PII: S0043-1354(97)00349-7 Wat. Res. Vol. 32, No. 5, pp. 1544±1552, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00 ARSENATE SORPTION BY FE(III)-DOPED ALGINATE GELS JOON H. MIN1 and JANET G. HERING2* Department of Civil and Environmental Engineering, University of California, Los Angeles, 5732 Boelter Hall, Los Angeles, CA 90095-1593, U.S.A. and 2California Institute of Technology, Environmental Engineering Science (138-78), Pasadena, CA 91125, U.S.A. 1 (First received February 1997; accepted in revised form August 1997) AbstractÐAlthough cationic metal contaminants can be eectively removed from wastewaters by treatment with biopolymers, application of biopolymers for the removal of anionic contaminants (such as As, Cr(VI), and Se) has been limited. The objective of this study was to examine the fundamental aspects of a possible remediation strategy for removal of anionic metal species employing the biopolymer alginic acid pretreated with Ca and Fe(III). Spherical gel beads (2 mm in diameter) were formed by dispensing the biopolymer solution dropwise into 0.1 M CaCl2; Ca beads were then washed and equilibrated with 0.1 M FeCl3 to achieve partial substitution of Fe(III) for Ca. The resulting Ca±Fe beads were found to be eective at removing As(V) from solution on a time scale of approximately 100 h. As(V) sorption was pH dependent; optimal removal and stability of the Ca±Fe beads was achieved at pH 4. At a given initial As(V) concentration, As(V) removal eciency increased with increasing Fe content (number of beads); at an initial As(V) concentration of 400 mg/l, up to 94% removal was achieved at pH 4 after 120 h. For a given Fe content, uptake of As(V) increased with increasing initial As(V) concentration until saturation was reached. Sorption data was modeled using a single type of As(V) binding site. Data on As(V) sorption as a function of dissolved As(V) concentration and Fe content were used to obtain and validate the site density and conditional anity constant for As(V) sorption. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐoxyanion, arsenic, arsenate, biopolymer, alginate, gel bead, Fe(III)-doped, removal, wastewaters INTRODUCTION Of the many toxic metals that may be present as contaminants in natural waters and wastewaters, a few (speci®cally arsenic, chromium(VI), and selenium) occur as oxyanions. Removal of arsenic is, in particular, an issue of increasing concern. Reevaluation of the current Maximum Contaminant Level (MCL) for arsenic is mandated under the 1996 re-authorization of the Safe Drinking Water Act. A decrease in the current standard (50 mg/l) would impact the required levels of treatment not only for potable water, but also for hazardous wastewaters and euents under the Resource Conservation and Recovery Act. Arsenic is a major contaminant of concern at many Superfund sites (Brewster and Passmore, 1994; Mariner et al., 1996). Arsenic containing wastestreams are generated in the microelectronics industry due to the use of arsenic in the form of gallium arsenide (GaAs) for the manufacturing of semiconductor devices (Gilles and Loehr, 1994; Vagliasindi and Poulsom, 1994). The average concentration of arsenic in *Author to whom all correspondence should be addressed [Tel: +1-626-3953644, Fax: +1-626-3952940, E-mail: [email protected]]. wastewater at facilities which produce GaAs has been estimated to be 2.4 mg/l (USEPA, 1982). For arsenic-contaminated wastestreams, various physical/chemical treatment technologies have been applied for contaminant removal. Arsenic-contaminated water collected during clean up at a former pesticide facility was treated in a full-scale process involving co-precipitation/adsorption with ferric chloride, ®ltration and carbon adsorption (Harper and Kingham, 1992). In pilot-scale studies using electrochemically generated Fe2+ and hydrogen peroxide, arsenic was eciently removed from contaminated waters at both a Superfund site and a wood-preserving facility (Brewster and Passmore, 1994). Other processes, such as adsorption onto activated carbon (Huang and Fu, 1984; Huang and Vane, 1989) or ¯y-ash (Diamadopoulos et al., 1993) have been tested at bench scale. Many of these processes generated a signi®cant quantity of sludges or other solid wastes; in some cases, these materials had to be disposed of as hazardous waste (Harper and Kingham, 1992). In some applications, selective removal of oxyanions may provide options for more economical treatment of wastestreams, for example, by reducing subsequent sludge disposal costs. Thus, this study was 1544 As(V) sorption by Fe(III) alginate gel designed to investigate the fundamental aspects of a possible remediation strategy for oxyanion removal using a negatively-charged biopolymer pretreated with metal cations. At the bench scale, biosorbents have been tested primarily for removal of heavy metal cations, which are removed preferentially to alkali and alkaline earth metals and anionic species (Brierley, 1990). Alginic acid has been shown to be eective at removing many cationic metals from solution including Pb2+ and Cu2+ (Deans and Dixon, (Hassan et al., 1993), Cu2+ (Jang et 1992), UO2+ 2 al., 1990), Cu2+, Zn2+, Cd2+, and Ni2+, (Jellinek and Sangal, 1972), Nd3+ and Yb3+ (Konishi et al., 1992), and 226 Ra (Torma et al., 1991). Pretreatment or doping of an anionic biosorbent, such as alginic acid, with cations allows (indirect) interaction between the biosorbent and anionic contaminants. Metal recovery (Co, Cu, As, Fe, Mg, Al, Ca) from an acidic (pH 2±3) cobalt ore leachate with alginic acid has been investigated by Jang et al. (1991). The removal of arsenic observed with this anionic biopolymer may be attributable to in situ metal doping in the presence of strongly binding cationic metals. In this study, the sorption properties of metaldoped anionic biopolymer were systematically examined using alginic acid, a linear, binary heteropolymer of mannuronic (M) and guluronic (G) residues (Fig. 1(a)). The proportions of MM-, GG- and MG-blocks vary with the source of the alginic acid. GG blocks have a strong anity for divalent metal ions and are responsible for gelation (Yotsuyanagi et al., 1990). Charged polysaccharides, such as sodium alginate, often form hydrogels in the presence of cations (Mikkelsen and Elgsaeter, 1995); the gel characteristics depend on the speci®c cation. In suf- 1545 ®ciently concentrated solutions of sodium alginate, divalent cations form junction zones by cross-linking functional groups of polymer chains to form a gel matrix, illustrated schematically in Fig. 1(b). In this study, Ca gel beads were further treated with Fe(III) to optimize both sorptive capacity for arsenate and the physical properties of the gel beads [Fig. 1(c)]. Arsenic removal was studied both with As(V), which occurs, within the pH range studied, principally as H2AsOÿ 4 , and with As(III), which occurs as the neutral species H3AsO4. MATERIALS AND METHODS Reagents and stock solutions Low viscosity grade sodium alginate (250 cps at 258C and 2% w/v) was obtained from Sigma and used without further puri®cation. Other chemicals were reagent grade and used as received. All solutions were prepared with water puri®ed by reverse osmosis and deionized using a Millipore System (Milli Q following Milli RO), referred to here as Milli Q water. The arsenate stock solution (1.000 g As/l) was prepared from sodium salt heptahydrate (Na2HAsO4 7H2O, Sigma) dissolved in Milli Q water. Secondary standard solutions for calibration samples (1.000 mg As/l) and spiking were freshly prepared for each experiment from the 1.000 g As/l stock solutions by dilution with Milli Q water. The arsenite stock solution was prepared from solid arsenic trioxide, As2O3 (Aldrich, A.C.S. primary standard) dissolved in 0.18 M (1.5% v/v) trace metal grade HCl (Fisher). The sulfate (0.010 M) and phosphate (0.001 M) stock solutions were prepared from Na2SO4 H2O and NaH2PO4 H2O, respectively, dissolved in Milli Q water. Calcium, copper, and iron solutions (0.100 M) used in gel synthesis were prepared from CaCl2 2H2O, CuCl2 2H2O, and FeCl3 6H2O, respectively, by dissolution of the salts in Milli Q water. Gel bead synthesis and washing The biopolymer powder (2.000 g) was dissolved in 100 ml of Milli Q water to obtain 2% w/v alginate solution. The solution was mixed on a wrist action shaker Fig. 1. Schematics of Ca±Fe gel bead formation with sodium alginate. (a) Sodium alginate chain showing conformation of M and G units. (b) Ca bead is produced by forming junctions with Ca (adapted from Jang et al., 1991). (c) Fe3+ displaces loosely bound Ca to produce Ca±Fe beads. 1546 Joon H. Min and Janet G. Hering (Burrell, Model 75) overnight until a yellowish brown, viscous solution was obtained. A peristaltic pump was used to dispense the polymer solution. At the end of the dispensing tube, a pipette tip (Bioplas, Fisher; ID 0.30 mm) was attached and positioned approximately 7 cm above the surface of the cation solution used for gel formation. The peristaltic pump was then programmed to dispense a drop of polymer solution approximately every 3 s into 1 l of 0.1 M cation solution (Ca, Cu or Fe chloride solution) in a beaker. A paddle stirrer unit (Phipps and Bird, Model 7790-400) was used to provide gentle stirring (60 rpm) when forming the gel beads, since the beads were reported to be susceptible to hydrodynamic forces (Ogbonna et al., 1991). The acid resistant polypropylene paddles (ColeParmer) were used as paddle assemblies for all mixing and bead washing. The beads were allowed to cure in cation solutions for 3 days and then removed from the cation solution and rinsed twice (either with Milli Q water for Ca and Cu beads or with 1 mM HCl for Fe and Ca±Fe beads). The beads were then transferred to a continuous washing system (015 ml/min) with in-line acid neutralizing unit. For preparation of Ca±Fe beads, washed Ca beads were transferred to 0.100 M FeCl3 solution for further doping of the beads with Fe(III). The beads were continuously washed for 3 days and stored in 1 mM HCl until used (up to 6 months) for As(V) sorption studies. To con®rm that the bead synthesis was reproducible, the size distribution of the beads was analyzed. Digitized photographs of the beads were used in combination with image analysis software (Optimetric, Optimas), which automatically counts and measures the diameter of each bead and calculates size distribution. Conditions and apparatus for batch sorption experiments For the batch experiments, 85 ml polycarbonate centrifuge tubes were used as reactors with 50 ml of solution as reaction volume. All of the batch experiments were done at pH 4 using acetate buer (0.5 mM) and background electrolyte of 5 mM NaNO3 unless otherwise speci®ed. The pH of the solution was adjusted with either HNO3 (0.1 N) or NaOH (0.1 N) solution, and the pH was measured with an Orion 720A pH meter. For each replicate in the batch experiments, 5 beads were equilibrated (unless otherwise speci®ed) in acetate buer solution at speci®ed pH for a day and transferred to the reaction bottles after the surface moisture of the beads was wiped o. Then, As(V) spikes (to ®nal concentrations of 1.00 mg/l to 10.0 mg/l) were added. The bottles were placed in a rack, which was mounted on a wrist action shaker for mixing. After the speci®ed reaction times, the beads were removed from the bottles to stop further reaction of arsenate with the beads, and the supernatants and/or gel beads were analyzed for As(V). All experiments were run in duplicate. Analytical measurements The total Fe in the beads was measured by disrupting the gel phase by sonication in an EDTA solution (Murata et al., 1993). The beads were transferred to a 50 ml volumetric ¯ask, 3 ml of 0.1 M EDTA was added and the ¯ask and contents were sonicated (Branson B-220, Smithkline) for 30 min. The ¯ask was then ®lled to volume with Milli Q water and ®ltered using 0.1 mm cellulose nitrate membrane ®lter (Sartorius). Fe analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Perkin-Elmer/Sciex Elan 5000A) was done by measuring the 57 Fe isotope. The aqueous samples were introduced at a rate of 1 ml/min from a Gilson 212B autosampler and a peristaltic pump to a cross-¯ow pneumatic nebulizer in a room temperature spray chamber. High purity Ni skimmer and sampling cones were used. Calibration standards were run in the same matrix (by disintegrating Ca beads with EDTA) to verify that matrix eects did not interfere with ICP-MS analysis. Both the initial As(V) and the dissolved As(V) concentration in the solution for each batch experiment were measured with ICP-MS for low initial As(V) concentrations (i.e., 1.00 to 400 mg/l) at 75 As without ®ltration. For initial As(V) concentrations higher than 400 mg/l, As(V) in the gel phase was measured by the method used for total Fe analysis in the beads with appropriate calibration standards. All samples were measured with ICPMS in 1% HNO3 solution. RESULTS AND DISCUSSION Eects of cation on As(V) sorption Initially, four gel bead types, Ca, Cu, Fe, and mixed Ca±Fe beads, were produced and tested for As(V) sorption. Among the beads tested, Ca±Fe beads showed highest As(V) removal at 24 h, and Fe and Ca±Fe beads showed comparable As(V) removal at 2 h (Fig. 2). Neither Cu beads nor Ca beads showed any signi®cant As(V) sorption even after 24 h. Even at an As(V) concentration of 5 mg/ l, the maximum uptake of As(V) by Ca beads was 0.013(20.001) mg per g of wet Ca bead compared with 1.57(20.03) mg per g of wet Ca±Fe beads. This absence of As(V) uptake by Ca beads demonstrates that As(V) accumulation in the pore volume of Fe and Ca±Fe beads by passive diusion is negligible compared with speci®cally bound As(V). This is consistent with the negligibly small water volume in the beads as compared with the total solution volume in the batch experiments. With Cu beads, the poor As(V) removal may be attributable to the formation of an impervious layer at the surface of the Cu beads which inhibits diusion of solutes into the beads as suggested by previous studies (Martinsen et al., 1989; Velings and Mestdagh, 1995). Although Fe beads were easier to produce than Ca±Fe beads, the physical integrity of the Fe beads was poor with extreme cracking and peeling at the surface. The mixed Ca±Fe beads, which were initially formed with Ca and then doped with Fe, showed good mechanical stability. In the Ca±Fe beads, Ca may act as primary gel forming cation providing a stable structure for the polymer network. The Fe then partly replaces the Ca in the gel matrix to provide favorable sorption sites for As(V). The average measured Fe content of a single Ca±Fe bead was 25.0 2 0.5 mg (0.452 0.01 mmol). All the subsequent experiments were performed with the Ca±Fe beads. As(V) sorption kinetics and eects of initial As(V) concentration Partitioning of As(V) between the solution and gel phase at pH 4 is shown as a function of time in Fig. 3. Excellent mass balance was obtained throughout the experiment by summing the As(V) sorption by Fe(III) alginate gel 1547 Fig. 2. 2 h (Q) and 24 h (q) As(V) sorption (normalized by wet bead weight) by four gel bead types produced with the same setup (2% sodium alginate solution, same size tip, gel formation in 0.1 M chloride salts) and background reaction conditions (0.5 mM acetic acid, 5 mM NaNO3, pH 4, 50 ml reaction volume) with 5 beads and 4 mg/l initial As(V) concentration. measured dissolved and sorbed As(V) concentration with an initial As(V) concentration of 400 mg/l (or total mass of 20 mg As(V)). The time scale for As(V) sorption was observed to be in the order of 100 h. With an initial As(V) concentration of 400 mg/l, increasing As(V) sorption was observed up to 93 h. At a 10-fold higher initial As(V) concentration, slow increase in the mass of As(V) sorbed was observed from 48 to 124 h (Fig. 4). Most As(V) uptake, however, occurred within 48 h; the mass of As(V) sorbed at 48 h is approximately 92% of that sorbed at 124 h. The eect of initial As(V) concentration on the As(V):Fe molar ratio attained was thus examined with a 120 h equilibration time. The ratio increased rapidly to 0.18:1 over a range in the initial As(V) concentration from 0 to 2 mg/l; a slow increase in the ratio was observed above an initial As(V) concentration of 4 mg/l until the plateau in As(V):Fe molar ratio of 0.26:1 was reached (Fig. 5). The Ca± Fe beads were nearly, if not fully, saturated with As(V) at this As(V):Fe molar ratio. In order to saturate the beads with arsenate and determine the maximum sorption capacity, it was Fig. 3. As(V) sorption kinetics: % As(V) in solution phase (w), % As(V) in gel phase (q), and % total As(V) in system (W); background reaction conditions as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per bead and 400 mg/l initial As(V) concentration (total mass As(V) 20 mg). 1548 Joon H. Min and Janet G. Hering Fig. 4. As(V) sorption kinetics for initial As(V) concentrations of 400 mg/l (r) (total mass As(V) 20 mg) and 4 mg/l (w) (total mass As(V) 200 mg); background reaction conditions as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per bead. necessary to employ conditions which may not be practical in treatment systems. The slow sorption observed with the current experimental conditions can be mitigated by increasing the number of beads or by decreasing the size of the beads, which can be accomplished by modifying the dispensing setup (Gilson and Thomas, 1995), to obtain a speci®c removal eciency on a given time scale. As(V) sorption model Sorption of As(V) by Ca±Fe beads over a wide range in initial As(V) concentrations could be modeled by assuming equilibrium partitioning of As(V) between the aqueous and gel phases, a uniform anity of As(V) for the Ca±Fe gel, and a maximum capacity of the Ca±Fe gel phase for As sorption as expressed by equation 1 fAsgsorb KAsdiss fAlgFegT 1 KAsdiss 1 where [As]diss is the equilibrium (i.e., ®nal) dissolved concentration of As(V) in mg/l, {As}sorb is the sorbed concentration in mg As/g Fe, {AlgFe}T is saturation (maximum) sorption capacity in mg As/g Fe, and K is the conditional anity constant in l/ mg. Values of K and {AlgFe}T were obtained by ®t- Fig. 5. Eect of initial As(V) concentration on As(V) sorption density; background reaction conditions as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per bead and 120 h equilibration time. As(V) sorption by Fe(III) alginate gel 1549 ting values of [As]diss and {As}sorb from Fig. 6(a) to equation 1 using a non-linear least square regression in the SigmaPlot (Jandel) software. Good agreement between the model and data was observed over the range in initial As(V) concentrations used in this study as shown in Fig. 6(a). At initial As(V) concentrations below 200 mg/l, however, the model consistently underpredicted the observed As(V) sorption at 120 h equilibration time [Fig. 6(b)]. The isotherm constants {AlgFe}T and K were found to be 352 mg As/g Fe and 1.68 l/mg, respectively, and the saturation As(V):Fe molar ratio predicted from the constant was 0.26 to 1 in agreement with the observed maximum value (Fig. 5). Eect of total Fe content on As(V) sorption Fig. 6. Model ®t (ÐÐÐ) and data for As(V) sorption by Ca±Fe beads; background reaction conditions as in Fig. 2 with 5 Ca±Fe beads of 25 mg Fe per bead and 120 h equilibration time. (a) 0±10 mg/l dissolved As(V) concentration. (b) Expanded view for 0±0.8 mg/l dissolved As(V) concentration. The total Fe content in the batch system was varied by using dierent number of beads, with an average amount of Fe in a single Ca±Fe bead of 25.0 2 0.5 mg (0.45 2 0.01 mmol). With an initial As(V) concentration of 400 mg/l, the percent As(V) removed in 120 h at pH 4 reached plateau at 94% with 20 beads (Fig. 7). The amount of As(V) removed by varying the total iron concentrations in the reactor volume, [Fe]T (g/l), can be easily predicted under conditions where the Ca±Fe beads are undersaturated with respect to As(V) sorption. Then, the concentration of free Fe sites in the gel phase, {AlgFe}, is fAlgFegT 1fAlgFeg Fig. 7. Eect of total Fe (number of Ca±Fe beads of 25 mg Fe per bead) on % As(V) removed (W) and mole of As(V) sorbed per mole of Fe (w); background reaction conditions as in Fig. 2 with 400 mg/l initial As(V) concentration and 120 h equilibration time. Lines show model ®ts for (ÐÐÐ) undersaturated case (equation 7) and ( ) not undersaturated case. 2 1550 Joon H. Min and Janet G. Hering The concentration of As(V) sorbed in the reactor volume, [As]sorb in mg/l, imental conditions, however, is not entirely negligible compared to the model saturation value (0.26:1). Therefore the result of model prediction accounting for sorbed As in the mass balance for alginate binding sites, that is Assorb fAsgsorb FeT KAsdiss fAlgFegT FeT AlgFeT 1AlgFe Assorb and fAsgsorb KAsdiss fAlgFeg1KAsdiss fAlgFegT 3 4 is used in the mass balance for the total As concentrations AsT Asdiss Assorb 5 Assorb 100 Assorb Asdiss KfAlgFegT FeT 100 KfAlgFegT FeT 1 With the values of K = 1.68 l/mg and {AlgFe}T=352 mg/g, the % As removed can be predicted by % Asremoved 592FeT 100 592FeT 1 is also shown in Fig. 7. The second prediction (not undersaturated case) shows a better ®t than the ®rst prediction (undersaturated case) at lower total Fe region where the As(V) sorbed is not negligible. Eects of pH on As(V) removal to obtain an expression for percent As removed. % Asremoved 8 7 Under these conditions, the Ca±Fe beads are undersaturated with respect to As(V) sorption with a As(V):Fe molar ratio below 0.1:1 compared with 0.26:1 for a saturated case. The model (equation 7) predicted arsenic removal to within 10% of the measured values for the entire range of total Fe studied. The model prediction for % As removed is shown with the data in Fig. 7. The As(V):Fe molar ratio of approximately 0.1:1 achieved at low total Fe contents under these exper- The pH was a predominant factor aecting the removal of As(V) under the conditions employed (Fig. 8). The Ca±Fe beads used were pre-equilibrated in various pH solutions before As(V) batch experiments to isolate the swelling or shrinking eects from the sorption process. With an initial As(V) concentration of 400 mg/l, a minimum in the residual dissolved As(V) concentration after a 120 h exposure to 5 Ca±Fe beads was observed at pH 3. A sharp increase in the residual, dissolved As(V) concentration was observed with a decrease in pH from 3 to 2 and a more gradual increase with increasing pH from 3 to 6.4. The poor As(V) removal by the gel at pH 2 may be due to partly to formation of the neutral species H3AsO4 (with pKa112.2), but also appears to be associated with instability of the Ca±Fe gel under these conditions. At pH 2, more than 60% of the total Fe (125 mg) in the Ca±Fe gel matrix leached to solution during pre-equilibrium and sorption experiments. Competition between proton and metal binding by alginic acid at pH values below 3 (the Fig. 8. Eect of pH on As(V) sorption (W) and Fe leaching (w); concentration of leached Fe were determined for un®ltered samples and could indicate colloidal iron; reaction conditions: 0.5 mM acetic acid, 5 mM NaNO3, 50 ml reaction volume with 5 Ca±Fe beads of 25 mg Fe per bead, 400 mg/l initial As(V) concentration, and 120 h equilibration time. As(V) sorption by Fe(III) alginate gel intrinsic pKa of alginic acid) has been observed (Jang et al., 1989, 1995). Leaching of Fe was less pronounced at pH 3 (approximately 15% leached). Leaching of Fe is not desirable since the sites for As(V) binding in the Ca±Fe beads would be decreased by this process. The iron released from the beads might also precipitate as an amorphous hydroxide solid; this colloidal phase iron might provide binding sites for As(V) competing with the alginate binding sites. The pH of 4 used in most of the experiments reported here provides favorable conditions both for Ca±Fe bead stability and As(V) sorption. Eects of competing ions and sorption of As(III) Co-occurring inorganic anions such as sulfate and phosphate may directly compete with As(V) for binding sites in the gel matrix and aect the extent of As(V) sorption. However, the presence of sulfate (0.52 mM) and phosphate (0.16 mM) (singly or in combination) did not aect As(V) sorption by Ca± Fe beads at a low initial As(V) concentration of 4 mg/l (0.053 mM); under these conditions, the available binding sites are in excess. The amount of As(V) sorbed in 24 h at pH 4 was 0.125 2 0.001 mg per 125 mg Fe (5 beads), as compared with 0.128 2 0.004 mg with sulfate, 0.122 2 0.001 mg with phosphate, and 0.123 2 0.006 mg with both. Under conditions where the binding sites approach saturation (i.e., at high initial concentrations of both arsenate and phosphate), the competitive eects were observed (Min, in preparation). In contrast to the signi®cant sorption of negatively-charged As(V) species by Ca±Fe beads, uncharged As(III) species were poorly removed under comparable conditions (Table 1). At initial As concentration in the mg/l range, the sorption densities observed with As(III) were <3% of those observed with As(V). At a lower initial As concentration (4 mg/l), low sorption densities were observed for both As(III) and As(V) with the sorption density for As(III) <25% that of As(V) (Table 1). The weaker interaction of As(III) as compared with As(V) with Ca±Fe beads is consistent with observed trends in As(III) and As(V) adsorption onto hydrous ferric oxide (Pierce and Moore, 1982; Wilkie and Hering, 1996). This comparison supports the hypothesis that As(V) sorption by the Ca±Fe beads is due to direct interaction between As(V) and Fe(III). 1551 CONCLUSIONS Bench scale tests were conducted to investigate the fundamental aspects of As(V) removal from contaminated water by use of biopolymeric materials, which are selective for oxyanions, economical compared to commercial anion-exchange resins, and environmentally benign. The experimental conditions studied here were chosen to elucidate the fundamental aspects of As(V) interaction with the metal-doped biosorbent and do not necessarily re¯ect optimal conditions for practical applications. Mixed Ca±Fe alginate beads were initially formed with Ca and then further treated with Fe(III) to optimize both sorptive capacity for As(V) and the physical properties of the gel beads. The As(V) sorption by Fe(III)-doped Ca alginate gel beads was found to be eective in removing As(V) over a wide concentration range (from mg to mg per liter). Up to 94% removal of As(V) from solution was achieved for an initial As(V) concentration of 400 mg/l by equilibration with 20 Ca±Fe beads at pH 4 for 120 h. Under these conditions, the As(V) sorption density in the Ca±Fe beads was approximately 10% of the saturation value. Slow sorption kinetics were observed with a time scale for sorption of approximately 100 h. The possible role of diusion within the gel matrix in governing sorption kinetics is under investigation. The eects of pH on As(V) sorption were studied over the pH range from 2 to 6.4. Poor As(V) sorption was observed at pH 2 with concomitant leaching of a signi®cant amount of Fe (>60%) leached from the gel matrix. Leaching of Fe decreased with increasing pH; an optimum combination of Ca±Fe beads stability and As(V) sorption was achieved at pH 4. In contrast with the ecient sorption of As(V) at pH 4, removal of As(III) from solution by Ca±Fe beads was negligible. The use of macro gel beads allowed simple modi®cation of total Fe in the system by changing the number of the beads and easy separation of spent beads from the treated solution. In practice, some of the limitations associated with slow sorption kinetics may be overcome by increasing the amount of biosorbent used. The sorption of other environmentally-signi®cant oxyanions (such as chromate, selenate and selenite) is also being examined and will be compared with that of As(V). AcknowledgementsÐThis work was supported by funding from the National Science Foundation (BES-9258431 and BES-9553208), the Fluor Foundation, and the Chevron Research and Technology Company. Table 1. Eects of oxidation state on As sorption mg As sorbed/g Fe Initial As concentration 4 mg/l 1 mg/l 4 mg/l As(III) 0.031 2 0.002 3.4 2 0.2 7.5 2 0.1 As(V) 0.132 20.001 121.8 21.3 261.4 22.4 0.5 mM acetic acid, 5 mM NaNO3, pH 4, 50 ml reaction volume, 5 beads, 24 h. REFERENCES Brewster M. D. and Passmore R. J. 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