Effect of Fluoride Pretreatment on the Solubility of Synthetic Carbonated Apatite

The purpose of this research was to address the following question: How is the solubility of fluoride-pretreated carbonated apatite (CAP) in aqueous acidic media related to the equilibrium solution fluoride and/or the CAP adsorbed fluoride levels? A CAP sample prepared by a precipitation method at 70°C containing approximately 6% carbonate was fluoride-treated (F adsorption from neutral aqueous solutions) to yield a ~1000 ppm F CAP and a ~3300 ppm F CAP. Metastable equilibrium solubility distributions were determined in acetate buffers at pH 5.0. Solution fluoride, calcium, phosphate, and pH were determined from the equilibrated solutions. The equilibrium solution fluoride levels were extremely low, e.g., as low as ~0.10 ppb to ~0.30 ppb at 50% dissolved for the two CAP preparations. The ~3300 ppm F CAP yielded a lower solubility than the ~1000 ppm F CAP (shift in the mean pKHAP value of 1.5–2 units). This can be attributed to the lower solution F? for the sample containing ~1000 ppm fluoride compared with the ~3300 ppm fluoride-containing CAP. These important findings suggest that a fluoride treatment simply may provide an adsorption fluoride depot for subsequent release, providing a solution fluoride effect upon the CAP solubility and not necessarily any intrinsic alteration of the mineral solubility.