Diffusion coefficients of alkaline cations in Bure mudrock

In this work, the diffusivities of alkaline cations (Li+, Na+, K+, Rb+ and Cs+) were measured in a mudrock sample from Bure (ANDRA site, Meuse/Haute-Marne, France). The material is a natural rock, mainly composed of interstratified illite/smectite, quartz and calcite. It was saturated with a Na-Cl-dominated synthetic solution with an ionic strength of 57 mM and a pH ∼8.0. The effective diffusion coefficients (De) for the cations were determined from their steady-state flux through mudrock slices at 23 °C (through-diffusion technique). HTO diffusion coefficients were systematically measured as well. Measured De for the cations were found to be higher than values predicted from water diffusion alone. Moreover, this observation appeared to depend on the considered species: the ratio between measured and calculated effective diffusion coefficients ranged between two for lithium and nearly one order of magnitude for rubidium and cesium. An interpretation with different models dealing with sorption–diffusion processes is proposed and discussed.