Energetics and kinetics of alkali ion exchange in analcime

Abstract

We employ density functional theory calculations to investigate the energetics and diffusion mechanisms of alkali metal ions (Na⁺, K⁺, Rb⁺, and Cs⁺) within the analcime framework. While Na⁺ preferentially adsorbs at the center of the 8-member ring, DFT calculations show that larger ions, such as Cs⁺, occupy the center of the cage due to steric constraints. The computed exchange energies indicate that exchange of Na⁺ by Cs⁺ is thermodynamically favorable. Migration energy barriers, determined using the climbing image nudged elastic band method, show that Na⁺ is mobile in analcime, diffusing through both 6-member rings and 8-member rings, with a low migration energy of about 0.3 eV; Cs⁺ diffusion primarily occurs through the 8-member ring pathway, with a high energy barrier of 1.35 eV, whereas the 6-member ring pathway presents even significantly higher barriers (∼3.37 eV), making it less accessible. These findings suggest that Cs⁺/Na⁺ exchange in analcime is thermodynamically feasible but kinetically hindered. Our work provides a theoretical foundation for the application of analcime in nuclear waste management and highlights the need for further experimental validation.