Abstract
The F--uptake behavior of octacalcium phosphate (OCP) was studied at a solid/solution ratio of 1 g/50 ml, initial fluoride concentrations of 200 and 1000 ppmF- and temperatures of 25-40°C, in comparison with that of other calcium salts.
At 200 ppmF-, OCP lowered the [F-] below 0.02 ppm at 25-40°C. Fluoride ions precipitated as fluorapatite (FAp). Similar removals of F- were obtained also by the use of monetite at 25-40°C and brushite at 40°C. An inactivation of brushite for F--uptake at 25-30°C suggested the F--uptake process brushite→monetite→ FAp. At 1000 ppmF- and 40°C, OCP could not lower the [F-] sufficiently since the total amount of F- was in excess of that needed for the reaction OCP→FAp, while monetite and brushite lowered the [F-] until around [F-] of the solubility of CaF2 as the successive reaction FAp→CaF2 proceeded. The rate-determining step for the reaction OCP→FAp was a diffusion process through the FAp dense product layer, and for the monetite→FAp it was a chemical reaction at the interface between monetite and FAp porous product layer. Activation energies were 12 kcal/mol for the former and 15 kcal/mol for the latter.
