Speciation-dependent microbial reduction of uranium within iron-coated sands

Transport of uranium within surface and subsurface environments is predicated largely on its redox state. Uranyl reduction may transpire through either biotic (enzymatic) or abiotic pathways; in either case, reduction of U(VI) to U(IV) results in the formation of sparingly soluble UO2 precipitates. Biological reduction of U(VI), while demonstrated as prolific under both laboratory and field conditions, is influenced by competing electron acceptors (such as nitrate, manganese oxides, or iron oxides) and uranyl speciation. Formation of Ca-UO2-CO3 ternary complexes, often the predominate uranyl species in carbonate-bearing soils and sediments, decreases the rate of dissimilatory U(VI) reduction. The combined influence of uranyl speciation within a mineralogical matrix comparable to natural environments and under hydrodynamic conditions, however, remains unresolved. We therefore examined uranyl reduction by Shewanella putrefaciens within packed mineral columns of ferrihydrite-coated quartz sand under conditions conducive or nonconducive to Ca-UO2-CO3 species formation. The results are dramatic. In the absence of Ca, where uranyl carbonato complexes dominate, U(VI) reduction transpires and consumes all of the U(VI) within the influent solution (0.166 mM) over the first 2.5 cm of the flow field for the entirety of the 54 d experiment. Over 2 g of U is deposited during this reaction period, and despite ferrihydrite being a competitive electron acceptor, uranium reduction appears unabated for the duration of our experiments. By contrast, in columns with 4 mM Ca in the influent solution (0.166 mM uranyl), reduction (enzymatic or surface-bound Fe(III) mediated) appears absent and breakthrough occurs within 18 d (at a flow rate of 3 pore volumes per day). Uranyl speciation, and in particular the formation of ternary Ca-UO2-CO3 complexes, has a profound impact on U(VI) reduction and thus transport within anaerobic systems.