Isotopic fractionation of chromium and uranium during Cr(VI) reduction by ascorbate and U(VI) reduction by sulfide

Uranium;Chromium;Isotope fractionation;reduction;sulfide;ascorbate;isotope geochemistry;isotope

Armstrong, Elizabeth ; Johnson ; Thomas J.

关键词: Uranium;    Chromium;    Isotope fractionation;    reduction;    sulfide;    ascorbate;    isotope geochemistry;    isotope;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/72921/Elizabeth_Armstrong.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Chromium and uranium naturally occur in a variety of Earth’s waters. Both elements canundergo valence state changes at surface conditions which control their chemical behavior innear surface settings. Reduction of hexavalent chromium (Cr(VI)) and uranium (U(VI)) greatlydecreases their solubility and mobility, and therefore their toxicity in groundwater. Redoxtransformations have been shown to induce predictable shifts in isotope ratios for both elements.As a result, Cr and U isotope ratios can be used to track the extent of reduction independent ofproblems related to dilution, advection, and adsorption that plague the standard concentration basedapproach.The magnitude of isotopic fractionation has been determined for various abiotic and bioticreduction reactions in the Cr and U systems; other reductants of interest remain to be studied.Using batch reactor experiments, this study quantifies the magnitude of isotopic fractionationassociated with reduction of Cr(VI) to Cr(III) by ascorbate and reduction of U(VI) to U(IV) bysulfide.The results of this study yielded isotopic fractionation factor values (ε) for reduction ofCr(VI) by ascorbate of -2.83‰ ±0.05‰ and -3.16‰ ±0.23‰ in two duplicate experiments.These results are closely similar to earlier experiments using organic reductants. In contrast,reduction of U(VI) via sulfide does not induce significant isotopic fractionation. The 238U/235U inthe remaining U(VI) appears to have increased by 0.17‰ after about 60% reduction, but theanalytical uncertainty was about 0.15‰.