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Phase Transformations, Ion-Exchange, Adsorption, and Dissolution Processes in Aquatic Fluorapatite Systems
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
2009 (English)In: Langmuir, Vol. 25, no 4, 2355-2362 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Chemical Society , 2009. Vol. 25, no 4, 2355-2362 p.
URN: urn:nbn:se:umu:diva-11581DOI: doi:10.1021/la803137uOAI: diva2:151252
Available from: 2009-01-16 Created: 2009-01-16 Last updated: 2010-01-21Bibliographically approved
In thesis
1. Solubility and Surface Complexation Studies of Apatites
Open this publication in new window or tab >>Solubility and Surface Complexation Studies of Apatites
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Apatites are a diverse class of phosphate minerals that are important in a great variety of natural and industrial processes. They are, for example, used as raw material in fertiliser production and in the remediation of metal-contaminated soils. Hydroxyapatite Ca5(PO4)3OH, (HAP) and fluorapatite Ca5(PO4)3F, (FAP) are similar to the biological apatite that is the main constituent of mammalian bone and teeth, and they are therefore promising materials for artificial bone and tooth implants.

This thesis is a summary of four papers with focus on dissolution and surface complexation reactions of HAP and FAP in the absence and presence of both organic ligands and the natural and commonly occurring iron oxide goethite (α-FeOOH).

The dissolution and surface complexation of HAP and FAP was investigated with a combination of different techniques. Potentiometric acid/base titrations and batch experiments were combined with X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectroscopy to generate dissolution and surface complexation models for both apatites. The results from these studies showed that both apatites form surface layers that are different from their bulk compositions when equilibrated in aqueous solutions. The modeling efforts predicted speciation of these surfaces as well as the concentration of the dissolution products in the solution.

The interaction between organic ligands and the apatite surfaces was also investigated and the results from this study show that the organic ligands form outer-sphere complexes on the apatite surfaces over a large pH interval, and that this adsorption enhances the dissolution of apatites.

The presence of goethite also enhances the dissolution of FAP as it acts as a sink for the phosphate released from FAP. Phase transformation in this system was detected using ATR-FTIR as the phosphate adsorbed to the goethite surface precipitates as FePO4 (s) after approximately 15 days of reaction time. This changes the speciation, and possibly also the bioavailability of phosphate in this two-mineral system.

Place, publisher, year, edition, pages
Umeå: Kemi, 2007. 56 p.
apatite, dissolution, calcium, phosphate, fluoride, goethite, surface complexation, speciation, outer-sphere complexation, FTIR, XPS, potentiometry
National Category
Other Basic Medicine
urn:nbn:se:umu:diva-1408 (URN)978-91-7264-425-0 (ISBN)
Public defence
2007-11-16, KB3B1, KBC, Umeå Universitet, Umeå, 10:00 (English)
Available from: 2007-10-29 Created: 2007-10-29 Last updated: 2010-01-21Bibliographically approved

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