Arsenic Adsorption and Oxidation at Manganite Surfaces. 1. Method

Apr 9, 2000 - of As(III) to As(V) was more rapid at pH 4 than at pH 6.3 and was unaffected by the presence of boric acid at 95 μM or 3 mM. .... React...
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Environ. Sci. Technol. 2000, 34, 2029-2034

Arsenic Adsorption and Oxidation at Manganite Surfaces. 1. Method for Simultaneous Determination of Adsorbed and Dissolved Arsenic Species V A N Q . C H I U †,‡ A N D J A N E T G . H E R I N G * ,§ Civil and Environmental Engineering Department, University of California, Los Angeles, Los Angeles, California 90095-1593, and Environmental Engineering Science (138-78), California Institute of Technology, Pasadena, California 91125

Arsenic occurs in the +III oxidation state as a metastable species in oxic waters. Under oxic conditions, As(III) is both more mobile in natural waters and less efficiently removed by water treatment processes than As(V). Other oxidants, however, can react with As(III) more rapidly than oxygen. The oxidation of As(III) by manganite occurs on the time scale of hours. Here, a method is introduced for the rapid determination of the total and dissolved concentrations of arsenic species in this heterogeneous system; adsorbed arsenic concentrations are calculated by difference. The oxidation reaction is quenched by the addition of ascorbic acid to effect the reductive dissolution of manganite and concomitant release of adsorbed As(III) and As(V) into solution. Once in solution, As(III) and As(V) are separated using anionexchange chromatography. Comparison of dissolved and total concentrations of As(III) and As(V) clearly illustrates that the overall conversion rate of As(III) to As(V) in this system would be overpredicted based solely on dissolved As(III) concentrations and underpredicted based solely on dissolved As(V) concentrations. The overall conversion of As(III) to As(V) was more rapid at pH 4 than at pH 6.3 and was unaffected by the presence of boric acid at 95 µM or 3 mM. However, the presence of 200 µM phosphate (at pH 4) decreased the overall rate of conversion of As(III) to As(V). Comparison of total and dissolved As(III) concentrations during the reaction time course demonstrates that the effects of pH and phosphate on adsorbed As(III) concentrations are generally consistent with these kinetic observations.

Introduction The mobility of arsenic in natural systems and the efficiency of its removal in treatment processes are strongly influenced by the oxidation state in which it occurs. Sorption of As(V) to mineral components of soils (1-3) and sediments (4-6) or in treatment processes to packed bed filter media (7-9) or coagulant flocs (10-12) is a dominant mechanism for * Corresponding author telephone: (626)395-3644; fax: (626)3952940; e-mail: [email protected]. † University of California, Los Angeles. ‡ Present address: Marine Biology Research Division, Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093. § California Institute of Technology. 10.1021/es990788p CCC: $19.00 Published on Web 04/09/2000

 2000 American Chemical Society

immobilization of arsenic in oxic systems. However, As(III) can also occur as a metastable species in oxic waters as a result of biologically mediated reduction of As(V) to As(III) (13-15) and the slow kinetics of the direct reaction of As(III) with dioxygen (16). Since As(III) is less strongly sorbed than As(V) to a variety of sorbents, arsenic is generally more mobile in the +III oxidation state than in the +V oxidation state, and immobilization of arsenic is enhanced by the oxidation of As(III) to As(V) (17, 18). The mobility and treatability of arsenic have become pressing concerns with the anticipated revision of the drinking water standard (currently 50 µg/L or 0.7 µM) to a value in the range of 20-2 µg/L; arsenic is a known human carcinogen (19). Oxidation of As(III) to As(V) by freshwater sediments has been attributed to the reaction of As(III) with manganese oxides; oxidation rates were comparable under N2 and air and were unaffected by the addition of HgCl2 as an inhibitor of biological activity (20). Treatment of lake sediments with hydroxylamine hydrochloride (to remove manganese oxides) decreased the extent of As(III) oxidation (21). Poorly crystalline birnessite (δ-MnO2) was observed to be an effective oxidant of As(III) (22). Further studies with birnessite, cryptomelane (R-MnO2), and pyrolusite (β-MnO2) indicated that differences in the rates of As(III) depletion (i.e., combined sorption and oxidation) could be attributed to the effects of the crystallinity and surface area of the various oxides (23). Later studies with birnessite demonstrated biphasic kinetics that were attributed to facile reaction of interlayer Mn followed by slower reaction with Mn in the octahedral sheets of the mineral (24). Oxidation rates [as determined by depletion of As(III) or release of As(V) to solution] exhibited little pH dependence (24, 25), but the rate of release of Mn(II) into solution decreased with increasing pH (25). The kinetics and mechanisms of the heterogeneous oxidation of As(III) by the manganese(III) oxide manganite (γ-MnOOH) have not been previously studied. Interpretation of such results can be complicated by the need to infer the progress of reactions occurring at surfaces from changes in dissolved concentrations of reactants and products. Direct determination of the concentrations of adsorbed arsenic species would substantially constrain these interpretations and thus provide significant insight into the kinetics and mechanisms of heterogeneous oxidation processes. The oxidation state of arsenic associated with kaolin, anatase (26), and goethite (27) surfaces has been quantitatively determined using X-ray absorption near-edge structure (XANES) spectroscopy. Such techniques offer substantial promise for future applications in kinetic studies. X-ray photoelectron spectroscopy (XPS) has been used to infer As(III) oxidation by following the reduction of manganese(III,IV) oxides (28). The concentrations of adsorbed species can be quantified by measuring the increase in the concentration of that species in solution after displacement with a competing adsorbate or exhaustive digestion (i.e., dissolution) of the solid. Excess phosphate has been used to desorb As(III) and As(V) from sediments, fly ash, and soils and to assess the extent of As(III) oxidation at the surface of clay minerals, but the displacement is slow (ca. 20 h) and often incomplete (3, 29, 30). Digestion with hot 4 M HCl has been used to release As(III) and As(V) from lacustrine sediments (31) and to follow the oxidation of As(III) to As(V) by birnessite (24), but the method is not applicable for studies of oxidation kinetics on the time scale of minutes to hours. VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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In this paper, we report on a simple technique that accomplishes rapid and complete dissolution of manganite and concomitant release of adsorbed arsenic species, As(III) and As(V), into solution. This technique allows for the simultaneous determination of total and dissolved As(III) and As(V) concentrations; adsorbed arsenic concentrations are calculated by difference. The technique has been applied to study the oxidation of As(III) by manganite.

Materials and Methods Standards and Reagents. Solutions were prepared with water from a Millipore Milli-Q 18 MΩ system. All glassware was cleaned by rinsing with hydroxylamine hydrochloride, soaking in 10% HCl, and rinsing with deionized water and then with Milli-Q water. As(III) and As(V) stock solutions were prepared as outlined by Wilkie and Hering (32). Nitric acid HNO3 (instrumental grade, EM Science), L-ascorbic acid C6H8O6 (Ultra, Sigma), H2O2 (ACS, EM Science), NH4OH (ACS, J. T. Baker), NaNO3 and MnSO4‚H2O (ACS, Mallinckrodt), hydroxylamine hydrochloride (ACS, Sigma), CH3COONa‚ 3H2O (ACS, Fisher), and Mn and As standards (ICP, VWR) were used without purification. Manganite (γ-MnOOH) Preparation. Manganite was prepared according to a modification of the method of Giovanoli and Leuenberger (33) as outlined by McArdell et al. (34) and Xyla et al. (35). Milli-Q water (for preparation of MnSO4 and NH3 solutions) and the prepared 0.06 M MnSO4 were continuously purged with nitrogen gas (N2) to remove dissolved oxygen. Purging was discontinued after 20.4 mL of 30% H2O2 was added to the 1 L of 0.06 M MnSO4 solution. Next, 300 mL of 0.2 M NH3 was added to the H2O2/MnSO4 solution while the solution was vigorously stirred. The resulting brown suspension was quickly heated to 95 °C and maintained at this temperature for 6 h. While still hot, the dark brown suspension was centrifuged and repeatedly washed with 1000 mL (total) of hot (ca. 80 °C) Milli-Q water. The solid suspension was then dried in a lyopholyzer, crushed, and stored in a freezer. Analytical Methods. pH was monitored with a Fisher model 25 pH meter calibrated with two or three buffered (pH ) 4, 7, and 10) solutions. A modified anion-exchange method (36) as described by Wilkie and Hering (32) was used for separation of As(III) and As(V). The anion-exchange resin (Dowex 1X8-100, Sigma) was obtained in the chloride form and converted to acetate form prior to being packed in PolyPrep 0.8 × 4 cm columns (Bio-Rad). For separation of As(III) and As(V), samples were acidified to pH ≈ 3.5 so that As(III) is fully protonated (H3AsO3, pKa1 ) 9.2) and passes through the column and that As(V) is partly deprotonated (H3AsO4, pKa1 ) 2.2, pKa2 ) 7.0) and is retained on the column; validation of this method is described elsewhere (37, 38). Total As was determined from the original sample, As(III) was determined from the column effluent, and As(V) was determined by difference. Collected samples for ICP-MS analysis were acidified to 1% (v/v) HNO3 by addition of concentrated instrumental grade acid; excess EDTA was added to keep Mn(II) in solution. Arsenic and manganese analyses were performed on a Perkin-Elmer Elan 5000 inductively coupled plasma-mass spectrometer. Duplicate analyses of As(III) in anion-exchange column effluents agreed within 9%. Manganite was characterized by X-ray powder diffraction; the observed diffraction pattern matched that reported for manganite in the Joint Commission on Powder Diffraction Standards (JCPDS 41-1378) database. The concentration of exchangeable sites was determined by fluoride adsorption measurement (39) at pH 4 to be 2.4 × 10-4 mol/g. Fluoride was measured with a fluoride ion selective combination electrode (Orion, model 9609BN). The average oxidation state of manganese in the solid was determined by oxalate 2030

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FIGURE 1. Sampling scheme for total and dissolved arsenic species. See text for details. titration (40) to be approximately 3.0. Previous XPS studies have shown manganite to be an mangenese(III) oxide (41). The nominal molecular weight (100 ( 3 g/mol) was determined from the total moles of Mn in a known weight of manganite used in the oxalate titration. The reported surface area (determined by BET measurements) for manganite produced by the same synthesis method is 30.5 m2/g (35). Release of Adsorbed Arsenic by Rapid Dissolution of Manganite. Ascorbic acid was used to reductively dissolve manganite and to liberate the adsorbed arsenic. Four milliliters of arsenic-manganite suspension (initial pH 4 or 6.3) was pipetted into a 15-mL centrifuge tube containing 10 mL of 0.1 M ascorbic acid (final pH ca. 2.9). The centrifuge tube was then capped and shaken vigorously for approximately 30 s or until the solution appeared to be completely clear. The pH was then readjusted to ca. pH 3.5 and set aside for 5 min-1 h for separation of arsenic species by anion exchange. Kinetic Experiments. Manganite suspensions were allowed to equilibrate in batch reactors (500-mL borosilicate glass bottles, KIMAX) for 1 h at 22 °C in a background electrolyte consisting of 0.01 M NaNO3 and 5 mM CH3COONa at pH 4 or pH 6.3 before addition of As(III). The desired pH was achieved by addition of 1.0 N HNO3 or NaOH. Acetate was chosen as a pH buffer since it has a weak tendency to form surface complexes with goethite (R-FeOOH) (42), which is a reasonable analogue for manganite. For competitive adsorbate experiments, borate or phosphate was also added to the particle suspensions 1 h before initiation of the kinetics experiments. Reactions were initiated (t ) 0) by the addition of an aliquot of 20 mM As(III) stock solution [to the desired final As(III) concentration]. The pH was monitored several times during the experiments; the observed drift was e0.1 pH unit due to buffering by acetate and manganite. During the kinetics experiments, subsamples were withdrawn over time and processed as shown in Figure 1. For dissolved As(III) and As(V), ca. 20-mL aliquots were removed with a plastic syringe and filtered through 0.2-µm cellulose acetate filter (Gelman Science). The first ca. 5 mL was wasted, and the remaining 15 mL was collected in a 15-mL polypropylene centrifuge tube (VWR). For total As(III) and As(V), a 4-mL aliquot of the suspension was pipetted into 10 mL of 0.1 M ascorbic acid and shaken vigorously for ca. 30 s. Both the resulting solution (for total As) and the filtrate (for dissolved

As) were processed within 1 h for separation of As(III) and As(V). As shown in Figure 1, concentrations of total As ([As]total), dissolved As ([As]diss), total As(III) ([As(III)]total), and dissolved As(III) ([As(III)]diss) were measured directly. However, quenching of the reactions (i.e., by filtration or dissolution) could not be performed at exactly the same time. Thus, the times corresponding to the measured concentrations of total and dissolved arsenic species were offset by a few minutes. To compensate for the time offset, concentrations of total As(III) and As(V) at times corresponding to the times of measured dissolved arsenic concentrations were obtained by fitting the [As(III)]total as a function of time to an equation of a sum of exponentials [e.g., [As(III)]total ) ae-bt + ce-dt where a-d are fitting parameters] and by fitting the [As(V)]total as a function of time to a second (or third) order logarithmic equation [e.g., [As(V)]total ) [As(V)]o + a ln(t) + b (ln(t))2 where a and b are fitting parameters]. The adsorbed and remaining arsenic species were calculated by difference as follows:

[As(III)]ads ) [As(III)]total - [As(III)]diss [As(V)]total ) [As]total - [As(III)]total [As(V)]diss ) [As]diss - [As(III)]diss [As(V)]ads ) [As(V)]total - [As(V)]diss ) [As]total [As(III)]total - [As]diss + [As(III)]diss

Results and Discussion Arsenic Oxidation by Manganite. Experiments were conducted to demonstrate that dissolved and adsorbed arsenic species could be quantified (either directly or by difference) during the heterogeneous oxidation of As(III) by manganite. The reaction is expected to obey the overall stoichiometry:

2γ-MnOOH(s) + H3AsO3 + 3H+ f 2Mn2+ + H2AsO4- + 3H2O (∆G° ) -166 kJ/mol) To follow changes in the concentrations of reactants and products occurring on a time scale of minutes to hours, the reaction was effectively quenched by removal of the oxidant (manganite) either by filtration or by reductive dissolution. In this paper, the effectiveness of this method is demonstrated, and the difficulties in establishing accurate As(III) oxidation rates based only on the measurement of dissolved species are illustrated. By using the method developed here, effects of pH and competing adsorbates on the sorption and oxidation of As(III) can be distinguished. A companion paper will develop a mechanistic interpretation of the results reported here (43). Validation of Conditions for Determination of Adsorbed Arsenic. The conditions for the rapid reductive dissolution of manganite (excess ascorbic acid at pH ≈ 2.9) were chosen based on available kinetic data for the reduction of manganese oxides by various organic reductants (44). Reduction of manganite (with oxidation of ascorbic acid, C6H8O6, to dehydroascorbic acid, C6H6O6) is thermodynamically favorable under these conditions:

2γ-MnOOH(s) + C6H8O6 + 4H+ f 2Mn2+ + C6H6O6 + 4H2O (∆G° ) -212 kJ/mol) with ∆G ≈ -186 kJ/mol for typical reaction conditions of 0.07 M ascorbic acid, pH 2.9, and 1.6 mM Mn(II). Reduction of As(V) is also energetically favorable:

FIGURE 2. Concentrations of Astotal (9), As(III)total (b), As(V)total (1), Asdiss (0), As(III)diss (O), and As(V)diss (3) as a function of time after addition of As(III) to a manganite (γ-MnOOH(s)) suspension. Conditions: [As]total ) 120 µM, [Mn]total ) 5.3 mM, pH ) 4.0, T ) 22 °C, [NaNO3] ) 0.01 M, and [CH3COONa] ) 5 mM.

H2AsO4- + C6H8O6 + H+ f H3AsO3 + C6H6O6 + H2O (∆G° ) -46 kJ/mol) At equilibrium (∆G ) 0), the predicted ratio of As(III):As(V) is >108:1 with [As]total ) 35 µM in the presence of 0.07 M ascorbic acid at pH 2.9. Thus, the method relies on the kinetic inertness of As(V) under these conditions. Preliminary testing demonstrated that no As(III) was formed during a 2-h period under these conditions (37). An additional consideration with the ascorbic acid reduction of manganite is the possibility that some colloidal manganite might remain even after the reaction appears complete (i.e., a colorless solution is obtained). Prior testing of the anion-exchange method for separation of (dissolved) As(III) and As(V) has indicated that colloidal arsenic species are not retained by the column; thus, colloidal As(V), if present, would pass through the column and be (incorrectly) identified as As(III) (45). However, when manganite was reacted with excess ascorbic acid for 1, 2, or 5 min (pH ) 2.9, [Mn]total ) 1.6 mM, and [H2C6H6O6] ) 0.07 M), less than 0.01 ( 0.005 µM manganese was retained on 0.025-µm filters (37). Thus, artifacts due to the presence of colloidal As(V) species would be minimal. Importance of Total Arsenic Species. In assessing the mobility and toxicity of arsenic, both the redox speciation of arsenic and its partitioning between the dissolved (mobile) and solid (immobile) phases must be considered. Previous studies of As(III) oxidation by manganese oxides have largely focused on the depletion of As(III) from solution and the release of As(V) to solution (20-23, 25). While this approach does address the redox speciation of mobile forms of arsenic, it relates only indirectly to the oxidation of As(III) occurring at the oxide surface. In cases where adsorption of either the reactant As(III) or the product As(V) is significant, measurement of only dissolved arsenic species will be insufficient for determination of oxidation rates. The importance of the distinction between dissolved and total arsenic species is illustrated in Figure 2. For a typical oxidation experiment at pH 4 with [As]total ) 120 µM and [Mn]total ) 5.3 mM, the concentration of dissolved As(III) decreases more rapidly than that of total As(III) while the concentration of dissolved As(V) increases more slowly than that of total As(V). Thus, the overall conversion rate of As(III) to As(V) in this system would be overpredicted based solely on the decrease in dissolved As(III) concentrations and underpredicted based solely on the increase in dissolved VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Concentrations of As(III)total (closed symbols) and As(III)diss (open symbols) as a function of time at pH 4 (b, O) and pH 6.3 (2, 4): (a) at t ) 0-65 min and (b) at t ) 0-400 min. Conditions: T ) 22 °C, [NaNO3] ) 0.01 M, [CH3COONa] ) 5 mM, and [As]total ) 25 µM with [Mn]total ) 3.9 mM at pH 4.0 and [Mn]total ) 4.2 mM at pH 6.3. As(V) concentrations. Note that the dissolved arsenic concentration decreases rapidly and reaches a constant value after approximately 10 min. Such behavior has been attributed, in previous studies, to rapid adsorption of As(III) followed by oxidation (23). However, Figure 2 clearly shows that As(V) is also formed on this shorter time scale but remains associated with the oxide surface. As shown in Figure 2, more than 92% conversion of As(III) to As(V) is achieved in this system within 3 h. However, neither the decrease in total As(III) or in dissolved As(III) exhibits first-order kinetic behavior. Such behavior would be expected if a steady state is maintained between adsorbed and dissolved As(III) as in the following simplified mechanism:

As(III)diss a As(III)ads f As(V)ads a As(V)diss where, for convenience, only the arsenic species are shown and the concentration and reactivity of the surface sites (both as sorbent and oxidant) are assumed to be constant over the course of the reaction. The data in Figure 2 suggest that a more sophisticated model is required to account for the retardation of the reaction observed at longer time scales; these reaction kinetics will be addressed in detail in a later paper (43). Effect of pH. Solution pH will affect both the charge of the manganite surface and the extent of adsorption of the reactant, As(III), and the products, Mn(II) and As(V), of the oxidation reaction. Electrostatic effects enhance cation adsorption and inhibit anion adsorption at pH values above the pHZPC ) 6.2 for the manganite solid (35). Thus (indirect) effects of pH on the rates of redox reactions occurring at the mineral surface reflect the pH dependence of adsorption (46). It is also possible that the rate of oxidation of the adsorbed As(III) may be directly influenced by the surface charge of the solid. The overall rate of conversion of As(III) (at an initial concentration of 25 µM) to As(V) by manganite and the adsorption of As(III) and As(V) at the manganite surface were examined at pH 4 (with [Mn]total ) 3.9 mM) and at pH 6.3 (with [Mn]total ) 4.2 mM). As shown in Figure 3, the total As(III) concentration decreased substantially more rapidly at pH 4 than at pH 6.3 despite the slightly higher total Mn concentration in the higher pH experiment. 2032

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FIGURE 4. Adsorbed As(III) concentrations (normalized to total Mn) as a function of dissolved As(III) concentrations and time at pH 4 (b) and pH 6.3 (2). Data correspond to those shown in Figure 3. To evaluate the effect of pH on adsorption of As(III), adsorbed As(III) concentrations (normalized to the Mn content of the solid) are plotted against both time and the dissolved As(III) concentrations determined during the time course of the reaction (Figure 4). Note that, because these data are derived from kinetic experiments, equilibrium cannot be assumed. As shown in Figure 4, more As(III) is adsorbed [for corresponding dissolved As(III) concentrations] at pH 4 than at pH 6.3. Equilibrium adsorption of As(III) onto both goethite (pHZPC ) 7.6) and hydrous ferric oxide (pHZPC ) 8.1) is only weakly pH dependent in the pH range of 4-7 and increases slightly over this pH range rather than decreasing as observed in Figure 4 (47-49). However, the kinetics of As(III) adsorption onto both of these solids has been shown to be pH dependent (47, 49). Thus, in the case of manganite, it may be that adsorbed As(III) concentrations are greater at pH 4 than at pH 6.3 at early reaction times because of pH-dependent As(III) adsorption kinetics. This pattern is consistent with the observation of faster As(III) oxidation at pH 4 than at pH 6.3. At longer reaction times, the persistence of adsorbed As(III) on the manganite surface (at pH 4) even as the dissolved As(III) is almost completely depleted suggests that As(III) desorption under these conditions is slow. Effect of Competing Adsorbates. The presence of a competing adsorbate can also influence both the extent of As(III) adsorption and the surface charge of the oxide surface. The latter effect will be significant only if a charged surface complex is formed on the manganite surface. Oxidation experiments were performed with As(III) (at an initial concentration of 29 µM) in the presence of boric acid [B(OH)3] at 95 µM or 3 mM or phosphate (H2PO4-) at 200 µM to examine these effects. Note that the concentration of manganite surface sites (ca. 94 µM based on fluoride adsorption) is in excess of the initial As(III) concentration but less than the added concentration of boric acid or phosphate. The addition of boric acid had no discernible effect on the overall rate of conversion of As(III) to As(V) by reaction with manganite at pH 4; 50% conversion of As(III) to As(V) was observed in 7, 7, or 8 min with 0, 95 µM, or 3 mM B(OH)3. In contrast, the addition of 200 µM phosphate at pH 4 caused a decrease in the overall rate of conversion of As(III) to As(V); 50% conversion required 22 min (Figure 5). Note that the data with boric acid in Figure 5 are essentially a control for the data with phosphate (the values of [As]total differ slightly in Figures 3 and 5).

Morgan for helpful discussions of this work and Mr. Brian R. King for assistance with sampling.

Literature Cited

FIGURE 5. As(III)total in the presence of H2PO4- (b) and B(OH)3 (2, 1) and As(III)diss in the presence of H2PO4- (O) and B(OH)3 (4, 3) as a function of time. Conditions: T ) 22 °C, [CH3COONa] ) 5 mM, [NaNO3] ) 0.01 M, [As]total ) 29 µM, and [Mn]total ) 3.9 mM at pH 4.0 with [B(OH)3] ) 95 µM (2, 4) or 3 mM (1, 3) or [H2PO4-] ) 200 µM.

FIGURE 6. Adsorbed As(III) concentrations (normalized to total Mn) as a function of dissolved As(III) concentrations and time at pH 4 in the presence of [B(OH)3] ) 3 mM (2) or [H2PO4-] ) 200 µM (b). Data correspond to those shown in Figure 5. The concentrations of adsorbed As(III) as a function of time and dissolved As(III) concentrations in the presence of boric acid and phosphate (Figure 6) are generally consistent with the kinetic observations. Adsorbed As(III) concentrations are decreased by the presence of phosphate, consistent with a competitive adsorption process. Again, the data with boric acid provide a control for the data with phosphate since the addition of boric acid had no effect on As(III) adsorption. Even in the presence of phosphate as a competing adsorbate, the manganese(III) oxide manganite, like the manganese(IV) oxides previously studied, is an effective oxidant for As(III). A novel method used to quantify total As(III) as well as dissolved As(III) concentrations during the reaction time course provides insight into the effects of solution composition on adsorbed As(III) concentrations and the correspondence between these effects and As(III) oxidation kinetics.

Acknowledgments This project was funded by the National Science Foundation (BES-9753074 and BES-98696136). We thank Prof. James J.

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Received for review July 14, 1999. Revised manuscript received February 15, 2000. Accepted February 21, 2000. ES990788P