Surface structure and composition of calcium hydroxyapatites substituted with Al(III), La(III) and Fe(III) ions

Synthetic calcium hydroxyapatite (CaHap) particles were modified by ionexchange with Al(III), La(III) and Fe(III) by the coprecipitation and immersion methods. The growth of CaHap particles was observed on substituting with these metal ions by coprecipitation. The exchange ratios of Ca(II) with the metal ions revealed that Fe(III) replaces Ca(II) as hydroxo ions and the substitution with La(III) accompanies OH−-deficiency. In the substitution with immersion the exchange ratios for all the metal ions were less than 1.5 of the theoretical ratio, verifying the exchange with metal hydroxo ions. IR spectra of the CaHap particles substituted with Al(III) by both the methods gave rise to several bands due to surface Al-OH besides the surface P-OH bands, whereas the CaHap substituted with La(III) showed no surface La-OH band. The surface Fe-OH was found on the CaHaps substituted by the coprecipitation but not on those substituted by the immersion.