title: Determination of hexavalent chromium reduction using Cr stable isotopes: isotopic fractionation factors for permeable reactive barrier materials.
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 (53)Cr/(52)Cr 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 Fe(0) 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 FeCO(3), 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.
authors.affiliation: Department of Geology, University of Illinois at Urbana-Champaign, 208 Natural History Building, 1301 West Green Street, Urbana, Illinois 61801, United States. abasu3@illinois.edu
authors.forename: Anirban
authors.initials: A
authors.lastname: Basu
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Johnson
mesh_terms: D000327:Adsorption; D002254:Carbonates / Q000737:chemistry; D005591:Chemical Fractionation / Q000379:methods*; D002857:Chromium / Q000032:analysis*; D002859:Chromium Isotopes; D004058:Diffusion; D052918:Environmental Restoration and Remediation; D005290:Ferric Compounds / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry; D007553:Isotope Labeling; D008903:Minerals / Q000737:chemistry; D010084:Oxidation-Reduction; D010539:Permeability
chemical_list: D002254:Carbonates; D002859:Chromium Isotopes; D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; D002857:Chromium; C094886:goethite; C074702:chromium hexavalent ion; D007501:Iron; C486982:siderite