Sorption of Arsenite, Arsenate, and Thioarsenates to Iron Oxides

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Sorption of Arsenite, Arsenate, and Thioarsenates to Iron Oxides and Iron Sulfides: A Kinetic and Spectroscopic Investigation R.-M. Couture,*,†,‡,§ J. Rose,∥ N. Kumar,∥ K. Mitchell,§ D. Wallschlag̈ er,⊥ and P. Van Cappellen§ ‡

Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Ecohydrology research group, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada ∥ Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE), Centre National de la Recherche Scientifique (CNRS), UMR 7330, Aix-Marseille University, 13545 Aix-en-Provence Cedex 4, France ⊥ Environmental & Resource Sciences Program and Department of Chemistry, Trent University, Peterborough, Ontario K9J 7B8, Canada §

S Supporting Information *

ABSTRACT: Sorption to iron (Fe) minerals determines the fate of the toxic metalloid arsenic (As) in many subsurface environments. Recently, thiolated As species have been shown to dominate aqueous As speciation under a range of environmentally relevant conditions, thus highlighting the need for a quantitative understanding of their sorption behavior. We conducted batch experiments to measure the time-dependent sorption of two S-substituted arsenate species, mono- and tetrathioarsenate, and compared it to the sorption of arsenite and arsenate, in suspensions containing 2-line ferrihydrite, goethite, mackinawite, or pyrite. All four As species strongly sorbed to ferrihydrite. For the other sorbents, binding of the thiolated As species was generally lower compared to arsenate and arsenite, with the exception of the near instantaneous and complete sorption of monothioarsenate to pyrite. Analysis of the X-ray absorption spectroscopy (XAS) spectra of sorbed complexes implied that monothioarsenate binds to Fe oxides as a monodentate, inner-sphere complex. In the presence of Fe sulfides, mono- and tetrathioarsenate were both unstable and partially reduced to arsenite. Adsorption of the thiolated As species to the Fe sulfide minerals also caused the substitution of surface sulfur (S) atoms by As and the formation of As−Fe bonds.

1. INTRODUCTION The reactivity, bioavailability, toxicity, and transport of arsenic (As) are linked to its chemical speciation. In iron (Fe)-rich environments, As sorption complexes to Fe minerals often play a critical role. In southern Asia, for instance, the release of As bound to Fe sulfides and Fe oxides present in sediments threatens the quality of groundwater used for drinking and irrigation.1 Hence, to assess the environmental fate of As, the sorption of aqueous As species to various Fe mineral phases should be fully understood. Sorption of arsenite ([HxAsIIIO3(3 − x)−](aq); AsIII) and arsenate ([HxAsVO4(3 − x)−](aq); AsV) by Fe oxides and hydroxides has been extensively studied2−7 and represented in geochemical models.2,8,9 Similarly, geochemical10−12 and spectroscopic13−16 studies have shown that As sorption to Fe sulfides is a potentially important As sequestration mechanism in anoxic environments, where FeIII oxides are unstable. However, AsIII and AsV sorption onto Fe sulfides has received far less attention, partly because of the experimental challenges inherent in maintaining strict oxygen-free conditions when conducting experiments with sulfides. To our knowledge, only one study provides values of equilibrium constants17 and isotherms for AsIII and AsV sorption onto disordered mackinawite (FeSm(s)) © 2013 American Chemical Society

in the pH range of 5.5−8.5. Despite recent advances in our understanding of the molecular-scale interactions between AsIII and Fe sulfides, there is still a lack of quantitative information to predict the macroscale behavior of As in the presence of Fe sulfides in subsurface environments. Although AsIII species generally dominate As speciation in reducing environments, there is mounting evidence indicating that thiolated AsV species, thioarsenates, can also exist in reducing Fe-rich18 or sulfide-rich waters.19 For example, tetrathioarsenate ([HxAsVS4](3 − x)−(aq), hereafter referred to as TTAsV) and monothioarsenate ([HxAsVSO3](3 − x)−(aq), hereafter referred to as MTAsV) can form through the oxidative dissolution of As-bearing sulfide minerals, such as orpiment (As2S3(s)) and arsenopyrite (FeAsS(s)),20 or possibly form directly in solution, for example, via the oxidation of AsIII by S0 at high S0/HS− and S/As ratios.21−23 Among thioarsenates, MTAsV is a remarkable species because it is thought to be relatively innocuous, as indicated by toxicity tests on Vibrio Received: Revised: Accepted: Published: 5652

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fisheri.24 MTAsV has been shown to dominate As speciation in various environmental systems, including the drainage channels of hot springs25,26 and natural Fe-rich groundwater.18 With the exception of a preliminary study20 revealing higher MTAsV mobility in goethite suspensions compared to AsIII and AsV, the effect of an increased replacement of O by S on the sorption behavior of AsV species has not been previously described. More information on the sorption behavior of thiolated As species is thus needed to assess the potential risk posed by As in the subsurface. Here, we compare the sorption of MTAsV to the sorption of aqueous As species with contrasting As/S ratios. Specifically, we exposed MTAsV, AsIII, AsV, and TTAsV to the following representative FeIII oxide and FeII sulfide minerals: 2-line ferrihydrite (2l-Fh), goethite (α-FeOOH), mackinawite (FeSm(s)), and pyrite (FeS2). These four Fe minerals were selected because of not only their ubiquity in the subsurface but also the existing data on sorption of AsIII and AsV to these mineral phases, as shown in Table SI-1 of the Supporting Information. We used the total aqueous As concentration as a function of time to extract equilibrium and kinetic parameters describing the sorption of As to the Fe minerals as well as X-ray absorption near-edge spectroscopy (XANES) and extended Xray absorption fine structure (EXAFS) spectroscopy to gain insight into the molecular structure of the As sorption complexes.

is commonly used when studying sorption of oxyanions to Fe minerals (see Table SI-1 of the Supporting Information); it is assumed to have no significant influence on the sorption reactions for pH ranging from 6 to 8.5.17 Suspensions of 2 g L−1 were prepared using the background solution in 150 mL serum bottles, amended with 1 mL of As solution to achieve initial aqueous As concentrations of 100 μM (7.5 mg L−1), crimp-sealed, and placed on a roller (Stovall Life Science). Subsamples were taken over a period of 48 h as follows: (i) immediately after injection of As (t = 0) and after 5 min, (ii) every 10 min during the first hour, (iii) every hour during 9 h, and (iv) at t = 24 and 48 h. The subsamples were syringefiltered (0.2 μm pore size polysulfone membranes, Acrodiscs) into 50 mL high-density polyethylene (HDPE) vials (Falcon). Subsamples were treated with NaOH/H2O2 32 and acidified with HCl (Aristar VWR) before analysis by flow injection hydride generation atomic fluorescence spectrometry (FI-HGAFS; PS Analytical Millennium Excalibur, U.K.). The analytical precision on As concentrations was ±6%, as determined from replicate analysis (n = 7) of the reference water SLRS-4 [National Research Council (NRC) Canada, certified As concentration of 9.1 ± 0.8 nM]. The limit of detection [3 standard deviations (SD)] of blank solutions was 3 nM. At the termination of the subsampling sequence, the suspensions were recovered and centrifuged at 4000g for 15 min. Measurement of pH of the supernatant confirmed that the pH of the suspensions remained at 7.00 ± 0.05 throughout all of the experiments. The solid was dried under a N2 atmosphere, homogenized, and sealed in amber 1.5 mL tubes (Eppendorf Safe-Lock). 2.2. X-ray Absorption Spectroscopy (XAS). Speciation of As associated with the final solids was examined by acquiring XANES and EXAFS spectra at the As K-edge (11 867 eV) on the bending magnet beamline BM-30b (FAME) at the European Synchrotron Radiation Facility (ESRF), in Grenoble, France. Precautions taken during sample handling are described in the Supporting Information. To limit As photo-oxidation/ reduction under the beam, analyses were performed at T < 15 K in a helium (He) cryostat and a motorized sample holder was used to illuminate a new spot on the sample for each acquisition. The X-ray energy resolution was maintained by a Si(111) monochromator, and the energy calibration was based on a Na2HAsVO4(s) standard. Between 6 and 15 spectra were collected in transmission mode using an ionization chamber (reference material) or in fluorescence mode (samples) using a 30 element array Ge solid-state detector (Canberra). Data reduction was performed using the software package EXAFS for Mac OS.33 EXAFS spectra were Fourier-transformed from k to R space using Kaiser−Bessel apodization windows. This procedure results in pseudo-radial distribution functions (RDFs) uncorrected for phase shifts, so that the positions are slightly shifted by ΔR ∼ −0.3 Å from the actual distances. Raw EXAFS spectra were least-squares fitted with a theoretical function to determine the structural and chemical parameters with the Artemis software,34 which uses theoretical standards from FEFF. 35 The validity of FEFF-derived amplitudes and phase functions for various backscattering atoms were ascertained by fitting pure reference compounds. The value of ΔE0 thus determined was then kept fixed to fit the samples. Structural parameters fitted for samples of unknown structure were distances between neighbors (Rj), number of closest neighbors (CNj), nature of atomic neighbors in the jth shell around the central atom, and the Debye−Waller factors

2. MATERIALS AND METHODS Unless otherwise specified, all manipulations were carried out under O2-free experimental conditions ( MTAsV > AsV; (2) αFeOOH, AsIII > AsV > TTAsV ≥ MTAsV; (3) FeSm(s), AsV ≥ AsIII > TTAsV ≫ MTAsV; and (4) FeS2, MTAsV > AsIII ≥ AsV ≫ TTAsV. Although the chemical properties of thioarsenates are still largely unknown,23 they clearly exhibit sorption properties that are distinct from those of the more widely studied arsenate and arsenite oxyanions. Given that the environmental relevance of thioarsenates is increasingly recognized,19−21,23−25,51,52 the results presented here provide much needed information on their potential mobility and their sorption mechanisms with Fe minerals. As shown above, in the presence of all Fe minerals but FeS2, MTAsV is the most mobile As species. In the presence of FeS2, however, aqueous MTAsV is removed to a greater extent than the other As species. Both MTAsV and FeS2 are found in geothermal systems and hydrothermal ore deposits, where AsIII is thought to convert to MTAsV, under slightly oxidizing conditions, as the activity of S increases.25 Spectroscopic evidence has shown that the co-precipitation of As with sedimentary Fe sulfide minerals may not be a dominant mechanism of As uptake in soils and sediments,53 while others have suggested that the presence of As could slow FeS2 formation.54 FeAsS is nevertheless fairly widespread, with As concentrations of up to 5 wt % in individual FeS2 grains, where As substitutes for S in the crystal structure.55 Although FeAsS formation is generally known to proceed at a high temperature,50 our results suggest that low-temperature FeAsS and arsenian pyrite formation could conceivably proceed through the reaction of Fe sulfide minerals with soluble As sulfur compounds, such as MTAsV. This mechanism for As sequestration in the Fe−As−S system requires further investigation, e.g., by performing FeS co-precipitation experiments in the presence of MTAsV and employing analytical techniques more specific to surface species, such as XPS.

predominantly in the reduced trivalent form (see Table SI-4 of the Supporting Information). While the As−O distance would limit the realistic number of neighboring O atoms in a single complex to 3 (±25%), the best fit of the EXAFS spectrum yields a CN value for As of 3.7. In addition, the Fe atoms found at 2.4 Å are associated with a CN of about 0.3. These results can be explained if some As atoms replace S at the FeSm(s) surface. If all of the As atoms were replacing S atoms in a single corner sharing, CN for Fe would be 1. A value of 0.3 thus suggests that only 30% of sorbed the As is substituting for S. It follows that the remaining As, likely bound to O, has an actual CN closer to 2.1 rather than 3.7. This discrepancy indicates that the value of CN bears an uncertainty of ±1.6 (i.e., ±43%). Arguably, the precise quantification of different sorption sites remains difficult solely based on a bulk technique, such as EXAFS. Surface-sensitive techniques, such as XPS, could potentially be used to further investigate thioarsenic sorption mechanisms. Nevertheless, our results allow us to conclude that As is partitioned between adsorbed and substituted at a corner site. For TTAsV sorption, the modeled As−S distance also indicates that a portion of sorbed As is reduced to the trivalent state (see Table SI-4 of the Supporting Information), as in the case of MTAsV. For both FeS(m)s and FeS2, TTAsV sorption results in the substitution of surficial S by As, as demonstrated by the presence of an As−Fe bond. At circumneutral pH, the kinetic lability of TTAsV27 and its reduction to trivalent As may explain the higher sorption capacity of FeS(m)s for TTAsV compared to MTAsV. Together, these results suggest that thioarsenics donate S atoms to the Fe sulfide, which changes As oxidation states and coordination. In terms of MTAsV sorption onto both FeSm(s) and FeS2, the EXAFS results suggest that the S atom, originally in the immediate vicinity of As, is replaced by one at a greater distance. Initially at a distance of 2.15 Å from the central As atom in the MTAsV molecule (see Table SI-3 of the Supporting Information), S is found at distances >3 Å in the sorption complexes. For the sorption onto FeS(m)s, we suggest that MTAsV is enriching the solid phase in S, possibly donating a S0 atom to FeS(m)s to form FeS2 as follows: HAs V SO32 −(aq) + FeSm(s) = As III O33 −(aq) + FeS2 + H+ (1)

Using ΔfG° values reported in the literature for the formation of MTAs V,22 FeS m(s),45 AsIII,46 and FeS 2 (using the S0 pathway45), a ΔrG value of −110.3 kJ mol−1 is calculated for this reaction, showing that this reaction is thermodynamically favorable at pH 7. For the sorption of MTAsV onto FeS2, a possible reaction would yield arsenian pyrite. On the basis of the previously reported simultaneous presence of MTAsV and thiosulfalte (S2O32−) during the oxidative dissolution of FeAsS,20 a possible reaction for the sorption of MTAsV onto pyrite would be HAs V SO32 −(aq) + FeS2 = FeAsS + S2 O32 −(aq) +

1 H 2(g) 2

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ASSOCIATED CONTENT

* Supporting Information

This reaction is also favorable, with a ΔrG value of −65.9 kJ mol−1, calculated using ΔfG° values reported for FeAsS (arsenian pyrite47) and S2O32−(aq).48 Although these mechanisms are simplifications of the complex surface chemistry in play at the interface of Fe sulfides, they are consistent with

S

Details on suspension preparation and sample transport and preparation to the beamline, results of XANES LCF, EXAFS parameters determined by shell fitting, and differences between 3- and 5-shell EXAFS fits compared to the noise level for 5657

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MTAsV sorption onto 2l-Fh, α-FeOOH, and FeSm(s). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +47-98-28-64-78. Fax: +47-22-18-52-00. E-mail: [email protected]. Present Address †

R.-M. Couture: Norwegian Institute for Water Research (NIVA), N-0411 Oslo, Norway. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank E. Torres-Sanchez and D. Borschneck for research assistance, as well as K. Mueller, C. Parsons, and C. Smeaton for editing the manuscript. We acknowledge the ESRF for provision of synchrotron facilities and thank D. Testemale for assistance at the beamline BM30b. R.-M. Couture was financially supported by a postdoctoral fellowship from the Fonds de recherche du QuébecNature et technologies (FRQNT). P. Van Cappellen acknowledges funding from the Canada Excellence Research Chair Program.

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