X-ray Absorption and X-ray Photoelectron Spectroscopic Study of

Environ. Sci. Technol. 2010, 44, 955–961

X-ray Absorption and X-ray Photoelectron Spectroscopic Study of Arsenic Mobilization during Mackinawite (FeS) Oxidation HOON Y. JEONG,† YOUNG-SOO HAN,‡ A N D K I M F . H A Y E S * ,‡ Pohang Accelerator Laboratory, Pohang University of Science and Technology (POSTECH), Pohang 790784, Korea, and Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109

Received August 25, 2009. Revised manuscript received November 8, 2009. Accepted December 9, 2009.

In this study we investigated the speciation of the solidphase As formed by reacting 2 × 10-4 M As(III) with 1.0 g/L mackinawite and the potential for these sorbed species to be mobilized (released into the aqueous phase) upon exposure to atmospheric oxygen at pH 4.9, 7.1, and 9.1. Before oxygen exposure, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) analyses indicated that As(III) was removed from the aqueous phase by forming As(0), AsS, and surface precipitates as thioarsenites at pH 4.9 and As(0) and thioarsenite surface precipitates at pH 7.1 and 9.1. When oxygen was introduced, XAS analysis indicated that As(0) and the surface precipitates were quickly transformed, whereas AsS was persistent. During intermediate oxygen exposure times, dissolved As increased at pH 4.9 and 7.1 due to the rapid oxidation of As(0) and the slow precipitation of iron (oxyhydr)oxides, the oxidation products of mackinawite. This indicates that oxidative mobilization is a potential pathway for arsenic contamination of water at acidic to neutral pH. The mobilized As was eventually resorbed by forming edgesharing and double-corner-sharing surface complexes with iron (oxyhydr)oxides.

Introduction High levels of arsenic threaten water quality and human health on a worldwide basis (1). In Bangladesh and India alone, over 70 million people are at risk from drinking arseniccontaminated waters (2, 3). The mobility and toxicity of arsenic are strongly dependent on its oxidation state and chemical form. Arsenite (As(III)) and arsenate (As(V)) are the two main forms of arsenic in soil and water, with the former stable under anoxic conditions and the latter dominant under oxic conditions (4). As(III) species are known to be more mobile and toxic than As(V) species (5). Thus, the mobility and toxicity of arsenic are expected to be strongly affected by the extant redox conditions. In sulfidic sediments, arsenic mobility is mainly controlled by reaction with iron sulfides (6-8). Among iron sulfides, mackinawite (FeS), an important precursor to pyrite (FeS2), * Corresponding author phone: (734) 763-9661; fax: (734) 7632275; e-mail: [email protected]. † POSTECH. ‡ University of Michigan. 10.1021/es902577y

 2010 American Chemical Society

Published on Web 12/30/2009

is ubiquitous in low-temperature anoxic sediments (9) and typically present in nanoparticulate forms (10). Under anoxic conditions, mackinawite sequesters arsenic through various sorption reactions including adsorption, precipitation, and coprecipitation (11-13). Arsenic may form outer-sphere surface complexes or arsenic sulfides (orpiment (As2S3) and realgar (AsS)) when reacted with mackinawite (11-13). In reaction with troilite (FeS) and pyrite, an arsenopyrite (FeAsS)like surface precipitate was observed (7). In anoxic sediments, arsenic uptake has been attributed to bulk precipitation of arsenic sulfides including arsenopyrite, orpiment, or realgar (8, 13, 14). However, dissolved arsenic concentrations are often undersaturated with respect to those of pure arsenic sulfides (6). Such diversity in arsenic sorption reactions reflects the complexity of arsenic speciation in sulfidic environments. Anoxic sediments are often mixed with the overlying oxic water as a result of various activities such as dredging, storms, and bioturbation. In such cases, iron sulfides are subject to oxidative dissolution, resulting in the release of Fe, S, and sorbed contaminants into water (15-17). Released arsenic can be resorbed via adsorption and/or coprecipitation with iron (oxyhydr)oxides, the oxidation products of iron sulfides (11, 18, 19). However, such resorption occurs only when iron (oxyhydr)oxides form in the vicinity of the released As, which is controlled by the relative rate of iron (oxyhydr)oxide formation to As mobilization. In this study, aerobic oxidation experiments were conducted using aqueous suspensions of mackinawite (FeS) previously reacted with As(III). Mackinawite is a potentially important scavenger for As in anoxic sediments (11-13). The oxidation state and speciation of the solid-phase arsenic were examined as a function of the oxidation time and pH using X-ray absorption spectroscopy (XAS). X-ray photoelectron spectroscopy (XPS) analysis was also performed for unoxidized samples to supplement XAS data. This was necessary due to the ambiguity in XAS identification of disordered arsenic sulfide phases (e.g., AsS and As2S3). For oxidized samples, XAS data alone were sufficient for distinguishing As sorption phases. X-ray diffraction (XRD) measurements were also performed to identify crystalline Fe, S, and As products. This work was undertaken to simulate oxidative transformation of mackinawite and sequestered As species in sulfidic sediments when exposed to oxygen and to provide a detailed understanding of changes in the solid-phase As speciation responsible for its sorption and mobilization as a function of the pH and oxidation time (extent). This work was also expected to ascertain whether oxidative mobilization of arsenic by dissolved oxygen is a potentially significant pathway for its accumulation in pore and sediment waters during episodic oxygen exposure events.

Experimental Section All oxygen-free experiments were conducted inside an anaerobic chamber with the atmospheric composition of 5% H2 in N2. Aqueous solutions were prepared using deoxygenated water purged with N2. Mackinawite was synthesized as described in ref 20. Arsenic(III)-reacted mackinawite batches were prepared by adding 2 × 10-4 mol of As(III) from a NaAsO2 stock solution and 1.0 g of mackinawite to 1 L of the deoxygenated water that was buffered at pH 4.9 with 0.1 M acetate buffer, pH 7.1 with 0.1 M 3-(N-morpholino)propanesulfonate (MOPS) buffer, and pH 9.1 with 0.1 M 2-(N-cyclohexylamino)ethanesulfonate (CHES) buffer. The ionic strength was adjusted to 0.1 M using VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Dissolved Species Concentrations and Mineralogy during Mackinawite (FeS) Oxidation oxidation time (h)

species

at pH 4.9 -3

at pH 7.1 -6

at pH 9.1

0

Fe (M) As (M) sulfate (mg/L) mineralogya

1.53 × 10 2.70 × 10-8 0.25 Mack

8.03 × 10 2.11 × 10-6 0.18 Mack

7.72 × 10-6 7.05 × 10-5 0.00 Mack

1


3.47 × 10-3 2.53 × 10-5 0.60 Mack

8.38 × 10-5 3.95 × 10-6 0.54 Mack + GRs

1.15 × 10-5 1.59 × 10-5 0.10 Mack

2


4.72 × 10-3 7.52 × 10-5 0.62 Mack + Sul

7.15 × 10-5 5.06 × 10-6 0.94 Mack + GRs + Lepi + Sul

1.01 × 10-6 7.22 × 10-6 0.11 Mack + Lepi + Sul

4


9.14 × 10-3 1.49 × 10-4 0.63 GRs + Sul

2.16 × 10-6 2.49 × 10-7 1.93 GRs + Lepi + Sul

8.71 × 10-7 3.28 × 10-6 0.11 Mack + Lepi + Sul

192


1.24 × 10-3 NDb 0.68 Goe + Sul

1.79 × 10-6 1.79 × 10-7 1.94 Lepi + Sul

6.33 × 10-7 2.67 × 10-6 0.54 Lepi + Sul

a Mack, GRs, Goe, Lepi, and Sul represent mackinawite (FeS), green rusts, goethite (R-FeOOH), lepidocrocite (γ-FeOOH), and elemental sulfur (S0), respectively. No crystalline As phase was found by XRD in any samples. b ND means not detected at the detection limit of ∼2.5 × 10-8 M.

NaCl. The initial As(III) and mackinawite concentrations in reaction batches were 2 × 10-4 M and 1 g/L, respectively. In separate experiments using nonbuffered batches, no significant buffer impact on As(III) sorption was observed. Prior to oxidation, As(III)-reacted batches were magnetically stirred inside the anaerobic chamber for 3 days, which was sufficient on the basis of previous studies of As(III) sorption by mackinawite (12, 21). Subsequently, the batches were removed from the anaerobic chamber, uncapped, and allowed to equilibrate with atmospheric oxygen by diffusion while being vigorously stirred in the dark. At specific intervals, 8 mL aliquots of suspensions were taken and transferred to the anaerobic chamber. After syringe-filtering the aliquots with a 0.02 µm filter, 4 mL of the filtrates were acidified with 4 mL of 10% HNO3 and analyzed for dissolved Fe and As by inductively coupled plasma coupled with mass spectroscopy and dissolved sulfate by ion chromatography (Table 1). Although the dissolved arsenic concentration may be underestimated by acidification due to precipitation of arsenic sulfides (22), comparison of acidified and untreated filtrates showed no difference beyond the analytical errors. Wet paste was obtained by centrifuging the remaining slurries at 2000 rpm for 5 min and decanting the supernatant. One portion of wet paste was dried inside the chamber and analyzed for the bulk mineralogy using X-ray diffraction (XRD) (Table 1), and the other portion was stored in airtight serum vials and kept frozen until XAS data collection. Defrosted pastes were mounted into Teflon sample holders and sealed with a double layer of Kapton tape inside an anaerobic chamber at the Stanford Synchrotron Radiation Laboratory. Each sample was brought outside the chamber when it was ready for data collection. Arsenic K-edge XAS spectra were collected at room temperature on beamline 10-2 or 11-2 (3 GeV, ∼100 mA) using an unfocused beam with a Si(220) double-crystal monochromator and a 13element solid-state Ge-array fluorescence detector or a Lytle detector. The sample chamber was continuously purged with He(g). Although photooxidation of As-sorbed samples was reported during XAS data collection (7, 23), no such reaction occurred in this study on the basis of comparison of individual 956

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XAS scans. XAS spectra were also collected at 15-20 K with a He cryostat for selected samples. Comparison of the XAS spectra obtained at room and cryogenic temperatures indicated no photocatalytic reduction of As. XAS spectra were also obtained for reference compounds including zerovalent arsenic (Alfa Aesar), AsS (Alfa Aesar), As2S3 (Alfa Aesar), aqueous As(III), and aqueous As(V). By XRD, zerovalent arsenic (As(0)) was found to be wellcrystalline, whereas AsS and As2S3 were disordered (Figure S1, Supporting Information). These reference compounds were further characterized using XAS and XPS (Table S1, Supporting Information). Both disordered arsenic sulfides have been used for comparison of their oxidation kinetics with those of crystalline realgar and orpiment (24, 25). In general, the precipitates first formed in saturated aquatic systems are amorphous or disordered, which slowly convert into more crystalline phases over time (26). Considering the short equilibration period (3 days), disordered AsS and As2S3 are more likely to control the precipitation of arsenic sulfides (if any). Furthermore, no arsenic-containing crystalline phase was identified by XRD (Table 1). XAS spectra were subjected to X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses as described in the Supporting Information. To accurately determine the absorption edges (i.e., inflection energies), energy calibration was performed for each scan using an As(0) foil with a step size of 0.3 eV around the As K-edge (11 867 eV). XPS analysis was performed for unoxidized samples. Wet pastes obtained by filtering with 0.22 µm nylon filters were freeze-dried and mounted on double-sided Cu tape in the anaerobic chamber. The samples, kept inside airtight containers, were transferred to the XPS chamber. During loading into the chamber, sample exposure to air was less than 1 min. XPS spectra were collected for samples and reference compounds on a Kratos Axis Ultra XPS with Al KR radiation. Reference compounds (purchased from Alfa Aesar) included As(0), AsS, As2S3, NaAsO2, and Na2HAsO4 · 7H2O. Surface charging effects were corrected using the adventitious carbon 1s spectral line at a binding energy of 284.6 eV. Narrow scans

FIGURE 1. First derivatives of arsenic K-edge XANES spectra for samples at pH 4.9 (a), 7.1 (b), and 9.1 (c) as well as reference compounds. The samples oxidized for 0, 1, 2, 4, and 192 h (all black) are arranged from bottom to top, and are enveloped by reference compounds: aqueous As(V) (red), aqueous As(III) (orange), disordered As2S3 (pink), disordered AsS (blue), arsenopyrite (green), and As(0) (gray). Arrows indicate the positions of two adsorption edges, with the lower edge close to that of aqueous As(III) and the higher one close to that of aqueous As(V). were obtained for As 3d regions. Details of XPS analysis are described in the Supporting Information.

Results and Discussion XANES Analysis. The first derivatives of XANES spectra for samples and reference compounds are compared in Figure 1. The positions of the absorption edges (the first-derivative maxima) are sensitive to the oxidation state, with higher absorption edge energy indicative of a higher oxidation state of arsenic. The absorption edges of the unoxidized samples at pH 4.9 and 7.1 (11 867.7 eV) are much lower than that of aqueous As(III) (11 870.9 eV) and even below that of disordered As2S3 (11 869.0 eV), indicating the initially added As(III) was extensively reduced by mackinawite to lower valent-state species at acidic to neutral pH. The absorption edge of the pH 9.1 unoxidized sample (11 868.8 eV) is close to that of disordered As2S3, suggesting the dominance of sulfur-coordinated As(III) species at basic pH. Attempts to fit the XANES spectra of the unoxidized samples to linear combinations of those of reference compounds were unsuccessful, likely due to an incomplete reference set (e.g., the unavailability of surface precipitates as thioarsenites, which comprised a portion of the sorbed As). Absorption edge energies were monitored as a function of the exposure time to atmospheric oxygen. At pH 4.9, the absorption edge shifted to higher energy (11 868.8 eV) during 0-1 h of oxygen exposure, but shifted back to lower energy (11 868.1 eV) during 1-2 h of oxygen exposure. This was not consistent with formation of a single As sorption species in the pH 4.9 unoxidized sample. Instead, it was likely due to formation of multiple As sorption phases with different resistances to oxidation. The proximity of the absorption edge obtained after 2 h of oxygen exposure to that of disordered AsS (11 868.1 eV) indicated that an AsS-like precipitate formed and remained at 2 h of exposure at pH 4.9. At g4 h of oxygen exposure, the absorption edge energies at pH 4.9 were close to that of aqueous As(V) (11 874.4 eV). At pH 7.1 and 9.1, the 1 h exposure to atmospheric oxygen resulted in two distinct adsorption edges (indicated by arrows in Figure 1), with the lower edge close to that of aqueous As(III) and the higher one close to that of aqueous As(V). Although two edges cannot be resolved at longer exposure times, the gradual shift of the edges to higher energies at pH 7.1 and 9.1 indicated the increasing contribution of As(V)

sorption species with exposure time. Due to thermodynamic unfavorability of arsenic (hydr)oxide precipitation under our experimental conditions, the observed resorption of the mobilized As would occur via surface complexation with iron (oxyhydr)oxides, the mackinawite oxidation products. EXAFS and XPS Analyses of Unoxidized Samples. The EXAFS spectra and corresponding Fourier transforms of samples are compared with those of reference compounds in Figures 2 and 3, respectively. Structural parameters obtained from the numerical fitting of samples and reference compounds are summarized in Tables 2 and S2 (Supporting Information), respectively. Crystalline realgar (AsS) exhibits characteristic doublets over k ) 6-10 Å-1 in the EXAFS spectrum due to strong As-As interaction at ∼3.5 Å (8, 27). Surprisingly, no such pattern is observed for disordered AsS due to the lack of the long-range order, making its EXAFS spectrum similar to that of disordered As2S3 (Figure 2). The lack of the doublet patterns in k-space and the weak second coordination shell in R-space for disordered AsS are likely due to structural disorder. Previously, disordered As2S3 was found to exhibit fewer interatomic interactions at long distances compared with crystalline orpiment (27). At k < ∼9 Å-1, the EXAFS spectra of all unoxidized samples are similar to those of disordered AsS and As2S3, suggesting sulfurcoordinated environments around As. The first coordination shells in the Fourier transforms of the unoxidized samples (Figure 3) are mainly characterized by As-S bonding at an interatomic distance of ∼2.26 Å. The resemblance of the EXAFS spectra between disordered AsS and As2S3 makes it challenging to definitively assign As-S bonding in the unoxidized samples to either phase. As discussed below, XPS analysis was used to assign As-S bonding in the unoxidized samples due to this ambiguity. In Figure 3, the first coordination shell peaks of the unoxidized samples, compared with those of disordered AsS and As2S3, are noticeably broader on the higher R side. This right shoulder feature is likely due to additional path(s) in the first coordination shells. In Figure 3, either arsenopyrite or As(0) may contribute to the right shoulder feature of the first coordination shell peaks. Attempts to include the AsFe interaction at 2.36 Å, characteristic of arsenopyrite (28), were unsuccessful, indicating the strong oscillations at low k in the EXAFS spectrum of arsenopyrite are incompatible with those of the unoxidized samples. However, the relatively VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. k3-weighted arsenic K-edge EXAFS spectra (k3χ(k)) of pH 4.9 (a), 7.1 (b), and 9.1 (c) as well as reference compounds. Solid lines are the experimental data; dashed lines are the numerical fits. The samples oxidized for 0, 1, 2, 4, and 192 h (all black) are arranged from bottom to top, and are enveloped by reference compounds: aqueous As(V) (red), aqueous As(III) (orange), disordered As2S3 (pink), disordered AsS (blue), arsenopyrite (green), and As(0) (gray).

FIGURE 3. Fourier transforms of pH 4.9 (a), 7.1 (b), and 9.1 (c) as well as reference compounds. Solid lines are the experimental data; dashed lines are the numerical fits. The samples oxidized for 0, 1, 2, 4, and 192 h (all black) are arranged from bottom to top, and are enveloped by reference compounds: aqueous As(V) (red), aqueous As(III) (orange), disordered As2S3 (pink), disordered AsS (blue), arsenopyrite (green), and As(0) (gray). Peak positions are uncorrected for phase shift. weak oscillations in the As(0) spectrum at k < ∼7 Å-1 are compatible. Inclusion of the As-As interaction at ∼2.52 Å, close to the As-As bonding distance in As(0) (29), significantly improves the numerical fits of the unoxidized samples. Furthermore, the As-As component at ∼2.52 Å in the XAS spectra of the unoxidized samples is markedly similar to that in the XAS spectrum of As(0) (Figures S2 and S3, Supporting Information). Lee et al. (30) reported biogenic arsenic sulfides with metallic conducting properties. In their study, the right shoulder feature at ∼2.2 Å (phase-uncorrected) in the As-S peaks was attributed to As(0) to explain the metallic conducting properties. As discussed later, XPS analysis also supports As(0) formation in the unoxidized samples. Unlike disordered AsS and As2S3, the unoxidized samples have apparent second coordination shells at ∼3.5 Å, causing 958

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their EXAFS spectra to significantly deviate from those of these arsenic sulfides at k > ∼9 Å-1 (Figures 2 and 3). These second shells can be assigned to either As-Fe or As-As bonding. The As-Fe bonding that may result from the surface complexation of thioarsenites with mackinawite is less likely responsible for the observed second shells given geometric considerations in Figure S4, Supporting Information. Bostick et al. (23) proposed surface precipitates in the form of trimeric arsenic sulfide (As3S3(SH)3) for As(III) uptake by PbS and ZnS. The As-S and As-As bonding distances for the surface precipitates formed on ZnS were ∼2.23 and ∼3.35 Å, and those on PbS were ∼2.51 and ∼3.66 Å (23). In this study, the bonding distances of As-S at ∼2.25 Å and As-As at ∼3.51 Å may also result from the surface precipitate on mackinawite. The As-S and As-As bonding distances for surface pre-

TABLE 2. EXAFS Fit Results for As(III)-Reacted Mackinawite during Oxidationa samples at pH 4.9 oxidation time (h) 0

shell 1

N

R(Å)

σ2(Å2)b

N

R(Å)

σ2(Å2)b

pair

N

R(Å)

σ2(Å2)b

As-S 2.5 2.25 0.0055 As-As 1.4 2.52 0.008 As-As 1.8 3.51 0.006 ∆E0 ) -9.03 eV, Rf ) 0.026


1

As-S

2.3

2.26

0.003

2

As-As

0.60

3.48

0.006

As(V)-O 2.4 1.69 As(III)-O 0.84 1.79 As-Fe 0.27 2.92 As-Fe 0.52 3.37 ∆E0 ) -11.24 eV, Rf )

0.0033 0.0037 0.0044 0.0048 0.051

As(V)-O 1.4 1.69 0.0033 As(III)-O 2.2 1.79 0.0037 As-Fe 0.33 2.95 0.0044 As-Fe 0.64 3.42 0.0048 ∆E0 ) -5.31 eV, Rf ) 0.054

∆E0 ) -7.43 eV, Rf ) 0.020


0.0033 0.0037 0.0044 0.0048 0.044


0.0033 0.0037 0.0044 0.0048 0.045 0.0033 0.0044 0.0048 0.043

∆E0 ) -7.97 eV, Rf ) 0.028 2

pair

samples at pH 9.1


2 1

pair

samples at pH 7.1

1

As-S

2.6

2.27

0.003

2

As-As

0.19

3.50

0.006

4

1 2

As(V)-O 3.6 1.69 0.0033 As-Fe 0.40 2.90 0.0044 As-Fe 0.87 3.33 0.0048 ∆E0 ) -9.67 eV, Rf ) 0.035

As(V)-O 3.8 1.69 As-Fe 0.36 2.94 As-Fe 0.78 3.36 ∆E0 ) -12.60 eV, Rf )

0.0033 0.0044 0.0048 0.044


192

1 2



0.0033 0.0044 0.0048 0.041

As(V)-O 3.8 1.69 0.0033 As-Fe 0.36 2.91 0.0044 As-Fe 0.61 3.38 0.0048 ∆E0 ) -8.96 eV, Rf ) 0.042

0.0033 0.0044 0.0048 0.049

a The amplitude-reduction factor (So2) was set at 0.92. ∆E0 and Rf indicate the energy shift and goodness of fit, respectively. b The Debye-Waller factors (σ2) were fixed during the numerical fit.

cipitates are strongly affected by the arrangement of the constituent metals and sulfurs in metal sulfides. For example, the surface precipitate formed on PbS (the metal sulfide with larger constituent metals) has longer bonding distances. In agreement with the structure of trimeric thioarsenites (27), the As-As coordination number in the second shells (NAs-As) at pH 4.9 and 7.1 is close to 2 (Table 2), indicating the dominance of trimeric thioarsenite clusters. However, the much smaller NAs-As at pH 9.1 indicates a mixture of monomeric and trimeric thioarsenites on the mackinawite surface. Arsenic 3d XPS spectra of the unoxidized samples are shown in Figure 4. Higher binding energy corresponds to a higher oxidation state while lower binding energy corresponds to a lower oxidation state (Table S1, Supporting Information). Consistent with XANES analysis, the binding energies of the unoxidized samples are lower at pH 4.9 and 7.1 than pH 9.1. The XPS spectrum of the unoxidized sample at pH 4.9 consists of 80.5 ( 20.8% As(0)-As(0) component and 19.5 ( 15.3% As(II)-S component. At pH 7.1, the experimental peak is decomposed into 69.5 ( 22.6% As(0)-As(0) component and 30.5 ( 17.0% As(III)-S component. These XPS results support our earlier conclusion that formation of As(0) leads to the low absorption energies of the unoxidized samples at pH 4.9 and 7.1 from XANES analysis (Figure 1) and the strong right shoulder feature in their first coordination shells from EXAFS analysis (Figure 3). Due to the weak peak intensity at pH 9.1, XPS analysis at this pH is rather qualitative. The XPS peak at pH 9.1 is mainly contributed to by As(III) species (e.g., As(III)-S and As(III)-O) with less than ∼8% As(0) species. Thus, the higher absorption energy at pH 9.1 compared with those at pH 4.9 and 7.1 results from less As(0) formation. Previously, Gallegos et al. (13) did not consider the possibility of As(0) formation in their EXAFS analysis of As(III)-reacted mackinawite. However, a recent XPS study of the samples prepared by Gallegos et al. (13) confirmed the presence of As(0) (31). Thermodynamic

FIGURE 4. Arsenic 3d XPS spectra of unoxidized samples. The experimental data and the corresponding overall fits are indicated by dashed lines and solid lines, respectively. The As(0)-As(0), As(II)-S, As(III)-S, and As(III)-O components are indicated by gray, blue, pink, and orange, respectively. Baselines are indicated by black dashed lines.

calculations made under our experimental conditions (Figure S5, Supporting Information) also support As(0) formation under highly reduced conditions. VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The XPS results also provide information on the nature of As-S bonding in the EXAFS spectra of the unoxidized samples. At pH 4.9, the observed As(II)-S component in the As 3d XPS spectrum indicates that the AsS phase is mainly responsible for As-S bonding in the EXAFS analysis. This is consistent with the closeness of the absorption edge of the pH 4.9 sample oxidized for 2 h to that of disordered AsS. At pH 7.1 and 9.1, the XPS peaks are contributed to by the As(III)-S component. Since precipitation of bulk arsenic sulfides is not expected at pH 7.1 and 9.1 (Figure S5, Supporting Information), surface precipitates as thioarsenites are more likely to account for the observed As-S bonding in the EXAFS analysis at these pH values. Prior to oxidation, As(III) is sequestered by forming discrete As solids and thioarsenite surface precipitates in reaction with mackinawite. While As(0), AsS, and the surface precipitates as thioarsenites are responsible for As(III) uptake at pH 4.9, As(0) and the surface precipitates form at pH 7.1 and 9.1. Given that no crystalline As phase was detected by XRD (Table 1), both As(0) and AsS were disordered, amorphous phases. EXAFS Analysis of Oxidized Samples. The EXAFS spectra and corresponding Fourier transforms of oxidized samples are shown in Figures 2 and 3, respectively. The fitting results are summarized in Table 2. The samples at pH 4.9 maintain As-S coordination shells even after 1-2 h of exposure to atmospheric oxygen. This is consistent with the persistent nature of AsS against oxidation under acidic conditions (24). At pH 7.1 and 9.1, no As-S interaction is observed in the oxidized samples, and the As-As bonding at ∼3.6 Å (characteristic of the surface precipitates) also disappears quickly upon exposure to atmospheric oxygen. Taken together, thioarsenite surface precipitates are more readily transformed under oxic conditions. Despite the presence of As-S shells, the As-As component at ∼2.52 Å is not observed in the pH 4.9 samples subjected to 1-2 h of oxygen exposure (see Table 2), suggesting that the As-As component is not structurally related to As-S bonding. This supports our interpretation that As(0), characterized by the As-As bonding at ∼2.52 Å, disappears more rapidly than AsS when exposed to oxygen. At all pH values examined, the As-As component at ∼2.52 Å is not observed in the oxidized samples, indicating the high instability of As(0) under oxic conditions. As can be seen in Table 1, arsenic is mobilized significantly at pH 4.9 and to some extent at pH 7.1. However, no such mobilization occurs at pH 9.1. From both XAS and XPS analyses, As(0) is observed to extensively form at pH 4.9 and 7.1 in reaction with mackinawite. Therefore, the increased dissolved As at these pH values can be attributed to the rapid oxidation of As(0). Furthermore, the relatively high dissolved Fe concentrations at pH 4.9 (Table 1) indicate that precipitation of iron (oxyhydr)oxides (the mackinawite oxidation products) at acidic pH occurs slowly, thus providing a pathway for As mobilization under oxidizing conditions. Except for the pH 4.9 samples subjected to 1-2 h of oxygen exposure, the EXAFS spectra of all other oxidized samples are similar to one another. Their first coordination shells are characterized by As(III)-O at 1.79 Å and As(V)-O at 1.69 Å, suggesting that the mobilized (released) As is resorbed by forming surface complexes with iron (oxyhydr)oxides. By XRD, the iron (oxyhydr)oxides formed are goethite (R-FeOOH) at acidic pH and lepidocrocite (γ-FeOOH) at neutral to basic pH, with green rusts as intermediate products (Table 1). The longer interatomic distance of As(III)-O compared with As(V)-O in oxidized samples is consistent with that observed in their aqueous As(III) and As(V) counterparts (Table S2, Supporting Information). In agreement with the gradual shift of the XANES absorption edges to higher energies at pH 7.1 and 9.1, the coordination number for the As(III)-O path 960

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(NAs(III)-O) decreases while that for the As(V)-O path (NAs(V)-O) increases with oxygen exposure time (Table 2). Adsorption of arsenic is strongly affected by its oxidation state (16, 19). For example, although the As(III) species is a weak adsorbate, the oxidized As(V) is strongly adsorbed by iron (oxyhydr)oxides (11, 16). Thus, the transition of As(III) to As(V) during mackinawite oxidation results in favorable resorption of the mobilized As. The second coordination shells of oxidized samples are divided into two subshells: As-Fe interactions at 2.88-2.95 and 3.32-3.42 Å. These As-Fe distances are much shorter than the distances reported for a monodentate surface complex (∼3.6 Å) (18, 19), but close to those reported for a bidentate mononuclear (edge-sharing) complex (2.80-2.83 Å) and a bidentate binuclear (double-corner-sharing) complex (3.25-3.29 Å) with FeO6 octahedra (32). The slightly longer As-Fe distances here reflect the distorted, elongated surface groups on iron (oxyhydr)oxides. Fendorf et al. (19) observed the dominance of bidentate-type surface complexes at high As loadings (10-2.05 mol of As sorbed/mol of Fe), which were even lower than those investigated here (10-1.79-10-1.65 mol of As sorbed/mol of Fe). Recently, the radial structure function (RSF) peak at ∼2.9 Å was attributed to the multiple As-O-O scattering within AsO4 tetrahedra, not the single As-Fe backscattering corresponding to edge-sharing surface complexes (33). However, attempts to include this multiple scattering path failed. Even when the potential involvement of the As-O-O multiple scattering was taken into account, edge-sharing surface complexes were also noted in reaction of As with iron (oxyhydr)oxides (34, 35). Therefore, both edgesharing and double-corner-sharing complexes account for the observed As-Fe interactions in this study. Such innersphere complexes provide strong sequestration paths for As in oxic environments. Environmental Implications. Iron sulfides are potentially important scavengers of arsenic in anoxic sediments (7, 11-13). When sulfidic sediments are exposed to oxygen, the release of sorbed As into the aqueous phase is of concern. This study illustrates that oxidative mobilization of arsenic previously reacted with mackinawite strongly depends on the pH and oxygen exposure time. At acidic pH, high levels of arsenic may be released to nearby waters due to high instability of As(0) under oxic conditions. Furthermore, the resorption of the mobilized arsenic is kinetically limited by slow precipitation of iron (oxyhydr)oxides at acidic pH. Elevated arsenic levels during acidic oxidation may last for extended periods at the mixing regions between anoxic sediments and oxic water (8, 36), where the redox conditions cause oxidative dissolution of mackinawite but do not favor precipitation of iron (oxyhydr)oxides. At basic pH, oxidative mobilization of arsenic does not occur due to less formation of the readily oxidizing As(0) phase and no significant dissolution of mackinawite (Table 1). Although reductive dissolution of iron (oxyhydr)oxides under anoxic conditions is generally considered the main path for arsenic accumulation in waters (2, 37), this study demonstrates that oxidative mobilization may also be important. It remains to be shown whether similar results would be found in environments in which non-acid volatile sulfides such as pyrite (FeS2) are reacted with arsenic.

Acknowledgments Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource (SSRL), a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. Funding for this work was provided by the

Department of the Army, Strategic Environmental Research and Development Program (SERDP Contract No. W912HQ04-C-0035), and the Korean Institute of Geoscience and Mineral Resources (KIGAM).

Supporting Information Available Details of XAS and XPS analyses, thermodynamic calculations for As solids, XANES absorption edge energies and XPS binding energies of reference compounds, EXAFS fit results of reference compounds, XRD patterns of reference compounds, EXAFS component analysis of unoxidized samples, geometry of thioarsenite surface complexes, and equilibrium speciation of As solids. This material is available free of charge via the Internet at http://pubs.acs.org.

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