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

In situ stabilization (inclusive of natural attenuation) of toxic metals and radionuclides is an attractive approach for remediating many contaminated DOE sites. By immobilizing toxic metals and radionuclides in place, the removal of contaminated water to the surface for treatment as well as the associated disposal costs are avoided. To enhance in situ remediaton, microbiological reductive stabilization of contaminant metals has been, and continues to be, actively explored. It is likely that surface and subsurface microbial activity can alter the redox state of toxic metals and radionuclides, either directly or indirectly, so they are rendered 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. 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. Imparting an important criterion on the probability that contaminants will undergo reductive stabilization, however, is the chemical and physical heterogeneity of the media. Our research first examined microbially induced transformation of iron (hydr)oxide minerals and their impact on contaminant attenuation. We revealed that in intricate cascade of geochemical reactions is induced by microbially produced Fe(II), and that during transformation contaminants such as U(VI) can be incorporated into the structure, and a set of Fe(II) bearing solids capable of reducing Cr(VI) and stabilizing resulting Cr(III). We also note, however, that common subsurface constituents such as phosphate can modify iron oxide transformation pathways and thus impact contaminant sequestration—affecting both Cr and U stabilization. We extended our work to explore factors controlling the sequestration of uranium in the subsurface, with a particular emphasis on mineralogic and geochemical complexity. We reveal that one of the primary factors controlling uranium reduction, via both biological and chemical pathways, is the aqueous speciation of U(VI). Specifically, ternary calcium-uranyl-carbonato complexes stabilize U(VI) relative to reduction. However, countering the lack of reduction, we note a novel sequestration pathway in which the U(VI), as the uranate ion, is incorporated into the structure of transformation iron oxides; magnetite and goethite, both products of Fe(II) induced transformation of ferrihydrite, harbor appreciable quantities of uranium. In sum, our results provide important information on predicting and potentially controlling the migration of chromium and uranium within the DOE complex