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 96 h. 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 P 4 ¯ 3 n . The refined unit-cell parameters of the mixed sodalites ranged from 8.88 to 9.15 Å 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− < NO3− < MnO4− and CO32− < SO42− < WO42− for the monovalent and divalent anions, respectively. The relationship between the ReO4− distribution coefficient and competing anion size was nonlinear. When the difference in ionic radius (DIR) between ReO4− and Xn − (n = 1 or 2) was greater than ~ 12%, then ReO4− incorporation into sodalite was insignificant. The results imply that anion size is the major factor that determines sodalite anion compositions. Given the similarity in chemical behavior and anion size, ReO4− serves as a suitable analogue for TcO4− under oxidizing conditions where both elements are expected to remain as oxyanions in the + 7 oxidation state.