Uranium Isotopes Tell a Fractionating Story
Uranium is a risk-driving contaminant at many DOE sites and its mobility in groundwater is influenced by both geochemical and biological processes. Methods are needed to identify which biogeochemical processes influence uranium mobility so that we can develop more robust contaminant transport models. Researchers at the University of Illinois, Pacific Northwest National Laboratory and Lawrence Berkeley National Laboratory have developed an isotopic method based on U-238/U-235 ratios that can be used to distinguish between microbe-mediated (preferentially U-238) versus chemical (either isotope) reduction of uranium in contaminated subsurface environments. In the laboratory, soluble uranium [U(VI)] can be reduced to an insoluble species [U(IV)] either enzymatically, by microorganisms, or chemically, by species such as Fe(II) or sulfide. To accurately model the transport of uranium in groundwater, methods are needed that discriminate between enzymatic and chemical reduction of uranium. At a field research site in Colorado, stimulation of subsurface microbial communities produces a decrease in the concentration of soluble uranium co-incident with an increase in uranium-reducing microorganisms and the production of chemical reductants such as Fe(II) and sulfide. Samples collected during these tests indicated a preferential shift in the U-238/U-235 ratios consistent with an enzymatic reduction process. The results indicate that isotopic methods can be used to distinguish between biotic and abiotic processes influencing uranium reduction under bioremediation conditions and/or natural attenuation conditions in the environment. The technique is important in the development of more robust models of contaminant transport in groundwater at uranium-contaminated sites.
C.J. Bopp IV, C.C. Lundstrom, T.M. Johnson, R.A. Sanford, P.E. Long, K.H. Williams. (2010) “Uranium 238U/235U Isotope Ratios as Indicators of Reduction: Results from an in situ Biostimulation Experiment at Rifle, Colorado, U.S.A.,” Environ. Sci. Technol. 44(15):5927-5933.