Effect of Aging on the Stability of Microbially Reduced Uranium in Natural Sediment

Reductive immobilization of uranium has been explored as a remediation strategy for the U-contaminated subsurface. Via the in situ biostimulation of microbial processes, hexavalent U is reduced to less soluble tetravalent species, which are immobilized within the sediment. Although the mineral uraninite (UO₂) was initially considered the dominant product of biological reduction, non-crystalline U­(IV) species (NCU­(IV)) are found to be abundant in the environment despite their greater susceptibility to oxidation and remobilization. However, it has been recently proposed that, through aging, NCU­(IV) might transform into UO₂, which would potentially enhance the stability of the reduced U pool. In this study, we performed column experiments to produce NCU­(IV) species in natural sediment mimicking the environmental conditions during bioremediation. Bioreduced sediment retrieved from the columns and harboring NCU­(IV) was incubated in static microcosms under anoxic conditions to allow the systematic monitoring of U coordination by X-ray absorption spectroscopy (XAS) over 12 months. XAS revealed that, under the investigated conditions, the speciation of U­(IV) does not change over time. Thus, because NCU­(IV) is the dominant species in the sediment, bioreduced U­(IV) species remain vulnerable to oxidation and remobilization in the aqueous phase even after a 12-month aging period.