Spectroscopic studies on the interaction of U(VI) with Bacillus sphaericus

We studied the interaction of U(VI) with vegetative cells, heat killed cells, spores, and decomposed cells of Bacillus sphaericus. The characterization of the formed complexes was performed by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fin structure spectroscopy (EXAFS). We observed no significant differences in the sorption behavior of vegetative and heat killed cells, whereas the spores showed a higher sorption of U(VI) (related to their dry weight). Regardless of the higher relative sorption of the spores of B. sphaericus, the fluorescence and EXAFS spectra of the vegetative cells, heat killed cells and spores were almost identical. Analysis of the data proved that U(VI) forms inner sphere complexes with organic bound phosphate groups on the cell surface. We observed no significant differences in the coordination numbers and the distances of the oxygen and phosphorus atoms in the inner coordination sphere. After eight weeks, the vegetative cells of B. sphaericus were completely decomposed. Lysing of the cell walls and activity of enzymes led to a release of various decomposition products. We found that large amounts of H2PO4- were released which caused a quantitative precipitation of bacterial U(VI) as UO2(H2PO4)(2). The H2PO4- was detected by Raman spectroscopy. The decomposed bacterial suspension showed the same fluorescence spectrum as UO2(H2PO4)(2) which differed significantly from those of the bacterial U(VI) surface complexes.