Nanocrystalline low-silica X zeolite as an efficient ion-exchanger enabling fast radioactive strontium capture

Abstract

NaA zeolite (Si/Al = 1.00) has been commercially applied for capturing radioactive ⁹⁰Sr²⁺ because of its high surface charge density, effectively stabilizing the multivalent cation. However, owing to its narrow micropore opening (4.0 Å), large micron-sized crystallites, and bulkiness of hydrated Sr²⁺, the Sr²⁺ exchange over NaA has been limited by very slow kinetics. In this study, we synthesized nanocrystalline low-silica X by minimizing a water content in a synthesis gel and utilizing a methyl cellulose hydrogel as a crystal growth inhibitor. The resulting zeolite exhibited high crystallinity and Al-rich framework (Si/Al of approximately 1.00) with the sole presence of tetrahedral Al sites, which are capable of high Sr²⁺ uptake and ion selectivity. Meanwhile, the zeolite with a FAU topology has a much larger micropore opening size (7.4 Å) and a much smaller crystallite size (∼340 nm) than NaA, which enable significantly enhanced ionexchange kinetics. Compared to conventional NaA, the nanocrystalline low-silica X exhibited remarkably increased Sr²⁺-exchange kinetics (> 18-fold larger rate constant) in batch experiments. Although both the nanocrystalline low-silica X and NaA exhibited comparable Sr²⁺ capacities under equilibrated conditions, the former demonstrated a 5.5-fold larger breakthrough volume than NaA under dynamic conditions, attributed to its significantly faster Sr²⁺-exchange kinetics.