Ion-exchange properties of ternary CaO–MgO–Al2O3spinels in pH-controlled aqueous radioactive waste solutions

Spinel ion-exchange materials, composed of CaO–MgO–Al2O3ternary systems were prepared via sol–gel reactions followed by sintering at different temperatures. Ca3MgAl4O10, Ca3Al2O6, CaMg2Al16O27, Ca2Mg2Al26O45, corundum, periclase, and monoxide lime were detected at room temperature, while CaAl2O4and MgAl2O4spinels were ascertained at 1,200–1,400 and 1,725–1,900°C, respectively. They were characterized using scanning electron microscopy, X-ray diffraction, X-ray fluorescence, FTIR, DTA–TGA, N2-gas adsorption, and laser diffraction particle size analyzer. We proved that they are chemically and thermally stable in their face-centered array spinel configurations with different surface, morphological, and topographical structures. They were used for removal of 134Cs+, 60Co2+, and 152−154Eu3+from waste solutions; preliminary kinetic and pH were investigated. Langmuir, Freundlich, Dubinin–Radushkevich, Temkin, Harkins–Jura, and Halsey isothermal prototypes were applied to describe the equilibria between adsorbed phase metal ions and those in solution at different concentrations. The adsorption of 134Cs+, 60Co2+, and 152−154Eu3+was critically affected by temperatures and ambient pH; spontaneous and exothermic nature of adsorption was found at specific pH values, beyond which the adsorption is restricted. The mechanism of adsorption was found to be chemical reaction combined with ion exchange rather than pure ion exchange.