The ion-exchange characteristics between Pb²⁺ ions of aqueous solutions containing various counter-anions (F^–, Cl^– and NO^–₃) and Ca²⁺ ions of synthetic hydroxyapatite samples have been investigated in detail under the conditions of low pH values (3.0, 4.0 and 5.0).

The ratio of removal of Pb²⁺ ions to the apatites increased with a decrease in pH from 5.0 to 3.0, but even at pH 5.0 the ratio of removal of Pb²⁺ ions increased from 10 to 60% as the reaction time increased from 2 h to 20 days.

Even at the low pH value of 3.0 the apatite structure in a solution contaning F^– or Cl^– ions was maintained via a concurrent ion-exchange effect of Pb²⁺ ions together with F^– or Cl^– ions, which are known to be exchangeable for OH^– ions of the apatite, while the apatite structure in the system containing NO^–₃ ions was destroyed by protons. This destruction is thought to be due to the fact that since NO^–₃ ions cannot be exchanged for OH^– ions, the retention of the apatite structure together with Pb²⁺ ions was made impossible by the loosening or dissolving effect of protons on the skeletal structure of the apatite at such a low pH as 3.0.

Moreover, the crystal structure of Pb²⁺ ion-exchanged apatite [Ca_3.6Pb_6.4(PO₄)₆(OH)₂], with a= 9.882(3)Å and c= 7.417(2)Å, has been investigated by the X-ray powder pattern-fitting method. The determined site occupancy factors of Pb²⁺ ions were almost the same, i.e. 0.75 and 0.57 for M1 and M2 (column site) sites, respectively.

These results show that Ca²⁺ ions in the apatite sample can easily be exchanged for Pb²⁺ ions almost without distinction between M1 and M2 sites, assisted by the loosening effect of protons even at room temperature. The hydroxyapatite can easily be converted into stable lead apatite in acidic solutions containing exchangeable anions such as F^– or Cl^– by a synergistic effect.