Immobilization of aqueous tetravalent cations of titanium and zirconium with calcium hydroxyapatite

In order to prepare calcium hydroxyapatite (CaHAp) containing a component with photo-catalytic activities such as Ti4+, two preparation procedures, coprecipitation and surface treatment, were investigated since it was recently reported that CaHAp containing Ti4+ species possessed rather greater activities than the corresponding oxide. The solid produced from the coprecipitation of Ca2+, Ti4+ and PO43− in aqueous phase was not homogeneous due to the precipitation of Ti-species in the solution during the preparation, indicating that a particle of the Ti-species was present in the bulk phase of the hydroxyapatite. Since it is sufficient that the Ti-species is present on the surface of CaHAp in respect to the employment as a photo-catalyst, the surface treatment of CaHAp with Ti4+ was examined under the dilution conditions of TiSO4 in acidic solution to avoid the precipitation of the Ti-species. The employment of pH = 1.1 was rather unfavorable due to the conversion of CaHAp to CaHPO4 while the conversion was suppressed by using the solution of pH = 1.7. When CaHAp was added to the homogeneous solution containing Ti4+ by the atomic ratio Ti4+/Ca2+ = 1/10, in which the former and latter were corresponded to molar number of Ca2+ in CaHAp added and that of Ti4+ in the solution, Ti4+ was completely immobilized within 30 mim. Similar immobilization behaviors were also observed in the immobilization of aqueous Zr4+ with CaHAp. It was pointed out that the surface of CaHAp treated with Ti4+ and Zr4+ was covered with Ca2+ species since re-precipitation of Ca2+ species occurred after the immobilization of those tetravalent cations.