Mechanisms and rates of uranium(VI) reactions with aqueous and ferrous sulfide (mackinawite) were studied under a range of geochemical conditions. The experimental conditions were selected to cover the variations in groundwater chemistry observed at the Old Rifle Uranium Mill Tailings Remedial Action (UMTRA) site (Colorado, USA). X-ray diffraction, X-ray absorption spectroscopy, and transmission electron microscopy showed formation of uraninite as the major U(VI) removal mechanism by the reaction with aqueous and ferrous sulfide.  Geochemical variables including pH, S(-II), Ca(II), U(VI), and carbonate concentrations had strong impacts on the rate of U reduction by aqueous sulfide. Thermodynamic calculations suggested that the initial U(VI) aqueous speciation had systematic relations with the U reduction rate by aqueous sulfide.  The results of this study show that abiotic U reduction by aqueous and ferrous sulfide can occur to a significant extent making it an important process to consider in evaluating U mobility in aquifer groundwater at contaminated sites under naturally achieved and stimulated sulfate reducing conditions.

Uranium(VI) reactions with aqueous and ferrous sulfide

243rd National Meeting (2012)

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