Understanding How Uranium Changes in Subsurface Environments
The U.S. Department of Energy has a long-term responsibility to contain uranium leaked into the environment at mining and processing sites. Uranium has a complex chemistry that determines whether it is immobilized or moves out of a contaminated area, potentially into water supplies. New research on the transformation of uranium (VI) to uranium (IV)—the most common oxidation states of the element—discovered that bacterial biomass in the ground impacts this transition. Studies were carried out at the Rifle (Colorado) Integrated Field Research Challenge site, by scientists from the SLAC National Accelerator Laboratory and Berkeley Lab, to determine how uranium (VI) exposed to natural conditions at the site behaved and to determine the underlying controlling biological and chemical mechanisms. The experiments showed that uranium (IV) unexpectedly was present both as a monomeric, biomass-associated uranium (IV) species and, to a much
lesser extent, as nanoparticles of uraninite (UO2). The researchers attribute the presence of the former to the binding of uranium (IV) to phosphate groups in biomass following the chemical transformation of uranium (VI) to uranium (IV) by reaction with iron sulfides or bacterial enzymes. Since a substantial portion of the uranium is found in this form, models of uranium transport in contaminated subsurface environments need to recognize the existence of multiple pathways for reduction of uranium (VI), including the
biological factors identified in this research.
Bargar, J. R., et al. 2013. “Uranium Redox Transition Pathways in Acetate-Amended Sediments,” Proceedings of the National Academy of Sciences USA, DOI: 10.1073/pnas.1219198110.