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Quantitative adsorption and local structures of gallium(III) at the water-alpha-FeOOH interface
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2006 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 22, no 5, 2096-2104 p.Article in journal (Refereed) Published
Abstract [en]

The adsorption of Ga(III) at the water−α-FeOOH (goethite) interface has been investigated by means of quantitative adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. Under the conditions studied, pH range 3−11 and surface coverages of 0.9−3.2 μmol/m2, Ga(III) was found to adsorb strongly to α-FeOOH, and the surface species were more resistant toward hydrolysis and formation of soluble Ga(OH)4- than either solid gallium hydroxides or soluble polynuclear complexes. The EXAFS measurements revealed the presence of octahedral Ga(III) complexes at the water−α-FeOOH interface, with practically no structural variations as a function of pH or total gallium concentration. Analysis of the first coordination shell required an anharmonic model indicating a distorted geometry of the GaO6 octahedra, with mean Ga−O distances at 1.96−1.98 Å. A method based on the continuous Cauchy wavelet transforms (CCWT) was used to identify backscattering atoms in the higher coordination shells. This analysis indicated predominately Fe backscattering, and the quantitative data fitting resulted in three Ga−Fe paths at 3.05, 3.2, and 3.55 Å, which correspond to two edge-sharing and one corner-sharing linkage, respectively. The collective results from EXAFS spectroscopy showed that Ga(III) adsorbs to Fe equivalent sites at the surface α-FeOOH as an extension of the rows of Fe octahedra in the bulk structure. This interpretation was further corroborated by a Ga−Fe−Fe multiple scattering path at 6.13 Å. The quantitative adsorption and proton data were modeled using a surface complexation formalism based on a 1 pKa constant capacitance model. In agreement with the EXAFS results, the model obtained included one predominating surface complex with the stoichiometry FeOGa(OH)2-0.5 and the stability constant log β(intr.) = −2.55 ± 0.04 (FeOH-0.5 + Ga3+ + 2H2O ↔ FeOGa(OH)2-0.5 + 3H+).

Place, publisher, year, edition, pages
Washington: American Chemical Society , 2006. Vol. 22, no 5, 2096-2104 p.
URN: urn:nbn:se:umu:diva-2522DOI: 10.1021/la052555jOAI: diva2:140690
Available from: 2007-09-06 Created: 2007-09-06 Last updated: 2011-03-17Bibliographically approved
In thesis
1. Coordination Chemistry of Monocarboxylate and Aminocarboxylate Complexes at the Water/Goethite Interface
Open this publication in new window or tab >>Coordination Chemistry of Monocarboxylate and Aminocarboxylate Complexes at the Water/Goethite Interface
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a summary of five papers with focus on adsorption processes of various monocarboxylates and aminocarboxylates at the water/goethite interface. Interaction of organic acids at the water/mineral interfaces are of importance in biogeochemical processes, since such processes have potential to alter mobility and bioavailability of the acids and metal ions.

In order to determine the coordination chemistry of acetate, benzoate, cyclohexanecarboxylate, sarcosine, MIDA (methyliminediacetic acid), EDDA (ethylenediamine-N,N’-diacetic acid) and EDTA (ethylenediamine-N,N’-tetraacetic acid) upon adsorption to the goethite (alpha-FeOOH) surface, a combination of quantitative measurements with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was utilized.

Over the pH range studied here (pH 3- 9) all ligands, except for sarcosine, have been found to form surface complexes with goethite. In general, theses were characterized as outer sphere surface complexes i.e. with no direct interaction with surface Fe(III) metal ions. Furthermore, two types of different outer-sphere complexes were identified, the solvent-surface hydration-separated ion pair, and hydration-shared ion pair. For the monocarboxylate surface complexes distinction between these two could be made. At high pH values the solvent-surface hydration-separated ion pair was the predominating complex, while at low pH the surface complex is stabilized through the formation of strong hydrogen bonds with the goethite surface. However, it was not possible to clearly separate between the two outer-sphere complexes for coordination of the aminocarboxylates with the surface of goethite. Additionally, EDDA also formed an inner-sphere surface complex at high pH values. The EDDA molecule was suggested to coordinate to the surface by forming a five membered ring with an iron at the goethite surface, through the amine and carboxylate groups.

Contrary to the other ligands studied, EDTA significantly induced dissolution of goethite. Some of the dissolved iron, in the form of the highly stable FeEDTA- solution complex, was indicated to re-adsorb to the mineral surface as a ternary complex. Similar ternary surface complexes were also found in the Ga(III)EDTA/goethite system, and quantitative and spectroscopic studies on adsorption of Ga(III) in presence and absence of EDTA showed that EDTA considerably effects speciation of gallium at goethite surface.

The collective results in this thesis show that the affinity of these ligands for the surface of goethite is primarily governed by their chemical composition and structure, and especially important are the types, numbers and relative position of functional groups within the molecular structure.

Place, publisher, year, edition, pages
Umeå: Kemi, 2007. 52 p.
Acetate, benzoate, adsorption, surface complex, ternary surface complexes, ATR-FTIR spectroscopy, EXAFS spectroscopy., cyclohexanecarboxylate, sarcosine, MIDA, EDDA, EDTA, gallium(III), goethite, mineral surface
National Category
Other Basic Medicine
urn:nbn:se:umu:diva-1337 (URN)978-91-7264-320-8 (ISBN)
Public defence
2007-09-28, KB3A9, KBC, Umeå Universitet, Umeå, 13:00
Available from: 2007-09-06 Created: 2007-09-06 Last updated: 2012-06-29Bibliographically approved

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