Experimental and Modeling Study of Retarded Diffusive Transport of Uranium(VI) in a Hanford Sediment

Uranium(VI) is a common contaminant at sites of nuclear material processing and uranium mining. U(VI) fate and transport is of great concern at the U.S. Department of Energy (DOE) Hanford Site due to the high probability of discharging into the nearby Columbia River. While large-scale U migration in the subsurface is generally determined by advection along permeable pathways, local scale transport in less permeable regions, such as rock matrices or fine-grained (e.g. clayey) soils is controlled by diffusion. In this work, a single reservoir diffusion cell (in-diffusion method) was designed to study U(VI) diffusion in the fully saturated silt/clay fraction of a composite Hanford sediment material, under strictly controlled chemical conditions (pH 8.0, I = 0.02M, PCO2=10-3.5atm, saturated with calcite). Time-variant concentrations of tritiated water tracer (3H2O) and U(VI) in the reservoir were monitored during the diffusion phase and used as a measure of diffusion flux into the porous medium of the cell. U(VI) profiles in the pore water and in the solid phase along the cell length were also obtained at the end of the diffusion phase, by using high speed centrifugation and a specially designed extraction procedure. The 3H2O data were used to determine the tortuosity factor of the cell sediment. A retarded diffusive transport model for U(VI) was developed and numerically solved assuming local equilibrium and using a Freundlich sorption isotherm that was independently obtained from batch sorption experiments. The effective diffusion coefficient of U(VI) was determined by fitting the modeling results to experimental data (Optimization Toolbox, Matlab). The U(VI) diffusion showed less sorption retardation than expected based on batch results, indicating either reduced retardation capacity of the sediment, or invalidity of the local equilibrium assumption due to kinetic processes such as intra-particle diffusion at the grain scale. An improved model using first-order kinetic rates to account for the disequilibrium is under development towards better of the retarded diffusion process for U(VI) species.