U(VI) Reduction by Biogenic and Abiotic Hydroxycarbonate Green Rusts: Impacts on U(IV) Speciation and Stability Over Time

Green rusts (GRs) are redox active Fe^II–Fe^III minerals that form in the environment via various biotic and abiotic processes. Although both biogenic (BioGR) and abiotic (ChemGR) GRs have been shown to reduce U^VI, the dynamics of the transformations and the speciation and stability of the resulting U^IV phases are poorly understood. We used carbonate extraction and XAFS spectroscopy to investigate the products of U^VI reduction by BioGR and ChemGR. The results show that both GRs can rapidly remove U^VI from synthetic groundwater via reduction to U^IV. The initial products in the ChemGR system are solids-associated U^IV–carbonate complexes that gradually transform to nanocrystalline uraninite over time, leading to a decrease in the proportion of carbonate-extractable U from ∼95% to ∼10%. In contrast, solid-phase U^IV atoms in the BioGR system remain relatively extractable, nonuraninite U^IV species over the same reaction period. The presence of calcium and carbonate in groundwater significantly increase the extractability of U^IV in the BioGR system. These data provide new insights into the transformations of U under anoxic conditions in groundwater that contains calcium and carbonate, and have major implications for predicting uranium stability within redox dynamic environments and designing approaches for the remediation of uranium-contaminated groundwater.