Competitive Effects of Cations on the Diffusion Properties of Strongly Sorbing Trace Cations in Compacted Illite and Opalinus Clay

Surface diffusion may enhance the diffusive fluxes of cationic species in argillaceous materials and lead to an underestimation of radioactive doses emanating from deep geological repositories for radioactive waste to the biosphere. The conceptual understanding of surface diffusion implies that competition effects between different cations would also affect diffusion and not only sorption. The present work provides for the first time diffusion data and simulations supporting this view. The effect of Ca2+ on the diffusion and sorption of a Co-57(2+) tracer was measured in compacted illite. The results could be quantitatively explained using an electrical double layer or a cation-exchange model for the uptake of cationic species, with both models involving mobile species at the planar surfaces. The observed effects were related to the presence of different sites for Co2+ uptake, exhibiting different surface mobilities and different affinities for Ca2+. The strong sorption sites, which dominate sorption at the conditions of the experiments, do not exhibit noticeable surface mobility and are not prone to competition with Ca2+. The effective diffusion coefficients are rather dominated by the cation species at the planar surfaces, which are affected by competition with Ca2+. The relevance of the illite model system for intact clay rock was demonstrated by a successful application of the illite source data to Co-57(2+) diffusion measurements in Opalinus Clay, in which the equilibrium pore water contained substantial amounts of alkaline earth cations. Our present findings enhance the conceptual understanding of sorption and diffusion of trace elements in compacted clay minerals and clay rocks.