Role of Iron Sulfide Phases in the Stability of Noncrystalline Tetravalent Uranium in Sediments

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U­(IV) species (NCU­(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU­(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU­(IV) in the environment is of crucial importance. Fe­(II) minerals (such as FeS) are often associated with U­(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU­(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU­(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU­(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU­(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.