The Effect of Bicarbonate on Autunite Dissolution in the Presence of Shewanella oneidensis under Oxygen Restricted Conditions - 16429
Uranium is a key contaminant at many U.S. Department of Energy sites that served a leading role in the nation's defense for over 50 years. Uranium contamination of soil and groundwater is of great environmental concern due to the toxicological properties of the uranyl species. In an effort to decrease the concentration of soluble uranium, tripolyphosphate injections were identified as a feasible remediation strategy for sequestering uranium in situ in contaminated groundwater at the Hanford Site (WA). The introduction of sodium tripolyphosphate into uranium-bearing porous media results in the formation of uranyl phosphate minerals (autunite) of general formula (X{sub 1-2}[(UO{sub 2})(PO{sub 4})]{sub 2-1}.nH{sub 2}O), where X is a monovalent or divalent cation. The stability of the uranyl phosphate minerals in the subsurface is a critical factor since it determines the long-term effectiveness of the sodium tripolyphosphate injection as a remediation strategy. The behavior of uranium and its mobility in the subsurface are affected by various factors such as the chemistry of pore water, groundwater and soil minerals, the presence of complex-forming ligands, and micro-organisms that thrive under these conditions. Bicarbonate presence in the aqueous phase facilitates uranium desorption from soil and sediments and promotes the release of U(VI) into the aqueous phase, thus increasing uranium mobility in natural waters. In neutral or basic pH conditions, uranium can readily complex with a wide variety of ligands such as carbonate and form soluble uranyl-carbonato complexes such as UO{sub 2}(CO{sub 3}){sub 2}{sup 2-} and UO{sub 2}(CO{sub 3}){sub 3}{sup 4-}. Bicarbonate complexes have been identified in contaminated pore water at the Hanford Site. Furthermore, the presence of bacteria can significantly affect uranium mobility since bacteria may bio-enhance the release of U (VI) from uranyl-phosphate minerals, thus liberating uranium in the aqueous phase in an effort to obtain phosphorous, a vital nutrient for their metabolism. The Columbia River at the site exhibits large stage variations, causing fluctuations in the water table. These water table fluctuations and multiple rise-and-fall cycles in the river created an oxic-anoxic interface in this region. Previous assessments of Hanford sediment samples collected from this area noted a decline in cultivable aerobic bacteria and suggested the presence of facultative anaerobic bacteria. Shewanella oneidensis MR1 is a common bacterial group in soils and Shewanella species have been found in the Hanford soil. Shewanella oneidensis is a facultative anaerobic species which also belongs to a group known as dissimilatory metal-reducing bacteria (DMRB), due to their ability to reduce metals as a part of their metabolic pathway. Therefore, understanding the role of Shewanella is a critical factor affecting the outcome of environmental remediation. The objective of this research was to demonstrate the significance of bacteria-uranium interactions and investigate the effect of bicarbonate in the aqueous phase on Ca-autunite dissolution in the oxygen-restricted conditions. Sterile 100-mL glass mixed bioreactors with weighed amounts of ground Ca-autunite particles were used for initial experimentation. These autunite-containing bioreactors were injected with bacterial cells after the autunite equilibrated with the media solution amended with 0 mM, 3 mM 5 mM and 10 mM concentrations of bicarbonate. Additional abiotic samples were prepared without bacterial inoculation to provide a control for the biotic samples. The concentration of U(VI) determined in the aqueous phase was in direct correlation to the concentration of bicarbonate present in the solution. It was noted that an increase in bicarbonate concentration led to higher U(VI) concentration in the aqueous phase. A sharp decrease in U(VI) concentration was observed in the bicarbonate-free samples, probably due to a microbe mediated reduction of U(VI) to U(IV). In the case of samples amended with 3 mM bicarbonate, a slight, yet significant, decrease in U(VI) was also observed compared to the abiotic control. However, compared to the abiotic controls, there was no change in U(VI) levels in the aqueous phase as a result of bacterial inoculation for the samples amended with 5 and 10 mM bicarbonate for a period of time as long as 50 days. These results imply that the uranyl-carbonate complexes (UO{sub 2}(CO{sub 3}){sub 2}{sup 2-} and UO{sub 2}(CO{sub 3}){sub 3}{sup 4-}) that are present in carbonate-rich environments might be interfering with the microbial reduction in the oxygen- restricted conditions; however, this conclusion requires further investigation. Finally, the data indicate that release of U, P and Ca from autunite was non-stoichiometric. In conclusion, the high stability of uranyl-phosphate complexes makes them a strong candidate for the remediation efforts to sequester uranium in the subsurface. Nevertheless, in a bicarbonate-rich environment, Ca-autunite has a high probability of dissolution and the presence of metal-reducing bacteria does not seem to impede this process in the presence of higher bicarbonate concentrations under oxygen restricted conditions. (authors)