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-VI 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 Uvl from synthetic groundwater via reduction to U-VI. The initial products in the ChemGR system are solids-associated U-VI-carbonate complexes that gradually transform to nanocrystalline uraninite over time, leading to a decrease in the proportion of carbonate-extractable U from similar to 95% to similar to 10%. In contrast, solid-phase U-VI 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-VI 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.