Mechanism of fluoride uptake by hydroxyapatite

Abstract

The mechanism of uptake of fluoride by synthetic hydroxyapatite was investigated in pre-equilibrated solution-hydroxyapatite systems containing initial concentrations of 3 or 10 ppm F in solution and trace amounts of 45Ca or 32P. Uptake occurred by precipitation of fluorapatite, by recrystallization of hydroxyapatite to fluorapatite, and by surface adsorption or exchange with surface hydroxyl. Precipitation from solution and adsorption or exchange occurred rapidly. Recrystallization was not apparent until more than 1 hr after addition of fluoride. At p H 7, recrystallization and surface adsorption or exchange were the primary mechanisms. Decreasing pH increased the magnitude of both precipitation and recrystallization. With high surface area apatites, adsorption or exchange was the major mechanism for fluoride uptake at neutral pH although recrystallization was also increased by increased surface area. In cases where fluoride was rapidly depleted from solution (low fluoride, low pH, high surface area), fluoride initially acquired by adsorption or exchange was re-utilized in the recrystallization process.