Pressure-Induced Metathesis Reaction To Sequester Cs

Abstract

We report here a pressure-driven metathesis reaction where Ag-exchanged natrolite (Ag₁₆Al₁₆Si₂₄O₈₀·16H₂O, Ag-NAT) is pressurized in an aqueous CsI solution, resulting in the exchange of Ag⁺ by Cs⁺ in the natrolite framework forming Cs₁₆Al₁₆Si₂₄O₈₀·16H₂O (Cs-NAT-I) and, above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the initial cation exchange, the precipitation of AgI occurs. Additional pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to a pollucite-related, highly dense, and water-free triclinic phase with nominal composition CsAlSi₂O₆. At ambient temperature after pressure release, the Cs remains sequestered in a now monoclinic pollucite phase at close to 40 wt % and a favorably low Cs leaching rate under back-exchange conditions. This process thus efficiently combines the pressure-driven separation of Cs and I at ambient temperature with the subsequent sequestration of Cs under moderate pressures and temperatures in its preferred waste form suitable for long-term storage at ambient conditions. The zeolite pollucite CsAlSi₂O₆·H₂O has been identified as a potential host material for nuclear waste remediation of anthropogenic ¹³⁷Cs due to its chemical and thermal stability, low leaching rate, and the large amount of Cs it can contain. The new water-free pollucite phase we characterize during our process will not display radiolysis of water during longterm storage while maintaining the Cs content and low leaching rate