Integrated sorption and diffusion model for bentonite, 1; Clay-water interaction and sorption modeling in dispersed systems

Tachi, Yukio  ; Ochs, M.*; Suyama, Tadahiro

The integrated sorption and diffusion (ISD) model for bentonite was developed to predict the long-term migration of radionuclides (RNs). The basic premise in the ISD model was to consistently use the same simple model design and parameters for describing RNs sorption as well as the clay surface chemistry. A 1-site non-electrostatic surface complexation model in combinations with 1-site ion exchange model was selected to keep model characteristics relatively robust when applied to compacted systems. Fundamental model parameters were evaluated from titration data for purified montmorillonite. The resulting model was then parameterized on the basis of selected sorption datasets for Np(V), Am(III) and U(VI), which cover key geochemical conditions such as pH, ionic strength and carbonate. The sorption trends for these RNs can be quantitatively described by the model considering a full suite of surface species including carbonate species which are consistent with aqueous RNs speciation.