Redox conditions are critical in governing the mobility and accumulation of uranium (U) in subsurface soils and groundwater. Mackinawite (FeS), produced during in situ bioremediation of U, can serve as an electron source or an oxygen scavenger and inhibit UO2 reoxidation upon oxygen intrusion once active bioreduction is halted. However, FeS may eventually become completely oxidized, allowing dissolved oxygen (DO) levels to rise, leading to oxidation and remobilization of U. The rate controlling redox reactions of Fe and U species during an anoxic to oxic transition will dictate the fate and transport of U in the subsurface, yet these have not been sufficiently characterized.  The present study investigated the kinetics and mechanism(s) of UO2 reoxidation and U(VI) transport in the presence of oxygen and nanoparticulate FeS using batch and flow-through reactors as a function of pH, calcium, and carbonate concentrations. FeS and UO2 oxidation products were characterized by transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). FeS reaction products, i.e., Fe(III) hydroxides and aqueous Fe(II), were examined for their influence on UO2 oxidative dissolution as FeS became depleted. This study illustrates that FeS serves as an effective oxygen scavenger to maintain UO2 stability until it drops below a threshold concentration. The newly formed Fe(III) hydroxides may subsequently enhance U dissolution, depending on Fe(III) species and solution pH. This study shows the critical role of geochemical conditions in controlling FeS transformation, which in turn strongly influences U remobilization in the subsurface environment.

Nano-FeS control of UO<sub>2</sub> reoxidation in simulated groundwater

247th National Meeting (2014)

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Nano-FeS control of UO2 reoxidation in simulated groundwater

Nano-FeS control of UO₂ reoxidation in simulated groundwater