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 X^n- (Cl^-, CO₃^2-, SO₄^2-, MnO₄^-, or WO₄^2-). 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^-/X^n- 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 (K_d), increased in the following order: Cl^-<NO₃^-<MnO₄^- and CO₃^2-<SO₄^2-<WO₄^2- 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 X^n- (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.