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Solubility and Surface Complexation Studies of Apatites
Umeå University, Faculty of Science and Technology, Chemistry.
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.
Keyword [en]
apatite, dissolution, calcium, phosphate, fluoride, goethite, surface complexation, speciation, outer-sphere complexation, FTIR, XPS, potentiometry
National Category
Other Basic Medicine
Identifiers
URN: urn:nbn:se:umu:diva-1408ISBN: 978-91-7264-425-0 (print)OAI: oai:DiVA.org:umu-1408DiVA: diva2:140931
Public defence
2007-11-16, KB3B1, KBC, Umeå Universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2007-10-29 Created: 2007-10-29 Last updated: 2010-01-21Bibliographically approved
List of papers
1. Dissolution, adsorption and phase transformation in the fluorapatite–goethite system
Open this publication in new window or tab >>Dissolution, adsorption and phase transformation in the fluorapatite–goethite system
2007 (English)In: Applied Geochemistry, ISSN 0883-2927, E-ISSN 1872-9134, Vol. 22, no 9, 2016-2028 p.Article in journal (Refereed) Published
Abstract [en]

An aqueous system containing fluorapatite (Ca5(PO4)3F), (FAP) and varying amounts of goethite (α-FeOOH) has been investigated. Batch experiments and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were used to monitor the dissolution products of FAP, as well as the adsorption, and phase transformation of phosphate at the goethite surface over a period of 129 days. The results show that the presence of goethite increases dissolution of FAP, mainly due to the high affinity of phosphate for the goethite surface: Besides monitoring the pH changes associated with this reaction, the concentrations of Ca2+ and fluoride were determined. Furthermore, the amount of phosphate adsorbed was quantified from ATR-FTIR spectra. In addition to adsorbed phosphate, phase transformations of goethite into a Fe phosphate phase (FePO4(s)) are seen in the samples with relatively high phosphate to goethite ratios (excess phosphate to available surface sites) equilibrated for 15–129 days.

An equilibrium model that takes into account (i) FAP dissolution, (ii) solution complexation, (iii) surface complexation of phosphate species onto goethite and (iv) possible phase transformation Ca5(PO4)3F–CaF2 and FeOOH–FePO4 was designed. This model was found to be in very good agreement with experimental observations and could thus be used to give qualitative and quantitative information about goethite promoted dissolution of FAP under other pH conditions than those studied in the present work.

Place, publisher, year, edition, pages
Elsevier, ScienceDirect, 2007
Identifiers
urn:nbn:se:umu:diva-16233 (URN)doi:10.1016/j.apgeochem.2007.05.001 (DOI)
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2017-12-14Bibliographically approved
2. A solubility and surface complexation study of a non-stoichiometric hydroxyapatite
Open this publication in new window or tab >>A solubility and surface complexation study of a non-stoichiometric hydroxyapatite
2009 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 73, no 2, 257-267 p.Article in journal (Refereed) Published
Abstract [en]

The dissolution and surface complexation of a non-stoichiometric hydroxyapatite (Ca8.4(HPO4)1.6(PO4)4.4(OH)0.4), (HAP) was studied in the pH range 3.5 – 10.5, at 25 ºC in 0.1 M Na(Cl). The results from well-equilibrated batch experiments, potentiometric titrations, and zeta-potential measurements were combined with information provided by Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The information from the analyses was used to design an equilibration model that takes in to account dissolution, surface potential, solution and surface complexation, as well as possible phase transformations. The results from the XPS measurements clearly show that the surface of the mineral has a different composition than the bulk and that the Ca/P ratio of the surface layer is 1.4 ± 0.1. This ratio was also found in solution in the batches equilibrated at low pH where the dominating reaction is dissolution. In the batches equilibrated at near neutral pH values, however, the Ca/P ratio in solution attains values as high as 25, which is due to re-adsorption of phosphate ions to the HAP surface. The total concentration of protons as well as the total concentration of dissolved calcium and phosphate in solution were used to calculate a model for the dissolution and surface complexation of HAP. The constant capacitance model was applied in designing the following surface complexation model.

Place, publisher, year, edition, pages
Elsevier, 2009
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-10985 (URN)10.1016/j.gca.2008.09.034 (DOI)
Available from: 2008-12-29 Created: 2008-12-29 Last updated: 2017-12-14Bibliographically approved
3. Solubility and Adsorption Characterisics of Non-Stoichiometric Surface Layers of Hydroxyapatite with some Organic Carboxylic Ligands: An ATR-FTIR Spectroscopic and Thermodynamic Modeling Study
Open this publication in new window or tab >>Solubility and Adsorption Characterisics of Non-Stoichiometric Surface Layers of Hydroxyapatite with some Organic Carboxylic Ligands: An ATR-FTIR Spectroscopic and Thermodynamic Modeling Study
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Identifiers
urn:nbn:se:umu:diva-2688 (URN)
Available from: 2007-10-29 Created: 2007-10-29 Last updated: 2012-05-04Bibliographically approved
4. Phase Transformations, Ion-Exchange, Adsorption, and Dissolution Processes in Aquatic Fluorapatite Systems
Open this publication in new window or tab >>Phase Transformations, Ion-Exchange, Adsorption, and Dissolution Processes in Aquatic Fluorapatite Systems
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
Identifiers
urn:nbn:se:umu:diva-11581 (URN)doi:10.1021/la803137u (DOI)
Available from: 2009-01-16 Created: 2009-01-16 Last updated: 2010-01-21Bibliographically approved

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