PNNL

Abstract

A diffusion cell method was developed to measure the effective aqueous diffusion coefficient for U(VI) under strictly controlled chemical conditions within the inter-particle pores of silt/clay sediment from the DOE Hanford site, WA. "Inward-flux” diffusion studies were conducted in which U(VI) concentrations in both aqueous and solid phases were measured as a function of distance into the cell under conditions of constant concentration at the cell boundaries. A sequential extraction method was developed to measure sorbed U(VI) content in the solid phase, while accounting for the non-negligible extractable background U(VI). U(VI) diffusion data were found to be consistent with a model that assumed that: 1) a single effective aqueous diffusion coefficient could be used to simulate the coupled diffusion of various aqueous U(VI) species, and 2) the local equilibrium assumption (LEA) is appropriate for modeling the effects of sorption under the given experimental conditions. An effective aqueous diffusion coefficient (De) of 1.6x10^-6 cm2/s was obtained under conditions of pH 8.0 and calcite saturation that are relevant to the subsurface conditions at some regions of the Hanford site. The developed experimental techniques provide a practical approach for measuring effective aqueous U(VI) diffusivity in sorptive porous media.