Oxidative dissolution of UO₂ in a simulated groundwater containing synthetic nanocrystalline mackinawite

The long-term success of in situ reductive immobilization of uranium (U) depends on the stability of U(IV) precipitates (e.g., uraninite) in the presence of natural oxidants, such as oxygen, Fe(III) hydroxides, or nitrite. Field and laboratory studies have implicated iron sulfide minerals as redox buffers or oxidant scavengers that may slow oxidation of reduced U(VI) solid phases. Yet, the inhibition mechanism(s) and reaction rates of uraninite (UO₂) oxidative dissolution by oxic species such as oxygen in FeS-bearing systems remain largely unresolved. To address this knowledge gap, abiotic batch experiments were conducted with synthetic UO₂ in the presence and absence of synthetic mackinawite (FeS) under simulated groundwater conditions of pH=7, PO2=0.02atm, and PCO2=0.05atm. The kinetic profiles of dissolved uranium indicate that FeS inhibited UO₂ dissolution for about 51h by effectively scavenging oxygen and keeping dissolved oxygen (DO) low. During this time period, oxidation of structural Fe(II) and S(-II) of FeS were found to control the DO levels, leading to the formation of iron oxyhydroxides and elemental sulfur, respectively, as verified by X-ray diffraction (XRD), Mössbauer and X-ray absorption spectroscopy (XAS). After FeS was depleted due to oxidation, DO levels increased and UO₂ oxidative dissolution occurred at an initial rate of r_m=1.2±0.4×10^-8molg^-1s^-1, higher than r_m=5.4±0.3×10^-9molg^-1s^-1 in the control experiment where FeS was absent. XAS analysis confirmed that soluble U(VI)-carbonato complexes were adsorbed by iron oxyhydroxides (i.e., nanogoethite and lepidocrocite) formed from FeS oxidation, which provided a sink for U(VI) retention. This work reveals that both the oxygen scavenging by FeS and the adsorption of U(VI) to FeS oxidation products may be important in U reductive immobilization systems subject to redox cycling events.