The reduction of soluble hexavalent uranium to tetravalent uranium was long assumed to solely produce insoluble uraninite. However, recent laboratory and field studies report the presence of noncrystalline U(IV) coordinated to phosphate or carbonate groups. Although these noncrystalline U(IV) species are likely to be more labile and susceptible to oxidation than uraninite, their reactivity has not been well characterized. In addition, because noncrystalline U(IV) can be associated with iron sulfide minerals in biostimulated aquifers, the stability and mobilization of the U(IV) products may be tightly coupled with the cycling of iron and sulfur. Whereas mackinawite (FeS) was previously recognized as an effective oxygen scavenger to retard uraninite oxidative dissolution, it is unclear whether FeS can provide the same effectiveness in inhibiting the oxidation of noncrystalline U(IV). The present study investigates the kinetics and mechanism of oxidation of noncrystalline U(IV), produced by <i>Shewanella oneidensis</i> strain MR-1, under simulated oxic groundwater conditions. Synthetic nanoparticulate FeS was evaluated for its role in affecting the mobilization of the U(IV) product in a flow-through system as a function of pH and carbonate concentration. The results demonstrate that noncrystalline U(IV) is mobilized at a faster rate than uraninite to form uranyl-calcium-carbonato complexes. Interestingly, rapid release of U occurs immediately after FeS addition without exhibiting an initial transitory inhibition stage as was observed during the oxidation of uraninite, although FeS effectively keeps DO levels low. Intermediate amorphous Fe(III) (hydr)oxides generated from FeS oxidation were characterized as potential oxidants accelerating the oxidation of noncrystalline U(IV). This study illustrates the strong interactions between iron sulfide and U(IV) species during redox transformation and implies the lability of biogenic U(IV) species in the subsurface when subjected to redox cycling events.

Rapid mobilization of noncrystalline U(IV) coupled with FeS oxidation

249th National Meeting (2015)

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