Cesium transport in Czech compacted bentonite: Planar source and through diffusion methods evaluated considering non-linearity of sorption isotherm

The aim of the presented study was to determine the influence of non-linear character of Cs sorption isotherm and dominant interlayer cation on the sorption and diffusion properties of the Czech compacted bentonite. Planar source and through diffusion experiments with Cs on the compacted bentonite were performed and evaluated by suitable methods accounting for the transient state of diffusion and the presence of separating filters. It was shown that the observed Cs concentration profiles can be appropriately simulated with the use of Langmuir isotherm. The fitted values of sorption parameters obtained from the evaluation of through diffusion experiments generally agreed with the values expected from the batch sorption experiments. The most important factor for the Cs diffusion was its total concentration in the experiment with Da values ranging from about 1⨯10−13 m2·s−1 at the Cs concentration 10−7 M to about 4⨯10−12 m2·s−1 at the Cs concentration 10−3 M. The observed discrepancies between the experimental data and the standard model based on Fickian diffusion were explained by the application of dual transport model using so-called surface diffusion theory. The importance of surface diffusion in through diffusion experiments increased with increasing Cs total concentration, which indicates that this effect is probably connected to the ion exchange mechanism on layer sites of bentonite.