Thermal transformation of Cs-clinoptilolite to CsAlSi5O12

In this study we have investigated the protocol to produce CsAlSi5O12 (CAS), a potential host for radioactive cesium, starting from clinoptilolite, a natural zeolite employed in cesium decontamination. A clinoptilolite-rich rock sampled in Sardinia (Italy) has been subjected to a beneficiation process, to obtain a powder with a higher zeolite content. The enriched material (90 wt.% clinoptilolite) has been previously Na-, then Cs-exchanged. Both forms have been analyzed by ICP-AES and ICP-MS. Thermal treatments of 2 hours between 800 and 1250°C have been performed on different aliquots of the Cs-exchanged material. Samples heated at T≥1150°C are composed basically only by CAS, as determined by XRD analyses, and their high crystallinity has been also confirmed by SEM observations. Ideally, the nucleation of CAS follows the reaction Cs6Al6Si30O72•nH2O → nH2O↑ + 6CsAlSi5O12. DTA analyses indicate that the transformation has occurred at 1145°C. TG analyses, coupled with XRD data, lead to exclude a cesium volatilization during the synthesis of CAS.