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

• Published in: Langmuir Vol. 25; no. 4; pp. 2355 - 2362
• Main Authors: Bengtsson, Åsa, Shchukarev, Andrei, Persson, Per, Sjöberg, Staffan
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17.02.2009
• Subjects: Adsorption; Apatites - chemistry; Biological Interfaces: Biocolloids, Biomolecular and Biomimetic Materials; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Hydrogen-Ion Concentration; Ion Exchange; Spectrum Analysis; Surface physical chemistry; Surface Properties; Thermodynamics; Titrimetry; Surface reaction; Calcium; X ray; Exchange reaction; Particle; Phase transformation; Halogenapatite; Proton; pH; Electrokinetic potential; Photoelectron spectrometry; Ion exchange; Release; Surface complex; Surface charge; Composition; Solubility product; Surface layer; Prediction; Ions; Suspension; Fluorides; Dissolution; Infrared spectrometry; Adsorption; Dialysis; Models; Surface area
• ISSN: 0743-7463; 1520-5827; 1520-5827
• DOI: 10.1021/la803137u

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 (∼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: ≡CaOH2 + + F− ⇋ ≡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.