Ion-exchange property and mechanism of magnesium–calcium hydroxyapatite solid solutions

Magnesium–calcium hydroxyapatite solid solution (MgCaHap) particles were immersed into solutions containing Ca2+ ions under different conditions where Ca2+ concentration, immersion period, solution pH and temperature were varied. Mg2+ ions in the particles were released into the solution accompanying the incorporation of Ca2+ ions from the solution into the particles by the treatment, which demonstrates that Mg2+ in MgCaHap was exchanged by Ca2+. The ion-exchange equilibrium was attained within a short time of 15–30 min. The ion-exchange was promoted by raising the Ca2+ concentration and solution temperature and by lowering the solution pH. The exchange of Mg2+ with Ca2+ was shown to take place by the dissolution of surface layer of the MgCaHap particles and the recrystallization of a new hydroxyapatite (Hap) layer on the particles.