Radionuclide-specific ion exchange on zeolite frameworks: A conceptual proposal

Abstract

Understanding the factors that govern the selective ion exchange abilities of different cations and the detailed atomic structures of the exchanged cations inside the nanocavities of zeolites remain an ongoing challenge. In this study, the precise locations of Cs⁺ and Sr²⁺ ions captured within NaA zeolite were analyzed using focused-ion-beam scanning electron microscopy and high-resolution (scanning) transmission electron microscopy. Using focused-ion-beam scanning electron microscopy and high-resolution (scanning) transmission electron microscopy, the study analyzes the adsorption and positioning of Cs⁺ and Sr²⁺ ions. The adsorption equilibrium and capacities of Cs⁺ and Sr²⁺ ions were achieved from experimental adsorption data using the NaA zeolite. The adsorption capacities (q_m) of Cs⁺ and Sr²⁺ ions were 1.01 mmol/g and 4.29 mmol/g, respectively. Theoretical estimations revealed the actual adsorption capacities of Cs⁺ and Sr²⁺, along with their positions within the zeolitic framework, both of which influence the cation exchange behavior. Unlike Cs⁺, Sr²⁺ ions were predominantly captured in the β cages, suggesting that the location of ion exchange within the zeolite framework may vary depending on the ionic radius and the effect of the cage structures. These results clarify the radionuclide capture in zeolites by revealing distinct ion exchange behaviors.