Photoinduced Oxidation of Arsenite to Arsenate on Ferrihydrite

Mar 1, 2011 - ATR-FTIR spectra were collected using a Smart Orbit ATR diamond accessory and a Nicolet 6700 spectrometer (Thermo Scientific) equipped w...
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Photoinduced Oxidation of Arsenite to Arsenate on Ferrihydrite Narayan Bhandari,† Richard J. Reeder,‡ and Daniel R. Strongin†,* † ‡

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794, United States

bS Supporting Information ABSTRACT: The photochemistry of an aqueous suspension of the iron oxyhydroxide, ferrihydrite, in the presence of arsenite has been investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATRFTIR), X-ray absorption near edge structure (XANES), and solution phase analysis. Both ATR-FTIR and XANES show that the exposure of ferrihydrite to arsenite in the dark leads to no change in the As oxidation state, but the exposure of this arsenite-bearing surface, which is in contact with pH 5 water, to light leads to the conversion of the majority of the adsorbed arsenite to the As(V) bearing species, arsenate. Analysis of the solution phase shows that ferrous iron is released into solution during the oxidation of arsenite. The photochemical reaction, however, shows the characteristics of a self-terminating reaction in that there is a significant suppression of this redox chemistry before 10% of the total iron making up the ferrihydrite partitions into solution as ferrous iron. The self-terminating behavior exhibited by this photochemical arsenite/ferrihydrite system is likely due to the passivation of the ferrihydrite surface by the strongly bound arsenate product.

1. INTRODUCTION Arsenic (As) is a toxic element and is classified as a Group I human carcinogen.1 Arsenic contamination in groundwater is widely recognized as a global health problem. Over 137 million people in more than 70 countries are affected by a high level of arsenic in drinking water, including the United States.2,3 High arsenic concentrations in natural environments originate from various natural processes that include geothermal sources,4 weathering of As-bearing minerals,3 microbial activities,5 and anthropogenic activities.3,6,7 In aqueous media, arsenic mostly occurs as the oxyanions of As(III) and As(V); arsenite and arsenate, respectively. Aqueous arsenite [pKa1 = 9.2] occurs primarily as a neutral species (H3AsO3), whereas arsenate [pKa1 = 2.2, pKa2 = 6.9, pKa3 = 11.5] mainly occurs as doubly (H2AsO4-) and singly (HAsO42-) protonated species in most natural environments.8 In the context of human health, arsenite has been suggested to be 25-60 times as toxic as arsenate.9 Because arsenite is less strongly sorbed than arsenate to a variety of sorbents, it shows a higher mobility in aqueous environments. Immobilization, therefore, can be enhanced by the oxidation of arsenite to arsenate.10 While both species form inner-spheres complexes on the surface of iron oxyhydroxide, arsenate typically shows a stronger binding.11-14 These prior studies have not experimentally observed redox chemistry that could, for example, convert arsenite into the less toxic arsenate form. However, investigations that have focused on the photochemistry of these solid/aqueous systems with an eye on facilitating the conversion of arsenite to arsenate have to the best of our knowledge not been pursued. We r 2011 American Chemical Society

do mention however that UV light-treated solutions containing aqueous Fe(II) and/or Fe(III) in the presence of organic complexing agents like oxalate 15 and citrate 16 have been studied in the context of arsenite oxidation, and more generally the photochemistry of arsenite on TiO2 has been the focus of prior studies.10 In the present study, we investigate the reaction of arsenite on the iron oxyhydroxide, ferrihydrite (Fh), in the absence and presence of light. Fh is an intrinsically nanocrystalline material that is ubiquitous to many natural systems.17,18 Researchers have shown that the high surface area and high surface reactivity associated with Fh makes it an effective material for the sequestration of many transition metals and metalloids (e.g., arsenic bearing species) in environmental settings.17,19 A recent in vitro study has shown that Fh has the potential to be an effective enterosorbent for arsenic that enters humans via contaminated drinking water.20 We show in the present contribution using solution based analytical techniques, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray absorption near edge structure (XANES) spectroscopy that the irradiation of aqueous suspensions of iron oxyhydroxide in the presence of arsenite with light facilitates the conversion of this arsenic oxyanion to arsenate and that the majority of the reaction product remains bound to the Fh surface. Received: November 10, 2010 Accepted: February 1, 2011 Revised: January 24, 2011 Published: March 01, 2011 2783

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2. EXPERIMENTAL SECTION Synthesis and Characterization of Ferrihydrite. Fh used in this study was the 2-line variety and prepared using a modified version 21 of the method developed by Cornell and Schwertmann.22 BET surface area measurements of synthesized Fh yielded average values of 320 ( 20 m2/g. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) data showed that the size and crystallinity of the synthesized Fh were consistent (Supporting Information) with what was expected for 2-line Fh.18,23 FTIR Measurements. ATR-FTIR spectra were collected using a Smart Orbit ATR diamond accessory and a Nicolet 6700 spectrometer (Thermo Scientific) equipped with a DTGS detector. ATR-FTIR photo-irradiation experiments were carried out by first drying a thin film of Fh (∼1 mg) on a diamond ATR element in a N2 environment. A Teflon flow cell (volume capacity ∼2 mL) that surrounded the dry film allowed the passage (at a rate of 30 mL/h) of arsenite solution. In all of these in situ FTIR experiments, data was collected as a single beam (resolution 4 cm-1) of 200 co-added scans. Irradiation of the sample during the ATR-FTIR experiments was carried out by directing the light from a 900 W high-pressure xenon lamp (Schoeffel Instruments, Westwood, NJ) to the reaction cell via a fiber optic cable. Batch Studies. Batch photoirradiation experiments were performed in a Pyrex vessel (400 mL) using 50 mg of ferrihydrite in a total volume of 200 mL. Initially, 50 mg of dry Fh was resuspended in 198 mL of 5 mM NaCl and sonicated for 30 min and stirred 30 min to obtain well suspended Fh particles, and the pH was maintained at 5 using an autotitrator (718 STAT Titrino, Metrohm) operating in pH-stat mode while being constantly stirred with a Teflon bar. A 1.65 mL volume of 50 mM arsenite solution was then added to the solution containing the Fh suspension. The Fh was allowed to equilibrate with the added arsenite for a period of 24 h. Solution prepared in this manner was used in both photochemical and dark (control) experiments. The incident radiation from the 900 W high-pressure xenon lamp used in the current study had a maximum light output at a wavelength of ∼600 nm (Supporting Information), and the power density impinging on the reaction solution during the batch and IR studies was measured to be ∼1450 and ∼15 W/m2, respectively. In all cases, pyrex glass was used as a cutoff filter for UV light,21 and experiments were performed at a constant temperature of 25 ( 2 °C. Considering the environmental relevancy, all experiments were performed in solutions that were exposed to the ambient atmosphere and hence contained dissolved oxygen and dissolved CO2. Solution Analyses. Collected samples were centrifuged and filtered through a Millipore filter (0.2 μm) and analyzed by spectrophotometric methods for the determination of dissolved Fe(II).24 Aqueous phase arsenic was analyzed using an ion chromatograph (IC, Dionex ICS-1000), with a detection limit of ∼5 μM, that was equipped with a Dionex IonPacAS22 (4 mm 250 mm) analytical column and a conductivity detector.25 The eluent was a mixture of 4.5 mM Na2CO3 and 1.4 mM NaHCO3 solutions. The amount of adsorbed As on Fh at a given reaction time was determined by subtracting the amount of aqueous phase arsenic species at that time from the initial concentration of arsenite. The IC calibration for As(V) used standard aqueous solutions of Na2HAsO4 in the concentration range of 5-1000 μM. The IC method under our conditions only could measure the solution concentration of arsenate. Arsenite concentrations

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were therefore determined after a solution of interest was oxidized to arsenate with excess hydrogen peroxide (24 h of oxidation time and presence of 100 μM Fe(II)). A plot of arsenate concentration (from standard solutions of Na2HAsO4) versus the concentration of arsenate resulting from the oxidation of arsenite by excess H2O2 yielded a straight line with a satisfactory linearity of R2 = 0.99 (Supporting Information). All of the chemicals that were used in this study that included sodium arsenite, sodium arsenate, ferric chloride, sodium hydroxide, hydrochloric acid, and hydrogen peroxide were obtained from Sigma Aldrich (analytical grade). XANES Analysis. Arsenic K-edge XANES spectra were collected at beamline X19A at the National Synchrotron Light Source, Brookhaven National Laboratory, in fluorescence mode using a passivated implanted planar silicon detector. The Si(111) double-crystal monochromator was calibrated using an arsenic reference, and was detuned by 30% for data collection to minimize harmonic contributions. Particular care was taken to avoid any potential beam-induced oxidation of arsenite because some arsenite oxidation on the surface of iron oxide during XANES analysis has been observed in previous studies.26 To ascertain the degree to which beam-induced arsenite oxidation occurred in our study, XANES spectra for an arsenite-sorbed Fh sample were repeatedly collected over a duration of 30 min. We observed no evidence of arsenic oxidation during this beam exposure. Therefore, we suspect in our experimental circumstance that beaminduced oxidation, if any, is below the detection limit of XANES. For collection of subsequent XANES spectra, the X-ray beam was positioned in a portion of the sample that had not previously been exposed to the X-ray beam. The photo-irradiated arsenite-Fh suspension, arsenite-, and arsenate-sorbed Fh control samples were individually centrifuged, washed once with pH 5 DI water, dried, and loaded into slots in Lucite holders, sealed with Kapton tape, and mounted at 45° relative to the incident X-ray beam. XANES spectra were also collected for arsenite and arsenate aqueous reference solutions. A linear function was fitted to the pre-edge region and subtracted from the raw XANES spectra. A single-point normalization at 11.915 keV was used, corresponding to the initial rise in a weak oscillation present in all spectra.

3. RESULTS AND DISCUSSION 3.1. ATR-FTIR during Photoirradiation. Experiments were carried out that investigated the surface chemistry of arsenite on the Fh surface in situ during irradiation. Figure 1 displays ATRFTIR experimental results obtained before and during the irradiation of Fh under constant flow conditions (∼30 mL/h) with a solution composition of 0.41 mM arsenite and 5 mM NaCl at pH 5. Under these flow conditions, the solution composition with regard to arsenite and pH was presumed to remain constant during the course of the experiments. The bottom spectrum was obtained after Fh was exposed to the flowing solution for 24 h under dark conditions and the remaining spectra were collected after various light exposure times. The ATR-FTIR spectrum obtained under dark conditions shows a vibrational mode at ∼770 cm-1 that is associated with adsorbed arsenite.27 After 0.5 h of light exposure, two new IR modes at 805 and 876 cm-1 start to grow in intensity and these modes are associated with arsenate, consistent with assignments in prior research.12,27 This experimental observation indicates that a fraction of arsenite has been oxidized to arsenate during this 0.5 h exposure to light. Modes 2784

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Figure 1. ATR-FTIR spectra showing arsenite oxidation during the photo irradiation of Fh in the presence of flowing arsenite (0.41 mM) as a function of time. The bottom spectrum was collected after a 24 h exposure to the arsenite, but before exposure to light. Subsequent spectra were collected after a 0.5, 1, 2, 3, 4, 5, 6, 7, and 8 h of light exposure (as indicated by arrow). Vertical dashed lines at 876 and 805 cm-1 indicate the peak positions associated with adsorbed arsenate and the dashed line at 770 cm-1 is associated with adsorbed arsenite. In the figure inset, spectra a and b are difference spectra. Spectra a and b were obtained by subtracting a clean Fh spectrum from an arsenite on an Fh spectrum prior to irradiation and from a spectrum obtained after an 8 h exposure to light, respectively. Vertical lines at 1360 and 1475 cm-1 indicate the peak positions of vibrational modes associated with adsorbed carbonate on Fh. The negative peaks show that carbonate is lost from the surface after Fh is exposed to arsenite and an additional amount is lost during the exposure to light.

associated with arsenate show a continuous increase in intensity until 7-8 h of irradiation, after which the arsenate derived modes remain relatively constant. We surmise from this experimental observation that after 7-8 h of irradiation the Fh surface becomes saturated with arsenate. A further examination of the data shown in Figure 1 shows that the absorbances of the arsenate-derived modes after 8 h of light exposure are greater than those associated with arsenite prior to irradiation. We believe that this increase in absorbance over the 0 to 8 h exposure is due at least in part to an increasing amount of adsorbed arsenate. We will quantify this increase later when we discuss our kinetic measurements, but we point out that quantifying the concentrations of the two species by IR is complicated by differences in the absorption cross sections of arsenite and arsenate.27 Furthermore, changes in the surface composition and structure of the Fh may play a role in at least partially controlling the surface coverage of arsenite and arsenate. The inset to Figure 1 exhibits difference spectra that emphasize changes in the surface concentration of carbonate, which is an intrinsic contaminate on Fh that has been exposed to carbonate either in solution or gaseous carbon dioxide in the ambient atmosphere.21,23 The difference spectrum (spectrum of interest minus clean ferrihydrite in pH 5 water) shows that there is a decrease in the carbonate concentration (i.e., appear as negative

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Figure 2. ATR-FTIR time dependent spectra showing the photooxidation of preadsorbed arsenite on Fh in 5 mM NaCl at pH 5 (no aqueous arsenite). The bottom spectrum was collected before arsenite/Fh was exposed to light. Subsequent spectra were collected after a 0.5,1, 2, 3, 4, 5, 6, and 7 h exposure to light. Vertical solid lines indicate the peak positions associated with adsorbed arsenate and the dashed line is associated with adsorbed arsenite.

absorptions) after Fh is exposed to 0.41 mM arsenite for 24 h (spectrum a in inset) and there is an additional loss of carbonate during the exposure to light (spectrum b in inset). This experimental observation is generally consistent with prior studies that have also shown that carbonate on Fh is displaced from the surface upon exposure to aqueous arsenite or arsenate in the absence of light.27,28 The additional loss of carbonate during exposure to light may be a result of the loss of iron (that was coordinated to carbonate) via Fe(II) dissolution and/or possibly is a result of the stronger binding of arsenate product (relative to arsenite or carbonate 27). In either case, it is likely that additional surface sites for the oxidation of arsenite become available as carbonate is lost from the surface. The experimental conditions (flowing arsenite) used to obtain the data presented in Figure 1 should largely preclude the possibility of the readsorption of arsenate from solution, if some fraction of arsenate formed on Fh during irradiation partitions into the aqueous phase. To further address the possibility of arsenate readsorption from solution, we investigated the photoreactivity of adsorbed arsenite on Fh surface at pH 5. For these experiments, Fh was exposed to 0.41 mM arsenite at pH 5 for 24 h and then the flowing arsenite-bearing solution was replaced with a flowing solution containing only 5 mM NaCl at the same pH. ATR-FTIR results for this experiment are presented in Figure 2. The 0-time spectrum (no light exposure) shows mode intensity at 770 cm-1, characteristic of adsorbed arsenite. As the irradiation time increases from 0 to 7 h, modes at 805 and 876 cm-1 appear that increase in intensity during the irradiation. The transformation from adsorbed arsenite to arsenate is largely complete after 7 h. These results show that the oxidation of arsenite to arsenate occurs on the surface of Fh, since solution phase arsenite was not present in this particular experiment. A comparison of the 0-time spectra in Figure 1 and Figure 2 shows that similar amounts of arsenite are present based on the absorbance peak maximum of the 770 cm-1 spectral feature for each experiment. However, after 7 h of irradiation the maximum of the 805 cm-1 peak associated with arsenate is greater when arsenite is present in the solution flowing over the 2785

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Figure 4. Normalized arsenic K-edge XANES spectra recorded for arsenite (dotted line) and arsenate (dashed line) adsorbed on to Fh in the dark, and for photoirradiated Fh/arsenite (solid line). In the inset, the linear combination fit (gray solid line) is compared to the experimental spectrum (dotted line) for the light-exposed arsenite treated Fh (see text). The linear combination fit shows 75-80% arsenate and 2025% arsenite, respectively, after an 8 h exposure to light. Figure 3. Amount of As adsorbed on Fh as a function of time in the absence of light (filled triangle) and in the presence of light (open triangle) in a 0.25 g L-1 Fh suspension at pH 5 with an initial arsenite addition of 1.64 mol kg-1. Arsenic adsorbed on Fh at time-zero on this plot is representative of the amount of adsorbed arsenic containing species that adsorbed on Fh after the mineral was individually exposed to either 0.41 mM of arsenite (open and filled triangle) or arsenate (filled square) for 24 h under dark conditions. The remaining data points show the amount of adsorbed arsenic containing species on the Fh during the exposure to light and in the dark. For each plot, error bars represent one standard deviation from the average value of three individual experiments.

Fh. We interpret this result as suggesting that if arsenite is present in the aqueous solution during irradiation it is capable of adsorbing and undergoing oxidation to arsenate. This experimental observation brings up the possibility that adsorption sites become available on Fh during the irradiation and these can further adsorb arsenite and facilitate its oxidation to arsenate. 3.2. Batch Studies and XANES Analysis of Fh after Reaction. In addition to the ATR-FTIR experiments that probed the redox state of the arsenic surface complexes on Fh, batch reactions were carried out to quantify the amount of arsenite oxidation as well as the nature of the aqueous and surface-bound product. As mentioned in the Experimental Section, Fh samples used for the batch reaction were first equilibrated with either 0.41 mM arsenite or arsenate for 24 h. Figure 3 plots the amount of As adsorbed on Fh as a function of time after this pre-equilibration period. Time-zero on this plot is associated with the amount of arsenite (filled and open triangles) or arsenate (filled squares) that adsorbs on Fh after the 24 h equilibration. The experimental results show that total amount of As adsorbed to Fh is enhanced significantly in the presence of light. Specifically, the total amount of As adsorbed to Fh during an 8 h exposure to light was 1.22 ( 0.03 mol kg-1, which is 58% higher than the amount of As (0.77 ( 0.02 mol kg-1) that adsorbs when Fh is exposed to the same amount of arsenite in the dark. This amount of As adsorbed on Fh in the presence of light (primarily arsenate) is slightly higher than the amount of As (in the form of arsenate) that adsorbs on Fh when the mineral is exposed to 0.41 mM arsenate in the dark (i.e., 1.13 mol kg-1). Taking into account the surface area of the Fh the As surface coverage is 2.40 μmol/m2 (As(III)) before exposure to light and 3.84 μmol/m2 (primarily As(V)) after exposure, values that are consistent with prior studies at similar solution conditions that have investigated arsenite/arsenate

adsorption on Fh.29,30 Dixit and Herring,29 for example, obtained a surface density of 2.33 and 3.5 μ mol/m2 (pH 4) for As(III) and As(V) on Fh, respectively. Of note is that, while this prior work reported similar surface densities, the solution concentrations of arsenite and arsenate were significantly lower (0.10 M) than the solution concentration of As-species (0.41 M) used in our study. This comparison suggests that the 2.40 μmol/m2 (As(III)) and 3.84 μmol/m2 (As(V)) densities obtained in our study are likely associated with a saturation coverage of arsenite and arsenate respectively on Fh. Furthermore, the prior studies showed that at a similar solution pH the adsorption of arsenate occurred more readily than arsenite and hence this difference is likely a reason for the increased loading of As (as arsenate) on the Fh during irradiation. As mentioned above, we also speculate that the increased adsorption of arsenic containing oxyanions during irradiation is in part due to the concomitant loss of carbonate shown by the ATR-FTIR results. The loss of carbonate likely results in an increased amount of adsorbed arsenite on Fh that can then be oxidized to arsenate in the presence of light. On the basis of ATR-FTIR the majority of the As-bearing species that was adsorbed on Fh during irradiation was arsenate. To better quantify the relative surface concentration of arsenate and arsenite after irradiation, a XANES analysis was carried out on Fh that was exposed to 0.41 mM arsenite and irradiated for 8 h. XANES spectra were also collected for control samples to characterize the adsorbed layer on Fh that resulted from the individual exposure of the mineral to arsenite and arsenate under dark conditions. Figure 4 exhibits these XANES data and shows that Fh samples that were individually exposed to arsenite and arsenate exhibit well-resolved edge structures. The energy difference between the absorption maxima associated with the two surface species is ∼3.7 eV, which is consistent with edge positions for As(III) and As(V) coordinated to oxygen, respectively.14 An analysis of photoirradiated arsenite-treated Fh indicates the presence of both arsenate and arsenite. A least-squares linear combination fit of the XANES spectrum, using the arsenite- and arsenate-treated Fh spectra, showed that the adsorbed surface species consisted of 75-80% arsenate and 25-20% arsenite. Hence, this XANES analysis shows that even after 8 h of irradiation there is still arsenite on the Fh surface. Interestingly, we observe only about ∼5-10 μM of arsenate in solution after 8 h of irradiation. Control experiments show no arsenate in solution if the initial arsenite/Fh system is kept in the 2786

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Figure 5. Concentration of soluble Fe(II) produced during the photo irradiation of an arsenite/Fh suspension after 24 h of equilibrium (referred to on the plot as 0 time) with aqueous arsenite in the dark (open square). Open triangles represent the amount of soluble Fe(II) produced when Fh with adsorbed arsenite was exposed to light in pH 5 water (no solution phase arsenite was present). The inset to the figure shows expanded views of data points corresponding to open circles, filled triangles, and filled squares. Open circles are associated with the irradiation of Fh in the absence of either adsorbed or solution phase arsenic bearing species (control experiment). Filled triangles are associated with the amount of soluble ferrous iron resulting from the exposure of Fh to 0.41 mM arsenite under dark conditions. Finally, the filled squares are associated with Fh that has a saturated layer of adsorbed arsenate and has been exposed to light in the presence of solution phase arsenite. All experiments used a 0.25 g L-1 loading of Fh and 5 mM NaCl.

dark or aqueous arsenite is exposed to light in the absence of Fh. These batch reaction results, taken along with the ATR-FTIR results, suggest that the vast majority of the arsenate product remains bound to the Fh surface and does not partition to any large degree into the solution phase. Figure 5 shows the concentration of aqueous ferrous iron as a function of time during the photoinduced oxidation of arsenite. The data show that during the irradiation of Fh in the presence of arsenite, there is a release of ferrous iron into solution, presumably due to the reduction of ferric iron in Fh to soluble ferrous iron during the concomitant oxidation of arsenite to arsenate. Data shown in the plot also reveal that there is an insignificant release of soluble Fe(II) when Fh is irradiated in the absence of the arsenite reducing agent. Also shown is an arsenite/Fh control experiment carried out in the dark, and under this condition there also is an insignificant amount of Fe(II) release into solution. To further test the notion that the buildup of surface bound arsenate product on the Fh surface inhibited its ability to oxidize arsenite (in the presence of light), we also carried out an experiment where Fh was first equilibrated with arsenate for 24 h. This pretreatment results in what we believe is a saturation surface coverage of arsenate on Fh (i.e., no more arsenate can be adsorbed under these conditions). The resulting Fh with adsorbed arsenate was washed and resuspended in 0.41 mM

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arsenite. Irradiation of this sample led to an insignificant amount of Fe(II) release into solution. Hence, the adsorbed layer of arsenate inhibited any adsorption of arsenite and essentially did not allow any photochemistry to occur. This result also supports our contention that the formation of an adsorbed layer of arsenate product after ∼6-8 h of irradiation of Fh in an arsenite solution inhibits any further oxidation of arsenite. We surmise that the plateau in the ferrous iron versus time plot that begins near 6 h is where the Fh surface becomes saturated with arsenate. Supporting this contention is the data presented in Figure 3 that shows that the amount of arsenate that adsorbs to Fh also reaches a maximum near 6 h. On the basis of an analysis of the mass of the Fh before and after a ∼8 h light exposure, only about 10% of the Fh mass underwent dissolution. A similar self-terminating behavior has been observed during the oxidation of arsenite to arsenate on birnessite (MnO2). Similar to the arsenite/Fh system, it was proposed in the arsenite/birnessite system that the stability of the adsorbed arsenate product layer led to the inhibition of arsenite adsorption from solution and its oxidation.31 Figure 5 also shows results from experiments that determined the amount of ferrous iron released into solution from Fh with a preadsorbed arsenite in pH 5 water (complements ATR-FTIR results in Figure 2). Data plotted in Figure 5 show that the amount of ferrous iron released is less than when arsenite is present in solution. After 6 h of irradiation of the preadsorbed arsenite (no arsenite in solution), ∼177 μM of ferrous iron resides in solution. When arsenite is present in solution with Fh during a similar exposure to light, the solution concentration of ferrous ions is ∼365 μM. Our ATR-FTIR and batch chemistry results show that the initial surface coverage of arsenite on both of these Fh surfaces is similar. Hence, if arsenite is present in solution during the irradiation there is an additional amount of arsenite that adsorbs and oxidizes (presumably in a concerted manner) and results in additional ferrous iron release (i.e., ∼188 μM). If we analyze Figure 3 further, we find that in concentration units the additional amount of arsenic adsorption that parallels the additional iron release is 88 μM. Hence the ferrous iron to arsenate product concentration ratio is ∼2.1:1. We can also further infer from Figure 3 that ∼63% of the total arsenic on Fh after 8 h of irradiation is due to the oxidation of the initial adsorbed arsenite that is present prior to irradiation. A similar analysis (considering only 75-80% of surface species as arsenate) for the total amount of ferrous and arsenite oxidation during 8 h of irradiation when aqueous arsenite is present yields a ferrous iron to arsenate product concentration ratio of ∼1.81.9:1. This ratio is close to the ratio of 2:1 that might be expected if the reduction of 2 ferric iron atoms is required to oxidize arsenite to arsenate. We mention that a prior study experimentally observed a phase transformation of Fh to goethite and lepidocrocite (observed by XRD) when the iron oxyhydroxide was exposed to aqueous Fe(II) species at a neutral pH.32 XRD of our samples before and after exposure to arsenite and light showed no evidence for any phase but Fh (Supporting Information). Our experimental observations taken as a whole support a surface picture that includes the direct oxidation of surface-bound arsenite and the concomitant reduction of Fe(III). Similar to conjecture in an earlier publication 21 we suggest the possibility that the iron binding the arsenite need not be the metal species that are reduced and desorb from the surface. Shuttling of electrons through the bulk of the mineral is a process believed to occur in low band gap semiconductors like iron 2787

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Environmental Science & Technology oxy/hydroxides 33 and hence the ferric iron that is reduced to ferrous iron may not be directly participating in the binding of the arsenite reactant. We emphasize, however, that the vast majority of the ferrous iron that does form during arsenite oxidation partitions into solution (considering the experimentally determined aqueous ferrous to arsenate product concentration ratio is close to 2:1), suggesting that this reduced iron species primarily forms at the outer surface of Fh. In closing, we note that developing an inexpensive and efficient arsenite oxidation method coupled with the removal of the toxic product is still an elusive goal. Generally, arsenic removal from contaminated water is achieved in two steps: 1) the oxidation of arsenite to arsenate and 2) the adsorption of the arsenate to a suitable adsorbent.16 In all remedial processes, the proper storage and disposal of arsenic-contaminated sludge becomes a relevant issue. Prior studies have shown that arsenic bearing Fh appears to be stable for many years at a slightly acidic pH and oxidizing conditions.34 Considering the present limitations of various As(III) oxidation processes,35 we believe that the irradiation of Fh in the presence of arsenite may be a potential method for the removal of arsenic because of its dual functions as an oxidant (in presence of light) and sorbent.

’ ASSOCIATED CONTENT

bS

Supporting Information. TEM image of the Fh used in the study, a plot of As(V) concentration (from standard solutions of Na2HAsO4) versus the concentration of As(V)*(resulting from oxidation of the standard solution of arsenite by excess H2O2) that was used as a standard curve for our As(III) concentration measurements, an emission spectrum of the lamp (passed through pyrex) used in this study, and XRD patterns of the Fh before and after reaction with arsenite in the presence of light. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.R.S. and R.J.R. greatly appreciate financial support from the National Science Foundation (NSF) Collaborative Research in Chemistry grant. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. ’ REFERENCES (1) Monographs, some drinking-water disinfectants and contaminants, including arsenic related nitrosamines. IARC (International Agency for Research on Cancer) 2004, 84. (2) Nordstrom, D. K. Worldwide occurrences of arsenic in ground water. Science 2002, 296 (5576), 2143–2145. (3) Smedley, P. L.; Kinniburgh, D. G. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17 (5), 517–568. (4) Wilkie, J. A.; Hering, J. G. Rapid oxidation of geothermal arsenic(III) in streamwaters of the Eastern Sierra Nevada. Environ. Sci. Technol. 1998, 32 (5), 657–662.

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