Cesium sorption reversibility and kinetics on illite, montmorillonite, and kaolinite

Understanding sorption and desorption processes is essential to predicting the mobility of radionuclides in the environment. We investigate adsorption/desorption of cesium in both binary (Cs+one mineral) and ternary (Cs+two minerals) experiments to study component additivity and sorption reversibility over long time periods (500days). Binary Cs sorption experiments were performed with illite, montmorillonite, and kaolinite in a 5mM NaCl/0.7mM NaHCO3 solution (pH8) and Cs concentration range of 10^-3 to 10^-11M. The binary sorption experiments were followed by batch desorption experiments. The sorption behavior was modeled with the FIT4FD code and the results used to predict desorption behavior. Sorption to montmorillonite and kaolinite was linear over the entire concentration range but sorption to illite was non-linear, indicating the presence of multiple sorption sites. Based on the 14day batch desorption data, cesium sorption appeared irreversible at high surface loadings in the case of illite but reversible at all concentrations for montmorillonite and kaolinite. A novel experimental approach, using a dialysis membrane, was adopted in the ternary experiments, allowing investigation of the effect of a second mineral on Cs desorption from the original mineral. Cs was first sorbed to illite, montmorillonite or kaolinite, then a 3.5-5kDalton Float-A-Lyzer® dialysis bag with 0.3g of illite was introduced to each experiment inducing desorption. Nearly complete Cs desorption from kaolinite and montmorillonite was observed over the experiment, consistent with our equilibrium model, indicating complete Cs desorption from these minerals. Results from the long-term ternary experiments show significantly greater Cs desorption compared to the binary desorption experiments. Approximately ~45% of Cs desorbed from illite. However, our equilibrium model predicted ~65% desorption. Importantly, the data imply that in some cases, slow desorption kinetics rather than permanent fixation may play an important role in apparent irreversible Cs sorption.