Adsorption of cesium and strontium on mesoporous silicas

Studies are underway on the adsorption reactions of metal ions in confined spaces at the solid-water interface, but it is unclear how the effects of confinement differ for different types of ions. We investigated the effect of the pore size on the adsorption of two cations with different valence, Cs⁺ and Sr²⁺, on mesoporous silicas with different pore size distributions. The amount of Sr²⁺ adsorbed per unit surface area did not differ significantly among the silicas, whereas that of Cs⁺ was particularly high for silicas with a larger fraction of micropores. The results of X-ray absorption fine structure analysis showed that both ions form outer-sphere complexes with the mesoporous silicas. The results of adsorption experiments were analyzed by fitting using a surface complexation model with the cylindrical Poisson-Boltzmann equation and optimized capacitance of the Stern layer for different pore sizes, and we found that the intrinsic equilibrium constant for the adsorption of Sr²⁺ is constant regardless of the pore size, whereas that of Cs⁺ increases as the pore size decreases. The decrease in the relative permittivity of water inside pores with a decrease of the pore size can be interpreted to cause a change in the hydration energy of Cs⁺ in the second coordination sphere upon adsorption. The reasons for the different confinement effects on the adsorption reactions of Cs⁺ and Sr²⁺ were discussed based on the distance of the adsorbed ions from the surface and the chaotropic and kosmotropic nature of Cs⁺ and Sr²⁺, respectively.