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Dissolution, adsorption and phase transformation in the fluorapatite–goethite system
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
Umeå University, Faculty of Science and Technology, Chemistry.
2007 (English)In: Applied Geochemistry, ISSN 0883-2927, 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. Vol. 22, no 9, 2016-2028 p.
Identifiers
URN: urn:nbn:se:umu:diva-16233DOI: doi:10.1016/j.apgeochem.2007.05.001OAI: oai:DiVA.org:umu-16233DiVA: diva2:155906
Available from: 2007-09-03 Created: 2007-09-03 Last updated: 2010-01-21Bibliographically approved
In thesis
1. Aqueous surface chemistry of Goethite: adsorption and desorption reactions involving phosphate and carboxylic acids
Open this publication in new window or tab >>Aqueous surface chemistry of Goethite: adsorption and desorption reactions involving phosphate and carboxylic acids
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Phosphorus is an essential element for all known forms of life. In the form of phosphate, this nutrient is retained in soils and sediments by sorption on mineral particles, clays and other soil constituents. In addition, phosphate precipitates with metal ions to form a range of phosphate containing minerals, and only a minor part of phosphate is found dissolved in soil solution. One way of releasing sorbed phosphate is through ligand exchange reactions with for example carboxylic acids. This thesis summarizes five papers and focuses on the interactions of phosphate and carboxylates at the water-goethite interface. Quantitative adsorption data and spectroscopic evaluations of the surface complexation were used collectively to gain a better understanding of these processes. In agreement with previous studies, it was found that the number of carboxylic groups is important to the competitive ability of the organic acids towards phosphate. However, it was also shown that the positions of the functional groups are highly relevant to this ability. Furthermore, partially protonated species were – because of hydrogen bond interactions - shown to be more competitive than fully deprotonated equivalents. Another central finding in this work is that competitive interactions do not necessarily involve ligand-exchange reactions between inner sphere surface complexes.  To study the lability of the complexes, desorption experiments were performed. Among the benzenecarboxylates, the order of increased lability matched that of the decreased ability to compete with phosphate for surface sites on the goethite. Also shown in this thesis is the ability of goethite to increase the dissolution of fluorapatite mainly through the high affinity of phosphate ions for the goethite surface.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2009. 39 p.
Identifiers
urn:nbn:se:umu:diva-25727 (URN)978-91-7264-774-9 (ISBN)
Distributor:
Kemi, 90187, Umeå
Public defence
2009-09-25, KB3B1, KBC, Umeå University, 13:00 (English)
Opponent
Supervisors
Available from: 2009-09-03 Created: 2009-09-01 Last updated: 2009-09-03Bibliographically approved
2. 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.
Keyword
apatite, dissolution, calcium, phosphate, fluoride, goethite, surface complexation, speciation, outer-sphere complexation, FTIR, XPS, potentiometry
National Category
Other Basic Medicine
Identifiers
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)
Opponent
Supervisors
Available from: 2007-10-29 Created: 2007-10-29 Last updated: 2010-01-21Bibliographically approved

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Bengtsson, ÅsaLindegren, MalinSjöberg, StaffanPersson, Per

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