Effect of Grain Sizes on Uranium(VI) Adsorption/Desorption Kinetics and Rate Additivity

Stirred-flow cell experiments were performed to investigate the grain-scale kinetics of uranyl [U(VI)] adsorption and desorption in a sediment collected from the 300 Area at US DOE Hanford Site, Washington. The sediment was separated into four size fractions (1-2mm, 0.2-1mm, 0.05-0.2mm, and less than 0.05 mm) to investigate their relative contribution to U(VI) adsorption/desorption in the sediment, to determine the rates of U(VI) adsorption/desorption kinetics, and to investigate the additivity of U(VI) adsorption/desorption processes from individual grain size fractions. The experiments were performed under dynamic flow conditions using a flow interruption technique and under a chemical condition mimicing the groundwater chemical composition at the Hanford 300 Area. The measured rates of U(VI) adsorption and desorption varied with sediment size, showing that a smaller size fraction yielded faster adsorption and slower desorption rates. The kinetic behavior of U(VI) adsorption/desorption was attributed to the diffusion-controlled U(VI) adsorption/desorption in the intragrain regions that displayed variable pore size and connectivity. The experimental data were used to evaluate two diffusion-based models of U(VI) adsorption/desorption kinetics. The parameterized kinetic models for individual grain-size fractions were used to predict the additive behavior that was then compared with the experimental data for the whole sediment. The surface area, intragrain pore size and volume distribution, and pore connectivity of the sediment and its size fractions were also measured to provide insights into the observed kinetic behavior.