Thermal stabilization of extraframework Cs+ in zeolite 13X

Abstract

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 °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 LIII-edge X-ray absorption spectroscopy (XAS) analysis, the Cs-Si(Al) interaction at ∼4.37 Å became stronger relative to the Cs-O interaction at ∼2.98 Å 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 °C, bulk-phase analyses such as XRD, scanning electron microscopy, and surface area measurement showed that the zeolite framework completely broke down to form non-porous phases. The XAS analysis further indicated that Cs+ was mainly incorporated into amorphous phases at 900 °C and pollucite at ≥ 1,000 °C. This was consistent with the markedly low Cs+ leaching for the ≥ 900 °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.