Abstract
The adsorption of fluoride ions onto hydroxyapatite powder from aqueous solutions is studied as a function of the concentration of fluoride ions the amount of powder, in well-equilibrated congruent solutions and in the absence of any dissolution at neutral pH. The exchange kinetics 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 fluoride occurs together with the adsorption of H 2PO − 4 and H + ions. For higher fluoride concentrations, once the solubility product of CaF 2 is well exceeded, this phase is deposited. When a further increase in the fluoride ion concentration occurs, the solutions remain supersaturated with respect to CaF 2 and only fluoride ions are adsorbed. The observed adsorption of protons and H 2PO − 4 ions at the interface excludes the formation of fluorapatite or fluorinated apatite but it may lead to the formation of brushite.
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