Institute: South Dakota
Year Established: 2011 Start Date: 2011-03-01 End Date: 2012-02-28
Total Federal Funds: $16,825 Total Non-Federal Funds: $33,650
Principal Investigators: Rajesh Sani
Abstract: Uranium (U) contamination is a global problem in aquifers, water supplies, and related ecosystems. A promising strategy for in situ remediation of U is via biostimulation of iron- and/or sulfate-reducing native bacterial species, that mediate reduction of soluble U(VI) leading to its precipitation as uraninite (UO2). Uraninite is generally regarded as the most desirable product of bioreduction because of its low solubility under reducing conditions. The most studied microbes are the dissimilatory metal- and sulfate-reducing bacteria (DMRB and DSRB, respectively) including strains of the genera Geobacter, Shewanella, and Desulfovibrio. They are representative of strains present in many U-contaminated sites. Biostimulation of DSRB for U remediation can be especially advantageous since DSRB can reduce U(VI) by direct enzymatic mechanisms as well as indirectly by sulfide production. My group’s results show that uraninite particles produced by the DSRB (Desulfovibrio desulfuricans G20) grown in a metal toxicity medium containing piperazine-1,4-bis(2-ethanesulfonic acid) buffer (PIPES, 30 mM, pH 7) were 3-5 nm-sized particles and resided both extracellularly and intracellularly (periplasm and cytoplasm). Surprisingly, 60% of the reduced U passed through 0.2 m membrane filters (Gelman Acrodisc). However, when G20 was grown in MTM/bicarbonate buffer medium, about 35% of the uraninite could pass through 0.2 m filters. These results show the presence of a significant amount (35-60%) of reduced U in the mobile phase, and raise several fundamental questions. For example: what fraction of biogenic uraninite appears to be soluble or in suspension with U associated with i) periplasmic and cytoplasmic regions of bacterial cells, ii) the surfaces of cells, iii) the aqueous phase, and iv) on colloids (e.g., iron sulfide nanoparticles). In U-contaminated sites, what controls the distribution of uraninite in mobile vs. immobile (mineral) phases? None of these questions has been answered previously. To address these basic science questions, the objectives of the proposed research are: i) Batch studies of U reduction with and without iron minerals to quantifying the partitioning of various phases of U. ii) Identification of Fe/S/U species during U bioreduction to provide solid-phase geochemistry. Characterization of biogenic U(IV) in batch systems will be carried out using the sulfate reducing bacterium, Desulfovibrio desulfuricans G20 (our model organism). Lactate will be used as the substrate for bacterial growth. To mimic natural conditions, we will use Fe(III)(hydr)oxide minerals (hematite and ferrihydrite) and quartz (α-SiO2, 212-300 m) as model redox-sensitive and -insensitive aquifer minerals, respectively. Size fractions will be obtained by passing the samples through membrane filters of various pore sizes (e.g., 0.6 nm to 200 nm). Samples of unfiltered, filtered oxidized, and unfiltered oxidized will be analyzed for U(VI) content using a Kinetic Phosphorescence Analyzer (KPA). In addition to various U analyses, secondary minerals formed during U bioreduction will be analyzed using a select combination of X-ray photoelectron spectroscopy, X-ray diffraction and micro-XRD, high-resolution transmission electron microscopy, selected-area electron diffraction, and Mauer spectroscopy techniques to allow the determination of elemental oxidation states in complex solid samples. By carefully and selectively combining these advanced techniques, we will provide solid-phase geochemistry that is critical to develop mathematical models that accurately predict the behavior of bioreduced U in natural settings. Finally these results will be applicable in developing strategies for water restoration and to reduce environmental and human health risks from U-contaminated sites.