Uptake of highly concentrated fluoride ions by hydroxyapatite.

Uptake of fluoride ions by hydroxyapatite (HAP) has been investigated. The adsorption characteristics of HAP in sodium fluoride and hydrofluoric acid differ remarkably from one another. At low fluoride concentration, the uptake of fluoride ions takes place through ion-exchange between OH- ions in the HAP particles and fluoride ions in solution. The extraordinarily high uptake of fluoride ions was observed in the wide concentration range of hydrofluoric acid where an uptake minimum was found at about 100 ppm of fluoride. The degree of crystallisation of HAP does not affect the uptake of hydrofluoric acid at higher concentration. A consideration of ion-exchange capacity suggests the second uptake mechanism at high hydrofluoric acid concentration. X-ray diffraction patterns of HAP treated with hydrofluoric acid show a new prominent peak at d=3.14 Å and differ from those of HAP, fluoroapatite, calcium fluorophosphate, and calcium fluoride.