Published: March 04, 2011

r 2011 American Chemical Society 2591 dx.doi.org/10.1021/es103535k | Environ. Sci. Technol. 2011, 45, 2591–2597

ARTICLE

pubs.acs.org/est

Acidification and Buffering Mechanisms in Acid Sulfate Soil Wetlandsof the Murray-Darling Basin, AustraliaFiona Glover,†Kerry L. Whitworth,‡ Peter Kappen,§Darren S. Baldwin,||Gavin N. Rees,|| John A. Webb,† andEwen 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

’ INTRODUCTION

The presence of significant quantities of reduced inorganicsulfur (RIS) species in the sediments of many inland waterwayshas recently been recognized as a serious environmental issue inAustralia’s Murray-Darling Basin.1,2 These materials have accu-mulated far beyond historical levels due to a combination ofprolonged unnaturally high water levels (due to river regulation),a decline in the frequency of flushing events associated with peakflows, and the ingress of saline water containing sulfate (e.g.,groundwater or irrigation drainage water).1,3 Recent severedrought in South-Eastern Australia, and some well-intentionedattempts to reintroduce ephemeral wetland hydrology throughmanaged drying events, have exposed these sulfidic sediments tooxygen, resulting in oxidation and, in some cases, acidification.1

While considerable research effort has focused on the oxidativebehavior of coastal acid sulfate soils (ASS) 4,5 and pyrite in acid minedrainage (AMD) 6 systems, there has been less work on the sulfidicsediments of inlandwaters. Detailed studies of these sediments in theMurray-Darling basin have revealed the presence of diverse Fe-Smineralogy, from X-ray amorphous monosulfidic ‘gels’ (e.g., mack-inawite; FeS) to more morphologically distinct iron disulfides (e.g.,

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 highlycontrasting responses to oxidation, and attempt to reconcile thepH changes that occur upon oxidation against the acid genera-tion potential of the RIS, and the buffering characteristics of thesediment. The results highlight the importance of sedimentbuffering in imparting resilience to these systems but show thateven for wetlands effectively devoid of sediment bufferingcapacity, the formation of acid-anion FeIII (oxyhydr)oxide pro-ducts 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, 2010Accepted: January 26, 2011Revised: January 13, 2011

ABSTRACT: The acid generation mechanisms and neutralizing capacities of sulfidic sediments fromtwo inland wetlands have been studied in order to understand the response of these types of systemsto 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 BBstrongly acidifies during oxidation the free acid generation is less than that expected from themeasured proton production and consumption processes, with additional proton consumptionattributed to the formation of an acid-anion (chloride) FeIII (oxyhydr)oxide product, similar toakagan�eite (Fe(OH)2.7Cl0.3). While such products can partially attenuate the acidification of thesesystems, resilience to acidification is primarily imparted by sediment ANC.

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River, Australia (Figure S1, Supporting Information). The sedi-ment and water chemistry of these wetlands is describedelsewhere.3,7 Both wetlands contain a sulfidic sediment zonebetween a lower clay-rich layer and an overlying oxidized crust(Figure S2, Supporting Information). Bottle Bend lagoon under-went an acidification event following partial drawdown in 2002,and the pH of the overlying water has remained below 3 sincethat time, with the electrical conductivity (EC) of the BB watercolumn varying in the range 20 000-200 000 μS cm-1.7 Theoverlying water in PB is hypersaline, with EC values exceeding200 000 μS cm-1. Acidification has not occurred in PB during therecent prolonged drought (2002-2009) with water column pHvalues remaining near-neutral despite drawdown.Mineralogical Characterization. The bulk elemental com-

position was determined by X-ray fluorescence (XRF). Domi-nant minerals present in sediment samples were characterized bypowder X-ray diffraction (XRD) using Cu KR radiation. X-rayabsorption near edge structure (XANES) and extended X-rayabsorption fine structure (EXAFS) spectra were recorded fordried powders of reduced and oxidized sulfidic materials fromboth BB and PB and reference minerals (pyrite, pyrrhotite,goethite and hematite; see Supporting Information for experi-mental details). Sulfidic material from BB was also analyzed byenvironmental scanning electron microscopy (ESEM). Thereduced sulfur speciation was analyzed by sequential measure-ment of the following: acid volatile sulfur (AVS; nominallymeasures dissolved sulfide and iron monosulfides, reported aspercent dry weight of S); elemental sulfur (S0); chromiumreducible sulfur (CRS; nominally measures all other reducedsulfur species). AVS was determined using the cold diffusiontechnique.8 S0 was extracted from samples after AVS extractionusing acetone9 and analyzed by HPLC (see below). Post-extrac-tion sediment was triple-rinsed with acetone, air-dried at ambienttemperature, 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: combinedPt-ring electrode), gas sparging, and sample collection. Sedimentwas suspended using an overhead stirrer, and all experimentswere conducted at 20 �C. For each experiment, 150 mL of purewater (18 MΩ cm; Milli-Q) was added to the reactor pot andpurged with a gas mixture containing 418 ppm CO2 in N2 for30min. Field-moist sulfidic sediment (15( 0.1 g) was added andthe suspension equilibrated until the pH had stabilized; oxidationwas initiated by changing the gas flow to instrument grade air.Throughout the oxidation experiment, pH and EHwere recordedand samples removed for analysis, at time intervals consummatewith the reaction kinetics (increasing time intervals). Oxidationexperiments were continued until deemed complete, as indicatedby 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 a0.22 μm nitrocellulose membrane for the analysis of iron(II),acid anions, base cations, and thiosulfate (methods below).Iron(II) and thiosulfate analyses were conducted immediatelyafter collection while other ionic analyses were performed at alater time (samples frozen). The second subsample was filteredthrough a 0.45 μm nylon filter membrane; the filtrate wasacidified (to 0.1 mol L-1 HCl) and frozen for later analysis oftotal Fe, Al, and Si. The filter cake was used to analyze elemental

sulfur (by acetone extraction) after first rinsing the cake with 10mL of 2 mol L-1 HCl and then 10 mL ofMilli-Q water to removesulfides.11

Anaerobic Buffering Titrations. The pH buffering proper-ties of BB and PB sediment were studied under the same generalconditions as described for oxidation experiments (100 g L-1 wetweight), 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 mineralacid (0.1 mol L-1 HCl) were made using an autotitrator(Metrohm 721Net). Samples were collected via a rubber septumperiodically 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 thio-sulfate were determined by ion chromatography; the anion andthiosulfate separation system was a 150 mm Metrosep A Supp 5column with an (isocratic) eluant containing 1 mmol L-1

NaHCO3, 3.2 mmol L-1 Na2CO3, and 2% acetone; the cationseparation system was a 100 mm Metrosep C2 column with an(isocratic) eluant containing 2 mmol L-1 HNO3. NationalInstitute of Standards and Technology (NIST)-traceable ICstandards were used for instrument calibration, with regularcheck 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 wasmeasured by HPLC (LiChrospher 100 RP18 (C18) column;methanol mobile phase; UV detection at 254 nm).13,14

Geochemical Calculations. Geochemical calculations werecarried out using Geochemist’s Work Bench (GWB v6.0) usingthe default thermodynamic data set.15

’RESULTS AND DISCUSSION

Sulfide Mineralogy. Data reported in Supporting Informa-tion: 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 adominance of quartz (major) and illite (minor) with no othermineral phases at significant levels. Sulfidic sediment from PBalso had quartz as a dominant phase but with significant amountsof gypsum (CaSO4) and halite (NaCl), as well as illite andaragonite (CaCO3) (both minor). The sulfide content was toolow in either system to determine the iron sulfide mineralogyfrom diffraction. Environmental scanning electron microscopy(ESEM) analysis of sulfidic sediment (Figure S4, SupportingInformation) from BB revealed an iron-sulfide phase on thesurfaces 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 isentirely iron monosulfide. Sequential extraction of this materialrevealed 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 asFeS, with the remaining 2/3 in nonsulfide minerals. Given thereducing conditions of the sediment FeIII(oxyhydr)oxides areunlikely to persist; the presence of clay (illite) suggests that theremaining Fe is likely in clay-bound forms. While some of thisclay-bound Fe could be interlayer FeII, the high salinity of themediumwould more likely lead to a dominance of base cations,16

with Fe restricted to clay lamellae, as either FeII or FeIII. For PBsulfidic sediment, the AVS (0.21( 0.03%), S0 (0.08%), and CRS

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(0.07%) analyses also suggest that FeS is a dominant componentof the RIS species. On the basis of the XRF-derived Fe content(1.68%), FeS can account for ∼25% of the total Fe. Less than10% of the sediment S content is in the form of FeS, with gypsumthe dominant S-containing mineral.Buffering Properties of Bottle Bend and Psyche Bend

Sulfidic Sediments. The pH buffering properties of BB andPB sulfidic sediments were investigated in anaerobic (acidi-metric) titrations (Figure 1a); essentially an analysis of bufferingprovided by the sediment minerals, porewater bicarbonate, andorganic materials. Very little buffering is observed in BB sediment

above the free acid line. A comparison of the acid consumptionby the BB sediment with the release of divalent ions (FeII, Ca2þ,and Mg2þ) is shown in Figure 1b. FeII release was the dominantbuffering component (data not shown), indicating that theobserved buffering is almost entirely due to FeS dissolution; FeII

release is close to that expected from the FeS content of thissediment (100 g L-1; AVS = 0.21%; moisture content=26%;ΔFeII = 9.5 meq L-1). Considerably more buffering is observedin the PB sediment, in an amount that is more than 1 order ofmagnitude greater than the acid generation potential. A detailedanalysis of the buffering properties of this sediment is shown inFigure 1c, including a buffering intensity profile, major aqueouscations, and a comparison of calcium concentrations with known

Figure 1. (a) Anaerobic acidimetric titration curves for BB and PBslurries; AVS arrows indicate the expected acid generation from thereduced sulfide content (as AVS). Also shown is the free acid (zerobuffering) line. (b) Acid consumption and release of divalent ions (FeII,Ca2þ, andMg2þ; plotted as equivalent concentrations) for BB sediment,as a function of pH. (c) Detailed study of the buffering properties of PBsediment, showing (i) buffering intensity (log(β/2.3)) as a function ofpH, 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 thatexpected based on solubility control by aragonite (CaCO3) at pH > 7,and gypsum (CaSO4) control at pH < 7.

Figure 2. Physical and chemical parameters monitored during oxida-tion of BB reduced sediment: (a) pH and EH, (b) sulfur species (FeScalcline 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 additionalbuffering required for observed pH. Also shown in plot e are the likelycontributions to buffering by Ca2þ (and Mg2þ) and Fe(OH)3.

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Ca-mineral phase behavior. The high buffering intensity at pH 7matches closely the formation of aqueous Ca2þ and is consistentwith the dissolution of a calcium carbonate phase (aragonite);below the dissolution edge, calcium concentrations are con-trolled by gypsum (CaSO4) precipitation. Smaller bufferingfeatures are observed at pH 6 and 4.5; these are attributed toorganic or clay-exchange buffering, with small increases in Ca2þ

and Mg2þ concentrations over this pH range. The detection ofFeII at low pH is consistent with FeS dissolution; there is noevidence for clay dissolution (dissolved aluminum formation) onthe time scale of this titration.Bottle Bend Oxidation Studies. The physical and chemical

parameters measured during the oxidation of BB sulfidic sedi-ment are shown in Figures 2a-e; background aqueous ioniccomposition is given in Table S3 (Supporting Information).Prior to oxidation, the pH of these suspensions was∼7; separateexperiments under anaerobic conditions showed that this pHcould be maintained for periods greater than 80 h, ultimatelydecreasing slowly due to air leakage. Oxidation of these suspen-sions led to a rapid pH decrease to ∼5 and a correspondingincrease in suspension EH. Continued oxidation resulted in aslower 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’) corre-sponds to a plateau in the elemental sulfur concentration whichultimately converts to sulfate as an end product. Thiosulfateconcentrations were low throughout the oxidation, but detect-able levels were observed at the onset of sulfate formation. Thechange in solution concentration of sulfate from an initial level of1 mmol L-1 to a final value of ∼5.0 mmol L-1 is close to thatexpected from the FeS (AVSmeasurement) and elemental sulfurcontent (100 g L-1; AVS = 0.21%; So = 0.034%; moisturecontent = 26%;ΔSO4

2- = 5.6 mmol L-1). Given the sample vari-ability we have used the measuredΔSO4

2- value (4.1 mmol L-1)determined from oxidation of this material to calculate the sulfurmole balance. Approximately 1/3 of the initial FeS phase remainsunoxidized during the reaction poise (Figure 2b); the oxidationof this material occurs after the conversion of elemental sulfur tosulfate and coincides with the transient formation of FeII insolution. The general behavior of the BB sulfide oxidation wasreproduced in replicate experiments (n = 3), although the lengthof the reaction poise period (plateau elemental sulfur) did varybetween experiments. A similar delay in So oxidation has alsobeen observed in the oxidation of coastal sulfidic materials.4,5

Given that there was no external inhibition ofmicrobial processesin these experiments, the delay in the oxidation of elementalsulfur may be due to a microbial induction time.5 The oxidationprocesses in BB sediment can be summarized by eqs 1-3.Oxidation Steps.

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

oxidation of the particle core and the later FeII pulse. Similarshell-core oxidation behavior has been observed for nanoparti-culate chalcopyrite 18 and is likely typical of nanoparticulatemetal sulfides. Possible alternative explanations include thedissolution of the FeIII (oxyhydr)oxide reaction products andreduction of the released aqueous FeIII by sulfur, or the dissolu-tion 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 pHcorresponding to the onset of this reaction is 4, above theprecipitation 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 ispossible,16 however under the high salinity conditions of BBsediment, very little interlayer FeII is expected; iron carbonateminerals 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 systemto ∼2. The final pH of ∼3 in the oxidation of BB sedimentreveals the existence of additional buffering that was not evidentin the titration of this material (see Figure 1b). Concentrations ofSi and Al measured during the oxidation of BB sediment(Figure 2d) indicate the dissolution of clay minerals (Al andSi), with Al dissolution providing some buffering during oxida-tion (assumed to beΔAl:ΔHþ = 1:3, i.e., all dissolved Al presentas Al3þ), although insufficient to account for the observed finalpH. At longer times the continued dissolution of Al leads to aslight pH increase. Taking into account the known protongeneration processes (FeII oxidation and So oxidation), and pro-ton consumption processes (clay (Al) dissolution), the addi-tional required buffering can be calculated (eqs 4 and 5) wheresubscripts refer to the commencement of oxidation (‘0’) and anytime (‘t’) during oxidation.

ΔtotalHþ;t ¼ 2� ð½SO2-4 �t - ½SO2-

4 �0Þ

- 2� ð½FeII�t - ½FeII�0Þ- 3� ð½AlIII�t - ½AlIII�0Þ ð4Þadditional bufferingt ¼ ΔtotalHþ;t - 10-pHt ð5Þ

Two buffering regions are evident (Figure 2e), the firstcorresponding to the initial rapid oxidation process and thesecond to the sulfur oxidation step. The first of these twoprocesses appears to be due to (trace) carbonate mineraldissolution, or clay exchange processes; a small increase in thesolution concentrations of Ca2þ and Mg2þ equivalent to theinitial FeII acid generation potential was observed in bothoxidation and anaerobic titration experiments (data not shown)(i.e.,ΔCa2þþΔMg2þ =ΔFeII). The second buffering process issomewhat greater (∼3 mmol L-1) and can be explained by theformation of an acid-anion FeIII (oxyhydr)oxide (presumablywith chloride given the dominance of this anion in the aqueousmatrix) as shown by eq 6.

FeðOHÞ3ðsÞ þ xCl-ðaqÞ þ xHþðaqÞ f FeðOHÞ3 - xClxðsÞ þ xH2O

ð6ÞSeveral lines of evidence support the formation of such a

phase, including (i) the observed EH-pH behavior during theoxidation, showing a shift from a∼177mV/pH slope to a∼120mV/pH slope (see below), (ii) a relative depletion of chloridefrom the aqueous solution (Figure S5, Supporting Information),

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and (iii) the observed formation of akagan�eite (β-Fe-(OH)2.7Cl0.3) in the oxidized crustal material collected fromthe BB site.20 The recorded EH and pH data pairs are plotted ona Pourbaix diagram (Figure 3; Fe-S-Cl-H2O system, se-lected phases removed; see Supporting Information for fulldescription). The data show control of the measured redoxpotential by the Fe(OH)3/Fe

2þ couple in the initial stages ofoxidation, followed by a clear decrease in slope after thereaction poise (pH ∼5). The data has been fitted to a nominalFe(OH)2Cl phase (a high Cl-content is required to account for the

observed EH-pH slope); a comparison with phase boundaryposition for akagan�eite is shown in Figure S621 (SupportingInformation).Psyche Bend Reactor Studies. The physical and chemical

parametersmeasured during the oxidation of PB sulfidic sediment areshown in Figures 4a-c (FeII, Al, and Si not detected; data notshown).Thebackground aqueous ionic composition is given inTableS3 (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 pHremains in the range 7.5-8.5 throughout the oxidation (Figure 4a).Sulfur speciation (Figure 4b,c) shows an initial rapid formation ofelemental sulfur, followed by a decay that coincides with thiosulfateformation in solution. Surprisingly, the dynamics of sulfate forma-tion do not match that expected from elemental sulfur andthiosulfate profiles. Using the same approach as for BB, we usethe ΔSO4

2- value to calculate an initial FeS plus elemental sulfurconcentration (∼7 mmol L-1), and therefore a sulfur molebalance; a significant intermediate sulfur oxidation product isindicated (Figure 4b).Despite the strong buffering in this system, the small

decrease in pH corresponding to sulfate generation indicatesthat the intermediate is not sulfite (sulfite to sulfate conversionis not acid generating). We are unable to identify this sulfurintermediate. The sulfur mole balance shows evidence of asimilar shell-core oxidation process as hypothesized for BBsediment. In this case,∼1/2 of the sulfidic material is unreactedafter the initial rapid process, indicating a larger particle size.

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

Figure 4. Physical and chemical parametersmonitored during oxidation ofPB reduced sediment: (a) pH and EH; (b) sulfur species (except SO4

2-).Also shown is a calculated amount of unreacted FeS plus undetected sulfurintermediate from S-mole balance; (c) SO4

2- concentrations.

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) sulfidicsediments.

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X-ray Absorption Spectroscopic Analysis of Reduced andOxidized Sulfidic Sediments. In oxide mineral mixtures, theamplitude and position of the Fe K-edge pre-peak can be used todetermine both the oxidation state and coordination environ-ment of Fe;22 the use of the K-edge pre-peak in the analysis ofoxide/sulfide mixtures is less well developed. Comparison of theprepeaks for reduced material from BB and PB (BB-reduced andPB-reduced; Figure 5) with that for reference iron sulfideminerals (FeS (pyrrhotite) and FeS2 (pyrite)) is consistent withsulfide character in these sediments (Figure 5). EXAFS analysisof the first Fe coordination shell (mixture of Fe-O and Fe-S)provides some evidence for the presence of mackinawite (FeS) inthese sediments (see discussion in Supporting Information andassociated data: Table S4 (EXAFS fit parameters: referencematerials); Table S5 (EXAFS fit parameters: BB and PBsediments); Figure S7 (radial distribution functions (RDFs)and fits to Fourier filtered spectra). Given that FeS is not thedominant form of iron in these sediments, the majority of thepre-edge peak is due to nonsulfide components; the peak shapeof the BB-reduced and PB-reduced spectra suggest that at leastpart of this additional Fe is present as FeIII, presumably protectedfrom reduction under natural conditions due to incorporationinto 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 expectedfrom conversion of FeS to FeIII (oxyhydr)oxide (eqs 1-). Thespectral changes due to the oxidation of FeS are superimposed ona background of nonsulfide Fe minerals. The results of this workprovide no information about FeII f FeIII conversion in thisnonsulfide fraction during oxidation; such processes can also beacid generating and are worthy of further investigation.16

Fe Geochemistry in Sulfidic Sediments. We have recentlyreported on an acidification mechanism in low-sulfur wetlands ofthe Murray River caused by the displacement of FeII from clayminerals by base cations, followed by FeIII hydrolysis as the acid-generating step.16 Clay minerals are present in BB and PBsediments; however, the high background salt level in thesesystems likely results in very little contribution of reduced iron tothe clay interlayer charge. Reduced iron in these systems isinstead largely stored in sulfide form, a consequence of microbialsulfate reduction. While the reduced iron speciation in low- andhigh-sulfur wetlands will likely be different, the proton genera-tion per mole of reduced iron is in principle the same(ΔHþ:ΔFeII = 2:1, provided a pure FeIII (oxyhydr)oxide isformed as a reaction product), as shown by eqs 7 and 8, whereMxþ refers to a displacing cation, and � S to the interlayerexchange sites on clay minerals.

� S2Feþ 2xMxþ þ 1

4O2 þ 5

2H2O

f2x� SxMþ FeðOHÞ3 þ 2Hþ ð7Þ

FeSþ 94O2 þ 5

2H2O f FeðOHÞ3 þ SO2-

4 þ 2Hþ ð8Þ

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

conditions required for acidification to occur (salinization andoxidation in the case of low-sulfur systems, and oxidation in thecase of high sulfur systems), and (ii) the rate of oxidation(strongly pH dependent in the case of displaced FeII, and limitedby surface reactions (e.g., passivation) in the case of iron sulfide).In this work we have shown that acid generation can be attenuatedby the formation of acid-anion FeIII (oxyhydr)oxides. Similarproducts are commonly observed as oxidization products of ironsulfides, particularly schwertmannite and jarosites in high sulfatesystems.2,3,23

Sulfidic Sediment Oxidation and Wetland Acid Neutraliz-ing Capacity (ANC). The acid generation potential of a wetlandsediment 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 ofsufficient ANC in BB may be due to hydrological phenomenasuch as groundwater flow-through.3 Pysche Bend, as an irrigationdrainage collection pond, is presumably a more closed system,allowing the ANC received from drainage water, and thatgenerated by iron reduction to be retained. It is this storage ofgenerated ANC that is our ongoing research interest, as this iscritical to the robustness of wetlands toward acidification andlikely to be influenced by the flow regime and therefore sensitiveto the management of these systems.

’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 fromBB and PB (Figure S3); ESEM image and EDX pattern for BBsulfidicmaterial (Figure S4); relative depletion of chloride duringoxidation of BB sediment (Figure S5); Pourbaix diagrams in Fe-S-Cl-H2O system with either (nominal) Fe(OH)2Cl or aka-gan�eite phase (Figure S6); fits to Fourier filtered EXAFS data forBB and PB reduced and oxidized sediments (Figure S7); XRFdata for BB and PB sulfidic sediment (Table S1); reduced sulfurspeciation (Table S2); ionic composition in BB and PB oxidationexperiments (Table S3); fitted Fe-O and Fe-S distances andnumber of neighbors for reference minerals (Table S4); fittedFe-O and Fe-S distances for reduced and oxidized sulfidicsediment from BB and PB (Table S5). This information isavailable 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.

’ACKNOWLEDGMENT

This project was funded by the National Water Commissionthrough its Raising National Water Standards Program. ThisAustralian Government program supports the implementation ofthe National Water Initiative by funding projects that areimproving Australia’s national capacity to measure, monitor,and manage its water resources. Additional funding wasreceived from the NSW Murray Wetlands Working Group andMurray-Darling Freshwater Research Centre. Access to the

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Australian Synchrotron was through the foundation investorscheme.

’REFERENCES

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