Elucidating Bioreductive Transformations within Physically Complex Media: Impact on the Fate and Transport of Uranium and Chromium

Uranium and chromium are two elements of particular concern within the DOE complex that, owing to their abundance and toxicity, appear well suited for biologically mediated reductive stabilization. Subsurface microbial activity can alter the redox state of toxic metals and radionuclides, rending them immobile. Furthermore, anaerobic bacterial metabolic products will help to buffer pulses of oxidation, typically from fluxes of nitrate or molecular oxygen, and thus may stabilize reduced contaminants from oxidative mobilization. Imparting an important criterion on the probability that contaminants will undergo reductive stabilization, however, is the physical nature along with the chemical and physical heterogeneity of the media. In our study we have been investigating the impact of chemical/mineralogical heterogeneity on uranium reduction, with an emphasis on iron transformations and resulting impacts on contaminant retention. We have, in particular, emphasized considering chemical/mineralogical and physical complexity on bioreduction of metals. Over the past year we have also made appreciable advances on discerning geochemical constraints on microbially mediated reduction of U(VI) and on means to discern spatial heterogeneity in operative biogeochemical reactions within soils and sediments