Unveiling the effects of ion substitutions and carbonation on acid/basic surface features of hydroxyapatites

Herein, hydroxyapatite and carbonate hydroxyapatite with calcium ions partially substituted by Mg2+ and Sr2+ ions were prepared by a wet-precipitation method and then calcined at different temperatures (from 300◦ to 600 ◦C). The effect of ion substitution and carbonation of hydroxyapatite on the surface properties of the obtained samples was studied as a function of thermal treatment, with particular focus on surface acidity and basicity, using a multi-technique approach. Adsorption microcalorimetric-volumetric gas–solid measurements with ammonia and sulfur dioxide probes, liquid–solid acid-base titrations with phenylethylamine and benzoic acid in cyclohexane, infrared spectroscopy, thermogravimetric analysis, N2 adsorption–desorption isotherms were complementarily used. All the samples studied, both with and without carbonation, exhibited the characteristic amphoteric behavior of hydroxyapatite materials, with a slight predominance of basic sites (B) over acidic ones (A), as reflected by an A/B ratio of approximately 0.77. Microcalorimetric measurements further confirmed the amphoteric nature of the samples, showing that although acidic sites were more numerous, the basic sites exhibited greater strength compared to the acidic ones. The results show that multiple ion doping does not disrupt the typical amphoteric character of hydroxyapatites and this supports the applicability of apatites derived from food industry bio-waste.