Reduction of U VI to U IV as the result of direct or indirect microbial activity is currently being explored for in situ remediation of subsurface U plumes, under the assumption that U IV solubility is controlled by the low-solubility mineral uraninite (U IV-dioxide). However, recent characterizations of U in sediments from biostimulated field sites, as well as laboratory U VI bioreduction studies, report on the formation of U IV species that lack the U=O 2=U coordination of uraninite, suggesting that phases other than uraninite may be controlling U IV solubility in environments with complexing surfaces and ligands. To determine the controls on the formation of such nonuraninite U IV species, the current work studied the reduction of carbonate-complexed U VI by (1) five Gram-positive Desulfitobacterium strains, (2) the Gram-negative bacteria Anaeromyxobacter dehalogenans 2CP-C and Shewanella putrefaciens CN32, and (3) chemically reduced 9,10-anthrahydroquinone-2,6- disulfonate (AH 2QDS, a soluble reductant). Further, the effects of 0.3 mM dissolved phosphate on U IV species formation were explored. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis demonstrated that the addition of phosphate causes the formation of a nonuraninite, phosphate-complexed U IV species, independent of the biological or abiotic mode of U VI reduction. In phosphate-free medium, U VI reduction by Desulfitobacterium spp. and by AH 2QDS resulted in nonuraninite, carbonate-complexed U IV species, whereas reduction by Anaeromyxobacter or Shewanella yielded nanoparticulate uraninite. These findings suggest that the Gram-positive Desulfitobacterium strains and the Gram-negative Anaeromyxobacter and Shewanella species use distinct mechanisms to reduce U VI.
ASJC Scopus subject areas
- Environmental Chemistry