Biostimulation of mine-impacted sediments is studied as a method for remediation of U by reduction of U(VI) to sparingly soluble UO<sub>2</sub>. We examine the simultaneous redox dynamics of U and As in anaerobic mine tailings sediments because As is commonly present at U mining and milling sites. Like U, the mobility of As is controlled by redox speciation with As(III) generally being more mobile than As(V). Batch sediment incubations were spiked with either U(VI), As(V), or both. No U(VI) reduction was observed during the initial 20 day period of dissimilatory iron reduction. Mackinawite (FeS) was observed to precipitate at the onset of sulfate reducing conditions in systems without U(VI). Abiotic experiments with added Fe(II) and sulfide indicate that the formation of mackinawite is necessary in order for U(VI) reduction to take place. This observation points to the primary mechanism of reduction being abiotic reduction by mackinawite alone or in combination with microbial sulfate reduction. Although some As(V) reduction was observed in all treatments, an increase in solid-associated As(III) was observed in systems where mackinawite precipitation occurred. Soil microcosms were stimulated with a variety of electron donors—acetate, lactate, and ethanol—in order to compare differences in the extent of U(VI) and As(V) reduction. The greatest extent of U(VI) reduction was observed in acetate amended systems while the greatest extent of As(V) reduction was observed in ethanol systems. Significant differences in microbial communities were observed between different electron donors and in systems with and without U(VI). Oxidation experiments were also performed and show that mackinawite protects reduced U and As from reoxidation. Our findings clearly show that remediation strategies for uranium polluted sediments need to consider toxic elements with contrasting redox behavior such as arsenic and that sulfate reducing conditions are required for reductive U immobilization.

Fate of U and As in biostimulated mine tailings sediments

249th National Meeting (2015)

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