Effects of anion substitution on the acid properties of hydroxyapatite

The influence of the anionic substitution and the thermal treatment on the acid/base properties of hydroxyapatite was studied by temperature-programmed desorption of ammonia (TPD-NH3). Stoichiometric hydroxyapatite, A-type and B-type carboapatites have been synthesized. Substitution of OH− by the CO32− (Carbo-A) modifies the acid strength distribution, increases the overall acidity and acid site density (μmol/m2). Substitution of the PO43− (B-site) by the CO32− did not lead to noticeable changes on the adsorption sites. These results suggest that the Ca2+ ions are the most significant sites for ammonia adsorption.