SURFACE-REACTIONS ON HYDROXYAPATITE IN THE PRESENCE OF FLUORIDE IONS .1. SATURATED AND CONGRUENT CONDITIONS

The adsorption of fluoride ions onto hydroxyapatite powder from aqueou s solutions is studied as a function of the concentration of fluoride ions and the amount of powder, in well-equilibrated congruent solution s and in the absence of any dissolution at neutral pH. The exchange ki netics of fluoride, calcium and protons are continuously followed with use of ion-selective electrodes. Neither the desorption of phosphate nor hydroxyl ions is observed during the fluoride adsorption process. For low fluoride ion concentrations, Langmuir-type adsorption of fluor ide occurs together with the adsorption of H2PO4- and H+ ions. For hig her fluoride concentrations, once the solubility product of CaF2 is we ll exceeded, this phase is deposited. When a further increase in the f luoride ion concentration occurs, the solutions remain supersaturated with respect to CaF2 and only fluoride ions are adsorbed. The observed adsorption of protons and H2PO4- ions at the interface excludes the f ormation of fluorapatite or fluorinated apatite but it may lead to the formation of brushite.