ROLE OF ADSORBED FLUORIDE IONS ON THE DISSOLUTION OF SYNTHETIC HYDROXYAPATITE

The dissolution kinetics at 37 degrees C and for constant pH values of 5.0 and 6.0 are studied for calcium hydroxyapatite (HAp) equilibrated at pH 7.0 in the presence of 1 or 10 ppm of F-. Experiments are realized using an automatic setup which allows a continuous record of proton uptake and calcium or fluoride activities in solution using specific electrodes. It is shown that the presence of fluoride ions affects the dissolution kinetics in two ways. Their presence at the HAp interface reduces the final solubility of HAp, but accelerates the initial dissolution rates. Both observations are fully interpreted using the HAp dissolution model recently developed. According to this model, apatite dissolution is auto-inhibited by the adsorption at the solid interface of calcium ions forming a cationic semi-permeable layer. Adsorbed fluoride ions interact with this natural cationic layer, decrease its ionic capacity and accelerate the dissolution process.