Thermal stabilization of extraframework Cs+ in zeolite 13X

Given the reversible nature of ion exchange, Cs+-exchanged zeolite 13X was thermally treated to safely retain extraframework Cs+. By thermal gravimetric-differential scanning calorimeter (TG-DSC) analysis, both endothermic and exothermic peaks, indicative of phase transitions, gradually shifted to higher temperatures at the higher surface loading of Cs+ (i.e., [Cs+](0)/[NaX](0)), indicating that extraframework Cs+ stabilized the zeolite framework. By thermal treatment at <= 800 degrees C, X-ray diffraction (XRD) showed that the samples with [Cs+](0)/[NaX](0) = 0.5 and 5 mmol/g maintained the faujasite framework. Nonetheless, as indicated by Cs L-III-edge X-ray absorption spectroscopy (XAS) analysis, the Cs-Si(Al) interaction at similar to 4.37 angstrom became stronger relative to the Cs-O interaction at similar to 2.98 angstrom with the increasing treatment temperature, indicating that thermally induced dehydration caused extraframework Cs+ to migrate from spacious sites to confined (less exchangeable) sites. Thus, the redistribution of extraframework Cs+ among exchange sites was responsible for the lower leaching with the increasing treatment temperature. On the other hand, by thermal treatment at >= 900 degrees C, bulk-phase analyses such as XRD, scanning electron microscopy, and surface area measurement showed that the zeolite framework completely broke down to form nonporous phases. The XAS analysis further indicated that Cs+ was mainly incorporated into amorphous phases at 900 degrees C and pollucite at >= 1,000 degrees C. This was consistent with the markedly low Cs+ leaching for the >= 900 degrees C-treated samples. Since pollucite can provide the greatest retainability of Cs+, the thermal treatment leading to its formation is recommended before storing Cs+ -exchanged zeolite 13X at nuclear waste repositories. (c) 2022ElsevierB.V. Allrightsreserved.

Automated summary

This study investigates the stability of Cs+-exchanged zeolite 13X using thermal treatment. The results show that extraframework Cs+ can stabilize the zeolite framework, but with increasing treatment temperature, Cs+ migrates from spacious sites to confined sites, resulting in lower leaching. During high-temperature treatment above 900℃, the zeolite framework is completely destroyed and nonporous phases are formed, with Cs+ mainly existing in amorphous phases and pollucite. Therefore, thermal treatment to form pollucite is recommended for storing Cs+-exchanged zeolite 13X in nuclear waste repositories to achieve maximum Cs+ retention.