Synthesis, Structure Determination, and Aqueous Durability of Cs₂ZrSi₃O₉

The structure of Cs₂ZrSi₃O₉ synthesized using a sol-gel method was determined from the Rietveld refinement of experimental powder X-ray diffraction data. The refinement confirmed that this compound was isostructural with wadeite: its structure was hexagonal (space group P6₃/m), and its lattice parameters were a = 7.2303(2) Å and c = 10.2682(4) Å. The aqueous durability of Cs₂ZrSi₃O₉ varied, depending on the solution conditions. In modified leach tests with buffered (pH 7) and unbuffered solutions, the 7-d cesium release rates were <1.2 × 10^-4 g·(m²·day)^-1, which indicated high aqueous durability. However, in unsaturated, unbuffered solutions with a pH of 9-10, the durability was much lower, with 7-d cesium release rates of 2.2 × 10^-3 g·(m₂·day)^-1. The ability of this material to retain cesium in aqueous environments can be explained by its condensed ring structure, in which the size of the channel openings is smaller than the diameter of a Cs₊ ion. However, dissolution of the network silicate occurred at high pH, which resulted in the release of cesium.