Mechanisms of Uranium Reduction In the Subsurface

Uranium (U) is a common contaminant in groundwater and soils at many hazardous waste sites in the United States. Uranium can exist in multiple oxidation states under common subsurface environmental conditions, with U(VI) species (e.g., UO₂(OH)⁺, UO₂(CO₃)₂^2-) being much more mobile than U(IV) species (e.g., UO₂(s)). Because complete removal of uranium from contaminated soils and water for ex-situ disposal is infeasible at most sites, in-situ stabilization through U(VI) reduction to insoluble U(IV) is considered one of the most important approaches for site remediation. Biostimulation, in which electron donors and/or nutrients are injected into the subsurface to stimulate indigenous organisms to enhance enzymatic activities for U(VI) reduction, has been actively explored for U(VI) reduction and immobilization. The reduction mechanisms, however, have not been fully understood. Sulfides could be produced from microbial reduction of sulfate during biostimualtion, but the contribution of biogenic sulfide species to the reduction of U(VI) is unclear. In this presentation, we will discuss the mechanisms of U(VI) reduction by hydrogen sulfide and iron sulfide in the anoxic environment. Our batch experimental results showed that the rate of U(VI) reduction was largely controlled by solution pH and carbonate concentration. In the presence of iron sulfide, a rapid uptake of U(VI) by FeS occurred within one hour under all pH conditions accompanied by simultaneous release of Fe(II); whereas the reduction of sorbed U(VI) took hours to over a week for completion. The reduction followed a pseudo-first-order kinetics and the rate was strongly dependent on the solution pH. Product analysis by X-ray photoelectron spectroscopy showed the formation of U₃O₈/UO₂ and polysulfide. The study suggests that sulfide species are capable of reducing U(VI), thus the abiotic reduction may contribute significantly to U(VI) immobilization in the biostimulated systems.