Effects of carbonate inclusion on fluoride ion removal by hydroxyapatite: A discussion from the viewpoint of hydroxyapatite dissolution

Abstract

Hydroxyapatite (HA) can be used for removing F-. Materials containing HA for removing F- can be obtained from biological wastes, and these HA might contain carbonate ions. The dissolution behavior of HA should affect the F- removal as fluorapatite (FA) formation at HA surface by the dissolution of HA and subsequent precipitation of FA results in the removal of F-. In this study, we examined the relationship between the dissolution and F- removal rates of carbonated HA (CHA). CHA samples with different carbonate contents were prepared by a hydrothermal process. The dissolution and F- removal rates were investigated by immersing the samples in pH 5 and 7 buffer solutions in the absence and presence of F-, respectively. The dissolution and F- removal rates were increased with an increase of carbonate-ion content in the samples. Dissolution and F- removal rates at pH 5 were larger than that at pH 7. The pseudo-second order kinetic model provides a good fit for the F- removal process. An increase in the kinetic constant of dissolution decreases the kinetic constant of the F- removal process for samples with high carbonate contents at pH 5.