Biogeochemical Factors Governing the Formation and Subsurface Stability of Reduced Molecular Uranium Species

Subsurface reduction of U(VI) can be mediated by microorganisms and their redox-active mineral byproducts, yielding relatively insoluble U(IV) species. Although uraninite is generally assumed to be the dominant product of biological U(VI) reduction, recent work by the SLAC SFA and others provide evidence that other forms of U(IV), such as molecular complexes, may also be produced. The microbially mediated in situ reduction of U(VI) is touted as a potential approach for the remediation of uraniumcontaminated DOE sites. The success of such a strategy is contingent upon the enduring stability of the biologically reduced uranium. The goal of this work is to elucidate and quantify the fundamental structure-composition-stability relationships of these species as part of the SLAC SFA focus on molecularscale processes of key importance to uranium behavior in the subsurface at DOE sites. Here we report key insights to factors controlling the formation of molecular U(IV) product(s) in pure culture and sediment columns. These species lack the U-U EXAFS shell characteristic of uraninite. We carried out lactate-stimulated uranium reduction experiments in columns packed with Rifle IFRC sediment augmented with Shewanella oneidensis MR-1 cells. Major findings of the work included a shift in the microbial community composition over time away from S. oneidensis in low G+C Gram-positive bacteria, as well as the formation of a molecular tetravalent uranium species lacking intermediate-range structure, as demonstrated by EXAFS. In addition, several bacterial species, including Desulfotomaculum reducens MI-1, Clostridium acetobutylicum and S. oneidensis, were found to be capable of producing molecular U(IV) as a product of U(VI) reduction in pure culture under specific geochemical conditions. In fact, geochemical conditions appear to exert significant control over the product of microbial U(VI) reduction since S. oneidensis produces uraninite under some geochemical conditions and molecular U(IV) under others. Finally, the reduction of U(VI) by redox-active biogenic Fe(II) phases also generates several distinct U(IV) products depending on the geochemical matrix. In some cases, EXAFS indicated the formation of a molecular U(IV) species, whereas in other instances, uraninite was formed. These findings reinforce the conclusion that uraninite is not the only product of direct and indirect biological U(VI) reduction, but that molecular tetravalent uranium species must also be considered when modeling uranium mobility in the subsurface. Our present research efforts are focused on evaluating the reactivity of molecular tetravalent uranium, as well as the conditions leading to the formation of such species as a result of biological U(VI) reduction. Identification of key factors controlling the formation of molecular U(IV) and biogenic uraninite and knowledge of their reactivity is leading to new geochemical models for uranium cycling in reducing sediments, and to a deeper and more defensible scientific basis for in situ reductive remediation technologies