The biogeochemical transformation of simple uranium(VI) oxyhydroxides to uranium(VI) phosphate solids.

摘要

The transformation of U(VI) oxyhydroxides to U(VI) phosphate minerals is a potential method of remediation for U contamination in the environment. In this work, the chemistry of the abiotic transformation pathway of metaschoepite, [(UO2)8O2(OH)12]·10H 2O(s), to meta autunite, Ca[(UO2)(PO4)] 2·6H2O(s), was studied using X-ray diffraction (XRD), and a conceptual geochemical model was developed based on our experimental results and equilibrium data available in the literature. By varying the concentration of inorganic phosphate, we found that the initial phosphate concentration governed the transformation pathway by controlling the precipitation of brushite, CaHPO4·2H2O(s), and solution pH.; The influence of the Gram positive bacterium Bacillus sphaericus on the transformation system was also studied. The primary goal of the work was to determine whether the presence of Gram positive cell surfaces facilitate the formation of U(VI) phosphate solids, possibly by serving as nucleation sites in the transformation of metaschoepite to meta autunite. To this end, we worked with non-viable bacteria fixed with formaldehyde prevent lysis and preserve the external cell surface.; High concentrations of B. sphaericus appeared to inhibit the transformation of metaschoepite. Reflections indicative of metaschoepite were evident far into the experiment whereas U(VI) phosphate formation was observed in the abiotic controls. When the concentration of B. sphaericus was significantly reduced, the inhibition to transformation was still observed. Interestingly, little evidence of U(VI) phosphate formation was observed in the abiotic controls of this system.; The systems studied herein were complicated by the potential for a variety of equilibrium processes. Sorption, nucleation, and precipitation are competing processes in both the biotic and abiotic systems. Though the complexity made the elucidation of specific mechanisms difficult, we expect that such competing processes will occur in geologic repositories. Thus, the potential for bacterial cell surfaces to influence the behavior of U in the environment must be considered in the design and performance assessment of geologic repositories for SNF and U mill tailings.