Article pubs.acs.org/est
Determination of Hexavalent Chromium Reduction Using Cr Stable Isotopes: Isotopic Fractionation Factors for Permeable Reactive Barrier Materials Anirban Basu* and Thomas M. Johnson Department of Geology, University of Illinois at Urbana-Champaign, 208 Natural History Building, 1301 West Green Street, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: Cr stable isotope measurements can provide improved estimates of the extent of Cr(VI) reduction to less toxic Cr(III). The relationship between observed 53Cr/52Cr ratio shifts and the extent of reduction can be calibrated by determining the isotopic fractionation factor for relevant reactions. Permeable reactive barriers (PRB) made of Fe0 and in situ redox manipulation (ISRM) zones effectively remediate Cr-contaminated aquifers. Here, we determine the isotopic fractionations for dominant reductants in reactive barriers and reduced sediments obtained from an ISRM zone at the US DOE’s Hanford site. In all cases, significant isotopic fractionation was observed; fractionation (expressed as ε) was −3.91‰ for Fe(II)-doped goethite, −2.11‰ for FeS, −2.65‰ for green rust, −2.67‰ for FeCO3, and −3.18‰ for ISRM zone sediments. These results provide a better calibration of the relationship between Cr isotope ratios and the extent of Cr(VI) reduction and aid in interpretation of Cr isotope data from systems with reactive barriers.
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α = R product /R reactant
INTRODUCTION Chromium (Cr) contamination, arising mostly from anthropogenic and sometimes, natural sources, is common in soils, groundwater and surface waters. Anthropogenic sources include industrial practices, such as leather tanning, chromium plating, pigment manufacturing, wood preservation, and the use of Cr as a corrosion-inhibitor in cooling towers,1 and natural sources, including leaching of Cr during weathering of ultramafic rocks.2 The toxicity of Cr is determined by its redox state. In aqueous systems, Cr occurs in two valence states; hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Under circumneutral pH conditions, Cr(VI) is soluble, highly mobile and toxic whereas Cr(III) is insoluble, strongly adsorbing and less toxic. Numerous abiotic reductants (e.g., Fe(II)-bearing minerals, aqueous Fe(II), Fe(II) sorbed onto iron oxides and hydroxides, sulfides), naturally occurring organic compounds, and microbes can reduce Cr(VI) in the subsurface. Reduction of Cr(VI) to Cr(III) as a means to immobilize Cr in a less toxic form in contaminated aquifers is a common remediation strategy.3 Cr isotope ratios provide a means to detect and perhaps quantify reduction. The reduction reactions fractionate Cr isotopes: Reaction products are enriched in lighter isotopes, and with progressive reduction the remaining reactant pool becomes enriched in heavier isotopes. The 53Cr/52Cr ratios measured in groundwater samples are used to quantify enrichment or depletion of 53Cr relative to 52Cr. The magnitude of the isotopic fractionation is measured by the fractionation factor, α: © 2012 American Chemical Society
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where Rproduct and Rreactant are Cr/ Cr in the Cr(III) product flux and in the remaining Cr(VI) pool, respectively. The isotopic fractionation during oxidation of Cr(III) to Cr(VI) is small (1 mM) of the reactants in their experiments. Also, in their study, continuous injection of high concentrations of aqueous Fe(II) and subsequent formation of green rust near the inlet could lead to very rapid diffusion-limited reduction of Cr(VI) which in turn would produce εeff of a very small magnitude. Interestingly, the ε values from our GR-SO4 experiments coincide roughly with the fractionation observed for the sediments studied by Berna et al.,44 which were inferred to contain green rust or similar phases. The isotopic fractionation for Cr(VI) reduction by FeCO3 is also weaker relative to that for dissolved Fe(II). This leads us to 5358
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nations of the extent of Cr(VI) reduction help assess the reductive capacity and performance of the existing PRB/ISRM zone and help diagnose problems. These results can be used to interpret Cr isotope data being collected at the Hanford ISRM barrier, and more generally improve our knowledge of Cr isotope fractionation in a variety of geochemical applications.
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ASSOCIATED CONTENT
S Supporting Information *
Details of experimental methods, including mineral synthesis, Cr(VI) batch incubations, MC-ICP-MS and concentration analysis procedures, and Cr(VI) concentration and δ53Cr data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (217) 333-2695. Fax: (217) 244-4996. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the U.S. Department of Energy, Office of Science (BER) in the Subsurface Biogeochemical Research Program under grant DE-FG02-07ER64405. We thank Jim Szecsody (Pacific Northwest National Lab) for providing sediment samples from the Hanford ISRM zone.
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