Ion-exchange of magnesium–calcium hydroxyapatite solid solution particles with Cd2+ ion

Magnesium–calcium hydroxyapatite solid solution (MgCaHap) particles with different atomic Mg/(Mg + Ca) ratios (XMg) from 0 to 0.28 were prepared by a wet method and were used as a cation-exchanger with Cd2+ ions in solution by batch and flow methods. The amounts of Mg2+ and Ca2+ ions released into the solution from the particles and that of Cd2+ ions incorporated into the particles from the solution were determined by the batch method. The ion-exchange with Cd2+ reached an equilibrium within ca. 30 min and did not change the morphology and crystal structure of MgCaHap particles. The exchanged amount per gram exhibited a maximum at XMg = 0.20 depending on the specific surface area of the particles. XMg value of the MgCaHap particles after the ion-exchange treatment was less than that before the treatment, that is, Mg2+ ions were more easily released from the MgCaHap particles than Ca2+ ions by the treatment. The ion-exchange heat per gram and per mole of exchanged ions obtained by the flow method were increased with an increase of XMg, indicating that the ion-exchange heat between Mg2+ and Cd2+ is more than that between Ca2+ and Cd2+.