Adsorption structures and bonding states of cesium and barium adsorbed on various sites of vermiculite.

The adsorption reaction on clay minerals is crucial for understanding the environmental behavior of various cations, including cesium (Cs). However, its details remain unclear because of multiple adsorption sites of the clay minerals, a significant difference between concentrations in the atomic-scale experiments and the actual environment, and difficulties of evaluating bonding states of the adsorbed cations. It is expected that systematic experiments at the atomic-scale with a wide concentration range and application of density functional theory (DFT) calculations overcome the problems and bring crucial insights to link laboratory experiment results with environmental sample analysis. This study obtained adsorption isotherms and conducted X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements at various concentrations of Cs or barium (Ba). Additionally, high-energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) measurements was performed for Cs to estimate its bonding state. To interpret the results, DFT calculations were also conducted. The results clarified the transition process of the adsorption site and local structures of adsorbed cations depending on the adsorption concentrations. Especially, the combination of EXAFS and DFT calculations succeeded in revealing the intermediate state between frayed edge site at a low concentration and collapsed interlayer site at a high concentration. Furthermore, HERFD-XANES and DFT calculations suggested the ionic bonding nature of Cs, regardless of the adsorption sites.