title: Surface passivation limited UO2 oxidative dissolution in the presence of FeS.
abstract: Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO2) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U(IV). The present study investigated the mechanism that regulates UO2 oxidative dissolution rate in a flow-through system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO2 were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U(VI) to preserve an intermediate U4O9 surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO2 was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U(VI) detachment as U(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U(VI) complexes and impeded the formation of U(VI) passivation layer. This study demonstrates the critical role of U(VI) surface layer formation versus U(VI) detachment in controlling UO2 oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions.
authors.affiliation: Department of Civil and Environmental Engineering, University of Michigan , Ann Arbor, Michigan 48109, United States.
authors.forename: Yuqiang
authors.initials: Y
authors.lastname: Bi
authors.forename: Kim F
authors.initials: KF
authors.lastname: Hayes
mesh_terms: D019149:Bioreactors; D002254:Carbonates / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry*; D060587:Groundwater / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D056951:Photoelectron Spectroscopy; D012995:Solubility; D013499:Surface Properties; D014501:Uranium / Q000737:chemistry*; D017974:Uranium Compounds / Q000737:chemistry*; D014875:Water Pollutants, Radioactive / Q000737:chemistry
chemical_list: D002254:Carbonates; D005296:Ferrous Compounds; D017974:Uranium Compounds; D014875:Water Pollutants, Radioactive; D014501:Uranium; C047385:uranium oxide; C022597:ferrous sulfide