Scale-dependent kinetics of uranium desorption from contaminated subsurface sediments

Column experiments were performed to investigate the scale-dependent desorption of uranyl [U(VI)] from a contaminated sediment collected from the Hanford 300 Area at the US Department of Energy (DOE) Hanford Site, Washington. The sediment was a coarse-textured alluvial flood deposit containing significant mass percentage of river cobble. U(VI) was, however, only associated with its minor, fine-grained (< 2mm) mass fraction. U(VI) desorption was investigated both from the field-textured sediment using a large column (80 cm length by 15 cm inner diameter), and from its < 2mm, U(VI)-associated mass fraction using a small column (10 cm length by 3.4 cm inner diameter). Dynamic advection conditions with intermittent flow and stop-flow events of variable durations were employed to investigate U(VI) desorption kinetics and its scale dependence. A multi-component kinetic model that integrated a distributed rate expression with surface complexation reactions successfully described U(VI) release from the fine-grained, U(VI)-associated materials. The field-textured sediment in the large column displayed dual domain, tracer-dependent mass transfer properties that affected the breakthrough curves of bromide, pentafluorobenzoic acid (PFBA), and tritium. The tritium breakthrough curve showed stronger non- equilibrium behavior than did PFBA and bromide, and required a larger immobile porosity to describe. The dual domain mass transfer properties were then used to scale the kinetic model of U(VI) desorption developed for the fine-grained materials to describe U(VI) release and reactive transport in the field-textured sediment. Numerical simulations indicated that the kinetic model that was integrated with the dual domain properties best described the experimental results. The kinetic model without consideration of the dual domain properties over-predicted effluent U(VI) concentrations. Overall, our results indicated that the kinetics of U(VI) release from the field-textured sediment were different from that of its fine-grained, U(VI)-associated mass fraction. However, the desorption kinetics measured on the U(VI)-containing mass fraction could be scaled to describe U(VI) reactive transport in the contaminated field-textured sediment after proper consideration of the physical transport properties of the sediment. The research also demonstrated a modeling approach to integrate geochemical processes into field scale reactive transport models.