Cation-Exchange Characteristics of Synthetic Hydroxyapatites for Zn2+, Cd2+ and Hg2+ Ions

The cation-exchange characteristics for Ca2+ ions in synthetic hydroxyapatites (HAP) [Ca₁0^. (PO4)6(OH)2] and 2B group ions (Zn2+, Cd2+ and Hg2+ ions) in aqueous solutions containing various counter anions (CI^- and NO3^- ions) have been investigated by a normal batch method at 25°C; the influence of H+ ions on these characteristics was determined. Zn2+ ions in aqueous solution were taken into the HAP by the cation-exchange reaction with a molar ratio of Zn2+/Ca2+-1.0. The apatite structure, however, was gradually destroyed as increasing amounts of Zn2+ ions were taken up into the HAP in the reaction with the highly concentrated aqueous solution at pH=4.0, and hydrated Zn3(PO4)2 molecules were finally produced (Fig. 4). Cd2+ ions in aqueous solution exchanged with Ca2+ ions in the HAP with a molar ratio of Cd2+/Ca2+-1.0 and yielded stable Cd2+ ions-exchanged apatites (Fig. 4). Hg2+ ions displayed different reaction behaviors toward the HAP for different counter anions (Figs. 2, 5 and 6). Hg2+ ions in HgCl2 aqueous solution was poor in the reactivity; on the other hand, Hg2+ ions in Hg(NO3)2 aqueous solution actively attacked the HAP molecules and decomposed them into Hg3(PO4)2 in the pH range below 4.0 (Fig. 4). The differences of the reactivity were ascribable to the formation of HgCl42^- anions in HgCl2 aqueous solution.