X-ray spectroscopic studies of microbial transformations of uranium

Several uranium compounds U-metal ({alpha}-phase), UO{sub 2}, U{sub 3}O{sub 8}, {gamma}-UO{sub 3}, uranyl acetate, uranyl nitrate, uranyl sulfate, aqueous and solid forms of 1:1 U:citric acid and 1:1:2 U:Fe:citric acid mixed-metal complexes, and a precipitate obtained by photodegradation of the U-citrate complex were characterized by X-ray spectroscopy using XPS, XANES, and EXAFS. XPS and XANES were used to determine U oxidation states. Spectral shifts were obtained at the U 4f{sub 7/2} and U 4f{sub 5/2} binding energies using XPS, and at the uranium M{sub V} absorption edge using XANES. The magnitude of the energy shift with oxidation state, and the ability to detect mixed-valent forms make these ideal techniques for determining uranium speciation in wastes subjected to bacterial action. The structure of 1:1 U:citric acid complex in both the aqueous and solid state was determined by EXAFS analysis of hexavalent uranium at the L{sub M} absorption edge and suggests the presence of a binuclear complex with a (UO{sub 2}){sub 2}({mu},{eta}{sup 2} {minus}citrato){sub 2} core with a U-U distance of 5.2 {angstrom}. The influence of Fe on the structure of U-citrate complex was determined by EXAFS and the presence of a binuclear mixed-metal citrate complex with a U-Fe distance of 4.8 {angstrom} was confirmed. The precipitate resulting from photodegradation of U-citrate complex was identified as an amorphous form of uranium trioxide by XPS and EXAS