A promising strategy for in-situ remediation of uranium (U) is through the biostimulation of iron- and sulfate-reducing native bacterial species, which mediate the reduction of soluble U(VI) leading to its precipitation as uraninite (UO2) mineral. Uraninite is generally regarded as the most desirable product of bioreduction because of its low solubility under reducing conditions. At U-contaminated sites, naturally-occurring Fe(III) reducing bacteria (FeRB) areconsidered as the primary control for the immobilization of U(VI) as uraninite; however, upon transitioning from Fe(III)-reducing to sulfate-reducing conditions, U remobilization is observed (Anderson et al. 2003).  These observations suggest that sulfate reducing conditions control the stability of bioreduced U; and the study of fate and transport of U reduced by SRB isvery important in the understanding of subsurface microbial reductive precipitation of U and other actinides. Here we report on the Desulfovibrio desulfuricans G20 which reduced U(VI) tonanometer-sized UO2 particles (1-5 nm) which were both intra- and extracellular, with higher initial U(VI) concentration resulting in higher amount of reduced U residing intracellularly as well as in aggregate form outside the cells.  The results of the UO2 nanoparticles produced by G20 cells under non-growth (with cells only) conditions have showed that 0-8% of bioreduced U existed in a mobile phase with initial concentrations of 150 and 900 uM U(VI) in PIPES or bicarbonate buffer. The aggregation of UO2 nanoparticles depended on the initial concentrations of U(VI) and the type of buffer used. On the other hand, under growth (liquidculture with metal toxic media (MTM; Sani et al. 2001)) conditions, a significant mass fraction of bioreduced U (28-65 %) existed as a mobile phase in the presence of different electrondonors (lactate or pyruvate) and acceptors (sulfate, thiosulfate, or fumarate). When the G20 culture was treated with 2 mM U(VI) in MTM in the presence of hematite, HR-TEM, EDS and SEAD data showed the coexistence of biogenic uraninite as well as iron sulfides within the periplasm and cytoplasm, as well as outside the cells. M�ssbauer spectroscopy data of the samples and its temperature behavior suggested that the iron sulfide precipitates could be mackinawite. The results however showed that still a significant mass fraction of bioreduced U(35-60%) existed as a mobile phase, raising several fundamental questions regarding long-term U reclamation stewardship. This study presents findings that would be helpful to quantitatively analyze the reduction of soluble U(VI) to bioreduced U nanoparticles and theroles of natural indigenous bacteria in appropriate long-term bioremediation of U(VI) present at subsurface contaminated sites.

Aggregation of biogenic U(IV) nanoparticles: Impact of different environmental conditions

247th National Meeting (2014)

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