The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-UVI-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulfurreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10- to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of UVI reduction to UIV. The reduction extent of the same mass of HUP to UIV was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of UVI reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U VI species formed after HUP dissolution, rather than to solid-phase UVI in the HUP mineral. Interestingly, the bioreduced UIV atoms were not immediately coordinated to other UIV atoms (as in uraninite, UO2) but were similar in structure to the phosphate-complexed UIV species found in ningyoite [CaU(PO 4)2·H2O]. This indicates a strong control by phosphate on the speciation of bioreduced UIV, expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.
ASJC Scopus subject areas
- Environmental Chemistry