Article pubs.acs.org/est
Thioarsenic Species Associated with Increased Arsenic Release during Biostimulated Subsurface Sulfate Reduction Valerie K. Stucker,*,†,∥ David R. Silverman,‡ Kenneth H. Williams,§ Jonathan O. Sharp,‡ and James F. Ranville† †
Chemistry and Geochemistry Department and ‡Civil and Environmental Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States § Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Introduction of acetate into groundwater at the Rifle Integrated Field Research Challenge (Rifle, CO) has been used for biostimulation aimed at immobilizing uranium. While a promising approach for lowering groundwater-associated uranium, a concomitant increase in soluble arsenic was also observed at the site. An array of field data was analyzed to understand spatial and temporal trends in arsenic release and possible correlations to speciation, subsurface redox conditions, and biogeochemistry. Arsenic release (up to 9 μM) was strongest under sulfate reducing conditions in areas receiving the highest loadings of acetate. A mixture of thioarsenate species, primarily trithioarsenate and dithioarsenate, were found to dominate arsenic speciation (up to 80%) in wells with the highest arsenic releases; thioarsenates were absent or minor components in wells with low arsenic release. Laboratory batch incubations revealed a strong preference for the formation of multiple thioarsenic species in the presence of the reduced precursors arsenite and sulfide. Although total soluble arsenic increased during field biostimulation, the termination of sulfate reduction was accompanied by recovery of soluble arsenic to concentrations at or below prestimulation levels. Thioarsenic species can be responsible for the transient mobility of sedimentassociated arsenic during sulfidogenesis and should be considered when remediation strategies are implemented in sulfatebearing, contaminated aquifers.
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INTRODUCTION The mobility of metals and metalloids in subsurface systems results from a complex interplay between hydrology, geochemistry, and microbiology, which must be considered by practitioners targeting contaminant attenuation. For example, in situ biostimulation, a contaminant remediation technique that enhances the activity of native microorganisms, can be applied to shift elemental speciation to favor the formation of less mobile phases, thereby limiting aqueous mobility. However, in the process of immobilizing one element, manipulation of redox conditions can have deleterious consequences for nontarget pollutants creating conditions favorable for their release to groundwater. Reductive immobilization in groundwater has been shown to be effective for lowering uranium concentrations to below the United States Environmental Protection Agency (USEPA) maximum contaminant level (MCL) of 30 μg/L during repeated experiments in an unconfined alluvial aquifer at the Rifle Integrated Field Research Challenge Site (Rifle site) in Rifle, CO.1−4 In this case, biostimulation was achieved through the in situ delivery of sodium acetate to shallow groundwater via injection wells to stimulate indigenous microorganisms, © 2014 American Chemical Society
driving the system into geochemically reducing conditions. Under these conditions, U(VI) was reduced to U(IV), which forms a number of low-solubility phases.5,6 While biostimulation effectively mitigated uranium transport, additional changes to subsurface biogeochemistry included a several-fold increase in total arsenic concentrations above prestimulation levels.7 Arsenic, present in surface and ground waters as a result of both natural and anthropogenic processes, is a toxic element and potential carcinogen.8 The USEPA MCL for arsenic is 10 μg/L or 0.13 μM;9 however, as a function of the background geology and status as a former ore milling site, the alternate contamination level for the Rifle site is 50 μg/L or 0.67 μM.10 Successful groundwater remediation at the Rifle site requires an understanding of arsenic mobilization and a strategy to keep both soluble uranium and arsenic below regulatory limits. Received: Revised: Accepted: Published: 13367
July 20, 2014 October 16, 2014 October 20, 2014 October 20, 2014 dx.doi.org/10.1021/es5035206 | Environ. Sci. Technol. 2014, 48, 13367−13375
Environmental Science & Technology
Article
Figure 1. Rifle site Plot C well diagram. Open circles represent acetate injection wells, gray circles represent bicarbonate injection wells, black circles represent downgradient monitoring wells, and black squares represent upgradient monitoring wells. The arrow indicates measured groundwater flow direction.
the onset of sulfidogenesis, exceeding the arsenic release associated with the reductive dissolution of Fe(III) minerals.7 Aqueous arsenic−sulfur compounds, or thioarsenic species, have recently been considered to be important arsenic species in reduced, sulfidic waters. Thioarsenites, As(III)−S−(O) compounds, have been formed in aqueous mixtures of arsenite and sulfide.34 Meanwhile, thioarsenates, As(V)−S−(O) compounds, are more typically observed in environmental samples.34−38 Where the oxidation state of thiolated arsenic compounds is uncertain, we have used the term “thioarsenic” herein. With seemingly contradictory observations of arsenic mobilization under sulfate reducing conditions, the results of the Rife biostimulation experiments7 should not be extrapolated broadly without a better mechanistic understanding of the processes involved. The objective of this study was to better understand the mobilization of arsenic observed during fieldscale biostimulation at Rifle, CO, particularly with respect to the impacts of iron and sulfur on arsenic release and thioarsenic formation.
In solutions near neutral pH, arsenic is typically present as As(V) or As(III) oxyanions, arsenate and arsenite, in oxic and anoxic environments, respectively.11 Arsenic can be mobilized due to the direct enzymatic reduction of sorbed arsenate to arsenite, which in some environments sorbs less strongly to Fe(oxy)hydroxides and Fe-sulfides than arsenate.12−16 Additionally, microbially mediated reductive dissolution of Fe- and Mnhydroxides can release sorbed and surface-associated arsenic into solution,16−18 which is also often reduced in the groundwater. As a result, arsenite is generally considered more mobile than arsenate in environmental systems; however, arsenic sorption to Fe-, Mn-, and Al-hydroxides is a complex process that depends on solution chemistry, competition for sorption sites, and the nature of the sorbent.14,19,20 Along with metal reduction, sulfate reduction has also been shown to impact the mobility of arsenic in subsurface environments.21,22 Many studies support the efficacy of inducing sulfate reducing conditions for the in situ immobilization of arsenic in or on As−S(−Fe) precipitates. In bench-scale bioreactors, arsenic concentrations decreased when sulfatereducing conditions were achieved due to arsenic sorption to iron sulfides23−25 and/or the formation of As−S minerals, such as arsenic sulfide, orpiment, and realgar.23,24,26 Arsenic was also immobilized under sulfate-reducing conditions through covalent bonding to organic sulfur compounds.27 Flow-through columns have indicated decreased arsenic concentrations through the formation of FeAsS through microbially induced sulfate reducing conditions.28 Zero-valent iron can be used to immobilize arsenic, and the addition of sulfide has been demonstrated to enhance arsenic removal.29 Though several studies show bioremediation effectively immobilized arsenic through coprecipitation with sulfides,30 biogenic sulfate reduction can also mobilize arsenic.31 Several factors have been identified that influence arsenic mobility, including availability of aqueous iron and sulfide, as well as the rate of iron oxide dissolution and iron sulfide precipitation.32 The presence of sulfide has increased arsenic concentrations from the dissolution of orpiment and arsenopyrite.33At the Rifle site, the highest arsenic concentrations were observed following
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EXPERIMENTAL SECTION Collection and Preservation of Rifle Field Samples. The Rifle site has been used to investigate remediation strategies over the past 12 years, and more extensive details can be found elsewhere.39 Background sulfate, iron, and arsenic concentration ranges (and means) at the site over the 12 year period were 4.98−18.43 (8.39) mM,
