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.