Perrhenate incorporation into binary mixed sodalites: The role of anion size and implications for technetium-99 sequestration

Perrhenate (ReO4−), as a TcO4− analogue, was incorporated into mixed-anion sodalites from binary solutions containing ReO4− and a competing anion Xn− (Cl−, CO32−, SO42−, MnO4−, or WO42−). Our objective was to determine the extent of solid solution formation and the dependence of competing ion selectivity on ion size. Using equivalent aqueous concentrations of the anions (ReO4−/Xn− molar ratio=1:1), we synthesized mixed-anion sodalites from zeolite and NaOH at 90°C for 96h. The resulting solids were characterized by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure (XANES) spectroscopy to determine crystal structure, chemical composition, morphology, and rhenium (Re) oxidation state. Rhenium in the solid phase occurred predominately as Re(VII)O4− in the sodalites, which have a primitive cubic pattern in the space group P4¯3n. The refined unit-cell parameters of the mixed sodalites ranged from 8.88 to 9.15Å and showed a linear dependence on the size and mole fraction of the incorporated anion(s). The ReO4− selectivity, represented by its distribution coefficient (Kd), increased in the following order: Cl−