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

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 Sr-90(2+) and explained the origin of this high selectivity by combining structural Rietveld refinements and density functional theory (DFT) calculations. Nuclear wastewater with a Sr-90(2+) radioactive concentration of 400 Bq L-1 (7.75 x 10(-2) ppt) and highly concentrated NaNO3 and NH4NO3 (0.03 M Na+ and 0.4 M NH4+, Na+/Sr-90(2+) = 3.48 x 10(10), and NH4+/Sr-90(2+) = 4.64 x 10(11)) was treated with NaA zeolite. The resultant Sr-90(2+) radioactive concentration was as low as 0.62 +/- 0.12 Bq L-1, close to the upper limit of Sr-90-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 Co-60 gamma 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 Sr-90(2+) 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.

Automated summary

In this work, researchers reported the exceptional selectivity of NaA zeolite towards radioactive Sr-90(2+) and explained its origin through structural refinements and density functional theory calculations. The zeolite showed high capture capacity, fast adsorption kinetics, and broad working pH range, making it a promising material for treating nuclear wastewater. The study also revealed the migration of Sr2+ ions within the zeolite structure during the ion-exchange process.