Origin of the exceptional selectivity of NaA zeolite for the radioactive isotope 90Sr2+

Abstract

In this work, we reported an exceptionally high selectivity of a NaA zeolite (for which the framework type code of LTA was assigned by the Structure Commission of the International Zeolite Association) towards radioactive ⁹⁰Sr²⁺ and explained the origin of this high selectivity by combining structural Rietveld refinements and density functional theory (DFT) calculations. Nuclear wastewater with a ⁹⁰Sr²⁺ radioactive concentration of 400 Bq L⁻¹ (7.75 × 10⁻² ppt) and highly concentrated NaNO₃ and NH₄NO₃ (0.03 M Na⁺ and 0.4 M NH₄⁺, Na⁺/⁹⁰Sr²⁺ = 3.48 × 10¹⁰, and NH₄⁺/⁹⁰Sr²⁺ = 4.64 × 10¹¹) was treated with NaA zeolite. The resultant ⁹⁰Sr²⁺ radioactive concentration was as low as 0.62 ± 0.12 Bq L⁻¹, close to the upper limit of ⁹⁰Sr-induced radioactive activity in drinking water, and the distribution coefficient, K_d, was as high as 421.56 ± 13.39 L g⁻¹. The NaA zeolite showed fast adsorption kinetics (ca. 5 min), high capture capacity (294.12 mg g⁻¹), a broad working pH range (4–12), and excellent radiation resistance. The crystallinity and performance of NaA zeolite were not affected by 1000 kGy ⁶⁰Co γ irradiation. Structural Rietveld refinements revealed that all Sr²⁺ ions are located at the center of the single 6-membered rings (s6rs) of the NaA zeolite. DFT calculations showed the free energy difference of exchanging different cations with Na⁺ located in the 8-membered ring (s8r) of the NaA zeolite, and predicted the sequence of Sr²⁺(−8.17 eV) > Ca²⁺(−6.17 eV) > K⁺(−2.13 eV) > Mg²⁺(−1.94 eV) > NH₄⁺(−0.65 eV) > Cs⁺(−0.11 eV). The strong tendency of Sr²⁺ replacing Na⁺ explains the exceptionally high selectivity of the NaA zeolite for ⁹⁰Sr²⁺ in high concentrations of Na⁺ and NH₄⁺. Along with the ion-exchange process, structural Rietveld refinements revealed that the Sr²⁺ ions at the s8rs center migrated to the s6rs center.