Redox conditions across the mineral-water interfaces are important in controlling immobilization and accumulation of uranium in soils and sediments. Reduction of dissolved U(VI) species to insoluble U(IV) precipitates has been shown to be effective in sequestration of uranium in the subsurface environment. However, reoxidation of reduced U(IV) solid phases to soluble U(VI) may occur when subjected to oxidants, such as oxygen and Fe(III). Naturally formed iron sulfide minerals (e.g., mackinawite) have been reported to slow oxidation of UO2, but few studies have examined the reactions of UO2 with iron sulfide or the protective nature of iron sulfide in inhibiting UO2 oxidation. The present study investigated the inhibition mechanism(s) and reaction rates of UO2 oxidative dissolution by oxygen and Fe(III) oxides in mackinawite-bearing systems using batch and flow-through reactors. The mackinawite oxidation products (i.e. iron hydroxides and elemental sulfur) were characterized by spectroscopic techniques. Iron hydroxides were further evaluated for facilitating UO2 oxidative dissolution as U(VI) sorbents and oxidants. Experiments were conducted as a function of pH, calcium and carbonate concentrations to assess the impact of groundwater conditions on UO2 oxidation kinetics. This study supports mackinawite as an effective oxidant scavenger against UO2 oxidation and illustrates the impact of mackinawite oxidation products on U reductive immobilization systems subject to redox cycling events.

Impact of iron sulfide minerals on UO<sub>2</sub> reoxidation in groundwater

245th National Meeting (2013)

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Impact of iron sulfide minerals on UO2 reoxidation in groundwater

Impact of iron sulfide minerals on UO₂ reoxidation in groundwater