Effects of fine structure changes of strontium hydroxyapatites on ion-exchange properties with divalent cations

Abstract

The ion exchange of various strontium hydroxyapatites [SrHAp; stoichiometric form, Sr₁₀(PO₄)₆(OH)₂] of different Sr/P ratios with divalent cations has been investigated and compared with those between the corresponding cations and calcium hydroxyapatites (CaHAp). SrHAp has superior exchange properties for divalent cations in comparison with those of CaHAp and the former solid with Sr/P equal to 1.73 has the highest capacity for ion exchange of the compositions examined. The addition of HCl to the exchanging solution enhanced the exchange of SrHAp with Pb²⁺, apparently influenced by the formation of strontium and lead chlorapatite during the exchange process. The extended X-ray absorption fine structure (EXAFS) analyses revealed that the ion-exchange characteristics are strongly influenced by the nearest-neighbour distances of the Sr—O bond of SrHAp or the Ca—O bond of CaHAp.