Environ. Sci. Technol. 2001, 35, 2026-2032
Electrochemical and Spectroscopic Study of Arsenate Removal from Water Using Zero-Valent Iron Media J A M E S F A R R E L L , * ,† J I A N P I N G W A N G , † PEGGY O’DAY,‡ AND MARTHA CONKLIN§ Department of Chemical and Environmental Engineering and Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, and Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287
This study investigated the mechanisms involved in removing arsenate from drinking water supplies using zerovalent iron media. Batch experiments utilizing iron wires suspended in anaerobic arsenate solutions were performed to determine arsenate removal rates as a function of the arsenate solution concentration. Corrosion rates of the iron wires were determined as a function of elapsed time using Tafel analysis. The removal kinetics in the batch reactors were best described by a dual-rate model in which arsenate removal was pseudo-first-order at low concentrations and approached zero-order in the limit of high arsenate concentrations. The presence of arsenate decreased iron corrosion rates as compared to those in blank 3 mM CaSO4 background electrolyte solutions. However, constant corrosion rates were attained after approximately 10 days elapsed, indicating that the passivation processes had reached steady state. The cathodic Tafel slopes were the same in the arsenate and the blank electrolyte solutions. This indicates that water was the primary oxidant for iron corrosion and that arsenate did not directly oxidize the iron wires. The anodic Tafel slopes were greater in the arsenate solutions, indicating that arsenate formed complexes with iron corrosion products released at anodic sites on the iron surfaces. Ion chromatography analyses indicated that there was no measurable reduction of As(V) to As(III). X-ray absorption spectroscopy analyses indicated that all arsenic associated with the zerovalent iron surfaces was in the +5 oxidation state. Interatomic arsenic-iron distances determined from EXAFS analyses were consistent with bidentate corner-sharing among arsenate tetrahedra and iron octahedra. Results from this study show that under conditions applicable to drinking water treatment, arsenate removal by zero-valent iron media involves surface complexation only and does not involve reduction to metallic arsenic.
Introduction Drinking water supplies contaminated with arsenic compounds are a worldwide health concern. Arsenic ingestion * Corresponding author phone: (520)621-2465; fax: (520)621-6048; e-mail:
[email protected]. † Department of Chemical and Environmental Engineering, University of Arizona. ‡ Department of Geological Sciences, Arizona State University. § Department of Hydrology and Water Resources, University of Arizona. 2026
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may deleteriously affect the gastrointestinal tract, cardiovascular system, and central nervous system (1, 2). Additionally, arsenic is classified as a Group A carcinogen by the United States Environmental Protection Agency (U.S. EPA) and may also have teratogenic effects (1, 2). The U.S. EPA has recently proposed to decrease the maximum contaminant level (MCL) for arsenic in drinking water from 50 to 10 µg/L (3). Approximately 5% of community water systems, currently serving 13 million people, have levels above 10 µg/L and will require additional treatment steps to meet this new standard (4). Arsenic is released into groundwater by both natural processes, such as the weathering of arsenic containing minerals, and through anthropogenic activities, such as mining and application of organo-arsenical pesticides. Under the redox conditions of natural water systems, arsenic is commonly found in both oxidized and reduced oxidation states (5). Zero-valent metallic arsenic is thermodynamically stable in water, and reduced forms of arsenic are often found in mineral solids, as in FeAsS(s) and FeAs2(s) (6). Under the pH and redox conditions of most groundwaters and surface waters, dissolved arsenic exists as the As(V) (arsenate) species, H2AsO4- and HAsO42-, and the As(III) (arsenite) species, H3AsO30 and H2AsO3- (7). Although the predominant arsenic oxidation state at neutral pH and redox potentials above ∼100 mV (SHE) should be As(V), empirical observations indicate that ratios of As(III) to As(V) may differ significantly from those predicted by equilibrium calculations (8). There are several methods in current use for removing arsenic from drinking water supplies. These methods include ion exchange, reverse osmosis, lime softening, and chemical precipitation with iron and aluminum salts (9). While each of these methods can be effective, there are serious drawbacks to each process. Removal of arsenic via ion exchange is inefficient due to competition for exchange sites by background electrolytes that may be present at considerably greater concentrations than the arsenic (9). Although reverse osmosis is able to remove arsenic to levels below 10 µg/L, the process is expensive and often requires pretreatment of the water to remove compounds that may foul the membrane (9). The most commonly used method for removing both As(III) and As(V) compounds in water treatment plants involves chemical precipitation using iron and aluminum salts (9). Ferric salts are more widely used than aluminum salts since they are more effective for removing As(III) compounds (10, 11). Removal of arsenic occurs through adsorption and coprecipitation during the formation of iron(III) hydroxides (12-14). This technique is highly pH dependent but can obtain removal levels of 0.5 mol of As(V) per mol of Fe(III) and 0.1 mol of As(III) for each mol of Fe(III) (10). Chemical coprecipitation using ferric chloride has been shown to be effective for removing As(V) to levels below 2 µg/L in a number of different source waters for pH values less than 8 (11). However, these levels are achieved only when there is a filtration step to remove colloidal arsenic (11, 15). Removal of As(III) by ferric chloride precipitation is less efficient and is more affected by water composition (11). The major limitation of precipitation as a removal mechanism is that it is most suited for large-scale water treatment facilities that include a clarification step in their process train. The objective of this research was to investigate an arsenic removal method suitable for small-scale treatment systems or individual well-head units. Arsenate removal in packed beds of iron filings may be especially useful in developing nations, such as Bangladesh, where there is 10.1021/es0016710 CCC: $20.00
2001 American Chemical Society Published on Web 04/06/2001
widespread contamination of the groundwater supplies with high levels of arsenic compounds (16). Arsenate Reactions with Iron and Iron Oxides. In recent years, there has been considerable interest in electrochemical processes for removing redox active metals and metalloids from contaminated waters (17). Many metallic groundwater contaminants may be removed from solution via reduction to lower valence states that are less water soluble. In ironmediated metal remediation, the iron may serve as an electron donor to reductively precipitate reduced metal species (16). In many cases, the reduced species may be more than 3 orders of magnitude less soluble than the oxidized form of the metal (5). Reduction of arsenate compounds to As(0) by corroding iron has been observed to occur in acid solutions (5). This process forms a metallic arsenic cement that partially protects the iron from further corrosion. At neutral pH, reduction of arsenate to metallic arsenic by zero-valent iron can be described by the half-cell reactions (5):
Fe2+ + 2e- ) Fe0
E° ) -0.440 V
HAsO42- + 7H+ + 5e- ) As0 + 4H2O
(1)
E° ) 0.499 V (2)
with the net cell potential for the reaction given by
E ) 0.939 V - 0.0295 log[Fe2+] + 0.0118 log[HAsO42-] - 0.0826 pH (3) For solution concentrations and pH values of practical interest in water treatment, the cell potential for eq 3 is positive, indicating that reduction of arsenate to metallic arsenic by zero-valent iron is thermodynamically favorable. In addition to removal processes involving reduction, iron media may also remove As(V) compounds by adsorption onto iron corrosion products. Previous investigators have shown that arsenate forms both mono- and bidentate complexes with ferric oxides (18, 19). In some cases, adsorption of arsenate to iron oxides may be irreversible due to formation of stable solid phases. This phenomenon may occur during aging of arsenate adsorbed to hydrous ferric oxide, which may produce the stable mineral scorodite, FeAsO4‚2H2O (6). Several investigators have reported that solid phases more stable than scorodite are formed when the molar ratio of Fe(III) to As(V) in the solid are greater than 2 (20). For example, Krause and Ettel (21) reported that the solubility of arsenate at pH 5 decreased with increasing iron content for Fe:As ratios ranging from 1.5 to 16. In that study, arsenate removal was attributed to formation of compounds corresponding to the formula FeAsO4‚Fe(OH)3. However, more recent investigators have reported that after sufficient aging of arsenate/iron hydroxide precipitates, no solid solutions involving iron oxides and arsenate are formed (22, 23). Electrochemical Analysis. The effect of potential on the rate of electrochemical reactions involving a corroding iron electrode can be described by a form of the Butler-Volmer equation as (24)
i ) icorr[e-βc(E-Ecorr) - eβa(E-Ecorr)]
(4)
where i is the net current, icorr is the corrosion current, E is the electrode potential, Ecorr is the free corrosion potential, and βc and βa are the cathodic and anodic Tafel slopes, respectively. The first term in brackets represents the rate of the cathodic reactions, while the second term represents the rate of the anodic reactions. Under open circuit conditions where the electrode potential is equal to Ecorr, i is equal to zero, and the aggregate rate of the anodic reactions is equal to the aggregate rate of the cathodic reactions. Under these
conditions, the corrosion rate of the electrode is equal to icorr. At electrode potentials sufficiently above or below Ecorr, eq 4 indicates that a plot of log |i| versus E will be linear. The linear regions of the polarization profiles have slopes equal to βa and βc and are known as the anodic and cathodic Tafel regions. The βa and βc values are indicative of the anodic and cathodic reactions occurring on the surface of the electrode. The icorr under open circuit conditions can be determined graphically by extrapolating the anodic and cathodic Tafel slopes to their point of intersection at Ecorr (24). Recently, Lackovic et. al (25) demonstrated the effectiveness of columns packed with sand and zero-valent iron filings for removing arsenate and arsenite from solution. Although high levels of removal were observed, they reported no reduction of As(V) or As(III). However, the redox conditions in their column systems were not measured, and thus the absence of reduction could not be mechanistically explained. The goal of this research was to investigate the mechanisms involved in arsenate removal from solution by zero-valent iron media and to determine the effect of arsenate on the corrosion behavior of zero-valent iron. Toward this end, batch experiments were performed to measure arsenate removal rates by iron wire electrodes. To elucidate the electrochemical reactions, corrosion rates, solution and iron potentials, and anodic and cathodic Tafel slopes were monitored during the course of the removal process. Reaction products of arsenate with iron filings and iron powder were characterized spectroscopically to identify the speciation of arsenic associated with the iron surfaces.
Materials and Methods Batch Experiments. Kinetic experiments were performed to determine the rate of arsenate removal by iron wires suspended in 3 mM CaSO4 background electrolyte solutions containing Na2HAsO4 at concentrations of 100 or 5000 µg/L as arsenic. The batch reactors contained 850 mL of solution at pH 7, a 3.75 cm long by 1.5 mm diameter iron wire of 99.9% purity (Aesar, Ward Hill, MA), a calomel reference electrode (EG&G, Oak Ridge, TN), and a stainless steel wire counter electrode. The small amount of iron relative to the large solution volume was selected to (i) minimize adsorption as a removal mechanism, (ii) allow the buildup of a thick layer of precipitated arsenic compounds on the iron surface, and (iii) reduce the effects of iron corrosion on the solution pH. The reactors were purged with ∼50 mL/min humidified nitrogen gas in order to agitate the solutions and maintain the solution potentials at -390 to -400 mV with respect to the standard hydrogen electrode (SHE). Total arsenic concentrations in the aqueous phase were determined with a graphite furnace atomic absorption (AA) spectrophotometer (Perkin-Elmer, San Jose, CA) using a published method (26). The aqueous arsenic oxidation states were determined via ion chromatography (IC) using an atomic fluorescence detector (PS Analytical, Kent, U.K.). The detection limits for the AA and IC methods (27) were ∼1 µg/L. The solution potentials were measured using a platinum wire electrode that was intermittently inserted through the sampling port on each reactor. All potentials are reported with respect to the SHE. Electrochemical Experiments. During the removal process, the iron wires were periodically polarized to determine their corrosion rate (icorr) and free corrosion potential (Ecorr). The icorr and Ecorr were measured by analysis of Tafel diagrams produced by polarizing the electrodes (150 mV with respect to their open circuit potentials (24). The polarization experiments were performed using an EG&G model 273A scanning potentiostat and EG&G M270 software. To determine if performing the Tafel scans affected the arsenate removal rates, a duplicate 5000 µg/L reactor was operated without the periodic polarizations. VOL. 35, NO. 10, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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X-ray Absorption Spectroscopy (XAS). Arsenic K-edge XAS was used to determine the oxidation state and molecular environment of the arsenic compounds associated with the surfaces of the iron. Two types of iron samples were analyzed in the XAS experiments. One sample for XAS analysis was prepared by reacting 10 g of iron powder (first shell: R ( 0.02 Å, N ( 30%. 2028
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FIGURE 1. Arsenate solution concentrations with least-squares model fits (solid lines) for arsenate removal by iron wires for initial arsenic concentrations of 100 and 5000 µg/L.
Results Batch Experiments. Aqueous arsenate concentrations as a function of elapsed time in the two batch reactors are shown in Figure 1. Filtered and unfiltered samples gave similar results, indicating that the removed arsenate was associated with the iron wires. Ion chromatography analyses indicated that the arsenic in solution was comprised of >99% As(V). The solution potentials ranged from -390 to -400 mV. In this potential range, equilibrium calculations indicate that As(0) should be the predominant arsenic oxidation state (5). Therefore, the predominance of As(V) indicates that the arsenic oxidation state was not in equilibrium with the solution. In both the arsenate and blank reactors, the bulk solution pH remained at the initial pH of ∼7. The constant pH is consistent with other investigations using zero-valent iron media in low ionic strength solutions (36). At low ionic strengths, the high electrical resistivity of the solution requires small separations between anodic and cathodic sites on the iron surface. This allows the Fe2+ released at anodic sites to combine with the OH- released at cathodic sites to precipitate as Fe(OH)2(s) on the iron surface (37). As illustrated by the model fits in Figure 1, the arsenate removal kinetics could be described by a combined zeroand first-order kinetic model, according to
-k0C dC ) dt k0/k1 + C
(5)
where C is the arsenate solution concentration, t is time, k0 is the zero-order rate constant, and k1 is the first-order rate constant. This form of kinetic expression is appropriate where reaction rates are first order in the limit of infinite dilution and zero order in the high concentration limit (38). The mechanisms responsible for this form of kinetic expression are explained in the Discussion section. Figure 2 shows an example of a Tafel scan used to determine the icorr and Ecorr values for the iron wires in the batch reactors. Iron corrosion rates for the blank 3 mM CaSO4 background electrolyte and 100 and 5000 µg/L arsenate solutions are compared in Figure 3a. Both reactors containing arsenate had decreased corrosion rates as compared to the blank electrolyte solution. The decrease in corrosion rates with increasing arsenate concentration and the nearly constant current in the 5000 µg/L solution after 3 days elapsed, despite declining concentrations of arsenate, suggest that arsenate does not directly oxidize the iron. The similar cathodic Tafel slopes in all three reactors also indicate that arsenate does not directly contribute to iron oxidation. As illustrated in Figure 3b for the blank and 5000 µg/L reactors, the βc values were ∼0.006 dec/mV, with or without arsenate in the solution. Since the value of βc is
FIGURE 2. Anodic and cathodic Tafel scans for the 100 µg/L reactor after 1 day elapsed. indicative of the cathodic reaction, the similar cathodic Tafel slopes confirm that water was the primary oxidant in the blank and arsenate containing solutions. The smaller corrosion rates in the presence of arsenate can only partially be attributed to the increase in potential of the iron associated with adsorption of arsenate, as shown in Figure 3b. According to eq 4 and the measured βc of 0.006 dec/mV in all reactors, an increase in potential of 25 mV should decrease the current in the arsenate reactors by only ∼15% as compared to the blank electrolyte solution. Since the actual decline in corrosion rates was greater than 50% (as indicated in Figure 3a), the presence of arsenate must have also affected the surface properties of the iron, which in turn affected the exchange current for the water reduction reaction. The smaller current in the arsenate reactors cannot be attributed to the anodic corrosion inhibition that accompanies precipitation of As(0) on corroding steel in acidic media (5). In fact, the βa in the arsenate solutions were always greater than those in the blank electrolyte solution, as illustrated in Figure 3c for the blank and 5000 µg/L reactors. An increase in anodic Tafel slope is often associated with solution species that form complexes with products of the anodic reaction (37). This suggests that arsenate forms complexes with Fe(II) or Fe(III) in the vicinity of anodic sites on the iron surface. XAS Analyses. Arsenic K-edge spectra of the powdered iron and two column samples are compared to those of reference compounds with different arsenic oxidation states in Figure 4a. All the experimental samples have XANES absorption maxima that coincide with that of scorodite (FeAsO4‚2H2O), clearly indicating the presence of only As(V). More reduced forms of arsenic [e.g., As(+3) in As2O3 and As(-1) in FeAsS] have absorption maxima that are distinctly shifted to lower energies. The absorption maximum for As(V) at ∼11 876 eV is typical of As(V)-oxygen coordination reported in other studies (13, 29, 39, 40). Quantitative analysis of the arsenic EXAFS of the column samples in Figure 4, panels b and c (iron filings 1 and 2) as compared to the reference compound scorodite indicates As(V) in tetrahedral coordination with oxygen (RAs-O ) 1.69-1.70 Å; NAs-O ) 4.24.4) and the presence of second-neighbor iron atoms beyond the oxygen shell, as indicated in Table 1. Fit discrepancies in the first oscillation of the EXAFS spectra at about 4.5 Å-1 in Figure 4b are similar to those in the scorodite model compound. A previous study showed that these features were associated with multiple scattering among arsenic and oxygen in tetrahedral coordination (41). Comparison of the EXAFS spectra indicates no evidence for the formation of a separate scorodite phase or any evidence for a lower arsenic oxidation state in the column samples. Interatomic distances derived from fits of the EXAFS spectra (Table 1) indicate two sets of As-Fe backscatterers beyond the first shell of four oxygen atoms coordinating
FIGURE 3. (a) Iron wire corrosion currents in the blank, 100, and 5000 µg/L batch reactors. (b) Free corrosion potentials for the iron wires in the three batch reactors. (c) Anodic and cathodic Tafel slopes in the blank and 5000 µg/L reactors. arsenic. Iron backscattering amplitudes are slightly higher in iron filing sample 2 as compared to iron filing sample 1, which suggests more disorder in the local atomic environment around arsenic in sample 1. The interatomic As-Fe distances in the iron filing samples are similar to those found for arsenate sorbed to ferrihydrite, goethite (R-FeOOH), and akaganeite (β-FeOOH) reported in previous laboratory studies (13, 14, 19, 29, 39) and to interatomic As-Fe distances reported by Savage et al. (42) in arsenate-bearing, goethiterich natural samples formed as weathering products. As shown in these previous studies, the As-Fe distances of 3.243.29 and 3.44-3.45 Å indicate bidentate corner-sharing among arsenate tetrahedra and iron octahedra. However, these As-Fe interatomic distances are not diagnostic of arsenate association with a particular phase because the local atomic geometry is similar among amorphous and crystalline iron oxyhydroxide phases (13, 14, 39).
Discussion The ferrous hydroxide produced by corroding iron in anaerobic solutions ages to form magnetite (Fe3O4) over a VOL. 35, NO. 10, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. (a) Normalized arsenic K-edge XANES spectra for samples of arsenate reacted with iron powder, iron filings 1 and 2, and reference samples arsenopyrite (FeAs(-I)S), arsenolite (As2(III)O3), and scorodite (As(V)FeO4‚2H2O). The energy of maximum absorption (dashed line) for the three samples is characteristic of the As(V) oxidation state. (b) Normalized EXAFS spectra for arsenic reacted with iron filing samples 1 and 2 (top spectra) as compared to the reference spectrum of scorodite. The solid lines represent experimental data, and the dashed lines represent least-squares best fits (numerical fit results are given in Table 1). (c) Fourier transforms of the spectra shown in panel b (uncorrected for backscatterer phase shift). The spectral differences between the iron filing samples and scorodite indicate that arsenic is not forming a scorodite-like local structure.
TABLE 1. Arsenic K-Edge EXAFS Fit Resultsa iron filing 1 iron filing 2
atom
N
R (Å)
σ2 (Å-1)
O Feb Feb O Feb Feb
4.4 2.3 2.2 4.2 2.1 1.9
1.69 3.24 3.44 1.70 3.29 3.45
0.0037 0.0056 0.0056 0.0033 0.0011 0.0011
crystallographic values ref compd
atom
Nd
R (Å) σ2 (Å-1)
scoroditec
O
4.0
1.68
0.0023
Fe
4.0
3.36
0.0058
atom
N
R (Å)
O O O O Fe Fe Fe Fe
1 1 1 1 1 1 1 1
1.669 1.676 1.684 1.685 3.338 3.346 3.363 3.387
a N represents the number of backscatters at distance R; σ2, the Debye-Waller term, is the disorder parameter for the absorberbackscatterer pair. b For Fe shells, N and R were varied independently during fitting; σ2 was linked as a single variable. c Scorodite spectrum collected by Foster et al. (29) was reanalyzed for this study. Interatomic distances for crystallographic values were calculated from the structure refinement of Kitahama et al. (51). d Parameter fixed during fitting on known value.
period of hours to days (37, 43). The Ecorr values measured for the iron throughout most of this investigation were in the range of -520 ( 5 mV. This potential corresponds to the equilibrium potential between zero-valent iron and magnetite at neutral pH (5), and suggests that the zero-valent iron was in contact with a magnetite phase. It is well-known that passive films on iron often have an Fe(II)/Fe(III) gradient consistent with an inner layer of magnetite and an outer 2030
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layer of maghemite (γ-Fe2O3), and both phases may be thermodynamically stable due to the redox potential gradient between the corroding iron and the bulk solution (43). Therefore, it is likely that complexation of arsenate with Fe(III) on the surface of magnetite or maghemite was the mechanism responsible for removing arsenate from solution in this investigation. Arsenate complexation with iron (III) oxyhydroxides is consistent with the kinetic model described by eq 5. In the limit of low arsenate concentration, the ratio of dissolved arsenate to available (i.e., uncomplexed) iron oxyhydroxides is sufficiently low that there is no competition between arsenate species for complexation sites. This situation results in removal kinetics that are first order in arsenate concentration. The apparent first-order kinetics may reflect the rate of complex formation,or may result from diffusional mass transfer limitations within the iron oxide phase, as suggested by other investigators (44). With increasing arsenate concentration, the ratio of aqueous arsenate to available complexation sites increases, leading to competition for complexation sites. With increasing competition, the rate of arsenate removal becomes limited by the generation rate of new sites for arsenate complexation. New complexation sites are generated by iron corrosion and the subsequent aging of Fe(OH)2(s) to produce oxyhydroxides containing Fe(III). Under this scenario, removal kinetics should become zero order with respect to arsenate in the limit of high aqueous concentration. Since As(V) is readily reduced to As(0) by iron in acidic media (5), several pH-dependent factors may be responsible for the absence of reduction in this investigation. At low pH values, Fe(OH)2(s), magnetite, and maghemite will not form on zero-valent iron. Therefore, the physical barrier limiting arsenate access to the zero-valent iron phase is essentially eliminated in low pH solutions. Although iron oxides coat
the zero-valent phase at neutral pH values, the oxides are porous, and only partially protect the underlying iron from attack by oxidants in solution (37). Therefore, aqueous arsenate would still have some access to the zero-valent iron phase in neutral media. This suggests that there is another pH-dependent factor that precludes arsenate reduction. The equilibrium potential for the As(V)/As(0) redox couple can be expressed as (5)
E ) 0.499 V - 0.0827 pH + 0.0118 log[HAsO42-] (6) The divalent anion, HAsO42-, was chosen as the arsenate species of interest since it is the predominant species at the elevated pH values commonly found adjacent to the surface of freely corroding iron (37). Equation 6 indicates that increasing pH and decreasing [HAsO42-] will decrease the potential at which As(V) may be reduced. According to eq 6, the equilibrium potential for As(V)/As(0) in a 5000 µg As/L solution in neutral pH, 3 mM CaSO4 (where the Davies equation activity coefficient for HAsO42- is 0.64 (45)) is approximately -130 mV. However, both the pH and the arsenate activity adjacent to the surface of the iron may be considerably different than their bulk solution values. The potential of zero surface charge is -370 mV for iron (46) and is in the range of -70 to +80 mV for mixed valent iron corrosion products at pH values relevant to this investigation (47). Since the iron and solution potentials in this study were significantly below these values, the iron and its associated corrosion products likely carried a negative surface charge, even in the absence of specific adsorption. In addition, specific adsorption of negatively charged arsenate species may have further contributed to development of a negative surface charge on the iron/iron oxide reactants (48). A negative charge creates a potential difference between the iron surface and the bulk solution (ψ). This potential difference decreases the activity of HAsO42- adjacent to the iron surface ([HAsO42-]s) according to (49)
[HAsO42-]s ) [HAsO42-]b exp
[2Fψ RT ]
(7)
where [HAsO42-]b is the bulk solution activity of HAsO42-. This surface charge effect on [HAsO42-]s is controlled by pH, since pH affects the charge on the arsenate species. In low pH solutions, surface charge does not affect [HAsO42-]s since the predominant arsenate species is uncharged at pH values less than 3.6. Equations 6 and 7 indicate that a negative surface potential will decrease the equilibrium potential of the As(V)/As(0) redox couple to a value more negative than -130 mV for the conditions of the 5000 µg/L experiment. Therefore, arsenate reduction may not have been thermodynamically favorable at the conditions near the iron surface, even though the bulk solution calculations indicate that reduction was favorable. Results from this study indicate that zero-valent iron is capable of removing arsenate from aqueous solutions to levels below 5 µg/L. However, the effectiveness of iron filings for arsenic removal will likely depend on the presence of other groundwater ions that also form complexes with iron oxides, such as phosphate, carbonate, and nitrate (50). Although the arsenate removal mechanism does not take advantage of the redox capabilities of the zero-valent iron, iron filings may still prove to be a useful adsorbent in canister treatment systems. Studies of arsenate adsorption on iron oxides indicate that freshly formed ferric hydroxide has a greater adsorption capacity per unit mass than aged oxides, such as akaganeite or goethite (43, 44). Therefore, the advantage of zero-valent iron may be that its corrosion continuously
generates high specific surface area iron oxides that are more reactive than aged materials.
Acknowledgments We thank Wayne Seames, Wendell Ela, and Nikos Melitas for their assistance. This project was made possible by Grant 2P42ES04940-11 from the National Institutes for Environmental Health Sciences of the National Institutes for Health, with funds from the U.S. Environmental Protection Agency. Funding for synchrotron data collection was provided by the National Science Foundation, EAR-9629276 (to P.O.). Work was done (partially) at SSRL, which is operated by the DOE, Office of Basic Energy Sciences.
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Received for review September 18, 2000. Revised manuscript received February 20, 2001. Accepted February 26, 2001. ES0016710
