The sorption behaviour of caesium on Opalinus Clay: A comparison between intact and crushed material

The sorption of Cs on crushed and intact (non-crushed) Opalinus Clay (OPA) was studied. A Cs sorption isotherm was measured in a synthetic OPA pore water covering an equilibrium concentration (CCs,eq) range between 10−2 M and 10−9 M. The isotherms measured on crushed (<63 μm) and intact OPA were in excellent agreement indicating that all sorption sites for Cs in intact OPA are available. The isotherms were modelled assuming that illite is the dominating clay mineral responsible for Cs sorption. The model described the results very well at low Cs concentrations, over-predicted the results in the mid-concentration range and under-predicted the data at the highest Cs equilibrium concentrations. Improvements in the mid-concentration range were obtained by lowering the image selectivity coefficient of the Type II sites by 0.4 log units. At the highest Cs concentration an improvement can be achieved by considering sorption of Cs on other clay minerals present in OPA. The main result from this study is that in the case of cation exchange processes the exchange sites in the intact material are all available. This corroborates the common practice in performance assessment (PA) of applying batch sorption data obtained on dispersed systems to the intact rock for predicting the retention behaviour of radionuclides.