Solution and Microbial Controls on the Formation of Reduced U(IV) Species

Authors

Maxim I. Boyanov, Argonne National Laboratory
Kelly E. Fletcher, Georgia Institute of Technology
Man Jae Kwon, Argonne National Laboratory
Xue Rui, University of Notre Dame
Edward J. O’Loughlin, Argonne National Laboratory
Frank E. Löffler, University of Tennessee
Kenneth M. Kemner, Argonne National Laboratory

Date of this Version

2011

Citation

Environ. Sci. Technol. 2011, 45, 8336–8344; dx.doi.org/10.1021/es2014049

Abstract

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 UIV 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=O₂=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 (AH₂QDS, 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 UIV species, independent of the biological or abiotic mode of UVI reduction. In phosphate-free medium, U^VI reduction by Desulfitobacterium spp. and by AH₂QDS resulted in nonuraninite, carbonate-complexed U^IV species, whereas reduction by Anaeromyxobacter or Shewanella yielded nanoparticulate uraninite. These findings suggest that the Grampositive Desulfitobacterium strains and the Gram-negative Anaeromyxobacter and Shewanella species use distinct mechanisms to reduce U^VI.