Influence of the Nb content and microstructure of sitinakite-type crystalline silicotitanates (CSTs) on their Sr2+ and Cs+ sorption properties

Radioactive wastewater contaminated with Cs-137 and Sr-90 are produced at different stages of the nuclear fuel cycle. Transferring these radionuclides to a stable solid matrix is crucial for safe wastewater management and long-term storage. Sitinakite Crystalline silicotitanates (CSTs) are among the most selective mineral ion exchangers for Cs and/or Sr depending on the pH of the medium and the CST chemical composition (Nb doping). To better understand how their microstructure affects their sorption properties, CSTs with different morphologies (consisting of submicronic to micronic particles) and Nb contents (from 0% to 25%) were synthesized and characterized in terms of their Sr2+ and Cs+ sorption properties at moderately basic pH. The Sr2+/Na+ and Cs+/Na+ equilibrium exchange diagrams obtained highlight the presence of two kind of sorption sites with different selectivities and sorption capacities. The first, kind of site, highly selective for Sr and less selective for Cs at moderately basic pH, was attributed to "tunnel" sites in the CST structure. The second one, less selective for Sr and Cs, seems to be a surface site. Although Nb doping was found to promote Cs sorption and hinder Sr sorption at moderately alkaline pH, the accessibility and selectivity of these two sites was found to depend more strongly on the microstructure of the CSTs. Results indicate that the more highly crystalline, micronic particles, with fewer structural defects, are more selective for Sr, but that the larger specific surface areas (greater number of surface sites) afforded by the submicronic particles are associated with larger sorption capacities.