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

Summary, in English

A synthetic fluorapatite was prepared that undergoes a phase transformation generated during a dialysis step. A surface layer with the composition Ca(9)(HPO(4))(2)(PO(4))(4)F(2) 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 OPO(3)H(2) Sites were assumed to have similar protolytic properties as in a corresponding nonstoichiometric HAP (Ca(8.4)(HPO(4))(1.6)(PO(4))(4.4)(OH)(0.4)) system. Besides a determination of the solubility product of Ca(9)(HPO(4))(2)(PO(4))(4)F(2), 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 (similar to 1 mM) that turned out to be dependent on the surface area of the particles. Furthermore, to explain the lowering of pH(iep) 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: CaOH(2)(+) + F(-) -> CaF + H(2)O. The resulting model also agrees with predictions based upon XPS and ATR-FTIR observations claiming the formation of CaF(2)(S) in the most acidic pH range.