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 ion-exchange 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.

Graphical abstract

Keywords

Sr²⁺ removal / low-silica X zeolite / nanocrystal / hydrogel / methyl cellulose

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

This research was financially supported by the institute of Civil Miltary Technology cooperation funded by the Defense Acquisition Program Administration and Ministry of Trade, Industry and Energy of Korea Government under grant No. 22-CM-BR-14.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-024-2449-6 and is accessible for authorized users.

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