Nano-FeS inhibits UO₂ reoxidation under varied oxic conditions

Bioreductive in situ treatment of U-contaminated groundwater can convert soluble U(VI) species to immobile reduced U(IV) solid phases such as UO ₂(s) to contain U movement. Once active bioremediation is halted, UO₂ may be subsequently reoxidized if oxidants such as oxygen enter the reducing zone. However, iron sulfide minerals that form during bioreduction may serve as electron sources or oxygen scavengers and inhibit UO₂ reoxidation upon oxygen intrusion. In this study, flow-through reactor experiments examined the abiotic kinetics of UO₂ oxidative dissolution in the presence of oxygen and nanoparticulate FeS as a function of pH, dissolved oxygen (DO) concentration, and FeS content. The UO₂ dissolution rates in the presence of FeS were over 1 order of magnitude lower than those in the absence of FeS under otherwise comparable oxic conditions. FeS effectively scavenged DO and preferentially reacted with oxygen, contributing to a largely unreacted UO₂ solid phase during an "inhibition period" as determined by X-ray absorption spectroscopy (XAS). The removal of DO by FeS was significant but incomplete during the inhibition period, resulting in surface-oxidation-limited dissolution and greater UO₂ dissolution rate with increasing influent DO concentration and decreasing FeS content. Although the rate was independent of solution pH in the range of 6.1-8.1, the length of the inhibition period was shortened by substantial FeS dissolution at slightly acidic pH. The reducing capacity of FeS was greatest at basic pH where surface-mediated FeS oxidation dominated.