Isotopic fractionation of chromium and uranium during abiotic and microbial Cr(VI) reduction and microbial U(VI) reduction

Reduction of hexavalent chromium (Cr(VI)) influences the chemical behavior of Cr in a variety of geochemical settings such as ancient and modern oceans, lakes and groundwater and surface water systems. Cr(VI) reduction immobilizes the toxic form of Cr to less toxic and insoluble Cr(III). Several abiotic (e.g., Fe(II)-minerals) and biological (e.g. bacteria) reductants readily reduce Cr(VI) to Cr(III). These reduction reactions fractionate Cr isotopes and alter the Cr stable isotope ratios. Cr stable isotope ratios can be used to track the extent of this reduction and thus can be used as a redox-indicator. The magnitude of the isotopic fractionation must be determined to connect observed Cr isotope ratios to the extent of reduction. This study determines the Cr isotopic fractionation factors for Cr(VI) reduction by Fe(II) phases (Fe(II-doped goethite, FeS, green rust, siderite and sediments from an active barrier) occurring at Permeable Reactive Barriers and by a group of metabolically diverse bacteria (G. sulfurreducens, Shewanella sp. (Neckar River), P stutzeri DCP-Ps1 and D. vulgaris). The results can be used to interpret Cr isotope data from a wide range of geochemical settings involving abiotic and microbial Cr(VI) reduction. Similar to Cr, the geochemical behavior of uranium (U) is also governed by its oxidation state. The oxidized species U(VI) is stable and extremely soluble under oxic surface conditions whereas the reduced species U(IV) is insoluble. Redox transformation causes U isotope fractionation; the resulting U isotope ratio changes are related to the extent of U(VI) reduction. This study determines the magnitudes of isotopic fractionation by a wide variety of bacteria (G. sulfurreducens PCA, G sulfurreducens Criddle, A. dehalogenans FRC-W, A. dehalogenans FRC-R5, Shewanella sp. (Neckar River) and Desulfitobacterium Viet1) with different metabolisms. The results reveal the nature of microbial U(VI) isotope fractionation and the controlling factors under nutrient-poor conditions. The results of this study can be used to detect and possibly quantify U(VI) reduction in U-contaminated aquifers. This study aids in the interpretation of the U isotope data from any environment involving microbial U(VI) reduction from modern oceans to rock records