Facile formation of pollucite in geopolymers: Implications for radioactive Cs immobilization

Abstract

Pollucite, CsAlSi2O6, is one of the few naturally occurring cesium minerals, and is considered as an efficient phase for long-term immobilization of radioactive cesium. The occlusion of cesium ions within pollucite structure is conventionally achieved via solid state syntheses, requiring temperatures exceeding 1000 °C, or hydrothermal processes which are typically preformed at 200–300 °C and elevated pressures. The current paper reports on the formation of pollucite within Cs-bearing geopolymers at near-ambient temperatures and pressures. By combining data from X-ray powder diffraction and 133Cs solid-state NMR measurements we are able not only to identify the various phases that are formed within crystalline-amorphous geopolymer matrices, but also follow changes in the distribution of cesium between the various phases as a function of geopolymer formulation. Based on these data we were able demonstrate that the extent of pollucite formation increases with the Si2O:Al2O3 ratio in the geopolymer formulation. The formation of crystalline pollucite domains within composite crystalline-amorphous geopolymer matrices may have important implications for the immobilization of radioactive cesium.