (pKa1 ) 9.2) from the contaminated groundwater. 3.2. pH Effect on As(III) Sorption. Figure 4 demonstrates the results of As(III) sorption on NHIZO with varying initial pH (pHi) ranging from 3.0 to 11.0 at 303 ( 1.6 K. The sorption percentages for As(III) of NHIZO have increased with increasing pHi from 3.0 to 7.0, and that decreases at pHi above 7.0-11.0. The highest sorption percentage obtained is at pHi 7.0 from the solute solution of 10.0 mg · L-1 concentration. However, that has not been reflected clearly from the tested As(III) solution of 5.0 mg · L-1 concentration, which is presumably due to the addition of less solute load compared to the active surface sites available for sorption on the solid. It has been found (Figure 4) that the As(III) removal percentage at pHi 7.0 is about 99.0 ( 2.0 and 93.5 ( 1.5, while that at pHi 11.0 is 66.0 ( 2.5 and 64.0 ( 3.0, respectively, from the test solutions of arsenic concentrations (mg · L-1) of 5.0 and 10.0. Thus, the As(III) removal efficiency of NHIZO has been found to be highest at pHi 7.0, which is considered the optimum pH for the present reaction. The result obtained agrees well with that reported for iron oxide coated cement (IOCC)10 and pillared clays and iron oxides.22 The less As(III)-sorption capacity of NHIZO noted at pHi > 9.0 is presumably due to the coulumbic repulsion between

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Figure 7. Plot of ln R versus ln t for As(III)-sorption data obtained on NHIZO at 303 K and at pH 7.0 ( 0.2. Figure 4. Effect of initial pH (pHi) on As(III) sorption by NHIZO at 303 K.

Figure 5. Plot of qt (mg · g-1) versus t (min) for the As(III)-sorption data obtained on NHIZO at 303 K and at pH 7.0 ( 0.2.

Figure 6. Pseudo-second-order plot of the As(III)-sorption data obtained on NHIZO at 303 K and at pH 7.0 ( 0.2.

(HO)2AsO- [pK1a for As(OH)3 ) 9.2] and the negative surfaces of NHIZO (pHzpc ) 6.8 ( 0.2) particles. 3.2. Kinetic Analysis. Figure 5 demonstrates the kinetic data of As(III) sorption by NHIZO obtained at pH 7.0 ( 0.2 and 303 ( 1.6 K. It is found that ∼90% of the sorbed As(III) has taken place in 0.5 h and the time taken to reach the equilibrium is about 2.0 h. The equilibrium time obtained for the present case has been found to be exactly half of that reported for nanocrystalline titanium dioxide.31 Using the pseudo-first-order36 (eq 1) and the pseudo-secondorder37 (eq 2) kinetic equations, the kinetic data (Figure 5) have been analyzed by linear least-squares fit method using an appropriate coordinate for each equation. log(qe - qt) ) log qe - (k1t) ⁄ 2.303

(1)

t ⁄ qt ) 1 ⁄ (k2qe2) + t ⁄ qe

(2)

where qe and qt are the sorption capacities (mg · g-1) at equilibrium and at any time, t (min), respectively, and k1 (min-1) and k2 (g · mg · min-1) are the related rate constants. The kinetic parameters evaluated from the slopes and intercepts of the linear plots obtained for the used kinetic equations are given in Table 1. From the correlation coefficients, it is observed that the experimental data have been found to fit better with the pseudo-second-order model (eq 2; r2 ) 1.00; Figure 6) than the pseudo-first-order model (eq 1; r2 ) 0.78; plots omitted). Furthermore, the equilibrium capacity (qe) obtained from the analysis of kinetic data with the pseudosecond-order model is found to be closer to the experimental value than the value obtained from the pseudo-first-order model. These two aspects prove that the kinetics of As(III)-sorption reaction is pseudo-second-order type. The pseudo-second-order rate constant (k2) decreases with increasing initial load of As(III) in solution per gram of NHIZO. This has suggested that the As(III) sorption is a more favorable process at lower than the higher solute concentration. The result that has been obtained is found to be similar to the result reported by Pena et al.31 using nanocrystalline titanium dioxide. 3.3. Diffusion Kinetics. Despite the As(III)-sorption reaction with NHIZO being of the pseudo-second-order type, the ratelimiting step may be controlled by the diffusion of solute from the particle surface to the interior sites (called intraparticle or pore diffusion) or by the transport of the solute from the bulk solution to the outer surface of the solid particle. Thus, to evaluate the actual rate-limiting step for the present sorption reaction, the data shown in Figure 5 have been analyzed by the kinetic equation10,38,39 given as follows: R ′ ) kttm

(3)

Taking the natural log on both sides, eq 3 becomes ln R ′ ) ln kt + m ln t

(4)

where R′ is the As(III) removal efficiency (%), t is the contact time (min), and kt and m are the constants. The plots of ln R′ versus ln t (Figure 7) show that the sorption kinetics can be divided into two linear phases; viz., (i) the first phase corresponds to the rapid removal of As(III) by NHIZO, and (ii) the second phase corresponds to a much slower process where the removal becomes almost constant, indicating equilibrium condition. Applying the preceding equation (eq 4) to the first linear phase, where ∼90% removal of As(III) took place, the values of ln kt and m as calculated, respectively, from the plots (Figure 7) are 2.56 and 0.58; and 3.11 and 0.41for the solute concentra-

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Figure 8. Nonlinear plots for (a) 288, (b) 303, and (c) 318 K of (9) Langmuir, (--) Freundlich, and ( · · · ) Redlich-Peterson models and linear plots of (d) Langmuir, (e) Freundlich, and (f) Redlich-Peterson models of As(III)-sorption isotherm equilibrium data obtained on NHIZO at pH 7.0 ( 0.2.

tions (mg L-1) 10.0 and 5.0 at 303 ( 1.6 K and at pH 7.0 ( 0.2. If the pore diffusion is the rate-limiting step, the value of m should be 0.5;38 that is, the rate of sorption should vary with the square root of time, t0.5. However, in the present case, the rate of sorption varies with t0.58 and t0.41 for 10.0 and 5.0 mg · L-1 As(III) solutions, respectively, which has indicated that the pore diffusion is not the rate-limiting step, and it is confirmed from the plots that those do not pass through the origin. This

has indicated that the rate-limiting step of the reaction might be controlled by the boundary layer or film diffusion. To confirm this, the pore-diffusion coefficient (DP, cm2 · s-1; eq 5) and the film-diffusion coefficient (DF, cm2 · s-1; eq 6) are calculated using the following equations:40,41 DP ) 0.03r02 ⁄ t0.5 DF ) 0.23r0 ∂ Cs ⁄ CLt0.5

(5) (6)

9908 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 1. Kinetic Model Parameters Evaluated for the Sorption of As(III) on NHIZO at pH 7.0 ( 0.2 and at a Temperature of 303 ( 1.6 K pseudo-first-order values initial concentration (mg · L-1)

k1 (min-1)

qe (mg · g-1)

5 10

2.60 × 10-2 2.20 × 10-2

0.80 1.52

Table 2. Pore and Film Diffusion Coefficient Calculated for the Sorption of As(III) on NHIZO at pH 7.0 ( 0.2 and at a Temperature of 303 ( 1.6 K initial concentration (mg · L-1)

DP (cm2 · s-1)

DF (cm2 · s-1)

5 10

1.10 × 10-8 8.40 × 10-9

3.70 × 10-6 3.0 × 10-6

where r0 is the radius (1.075 × 10-2 cm) of the agglomerated particles, ∂ is the film thickness (10-3 cm),40 and Cs and CL are the solid- and liquid-phase solute concentrations (mg · g-1), respectively. If the value of Dp calculated ranges from 10-11 to 10-13 cm2 · s-1, the sorption reaction should be controlled by the pore diffusion; and if the value of DF calculated ranges from 10-6 to 10-8 cm2 · s-1, the sorption should be controlled by the film or boundary-layer diffusion.40,42 For the present reaction, the Dp and DF values (cm2 · s-1) calculated (Table 2) have been found around 10-8-10-9 and 10-6, respectively, which has suggested that the film or boundary layer diffusion is the ratelimiting step. This is similar to the conclusion that had been drawn by Kundu and Gupta for the As(III) sorption from the aqueous solution on iron oxide coated cement.10 3.5. Isotherm Modeling. Parts a-c of Figure 8 show the equilibrium isotherm data as points for the As(III) sorption on NHIZO obtained at the temperatures ((1.6 K) 288, 303, and

Figure 9. Dubinin-Radushkevich (D-R) isotherm plot of As(III)-sorption data obtained on NHIZO at studied three different temperatures and at pH 7.0 ( 0.2.

Figure 10. Fo¨urier transform infrared (FTIR) spectra of (a) pure NHIZO and (b) As(III)- sorbed NHIZO.

pseudo-second-order values r2

k2 (g mg · min-1)

qe (mg · g-1)

r2

0.78 0.78

7.74 × 10-2 2.94 × 10-2

2.41 4.94

1.00 1.00

318 and at pH 7.0 ( 0.2. To evaluate the nature of the sorption, the data have been analyzed by the following nonlinear and linear isotherm model equations, viz., the Langmuir43 (eqs 7 and 8), the Freundlich44 (eqs 9 and 10), and the RedlichPeterson45 (eqs 11 and 12) models: Langmuir: qe)(qmKaCe) ⁄ (1 + KaCe) (nonlinear form) (7) (Ce ⁄ qe) ) 1 ⁄ (Kaqm) + Ce ⁄ qm (linear form) (8) Freundlich: qe)KFCe(1⁄n) (nonlinear form)

(9)

log qe)log KF + (1 ⁄ n) log Ce (linear form)(10) Redlich-Peterson: qe ) (ACe) ⁄ (1 + BCeg) (nonlinear form) (11) (Ce ⁄ qe) ) (1 ⁄ A) + (B ⁄ A)Ceg (linear form) (12)

where Ce and qe have their usual significance as noted earlier and qm and Ka are the Langmuir constants related to monolayer sorption capacity (mg · g-1) and sorption equilibrium constant (L · g-1), respectively. KF and n are the Freundlich constants related to sorption capacity (mg · g-1) and intensity, respectively. A (L · g-1), B (L · mg-1), and g ()0-1.0) are the Redlich-Peterson constants. The isotherm data shown as points (Figure 8a-c) have been analyzed with the preceding equations (eqs 7-12) by both the nonlinear and the linear methods using the Origin computer software spread sheet, and the data fits are shown in parts a-c and d-f of Figure 8, respectively. The related isotherm parameters evaluated from the nonlinear and the linear analyses of the data are given in Table 3 and Table 4, respectively. On the basis of either the statistical error χ2 or the linear regression coefficient (r2) values (Table 3), it could be said that the fits of the present isotherm data are found to be equally good with the Langmuir and the Redlich-Peterson models and the fits have been found to be better than the Freundlich fit. The order of the data fit with the models used for the analysis as obtained from the nonlinear and the linear analyses is as follows: Langmuir ≈ Redlich-Peterson > Freundlich. The threeparameter Redlich-Peterson isotherm model includes the characteristics of both the Langmuir and the Freundlich models, respectively, for g ) 1 and g ) 0. The g values obtained (Tables 3 and 4) for the best fit of the Redlich-Peterson model suggested the predominance of the Langmuir characteristics on the As(III) sorption by NHIZO, and the later decreases with increasing temperature on the present sorption reaction. The Langmuir monolayer sorption capacity (qm, mg · g-1) value obtained from both the nonlinear and the linear methods of analysis are found to be high, and the values increase with increasing temperature on the reaction (Tables 3 and 4). The room-temperature (303 K) qm (mg · g-1) value of the present material has been found to be higher than either hydrous ferric oxide23 (33.33 mg · g-1) or hydrous zirconium oxide25 (21.74 mg · g-1). To access the As(III)-sorption performance of NHIZO, the room temperature qm (mg · g-1) value has been compared (Table 5) with some reported data. It has found that NHIZO is

Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9909 Table 3. Isotherm Parameters Estimated from the Nonlinear Analyses of Equilibrium As(III)-Sorption Data on NHIZO at pH 7.0 ( 0.2 and at Three Different Studied Temperatures temperature ((1.6 K) isotherm model

isotherm parameters estimated -1

qm (mg · g ) Ka (L · mg-1) χ2 r2 KF (mg · g-1) n χ2 r2 A (L · g-1) B (L · mg-1) g χ2 r2

Langmuir

Freundlich

Redlich-Peterson

Table 4. Isotherm Parameters Estimated from the Linear Analyses of Equilibrium As(III)-Sorption Data on NHIZO at pH 7.0 ( 0.2 and at Three Different Studied Temperatures isotherm model Langmuir Freundlich Redlich-Peterson

temperatures ((1.6 K)

isotherm parameters estimated -1

qm (mg · g ) Ka (L · mg-1) r2 KF (mg · g-1) n r2 A (L · g-1) B (L · mg-1) g r2

288

303

318

60.20 0.06 1.00 4.58 1.87 0.94 4.00 0.07 0.98 1.00

64.50 0.08 1.00 5.73 1.98 0.96 5.40 0.10 0.96 1.00

71.90 0.08 0.99 6.48 1.97 0.95 8.60 0.23 0.88 1.00

a better As(III) scavenging material than many others in the drinking water pH range. 3.6. Thermodynamic Parameters. The thermodynamic parameters, viz., the Gibbs free energy change (∆G°), standard enthalpy change (∆H°), and standard entropy change (∆S°), have been estimated. The Gibbs free energy change is related to the equilibrium constant (Ka) by the following equation: ∆G°)-RT ln Ka

(13)

where R is the molar gas constant (kJ · mol-1 · K-1) and T, the absolute temperature (K). According to thermodynamics, the Gibbs free energy change is related to the enthalpy change and entropy change in their standard state by the following equation: ∆G° ) ∆H° - T∆S° (14) The values of ∆H° and ∆S° can be evaluated from the intercept and the slope, respectively, of the linear plots of ∆G° (kJ · mol-1) versus T (K). In the present case, the equilibrium constant (Ka) used to calculate the standard Gibbs free energy change (∆G°, kJ · mol-1) is the Langmuir equilibrium constant (Ka, L · mg-1) since it is the best-fit model. The calculated ∆G° values have been plotted against the T (plot omitted) and, the ∆H° and ∆S° values are evaluated from the slope and intercept of the plot. The thermodynamic parameters estimated (Table 6) using the Langmuir equilibrium constant value (obtained from the linear and nonlinear method of analysis of data) have shown that (i) the ∆G° values are negative, (ii) the ∆H° value is positive, and (iii) the ∆S° value is positive. Thus, it can be concluded that the present sorption reaction was (i) spontaneous at the studied temperatures, (ii) endothermic in nature, and (iii) the randomness increases at the solid-liquid interface due to the release of aqua molecules when the hydrated As(III) is being sorbed from the aqueous solution by the solid.

288

303

318

60.7 ( 0.5 0.06 ( 0.01 3.22 0.99 10.50 ( 2.70 3.02 ( 0.50 46.04 0.92 3.40 ( 0.50 0.05 ( 0.02 1.04 ( 0.06 3.49 0.99

66.5 ( 1.8 0.06 ( 0.01 3.99 0.99 11.40 ( 2.60 3.01 ( 0.50 43.68 0.94 3.70 ( 0.60 0.05 ( 0.02 1.02 ( 0.06 4.59 0.99

72.2 ( 2.8 0.07 ( 0.01 10.05 0.99 12.60 ( 2.50 2.96 ( 0.40 39.24 0.95 6.80 ( 1.90 0.16 ( 0.10 0.89 ( 0.06 8.25 0.99

3.7. Energy of Sorption. The equilibrium isotherm data shown as points in Figure 8a-c have been analyzed by the Dubinin-Radushkevich (D-R) equation (eq 15) to evaluate the sorption energy. ln Qe)ln Qm - KDRε2

(15)

where Qe and Qm are the equilibrium and saturated sorption capacities in moles per kilogram, respectively, and KDR, a constant related to the free energy (mol2 · kJ-2) of sorption. ε is the Polanyi potential and expressed by ε ) RT ln{1 + (1 ⁄ Ce)}

(16)

where Ce, R, and T have their usual meaning and are described elsewhere. The Qm and KDR parameters, respectively, have been evaluated from the intercepts and slopes of the plots of ln Qe versus ε2 (Figure 9), and the values are shown in Table 7. If the mean free energy (E, kJ · mol-1) of sorption, which can be calculated by computation of KDR in the following relation (eq 17), is in the range of 8.0-16.0, the sorption should be of a chemical nature.50,51 E ) (-2KDR)-0.5

(17)

The mean free energy (E) values calculated at the three different studied temperatures are also shown in Table 7. The E values (kJ · mol-1) for the present reaction have been found in the range between 4.64 and 5.20 (less than 8.0 kJ · mol-1). This indicates the physisorption of the solute in the present sorption reaction, which is similar to the result that had been reported by the some other authors.10,15 Thus, the plausible mechanism of As(III) sorption by NHIZO described below is of the outer sphere surface complex type.

The mechanism suggested agrees well with the insignificant pH change of the batch equilibrium solution. To confirm the sorption mechanism depicted above, the FTIR analysis was conducted for both pure NHIZO and As(III)-NHIZO (Figure 10). It has been found that the OH stretching frequency of NHIZO at 3400 cm-1 shifts to about 3373 cm-1 for As(III)sorbed (32.8 (mg of As) · g-1) material. This shift of OH stretching mode to the lower frequency side by ∼27 cm-1 has indicated that the As(III) species is sorbed by NHIZO with weak hydrogen bonding.

9910 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 Table 5. Comparative Assessment of Langmuir Sorption Capacity of the Present Oxide with Selected Reported Values adsorbent

type of water

pH

concn range (mg · L-1)

qma (mg · g-1)

ref

present oxide iron(III) oxide loaded melted slag (IOLMS) goethite oxisol gibbsite nanoscale zerovalent iron (NZVI) titanium dioxide loaded amberlite XAD-7 resin GAC iron(III) oxide impregnated GAC nano-TiO2 crystalline hydrous ferric oxide hydrous stannic oxide crystalline hydrous titanium dioxide

aqueous solution waste water waste water waste water waste water ground water drinking/Wwaste water drinking/waste water drinking/waste water drinking water aqueous solution aqueous solution aqueous solution

7.0 2.5 5.5 5.5 5.5 7.0 5.0-10.0 7.0 7.0 7.0 7.0 7.0 7.0

5 - 350 20 - 200 10 - 1000 10 - 1000 10 - 1000

64.5 - 66.5 2.9-30.1 7.50 2.6 3.3 3.47 9.74 0.09 4.5 59.9 25 15.9 31.7

present work 46 47 47 47 29 48 49 49 31 23 26 24

a

0 - 375 1.0 1.0 5.0 - 250.0 1.0 - 100.0 5.0 - 250

qm ) Langmuir sorption capacity.

Table 6. Thermodynamic Parameters Evaluated for the As(III)-Sorption on NHIZO at pH 7.0 ( 0.2 linear analysis

nonlinear analysis

temp ((1.6 K)

∆G° (kJ · mol-1)

∆H° (kJ · mol-1)

∆S° (kJ · mol-1 · K-1)

∆G° (kJ · mol-1)

∆H° (kJ · mol-1)

∆S° (kJ · mol-1 · K-1)

288 303 318

-20.31 -21.76 -23.04

+5.83

+0.09

-20.22 -21.11 -22.49

+1.68

+0.08

Table 7. Dubinin-Radushkevich (D-R) Isotherm Parameters Evaluated for the As(III) Sorption on NHIZO at pH 7.0 ( 0.2

Table 8. Comparison of Cost of NHIZO with Some Other Sorbent Materials for Commercialization

As(III) parameters

288 ( 1.6 K

303 ( 1.6 K

318 ( 1.6 K

r2 E (kJ · mol-1) KDR × 10-2 (mol2 · kJ-2) Qm (mol · kg-1)

0.98 4.64 2.32 0.66

0.91 5.17 1.87 0.64

0.97 5.18 1.86 0.76

3.8. Desorption Test. Desorption of arsenic from the As(III)sorbed (32.8 (mg of As) · g-1) NHIZO was tested with increasing alkali (NaOH or KOH) concentration from 0.1 to 2.0 M, and, the results obtained are shown in Figure 11. It has been found that the percentage of arsenic desorption was nearly the same as that of a given concentration of either NaOH or KOH and, that has increased with increasing alkali concentration. Results have shown (Figure 11) that the maximum desorption of arsenic could be up to nearly 80% from the sorbed material. However, the use of higher alkali concentration (>1.5 M) led to the dissolution of some iron oxide parts of the material. 3.9. TCLP Test of As-NHIZO. According to the U.S. EPA,35 if the leached arsenic concentration from the contaminated solid waste is g5.0 mg · L-1, the solid waste should be marked as the hazardous waste, and that should need special precaution for disposal of the waste material. The results on the TCLP test of the present arsenic sorbed material (32.8 (mg of As) · g-1) have shown that the leached arsenic concentration in the used fluid (acetate buffer, pH ) 4.93 ( 0.05) is found to be 0.05 ( 0.01 mg · L-1, which is about 100 times lower than

Figure 11. Desorption of arsenic from the As(III)-sorbed NHIZO.

adsorbent present oxide hydrous ferric oxide hydrous zirconium oxide hydrous titanium oxide activated carbon hydrous stannic oxide

product cost cost(/mol of As(III) adsorbed) (dollars · kg-1) (dollars · mol · (kg-1 of solid)) 25 20 60 136 7.6 9

28.7 60.6 153.5 323.8 6333.0 42.5

the specified limit. Thus, the arsenic rich solid waste is nonhazardous material and can be disposed off safely in the surface landfill. 3.10. Estimated Product Cost. The cost calculation of the present product (NHIZO) has been made based on the sequential reactions used for the preparation. Taking the price of the used reagents from the standard price list for ordinary grade chemicals, the estimated cost obtained including 10% additional amount has been found to be about $25.0/(kg of the material). To estimate the economical viability for commercialization, it has been compared (Table 8) with some other available sorbent materials. On the basis of the efficiency, it is said that NHIZO could be an efficient cost-effective inorganic sorbent material for the As(III)-contaminated water treatment. 4. Conclusion Synthesis of hydrous Fe(III)-Zr(IV) mixed oxide (NHIZO) is a simple and low-temperature green method. The material is the agglomerate of nanocrystallite (dimension, 16-21 nm) and hydrated mixed oxide. The systematic As(III) sorption by this material has showed that the optimum pH and equilibrium contact time are ∼7.0 ( 0.2 and 2.0 h, respectively. The pseudosecond-order equation describes the kinetic data (pH, 7.0 ( 0.2; temperature, 303 ( 1.6 K) well. The equilibrium data (pH, 7.0 ( 0.2; temperatures ((1.6 K), 288, 303, and 318) fit very well with both the Langmuir and the Redlich-Peterson isotherm models. The Langmuir monolayer sorption capacity is 65.5 ( 1.0 (mg of As) · (g-1 of NHIZO) at 303 K, and that increases with increasing temperature. The sorption reaction is spontaneous (∆G° ) negative) and endothermic (∆H° ) positive), and that takes place with increasing entropy. The energy (kJ · mol-1)

Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 9911

of As(III) sorption (4.64-5.20) and the FTIR analysis have suggested that the As(III) sorption by NHIZO is physisorption in nature. The arsenic sorbed material could be regenerated (∼80%) with 2.0 M alkali solution. The TCLP test shows the nonhazardous nature of arsenic sorbed material. Acknowledgment We are thankful to the Head, Department of Chemistry, and the Principal, Presidency College, Kolkata, India, for providing laboratory facilities. K.G. is also grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for a fellowship. Nomenclature B ) Redlich-Peterson constant (L · mg-1) Ce ) equilibrium concentration (mg · L-1) Cf ) filtered solution arsenic concentrations (mg · L-1) Ci ) initial solution concentration (mg · L-1) CL ) liquid-phase solute concentration (mg · g-1) Cs ) solid-phase solute concentration (mg · g-1) DF ) film diffusion coefficient (cm2 · s-1) Dp ) pore diffusion coefficient (cm2 · s-1) ∂ ) film thickness (10-3 cm) E ) mean free energy (kJ · mol-1) ε ) Polanyi potential ∆G° ) free energy change (kJ · mol-1) ∂G° ) free energy change (kJ · mol-1) g ()0-1.0) ) Redlich-Peterson constant ∆H° ) enthalpy change (kJ · mol-1) Ka ) Langmuir constant (L · mg-1) KDR ) constant related to free energy (mol2 · kJ-2) KF ) Freundlich constant (mg · g-1) k1 ) pseudo-first-order rate constant (min-1) k2 ) pseudo-second-order rate constant (g · mg · min-1) kt ) constant L ) Redlich-Peterson constant (L · g-1) m ) constant n ) Freundlich constant Qe ) equilibrium sorption capacity (mol · kg-1) Qm ) saturation sorption capacity (mol · kg-1) qe ) equilibrium sorption capacity (mg · g-1) qm ) Langmuir monolayer capacity (mg · g-1) qt ) the sorption capacity at any time (t) (mg · g-1) R ) molar gas constant (kJ · mol-1 · K-1) R′ ) the removal percentage (%) r0 ) mean radius of agglomerated particles (cm) ∆S° ) entropy change (kJ · mol-1) T ) absolute temperature (K) t ) time (min) V ) solution volume (L) w ) mass (g) of solid added

Literature Cited (1) Hu, C. G.; Li, Y.; Liu, J. P.; Zhang, Y. Y.; Bao, G.; Buchine, B.; Wang, Z. L. Sonochemical Synthesis of Ferromagnetic Core-Sell Fe3O4FeP Nanoparticles and FeP Nanoshells. Chem. Phys. Lett. 2006, 428, 343. (2) Bakoyannakis, D. N.; Deliyanni, E. A.; Zouboulis, A. I.; Matis, K. A.; Nalbandian, L.; Kehagias, Th. Akaganeite and Goethite-type Nano-crystals: Synthesis and Characterization. Microporous Mesoporous Mater. 2003, 59, 35. (3) Zhang, W. X. Nano-scale Iron Particles for Environmental Remediation: An Overview. J. Nanopart. Res. 2003, 5, 323. (4) Smedley, P. L.; Kinniburgh, G. A Review of the Source, Behavior and Distribution of Arsenic in Natural Waters. Appl. Geochem. 2002, 17, 517.

(5) McArthur, J.; Ravenscroft, P.; Safiullah, S.; Thirwall, M. F. Arsenic in Groundwater: Testing Pollution Mechanisms for Sedimentary Aquifers in Bangladesh. Water Res. 2001, 37, 109. (6) Harvey, C. F.; Swartz, C. H.; Baduzzaman, M.; Keon-Blute, A. B.; Yu, W.; Ali, M. A.; Ray, J.; Beckie, R.; Niedon, V.; Brabander, D.; Oates, P. M.; Asfaque, K. N.; Islam, S.; Hemond, H. F.; Ahmed, M. F. Arsenic Mobility and Groundwater Extraction in Bangladesh. Science 2002, 298, 1602. (7) Mondal, P.; Majumder, C. B.; Mohanty, B. Laboratory Based Approaches for Arsenic Remediation from Contaminated Water: Recent Developments. J. Hazard. Mater. 2006, B137, 464. (8) Mohan, D.; Pittman, C. U., Jr. Arsenic Removal from Water/ Wastewater using AdsorbentssA Critical Review. J. Hazard. Mater. 2007, 147, 1. (9) Kundu, S.; Gupta, A. K. Investigation on the Adsorption Efficiency of Iron Oxide Coated Cement (IOCC) towards As(V)sKinetics, Equilibrium and Thermodynamics Studies. Colloids Surf. A 2006, 273, 121. (10) Kundu, S.; Gupta, A. K. Adsorption Characteristic of As(III) from Aqueous Solution on Iron Oxide Coated Cement (IOCC). J. Hazard. Mater. 2007, 142, 97. (11) Wilkie, J. A.; Hering, J. G. Adsorption of Arsenic onto Hydrous Ferric Oxide: Effects of Adsorbate/Adsorbent Ratios and Co-occurring Solutes. Colloids Surf. A 1996, 107, 97. (12) Driehaus, W.; Jekel, M.; Hilderbrandt, U. Grannular Ferric HydroxidesA New Adsorbent for the Removal of Arsenic from Natural Water. J. Water Supply: Res. Technol.—AQUA 1998, 47, 30. (13) Raven, K. P.; Jain, A.; Loeppert, R. H. Arsenite and Arsenate Adsorption on Ferrihydrite: Kinetics, Equilibrium, and Adsorption Envelopes. EnViron. Sci. Technol. 1998, 32, 344. (14) Altundogan, H. S.; Altundogan, S.; Tumen, F.; Bildik, M. Arsenic Adsorption from Aqueous Solution by Activated Red Mud. Waste Manage. 2002, 22, 357. (15) Singh, T. S.; Pant, K. K. Equilibrium, Kinetics and Thermodynamics Studies for Adsorption of As(III) on Activated Alumina. Sep. Purif. Technol. 2004, 36, 139. (16) Lin, T. F.; Wu, J. K. Adsorption of Arsenite and Arsenate within Activated Alumina Grains: Equilibrium and Kinetics. Water Res. 2001, 35, 2049. (17) Katsoyiannis, I. A.; Zouboulis, A. I. Removal of Arsenic from Contaminated Water Sources by Sorptoin onto Iron Oxide-Coated Polymeric Materials. Water Res. 2002, 36, 5141. (18) Thirunavukkarasu, O. S.; Viraraghavan, T.; Subramanian, K. S. Arsenic Removal from Drinking Water Using Iron Oxide Coated Sand. Water, Air Soil Pollut. 2003, 142, 95. (19) Zeng, L. Arsenic Adsorption from Aqueous Solution on an Fe(III)Si Binary Oxide Adsorbent. Water Qual. Res. J. Canada 2004, 39, 269. (20) Kuriakose, S.; Singh, T. S., III. Adsorption of As(III) from Aqueous Solution onto Iron Oxide Impregnated Activated Alumina. Water Qual. Res. J. Canada 2004, 39, 260. (21) Hlavay, J.; Polyak, K. Determination of Surface Properties of Iron Hydroxide-Coated Alumina Adsorbent for Removal of Arsenic from Drinking Water. J. Colloid Interface Sci. 2005, 284, 71. (22) Lenoble, V.; Bouras, O.; Deluchat, V.; Serpaud, B.; Bollinger, J. C. Arsenic Adsorption onto Pillared Clays and Iron Oxides. J. Colloid Interface Sci. 2002, 255, 52. (23) Manna, B. R.; Dey, S.; Debnath, S.; Ghosh, U. C. Removal of Arsenic from Ground Water using Crystalline Hydrous Ferric Oxide (CHFO). Water Qual. Res. J. Canada 2003, 38, 193. (24) Manna, B. R.; Dasgupta, M.; Ghosh, U. C. Studies on Crystalline Hydrous Titanium Oxide (CHTO)sAs Scavenger of Arsenic(III) from Natural Water. J. Water Supply: Res. Technol.—AQUA 2004, 53, 483. (25) Manna, B. R.; Debnath, S.; Hossain, J.; Ghosh, U. C. Trace ArsenicContaminated Groundwater Upgradation Using Hydrated Zirconium Oxide (HZO). J. Ind. Pollut. Control 2004, 20, 247. (26) Manna, B. R.; Ghosh, U. C. Adsorption of Arsenic from Aqueous Solution by Synthetic Hydrous Stannic Oxide. J. Hazard. Mater. 2007, 144, 522. (27) Ghosh, U. C.; Bandhyopadhyay, D.; Manna, B. R., III. Hydrous Iron (III)–Tin (IV) Binary Mixed Oxide: Arsenic Adsorption Behavior. Water Qual. Res. J. Canada 2006, 41, 198. (28) Manna, B. R.; Ghosh, U. C. Pilot-Scale Performance of Arsenic and Iron Removal from Contaminated Groundwater. Water Qual. Res. J. Canada 2005, 40, 82. (29) Kanel, S. R.; Manning, B.; Charlet, L.; Choi, H. Removal of Arsenic(III) from Groundwater by Nanoscale Zerovalent Iron. EnViron. Sci. Technol. 2005, 39, 1291. (30) Yuan, C.; Lien, H.-L. Removal of Arsenate from Aqueous Solution using Nanoscale Iron Particles. Water Qual. Res. J. Canada 2006, 41, 210.

9912 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 (31) Pena, M. E.; Korfiatis, G. P.; Patel, M.; Lippincott, L.; Meng, X. Adsorption of As(V) and As(III) by Nanocrystalline Titanium Dioxide. Water Res. 2005, 39, 2327. (32) Jing, C.; Meng, X.; Liu, S.; Baidas, S.; Patraju, R.; Christodoulatos, C.; Korfiatif, G. P. Surface Complexation of Organic Arsenic on Nanocrystalline Titanium Dioxide. J. Colloid Interface Sci. 2005, 290, 14. (33) Standard Methods for the Examination of Water and Wastewater, 20th ed.; Clesceri, L. S., Greenberg, A. E., Eaton, A. D. , Eds.; American Public Health Association (APHA), American Water Works Association (AWWA), Water Environment Federation (WEF): Washington, DC, 1998; pp 4-82. (34) Babic, M. B.; Milonjic, M. J.; Kaludierovic, B. V. Point of Zero Charge and Intrinisic Equilibrium Constants of Activated Carbon Cloth. Carbon 1999, 37, 477. (35) 40 Codes of Regulations; U.S. Environmental Protection Agency: Washington, DC, 1992; Part 261, p 31. (36) Lagergren, S. About the Theory of So-Called Adsorption of Solute Substances. K. SVen. Vetenskapsakad. Handl. 1898, 24, 1. (37) Ho, Y. S.; McKay, G. Kinetic Model for Lead(II) Sorption onto Peat. Adsorp. Sci. Technol. 1998, 16, 243. (38) Crank, J. The Mathematics of Diffusion; Clarendon Press: Oxford, U.K., 1979. (39) Lalvani, S. B.; Wiltwski, T.; Hubner, A.; Weston, A.; Mandich, N. Removal of Hexavalent Chromium and Metal Cations by a Selective and Novel Carbon Adsorbent. Carbon 1998, 36, 1219. (40) Helffrich, F. Ion-Exchange; McGraw-Hill: New York, 1962. (41) Weber, T. W.; Chakravorty, R. K. Pore and Solid Diffusion Models for Fixed Bed Adsorbents. J. Am. Soc. CiV. Eng. 1974, 20, 228. (42) MichelsonL. D. GideonP. G. PaceE. G. KutalL. H. RemoVal of Soluble Mercury from Wastewater by Complexing Techniques, Bulletin No.

74; Industry Office of Water Research and Technology, U.S. Department of Energy: Washington, DC, 1975. (43) Langmuir, I. The Conustitution and Fundamental Properties of Solids and Liquids. J. Am. Chem. Soc. 1916, 38, 2221. ¨ ber Die Adsorption in Lo¨sungen. Z. Phys. (44) Freundlich, H. M. F. U Chem. 1906, 57A, 385. (45) Redlich, O.; Peterson, D. L. A Useful Adsorption Isotherm. J. Phys. Chem. 1959, 63, 1024. (46) Zhang, F.-S.; Itoh, H. Iron Oxide-Loaded Slag for Arsenic Removal from Aqueous System. Chemosphere 2005, 60, 319. (47) Ladeira, A. C. Q.; Ciminelli, V. S. T. Adsorption and Desorption of Arsenic on an Oxisol and Its Constituents. Water Res. 2004, 38, 2087. (48) Tatineni, B.; Hideyuki, M. Adsorption Characteristics of As(III) and As(V) with Titanium Dioxide Loaded Amberlite XAD-7 resin. Anal. Sci. 2002, 18, 1345. (49) Reed, B. E.; Vaughan, R.; Jiang, L. As(III), As(V), Hg and Pb Removal by Fe-Oxide Impregnated Activated Carbon. J. EnViron. Eng. 2000, 126, 869. (50) El-kamash, A. M.; Zaki, A. A.; Abed, El.; Geleel, M. Modeling Batch Kinetics and Thermodynamics of Zinc and Cadmium Ions Removal from Waste Selwian Using Synthetic Zeolite A. J. Hazard. Mater. 2005, 127, 211. (51) Krishna, B. S.; Murty, D. S. R.; Prakash, B. S. J. Thermodynamics of Chromium(VI) Anionic Species Sorption onto Surfactant-Modified Montmorillonite Clay. J. Colloid Interf. Sci. 2000, 229, 230.

ReceiVed for reView February 5, 2008 ReVised manuscript receiVed August 20, 2008 Accepted September 22, 2008 IE8002107