Article pubs.acs.org/est
Sulfate Availability Drives Divergent Evolution of Arsenic Speciation during Microbially Mediated Reductive Transformation of Schwertmannite Edward D. Burton,*,† Scott G. Johnston,† Peter Kraal,† Richard T. Bush,† and Salirian Claff†,‡ †
Southern Cross GeoScience, Southern Cross University, Lismore, NSW 2480, Australia Crop, Environment and Livestock Division, Japan International Research Center for Agricultural Sciences, Tsukuba, Ibaraki 305-8686, Japan
‡
S Supporting Information *
ABSTRACT: The effect of SO42− availability on the microbially mediated reductive transformation of As(V)-coprecipitated schwertmannite (Fe8O8(OH)3.2(SO4)2.4(AsO4)0.004) was examined in long-term (up to 400 days) incubation experiments. Iron EXAFS spectroscopy showed siderite (FeCO3) and mackinawite (FeS) were the dominant secondary Fe(II) minerals produced via reductive schwertmannite transformation. In addition, ∼25% to ∼65% of the initial schwertmannite was also transformed relatively rapidly to goethite (αFeOOH), with the extent of this transformation being dependent on SO 4 2− concentrations. More specifically, the presence of high SO 4 2− concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO42− also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO42− reduction. In contrast, where SO42− was less available, there was near-complete reduction of schwertmanniteand goethite-derived Fe(III) as well as solid-phase As(V). As a result, under low SO42− conditions, almost no Fe(III) or As(V) remained toward the end of the experiment and arsenic solid-phase partitioning was controlled mainly by sorptive interactions between As(III) and mackinawite. These As(III)−mackinawite interactions led to the formation of an orpiment (As2S3)-like species. Interestingly, this orpiment-like arsenic species did not form under SO42−-rich conditions, despite the prevalence of dissimilatory SO42− reduction. The absence of an arsenic sulfide species under SO42−-rich conditions appears to have been a consequence of schwertmannite persistence, combined with the preferential retention of arsenic oxyanions by schwertmannite. The results highlight the critical role that SO42− availability can play in controlling solid-phase arsenic speciation, particularly arsenic−sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems.
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anoxia in, for example, reflooded ASS wetlands,4,5,26,27 mine-pit lake sediments,28−30 and AMD-treatment wetlands.31 In schwertmannite-bearing systems, aqueous SO42− concentrations can range from sub-mM to >100 mM.8,32,33 This variation in SO42− may strongly influence anoxic biologically mediated redox processes. In particular, a greater supply of SO42− can increase the rates and ultimate extent of dissimilatory SO42− reduction.34 Sulfate reduction may, in turn, alter the evolution of iron and arsenic speciation,35−37 potentially leading to divergence between SO42−-rich and SO42−-poor systems. At present, our understanding of how variable SO42− availability influences the anoxic transformation of schwert-
INTRODUCTION Arsenic often poses a serious threat to water resources in acid sulfate soil (ASS) and acid mine drainage (AMD) environments.1−6 Schwertmannite (Fe8O8(OH)8−2x(SO4)x, where x usually spans 1 to 1.75) is a poorly crystalline Fe(III)oxyhydroxysulfate, which occurs widely in such environments.7−13 Schwertmannite is capable of sequestering arsenic via sorption and coprecipitation reactions.14,15 As a result, schwertmannite represents an important mineralogical control on arsenic mobility in ASS- and AMD-affected aquatic systems.4,6,16,17 Schwertmannite stability has been extensively evaluated under oxic conditions.7,8,18−22 Likewise, the speciation and mobility of schwertmannite-associated arsenic has also been widely evaluated under oxic conditions.6,14−17 In contrast, less research has addressed the behavior of schwertmannite and associated arsenic under anoxic conditions.4,23−25 This is important as schwertmannite may be subjected to prolonged © 2013 American Chemical Society
Received: Revised: Accepted: Published: 2221
September 23, 2012 December 13, 2012 February 4, 2013 February 4, 2013 dx.doi.org/10.1021/es303867t | Environ. Sci. Technol. 2013, 47, 2221−2229
Environmental Science & Technology
Article
species).The supernatant solution was filtered to 300 mV). By day 8, pH had increased to ∼6.5 and Eh had decreased to ∼100 mV in both the low and high SO42− treatments (Figure 1a and b). From day 8 onward, the pH and Eh in both treatments remained relatively constant at ∼6.4−6.5 and 100−150 mV, respectively. Aqueous-Phase Sulfate, Iron, and Arsenic Concentrations. In the low SO42− treatment, aqueous SO42− increased from 0 mM initially to ∼20 mM at day 8 (Figure 1c). In contrast, aqueous SO42− in the high SO42− treatment decreased from 100 to ∼90 mM over the initial 8-day period (Figure 1c). In the low SO42− treatment, aqueous SO42− had decreased to 5, whereas dissimilatory Fe(III) reduction occurs readily at low pH.30 Therefore, the initial increase in aqueous Fe2+ under acidic conditions can be attributed to the reductive dissolution of schwertmannite: CH3COOH + Fe8O8(OH)6 (SO4 )(s) + 12H+ → 2HCO3− + 8Fe 2 + + SO4 2 − + 10H 2O
(1)
This process consumes acidity and would have forced the observed increase in pH during the first few days of the experiment. The increase in pH is likely to have then facilitated the onset of dissimilatory SO42− reduction, which can be represented as
Figure 4. (a) Arsenic EXAFS spectra and the (b) corresponding Fourier transform for reference standards and solid-phase samples from the high and low sulfate treatments. For the experimental samples, data are shown as a solid line while the linear combination fit (LCF) is shown as a dashed-line. The LCF for the high sulfate treatment is 37% arsenate and 63% arsenite and for the low sulfate treatment is 53% arsenite and 47% orpiment.
CH3COOH + SO24 − → 2HCO−3 + H 2S
(2)
SO42−
H2S is a major product of reduction, yet aqueous H2S remained
