PNNL

Abstract

The accessibility of precipitated, intragrain U(VI) in a contaminated sediment to microbial reduction was investigated to ascertain geochemical and microscopic transport phenomena controlling U(VI) bioavailability. The sediment was collected from the US DOE Hanford site, and contained uranyl precipitates in a form of Na-boltwoodite within the mm-sized granitic lithic fragments in the sediment. Microbial reduction was investigated in a culture of a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1, in bicarbonate solutions at pH 6.8 buffered by PIPES. Measurements of uranium concentration, speciation, and valence in aqueous and solid phases indicated that microbial reduction of intragrain U(VI) proceeded by two mechanisms: 1) sequentially coupled dissolution of intragrain uranyl precipitates, diffusion of dissolved U(VI) out of intragrain regions, and microbial reduction of dissolved U(VI); and 2) U(VI) reduction in the intragrain regions by soluble, diffusible biogenic reductants. The bioreduction rate in the first pathway was over 3 orders of magnitude slower than that in comparable aqueous solutions containing aqueous U(VI) only. The slower bioreduction rate was attributed to: 1) the release of calcium from the desorption/dissolution of calcium-containing minerals in the sediment, which subsequently altered U(VI) aqueous speciation and slowed U(VI) bioreduction and 2) alternative electron transfer pathways that reduced U(VI) in the intragrain regions and changed its dissolution and solubility behavior. The results implied that the overall rate of microbial reduction of intragrain U(VI) will be influenced by the reactive mass transfer of U(VI) and biogenic reductants within intragrain regions, and geochemical reactions controlling major ion concentrations that affect U(VI) aqueous speciation and microbial activity.