Microbial Reduction of Arsenic-Doped Schwertmannite by Geobacter

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Microbial Reduction of Arsenic-Doped Schwertmannite by Geobacter sulf urreducens Richard S. Cutting,†,* Victoria S. Coker,† Neil D. Telling,†,‡ Richard L. Kimber,† Gerrit van der Laan,§,† Richard A. D. Pattrick,† David J. Vaughan,† Elke Arenholz,∥ and Jonathan R. Lloyd† †

Williamson Research Centre for Molecular Environmental Science, and School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, U.K. ‡ Institute for Science & Technology in Medicine, Guy Hilton Research Centre, Keele University, Stoke-on-Trent, U.K. § Magnetic Spectroscopy Group, Diamond Light Source, Didcot, Oxfordshire, U.K. ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States S Supporting Information *

ABSTRACT: The fate of As(V) during microbial reduction by Geobacter sulfurreducens of Fe(III) in synthetic arsenic-bearing schwertmannites has been investigated. During incubation at pH7, the rate of biological Fe(III) reduction increased with increasing initial arsenic concentration. From schwertmannites with a relatively low arsenic content (0.79 wt %) resulted in the formation of goethite. At no stage during the bioreduction process did the concentration of arsenic in solution exceed 120 μgL1, even for a schwertmannite with an initial arsenic content of 4.13 wt %. This suggests that the majority of the arsenic is retained in the biominerals or by sorption at the surfaces of newly formed nanoparticles. Subtle differences in the As K-edge XANES spectra obtained from biotransformation products are clearly related to the initial arsenic content of the schwertmannite starting materials. For products obtained from schwertmannites with higher initial As concentrations, one dominant population of As(V) species bonded to only two Fe atoms was evident. By contrast, schwertmannites with relatively low arsenic concentrations gave biotransformation products in which two distinctly different populations of As(V) persisted. The first is the dominant population described above, the second is a minority population characterized by As(V) bonded to four Fe atoms. Both XAS and XMCD evidence suggest that the latter form of arsenic is that taken into the tetrahedral sites of the magnetite. We conclude that the majority population of As(V) is sorbed to the surface of the biotransformation products, whereas the minority population comprises As(V) incorporated into the tetrahedral sites of the biomagnetite. This suggests that microbial reduction of highly bioavailable As(V)-bearing Fe(III) mineral does not necessarily result in the mobilization of the arsenic.

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INTRODUCTION Schwertmannite (Fe8O8(OH)8−2x(SO4)x with 1 ≤ x ≤ 1.75) [4] is a poorly crystalline, high surface area iron oxyhydroxysulfate mineral which precipitates from the acidic (pH 3 - 4.5) high-sulfate (1000−3000 mg L−1) waters that issue from mine workings, waste dumps and tailings ponds.1,2 It is also ubiquitous in coastal lowland acid-sulfate soils.3,4 Schwertmannite is capable of sequestering a range of metals from aqueous solution2,5 and has been reported to remove arsenic from waters in acid streams6,7 and waters percolating through mine tailings.8 In acidic, oxidizing environments, adsorbed, or coprecipitated species associated with schwertmannite are regarded as immobile.9 However, temporal variation in geochemical conditions, subsequent transport, burial and microbial action may promote desorption/ dissolution reactions, resulting in the release of contaminants, including arsenic.9 Both the Fe(III) and SO42‑ in schwertmannite have the potential to serve as terminal electron acceptors for © 2012 American Chemical Society

bacterial metabolism, and although it has been previously reported that Fe(III) bound within synthetic schwertmannite is not available for reduction by Geobacter species;9 however, there are reports of microbial reduction using other organisms.10,11 Since dissimilatory iron reducing bacteria (DIRB) can respire using the Fe(III) from schwertmannite, the metabolic activity of these organisms could promote mineral dissolution, releasing adsorbed or coprecipitated contaminants. The impact of this process could be mitigated in part by the metabolic activities of sulfate-reducing bacteria (SRB); they have the potential to respire using the SO42‑ liberated from the schwertmannite, thereby generating sulphide Received: Revised: Accepted: Published: 12591

January 30, 2012 September 18, 2012 October 8, 2012 October 8, 2012 dx.doi.org/10.1021/es204596z | Environ. Sci. Technol. 2012, 46, 12591−12599

Environmental Science & Technology

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precipitates were then dried anaerobically under N2−H2 (98:2) gas prior to characterization. Analytical Techniques. X-ray diffraction (XRD) measurements were performed using a Bruker instrument (D8Advance) with a Cu-Kα1 source. Diffraction data were acquired over a 2θ range of 15−70° with a counting time of 4 s and a step size of 0.02°. The relative proportions of goethite and magnetite were estimated by comparison of the full width at half-maximum height of the (110) and (220) reflections respectively, with the latter multiplied by a weighting factor of 3.3. 27 The bulk elemental compositions of powder samples, both before and after incubation with G. sulf urreducens, were determined by X-ray fluorescence (XRF) analysis using a Panalytical Axios instrument equipped with a rhodium anode. Spectra were analyzed using a commercial software package (SuperQ-4) and elemental compositions were calculated as a weight percent of the corresponding oxide (Fe2O3 for iron and SO42‑ for sulfur, although As is reported as elemental arsenic). The concentrations of iron and arsenic in solution were determined using ICP-AES (Perkin-Elmer, Optima 5300DV) from acidified samples (2% HNO3 Analar, BDH) that were stored in the dark at 4 °C. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were acquired using a Phillips/FEI CM200 microscope equipped with a field emission gun, EDX system (Oxford Instruments UTW ISIS) and a Gatan imaging filter. An operating beam voltage of 200 keV was employed and a droplet of each of the dispersions was placed on holey carbon grids (Agar Scientific) and allowed to dry prior to imaging. X-ray photoelectron spectroscopy (XPS) data were recorded using a Kratos instrument (Axis Ultra) employing a monochromated Al-Kα X-ray source and an analyzer pass energy of 20 eV, resulting in a total energy resolution of ca. 0.9 eV. Samples were loaded into the spectrometer via a dry nitrogen glovebox to avoid exposure to atmospheric oxygen. Photoelectron binding energies (BE) were referenced to C 1s adventitious contamination peaks set at 285 eV BE. (See Supporting Information for details of analyzer calibration and data fitting). X-ray absorption (XAS) spectra were obtained on beamline 4.0.2 at the Advanced Light Source (Berkeley, CA) using an octopole magnet end station.28 Precipitates were analyzed to assess the effect of arsenate and the presence of AQDS on the speciation of iron. X-ray magnetic circular dichroism (XMCD) spectra of iron (FeL2,3) were also obtained to give information on the site occupancies of iron in “magnetic” products of biogenic synthesis (see refs 29−31 for details). The specific surface areas of the schwertmannite precipitates were measured using the Brunauer−Emmett−Teller (BET) N2 adsorption method.32 Nitrogen adsorption/desorption isotherms were recorded using a commercial instrument (Micromeritics Gemini), with helium as the carrier gas at liquid nitrogen temperature (77 K). Prior to measurement, the powder samples were degassed in a stream of N2 at 50 °C for 24 h (Micromeritics Flowprep 060). The specific surface areas of samples were calculated using the multipoint analysis method.

and resulting in localized reprecipitation of released metals as insoluble sulfides. In the present study, we have determined the fate of arsenic during the extended incubation of AsO43‑-doped synthetic schwertmannites with the Fe(III)-reducing bacterium Geobacter sulfurreducens using environmentally relevant arsenic concentrations up to 5 wt %.12 Previous studies have shown that this organism is not able to conserve energy for growth via the dissimilatory reduction of As(V) but is able to tolerate up to 500 μM As(V) in solution, despite an absence of genes encoding a classical arsenic resistance operon.13 We have chosen to perform these experiments at circum-neutral pH, bearing in mind that weathering processes may buffer groundwater to around neutral pH depending on the rocktypes present (see Blodau (2006) for reviews14). These experiments also allow the study of the reduction of schwertmannite by Geobacter species, identified in schwertmannite-containing sediments that were highly susceptible to bioreduction.15 The detailed characterization of biomineral products from these incubation experiments, in conjunction with the analysis of associated liquids, has allowed the impact of a range of initial arsenic concentrations to be assessed. The present work builds upon a recent body of research published by other authors and including abiotic and microbial reduction experiments (see, for example, refs 16−21).

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MATERIALS AND METHODS As-Bearing Schwertmannite Synthesis. Schwertmannite was synthesized using a modified version of the method described by Schwertmann and Cornell,22 as described previously.23 Solutions were prepared in which AsO43‑ ions replaced varying proportions of SO42‑ ions that had an initial concentration of 1000 mg L−1. Schwertmannites were precipitated from solutions with AsO43‑ concentrations of 0 mg L−1, 5 mg L−1, 20 mg L−1, 50 mg L−1, and 100 mg−1. The resulting suspensions were purified by discarding any supernatant and centrifuging the remaining materials, which were then redispersed in 18.2MΩ water using an ultrasonic probe (Misonix, Microson XL). The precipitates were then freezedried and stored in sterile vessels in the dark and under N2 atmosphere. Aliquots of each synthetic schwertmannite powder were removed for analysis using X-ray fluorescence (XRF) and X-ray diffraction (XRD) techniques, prior to use in bacterial reduction experiments. Bioreduction Experiments. G. sulf urreducens was grown under anaerobic conditions at 30 °C in modified fresh water medium.24 Sodium acetate (20 mM) and fumarate (40 mM) were added as the electron donor and acceptor, respectively. All manipulations were performed under an atmosphere of N2− CO2 (80:20). Late log-phase cultures of G. sulfurreducens were harvested by centrifugation and washed twice in bicarbonate buffer (NaHCO3; 30 mM, pH 7.1). Aliquots of the washed cell suspension (0.3 mL) were added to sealed anaerobic bottles containing bicarbonate buffer (30 mM), sodium acetate (20 mM), anthraquinone-2,6-disulfonate (AQDS; 10 μM) and 0.1g of arsenic-bearing schwertmannite where appropriate, to give a total volume of 10 mL. The final concentration of bacteria was 0.6 mg protein per mL. Bottles were incubated at 20 °C in the absence of light. Analyses were carried out at regular intervals to determine the concentration of 0.5 M HCl-extractable Fe(II) using the ferrozine method.25,26 Following 170 h of incubation, the precipitates were washed in anaerobic deionized water and separated from the supernatant by centrifugation. The biogenic

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RESULTS AND DISCUSSION Characterization of As-Bearing Schwertmannite Starting Materials. The As-bearing schwertmannites synthesized in this study were all poorly crystalline (see Figure.1) and 12592

dx.doi.org/10.1021/es204596z | Environ. Sci. Technol. 2012, 46, 12591−12599


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discernible effect on the morphology of the schwertmannite, although the average particle size decreases as a function of increasing As content. BET-N2 analyses showed that the As-bearing schwertmannites have similar specific surface areas to those reported previously by refs 34,35, within the range of 100−300 m2g−1. For the schwertmannite that contains no arsenic, XRF analyses gave values of 79.9 wt % Fe (as Fe2O3) and 16.7 wt % S (as SO42‑) in agreement with values published elsewhere.36 The arsenic concentrations of the other schwertmannites varied from 0.30 wt % to 4.13 wt % for materials precipitated from solutions with initial arsenic concentrations of 5 mg L−1 to 100 mg L−1, respectively. XRF measurements showed that the concentration of sulfur (present as SO42‑) decreased with increasing arsenic concentration, indicating that the latter replaced the former. The arsenic concentrations for the schwertmannites investigated here are summarized in Table 1 (also see Supporting Information; Figure S1). Table 1. Initial Arsenic Concentrations of the Synthetic AsBearing Schwertmannite Starting Materials (wt%)(a) along with Specific Surface Areas of the As-Schwertmannite Starting Materials (m2g−1)(b), Maximum Acid Extractable (0.5 M HCl) Fe2+ Content (mM) Generated When AsSchwertmannites Were Incubated for 170 h with G. sulf urreducens in the Presence of Acetate(c) and Initial Rates of Bacterial Fe(III) Reduction (Mm−2min−1) for the AsSchwertmannites in the Presence of Acetate(d)

sample

(a) arsenic concentration in schwertmannite starting materials (wt%)

(b) Specific surface area of Asschwertmannite (m2g−1)

maximum biogenic 2+ Fe content acetate (mM)

1 2 3 4 5

0 0.30 0.79 2.30 4.20

161.5 186.0 197.3 203.6 212.2

6.0 11.8 12.3 12.8 13.5

(c)

(d)

initial rate of Fe(III) reduction acetate only (Mm‑2min−1) 2.0 2.4 2.1 2.0 1.6

× × × × ×

10−5 10−5 10−5 10−5 10−5

XPS survey spectra show that, other than adsorbed adventitious carbon, the only elements present in the samples were iron, oxygen, sulfur and arsenic. High-resolution XPS data showed that, in all of the schwertmannites, the principal Fe2p3/2 peak occurs at 711.7 eV BE (see Supporting Information; Figure S2(b)), with an additional relatively intense satellite at 720.0 eV BE, all characteristic of Fe(III).37 The observed Fe2p spectra, and the absence of any satellite feature at ∼715 eV BE, show that there was no Fe(II) present in the near-surface regions of any of the synthetic schwertmannites prior to their exposure to G. sulf urreducens. Bioreduction of As-Bearing Schwertmannites. Fe(III) in all of the synthetic schwertmannite samples investigated here proved susceptible to bioreduction resulting in the accumulation of 0.5 M HCl acid extractable Fe(II) when incubated with resting cell cultures of G. sulf urreducens (see Figure 2). For all samples, the extractable Fe(II) concentrations reached a steady state maximum after 70 h of incubation, although the absolute concentrations of this Fe(II) measured varied depending on the initial arsenic content of the schwertmannite (see Figure 2 and Table 1). The concentration of 0.5 M HCl acid-extractable Fe(II) increased with increasing arsenic, reaching a maximum concentration in solution of 13.5 mM for schwertmannite that initially contained 4.13 wt % As. This

Figure 1. XRD data obtained from As-bearing schwertmannites with initial arsenic concentrations of (i) 0 wt %, (ii) 0.3 wt %, (iii) 0.79 wt %, (iv) 2.3 wt %, and (v) 4.13 wt %, and their respective biotransformation products obtained following 170 h of incubation with G. sulf urreducens in the presence of acetate. In each set of two traces, the lower trace is from the sample prior to incubation, while the upper trace is the sample incubated in the presence of acetate.

structurally similar to previously studied natural schwertmannites.2 TEM images of schwertmannite that contained no arsenic showed a mixture of randomly orientated “whiskers”, ∼30 nm in length and less than ∼5 nm in width. Individual whiskers were polycrystalline and lacked clearly defined lattice fringes. SAED patterns were dominated by diffuse, discontinuous rings (see23) that were consistent with the peaks seen in the corresponding XRD data (see Figure 1) and with values reported previously.33 Comparison of numerous TEM images showed that varying the concentration of arsenic has no 12593



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that the biotransformation process did not result in the liberation of appreciable arsenic into solution (see Figure 3).

Figure 3. Total As in solution measured by ICP-AES at three time points (t = 0, 48, and 170 h) obtained during the incubation of the synthetic As-schwertmannites that initially contained 0, 0.3 wt %, 0.79 wt %, 2.3 wt % and 4.13 wt % of arsenic, with G. sulfurreducens in the presence of acetate. Data obtained from controls that contained no added cells are also shown.

Figure 2. Graphs showing extractable Fe(II) (0.5 M HCl) concentrations as a function of time obtained during the incubation of the synthetic As-schwertmannites that initially contained (a) 0 wt %, (b) 0.3 wt %, (c) 0.79 wt %, (d) 2.3 wt %, and (e) 4.13 wt % of arsenic with G. sulf urreducens in the presence of acetate. Data obtained from controls that contained no added cells are also shown.

For systems that contained schwertmannite with a relatively low initial arsenic concentration (0.3 wt %), biotransformation resulted in an aqueous arsenic concentration of only 9.0 μg L−1 after 170 h incubation in the presence of the electron donor acetate. By contrast, incubation of the schwertmannite with a high initial arsenic concentration (4.13 wt %) under identical conditions resulted in an aqueous arsenic concentration of 33.0 μg L−1 compared with control samples that contained no added cells in which the concentration of aqueous arsenic exceeded 200 μgL−1 (see Figure 3). These data suggest that, rather than liberating arsenic, the bioreduction process actually promotes arsenic retention and prevents “build-up” of soluble arsenic in aqueous solution. A similar process was seen to occur through both the aging of schwertmannite in the presence of arsenic over several months and through the Fe(II) catalyzed transformation of schwertmannite, where new mineral phases formed and sequestered the contaminant.18,20 This finding has important implications for mechanisms of arsenic sequestration as discussed in more detail below. Characterization of the Biogenic Fe Minerals. XRD data show that the initial concentration of arsenic has an impact on the Fe biomineralization process (Figure 1). For schwertmannites that contained either no arsenic or a relatively low arsenic concentration (0.3 wt %, 0.79%), extended incubation (170 h) with G. sulf urreducens resulted in materials that gave XRD traces which were dominated by the (311) and (220) reflections of magnetite (see Figures 1(i), (ii), (iii)). The incubation of schwertmannite that contained 2.3 wt % of arsenic yielded a mixture of magnetite (80%) and goethite (20%) (see Figure 1(iv)), whereas for schwertmannite, with an initial arsenic concentration of 4.13 wt %, goethite was the only crystalline phase detected (see Figure 1(v)). This apparent contradiction suggests that the Fe(II) produced by bioreduction is sorbed to the surfaces of the solids present in these examples, as further discussed below.

suggests that the replacement of SO42‑ by AsO43‑ in the schwertmannite structure renders it more susceptible to direct bacterial reduction, most likely through a disruption of the structure of the schwertmannite, as noted previously.21,38 Comparison of BET-N2 data (Table 1) indicates that surface areas of the As-doped schwertmannites increased as a function of increasing As content. TEM images showed that the increase in surface area was accompanied by a decrease in the size of the schwertmannite particles. These features may be a result of disruption of the schwertmannite structure in response to an increase in the AsO43‑:SO42‑ ratio, leading to further changes in the crystallinity and stability of this poorly crystalline mineral. The rates of bacterial Fe(III) reduction, normalized to account for differences in initial surface area, show little variation in response to increasing As concentration. The minimum value of 2.0 × 10−5 mM m−2min−1 was obtained for the schwertmannite that contained no arsenic, whereas the maximum value of 2.4 × 10−5 mM m−2min−1 was determined for schwertmannite that initially contained 0.30 wt % of arsenic (see Table 1). Analysis of the solutions found that there was release of 65% of the sulfate over the first 24 h of incubation (see Supporting Information; Figure S3) indicating a partial breakdown in the schwertmannite structure. However, there was nearly 20% more sulfate released into solution when bacteria were present. This could be due to the additional breakdown of schwertmannite by bacteria during the course of Fe(III)reduction. XRD (see Supporting Information; Figure S4) of an abiotic control indicates that the material remains a low crystallinity solid over the time scale of the experiments. Extended incubation of the As-bearing schwertmannites (including controls that contained no added cells) showed 12594



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Table 2. Bulk Arsenic Concentration (%) Relative to Total Fe Content of As-Bearing Schwertmannite Starting Materials(a), and of the Biotransformation Products Obtained Following 170 h Incubation with G. sulf urreducens in the Presence of Acetate(b) Determined by XRFa (a)

Sample 1 2 3 4 5

Bulk %As relative to total Fe As-schwertmannite starting material (XRF) 0 0.45 0.96 2.94 4.82

bulk %As relative to total Fe 170 h acetate (XRF)

surface Fe(III):Fe(II) As-schwertmannite starting materials (XPS)

(d) surface Fe(III):Fe(II) 170 h acetate (XPS)

0 0.43 0.92 2.91 4.81

100:0 100:0 100:0 100:0 100:0

79:21 84:16 82:17 86:14 89:11

(b)

(c)

(e)

(h) surface %As relative to total surface %As Fe As-schwertmannite relative to total Fe starting material (XPS) 170 h acetate (XPS)

0 2 6 12 25

0 5 8 15 21

a

Surface Fe(III):Fe(II) ratios of the As-schwertmannite starting materials(c) and the biotransformation products obtained following 170 h incubation with G. sulfurreducens in the presence of acetate(d), are also shown as determined by XPS. Surface arsenic concentrations (%) are given relative to total Fe content of the As-schwertmannite starting materials(e), the biotransformation products obtained following 170 h incubation with G. sulfurreducens in the presence of acetate as determined by XPS.

High-resolution XPS data show that arsenic is associated with the near-surface regions (to depths of ∼60 Å) of all of the biotransformation products formed when As-bearing schwertmannites are incubated with G. sulfurreducens (see Supporting Information; Figure 2(c) and (e)). In all instances, the As3d peak occurs at 45.8 eV BE, which is identical to the position of the As3d features associated with the starting materials (see Supporting Information; Figure S2(a)) and is consistent with previously reported values for As(V).42−44 Thus, as expected for processes involving G. sulfurreducens,13 we conclude that the oxidation state of arsenic remains unchanged throughout the biotransformation process, with all arsenic present as As(V).42−44 Based on the lack of a binding energy shift, we also conclude that As(V) occurs in a similar chemical environment, regardless of whether it is associated with the As-bearing schwertmannite starting materials, biogenic magnetite or biogenic goethite. Comparison of bulk and surface arsenic contents determined using XRF and XPS methods clearly show that the arsenic associated with an As-bearing schwertmannite starting material is concentrated in the near-surface regions of the particles (see Table 2). Bioreduction of As-bearing schwertmannites by G. sulf urreducens also yields products in which the arsenic is concentrated in the near-surface regions (see Table 2); this is accompanied by a corresponding increase in the concentration of aqueous arsenic during incubation (see Figure 3). All of the biotransformation products produced during this study exhibit depleted bulk arsenic concentrations relative to the As-bearing schwertmannite starting materials, although the level of depletion is comparatively small and is accompanied by enrichment in near-surface arsenic. Arsenic not present in the solid phase mineral after biotransformation has taken place is assumed to have been released into aqueous solution, although this process must represent a relatively minor pathway since the concentration of aqueous arsenic did not exceed 120 μgL−1 at any stage (see Figure 3). Fe L 2,3 -edge XAS and XMCD data obtained from biotransformation products are consistent with the XPS data discussed above. Fe L3-edge XAS spectra develop a pronounced shoulder at ∼708 eV as the arsenic concentration of the schwertmannite starting material increases (see Figures 4(a) and (c)). Previous studies have shown that this shoulder arises as a result of the presence of Fe3+.45 The increasing intensity of this feature as a function of increasing initial arsenic content, reflects an increasing proportion of Fe(III) within the biotransformation products. Spectra from the biotransforma-

TEM images of the biotransformation products were consistent with the XRD data. Incubation of a schwertmannite that initially contained 0.3 wt % of arsenic yielded crystalline magnetite, with particles of a roughly spherical morphology and an approximate diameter of 20−30 nm (see Supporting Information; Figure S5(a)). This material is indistinguishable from the biotransformation product obtained when schwertmannite that contained no arsenic was incubated under identical conditions.23 By contrast, TEM images of the products obtained when schwertmannite that initially contained 4.13 wt % arsenic was incubated under identical conditions show acicular laths ∼150 nm in length and ∼40 nm in width (see Supporting Information; Figure S5(b)). Detailed analysis of Fe2p XPS spectra obtained from the same biotransformation products revealed subtle differences in the surface (or near-surface) Fe(III):Fe(II) ratios, with values of 79:21 and 84:16, respectively, when schwertmannites that contained no arsenic and 0.3 wt % arsenic were incubated with acetate (see Table 2). Fitting of the Fe2p XPS data for the biotransformation products obtained from experiments with schwertmannite containing 0.79 wt % As gave Fe(III):Fe(II) ratios of 82:17 (see Table 2 and Supporting Information; Figure S1(d) and (f)). The schwertmannites that contain higher initial arsenic concentrations of 2.3 wt % and 4.13 wt % produced biotransformation products with Fe(III):Fe(II) ratios of 86:14 and 89:11. However, despite the accumulation of Fe(II) at the surfaces of the transformation products, the concentration of this species appears to be insufficient to bring about the onset of magnetite formation.39 Rather, conditions favor the formation of a layer of sorbed biogenic Fe(II), which appears to passivate the mineral surface to some extent, such that the reducing power (e− flux) is insufficient to drive the conversion of goethite to magnetite,40,41 (see Table 2 and Supporting Information; Figure S1(d) and (f)). In keeping with previous studies, abiotic controls indicated that the loss of sulfate and arsenate from the schwertmannites occurred relatively rapidly under the experimental conditions employed here (hours) (See Supporting Information; Figure S3).20 However, XRD analysis of residual solid sampled from abiotic controls at different stages of the experiment indicated that neither goethite nor magnetite formed on the time scale of these studies. Fate of Arsenic in Postreduction Iron Biominerals. Despite the variability in both the bulk mineralogy and the near-surface Fe(III):Fe(II) ratios of the biotransformation products, the As3d XPS peak position remains unchanged. 12595



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the Fe cations compared to biogenic magnetite produced in the absence of As (see Figure S6). Fitting of the data using calculated spectra31,47 allowed the relative proportions of each component to be quantified. Site occupancy data presented in Table 3 shows that the ratio of Fe on Td to Oh sites does not change appreciably through the sample series. However, the Fe2+:Fe3+ ratio decreases from 0.66 for biogenic pure magnetite to values varying between 0.49 and 0.58 with the addition of As. The decrease in the Fe2+:Fe3+ ratio is nonlinear with respect to increasing As concentration, suggesting that more than one factor affects the structure of the magnetite and causes these changes. Examination of XANES spectra (see Figure 5) and

Figure 4. (a) XAS data and (b) XMCD spectra measured over the FeL2,3 absorption edges for biogenic precipitates obtained by incubating synthetic As-schwertmannites with G. sulfurreducens for 170 h with acetate only. The spectra are obtained from samples with schwertmannite arsenic concentrations that increase from the bottom to top of each figure and are, respectively: 0, 0.3, 0.79, 2.3, and 4.13 wt %.

tion products of schwertmannites with higher initial arsenic concentrations resemble those obtained from goethite46 in agreement with XRD data (see Figure 1). Thus, we conclude that this shoulder reflects increasing Fe(III) content in response to increased initial arsenic concentration. This interpretation is in line with the decrease in the normalized amplitude of the Fe L2,3-edge XMCD spectra, which have line shapes consistent with the presence of magnetite47 and are associated with increasing initial arsenic content (see Figure 4(b)). The shoulder on the Fe L3-edge XAS was not clearly distinguishable from the primary Fe adsorption peak until an initial arsenic concentration of 4.13 wt % was reached (see Figure 4(a)). The increasing intensity of this shoulder coincides with a decrease in the normalized amplitude of the Fe L2,3-edge XMCD spectra associated with magnetite (see Figure 4(b)). In keeping with XRD results, XAS and XMCD data suggest that the formation of biogenic magnetite by G. sulfurreducens is inhibited by greater initial arsenic concentrations. This produces an increase in the proportion of goethite relative to magnetite within the biomineralization products, with excess biogenic Fe(II) presumably forming an adsorbed layer at the surfaces of new formed biogenic precipitates. For schwertmannite samples that contained no or relatively low initial concentrations (0.3 wt %) of arsenic, magnetite forms readily during incubation with G. sulfurreducens (see Figures 1(i),(ii)). Comparison of Fe L2,3-edge XMCD data obtained from the biotransformation products of these schwertmannites shows a change in the site occupancies of

Figure 5. Normalized As K-edge XANES spectra of (i) sodium arsenate, and (ii) sodium arsenite standards (from Coker et al., 2006) and for the end point of the incubation of synthetic Asschwertmannites that initially contained (iii) 0.79 wt % and (iv) 4.2 wt % of arsenic with G. sulf urreducens in the presence of acetate.

EXAFS data (Table 4 and Supporting Information Figure S7) obtained from samples with initially 0.79 and 4.2 wt % As suggest a reason for the observed differences in XMCD results. In Figure 5, the As K-edge XANES spectra obtained from biotransformation products formed from schwertmannites with initial As concentrations of 0.79 wt % and 4.2 wt %, are compared with standard arsenite and arsenate spectra. The spectrum obtained from the biotransformation of schwertmannite with an initial As concentration of 0.79 wt % exhibits two additional features at energies above the main K-edge peak, and located at 11882 and 11894 eV. These feature are absent in the equivalent spectrum obtained from the biotransformation products of schwertmannite with an initial As concentration of

Table 3. Occupancies of the Lattice Site of Magnetites Formed by the Bioreduction of As-Bearing Schwertmannites by G. sulf urreducens, as Determined by Fitting the Series of XMCD Spectra Shown in Figure 5a site occupancy sample

As concentration in schwertmannite (wt%)

1 2 3 4

0 0.30 0.79 2.30

Fe

2+

Oh

1.19 0.99 1.10 1.05

Fe3+ Td

Fe3+ Oh

total Fe cations

Fe2+:Fe3+ ratio

Td:Oh ratio

0.89 0.94 0.91 0.91

0.92 1.07 1.00 1.05

3.00 3.00 3.00 3.00

0.66 0.49 0.58 0.54

0.42 0.45 0.43 0.43

It was assumed that the number of cations remains fixed throughout the series (at 3.0). Also shown are the ratios of Fe2+ to Fe3+ and tetrahedral to octahedral site occupancies.

a

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of schwertmannite reduction suggest that, for comparatively low arsenic concentrations (0.3 wt %), As(V) can replace iron in the magnetite lattice. For schwertmannites with higher initial arsenic concentrations, goethite is formed and it appears that this As is sorbed to the mineral surface. In this case, XMCD cannot be used to probe site occupancies as it is a method dependent on the presence of magnetic ordering. Hence, we cannot discount the possibility that a proportion of the arsenic present in the starting material occupies sites in the goethite structure. Most significantly, only small quantities of arsenic are released to aqueous solution during the biotransformation of As-bearing schwertmannites (≤120 μgL−1) relative to corresponding control samples. The question of structural incorporation versus surface adsorption of arsenic remains of critical importance.

Table 4. Results Obtained from the Fitting of As K-Edge EXAFS Spectra of Bioreduced Samples Containing Initially 0.79 and 4.20 wt % Asa sample

initial As concentration in schwertmannite

scatterer

N

r (Å)

2σ2 (Å2)

R factor

3 3 5 5

0.79 0.79 4.20 4.20

O Fe O Fe

4 4 4 2

1.68 3.43 1.68 3.38

0.004 0.017 0.005 0.023

27.2 23.4

a

Shown are the types and numbers (N) of scatterers, their radial distances from the central absorber (r in Å), the Debye-Waller factor (2σ2 in Å2) and a goodness of fit parameter (R-factor).

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4.2 wt %, indicating that the As(V) is not bonded in the same way for both samples. However, the second shell information differs. For the schwertmannite with an initial As concentration of 4.2 wt %, it reveals a single population of As species bonded to only two Fe atoms, at a distance of 3.38 Å. This is consistent with an inner sphere bidentate arsenate complex corner-sharing with iron octahedral,48−50 and indicates that the overwhelming majority of the As(V) is sorbed to the surface of the biotransformation products. The lack of improvement in fit obtained following the addition of a shell of oxygens at ca. 1.79 Å or sulfurs at ca. 2.25 Å, supports this interpretation as any reduced As species must therefore be present at low concentrations. By contrast, the spectrum obtained from the biotransformation products of schwertmannite with a lower initial As concentration (0.79 wt %) indicates that As(V) is bonded to four Fe atoms, at a bond distance of 3.43 Å. It is possible that some of the As(V) is incorporated into the tetrahedral sites of the magnetite, as previously reported.51 The site occupancy data obtained from the XMCD results (Table 2) suggest a deficiency in occupation of the tetrahedral sites, which could be related to uptake of As into those sites of the magnetite. Our data suggest that biogenic nanomagnetite has a maximum capacity with respect to As(V) incorporation. This threshold value may be somewhat lower than the As concentration associated with schwertmannite with an initial As concentration of 0.79 wt %, for which two As(V) populations contribute toward the EXAFS spectrum. For schwertmannite with a higher initial As concentration of 4.2 wt %, the As EXAFS signal obtained from the biotransformation products is dominated by the sorbed As(V) component, masking any contribution from the minority population of incorporated As(V). The findings of this study have implications for understanding aspects of the behavior of arsenic in the natural environment. As the arsenic concentration of the synthetic schwertmannite is increased, the nature of the biomineralization products varies, with goethite being produced in increasing amounts relative to magnetite. In natural systems, several factors in addition to arsenic concentrations may alter the biomineralization end points. These could include very high concentrations of Fe(III) which could potentially maintain oxidizing conditions, inhibiting the formation of magnetite, and also the pH of the system under study, which could impact on both the microbial communities present and the biogeochemical processes taking place. For schwertmannite with an initial As concentration of 0.3 wt %, magnetite readily forms when incubated with acetate. Analysis of XAS and XMCD data obtained from the products

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S7 are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of EPSRC and BBSRC in funding this research via grants EP/D058767/1 and BB/E004601/1 is gratefully acknowledged. Special thanks are due to Dr. Paul Wincott and Ms. Catherine Davies for advice and assistance with aspects of this work. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.

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REFERENCES

(1) Bigham, J. M.; Carlson, L.; Murad, E. Schwertmannite, a new iron oxyhydroxysulfate from Pyhasalmi, Finland, and other localities. Miner. Mag. 1994, 58, 641−648. (2) Acero, P.; Ayora, C.; Torrento, C.; Nieto, J. M. The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite. Geochim. Cosmochim. Acta 2006, 70, 4130−4139. (3) Burton, E. D.; Bush, R. T.; Sullivan, L. A. Sedimentary iron geochemistry in acidic waterways associated with coastal lowland acid sulfate soils. Geochim. Cosmochim. Acta 2006, 70 (22), 5455−5468. (4) Collins, R. N.; Jones, A. M.; Waite, T. D. Schwertmannite stability in acidified coastal environments. Geochim. Cosmochim. Acta 2012, 74, 482−496. (5) Waychunas, G. A.; Xu, N.; Fuller, C. C.; Davis, J. A.; Bigham, J. M. XAS study of AsO43‑ and SeO42‑ substituted schwertmanites. Phys. B (Amsterdam, Neth.) 1995, 209, 481−483. (6) Coutin-Nomade, A.; Bril, H.; Neel, C.; Lenain, J. F. Arsenic in iron cements developed within tailings of a former metalliferous mineEnguiales, Aveyron, France. Appl. Geochem. 2003, 18, 395− 408. (7) Fukushi, K.; Sasaki, M.; Sato, T.; Yanase, N.; Amano, H.; Ikeda, H. A natural attenuation of arsenic in drainage from an abandoned arsenic mine dump. Appl. Geochem. 2003, 18, 1267−1278. (8) Dold, B.; Fontbote, L. A mineralogical and geochemical study of element mobility in sulfide mine tailings of Fe oxide Cu-Au deposits from the Punta del Cobre belt, northern Chile. Chem. Geol. 2002, 189, 135−163.

12597



Article

(29) Coker, V. S.; Pearce, C. I.; Pattrick, R. A. D.; van der Laan, G.; Telling, N. D.; Charnock, J. M.; Arenholz, E.; Lloyd, J. R. Probing the site occupancies of Co, Ni, and Mn substituted biogenic magnetite using XAS and XMCD. Am. Mineral. 2008, 93, 1119−1132. (30) van der Laan, G.; Thole, B. T. Strong magnetic X-ray dichroism in 2p absorption spectra of 3d transition metal ions. Phys. Rev. B 1991, 43, 13401−13411. (31) van der Laan, G.; Kirkman, I. W. The 2p absorption spectra of 3d transition metal compounds in tetrahedral and octahedral symmetry. J. Phys.: Condens. Matter 1992, 4, 4189−4204. (32) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 1938, 60 (2), 309−319. (33) Loan, M.; Cowley, J. M.; Hart, R.; Parkinson, G. M. Evidence on the structure of synthetic schwertmannite. Am. Mineral. 2004, 89, 1735−1742. (34) Bigham, J. M.; Schwertmann, U.; Carlson, L.; Murad, E. A poorly crystalized oxyhydroxysulfate of iron formed by bacterial oxidation of Fe(II) in acid-mine waters. Geochim. Cosmochim. Acta 1990, 54 (10), 2743−2758. (35) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurences and Uses, 2nd ed.; Wiley-VCH: Weinheim, 2003. (36) Jonsson, J.; Persson, P.; Sjoberg, S.; Lovgren, L. Schwertmannite precipitated from acid mine drainage: Phase transformation, sulphate release and surface properties. Appl. Geochem. 2005, 20 (1), 179−191. (37) McIntyre, N. S.; Zetaruk, D. C. X-ray photoelectron spectroscopic studies of iron oxides. Anal. Chem. 1977, 49, 1521− 1529. (38) Regenspurg, S.; Peiffer, S. Arsenate and chromate incorporation in schwertmannite. Appl. Geochem. 2005, 20, 1226−1239. (39) Hansel, C. M.; Benner, S. G.; Nico, P.; Fendorf, S. Structural constraints of ferric (hydr)oxides on dissimilatory iron reduction and the fate of Fe(II). Geochim. Cosmochim. Acta 2004, 68, 3217−3229. (40) Coker, V. S.; Bell, A. M. T.; Pearce, C. I.; Pattrick, R. A. D.; van der Laan, G.; Lloyd, J. R. Time-resolved synchrotron X-ray powder diffraction study of magnetite formation by the Fe(III)-reducing bacterium Geobacter sulfurreducens. Am. Mineral. 2008, 93, 540−547. (41) Bell, A. M. T.; Coker, V. S.; Pearce, C. I.; Pattrick, R. A. D.; van der Laan, G.; Lloyd, J. R. Time-resolved synchrotron X-ray powder diffraction study of biogenic nanomagnetite. Z. Kristallogr. 2007, 423− 428. (42) Kim, E. J.; Batchelor, B. X-ray photoelectron spectroscopic investigation of interactions of arsenic with microwave synthesized pyrite as a function of pH. Environ. Eng. Sci. 2009, 26 (12), 1785− 1793. (43) Ramos, M. A. V.; Yan, W.; Li, X.-q.; Koel, B. E.; Zhang, W.-x. Simultaneous oxidation and reduction of arsenic by zero-valent iron nanoparticles: Understanding the Significance of the core-shell structure. J. Phys. Chem. C 2009, 113 (33), 14591−14594. (44) Nesbitt, H. W.; Muir, I. J.; Prarr, A. R. Oxidation of arsenopyrite by air and air-saturated, distilled water, and implications for mechanism of oxidation. Geochim. Cosmochim. Acta 1995, 59 (9), 1773−1786. (45) Brotton, S. J.; Shapiro, R.; van der Laan, G.; Guo, J.; Glans, P. A.; Ajello, J. M., Valence state fossils in Proterozoic stromatolites by Ledge X-ray absorption spectroscopy. J. Geophys. Res., [Biogeosci.] 2007, 112 (G3). (46) Liu, P.; Kendelewicz, T.; Brown, G. E.; Nelson, E. J.; Chambers, S. A. Reaction of water vapor with alpha-Al2O3(0001) and alphaFe2O3(0001) surfaces: Synchrotron X-ray photoemission studies and thermodynamic calculations. Surf. Sci. 1998, 417 (1), 53−65. (47) Pattrick, R. A. D.; van der Laan, G.; Henderson, C. M. B.; Kuiper, P.; Dudzik, E.; Vaughan, D. J. Cation site occupancy in spinel ferrites studied by X-ray magnetic circular dichroism: Developing a method for mineralogists. Eur. J. Mineral. 2002, 14, 1095−1102. (48) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A. Surfacechemistry of ferrihydrite Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochim. Cosmochim. Acta 1993, 57, 2251−2269.

(9) Jones, E. J. P.; Nadeau, T. L.; Voytek, M. A.; Landa, E. R. Role of microbial iron reduction in the dissolution of iron hydroxysulfate minerals. J. Geophy. Res., [Biogeosci.] 2006, 111, G01012. (10) Küsel, K.; Dorsch, T.; Acker, G.; Stackebrandt, E. Microbial reduction of Fe(III) in acidic sediments: Isolation of Acidiphilium cryptum JF-5 capable of coupling the reduction of Fe(III) to the oxidation of glucose. Appl. Environ. Microbiol. 1999, 65, 3633−3640. (11) Regenspurg, S.; Gossner, A.; Peiffer, S.; Kusel, K. Potential remobilization of toxic anions during reduction of arsenated and chromated schwertmannite by the dissimilatory Fe(III)-reducing bacterium Acidiphilum cryptum JF-5. Water, Air, Soil Pollut.: Focus 2002, 2, 57−67. (12) Courtin-Nomade, A.; Grosbois, C. c.; Bril, H.; Roussel, C. Spatial variability of arsenic in some iron-rich deposits generated by acid mine drainage. Appl. Geochem. 2005, 20 (2), 383−396. (13) Islam, F. S.; Pederick, R. L.; Gault, A. G.; Adams, L. K.; Polya, D. A.; Charnock, J. M.; Lloyd, J. R. Interactions between the Fe(III)reducing bacterium Geobacter sulf urreducens and arsenate, and capture of the metalloid by biogenic Fe(II). Appl. Environ. Microbiol. 2005, 71, 8642−8648. (14) Blodau, C. A review of acidity generation and consumption in acidic coal mine lakes and their watersheds. Sci. Total Environ. 2006, 369, 307−332. (15) Adams, L. K.; Harrison, J. M.; Lloyd, J. R.; Langley, S.; Fortin, D. Activity and diversity of Fe(III)-reducing bacteria in a 3000-yearold acid mine drainage site analogue. Geomicrobiol. J. 2007, 24, 295− 305. (16) Burton, E. D.; Bush, R. T.; Sullivan, L. A.; Mitchell, D. R. G. Schwertmannite transformation to goethite via the Fe(II) pathway: Reaction rates and implications for iron-sulfide formation. Geochim. Cosmochim. Acta 2008, 72 (18), 4551−4564. (17) Blodau, C.; Gatsek, C. Chemical controls on iron reduction in schwermannite-rich sediments. Chem. Geol. 2006, 235, 366−376. (18) Burton, E. D.; Johnston, S. G.; Watling, K.; Bush, R. T.; Keene, A. F.; Sullivan, L. A. Arsenic effects and behavior in association with the Fe(II)-catalyzed transformation of schwertmannite. Environ. Sci. Technol. 2010, 44 (6), 2016−2021. (19) Paikaray, S.; Goettlicher, J.; Peiffer, S. Removal of As(III) from acidic waters using schwertmannite: Surface speciation and effect of synthesis pathway. Chem. Geol. 2011, 283 (3−4), 134−142. (20) Paikaray, S.; Peiffer, S. Abiotic schwertmannite transformation kinetics and the role of sorbed As(III). Appl. Geochem. 2012, 27 (3), 590−597. (21) Carlson, L.; Bigham, J. M.; Schwertmann, U.; Kyek, A.; Wagner, F. Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: A comparison with synthetic analogues. Environ. Sci. Technol. 2002, 36 (8), 1712−1719. (22) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory; VCH Publishers Inc: New York, 1991. (23) Cutting, R. S.; Coker, V. S.; Fellowes, J. W.; Lloyd, J. R.; Vaughan, D. J. Mineralogical and morphological constraints on the reduction of Fe(III) minerals by Geobacter sulf urreducens. Geochim. Cosmochim. Acta 2009, 73 (14), 4004−4022. (24) Lloyd, J. R.; Leang, C.; Hodges Myerson, A. L.; Coppi, M. V.; Cuifo, S.; Methe, B.; Sandler, S. J.; Lovley, D. R. Biochemical and genetic characterization of PpcA, a periplasmic c-type cytochrome in Geobacter sulfurreducens. Biochem. J. 2003, 369, 153−161. (25) Stookey, L. L. Ferrozine-A new spectrophotometric reagent for iron. Anal. Chem. 1970, 42 (7), 779−781. (26) Lovley, D. R.; Phillips, E. J. P. Organic matter mineralization with reduction of ferric iron in anaerobic sediments. Appl. Environ. Microbiol. 1986, 51, 683−689. (27) Schwertmann, U.; Murad, E. The influence of aluminum on iron oxides: XIV. Al-substituted magnetite synthesized at ambient temperatures. Clays Clay Miner. 1990, 38 (2), 196−202. (28) Arenholz, E.; Prestemon, S. O. Design and performance of an eight-pole resistive magnet for soft X-ray magnetic dichroism measurements. Rev. Sci. Instrum. 2005, 76 (8), 083908/1−8. 12598



Article

(49) Foster, A. L.; Brown, G. E.; Tingle, T. N.; Parks, G. A. Quantitative arsenic speciation in mine tailings using X-ray absorption spectroscopy. Am. Mineral. 1998, 83, 553−568. (50) Farquhar, M. L.; Charnock, J. M.; Livens, F. R.; Vaughan, D. J. Mechanisms of arsenic uptake from aqueous solution by interaction with goethite, lepidocrocite, machinawite, and pyrite: An x-ray absorption spectroscopy study. Environ. Sci. Technol. 2002, 36, 1757−1762. (51) Coker, V. S.; Gault, A. G.; Pearce, C. I.; van der Laan, G.; Telling, N. D.; Charnock, J. M.; Polya, D. A.; Lloyd, J. R. XAS and XMCD evidence for species-dependent partitioning of arsenic during microbial reduction of ferrihydrite to magnetite. Environ. Sci. Technol. 2006, 40, 7745−7750.

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Microbial Reduction of Arsenic-Doped Schwertmannite by Geobacter

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