Coupled Biogeochemical Processes Governing the Stability of Bacteriogenic Uraninite and Release of U(VI) in Heterogeneous Media: Molecular to Meter Scales. Final Report

Bacteriogenic UO2 will be an important if not dominant source of mobile U(VI) at DOE sites where subsurface reductive bioremediation of U(VI) has been conducted. The chemical stability of bacteriogenic UO2 is the seminal issue governing the success of its use as an 'in-situ waste form' in the subsurface. Identification of reactions and rates controlling oxidative transformation of UO2 and the subsequent release of U(VI) are fundamental to constructing reactive transport models for this key contaminant of concern. The objectives of this project are: (I) characterization of the molecular scale structures, stability, and dissolution kinetics of bacteriogenic U(IV) products produced under conditions representing those in contaminated environments; (II) characterization of bacteriogenic UO2 oxidation in the presence of ongoing bacterial Mn oxide formation in controlled laboratory experiments; and (III) investigation of UO2 oxidation by O2 and bacteriogenic Mn oxides in column experiments using field sediments. This project brings together three universities in an interdisciplinary team focused on molecular scale processes, which will use an integrated suite of techniques to address the objectives: synchrotron-based XAS/XRD/SAXS, laboratory-scale kinetics experiments, and column experiments.