Alkaline earth ion exchange study of pure silica LTA zeolites using periodic first-principles calculations

Abstract

Experiments show that zeolites, such as Linde Type A (LTA), are promising materials for radioactive decontamination processes. In this work, the thermodynamic properties associated with alkaline earth ion (Ca²⁺, Sr²⁺ and Ba²⁺) exchange in pure silica LTA was studied using periodic first-principles calculations. The adsorption energies of alkaline earth ions compared with that of Na⁺ were investigated. The driving forces for alkaline earth exchange and related isotherms were calculated, and analyzed as a function of the electron chemical potential. The results demonstrate that Na⁺ in a pure silica LTA can completely be removed by Ba²⁺ and almost completely be removed by Sr²⁺ from a stream, but cannot be effectively exchanged by Ca²⁺. We also showed that electron-donating dopants should suppress Sr²⁺ and Ba²⁺ ion exchange, and that there is a substantial preference for incorporating Ba²⁺ over Sr²⁺. Lastly, the substantial difference between the adsorption energies of the ions in an assumed vacuum and those computed in water suggests that a solvation model is needed for accurate representation of ion adsorption in an LTA zeolite.