An integrated sorption-diffusion model for the calculation of consistent distribution and diffusion coefficients in compacted bentonite

A thermodynamic sorption model and a diffusion model based on electric double layer (EDL) theory are integrated to yield a surface chemical model that treats porewater chemistry, surface reactions, and the influence of charged pore walls on diffusing ions in a consistent fashion. The relative contribution of Stern and diffuse layer to the compensation of the permanent surface charge represents a key parameter; it is optimized for the diffusion of Cs in Kunipia-F bentonite, at a dry density of 400 kg/m³. The model is then directly used to predict apparent diffusivities (D_a) of Cs, Sr, Cl^-, I^- and TcO₄^- and corresponding distribution coefficients (K_d) of Cs and Sr in different bentonites as a function of dry density, without any further adjustment of surface chemical and EDL parameters. Effective diffusivities (D_e) for Cs, HTO, and TcO₄^- are also calculated. All calculated values (D_a, D_e, K_d) are fully consistent with each other. A comparison with published, measured data shows that the present model allows a good prediction and consistent explanation of (i) apparent and effective diffusivities for cations, anions, and neutral species in compacted bentonite, and of (ii) K_d values in batch and compacted systems.