Effect of thermal treatment on carbonated hydroxyapatite: Morphology, composition, crystal characteristics and solubility

Thermal treatment is one of the common methods used in the recovery of biological apatite from bones and teeth. However, the effect of this on its physicochemical and biological properties has not yet been given full consideration. As a simplified research model with characteristics assumed to be similar to biological apatite, carbonated hydroxyapatite was used to study the effect of thermal treatment on its physicochemical properties. Carbonated hydroxyapatite was prepared and heated at 600–1000 °C. The morphology, chemical composition, crystal characteristics, functional groups and solubility of the as-prepared and heated samples were determined. Crystal growth and aggregation were observed for materials heated at 800 °C and above. With increase of heating temperature, carbonate and water content decreased, while the crystallinity increased. The solubility of as-prepared material and those heated at 600 and 700 °C was higher than the reference curve for hydroxyapatite, while that of the rest overlapped the latter. Such changes indicate that the preparation approaches involving high temperature may not be appropriate for the recovery of ‘biological’ apatite; and that the importance of carbonate to the apparent solubility of calcium phosphate must be taken into account in all such work.