Sol–Gel Derived Benzo-Crown Ether-Functionalized Silica Gel for Selective Adsorption of Ca2+ Ions

Benzo-crown ether-functionalized silica (BCES) was prepared by the combination of the sol–gel process and the Schiff base reaction, characterized by Fourier transmission infrared spectroscopy, scanning electron microscopy, and N2 adsorption–desorption, and used for the selective adsorption of Ca2+ ions. The adsorption amounts of the BCES for Ca2+ ions were independent of pH in the range of 5–10. Adsorption equilibrium was performed within 60 min. The adsorption capacity of the BCES for Ca2+ ions was found to be 40.6 mg g–1 at 298 K. The high selectivity of the BCES for Ca2+ ions with respect to the other alkali and alkaline earth metals was attributed to the high affinity between Ca2+ ions and crown ether. The adsorption/desorption cycle of the BCES could be up to five times. Langmuir and pseudo-second-order equations provided good correlation for the adsorption of Ca2+ ions. The thermodynamics of Ca2+ ions/BCES system indicates spontaneous and endothermic nature.