Acidification and Buffering Mechanisms in Acid Sulfate Soil Wetlands

Mar 4, 2011 - Detailed studies of these sediments in the Murray-Darling basin have revealed the presence of diverse Fe−S mineralogy, from X-ray ...
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Acidification and Buffering Mechanisms in Acid Sulfate Soil Wetlands of the Murray-Darling Basin, Australia Fiona Glover,† Kerry L. Whitworth,‡ Peter Kappen,§ Darren S. Baldwin,|| Gavin N. Rees,|| John A. Webb,† and Ewen Silvester*,^ †

Department of Agricultural Sciences, La Trobe University, Bundoora, Australia, 3086 Murray-Darling Freshwater Research Centre (MDFRC), La Trobe University, Albury-Wodonga Campus, Victoria, Australia, 3690 Centre for Materials and Surface Science, §Department of Physics, La Trobe University, Bundoora, Australia, 3086 Murray-Darling Freshwater Research Centre (MDFRC), CSIRO Land and Water, La Trobe University, Albury-Wodonga Campus, Victoria, Australia, 3690 ^ Department of Environmental Management and Ecology (DEME), La Trobe University, Albury-Wodonga Campus, Victoria, Australia, 3690

)

‡

bS Supporting Information ABSTRACT: The acid generation mechanisms and neutralizing capacities of sulfidic sediments from two inland wetlands have been studied in order to understand the response of these types of systems to drying events. The two systems show vastly different responses to oxidation, with one (Bottle Bend (BB) lagoon) having virtually no acid neutralizing capacity (ANC) and the other (Psyche Bend (PB) lagoon) an ANC that is an order of magnitude greater than the acid generation potential. While BB strongly acidifies during oxidation the free acid generation is less than that expected from the measured proton production and consumption processes, with additional proton consumption attributed to the formation of an acid-anion (chloride) FeIII (oxyhydr)oxide product, similar to akaganeite (Fe(OH)2.7Cl0.3). While such products can partially attenuate the acidification of these systems, resilience to acidification is primarily imparted by sediment ANC.

’ INTRODUCTION The presence of significant quantities of reduced inorganic sulfur (RIS) species in the sediments of many inland waterways has recently been recognized as a serious environmental issue in Australia’s Murray-Darling Basin.1,2 These materials have accumulated far beyond historical levels due to a combination of prolonged unnaturally high water levels (due to river regulation), a decline in the frequency of flushing events associated with peak flows, and the ingress of saline water containing sulfate (e.g., groundwater or irrigation drainage water).1,3 Recent severe drought in South-Eastern Australia, and some well-intentioned attempts to reintroduce ephemeral wetland hydrology through managed drying events, have exposed these sulfidic sediments to oxygen, resulting in oxidation and, in some cases, acidification.1 While considerable research effort has focused on the oxidative behavior of coastal acid sulfate soils (ASS) 4,5 and pyrite in acid mine drainage (AMD) 6 systems, there has been less work on the sulfidic sediments of inland waters. Detailed studies of these sediments in the Murray-Darling basin have revealed the presence of diverse Fe-S mineralogy, from X-ray amorphous monosulfidic ‘gels’ (e.g., mackinawite; FeS) to more morphologically distinct iron disulfides (e.g., r 2011 American Chemical Society

pyrite; FeS2),3 similar to that observed in coastal systems.4,5 Inland wetland buffering capacities are, however, more spatially variable,1 leading to a wide range of susceptibilities to acidification.3 In this work we compare two sulfidic wetlands with highly contrasting responses to oxidation, and attempt to reconcile the pH changes that occur upon oxidation against the acid generation potential of the RIS, and the buffering characteristics of the sediment. The results highlight the importance of sediment buffering in imparting resilience to these systems but show that even for wetlands effectively devoid of sediment buffering capacity, the formation of acid-anion FeIII (oxyhydr)oxide products can partially attenuate acidification.

’ EXPERIMENTAL PROCEDURES Field Sites. Sediments were collected from oxbow lakes (Bottle Bend and Psyche Bend lagoons) adjacent to the Murray Received: October 30, 2010 Accepted: January 26, 2011 Revised: January 13, 2011 Published: March 04, 2011 2591

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Environmental Science & Technology River, Australia (Figure S1, Supporting Information). The sediment and water chemistry of these wetlands is described elsewhere.3,7 Both wetlands contain a sulfidic sediment zone between a lower clay-rich layer and an overlying oxidized crust (Figure S2, Supporting Information). Bottle Bend lagoon underwent an acidification event following partial drawdown in 2002, and the pH of the overlying water has remained below 3 since that time, with the electrical conductivity (EC) of the BB water column varying in the range 20 000-200 000 μS cm-1.7 The overlying water in PB is hypersaline, with EC values exceeding 200 000 μS cm-1. Acidification has not occurred in PB during the recent prolonged drought (2002-2009) with water column pH values remaining near-neutral despite drawdown. Mineralogical Characterization. The bulk elemental composition was determined by X-ray fluorescence (XRF). Dominant minerals present in sediment samples were characterized by powder X-ray diffraction (XRD) using Cu KR radiation. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were recorded for dried powders of reduced and oxidized sulfidic materials from both BB and PB and reference minerals (pyrite, pyrrhotite, goethite and hematite; see Supporting Information for experimental details). Sulfidic material from BB was also analyzed by environmental scanning electron microscopy (ESEM). The reduced sulfur speciation was analyzed by sequential measurement of the following: acid volatile sulfur (AVS; nominally measures dissolved sulfide and iron monosulfides, reported as percent dry weight of S); elemental sulfur (S0); chromium reducible sulfur (CRS; nominally measures all other reduced sulfur species). AVS was determined using the cold diffusion technique.8 S0 was extracted from samples after AVS extraction using acetone9 and analyzed by HPLC (see below). Post-extraction sediment was triple-rinsed with acetone, air-dried at ambient temperature, and ground to a powder for CRS measurement (Environmental Analysis Laboratory, Southern Cross University, Lismore, Australia10). Batch Reactor Oxidation Experiments. Reactions were carried out in 170 mL reactor pots with a multiport cap for: electrode placement (pH: Metrohm Aquatrode; EH: combined Pt-ring electrode), gas sparging, and sample collection. Sediment was suspended using an overhead stirrer, and all experiments were conducted at 20 C. For each experiment, 150 mL of pure water (18 MΩ cm; Milli-Q) was added to the reactor pot and purged with a gas mixture containing 418 ppm CO2 in N2 for 30 min. Field-moist sulfidic sediment (15 ( 0.1 g) was added and the suspension equilibrated until the pH had stabilized; oxidation was initiated by changing the gas flow to instrument grade air. Throughout the oxidation experiment, pH and EH were recorded and samples removed for analysis, at time intervals consummate with the reaction kinetics (increasing time intervals). Oxidation experiments were continued until deemed complete, as indicated by a constant pH and sulfate concentration. On each sampling occasion, two separate 2.5 mL subsamples were collected. One of the subsamples was filtered through a 0.22 μm nitrocellulose membrane for the analysis of iron(II), acid anions, base cations, and thiosulfate (methods below). Iron(II) and thiosulfate analyses were conducted immediately after collection while other ionic analyses were performed at a later time (samples frozen). The second subsample was filtered through a 0.45 μm nylon filter membrane; the filtrate was acidified (to 0.1 mol L-1 HCl) and frozen for later analysis of total Fe, Al, and Si. The filter cake was used to analyze elemental

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sulfur (by acetone extraction) after first rinsing the cake with 10 mL of 2 mol L-1 HCl and then 10 mL of Milli-Q water to remove sulfides.11 Anaerobic Buffering Titrations. The pH buffering properties of BB and PB sediment were studied under the same general conditions as described for oxidation experiments (100 g L-1 wet weight), but under anaerobic conditions throughout the titration, maintained by a positive pressure of N2/CO2 (418 ppm) gas, preconditioned in a sacrificial sulfidic slurry. Additions of mineral acid (0.1 mol L-1 HCl) were made using an autotitrator (Metrohm 721 Net). Samples were collected via a rubber septum periodically for analysis of base cations, totalAl and iron(II). Analytical Methods. Iron(II) in solution was determined by visible spectrophotometry using bathophenanthroline (ε = 23 000 mol-1 L cm-1).12 Acid anions, base cations, and thiosulfate were determined by ion chromatography; the anion and thiosulfate separation system was a 150 mm Metrosep A Supp 5 column with an (isocratic) eluant containing 1 mmol L-1 NaHCO3, 3.2 mmol L-1 Na2CO3, and 2% acetone; the cation separation system was a 100 mm Metrosep C2 column with an (isocratic) eluant containing 2 mmol L-1 HNO3. National Institute of Standards and Technology (NIST)-traceable IC standards were used for instrument calibration, with regular check samples for quality control. Total concentrations of Fe, Al, and Si were determined by ICP-OES or (in the case of Fe) atomic absorption spectrometry (AAS). Elemental sulfur was measured by HPLC (LiChrospher 100 RP18 (C18) column; methanol mobile phase; UV detection at 254 nm).13,14 Geochemical Calculations. Geochemical calculations were carried out using Geochemist’s Work Bench (GWB v6.0) using the default thermodynamic data set.15

’ RESULTS AND DISCUSSION Sulfide Mineralogy. Data reported in Supporting Information: Figure S3 (X-ray diffraction (XRD)); Table S1 (XRF); Table S2 (reduced sulfur speciation). Powder X-ray diffraction (Figure S3) of sulfidic sediment collected from BB revealed a dominance of quartz (major) and illite (minor) with no other mineral phases at significant levels. Sulfidic sediment from PB also had quartz as a dominant phase but with significant amounts of gypsum (CaSO4) and halite (NaCl), as well as illite and aragonite (CaCO3) (both minor). The sulfide content was too low in either system to determine the iron sulfide mineralogy from diffraction. Environmental scanning electron microscopy (ESEM) analysis of sulfidic sediment (Figure S4, Supporting Information) from BB revealed an iron-sulfide phase on the surfaces of the dominant quartz and clay particles. The AVS value of 0.21 ( 0.05% S for sulfidic sediment from BB corresponds to a FeS content of 0.58%, assuming the AVS is entirely iron monosulfide. Sequential extraction of this material revealed very little elemental sulfur (0.034%) or iron disulfide (CRS = 0.02%). The total Fe content in BB sulfidic sediment was ∼1.2% Fe (by XRF), so approximately 1/3 of Fe is present as FeS, with the remaining 2/3 in nonsulfide minerals. Given the reducing conditions of the sediment FeIII(oxyhydr)oxides are unlikely to persist; the presence of clay (illite) suggests that the remaining Fe is likely in clay-bound forms. While some of this clay-bound Fe could be interlayer FeII, the high salinity of the medium would more likely lead to a dominance of base cations,16 with Fe restricted to clay lamellae, as either FeII or FeIII. For PB sulfidic sediment, the AVS (0.21 ( 0.03%), S0 (0.08%), and CRS 2592

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Figure 1. (a) Anaerobic acidimetric titration curves for BB and PB slurries; AVS arrows indicate the expected acid generation from the reduced sulfide content (as AVS). Also shown is the free acid (zero buffering) line. (b) Acid consumption and release of divalent ions (FeII, Ca2þ, and Mg2þ; plotted as equivalent concentrations) for BB sediment, as a function of pH. (c) Detailed study of the buffering properties of PB sediment, showing (i) buffering intensity (log(β/2.3)) as a function of pH, and the likely origin of weaker buffering features, (ii) cation (Ca2þ, Mg2þ, FeII, and AlIIItot) concentrations in solution as a function of pH, and (iii) comparison of measured calcium concentrations with that expected based on solubility control by aragonite (CaCO3) at pH > 7, and gypsum (CaSO4) control at pH < 7.

(0.07%) analyses also suggest that FeS is a dominant component of the RIS species. On the basis of the XRF-derived Fe content (1.68%), FeS can account for ∼25% of the total Fe. Less than 10% of the sediment S content is in the form of FeS, with gypsum the dominant S-containing mineral. Buffering Properties of Bottle Bend and Psyche Bend Sulfidic Sediments. The pH buffering properties of BB and PB sulfidic sediments were investigated in anaerobic (acidimetric) titrations (Figure 1a); essentially an analysis of buffering provided by the sediment minerals, porewater bicarbonate, and organic materials. Very little buffering is observed in BB sediment

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Figure 2. Physical and chemical parameters monitored during oxidation of BB reduced sediment: (a) pH and EH, (b) sulfur species (FeScalc line corresponds to a calculated amount of unreacted FeS), (c) FeII and total Fe in solution, (d) total Al and total Si in solution, and (e) calculated acid generation (ΔtotalHþ) and calculated additional buffering required for observed pH. Also shown in plot e are the likely contributions to buffering by Ca2þ (and Mg2þ) and Fe(OH)3.

above the free acid line. A comparison of the acid consumption by the BB sediment with the release of divalent ions (FeII, Ca2þ, and Mg2þ) is shown in Figure 1b. FeII release was the dominant buffering component (data not shown), indicating that the observed buffering is almost entirely due to FeS dissolution; FeII release is close to that expected from the FeS content of this sediment (100 g L-1; AVS = 0.21%; moisture content=26%; ΔFeII = 9.5 meq L-1). Considerably more buffering is observed in the PB sediment, in an amount that is more than 1 order of magnitude greater than the acid generation potential. A detailed analysis of the buffering properties of this sediment is shown in Figure 1c, including a buffering intensity profile, major aqueous cations, and a comparison of calcium concentrations with known 2593

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Environmental Science & Technology Ca-mineral phase behavior. The high buffering intensity at pH 7 matches closely the formation of aqueous Ca2þ and is consistent with the dissolution of a calcium carbonate phase (aragonite); below the dissolution edge, calcium concentrations are controlled by gypsum (CaSO4) precipitation. Smaller buffering features are observed at pH 6 and 4.5; these are attributed to organic or clay-exchange buffering, with small increases in Ca2þ and Mg2þ concentrations over this pH range. The detection of FeII at low pH is consistent with FeS dissolution; there is no evidence for clay dissolution (dissolved aluminum formation) on the time scale of this titration. Bottle Bend Oxidation Studies. The physical and chemical parameters measured during the oxidation of BB sulfidic sediment are shown in Figures 2a-e; background aqueous ionic composition is given in Table S3 (Supporting Information). Prior to oxidation, the pH of these suspensions was ∼7; separate experiments under anaerobic conditions showed that this pH could be maintained for periods greater than 80 h, ultimately decreasing slowly due to air leakage. Oxidation of these suspensions led to a rapid pH decrease to ∼5 and a corresponding increase in suspension EH. Continued oxidation resulted in a slower pH decrease until a final pH of ∼3 was attained after 100 h (Figure 2a). The initial rapid reaction was accompanied by an increase in elemental sulfur (Figure 2b) and a decrease in dissolved FeII (Figure 2c); the slower pH decrease (reaction ‘poise’) corresponds to a plateau in the elemental sulfur concentration which ultimately converts to sulfate as an end product. Thiosulfate concentrations were low throughout the oxidation, but detectable levels were observed at the onset of sulfate formation. The change in solution concentration of sulfate from an initial level of 1 mmol L-1 to a final value of ∼5.0 mmol L-1 is close to that expected from the FeS (AVS measurement) and elemental sulfur content (100 g L-1; AVS = 0.21%; So = 0.034%; moisture content = 26%; ΔSO42- = 5.6 mmol L-1). Given the sample variability we have used the measured ΔSO42- value (4.1 mmol L-1) determined from oxidation of this material to calculate the sulfur mole balance. Approximately 1/3 of the initial FeS phase remains unoxidized during the reaction poise (Figure 2b); the oxidation of this material occurs after the conversion of elemental sulfur to sulfate and coincides with the transient formation of FeII in solution. The general behavior of the BB sulfide oxidation was reproduced in replicate experiments (n = 3), although the length of the reaction poise period (plateau elemental sulfur) did vary between experiments. A similar delay in So oxidation has also been observed in the oxidation of coastal sulfidic materials.4,5 Given that there was no external inhibition of microbial processes in these experiments, the delay in the oxidation of elemental sulfur may be due to a microbial induction time.5 The oxidation processes in BB sediment can be summarized by eqs 1-3. Oxidation Steps.

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oxidation of the particle core and the later FeII pulse. Similar shell-core oxidation behavior has been observed for nanoparticulate chalcopyrite 18 and is likely typical of nanoparticulate metal sulfides. Possible alternative explanations include the dissolution of the FeIII (oxyhydr)oxide reaction products and reduction of the released aqueous FeIII by sulfur, or the dissolution of FeII-containing minerals (e.g., FeII exchanged clays, siderite (FeCO3)).4 The temporal behavior of So, SO42-, and FeII is consistent with the So-FeIII(aq) reaction, but the pH corresponding to the onset of this reaction is 4, above the precipitation edge of even poorly crystalline forms of FeIII (oxyhydr)oxides,19 and FeIII is not observed in solution (Figure 2c). Acid displacement of clay-exchanged FeII is possible,16 however under the high salinity conditions of BB sediment, very little interlayer FeII is expected; iron carbonate minerals are very unlikely in this low buffering sediment. Based on the initial FeII concentration and the measured ΔSO42- value, the total acid production is predicted to be ∼9 mmol L-1, sufficient to lower the pH in an unbuffered system to ∼2. The final pH of ∼3 in the oxidation of BB sediment reveals the existence of additional buffering that was not evident in the titration of this material (see Figure 1b). Concentrations of Si and Al measured during the oxidation of BB sediment (Figure 2d) indicate the dissolution of clay minerals (Al and Si), with Al dissolution providing some buffering during oxidation (assumed to be ΔAl:ΔHþ = 1:3, i.e., all dissolved Al present as Al3þ), although insufficient to account for the observed final pH. At longer times the continued dissolution of Al leads to a slight pH increase. Taking into account the known proton generation processes (FeII oxidation and So oxidation), and proton consumption processes (clay (Al) dissolution), the additional required buffering can be calculated (eqs 4 and 5) where subscripts refer to the commencement of oxidation (‘0’) and any time (‘t’) during oxidation. 2ΔtotalHþ ;t ¼ 2  ð½SO24 t - ½SO4 0 Þ

- 2  ð½FeII t - ½FeII 0 Þ - 3  ð½AlIII t - ½AlIII 0 Þ

ð4Þ

additional bufferingt ¼ ΔtotalH þ ;t - 10-pHt

ð5Þ

Two buffering regions are evident (Figure 2e), the first corresponding to the initial rapid oxidation process and the second to the sulfur oxidation step. The first of these two processes appears to be due to (trace) carbonate mineral dissolution, or clay exchange processes; a small increase in the solution concentrations of Ca2þ and Mg2þ equivalent to the initial FeII acid generation potential was observed in both oxidation and anaerobic titration experiments (data not shown) (i.e., ΔCa2þ þ ΔMg2þ = ΔFeII). The second buffering process is somewhat greater (∼3 mmol L-1) and can be explained by the formation of an acid-anion FeIII (oxyhydr)oxide (presumably with chloride given the dominance of this anion in the aqueous matrix) as shown by eq 6. þ FeðOHÞ3ðsÞ þ xClðaqÞ þ xHðaqÞ f FeðOHÞ3 - x ClxðsÞ þ xH2 O

We interpret the sulfur dynamics in terms of an initial surface passivation process, typical of sulfide mineral oxidation,17 but with an amplified surface reaction due to the likely small (nanoparticulate) size of the FeS phase. This conceptual model explains the delayed

ð6Þ Several lines of evidence support the formation of such a phase, including (i) the observed EH-pH behavior during the oxidation, showing a shift from a ∼177 mV/pH slope to a ∼120 mV/pH slope (see below), (ii) a relative depletion of chloride from the aqueous solution (Figure S5, Supporting Information), 2594

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Figure 3. EH-pH diagram for the Fe-S-Cl-H2O system, showing the measured data points during oxidation of BB sediment, and the stability field for a nominal Fe(OH)2Cl phase that would correspond to the observed change in EH vs pH dependence.

Figure 5. Fe K-edge prepeaks (normalized to K-edge step height) for (a) reference minerals hematite (R-Fe2O3), goethite (R-FeOOH), pyrrhotite (FeS) and pyrite (FeS2), and (b) reduced (BB-reduced; PB-reduced) and oxidized (BB-oxidized, PB-oxidized) sulfidic sediments.

Figure 4. Physical and chemical parameters monitored during oxidation of PB reduced sediment: (a) pH and EH; (b) sulfur species (except SO42-). Also shown is a calculated amount of unreacted FeS plus undetected sulfur intermediate from S-mole balance; (c) SO42- concentrations.

and (iii) the observed formation of akaganeite (β-Fe(OH)2.7Cl0.3) in the oxidized crustal material collected from the BB site.20 The recorded EH and pH data pairs are plotted on a Pourbaix diagram (Figure 3; Fe-S-Cl-H2O system, selected phases removed; see Supporting Information for full description). The data show control of the measured redox potential by the Fe(OH)3/Fe2þ couple in the initial stages of oxidation, followed by a clear decrease in slope after the reaction poise (pH ∼5). The data has been fitted to a nominal Fe(OH)2Cl phase (a high Cl-content is required to account for the

observed EH-pH slope); a comparison with phase boundary position for akaganeite is shown in Figure S621 (Supporting Information). Psyche Bend Reactor Studies. The physical and chemical parameters measured during the oxidation of PB sulfidic sediment are shown in Figures 4a-c (FeII, Al, and Si not detected; data not shown). The background aqueous ionic composition is given in Table S3 (Supporting Information); the high sulfate background (Figure 4b) is due to the presence of gypsum in the PB sediment. Consistent with the higher buffering capacity of this material, the pH remains in the range 7.5-8.5 throughout the oxidation (Figure 4a). Sulfur speciation (Figure 4b,c) shows an initial rapid formation of elemental sulfur, followed by a decay that coincides with thiosulfate formation in solution. Surprisingly, the dynamics of sulfate formation do not match that expected from elemental sulfur and thiosulfate profiles. Using the same approach as for BB, we use the ΔSO42- value to calculate an initial FeS plus elemental sulfur concentration (∼7 mmol L-1), and therefore a sulfur mole balance; a significant intermediate sulfur oxidation product is indicated (Figure 4b). Despite the strong buffering in this system, the small decrease in pH corresponding to sulfate generation indicates that the intermediate is not sulfite (sulfite to sulfate conversion is not acid generating). We are unable to identify this sulfur intermediate. The sulfur mole balance shows evidence of a similar shell-core oxidation process as hypothesized for BB sediment. In this case, ∼1/2 of the sulfidic material is unreacted after the initial rapid process, indicating a larger particle size. 2595

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X-ray Absorption Spectroscopic Analysis of Reduced and Oxidized Sulfidic Sediments. In oxide mineral mixtures, the

amplitude and position of the Fe K-edge pre-peak can be used to determine both the oxidation state and coordination environment of Fe;22 the use of the K-edge pre-peak in the analysis of oxide/sulfide mixtures is less well developed. Comparison of the prepeaks for reduced material from BB and PB (BB-reduced and PB-reduced; Figure 5) with that for reference iron sulfide minerals (FeS (pyrrhotite) and FeS2 (pyrite)) is consistent with sulfide character in these sediments (Figure 5). EXAFS analysis of the first Fe coordination shell (mixture of Fe-O and Fe-S) provides some evidence for the presence of mackinawite (FeS) in these sediments (see discussion in Supporting Information and associated data: Table S4 (EXAFS fit parameters: reference materials); Table S5 (EXAFS fit parameters: BB and PB sediments); Figure S7 (radial distribution functions (RDFs) and fits to Fourier filtered spectra). Given that FeS is not the dominant form of iron in these sediments, the majority of the pre-edge peak is due to nonsulfide components; the peak shape of the BB-reduced and PB-reduced spectra suggest that at least part of this additional Fe is present as FeIII, presumably protected from reduction under natural conditions due to incorporation into mineral structures (e.g., clay lamellae). The Fe K-edge prepeaks for the oxidized forms of BB and PB sediments (Figure 5) show a clear shift to higher energy, consistent with a more FeIII-dominated mineralogy as expected from conversion of FeS to FeIII (oxyhydr)oxide (eqs 1-). The spectral changes due to the oxidation of FeS are superimposed on a background of nonsulfide Fe minerals. The results of this work provide no information about FeII f FeIII conversion in this nonsulfide fraction during oxidation; such processes can also be acid generating and are worthy of further investigation.16 Fe Geochemistry in Sulfidic Sediments. We have recently reported on an acidification mechanism in low-sulfur wetlands of the Murray River caused by the displacement of FeII from clay minerals by base cations, followed by FeIII hydrolysis as the acidgenerating step.16 Clay minerals are present in BB and PB sediments; however, the high background salt level in these systems likely results in very little contribution of reduced iron to the clay interlayer charge. Reduced iron in these systems is instead largely stored in sulfide form, a consequence of microbial sulfate reduction. While the reduced iron speciation in low- and high-sulfur wetlands will likely be different, the proton generation per mole of reduced iron is in principle the same (ΔHþ:ΔFeII = 2:1, provided a pure FeIII (oxyhydr)oxide is formed as a reaction product), as shown by eqs 7 and 8, where Mxþ refers to a displacing cation, and  S to the interlayer exchange sites on clay minerals.

2  Sx M þ FeðOHÞ3 þ 2H þ x

9 5 þ FeS þ O2 þ H2 O f FeðOHÞ3 þ SO24 þ 2H 4 2

’ ASSOCIATED CONTENT

bS

Supporting Information. XAS experimental description; site map (Figure S1); photographs of BB and PB sediments (Figure S2); XRD patterns for reduced sulfidic sediment from BB and PB (Figure S3); ESEM image and EDX pattern for BB sulfidic material (Figure S4); relative depletion of chloride during oxidation of BB sediment (Figure S5); Pourbaix diagrams in FeS-Cl-H2O system with either (nominal) Fe(OH)2Cl or akaganeite phase (Figure S6); fits to Fourier filtered EXAFS data for BB and PB reduced and oxidized sediments (Figure S7); XRF data for BB and PB sulfidic sediment (Table S1); reduced sulfur speciation (Table S2); ionic composition in BB and PB oxidation experiments (Table S3); fitted Fe-O and Fe-S distances and number of neighbors for reference minerals (Table S4); fitted Fe-O and Fe-S distances for reduced and oxidized sulfidic sediment from BB and PB (Table S5). This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Phone: 61 2 6024 9878; fax: 61 2 6024 9888; e-mail: e.silvester@ latrobe.edu.au.

2 1 5  S2 Fe þ M xþ þ O2 þ H2 O x 4 2 f

conditions required for acidification to occur (salinization and oxidation in the case of low-sulfur systems, and oxidation in the case of high sulfur systems), and (ii) the rate of oxidation (strongly pH dependent in the case of displaced FeII, and limited by surface reactions (e.g., passivation) in the case of iron sulfide). In this work we have shown that acid generation can be attenuated by the formation of acid-anion FeIII (oxyhydr)oxides. Similar products are commonly observed as oxidization products of iron sulfides, particularly schwertmannite and jarosites in high sulfate systems.2,3,23 Sulfidic Sediment Oxidation and Wetland Acid Neutralizing Capacity (ANC). The acid generation potential of a wetland sediment is balanced against the capacity of the wetland (sediment and water column) to neutralize the generated acid. Given that the generation of reduced iron produces ANC (essentially the reverse reaction of eqs 7 or 8), the absence of sufficient ANC in BB may be due to hydrological phenomena such as groundwater flow-through.3 Pysche Bend, as an irrigation drainage collection pond, is presumably a more closed system, allowing the ANC received from drainage water, and that generated by iron reduction to be retained. It is this storage of generated ANC that is our ongoing research interest, as this is critical to the robustness of wetlands toward acidification and likely to be influenced by the flow regime and therefore sensitive to the management of these systems.

ð7Þ ð8Þ

The generation of potential acidity in these wetlands is therefore controlled by the reduction of iron, rather than the speciation of the reduced iron. The main differences between the acid-forming potential of low- and high-sulfur wetlands will be (i) the

’ ACKNOWLEDGMENT This project was funded by the National Water Commission through its Raising National Water Standards Program. This Australian Government program supports the implementation of the National Water Initiative by funding projects that are improving Australia’s national capacity to measure, monitor, and manage its water resources. Additional funding was received from the NSW Murray Wetlands Working Group and Murray-Darling Freshwater Research Centre. Access to the 2596

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Environmental Science & Technology Australian Synchrotron was through the foundation investor scheme.

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