On the possibility to modify the CsAlSi5O12/CsAlSi2O6 ratio during the thermal transformation of Cs-clinoptilolite

Abstract

Clinoptilolite is a natural zeolite employed in Cs+ removal, and its cesium form transforms, by heating, into a matrix composed mainly by CsAlSi5O12 (CAS) with smaller quantities of glass and CsAlSi2O6 (pollucite). Both crystalline compounds were proposed as host for 137Cs, albeit pollucite is considered superior. This work shows how, by mixing Cs-clinoptilolite and amorphous hydrated aluminum hydroxide (L), it is possible to modify the solid-state transformation of the zeolite and favor the crystallization of pollucite over CAS. Cs-clinoptilolite, L and several mixtures of the two materials were preliminary subjected to thermal and XRD analyses. Muffle treatments of 32 h at 1200 °C and 8 h at 1300 °C, followed by Q-XRD analyses, were performed on some materials. Unlike CAS, pollucite always crystallized. The greater the quantity of L in the mixture, the lower the ratio between the quantities of CAS and pollucite formed. In the range 1200–1300 °C, the CAS/pollucite ratio decreases with temperature. Al2O3-polymorphs (mainly corundum) and glass were ubiquitous in the fired mixtures, whereas mullite was not. By varying the content of L in the mixture, the distribution of cesium among the Cs-bearing phases varied, and for some compositions most of the cesium was trapped inside pollucite.