SURFACE-REACTIONS OF HYDROXYAPATITE IN THE PRESENCE OF FLUORIDE IONS .2. EFFECTS OF CALCIUM AND PHOSPHATE IN SATURATED SOLUTIONS

The adsorption of fluoride ions onto hydroxyapatite powder from aqueous solutions is studied as a function of the concentrations of fluoride, calcium and phosphate ions in well-equilibrated conditions. The fluoride, calcium and proton adsorption kinetics are followed continuously with ion-selective electrodes. Under all conditions, the addition of fluoride induces an increase in the pH due to the adsorption of H2PO4- ions. This may favour the interfacial formation of brushite. Fluoride isotherms are characterized by a Langmuir-type plateau in which fluoride, calcium and phosphate ions are adsorbed without coupling and with no evidence of solid phase formation. At higher fluoride concentration, fluoride is adsorbed, together with precipitation of a definite amount of CaF2. In the presence of increasing amounts of calcium, CaF2 precipitation occurs at a constant fluoride concentration although the solutions are saturated well before this happens. Conversely, the presence of phosphate ions promotes the interfacial formation of fluoridated apatite.