The Kinetics and Thermodynamics of Sorption of Chromium(VI) onto

The percentage removal of Cr(VI) decreased with increasing ionic strength. ... Suzuki et al.9 used silica gel as a polymer support and grafted with a ...
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Ind. Eng. Chem. Res. 2001, 40, 2693-2701

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The Kinetics and Thermodynamics of Sorption of Chromium(VI) onto the Iron(III) Complex of a Carboxylated Polyacrylamide-Grafted Sawdust Maya R. Unnithan and T. S. Anirudhan* Department of Chemistry, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, India

A novel adsorbent was prepared and its adsorption properties for Cr(VI) were studied. The iron(III) complex of a carboxylated polyacrylamide-grafted sawdust has been found to be an effective adsorbent for the removal of Cr(VI) from aqueous systems. Experiments were carried out as a function of concentration of Cr(VI), agitation period, agitation speed, pH, ionic strength, and temperature. Maximum removal (>99.0%) was observed at an initial concentration of 25.0 mg/L in the pH range 2.0-3.0. Coordination unsaturated sites for the iron(III) complex of polymer were considered to be the adsorption sites for Cr(VI) species, with the predominant species being HCrO4-. Rate constants as a function of concentration and temperature were evaluated with the help of a proposed second-order kinetic model. The percentage removal of Cr(VI) decreased with increasing ionic strength. The L-type adsorption isotherm obtained for the adsorbent indicated a favorable process. Adsorption isothermal data could be interpreted by the Langmuir and Freundlich equations. The uptake of Cr(VI) on adsorbent increased from 144.20 mg/g at 20 °C to 172.74 mg/g at 60 °C. Thermodynamic parameters such as ∆G°, ∆H°, and ∆S° for the adsorption process were calculated. The isosteric heat of adsorption was also determined at various surface loadings of the adsorbent used. Simulated Cr(VI) electroplating wastewaters were also treated by the sorbent to demonstrate its efficiency in removing Cr(VI) from wastewater with other ions. Desorption studies showed that over 95.5% of Cr(VI) can be desorbed from the adsorbent using 0.1 M NaOH. Introduction Chromium, which is known to be a carcinogen, has various applications in a variety of industries such as textiles, fertilizers, leather tanning, electroplating, and metal finishing. Chromium can be present in solution in various chemical forms. Electroplating and metal finishing wastes primarily contain Cr(VI) species, whereas textile and tanning waste contain either Cr(VI) or Cr(III) species.1 Cr(VI) is about 100 times more toxic than Cr(III). Because of this high toxicity, it is imperative to reduce the concentration levels of Cr(VI) in effluents and promote its recycling and reuse. According to the U.S. Environmental Protection Agency, the permissible limit of Cr(VI) in effluent is 0.05 mg/L. This problem of Cr ions in the environment is made more prominent by the increasing discharge of Cr-containing wastes. It is reported that the Italian tannery industries located in Naples discharge 40 Mm3 of Cr(VI)-containing wastewater (500 mg of Cr/L) and 280 000 tons (1-5% Cr) of dry sludges per year.2 In India, about 150 000 ton/year of Cr-containing sludges is released into the environment from tannery industry effluents.3 The conventional means for treating wastewaters containing Cr(VI) are chemical reduction, precipitation, evaporation, ion exchange, and adsorption. Considerable research has been done on the removal of Cr(VI) from wastewaters using activated carbon adsorption.4 The use of low-cost materials such as coal, moss peat, feldspar, and sawdust5-8as possible media for Cr(VI) removal from wastewaters has been highlighted re* To whom correspondence should be addressed. E-mail: [email protected].

cently. New research in this direction indicates that surface modification of the adsorbent materials enhances the adsorption efficiency of the adsorbents. Graft polymerization on solids followed by functionalization is now widely used for the surface modification of adsorbent materials. Suzuki et al.9 used silica gel as a polymer support and grafted with a chloromethylstyrene divinylbenzene copolymer onto its surface through the adsorption and subsequent polymerization of the vinyl monomers. The polymer-grafted silica gel thus formed could successively be modified with a wide variety of functional groups, e.g., diethyl iminodiacetate was introduced onto the polymer surface through a reaction of the ligand with the chloromethyl group. These types of adsorbent materials have a sufficient mechanical strength for use in the usual experimental procedures. Materials such as iron(III) oxide,10 chitosan,11 and biomass12 have also been used as polymer supports for the preparation of adsorbent materials having different functional groups. In a previous paper,13 a report was given on the adsorption of Hg(II), Pb(II), and Cd(II) ions on polyacrylamide-grafted sawdust having -COOH functional groups. Recently, the use of polymerized coconut coir with an amine functionality14 and polymerized sawdust with a -NH3+Cl- functional group15 for Cr(VI) removal has been reported. Takeshita et al.16 investigated the applicability of the iron(III) complex of a commercial macroreticular polystyrene divinyl benzene (DVB) based ion-exchange resin with a carboxylate functionality, Amberlite IRC-50, for the removal of arsenate, arsenite, and phosphate ions from aqueous solutions. Experimental studies have demonstrated that iron(III) ions, which are coordinately hexavalent, were

10.1021/ie0009740 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/08/2001

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Figure 1. Preparation of carboxylated polyacrylamide-grafted sawdust and its iron(III) complex.

bound to the chelating resin under so-called “coordination unsaturated” conditions, leaving two coordination sites as adsorption sites for the coordinating anions.17 The adsorption behavior of Cr(VI) on iron(III)-loaded chelating resins on either commercial synthetic polymerbased or natural lignocellulosic-based polymer has not yet been reported. In the present investigation, a polyacrylamide-grafted sawdust (lignocellulosic matrix) with a carboxylate functionality was used to prepare the iron(III) complex of a chelating resin. The possible use of this adsorbent material for the removal of Cr(VI) from aqueous solutions and wastewaters was then explored by studying its adsorption properties under kinetic and equilibrium conditions. Experimental Section Preparation of Adsorbent. The sawdust (SD) of rubber wood (Havea Braziliansis) was grafted with polyacrylamide using the procedure described by Raji and Anirudhan.13,15 For this procedure, about 20.0 g of dried SD (1) was treated with 300 mL of a solution containing 5.0 g of N,N′-methylenebisacrylamide (2) and peroxydisulfate (2.0 g). Next, 7.5 g of acrylamide (3) was added, and the mixture was refluxed at 70 °C. The polyacrylamide-grafted sawdust (PGSD) was washed with water and dried at 80 °C. To convert it into the desired carboxylate-functionalized polymer product, PGSD was first refluxed with ethylenediamine [(en)2] in toluene and then with succinic anhydride in 1,4dioxane at pH 4.0 using the method described elsewhere.13,18 After reaction, carboxylic-acid-bound PGSD (PGSD-COOH) was separated, washed with 1,4-dioxane, washed with ethanol, and dried. PGSD-COOH was sieved to obtain -80 +230 mesh size particles. The physical, surface, and structural properties of PGSDCOOH were reported in a previous paper.13 The structure of PGSD-COOH is represented in Figure 1. The amount of carboxylate group in PGSD-COOH was found to be 2.03 mmol/g. The iron(III) complex of PGSD-COOH was prepared by the following procedure.16 Typically, 15.0 g of the sodium form of PGSD-COOH, which was conditioned by successive washings with 1.0 M HCl and NaOH solutions, was treated with 200 mL of 0.03 M sodium

acetate-acetic acid (buffer pH 3.0) containing 10.0 g of iron(III) chloride hexahydrate. The resulting product, the iron(III) complex of PGSD-COOH, was rinsed with distilled water until the washing were free of Fe(III) ion and finally dried in an oven overnight at 60 °C. Adsorbent Characterization. The iron content in the adsorbent was determined spectrophotometrically with the acid eluate. We measured the content of iron in the adsorbent material many times; the content value range was 1.39-1.44 mmol/g. To determine whether any dissolution of iron from the adsorbent occurred during the adsorption and desorption process, supernatant solutions was analyzed for both Fe(II) and Fe(III).19 It was observed that iron(III) is strongly bound to the resin so that it is not easily released from the adsorbent, even in the presence of alkali and alkaline earth metal cations above pH 3.0. The surface area of the adsorbent was determined using the methylene blue (MB) adsorption method described by Viladkar et al.20 The adsorption of MB in aqueous solution was carried out in closed glass container into which 100 mg of adsorbent and 50 mL of MB solution [(0.5-4.0) × 10-5 M] were introduced. The suspension was shaken for 24 h at 30 °C using a water bath shaker. After equilibrium was reached, the concentration of MB in solution was measured spectrophotometrically at 660 nm. The specific surface area of the adsorbent was calculated using the following expression

Ss ) (MfN/105)Am × 10-20 (m2/g)

(1)

where Mf is the amount of MB (mmol) adsorbed per 100 g of adsorbent when the surface is completely covered with a monolayer of MB, N is Avogadro’s number, and Am is the cross-sectional area per molecule on the surface (130A°). The value of Mf was obtained from the MB adsorption isotherm (BET plot), and substitution of this value into the above expression gave the value of the specific surface area of the adsorbent. The zero-point charge (pHzpc) is defined as the pH of the suspension at which surface charge density σ0 (C/ cm2) ) 0. σ0 as a function of pH was determined using potentiometric titration method.21 The water content of the adsorbent was also determined by measuring the weight loss after drying to constant weight at 110 °C. The apparent density was determined using nitrobenzene as the displacing liquid in a specific gravity bottle. Hereafter, the iron(III) complex of PGSD-COOH is designated as S-Fe(OH2)2 (Figure 1). The characteristics of the S-Fe(OH2)2 are as follows: surface area, 68.7 m2/g; pHzpc, 9.2; apparent density, 1.68 g/mL; porosity, 0.44 mL/g; water content, 6.47 mmol/g; iron content, 1.42 mmol/g; and polymer content, 21.8%. Adsorption Studies. Batch-mode adsorption studies were carried out by placing 100 mg of the adsorbent and 50 mL of a Cr(VI) solution of the desired concentration at pH 3.0 into a 100-mL stoppered bottle and agitating the mixture using a temperature-controlled water bath shaker. Adsorbent was separated from the adsorption medium at the end of each adsorption experiments, and the concentration of Cr(VI) in the aqueous phase was measured spectrophotometrically through the development of a purple-violet color with 1,5-diphenyl carbazide in acid solution.22 The amount of Cr(VI) adsorbed was calculated by subtracting the concentration of the Cr(VI) remaining in solution from the initial concentration. Studies regarding the effect of the initial Cr(VI)

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Figure 2. Effect of pH on the removal of Cr(VI) by S-Fe(OH2)2.

concentration, pH, agitation time, ionic strength, agitation speed, and temperature were carried out. The pH study was carried out by agitating 50 mL of 25.0 and 50.0 mg/L Cr(VI) solutions with 100 mg of adsorbent at different pH values (2.0-10.0) for 4 h. The pH of the solution was adjusted using 0.1 M NaOH and HCl. The remaining Cr(VI) was estimated spectrophotometrically. Adsorption isothermal studies were also carried out at pH 3.0 using the initial concentration range between 50.0 and 750.0 mg/L at temperatures of 20, 30, 40, 50, and 60 °C. Desorption studies were carried out by mixing 100 mg of Cr(VI)-loaded adsorbent and 50 mL aqueous solution of NaOH and agitating for 6 h at 200 rpm. The adsorbent was separated and gently washed with distilled water to remove any unloaded Cr(VI). The amount of desorbed Cr(VI) in the aqueous solution was estimated spectrophotometrically as described earlier. Results and Discussion Effect of pH. Figure 2 presents the removal of Cr(VI) as a function of pH over the range of 2.0-10.0 for S-Fe(OH2)2. The removal of Cr(VI) was pH-dependent. It can be seen that the optimum pH at which maximum removal of Cr(VI) occurred is observed over the pH range 2.0-3.0. Above this pH range, adsorption gradually falls with increasing basicity of the medium. At pH 10.0, about 40.9 and 26.8% removals were observed for initial concentrations of 25.0 and 50.0 mg/ L, respectively; the corresponding values at pH 4.0 were found to be 89.8 and 77.0%. The maximum removal of 99.2 and 90.0% was observed at pH 3.0 for the initial concentration of 25.0 and 50.0 mg/L, respectively, and below this pH, there was no significant increase in Cr(VI) removal between 3.0 and 2.0. The effect of pH on Cr(VI) removal by S-Fe(OH2)2 seems to depend on the species of Cr(VI) in solution and the adsorption reaction. The pHzpc of S-Fe(OH2)2 was found to be 9.2, and below this pH, the surface charge of the adsorbent is positive. At pH 3.0 the predominant Cr(VI) species23 is HCrO4-, and therefore, the uptake of Cr(VI) in the pH range of 2.0-3.0 occurs via the ligand-exchange mechanism. The adsorption of HCrO4- ion can be considered to be a ligand-exchange reaction between the coordinated water and HCrO4- ions. The reaction can be written schematically as

Figure 3. Effect of agitation period and initial concentration on the removal of Cr(VI) by S-Fe(OH2)2.

S-Fe(OH2)2 + HCrO4- h S-Fe(HCrO4)- + 2H2O (2) The decrease in removal of metal ions at higher pH is apparently due to the higher concentration of OHions present in the reaction mixture, which compete with Cr(VI) species23 (CrO42-) for the adsorption sites. As the adsorbent surface (pH > pHzpc) is negatively charged as well, the increasing electrostatic repulsion between negatively charged Cr(VI) species and negatively charged sorbent particles would also lead to a decrease in the adsorption of Cr(VI) ions. Effect of Initial Concentration and Agitation Period. The removal of Cr(VI) by adsorption on S-Fe(OH2)2 increases with time and attains a maximum value at 120 min; thereafter, it remains constant (Figure 3). The amount of Cr(VI) adsorbed increased from 21.0 mg/g (84.0%) to 55.2 mg/g (73.6%) with an increase in the initial concentration from 50.0 to 150.0 mg/L. It can also be seen from Figure 3 that the amount of Cr(VI) adsorbed on the solid surface at lower concentrations of Cr(VI) is smaller than the amount adsorbed when higher concentrations are used. However, the percentage removal is greater with lower initial concentrations and smaller with higher initial concentrations. This shows that Cr(VI) removal is highly concentrationdependent. With increasing concentration, the ratio of the initial number of moles of metal ions to the available adsorption sites of adsorbent becomes lower, and hence, the extent of metal removal depends on the initial concentration. In all cases, equilibrium was achieved in the system in 120 min and was independent of the Cr(VI) concentration. Adsorption Dynamics. Determination of the kinetic parameters and explanation of the mechanism in heterogeneous system is often a complex procedure, as surface effects can be superimposed on chemical effects. The adsorption of Cr(VI) from liquid phase to solid phase can be considered as a second-order reaction, i.e., the adsorption rate constant, kad, was obtained by assuming24 that the desorption rate is small compared to adsorption rate; the surface site concentration is expressed in units of the Cr(VI) concentration; the rate of adsorption whereas be approximated as equal to the rate of decrease in the concentration of the sorbate solution, i.e., kad ) -dc/dt; and there is a monolayer of

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Figure 5. Effect of ionic strength on the adsorption of Cr(VI) onto S-Fe(OH2)2.

Figure 4. Second-order kinetic plots for the adsorption of Cr(VI) on S-Fe(OH2)2 at different (A) concentrations and (B) temperatures. Table 1. Second-Order Rate Constants for the Adsorption of Cr(VI) on S-Fe(OH2)2 variable

kad (L mg-1 min-1)

r

concentration (mg/L) 50 100 150

7.49 × 10-4 1.49 × 10-4 9.10 × 10-5

0.9917 0.9784 0.9881

temperature (°C) 30 40 50 60

1.53 × 10-4 1.88 × 10-4 2.65 × 10-4 4.32 × 10-4

0.9922 0.9935 0.9968 0.9992

metal ions on the surface of the adsorbent, i.e., adsorption follows the Langmuir equation. The following integrated rate expression24 is used to calculate the second-order rate constant (kad) values

1 1 ) kadt C C0

(3)

where C0 is the initial bulk concentration of adsorbate solution and C is the concentration in the aqueous phase at time t. The rate constants of adsorption at different concentrations and temperatures were determined from the slope of the plot of 1/C vs t (Figure 4) using a regression analysis. The data showed good compliance with the proposed second-order equation. Indeed, the regression coefficients for the linear plots were greater than 0.97. The values of kad are shown in Table 1. The rate constant increases with decreasing concentration

and increasing temperature. The kad values vary from 7.49 × 10-4 to 9.10 × 10-5 L mg-1 min-1 as C0 varies from 50.0 to 150.0 mg/L. The values of kad were found to increase from 1.53 × 10-4 to 4.32 × 10-4 L mg-1 min-1 for an increasing temperature from 30 to 60 °C. The findings clearly indicate that higher temperature favors Cr(VI) removal by adsorption on S-Fe(OH2)2. An examination of the temperature effect on the rate at which Cr(VI) is adsorbed from solution also allows for an evaluation of the activation energy, Ea, for the sorption reaction. Ea for adsorption was determined using the Arrhenius equation. The Ea value as calculated from the slope of the ln kad vs 1/T plot was found to be 28.835 kJ/mol. The relatively low Ea value suggests that Cr(VI) adsorption is an activated or diffusion-controlled process.25,26 Because of the porous nature of the adsorbent, the intraparticle diffusion (pore diffusion) is supposed to be the rate-limiting step. Knocke and Hemphill26 have stated that, because diffusion is an endothermic process, the rate of sorption will increase with increased solution temperature when pore diffusion is the rate-limiting step. Hence, the results of this study of the temperature effect indicate that Cr(VI) sorption on S-Fe(OH2)2 is controlled by pore diffusion. Effect of Ionic Strength. The removal of Cr(VI) from the solution with a 50.0 mg/L initial concentration decreases from 89.9 to 17.6% as the ionic strength is increased from 0.001 to 0.1 M (Figure 5). It can be noted that the salt concentration primarily influences Cr(VI) adsorption. According to the surface chemistry theory developed by Guoy Chapman, when solid adsorbent is in contact with sorbate species in solution, the latter are surrounded by an electrical diffused double layer, the thickness of which is significantly expanded by the presence of NaNO3.27 Such expansion inhibits the adsorbent particles and Cr(VI) species from approaching each other more closely and, through the decreased electrostatic attraction, leads to the decreased uptake of Cr(VI) ions. Effect of Agitation Speed. The percentage removals of Cr(VI) from a solution with an initial concentration of 100.0 mg/L by S-Fe(OH2)2 were found to be 71.2, 76.0, 79.9, and 82.3% at 100, 200, 300, and 400 rpm, respectively. The results indicate that the external adsorption of Cr(VI) onto adsorbent is controlled by the degree of agitation. Increasing the agitation decreases the boundary layer resistance to mass transfer in the

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Figure 6. Adsorption isotherms of Cr(VI) onto S-Fe(OH2)2.

bulk and increases the driving force of Cr(VI) ions toward adsorption. The process is influenced by the concentration gradient between the boundary layer and the solution and the thickness of the diffusion layer, which is a function of the agitation process.28 Another cause could be the kinetic energy gained by the sorbate species during agitation. With increasing agitation speed, the Cr(VI) species might become activated through a gain in kinetic energy and easily cross the potential barrier. Adsorption Isotherm. Adsorption isotherms for 20, 30, 40, 50, and 60 °C were obtained at pH 3.0 by varying the initial concentration of Cr(VI) (50.0-750.0 mg/L). The adsorption isotherms thus obtained are depicted in Figure 6. As can be seen from Figure 6 from the asymptotic nature of the curves, these isotherms can be classified as L type according to the Giles classification system.29 At higher adsorbate concentrations, a saturation limit of S-Fe(OH2)2 is observed, i.e., as Ce approaches infinity, qe approaches complete monolayer formation. The adsorption isotherms in Figure 6 tend to define a plateau; therefore, it seems reasonable to suppose that, for the experimental conditions used, the formation of a complete monolayer of Cr(VI) ions covering the adsorbent is possible and the curves lead to a constant value of qe. When a saturation-type curve represents the ion exchange fraction, the Langmuir equation can be used to calculate the maximum adsorption capacity Qo (mg/g) and the energy of adsorption b (L/mg), which are given by

Ce Ce 1 ) o + qe Q b Q°

(4)

where Ce and qe are the equilibrium sorbate concentrations in the aqueous (mg/L) and solid phases (mg/g), respectively. Figure 7 depicts the results obtained by plotting Ce/qe vs Ce for the adsorption of Cr(VI) on S-Fe(OH2)2 at the different temperatures studied, with a correlation coefficient (r2) that is greater than 0.99 in all cases. The values of Qo and b were computed from the least-squares method applied to the straight line in Figure 7. The results are given in Table 2. The values of Qo and b increase, respectively, from 144.20 mg/g and 9.409 × 10-3 L/mg at 20 °C to 172.74 mg/g and 35.088 × 10-3 L/mg at 60 °C. Although a direct comparison of S-Fe(OH2)2 with other adsorbent materials is difficult, because of the different applied experimental conditions, it was found,

Figure 7. Langmuir isotherm plots for the adsorption of Cr(VI) onto S-Fe(OH2)2.

in general, that the adsorption capacity of S-Fe(OH2)2 for Cr(VI) using equilibrium experiments at 30 °C, determined to be 152.98 mg/g of S-Fe(OH2)2 is higher than those of bituminous coal5 (2.04 mg/g), Sphagnum moss peat6 (119.0 mg/g), rice husk carbon30 (21.0 mg/g), commercial activated carbon30 (13.8 mg/g), ground nut husk carbon31 (4.0 mg/g), and Feldspar32 (24.0 mg/g). The equilibrium adsorption data were also fitted to the Freundlich equation. Figure 8 shows that the adsorption of Cr(VI) by the S-Fe(OH2)2 follows a Freundlich adsorption isotherm, which satisfies an equation of the type

1 log qe ) log Ce + log KF n

(5)

where KF and 1/n are constants that are considered to be relative indicators of adsorption capacity and adsorption intensity, respectively. The values of these constants at different temperatures are listed in Table 2. It is clear that KF increases with increasing temperature, showing that the rate of adsorption also increases with increasing temperature. Values of 0.1< 1/n 95.0%) is adsorbed onto S-Fe(OH2)2. Test with Simulated Industrial Wastewaters. Two simulated Cr(VI) electroplating wastewater samples8,38 (composition given in Table 4) were also treated with S-Fe(OH2)2 to demonstrate its adsorption potential and utility in removing metal from wastewater in the presence of other ions. It is evident that, for the

quantitative removal of Cr(VI) from 50 mL of simulated wastewaters containing 50.0 and 300.0 mg/L of Cr(VI) in the presence of other ions, minimum adsorbent dosages of 50 and 200 mg are sufficient for the removal of 65.3 and 70.5%, respectively, of the total Cr(VI) which is in good agreement with the results obtained from the batch experiments mentioned above. Figure 12 demonstrates that the treatment of Cr(VI) in wastewater is not significantly different from the results predicted on the basis of batch experiments using Cr(VI) only. The complete removal of Cr(VI) from 50 mL of samples 1 and 2 was achieved by 250 and 900 mg adsorbent doses, respectively. The adsorbed Cr(VI) was removed using 0.1 M NaOH. Above 92.0 and 85.0% recoveries were

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Table 5. Comparison between m/V Values Calculated and Used initial concentration of Cr(VI) (mg/L) 50 75 100 150 200 300 500 750

m/V values (g/L) calculated used 2.20 1.94 1.96 1.97 1.98 1.95 1.97 2.03

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

achieved in samples 1 and 2, respectively, using 0.1 M NaOH. Design Application for Batch Adsorber. In a typical agitated batch adsorber, the wastewater is agitated with adsorbent in a treatment tank at constant speed for a set time. After attaining equilibrium, the slurry is filtered to separate the spent adsorbent and adsorbate from the liquid. The system can be made multistage by providing additional tanks and filters; if continuous operation is required, centrifuges or continuous rotary filters can be substituted for the filter press. The adsorbent/solution ratio is essential for industrialscale design of the technical system. Attempts have also been made to develop a model for predicting the adsorbent dose/solution ratio required to purify the wastewater and duration of replacement of adsorbent in a batch reactor. Consider V liters of solution with a Cr(VI) concentration that is to be reduced from C0 to C1 mg/L. The amount of sorbent is m (g), and the surface loading of Cr(VI) changes from q0 to q1 mg/g of sorbent. When fresh adsorbent is used, q0 ) 0, and mass balance equates the metal removed from the liquid phase to that adsorbed by the adsorbent39

V(C0 - C1) ) m(q1 - q0) ) mq1

(10)

At equilibrium

C1 ) Ce and q1 ) qe The Langmuir equation (eq 4) applied to eq 10 now gives

m C0 - Ce ) o V Q bC

(11)

e

1 + bCe Equation 11 can help engineers to determine the sorbent/solution ratio (g/L) for a given change in initial concentration from C0 to Ce. To test the robustness of eq 11, values of m/V for a selected sets of C0 and Ce values were calculated using this equation. A comparison of the used and predicted values of m/V for the sample values of C0 and Ce is shown in Table 5. The small variation in the calculated and used m/V values seems to be due to the approximation in the Qo and b values in eq 4. Additional research is also warranted to test the applicability of the equation with respect to real industrial wastewaters and other varying experimental conditions. Cost Estimation. Macroporous strong base anion (SBA) resins with a quaternary functionality [R-N+(CH3)3] and polymer matrixes of either polystyrene divinylbenzene (DVB) or polyacrylic DVB are wellknown for their ability to remove a variety of anions from aqueous solutions and are commonly used as ion

exchangers in many water treatment applications. These ion-exchange resins are very expensive and are available for $60-170 per kg of resin. Baes et al.14 compared the adsorption potential of a commercial chloride form of SBA, Amberlite IRA-900 produced by Rohn and Haas, Philadelphia, PA, with that of an amine-modified coconut coir exchanger for the removal of Cr(VI) from aqueous solutions. Even though the adsorption potential of former is greater, the cost of this commercial material is relatively very high ($65.0 per kg of resin). Studies have been reported on the use of SBA exchangers based on cellulose and lignocellulosic matrixes for the removal of Cr(VI) and other contaminant anions efficiently, which only cost about 1/3 of the price of polystyrene DVB or polyacrylic DVB resins.14,40 The raw material used in the present study, sawdust, is available from the timber industry free of cost, as a waste, and including expenses for transport, chemicals for surface modifications, electrical energy, etc., the final product S-Fe(OH2)2 would cost approximately $16.0 per kg. The relative cost of this material is lower than that of cellulose-based ion exchangers and more so than that of synthetic polymer-based materials. Conclusions From the overall results on the adsorption of Cr(VI) on S-Fe(OH2)2, the following conclusions can be drawn. The study clearly establishes that this adsorbent is very effective for the removal of Cr(VI) from aqueous solutions. The adsorption was found to depend on the Cr(VI) concentration, pH, ionic strength, agitation speed, and temperature. The maximum removal (99.2%) was observed at 30 °C and pH 3.0 with an initial concentration of 25.0 mg/L of Cr(VI) and a 2 g/L adsorbent dose. The process of uptake of Cr(VI) follows second-order kinetics. The adsorption capacity increases with decreasing ionic strength and increasing agitation speed and temperature. A ligand-exchange reaction between the coordinated water and the chromate ions (HCrO4-) is the major removal mechanism involved. The isothermal data obey the Langmuir and Freundlich equations for the present system, and the thermodynamic properties were calculated. The isosteric heat of adsorption was also determined and was found to vary with surface loading. More than 95.0% desorption was achieved using 0.1 M NaOH. The process of Cr(VI) removal from simulated electroplating industry wastewaters was examined. Additional research is warranted to evaluate the adsorption efficiency of this adsorbent with respect to various real industrial wastewaters. Acknowledgment The authors are thankful to the Head of the Department of Chemistry, University of Kerala, Thiruvananthapuram, India, for providing the laboratory facilities. Literature Cited (1) Huang, C. P.; Wu, M. H. Chromium Removal by Carbon Adsorption. J. Water Pollut. Control Fed. 1975, 47, 2437-2445. (2) Petruzzelli, D.; Tiravanti, G.; Santori, M.; Passino, R. Chromium Removal and Recovery from Tannery Wastes: Laboratory Investigations and Field Experience on a 10 m3/d Demonstration Plant. Water Sci. Technol. 1994, 30, 225-233. (3) Knott, M. Toxic Tannery Sludge Made as Safe as Houses. New Sci. 1996, 149, 22-26.

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Received for review November 17, 2000 Revised manuscript received February 23, 2001 Accepted March 11, 2001 IE0009740