Sunday, July 24, 2011
Grand Ballroom, 5th fl (Sheraton New Orleans)
Uraninite is generally regarded as the most desirable product of U bioreduction because of its low solubility under reducing conditions. However, our results show the presence of a significant amount (35-60%) of reduced U in the mobile phase, and raise several fundamental questions. What fraction of biogenic uraninite is i) associated with the surfaces of bacterial cells, ii) without cells, iii) in the periplasmic or cytoplasmic regions of cells, or iv) associated with colloids (e.g., iron sulfide nanoparticles). In U-contaminated sites, what controls the distribution of uraninite in mobile vs. immobile (mineral) phases? To address these basic science questions, we characterized biogenic U(IV) in batch systems produced by Desulfovibrio desulfuricans G20. We also used Fe(III)(hydr)oxide minerals (hematite and ferrihydrite) and quartz (α-SiO2, 212-300 mm) as model redox-sensitive and -insensitive aquifer minerals, respectively. Size fractions of U in various batch experiments were obtained by ultrafiltration using Nanosep devices. Samples of unfiltered, filtered oxidized, and unfiltered oxidized uraninite were analyzed for U(VI) content using a Kinetic Phosphorescence Analyzer. Results showed that aqueous phase U(IV) concentrations depended on the initial U(VI) concentration used. TEM results showed that uraninite particles were present adsorbed on the cell surfaces and inside the cells. Secondary minerals formed were also analyzed using X-ray photoelectron spectroscopy, X-ray diffraction, micro-XRD, and Mössbauer spectroscopy. These results improve our understanding of the fate and transport of bioreduced uranium, and are applicable in the assessment of long-term sequestration of U in the environment.