Impact of silica on the reductive transformation of schwertmannite and the mobilization of arsenic

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Geochimica et Cosmochimica Acta 96 (2012) 134–153 www.elsevier.com/locate/gca

Impact of silica on the reductive transformation of schwertmannite and the mobilization of arsenic Edward D. Burton ⇑, Scott G. Johnston Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia Received 30 March 2012; accepted in revised form 5 August 2012; available online 19 August 2012

Abstract Schwertmannite is an important Fe(III) mineral in acid–sulfate soil and acid–mine drainage environments because it is both widespread and highly reactive towards trace elements, such as As. Transformation of schwertmannite to more crystalline phases, such as goethite, may strongly influence As mobility. However, previous research suggests that the rate and extent of schwertmannite transformation can be strongly retarded by the presence of Si – a ubiquitous species in natural waters. The present study examines the impact of Si on reductive transformation of schwertmannite and the associated behavior of Fe and coprecipitated As. Synthetic As(V)-coprecipitated schwertmannite (Fe8O8(OH)4.2(SO4)1.9(AsO4)0.0005) was subjected to microbially-mediated reducing conditions for 126 days in the presence of three environmentally-relevant Si concentrations (0, 1.9 and 9.5 mM Si). In addition, complementary sorption experiments and short-term abiotic mineral transformation experiments were conducted to examine the interactive impacts of Si and Fe2+ on schwertmannite stability. Sorption experiments revealed negligible Si sorption to schwertmannite under acidic conditions, with Si sorption only being important towards near-neutral pH. In the 126 day biotic incubations, the onset of reducing conditions in the initially acidic schwertmannite suspensions stimulated dissimilatory Fe(III) reduction, producing Fe2+ and simultaneously causing pH to increase to 6.5. Sorption of Si to the schwertmannite surface at this near-neutral pH partially retarded the rate of Fe2+-catalyzed transformation of schwertmannite to goethite. However, the effect of Si was minor under microbially-reducing conditions, with Fe2+ catalyzing rapid schwertmannite transformation even in the presence of abundant Si. Our short-term abiotic experiments demonstrate that the limited effect of Si is a consequence of Fe2+ being produced concurrently with increases in pH. This allows Fe2+-schwertmannite interactions to proceed prior to the formation of surface-passivating Si species. The Fe2+ catalyzed transformation of As(V)-coprecipitated schwertmannite to goethite caused a major increase in PO3 4 -extractable As, but had little effect on aqueous As concentrations. The reduction of Fe(III) and the subsequent onset of dissimilatory SO2 4 reduction led to formation of siderite (FeCO3) and mackinawite (FeS), respectively. The reduction of As(V) to As(III) was associated with the Sidependent mobilization of As into the aqueous-phase. There was a concurrent decrease over time in the concentrations of PO3 4 -extractable As, which occurred independent of Si concentrations and appeared to be related to formation of siderite and mackinawite. The findings from this study provide new insights into the evolution of iron mineralogy and associated arsenic mobility following the establishment of reducing conditions in schwertmannite- and Si-rich environments. Ó 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Schwertmannite (Fe8O8(OH)8-2x(SO4)x, where x typically spans 1–1.75) is a poorly-ordered Fe(III)-oxyhydroxy-

⇑ Corresponding author.

E-mail address: [email protected] (E.D. Burton). 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.08.007

sulfate mineral which forms in Fe- and SO2 4 -rich waters at pH 3–4 (Bigham et al., 1990, 1996a; Bigham and Nordstrom, 2000). Schwertmannite is a common and often abundant Fe mineral in acid–sulfate soil (ASS) and acid–mine drainage (AMD) environments (Bigham et al., 1996b; Regenspurg et al., 2004; Acero et al., 2006; Burton et al., 2006; Kumpulainen et al., 2007). Given the key role of Fe biogeochemistry in such environments, there is strong

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interest in unraveling the fundamental controls on the stability and reactivity of schwertmannite. In acid–sulfate environments, the formation and fate of schwertmannite directly influences the geochemical behavior of Fe and S (Peine et al., 2000; Murad and Rojik, 2003; Regenspurg et al., 2004; Burton et al., 2006; Peretyazhko et al., 2009; Johnston et al., 2011a). In turn, Fe and S transformations play a critical role in both the generation and consumption of acidity (Peine et al., 2000; Blodau, 2006; Blodau and Gatzek, 2006; Blodau and Knorr, 2006; Knorr and Blodau, 2006). Schwertmannite also possesses a high surface reactivity which can strongly affect trace metal and metalloid mobility (Schroth and Parnell, 2005; Acero et al., 2006; Burton et al., 2008a; Burgos et al., 2011). For example, sorption to schwertmannite can strongly decrease As solubility and mobility in ASSand AMD-affected systems (Regenspurg and Peiffer, 2005; Burton et al., 2009a; Johnston et al., 2011b; Liao et al., 2011; Paikaray et al., 2011). Schwertmannite is metastable. It transforms over time via dissolution and re-precipitation to more crystalline Fe(III) hydr(oxides), such as goethite (aFeOOH) (Bigham et al., 1996b). In oxic and acidic environments, schwertmannite transformation proceeds very slowly requiring years to decades (Bigham et al., 1996b; Regenspurg et al., 2004; Jonsson et al., 2005; Acero et al., 2006). The simple hydrolytic transformation of schwertmannite to goethite is generally accelerated towards higher pH, yet still requires hundreds of days under near-neutral conditions (Regenspurg et al., 2004; Schwertmann and Carlson, 2005; Knorr and Blodau, 2007; Paikaray and Peiffer, 2010). Therefore, schwertmannite tends to accumulate and persist in oxic and acidic environments, such as within drained ASS landscapes (Sullivan and Bush, 2004; Collins et al., 2010). A large amount of research has addressed schwertmannite behavior under oxidizing conditions (Bigham et al., 1996b; Acero et al., 2006; Kumpulainen et al., 2008; Johnston et al., 2011b). In contrast, the stability and fate of schwertmannite under reducing conditions has received relatively little attention. This is significant as there are many important environmental scenarios under which schwertmannite may be subjected to prolonged reducing conditions. For example, reducing conditions can rapidly develop when previously drained ASS wetlands are reflooded (Burton et al., 2008a, 2011a; Johnston et al., 2005, 2009, 2011a; Keene et al., 2011). This example is of particular importance because wetland re-flooding is widely recommended for the remediation of severely degraded ASS settings (Tulau, 2002). Schwertmannite may also be subjected to reducing conditions in a range of AMD settings, including mine-pit lake sediments and organic-rich constructed wetland soils (Peine et al., 2000; Blodau and Peiffer, 2003; Gagliano et al., 2004; Blodau, 2006; Blodau and Gatzek, 2006). The establishment of reducing conditions can affect schwertmannite in a number of ways. Firstly, schwertmannite may be subject to reductive dissolution through the activity of dissimilatory iron(III)-reducing microorganisms (Kusel and Dorsch, 2000; Regenspurg et al., 2002). Reductive dissolution of schwertmannite releases Fe2+ and SO2 4 ,

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and might also liberate schwertmannite-bound trace elements, such as As (Regenspurg et al., 2002; Burton et al., 2010). Interactions between the released Fe2+ and remaining schwertmannite may then catalyze the anoxic transformation of schwertmannite to goethite and/or lepidocrocite (cFeOOH) (Burton et al., 2007, 2008b, 2010; Jones et al., 2009). Indeed, experiments examining pure schwertmannite in simple electrolyte solutions have shown that the Fe2+catalyzed transformation can be extremely rapid at nearneutral pH, with complete transformation occurring within hours to days (Burton et al., 2008b). Recently, some authors have questioned the environmental relevance of the potentially rapid Fe2+-catalyzed transformation of schwertmannite, due to the ubiquitous occurrence of species which may inhibit such transformation. For example, Jones et al. (2009) found that silicic acid, a common and abundant oxyanion in ASS environments, completely inhibited Fe2+-catalyzed schwertmannite transformation. Collins et al. (2010) invoked this stabilizing effect to explain the apparent in situ persistence of schwertmannite in a drained, agricultural ASS landscape. Collins et al. (2010) also suggested that Si coverage at the schwertmannite surface may render schwertmannite nonreactive towards the action of dissimilatory iron(III) reducing microorganisms. However, at present, the impact of Si on schwertmannite stability has been examined systematically under only a very limited set of simple experimental conditions (Jones et al., 2009). In particular, no previous studies have addressed Si-schwertmannite interactions under controlled experimental conditions designed to mimic key aspects of in situ reducing environments. As a result, our understanding of how the presence of varying Si concentrations affects the reductive transformation of schwertmannite in field settings is severely limited. In addition to potentially influencing Fe(III) mineralogical transformations, Si can also affect As sorption–desorption behavior (Swedlund and Webster, 1999; Luxton et al., 2006, 2008). Orthosilicate species (H4 SiO04 and H3 SiO 4 ) and polymeric Si species exhibit a strong affinity for Fe(III) (hydr)oxide surfaces and can compete with As for available sorption sites. The impact of Si on interactions between As and common Fe(III) hydr(oxides), including goethite and ferrihydrite, have been quantified previously (e.g. Swedlund and Webster, 1999; Luxton et al., 2006, 2008). However, the impact of Si on interactions between As and schwertmannite under reducing conditions have not been addressed. Given the ubiquity of Si in ASS and AMD environments, the potential effect of Si on Asschwertmanite interactions represents an important area requiring further research. In this contribution, we examine the impact of Si on the reductive transformation of schwertmannite and the subsequent fate of Fe, SO2 and co-precipitated As. We sub4 jected synthetic As(V)-coprecipitated schwertmannite to reducing conditions in a long-term incubation experiment designed to simulate organic-rich flooded acid–sulfate wetland soils. To ensure applicability to field settings, the incubation experiments featured artificial groundwater with a broad range of environmentally relevant Si concentrations, the presence of a natural humic acid as well as a source of

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labile organic C. We also carried out a series of complementary short-term abiotic experiments specifically examining the interactive impact of Si and Fe2+ on schwertmannite stability. The results significantly advance our understanding of how the development of reducing conditions in ASS and AMD environments influences schwertmannite stability, Fe mineralization pathways, and the speciation and partitioning of arsenic. 2. METHODS 2.1. General methods All laboratory glass-ware was cleaned by soaking in 5% (v/v) HNO3 for at least 24 h, followed by repeated rinsing with deionized water. All chemicals were analytical reagent grade and all reagent solutions were prepared with deionized water (milliQ). Results for solid-phase analyses are presented on a dry weight basis, unless noted otherwise. Deoxygenated solutions were prepared by purging with high-purity N2 for at least 1 h. As stated by Butler et al. (1994), purging for this duration with high-purity N2 is an effective procedure to scrub deionised water of dissolved O2. The production of deoxygenated solutions was verified with the use of a Hach LDO sensor. In order to achieve strictly anoxic experimental conditions, the experiments and anoxic sample preparation steps employed in this study were carried out with the use of an anaerobic chamber (1– 5% H2 in N2), maintained under O2-free conditions with a Pd catalyst. 2.2. Synthesis of schwertmannite with co-precipitated As(V) Schwertmannite, containing coprecipitated As(V), was synthesized by firstly dissolving 1.5 kg of FeSO47H2O and 250 mg of Na2HAsO47H2O in 50 L of water, and then adding 800 mL of a 30% H2O2 solution in order to oxidize aqueous Fe2+ (Regenspurg et al., 2004). The resulting suspension was allowed to settle overnight, and the supernatant was subsequently replaced with deionised water. This rinsing procedure was repeated 5 times in order to remove soluble ions, prior to air-drying the final concentrated slurry. The resulting dry material was finely ground using a mechanical ring-mill. 2.3. Long-term incubation experiment In brief, this experiment involved the long-term incubation of synthetic As(V)-coprecipitated schwertmannite (inoculated with natural schwertmannite-rich soil) at room temperature (20 ± 1 °C) under anoxic conditions. A constant weight of synthetic schwertmannite (1.00 g) was weighed into a series of 100 mL glass serum vials. A 70 mL volume of artificial groundwater was added to each of the glass vials. The artificial groundwater contained 2 mM HCl, 5 mM CaCl2, 10 mM KCl, 10 mM MgCl2, 50 mM Na2SO4, 0.1 mM KH2PO4, 1 mL L1 Wolfe’s mineral solution (Balch et al., 1979), 1 g L1 yeast extract, 0.5 g L1 Aldrich humic acid and 2 g L1 glucose, with either 0 mM Si, 1.9 mM Si or 9.5 mM Si (added from a

Na4SiO4 solution; and herein termed low, medium and high Si, respectively). This composition was designed to mimic natural groundwater within recently flooded organic-rich wetland soil (Johnston et al., 2010, 2011a; Burton et al., 2011a). In order to facilitate the growth of an assemblage of natural microorganisms, the schwertmannite-groundwater suspensions were inoculated with a small volume (200 lL) of 1:10 soil:water suspension prepared from surface soil freshly collected from the Tuckean Swamp, eastern Australia. For additional details of acid–sulfate soils within the Tuckean Swamp please refer to Sullivan and Bush (2004) and references therein. The unsealed 100 mL glass vials containing the schwertmannite-groundwater suspensions were then transferred into an anaerobic chamber. The headspace in these unsealed vials was allowed to deoxygenate via equilibration with the O2-free atmosphere for 16 h. The vials were then fitted with crimp-sealed, butyl-rubber stoppers and sealed under O2-free conditions within the anaerobic chamber. Although the initial artificial groundwater within the sealed vials may have contained some residual dissolved O2, the results discussed below indicate that this was rapidly removed via microbial respiration (thereby ensuring O2-free conditions and subsequent anaerobic microbial metabolism). The schwertmannite suspensions were allowed to incubate at room temperature (20 ± 1 °C), with regular shaking, for periods ranging from 2 to 126 days. In order to re-supply microbially consumed organic C, an additional 1 mL of an anoxic 140 g L1 glucose solution was injected into the remaining vials at day 43. This injection of additional glucose was equivalent to the amount of glucose present in the initial artificial groundwater. At each sampling time, triplicate vials were sacrificed for analysis of selected aqueous- and solid-phase properties. The sampling process involved vigorously shaking each vial to ensure a homogeneous suspension, followed by immediate withdrawal (via a stainless steel needle and polypropylene syringe) of a 10 mL subsample of the homogenized suspension for selective extraction of reduced inorganic S species. During sample withdrawal a constant positive pressure was maintained within the reaction vial via a second needle delivering high-purity N2. This procedure was repeated a second time for determination of 0.5 M HClextractable Fe(II) and Fe(III). A third 10 mL aliquot of the suspension was also withdrawn and filtered to <0.45 lm for determination of selected aqueous-phase properties. The suspension remaining in each vial was then transferred (under O2-free conditions within an anaerobic chamber) to 50 mL polypropylene centrifuge vials. These vials were sealed with screw-caps and centrifuged to separate the solid-phase for further characterisation. The solid-phase was rinsed once under O2-free conditions with deoxygenated, deionised water before being allowed to dry under O2-free conditions within an anaerobic chamber. The drying procedure was largely complete within 24 h, due to the presence of a large reserve of dessicant within the anaerobic chamber. Although we acknowledge the possibility of some transformation of highly reactive phases during

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anoxic sample drying, this approach has been employed in several previous studies examining reductive Fe(III) hydr(oxide) transformations (Hansel et al., 2003, 2011; Zachara et al., 2011). Once dry, the solid-phase material was finely ground in an agate mortar and pestle and stored within butyl-rubber sealed serum vials. Sample grinding and storage was completed within an anerobic chamber under O2-free conditions. 2.4. Silica sorption to schwertmannite The sorption of Si to schwertmannite was examined by suspending 0.5 g of the synthetic schwertmannite in 35 mL of a 50 mM Na2SO4 solution within a series of 50 mL polypropylene centrifuge vials. The resulting suspension was adjusted to either pH 3.5 or 6.5 by drop-wise addition of 1 M HCl or 1 M NaOH. The solid/solution ratio, pH conditions and Na2SO4 background solution were selected to ensure compatibility with the long-term incubation experiment. A small volume of a 1 M Si solution (prepared by dilution of a 28% Na4SiO4 solution in water) was then added to the schwertmannite suspensions, in order to yield Si loadings of up to 16 mM. After addition of Si, the pH of the schwertmannite suspension was re-adjusted (if necessary) to pH 3.5 or 6.5. The schwertmannite suspension was then allowed to equilibrate with the added Si by shaking for 24 h. After equilibration, the aqueous phase was separated by filtration to <0.45 lm and was analysed for Si as described below. The amount of sorbed Si was calculated by the difference between the initial aqueous Si concentration and the equilibrated aqueous Si concentrations. Potential sorption of Si to the vial walls was examined in vials which received no schwertmannite and was found to be negligible (i.e. the initial and final aqueous Si concentrations in vials without schwertmannite differed by <5%). X-ray diffraction analysis of solid phase material resulting from the Si sorption experiments confirmed that schwertmannite remained as the only solid phase present. 2.5. Effect of Si on the abiotic Fe2+-catalyzed transformation of schwertmannite The effect of Si on the abiotic Fe2+-catalyzed transformation of schwertmannite was examined under O2-free conditions in a series of short-term (i.e. 8 day) experiments. These experiments involved suspending samples of the synthetic schwertmannite (1.00 g) in 70 mL of anoxic artificial groundwater within 100 mL glass, O2-free, crimp-sealed serum vials for 8 days. Preparation of the schwertmannite suspensions, including dispensing deoxygenated stock Fe2+ and Si solutions and sealing of the serum vials was completed under O2-free conditions within an anaerobic chamber. The artificial groundwater composition was identical to that used in the long-term incubation experiment (including the presence of humic acid), except that it did not contain glucose or yeast extract and that 0.05 M MOPS (2-morpholinoethanesulfonic acid) was added as a pH buffer. This relatively complex artificial groundwater was used to ensure

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that the results from the short-term abiotic experiment were comparable to the long-term incubation experiment, and (ultimately) applicable to natural field settings. The experimental design involved amending triplicate vials with 0, 5, 10 or 20 mM Fe2+ (from a deoxygenated 1 M FeCl2 solution) and either 0 mM Si (Run 1) or 9.5 mM Si (Runs 2 and 3) (from a deoxygenated 1 M Na4SiO4 solution). In this experiment, three separate experimental runs were completed concurrently. In Run 1 (low Si) and Run 2 (high Si), the pH of the initial Si-bearing schwertmannite suspensions were raised to pH 6.5 by the drop-wise addition of 2 M NaOH. Once at a stable pH of 6.5, a small volume of an anoxic 1 M FeCl2 solution was added in order to achieve the desired initial aqueous Fe2+ concentration. This order of pH adjustment and Fe2+ introduction was subtly different in Run 3, in which the appropriate volume of 1 M FeCl2 solution was added prior to pH adjustment. In the case of Run 3, 9.5 mM Si and variable Fe2+ concentrations were present in solution at the pH of the initial schwertmannite suspension (approx. pH 3), prior to slowly raising the suspension pH to 6.5. For all experimental runs, the schwertmannite suspensions were allowed to interact with the desired Si and Fe2+ loadings for 8 days at pH 6.5 under O2-free conditions, with gentle agitation (100 rpm on an orbital shaker) to minimize diffusion effects. 2.6. Analytical methods Aqueous samples were filtered to <0.45 lm using enclosed syringe-driven filter units (to minimize atmospheric exposure). The filtrate pH and Eh were measured as described in Burton et al. (2007). Aqueous H2S was preserved with ZnOAc and analysed by the methylene blue method of Cline (1969). Alkalinity was determined using the method of Sarazin et al. (1999). Aliquots of filtrate were added to 1,10-phenanthroline solutions for Fe2+ determination (APHA, 2005). Total aqueous Fe was also determined using this method, following pre-reduction of Fe3+ by hydroxylamine hydrochloride. Analysis of aqueous SO2 4 and acetate was performed via ion chromatography using a Metrosep A Supp4-250 column, an RP2 guard column and eluent containing 2 mM NaHCO3, 2.4 mM Na2CO3 and 5% acetone, in conjunction with a Metrohm MSM module for background suppression. Aqueous Si was determined via inductively coupled plasma – atomic emission spectrometry (ICP-AES; Perkin-Elmer DV4300), with a detection limit of <0.01 mM. Samples for total aqueous As analysis were preserved by adding 8 mL of sample to 2 mL of a solution of 0.1 M HNO3 and 15% H2O2. This oxidation step is necessary to avoid the potential formation of arsenic sulfide precipitates upon standard acidification of sulfidic, As-bearing solutions (Smieja and Wilkin, 2003; Samanta and Clifford, 2006). Total aqueous As was determined by inductively coupled plasma – mass spectrometry (ICP-MS) using a Perkin–Elmer Optima 4000/30DV instrument, with a detection limit of <0.01 lM. The As ICP-MS analytical data were corrected for interference from 40Ar35Cl using the isotope ratio approach.

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Acid-volatile sulfide (AVS) was extracted from 10 mL of homogenized suspension (withdrawn from the long-term incubation experiment) by addition of a 5 mL solution of 6 M HCl/0.1 M ascorbic acid (as described by Burton et al., 2007). The evolved H2S(g) was trapped in 7 mL of 3% Zn acetate in 2 M NaOH, and subsequently quantified via iodometric titration (Hsieh et al., 2002). Elemental S (S(0)) and pyrite-bound S were then sequentially extracted according to the methods described in Burton et al. (2011a). The results of these sequential extractions showed that S(0) was of minor importance compared to AVS, and pyrite-S was negligible. Therefore, S(0) and pyrite will not be discussed further. Reactive Fe(II) and Fe(III) fractions were determined over the course of the long-term incubation experiment with the use of 0.5 M HCl extractions (Kostka and Luther, 1994). These extractions were initiated by adding 10 mL of homogenized suspension, withdrawn from the schwertmannite incubation vials, to 10 mL of deoxygenated 1 M HCl. Aliquots of 0.45 lm-filtered extract were analysed for Fe(II) and total Fe by the 1,10-phenanthroline method (APHA, 2005). The concentration of reactive Fe(III) was determined by difference between total extracted Fe and extracted Fe(II). Phosphate-extractable As(III) and As(V) was determined using the method of Keimowitz et al. (2005). Approximately 0.2 g of the dry solid-phase was weighed into 50 mL polypropylene vials under an anoxic atmosphere (within an anaerobic chamber). A 20 mL volume of a deoxygenated solution of 1 M NaH2PO4, 0.1 M ascorbic acid, and adjusted to pH 5 with NaOH was added to each sample. The vials were sealed with gastight screw-caps and mixed for 24 h on an orbital shaker. The suspensions were then centrifuged for 10 min at 5000 rpm, and the aqueous phase separated by filtration to <0.45 lm. Phosphate-extractable As(III) and total As were quantified by hydride-generation atomic absorption spectrometry (HG-AAS) using a Varian AA280FS spectrometer, a VGA77 continuous-flow hydride generation system and an electrothermally-heated quartz cell. For determination of total As, 5 mL of extract solution was mixed with 5 mL of 10% potassium iodide and 10% ascorbic acid in 5 M HCl to pre-reduce As(V) to As(III). For As(III) determination, 5 mL of extract solution was mixed with 5 mL of a pH 5 buffer solution consisting of 25% citric acid monohydrate and 11% NaOH. With the Varian VGA-77 system, a final reduction of As(III) to arsine (AsH3) was achieved using a reductant solution of 0.6% NaBH4 and 0.5% NaOH and an acid solution of 5 M HCl. In the selective determination of As(III), this acid solution was replaced with the citric acid buffer solution described above. Mineralogy was determined on dry solid-phase samples by X-ray diffraction (XRD) using a Bruker D4 Endeavor fitted with a Co X-ray source and Lynx-Eye fast detector. The anoxically dried samples were mounted into XRD sample holders under O2-free conditions within an anerobic chamber. Individual samples were then transferred from the anaerobic chamber and immediately scanned from 10 to 80° 2h with a 0.05° 2h step-size and a 2 s count-time (full scan duration = 46 min). Although no special precautions

(other than immediate analysis with a fast scan rate) were taken to avoid O2 exposure, a comparison of repeated scans indicated that minimal oxidation occurred during XRD data collection. Several previous studies have demonstrated that this XRD approach can reliably identify Fe(II)-bearing phases (e.g. green-rusts and Fe(II) sulfides) that are produced during reductive transformation of Fe(III) hydr(oxides) (Kukkadapu et al., 2004; Jeong et al., 2008, 2010; Zachara et al., 2011). The XRD patterns were evaluated using EVA software (DIFFRAC-plus evaluation package, Bruker AXS, Karlsruhe, Germany). The micro-morphology and elemental composition of mineral specimens in selected samples was examined by analytical scanning electron microscopy (SEM). Samples for SEM examination were mounted on aluminum stubs within an anaerobic chamber, coated with carbon, and the elemental composition and morphology of selected specimens determined using a Leica 440 SEM with an ISIS energy dispersive X-ray (EDX) microanalysis system. Solid-phase samples collected at the end of the 126 day anoxic incubation experiment were further examined by Fe K-edge X-ray absorption spectroscopy (XAS). Samples for Fe XAS were mounted (under O2-free conditions) between two layers of Kapton tape. The mounted samples then stored within butyl-rubber sealed, serum vials (which were themselves stored within an anaerobic chamber). The Fe XAS data were collected under an ambient atmosphere at room temperature on beamline 17C at the National Synchrotron Radiation Research Centre (NSRRC) in Hsinchu, Taiwan. During Fe XAS data collection, the samples were protected from oxidation by being sealed between layers of Kapton tape. Repeated X-ray absorption near-edge structure (XANES) scans on mackinawite and siderite reference standards as well as selected samples indicated negligible oxidation of Fe(II)-bearing phases during XAS data collection. Duplicate spectra were collected in fluorescence mode using an ionization chamber, with the X-ray energy being calibrated against an in-line Fe(0) foil. The ATHENA program was used for standard background subtraction and edge-height normalization (Ravel and Newville, 2005). Iron speciation in the experimental samples was quantified by linear combination fitting of the first-derivative XANES spectra and the k3-weighted extended X-ray absorption fine structure (EXAFS) oscilla˚ 1 range. Synthetic goethite, siderite tions in the 2–12 A and mackinawite (i.e. the only minerals present according to XRD) were used as reference standards. Goethite and mackinawite were synthesized using standard methods (Schwertmann and Cornell, 2000; Wolthers et al., 2005). Published methods for siderite synthesis yield highly reactive material, prone to rapid alteration during handling and transport (Wiesli et al., 2004; Jonsson and Sherman, 2008). In order to avoid this potential problem, we developed a new procedure for synthesizing relatively stable, crystalline siderite. The siderite synthesis procedure involved adding 50 g of NaHCO3 to 1 L of deoxygenated water that was heated to 90 °C. After dissolution of the NaHCO3, 60 g of FeCl24H2O was added as a solid powder. The solution was continuously purged with high-purity N2 during addition of both NaHCO3 and FeCl24H2O, in

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order to prevent Fe2+ oxidation. The resulting pale beige suspension was held at 90 °C for 1 h (with continuous N2 purging) before being rinsed multiple times with methanol and allowed to dry at ambient temperature. The final rinsing and drying steps were conducted in an anaerobic chamber under O2-free conditions. The mineralogy of the resulting material was confirmed as siderite by powder XRD as described above. Exposure of this material to air for up to 3 days caused no change in the XRD pattern, reflecting the formation of highly crystalline siderite that is relatively resistant to oxidation. Solid-phase S speciation in samples collected at the end of the long-term incubation experiment was examined by S K-edge XANES spectroscopy. The XANES data were collected at room temperature under a high-purity He atmosphere on beam-line 16A at the NSRRC in Taiwan. Details of sample handling, mounting and energy calibration in addition to data collection have been previously described (Burton et al., 2009b). In brief, all samples and airsensitive reference standards were mounted between layers of 2 lm X-ray transparent film under O2-free conditions within an anaerobic chamber. The mounted samples were stored and transported within O2-free sealed serum vials, with the mounted samples being exposed to atmospheric O2 only very briefly while being transferred directly from their serum storage vials into the He-purged, beamline sample chamber. Speciation of solid-phase S was achieved by linear combination fitting against selected reference standards using the ATHENA program (Ravel and Newville, 2005). For full details of the S standards, refer to Burton et al. (2009b). Arsenic K-edge XANES spectra were collected at 10– 15 K in fluorescence mode using a 100 element array Ge solid-state detector on the XAS Beamline at the Australian Synchrotron. Sample preparation and storage was performed under strictly O2-free conditions, as described above for Fe XAS. During collection of As XANES data, the samples were maintained under an atmosphere of high-purity Ar, with the Kapton-sealed samples being exposed to the atmosphere only briefly prior to insertion into the XAS cryostat. The X-ray energy was calibrated against an in-line Au foil, with the energy calibration and merging of multiple scans being performed with the software Average, version 2. The XANES spectra were used quantitatively to determine the As oxidation state in samples collected over the duration of the long-term incubation experiment. This was achieved by linear combination fitting of the As K-edge XANES spectra of experiment samples with contributions from Na2HAsVO47H2O, NaAsIIIO2, orpiment (As2S3) and arsenopyrite (FeAsS) using the ATHENA program.

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nite with the AsV O3 oxyanion incorporated into the 4 channels within the schwertmannite structure. Digestion of the synthetic schwertmannite according to Burton et al. (2008b) followed by analysis of Fe, SO2 4 and As revealed 8.7 mmol g1 Fe(III), 2.4 mmol g1 SO2 with only 4 0.64 lmol g1 As. The Fe, SO2 4 and As content indicate a composition of Fe8O8(OH)4.2(SO4)1.9(AsO4)0.0005 for the synthetic schwertmannite employed in this study. The stoichiometric ratio of Fe to SO2 4 agrees with previous reports for synthetic schwertmannite (Regenspurg et al., 2004; Regenspurg and Peiffer, 2005; Burton et al., 2008b, 2009a, 2010). The schwertmannite As content is very low, but consistent with natural schwertmannite that forms in non-contaminated ASS environments. For example, Johnston et al. (2010) report a range of 0.48 ± 0.34 lmol g1 As for schwertmannite-rich material collected at the East Trinity site in north-eastern Australia. 3.2. Silica sorption to schwertmannite Curves describing Si sorption to the synthetic schwertmannite used in this study are shown in Fig. 1. We examined Si sorption under two pH conditions: (1) pH 3.5, which represents the initial pH in the long-term incubation experiments and (2) pH 6.5, which approximates the final, steady-state pH attained in the long-term experiments. The inset plot in Fig. 1 shows that Si exhibited negligible affinity for the schwertmannite surface at pH 3.5, possibly due to a dominance of aqueous polymeric Si or formation of amorphous SiO2 colloids at low pH (Jones et al., 2009). In contrast, at pH 6.5, Si displayed a much greater sorption affinity, with the data being well described by a Langmuir sorption isotherm: q¼

Cmax K L C ðaqÞ ð1 þ K L C ðaqÞ Þ

3. RESULTS AND DISCUSSION 3.1. Initial schwertmannite properties X-ray diffractometry confirmed that the synthesis procedure yielded schwertmannite (data not shown). According to Regenspurg and Peiffer (2005), synthesis of schwertmannite from solutions containing As(V) produce schwertman-

Fig. 1. Sorption curve describing Si sorption to schwertmannite at pH 6.5 (symbols denote data, whilst the solid-line shows the corresponding fit to the Langmuir equation). The inset plot shows the relationship between initial aqueous Si and final (equilibrated) aqueous Si concentrations at both pH 6.5 and pH 3.5. The dashed 1:1 line in the inset plot indicates zero sorption of Si to the schwertmannite.

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where q and C(aq) denote the sorbed and aqueous Si concentrations at equilibrium, KL is the Langmuir sorption intensity and Cmax is the sorption capacity. At pH 6.5 (and normalizing for the schwertmannite Fe content), it was found that KL = 3.7 L molSi1 and that Cmax = 0.11 mmolSi mmolFe1. This value for Cmax supports Jones et al. (2009), who also found a Cmax of 0.11 mmolSi mmolFe1 for the sorption of Si to synthetic schwertmannite at pH 6.5 (based on a reported maxima for Si sorption of 0.286 mmolSi L1 in the presence of schwertmannite at a concentration of 2.5 mmolFe L1). 3.3. Long-term incubation experiment Fig. 2 shows changes over time in selected aqueousphase components in the Low, Medium and High Si treatments. The initial aqueous-phase was acidic (pH 3.5) with a relatively high concentration of sulfate (50 mM), consistent with the in situ characteristics of acid-sulfate waters. Many of the aqueous-phase properties changed dramatically over the initial 8 days under all Si treatments. However, the presence of strongly contrasting aqueous Si concentrations had relatively little impact on other species during this initial period. During the initial 8 days, pH increased from 3.5 to 6.5 and Eh decreased to 100 mV (Fig. 2). There was simultaneous accumulation of relatively high concentrations of

acetate (15 mM), HCO3 (20 to 30 mM) and Fe2+ (35 to 40 mM). The production of Fe2+ can be attributed to dissimilatory Fe(III)-reduction under anoxic conditions; a process which would have also driven the concurrent increase in pH. The initial linear accumulation of Fe2+ is interesting as it is commonly thought that sugars, such as glucose, are utilized in a sequential microbial food-chain whereby fermentative bacteria initially produce fatty-acids (such as acetate), which are subsequently utilized by Fe(III)-reducing bacteria. However, the production of Fe2+ without any apparent delay suggests that reduction of schwertmannite-derived Fe(III) may have been initially coupled directly to glucose oxidation. Such behavior has been documented by Kusel and Dorsch (2000) for non-fermentative, Fe(III)-reducing Acidiphilium species in acidic, schwertmannite-rich sediments. It is also possible that H2 in the headspace of the incubation vials may have been used as an electron donor during the initial onset of microbial Fe(III)-reduction. Fig. 3 shows changes in 0.5 M HCl-extractable Fe as total Fe, Fe(II) and Fe(III) over the course of the incubation experiment. In accord with the pore-water data, the impact of variable Si concentrations on temporal trends in extractable Fe was relatively minor. All Si treatments showed an initial rapid decrease in 0.5 M HCl-extractable Fe(III) and a concurrent increase in extractable Fe(II) (Fig. 3). The disappearance of extractable Fe(III) and production of Fe(II)

Fig. 2. Effect of high, medium and low Si treatments on the temporal evolution of pH, Eh and selected aqueous-phase components in the long-term incubation experiment. The vertical gray line denotes injection of additional glucose at day 43. Data points are the means of triplicate incubation vials (the relative standard deviation of triplicate vials was generally <10%).

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Fig. 3. Temporal evolution of 0.5 M HCl-extractable total Fe, Fe(II) and Fe(III) for the low, medium and high Si treatments in the long-term incubation experiment. Data points are the means of triplicate incubation vials (the relative standard deviation of triplicate vials was generally <10%).

are consistent with dissimilatory reduction of schwertmannite-derived Fe(III). Linear regression of 0.5 M HClextractable Fe(II) over the initial 8 days indicates that Fe(III)-reduction proceeded at rates of 8.0 mM day1, 8.2 mM day1 and 8.9 mM day1 in the low, medium and high Si treatments, respectively (all r2 > 0.96). Although these rates are not significantly different (P > 0.05), the general trend of this data suggests that sorbed Si may have somewhat enhanced the rates of Fe(III)-reduction. The Fe(III)-reduction rates presented above are based on the observed rates of Fe(II) production. Conversely, linear regression of the decrease in 0.5 M HCl-extractable Fe(III) over the initial 8 days of the experiment indicate that reactive Fe(III) was consumed at rates of 14.7 mM day1, 14.1 mM day1 and 12.1 mM day1 in the low, medium and high Si treatments, respectively (all r2 > 0.95). These rates of reactive Fe(III) consumption are intriguing because they are consistently greater than the rates of Fe(II)-production. In fact, the rates of Fe(II) production during the initial 8 days amount to only 54%, 58% and 73% of the corresponding rates for reactive Fe(III) consumption in the low, medium and high Si treatments, respectively. The finding that the rates of extractable Fe(III) consumption exceeded extractable Fe(II) production suggests that a portion of extractable Fe(III) may have been consumed via a pathway other than reduction to Fe(II). This

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is consistent with the observed decrease in the 0.5 M HClextractable total Fe concentrations during the initial 8 to 11 days. The decrease in extractable total Fe implies a partial transformation of solid-phase Fe(III) from a highly reactive phase (i.e. 0.5 M HCl-extractable) to a less reactive phase. Whilst a decrease in extractable total Fe was apparent for all Si treatments, the magnitude and initial rate of the decrease was lowest in the high Si treatment (Fig. 3). Fig. 4 shows XRD patterns for solids collected at days 2–126 in the long-term incubation experiment. At day 2, the XRD patterns are indicative of schwertmannite with some minor quartz (introduced via the inoculation with natural soil). However, the XRD patterns at day 5 and day 8 clearly show the accumulation of goethite at the expense of schwertmannite (Fig. 4). This mineral transformation was also apparent from SEM examination, which revealed replacement of spherical schwertmannite particles with smaller acicular goethite crystals between day 2 and 8 (Fig. 5). While schwertmannite is solubilized by the 0.5 M HCl extraction employed in this study, the much greater stability and crystallinity of goethite makes it relatively recalcitrant towards extraction with 0.5 M HCl. Hence, the XRD patterns indicate that the disparity between the rates of extractable Fe(III) disappearance and Fe(II) production over the early stages of the experiment can be attributed to the transformation of schwertmannite to goethite. The XRD and extraction data suggest that the rate of schwertmannite transformation to goethite was slowed marginally by the presence of high Si concentrations. For example, comparison of the XRD patterns collected at day 5 suggest less goethite formation in the high Si treatment compared to the low Si treatment (Fig. 4). However, it is also clear that the transformation of schwertmannite to goethite still occurred over a period of just a few days (i.e. from day 2 to day 8; Fig. 4) to a substantial degree within the high Si treatment. This important reductive mineralogical shift transpired despite the substantial differences in Si concentrations between the high and low Si treatments. The initial rapid increase in pore-water pH, acetate, 2 2+ HCO and the decrease in Eh and extract3 , SO4 and Fe able Fe(III) was followed by an intermediate phase characterized by the onset of dissimilatory SO2 4 reduction. This is evident by aqueous SO2 4 decreasing from 68 mM at day 8 for all Si treatments to mean concentrations at day 35 of 48, 50 and 56 mM for the low, medium and high Si treatments, respectively (Fig. 2). The magnitude of this decrease in aqueous SO2 4 was reflected in the increase in AVS which, by day 35, had accumulated to mean concentrations of 18.6, 15.7 and 13.4 mM in the low, medium and high Si treatments, respectively (Fig. 6). Along with the decrease in aqueous SO2 4 , there was also a decrease in aqueous Fe2+ between day 8 and day 35 (Fig. 2). The concurrent removal of both aqueous SO2 4 and Fe2+ is consistent with the formation of an iron sulfide phase. In accord, the day 28 XRD patterns for the high and ˚ , consislow Si treatments displayed a broad peak at 0.5 A tent with the (0 0 1) spacing in mackinawite (FeS; Rickard and Morse, 2005) (Fig. 4). However, the magnitude of the decrease in aqueous Fe2+ (approx. 25–30 mM) exceeded

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Fig. 4. XRD patterns showing changes in mineralogy over time for the low and high Si treatments in the long-term incubation experiment. The peaks are labeled as Sch for schwertmannite, G for goethite, M for mackinawite, S for siderite and Q for quartz. The vertical bars to the left of each pattern denote 1000 counts.

the corresponding increase in AVS (Fig. 6). Furthermore, by day 35, the AVS concentrations were also much lower than the corresponding 0.5 M HCl-extractable Fe(II) concentrations, which spanned 77 mM for low and medium Si treatments to 90 mM for the high Si treatment (Fig. 3). The fact that 0.5 M HCl-extractable Fe(II) exceeded AVS at day 35 by approximately 60–80 mM points to the formation of a considerable non-sulfidic solid-phase Fe(II) pool. The XRD patterns in Fig. 4 show that this accumulation of non-sulfidic Fe(II) can be attributed to the formation of siderite (FeCO3). By day 42, the concentrations of aqueous SO2 4 and solid-phase AVS had stabilized (Figs. 2 and 6), suggesting electron donor depletion. In agreement, by this stage of the experiment, aqueous acetate concentrations had decreased to <0.1 lM (Fig. 2). Comparison of the decrease

in acetate concentrations over the period from day 5 to day 42 (a decrease of 14–17 mM acetate) with the corresponding decrease in aqueous SO2 4 reveals a stoichiometric relationship that is very close to unity (Fig. 2). This is consistent with dissimilatory SO2 reduction coupled to the 4 oxidation of acetate:   CH3 COO þ SO2 4 ! 2HCO3 þ HS

In order to overcome the stagnating effect of electron donor depletion, additional glucose was added to the reactors on day 43. This caused a spike in acetate concentrations, with mean concentrations at day 49 spanning 13 mM in the low Si treatment to 18 mM in the high Si treatment (Fig. 2). The addition of glucose at day 43 also stimulated further dissimilatory SO2 reduction, leading 4 to an increase in AVS from approx. 15 mM at day 42 to

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Fig. 6. Formation of AVS (acid-volatile sulfide) in the low, medium and high Si treatment during the long-term incubation experiment. The vertical gray line denotes injection of additional glucose at day 43. Data points are the means of triplicate incubation vials (the relative standard deviation of triplicate vials was generally <10%).

126. Fig. 7 shows Fe K-edge XAS spectra for day 126 samples collected from the high and low Si treatments. The XANES and EXAFS spectra show that the final solidphase Fe speciation comprised a mixture of mackinawite, siderite and goethite (Table 1). This finding agrees with the XRD results shown in Fig. 4. SEM examination revealed that, by day 126, siderite existed as relatively large (several lm) rhomboidal crystals whereas mackinawite occurred as fine (sub-lm) platy aggregates (Fig. 5). Fig. 8 shows S K-edge XANES spectra for solid-phase samples collected at day 126 from the high and low Si treatments, in comparison to selected reference species. The spectra clearly reveal that mackinawite dominates the final solid-phase S speciation (Fig. 8). This dominance of mackinawite was confirmed by linear combination fitting, which indicated that mackinawite comprised >95% of total S at day 126 (Table 1). The absence of a contribution from solid-phase sulfate to the S XANES spectra provides a further line of evidence showing the complete disappearance of schwertmannite during the incubation experiment. 3.4. The mobility of coprecipitated arsenic

Fig. 5. Scanning electron microscopy images of material collected from the high Si treatment of the long-term incubation experiment: (a) the synthetic schwertmannite at day 2, (b) acicular goethite crystals at day 8, (c) rhomboidal siderite at day 126 and (d) platy aggregates of mackinawite at day 126.

approx. 35 mM at day 54 (Fig. 6). The HCO 3 and H2S concentrations also increased following the addition of glucose at day 43, with average concentrations of 28 mM HCO3 and 3 lM H2S by day 54 (Fig. 2). The aqueous- and solid-phase properties changed relatively little as the experiment progressed from day 54 to

Fig. 2 shows the temporal evolution of aqueous As concentrations during the long-term incubation experiment. During the initial 20 days, aqueous As increased steadily up to approx. 0.05 lM, with negligible difference between the varying Si treatments. For the low Si and medium Si treatments, this trend of steadily increasing aqueous As concentrations continued towards day 80. By this stage of the experiment, both the low and medium Si treatments had aqueous As concentrations of approx 0.1 lM. In contrast, by day 80, aqueous As within the high Si treatment had increased to just over 0.2 lM. By the end of the long-term incubation experiment, the aqueous As concentrations followed a trend that was dependent on the corresponding aqueous Si concentrations. For example, at day 126, aqueous As was present at mean concentrations of 0.10 lM, 0.16 lM and 0.27 lM in the low, medium and high Si treatments, respectively. The observed increase in aqueous As with increasing Si agrees with previous work showing that Si competes for surface complexation sites, particularly when As is present as As(III)

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Fig. 7. Iron K-edge first-derivative XANES spectra (left) and k3-weighted EXAFS (spectra) for high and low Si solid-phase samples collected at the completion of the long-term incubation experiment, in comparison to reference spectra. For the high and low Si spectra, data are shown as symbols, while the linear combination fit is shown as a solid line.

Table 1 Results for linear combination fitting of Fe EXAFS, Fe XANES and S XANES spectra for the low and high Si treatments after incubation for 126 days. Iron XANES

Low Si High Si

Iron EXAFS

Sulfur XANES

FeS

FeCO3

FeOOH

R-factor

FeS

FeCO3

FeOOH

R-factor

FeS

Humic

R-factor

56 65

28 23

16 12

0.0027 0.0123

58 65

32 27

10 8

0.084 0.112

96 98

4 2

0.012 0.004

oxyanions (Swedlund and Webster, 1999; Luxton et al., 2006, 2008). Although the aqueous As concentrations increased over time, the vast majority of As (between 97% and 99% of total As) remained in the solid-phase. In order to examine changes over time in the nature of this solid-phase As, we quantified the concentration and oxidation state of PO3 4 extractable As. Fig. 9 shows that the PO3 4 extractable As concentrations were initially very low in the As(V)-coprecipitated schwertmannite. In absolute terms, the initial concentration of PO3 extractable As was just 0:003 lmolAs 4 mmol1 Fe , compared to a total As concentration of 3 0:074 lmolAs mmol1 Fe . This very low degree of PO4 extractability is consistent with AsV O3 being an initial 4 coprecipitate (partly replacing SO2 within the schwert4 mannite channel structure; Regenspurg and Peiffer, 2005), rather than being merely adsorbed to the schwertmannite surface. Over the initial 8 day period of the incubation experiment, there was a substantial increase in PO3 4 extractable As for all Si treatments (Fig. 9). For example, PO3 4 extractable As increased from just 4% of total As initially to a mean of approximately 50% across all treatments by day

8. This initial increase was almost entirely in the form of As(V), with PO3 extractable As(III) comprising only 4 about 15% of total PO3 4 extractable As at day 8 (Fig. 9). Significantly, the presence of varying concentrations of Si appeared to have little impact on the evolution of PO3 4 extractable As (Fig. 9). The XRD patterns (Fig. 4) and the trends in 0.5 M HClextractable Fe(III) (Fig. 3) demonstrate that transformation of As(V)-coprecipitated schwertmannite to goethite occurred within the initial 8–11 day period. This mineralogical transformation appears to have driven the corresponding rapid increase in PO3 extractable As 4 (Fig. 9). This can be attributed to the Fe2+-catalyzed dissolution of As(V)-coprecipitated schwertmannite and its replacement by goethite; a mineral that does not form a coprecipitate with As, but instead retains As by formation of a surface-complex. Following the initial rapid increase, there was a subsequent decrease from day 20 to 126 in PO3 4 extractable As across all Si treatments (Fig. 9). This decrease in extractable As broadly corresponded with the onset of dissimilatory 2 SO2 4 reduction. Under SO4 reducing conditions, it is conceivable that As could have been rendered non-extractable

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Fig. 9. Temporal evolution of PO3 4 -extractable total As, As(V) and As(III) for the low, medium and high Si treatments in the longterm incubation experiment. The vertical gray line denotes injection of additional glucose at day 43.The inset plot in the lower panel shows the increase in the % of PO3 4 -extractable As(III) over the initial 20 days. Data points are the means of triplicate incubation vials (the relative standard deviation of triplicate vials was <15%).

Fig. 8. Sulfur K-edge XANES spectra for high and low Si solidphase samples collected at the completion of the long-term incubation experiment in comparison to spectra for selected reference standards. For the high and low Si spectra, data are shown as symbols, while the linear combination fit is shown as a solid line. The vertical gray line highlights the peak absorbance intensity corresponding to mackinawite-bound S(-II).

3.5. Impact of Si on Fe2+-catalyzed schwertmannite transformation

via the formation of solid As sulfide phases, such as orpiment or realgar. Indeed, the formation of an As sulfide phase under SO2 4 reducing conditions has been proposed as an important control on subsurface As behavior (Kirk et al., 2004). It also appears that such species can form, at least on a local atomic scale, through sorption of As to the mackinawite surface (Gallegos et al., 2007). The formation of a solid sulfide-coordinated As phase is readily detectable via As K-edge XANES, since arsenic sulfides (e.g. orpiment and arsenopyrite) have edge energies that are lower than that of arsenite (e.g. Burton et al., 2011b). However, the As XANES data show that a solid As sulfide phase did not form during the longterm incubation experiment (Fig. 10). Rather, a reduction of As(V) to As(III) occurred over the experiment duration, which may be attributed to microbially-mediated dissimilatory reduction of As(V) (Islam et al., 2004). The XANES results indicate broadly similar amounts of As(III) formation in the low and high Si treatments (i.e. within approximately ± 10%), whereby As(III) comprised about 60–80% of total As by day 126 (Fig. 11).

The results described above show that schwertmannite transformed rapidly during the initial stages of the longterm incubation experiment, and that this transformation strongly affected the extractability of solid-phase As. Significantly, the transformation of schwertmannite to goethite occurred rapidly even in the high Si treatment. To further examine the factors contributing to the observed rapid transformation of schwertmannite to goethite, we conducted a series of controlled abiotic experiments where we manipulated Fe2+ and Si loadings. XRD patterns for the solid-phase resulting from the short-term abiotic experiment are presented in Fig. 12. In the absence of Fe2+, schwertmannite was found to persist at pH 6.5 over the 8 day reaction period under both low and high Si conditions. Under low Si conditions (i.e. Run 1), the addition of 5 mM Fe2+ resulted in some transformation of schwertmannite to goethite, with the extent of this transformation increasing at higher Fe2+ concentrations. In addition to goethite, at higher Fe2+ concentrations, lepidocrocite was also produced under low Si conditions. Lepidocrocite formation under strictly O2-free conditions has been observed previously as a result of Fe2+ catalyzed schwertmannite transformation in the absence of Si and

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Fig. 11. Accumulation of solid-phase As(III) during the long-term incubation experiment, based on linear combination fitting of As K-edge XANES spectra. The vertical gray line denotes injection of additional glucose at day 43.

Fig. 10. Arsenic K-edge XANES spectra for samples collected at times ranging from 8 days to 126 days in the high and low Si treatment of the long-term incubation experiment in comparison to reference arsenite and arsenate species. For the low and high Si spectra, data are shown as symbols with the corresponding linear combination fit shown as a solid line.

HCO 3 (Burton et al., 2008a, 2008b; Jones et al., 2009). The formation of goethite and lepidocrocite over an 8 day period is attributable to Fe2+ acting to destabilize the schwertmannite structure and catalyze Fe(III) recrystallization (Burton et al., 2007, 2008b; Jones et al., 2009). The XRD patterns in Fig. 12 show that high Si concentrations generally decreased the extent of abiotic Fe2+-catalyzed schwertmannite transformation. This is consistent

with the observed slightly lower degree of goethite formation in the high Si treatment during the early stages of the long-term incubation experiment (Fig. 4). In Run 2 of the 8-day abiotic experiment (where Si = 9.5 mM), there was only very minor goethite/lepidocrocite formation even at the highest Fe2+ concentration of 20 mM (Fig. 12). Importantly, the results were quite different for Run 3 (also 9.5 mM Si), in which the XRD patterns show effectively complete transformation of schwertmannite to goethite in the presence of 20 mM Fe2+ (Fig. 12). Runs 2 and 3 both contained 9.5 mM Si, yet differed with regard to the timing of Fe2+ introduction. In Run 2, the pH was raised to 6.5 thereby allowing Si to pre-sorb to the schwertmannite surface prior to the introduction of Fe2+. The XRD results suggest that this pre-sorption led to formation of a surface-passivating Si species, which stabilized schwertmannite against transformation. Conversely, in Run 3, Fe2+ was added to the Si-rich schwertmannite suspensions under their initially acidic pH prior to raising the pH to 6.5. Evidently, having Fe2+ and Si simultaneously present as the suspension pH increased from acidic to nearneutral allowed a vastly greater extent of Fe2+-catalyzed schwertmannite transformation compared to when Fe2+ was introduced after Si pre-sorption (Fig. 12). The results from Run 3 are consistent with the rapid Fe2+-catalyzed transformation of schwertmannite that was observed in the long-term incubation experiment. Fe2+ production in the incubation experiment was driven by dissimilatory Fe(III)-reduction, which also produced concurrent increases in pH. Hence, in the incubation experiments, Fe2+ and Si were simultaneously present as the pH increased to 6.5. Therefore, Fe2+ had the opportunity to interact with the schwertmannite surface during the development of the near-neutral pH conditions that are required for significant sorption of Si. This meant that reactive sites on the schwertmannite surface were not blocked by presorbed Si prior to the production of Fe2+. 4. GENERAL DISCUSSION AND ENVIRONMENTAL IMPLICATIONS 4.1. Schwertmannite stability and reductive iron mineralisation Dissolved silica is typically present in natural waters at concentrations between 0.1 to 1.2 mM. However,

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Fig. 12. XRD patterns of the solid-phase produced by suspending synthetic schwertmannite in artificial groundwater for 8 days at pH 6.5 in the presence of 0 mM Si (Run 1) or 9.5 mM Si (Runs 2 and 3) and varying initial concentrations of aqueous Fe2+ (0 to 20 mM). In Run 2, aqueous Fe2+ was added after addition of 9.5 mM Si and after raising the suspension pH to 6.5. In Run 3, aqueous Fe2+ was also added after addition of 9.5 mM Si but prior to raising the suspension pH to 6.5. The XRD peaks are labeled as G for goethite and L for lepidocrocite. The vertical bars to the left of each pattern denote 1000 counts.

acid–sulfate waters (especially ASS pore-waters) can become enriched in dissolved Si due to the proton-promoted dissolution of alumino-silicate clays. For example, Preda and Cox (2004) reported that dissolved silica increased up to 4.6 mM during oxidation and acidification of pyritebearing sediment. Likewise, Collins et al. (2010) found Si concentrations of up to 1.8 mM in shallow groundwater within ASS profiles in eastern Australia. In the present study, we employed Si concentrations that bracket the range of concentrations observed in ASS environments. Schwertmannite precipitates from Fe- and SO2 4 -rich waters at pH 3 and 4. The sorption results presented herein show that, under such acidic conditions, Si does not sorb significantly to the schwertmannite surface. As a partial consequence, Si does not form a coprecipitate with schwertmannite during its precipitation from acidic solutions (Jones et al., 2009). This contrasts with the well studied formation and stability of Si-coprecipitated ferrihydrite, which (like schwertmannite) is also a poorly crystalline, meta-stable ferric mineral. Importantly, insignificant Si sorption at low pH means that any potential impact of Si on schwertmannite stability must therefore be related to sorptive interactions at higher pH. The sorption experiment revealed that Si displays a moderate affinity for sorption to the schwertmannite surface under near-neutral pH conditions. Although schwertmannite forms under acidic conditions, schwertmanniterich soil and sediment can rapidly develop higher pH following the onset of reducing conditions (Peine et al., 2000; Blodau and Gatzek, 2006; Burton et al., 2006, 2007, 2008a, 2008b; Johnston et al., 2005, 2011a). In particular,

several studies demonstrate that the flooding of organicrich soil in ASS landscapes triggers substantial increases in porewater pH (Burton et al., 2007, 2008a, 2008b, 2011a; Johnston et al., 2009, 2011a). This pH increase is driven primarily by proton consumption and alkalinity generation due to dissimilatory Fe(III) reduction (Johnston et al., 2012). Hence, there is typically a dramatic increase in porewater Fe2+ concentrations as pH increases from acidic to near-neutral following flooding of organic-rich ASS materials. Jones et al. (2009) found that, at near-neutral pH, the presence of just 1 mM Si prevented the Fe2+-catalyzed transformation of schwertmannite. In line with this previous work, we initially anticipated that high concentrations of Si would inhibit schwertmannite transformation to goethite, thereby altering reductive mineralization pathways and affecting the associated behavior of coprecipitated As. However, the long-term incubation experiment revealed that Si had relatively little impact, at least under the fieldrelevant experiment conditions employed in the present study. The long-term incubation experiment demonstrated that schwertmannite was rapidly transformed to goethite in both the low and high Si treatments, with this transformation causing a large increase in the extractability of solid-phase As. It may appear that the observed fast transformation of schwertmannite in the presence of high Si concentrations directly contradicts Jones et al. (2009), who found that Si completely inhibited the Fe2+-catalyzed transformation. In fact, this apparent discrepancy can be reconciled by considering key differences between the experimental condi-

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(Table 2). In the case of such high surface loadings, significant SiO4 polymerization would have occurred across the schwertmannite surface, thereby completely preventing surface accessibility for subsequently added Fe2+. In contrast, the experiments described here employed a lower Si to schwertmannite ratio, amounting to occupancy of 70% of the available Si sorption sites. Not surprisingly, this comparison suggests that the degree of Si sorption site occupancy affects the extent of Fe2+-catalyzed schwertmannite transformation. However, our results also clearly show that, at a constant high degree of Si sorption site occupancy (i.e. 70% of sorption capacity), higher Fe2+ concentrations contribute to greater degrees of schwertmannite transformation. The requirement for high Fe2+ concentrations to catalyze schwertmannite transformation in the presence of Si is consistent with the persistence of schwertmannite in an intensively-drained agricultural ASS landscape as reported by Collins et al. (2010). In this earlier study, porewater Fe2+ attained maximum in situ concentrations of only 1 mM with most samples having much lower concentrations. This naturally led Collins et al. (2010) to conclude that reductive dissolution of schwertmannite was severely limited at their study site. This limitation is not surprising given that (1) the organic-rich surface layers at the study site examined by Collins et al. were well-drained and thus non-reducing; and (2), although periodically waterlogged, the schwertmannite-bearing subsurface soil layers were

tions employed by Jones et al. (2009) and those employed in the present study. Firstly, Jones et al. (2009) allowed Si to pre-sorb to the schwertmannite surface at pH 6.5, prior to introducing Fe2+. This pre-sorption of Si at near-neutral pH passivates the schwertmannite surface against subsequent Fe2+ catalyzed transformation. In contrast, our abiotic short-term experiment demonstrates that when pH is raised to 6.5 with both Fe2+ and Si present in solution, the pre-sorption of Si does not occur prior to schwertmannite-Fe2+ interactions – thus allowing the Fe2+ catalyzed transformation to proceed. Secondly, both of the high Si runs from the short-term abiotic experiment showed that schwertmannite persisted when Fe2+ was added at a concentration of 5 mM or less. This addition of aqueous Fe2+ corresponds to a relatively low Fe2+: Si ratio of 0.5 (Table 2). Jones et al. (2009) found that pre-sorbed Si inhibited schwertmannite transformation when Fe2+ was added at broadly similar Fe2+: Si ratios (0.1–1), yet at a lower aqueous Fe2+ concentration (just 1 mM; Table 2). This suggests that the impact of Si on the Fe2+-catalyzed transformation of schwertmannite may be more closely related to absolute aqueous Fe2+ concentrations than to high relative Fe2+ to Si ratios. Thirdly, differences in the degree of surface site occupancy may influence the impact of Si on schwertmannite transformation. Examination of the conditions employed by Jones et al. (2009) reveals very high Si to schwertmannite ratios, which amount to 100% sorption site occupancy

Table 2 Summary of experiment conditions and observed mineralogical results for the impact of Si on the Fe2+-catalyzed transformation of schwertmannite. The presented data includes experimental conditions and mineralogical results for the initial 8 days of the biotic incubation experiment, the short-term abiotic experiments and the only previous study to examine the impact of Si on the Fe2+-catalyzed transformation of schwertmannite (Jones et al., 2009). Schw. (mM as Fe(III)

Fe2+ (mM)

Fe2+: SchwA

Fe2+: Si

SchwA: Si

Reaction period (days)

Final mineralogy at the end of the reaction period

Initial stage (day 0–8) of the long-term biotic incubation experiment 124 0–40 0 0–0.3 n/a n/a 0 124 0–40 9.5 0–0.3 0–4 13 70

8 8

Goethite Goethite

Short-term abiotic experiment – Run 1 124 0 0 0 124 5 0 0.04 124 10 0 0.08 124 20 0 0.16

n/a n/a n/a n/a

n/a n/a n/a n/a

0 0 0 0

8 8 8 8

Schwertmannite (no transformation) Schwertmannite, minor Goethite Goethite, Schwertmannite Goethite, Lepidocrocite

Short-term abiotic experiment – Run 2 124 0 9.5 0 124 5 9.5 0.04 124 10 9.5 0.08 124 20 9.5 0.16

0 0.53 1.05 2.10

13 13 13 13

70 70 70 70

8 8 8 8

Schwertmannite Schwertmannite Schwertmannite Schwert., minor

Short-term abiotic experiment – Run 3 124 0 9.5 0 124 5 9.5 0.04 124 10 9.5 0.08 124 20 9.5 0.16

0 0.53 1.05 2.10

13 13 13 13

70 70 70 70

8 8 8 8

Schwertmannite (no transformation) Schwertmannite (no transformation) Schwertmannite, Goethite Goethite

Previous work (Jones et al., 2009) 2.5 1 0 0.4 2.5 1 1 0.4 2.5 1 10 0.4

n/a 1 0.1

n/a 0.25 2.50

0 100 100

7 7 7

Lepidocrocite Schwertmannite (no transformation) Schwertmannite (no transformation)

A

Si (mM)

Si surface coverage (%)

Schw denotes the schwertmannite concentration expressed as mmol of Fe(III) per L.

(no transformation) (no transformation) (no transformation) Goethite & Lepicroc.

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mineral horizons with negligible biologically-labile organic matter. Despite these vitally important site features, Collins et al. (2010) suggested that the limited reductive dissolution of schwertmannite may have been due to Si surface-coatings. The results of the long-term incubation study described here suggest that Si actually has negligible retarding effect on the reductive dissolution of schwertmannite under anoxic, organic-rich conditions. It is important to note that the conditions employed in the long-term incubation experiment were specifically designed to be consistent with in situ conditions observed in re-flooded ASS wetlands. A realistic general composition for the artificial groundwater was employed, including the presence of a natural humic acid. Accordingly, the observed trends in aqueous-phase properties are also comparable to in situ observations obtained from flooded ASS landscapes. For example, although aqueous Fe2+ reached very high concentrations (up to 40 mM) during the early stages of the incubation experiment, equivalent or higher Fe2+ concentrations have been repeatedly observed in flooded, organic-rich ASS materials (Burton et al., 2008a, 2011a; Johnston et al., 2011a, 2011b). This is important, as the development of high Fe2+ concentrations via partial schwertmannite reductive dissolution appears to be a prerequisite for rapid schwertmannite transformation in the presence of abundant Si. The reductive mineralization pathways that transpired during the long-term incubation experiment bear a strong resemblance to pathways that occur following a shift from oxidizing to reducing conditions in organic-rich ASS environments. This resemblance is exemplified in the evolution of iron mineralogy in benthic sediments that accumulate in waterways receiving acidic, Fe- and SO2 4 -rich inputs from ASS drainage (Burton et al., 2006, 2008b). These sediments are characterized by deposition of acidic schwertmanniterich surface layers, under which lies near-neutral material rich in porewater Fe2+ and goethite (formed diagenetically via in situ schwertmannite transformation). The subsurface near-neutral sediments support dissimilatory SO2 reduc4 tion and are characterized by the concurrent formation of siderite and mackinawite (Burton et al., 2006); as also observed during the intermediate and final stages of the long-term incubation experiment. In essence, the previously reported in situ transformation of schwertmannite to goethite in the presence of abundant porewater Fe2+, and the subsequent accumulation of mackinawite and siderite closely match the results of the incubation experiment described here. 4.2. Arsenic mobility and fate during reductive transformation of schwertmannite The reductive dissolution of Fe(III) minerals has been widely implicated as a driver of arsenic mobilization in flooded soils and reducing groundwater systems (Smedley and Kinniburgh, 2002; Burton et al., 2008a; Johnston et al., 2010). However, despite substantial reductive dissolution of schwertmannite during the initial stages of the incubation experiment, there was very little concurrent arsenic dissolution (Fig. 2). In fact, comparison of the evolution

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of aqueous Fe2+ and As over the course of the incubation experiment clearly indicates decoupling between Fe and As solubility. In this case, Fe2+ rapidly increased during the early stages of experiment and then decreased over time, whereas As remained low initially before steadily increasing (Fig. 2). Decoupling of As and Fe is increasingly recognized as an important feature of As behavior in reducing environments (Islam et al., 2004; van Geen et al., 2004; Pedersen et al., 2006; Burnol et al., 2007; Burton et al., 2008a). Such decoupling has been partly attributed to arsenic redox transformations, and the contrasting sorption behavior of As(V) and As(III). Burnol et al. (2007) found that the release of As from As(V)-bearing ferrihydrite was due to As(V) reduction and As(III) desorption, rather than Fe(III) reductive dissolution. The present results are broadly consistent with this concept, whereby the observed increases in aqueous As generally followed the corresponding increases in solid-phase As(III), which can be attributed to dissimilatory As(V) reduction (Figs. 2 and 11). However, our results also show that this relationship is impacted by the presence of Si, highlighting the importance of oxyanion competition for available sorption sites. Decoupling between Fe and As solubility can also occur when Fe(III) reduction leads to formation of secondary Fe(II) phases (van Geen et al., 2004; Burnol et al., 2007; Burton et al., 2008a). In the present study, siderite began to form following the initially rapid onset of dissimilatory Fe(III)-reduction and the accumulation of aqueous Fe2+ and HCO 3 . The results indicate that goethite retained almost all As(V) that was available from the initial rapid reductive dissolution and transformation of schwertmannite. Compared with goethite, siderite has a very low sorption affinity for As (Charlet et al., 2011), and therefore its formation would have removed Fe2+ from solution but had negligible effect on As. Siderite formation is therefore likely to have contributed to the observed decoupling between aqueous Fe and As in the present study. The transformation of As(V)-coprecipited schwertmannite to goethite, while not releasing aqueous As, was associated with a major increase in the extractability of solidphase As. This can be attributed to a shift in the mode of AsV O3 binding, from an occluded co-precipitate within 4 the schwertmannite channel structure to an adsorbed species on the goethite surface. What is more difficult to explain is the observed subsequent decrease over time in the PO3 4 extractable As concentrations. Such behavior is consistent with an “aging” effect, whereby there is an increase in the relative strength of sorption with increasing residence time. However, O’Reilly et al. (2001) demonstrated that there was no change in As extractability by PO3 when 4 As-sorbed goethite was allowed to age for up to 12 months. This suggests that the observed decrease in PO3 4 extractable As is unlikely to be due to an increase over time in the strength of As sorption to goethite. The observed decrease in PO3 4 extractable As broadly corresponded with the onset of dissimilatory SO2 4 reduction and the formation of mackinawite. This relationship suggests that arsenic may have been sequestered via sorption to mackinawite. However, comparison of the evolution

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of AVS and PO3 4 extractable As reveals a clear discord. In particular, AVS increased in 2 step-like increments (Fig. 6), yet extractable As displayed a steady decrease from day 14 to 126 (Fig. 9). This discrepancy indicates that the decrease in PO3 4 extractable As may not be due to arsenic re-distribution from goethite to mackinawite. Furthermore, even if such re-distribution did occur, there would have needed to be a mechanism of uptake of As at the mackinawite surface that rendered the sorbed As non-extractable with PO3 4 . Such non-extractability would be consistent with the formation of an As sulfide-like precipitate on the mackinawite surface, as observed by Gallegos et al. (2007). However, our XANES data demonstrate that this certainly did not occur in the present study, and hence it cannot be invoked to explain the decrease in PO3 4 extractable As. The steady decrease in PO3 4 extractable As from day 14 to 126 closely matched the corresponding decrease in aqueous Fe2+. In fact, the concentrations of PO3 4 extractable As and aqueous Fe2+ were strongly correlated (r = 0.87) during this period. This relationship suggests that the decreased extractability of solid-phase As was associated with removal of Fe2+ from solution. The XRD results demonstrate that Fe2+ removal occurred via precipitation of siderite and mackinawite, yet As sorption by these secondary Fe(II) phases probably does not explain the decrease in As extractability. This is mainly because any As adsorbed by siderite or mackinawite would still be displaced by the 1 M PO3 4 extractant used here. However, it is likely that nucleation and subsequent growth of siderite crystals and (possibly) mackinawite nanoparticles occurred on the preexisting goethite crystals. This would have progressively decreased the accessibility (and hence extractability) of As complexed at the goethite surface. While consistent with the present data-set, future research is required to confirm the importance of mackinawite and/or siderite formation in driving the observed decreases in As extractability. 5. CONCLUSIONS Schwertmannite is a critically important Fe(III) mineral in ASS and AMD environments because of its high reactivity and widespread occurrence. This study has investigated the impact of Si on Fe, S and As behavior during the reductive transformation of schwertmannite under conditions that are similar to those observed in flooded, organic-rich ASS wetlands. The results confirm that sorption of Si to schwertmannite at near-neutral pH slows down the rate of Fe2+-catalyzed transformation of schwertmannite to goethite. However, this effect is minor in organic-rich systems, where the reductive dissolution of schwertmannite drives increases in pH and the simultaneous development of high Fe2+ concentrations. The present study demonstrates that this concurrent increase in pH (to 6.5) and Fe2+, to concentrations equivalent to those observed in organic-rich ASS wetlands, can catalyze schwertmannite transformation even in the presence of abundant Si. The results also highlight the need for further research into the interplay between pH, high Fe2+ concentrations and Si sorption site coverage as factors governing the degree of Fe2+-catalyzed schwertmannite transformation.

Transformation of As(V)-coprecipitated schwertmannite to goethite causes a major increase in the extractability of solid-phase As. However, in environments containing only modest total As concentrations (e.g. typical ASS wetlands), this process alone is unlikely to translate into significant As mobilization due to efficient retention of arsenic by the newly formed goethite. In contrast, the reduction of As(V) to As(III) appears to be associated with Si-dependent mobilization of As into the aqueous phase. Although higher Si concentrations were associated with higher aqueous As concentrations, our results indicate that Si had little impact on the extractability and speciation of solid-phase As. The formation of siderite and mackinawite appear to contribute to progressive decreases over time in the relative availability of solid-phase As, as estimated by PO3 4 extractable As. Future research is needed to resolve the exact role of siderite and mackinawite in contributing to the enhanced retention of strongly-bound arsenic. ACKNOWLEDGEMENTS Research expenses and salary support for Ed Burton were provided by the Australian Research Council (Grant number DP110100519). Salary support for Scott Johnston was provided by the Australian Research Council (Grant number FT110100130). The arsenic XAS components were supported by the Australian Synchrotron Research Program (Grant number AS112/XAS3855). We thank Dr Chris Glover for his technical assistance and expert advice with XAS data collection at the Australian Synchrotron. The iron and sulfur XAS components were made possible by support from the National Synchrotron Radiation Research Centre in Taiwan (Grant number 2011-2-002-1). We thank Dr. J.-F. Lee and Dr. L.-Y. Jang of the NSRRC for providing collaborative assistance and advice with collection of the iron and sulfur XAS data, respectively.

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