Spherical calcium-doped D152 resin (Ca/D152) was made in the same manner as that in the previous study [6]. Nickel-doped Ca/D152 resin (Ni–Ca/D152) was prepared by exchanging Ca2+ in Ca/D152 with Ni2+ in a nickel nitrate solution containing 75% ethanol (Ni–C2H5OH) or 0.1 M NH4Cl and 0.1 M NH3 Ni2þNH3 –NH4 for 24 h at ambient temperature