Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation

The reactivity of disordered, noncrystalline U­(IV) species remains poorly characterized despite their prevalence in biostimulated sediments. Because of the lack of crystalline structure, noncrystalline U­(IV) may be susceptible to oxidative mobilization under oxic conditions. The present study investigated the mechanism and rate of oxidation of biogenic noncrystalline U­(IV) by dissolved oxygen (DO) in the presence of mackinawite (FeS). Previously recognized as an effective reductant and oxygen scavenger, nanoparticulate FeS was evaluated for its role in influencing U release in a flow-through system as a function of pH and carbonate concentration. The results demonstrated that noncrystalline U­(IV) was more susceptible to oxidation than uraninite (UO₂) in the presence of dissolved carbonate. A rapid release of U occurred immediately after FeS addition without exhibiting a temporary inhibition stage, as was observed during the oxidation of UO₂, although FeS still kept DO levels low. X-ray photoelectron spectroscopy (XPS) characterized a transient surface Fe­(III) species during the initial FeS oxidation, which was likely responsible for oxidizing noncrystalline U­(IV) in addition to oxygen. In the absence of carbonate, however, the release of dissolved U was significantly hindered as a result of U adsorption by FeS oxidation products. This study illustrates the strong interactions between iron sulfide and U­(IV) species during redox transformation and implies the lability of biogenic noncrystalline U­(IV) species in the subsurface environment when subjected to redox cycling events