Reduction of U(VI) by Microorganisms: Formation of Nanoparticulate Uraninite and Other Products

Microbially-mediated U(VI) reduction leads to the formation of various U(IV) species, including a U(IV) oxide, uraninite (UO2). Biogenic uraninite produced by Shewanella oneidensis MR-1 was nanoparticulate (~ 3 nm in diameter) and displayed remarkable order and structural homology to stoichiometric UO2. The nanoparticles are produced both in the bacterial periplasmic space as well as extracellularly. A systematic study of the effect of phylogenetic diversity on U(VI) reduction revealed that , under the same chemical conditions, there was little difference in the U(IV) product regardless of the physiological and metabolic characteristics of the bacteria. However, the product of this reduction depended strongly on the geochemical conditions in which the biological process took place. At low cation and phosphate concentrations, nanoparticulate UO2 was the dominant product. In contrast, the presence of increasing concentrations of divalent cations promoted the formation of non-uraninite products. These included U(IV) associated with phosphate in a surface complexation arrangement as well as nanoparticulate crystalline and possibly amorphous U(IV)-phosphate phases. We refer to the non-uraninite products as 'monomeric U(IV)' due to the absence of a characteristic U-U shell in the EXAFS spectra of these compounds. We developed a method to easily quantify the relative contribution of monomeric U(IV) and UO2 in samples containing U(IV). We found that, under all the conditions tested, a measurable contribution of monomeric U(IV) was recorded. These findings suggest a more complex speciation of biogenic U(IV) than previously envisaged and provide fertile ground for the investigation of U(IV) geochemistry