Cation-exchange disks were swollen with methanol, and then ZrCa-CEDs were prepared by successively passing 2.5 mL of deionized water, 2.5 mL of a 0.05 mol/L Zr solution, 2.5 mL of deionized water, 2.5 mL of a 0.01 mol/L Ca solution, and 2.5 mL of deionized water through the cation-exchange disk.