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 degrees 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) O-4(-) in the sodalites, which have a primitive cubic pattern in the space group P (4) over bar 3n. The refined unit-cell parameters of the mixed sodalites ranged from 8.88 to 9.15 angstrom 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 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 similar to 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. (C) 2014 Elsevier B.V. All rights reserved.