Remediation and long-term stewardship of uranium-contaminated sediments and groundwaters are critical problems at a number of DOE facilities and mining sites.  Some remediation strategies based on in-situ bioreduction of U are potentially effective in significantly decreasing U concentrations in groundwaters.  However, a number of basic processes require understanding in order to identify environments where reduction-based U stabilization is more likely to succeed.  Our current research targets several of these issues including: (1) effects of organic carbon (OC) forms and supply rates on stability of bioreduced U, (2) the roles of Fe- and Mn-oxides as potential U oxidants in sediments, and (3) microbial community changes in relation to U redox changes.  Most of our studies are being conducted on historically U-contaminated sediments from Area 2 of the Field Research Center, Oak Ridge National Laboratory, in flow-through columns.

The rate of OC supply is a critical factor in U reduction, not only in determining the rate of electron donor supply, but also in determining the resulting concentration of aqueous (bi)carbonate generated by microbial respiration.  As shown in our previous ERSP research, increased (bi)carbonate concentrations drive aqueous U(VI) concentrations to higher levels through formation of stable U(VI) carbonato complexes, including Ca2UO2(CO3)3(aq).  Thus, the influence of OC supply rate on U reduction is more complex than previously assumed. Redox transformations of U are being tested in new FRC2 sediment columns supplied with OC at rates ranging from 0 to 580 mM (kg sediment)-1 year-1.  These columns are being supplied with either lactate or acetate, and have been running for over 400 days.  The effluent U concentrations do in fact show complex but very reproducible, nonlinear dependence on the OC supply rate, consistent with OC oxidation having dual impacts of driving reduction of U as well as formation of U(VI)-carbonato complexes.  This portion of our study is also showing that lactate and acetate have the same geochemical impact on effluent U concentrations (and all other measured chemical species), when compared on the basis of supply rate of C.

We identified several factors that point to a residual reactive Fe(III) fraction in these sediments that likely serves as the terminal electron acceptor for U reoxidation.  We are conducting even longer-term column incubations targeted at completely reducing the reactive Fe(III) fraction in sediments, micro- X-ray absorption spectroscopy for determining distributions of Mn, Fe, and U oxidation states in sediments at various stages of OC-stimulated bioreduction, and use of chemical methods for determining concentrations of Fe(II) and Fe(III) in sediments and pore waters. 

We analyzed the structure of the stimulated microbial communities in columns receiving ten different OC supply treatments at two time points; during a phase of net U-reduction and during a later phase of U-reoxidation and remobilization. Community analysis using a high-density 16S microarray (16S Phylochip) indicates that OC supply rate is the primary determinant of the bacterial community composition and that significant shifts in community dynamics occur between the U-reduction and remobilization phases.

Schematic showing high-density DNA microarray (PhyloChip) approach. Either 16S rRNA gene amplicons (to follow changes in biomass) or 16S rRNA molecules (to follow changes in activity) can be labeled and hybridized to the array.