Bioreduction of hydrogen uranyl phosphate: Mechanisms and U(IV) products

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-U^VI-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 U^VI reduction to U^IV. The reduction extent of the same mass of HUP to U^IV 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 U^VI 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 U^VI in the HUP mineral. Interestingly, the bioreduced U^IV atoms were not immediately coordinated to other U^IV atoms (as in uraninite, UO₂) but were similar in structure to the phosphate-complexed U^IV species found in ningyoite [CaU(PO ₄)₂·H₂O]. This indicates a strong control by phosphate on the speciation of bioreduced U^IV, expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.