Uranium(VI) diffusion in low-permeability subsurface materials

Uranium(VI) diffusion was investigated in a fine-grained saprolite sediment that was collected from U.S. Department of Energy (DOE) Oak Ridge site, TN, where uranium contamination in groundwater is a major environmental concern. U(VI) diffusion was studied in a diffusion cell with one cell end in contact with a large, air-equilibrated electrolyte reservoir. The pH, carbonate and U(VI) concentrations in the reservoir solution were varied to investigate the effect of solution chemical composition and uranyl speciation on U(VI) diffusion. The rates of U(VI) diffusion were evaluated by monitoring the U(VI) concentration in the reservoir solution as a function of time; and by measuring the total concentration of U(VI) extracted from the sediment as a function of time and distance in the diffusion cells. The estimated apparent rate of U(VI) diffusion varied significantly with pH, with the slowest rate observed at pH 7 as a result of strong adsorptive retardation. The estimated retardation factor was generally consistent with a surface complexation model. Numerical simulations indicated that a species-based diffusion model that incorporated both aqueous and surface complexation reactions was required to describe U(VI) diffusion in the low permeability material under variable geochemical conditions. Our results implied that low permeability materials will play an important role in storing U(VI) and attenuating U(VI) plume migration at circumneutral pH conditions, and will serve as a long-term source for releasing U(VI) back to the nearby aquifer during and after aquifer decontamination. (orig.)