Phase Transformations, Ion-Exchange, Adsorption, and Dissolution Processes in Aquatic Fluorapatite Systems

A synthetic fluorapatite was prepared that undergoes a phase transformation generated during a dialysis step. A surface layer with the composition Ca9(HPO4)2(PO4)4F2 is formed, which is suggested to form as one calcium atom is replaced by two protons. A surface complexation model, based upon XPS measurements, potentiometric titration data, batch experiments, and zeta-potential measurements was presented. The CaOH and OPO3H2 sites were assumed to have similar protolytic properties as in a corresponding nonstoichiometric HAP (Ca8.4(HPO4)1.6(PO4)4.4(OH)0.4) system. Besides a determination of the solubility product of Ca9(HPO4)2(PO4)4F2, two additional surface complexation reactions were introduced; one that accounts for a F/OH ion exchange reaction, resulting in the release of quite high fluoride concentrations (approximately 1 mM) that turned out to be dependent on the surface area of the particles. Furthermore, to explain the lowering of pHiep from around 8 in nonstoichiometric HAP suspensions to about 5.7 in FAP suspensions, a reaction that lowers the surface charge due to the readsorption of fluoride ions to the positively charged Ca sites was introduced: triple bond CaOH2++F-<-->triple bond CaF+H2O. The resulting model also agrees with predictions based upon XPS and ATR-FTIR observations claiming the formation of CaF2(s) in the most acidic pH range.