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

Armstrong, Elizabeth

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https://hdl.handle.net/2142/72921 Copy

Description

Title

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

Author(s)

Armstrong, Elizabeth

Issue Date

2015-01-21

Director of Research (if dissertation) or Advisor (if thesis)

Johnson, Thomas J.

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

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

Abstract

Chromium and uranium naturally occur in a variety of Earth’s waters. Both elements can undergo valence state changes at surface conditions which control their chemical behavior in near surface settings. Reduction of hexavalent chromium (Cr(VI)) and uranium (U(VI)) greatly decreases their solubility and mobility, and therefore their toxicity in groundwater. Redox transformations 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 of problems related to dilution, advection, and adsorption that plague the standard concentration based approach. The magnitude of isotopic fractionation has been determined for various abiotic and biotic reduction 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 fractionation associated with reduction of Cr(VI) to Cr(III) by ascorbate and reduction of U(VI) to U(IV) by sulfide. The results of this study yielded isotopic fractionation factor values (ε) for reduction of Cr(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 in the remaining U(VI) appears to have increased by 0.17‰ after about 60% reduction, but the analytical uncertainty was about 0.15‰.

Graduation Semester

2014-12

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http://hdl.handle.net/2142/72921

Copyright and License Information

Copyright 2014 Elizabeth Armstrong

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