PNNL

Abstract

Perovskite-based ceramic composites were developed as potential waste form materials for immobilizing Cs and I with high waste loadings and chemical durability. The perovskite Cs3Bi2I9 as the host phase for Cs and I was synthesized by a cost-effective solution process at low temperatures, and embedded into a highly durable hydroxyapatite matrix to form dense ceramic composite waste forms by spark plasma sintering. The chemical durabilities of the Cs3Bi2I9- hydroxyapatite composite pellets were investigated by static and semi-dynamic leaching testing. The Cs and I were incongruently released from the matrix for both pure Cs3Bi2I9 and composite structures (Cs being released faster than I on a normalized basis), which can be explained by the difference in strengths between Cs-I and Bi-I bonds as well as the formation of insoluble micrometer-sized BiOI precipitates. Hydroxyapatite in the composite structure of Cs3Bi2I9+HA acts an effective physical barrier that can significantly reduce the elemental release rates of Cs and I as compared to pure Cs3Bi2I9, and thus greatly improve the chemical durability of composite waste forms by a factor of 50. The activation energies of elemental releases based on dissolution and diffusion-controlled mechanisms were determined with significantly higher energy barriers for dissolution from the composite versus that of monolithic Cs3Bi2I9. The ceramic-based composite waste forms exhibit excellent chemical durabilities and waste loadings, commensurate with the state-of-the-art glass-bonded perovskite composites for iodine and cesium immobilization.