Sol–Gel Derived Benzo-Crown Ether-Functionalized Silica Gel for Selective Adsorption of Ca<sup>2+</sup> 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 N<sub>2</sub> adsorption–desorption, and used for the selective adsorption of Ca<sup>2+</sup> ions. The adsorption amounts of the BCES for Ca<sup>2+</sup> 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 Ca<sup>2+</sup> ions was found to be 40.6 mg g<sup>–1</sup> at 298 K. The high selectivity of the BCES for Ca<sup>2+</sup> ions with respect to the other alkali and alkaline earth metals was attributed to the high affinity between Ca<sup>2+</sup> 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 Ca<sup>2+</sup> ions. The thermodynamics of Ca<sup>2+</sup> ions/BCES system indicates spontaneous and endothermic nature.