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 degrees C containing approximately 6% carbonate was fluoride-treated (F adsorption from neutral aqueous solutions) to yield a approximately 1000 ppm F CAP and a approximately 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 approximately 0.10 ppb to approximately 0.30 ppb at 50% dissolved for the two CAP preparations. The approximately 3300 ppm F CAP yielded a lower solubility than the approximately 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 approximately 1000 ppm fluoride compared with the approximately 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.