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Molecular perspectives on goethite dissolution in the presence of oxalate and desferrioxamine-B
Umeå University, Faculty of Science and Technology, Department of Chemistry.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Iron, an essential nutrient, is primarily present in soils in the form of iron-bearing minerals characterized with low solubilities. Under iron deficient conditions, some plants and microorganisms exude a mixture of iron-complexing agents, including carboxylates and siderophores, that can cause minerals to dissolve and increase iron solubility. Siderophores are chelating agents with functional groups such as hydroxamate, catecholate, or α-hydroxycarboxylate, that have high selectivity and specificity for Fe(III). This thesis is focused on adsorption/dissolution processes at the surface of a common soil mineral, goethite(α-FeOOH), in the presence of oxalate and a trihydroxamate siderophore, desferrioxamine-B (DFOB) at pH 4 and/or 6 in the absence of visible light. In order to characterize these processes at a molecular level and to understand the reaction mechanisms, a combination of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy and quantitative solution phase measurements were applied.

In the oxalate-goethite system, four surface species were detected: 1) an electrostatically attracted outer-sphere complex, 2) a hydrogen bonded outer-sphere complex, 3) an inner-sphere oxalate coordinated to surface iron and 4) a ternary type A complex formed during a dissolution-readsorption process. Addition of Al(C 2O 4 ) 3 3-or Ga(C 2 O 4 ) 3 3- to a goethite suspension resulted in the formation of an additional surface complex - oxalate coordinated to Al or Ga in a ternary type A complex.

In the DFOB-goethite system, DFOB is subjected to surface-mediated hydrolysis followed by the reduction of Fe(III) as evidenced by the release of acetate and a nitroso-DFOB fragment into the aqueous phase. It is postulated that Fe(II) is not detected in the solution phase due to its adsorption at the surface. At low surface coverage, a small fraction of dissolved FeHDFOB + complex is also likely to form ternary surface complexes and hydrolyze. These observations suggest that DFOB-promoted dissolution of goethite may proceed not only via purely ligand-exchange reactions, but also through reductive pathways.

In the oxalate-DFOB-goethite system, the dissolution rates are greater than the sum of the dissolution rates in the single-ligand systems. Results presented demonstrate that this synergistic effect is due to the formation of the above mentioned ternary oxalate surface complex via dissolution and readsorption. Iron in this ternary complex is more labile than iron in the crystal lattice and thus more readily accessible for other complexing agents, e.g. siderophores.

The results presented in this thesis provide a molecular-level view of ligand-promoted mineral dissolution in the presence of small carboxylates and/or siderophores, which improves our fundamental understanding of the role of surface complexation in mineral dissolution and iron bioavailability.

Place, publisher, year, edition, pages
Umeå: Umeå University, Department of Chemistry , 2011. , 46 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-46605ISBN: 978-91-7459-208-5 (print)OAI: oai:DiVA.org:umu-46605DiVA: diva2:439370
Public defence
2011-09-29, KBC-huset, KB3A9, Umeå universitet, Umeå, 10:00
Opponent
Supervisors
Available from: 2011-09-08 Created: 2011-09-07 Last updated: 2011-09-07Bibliographically approved
List of papers
1. Formation of ternary metal-oxalate surface complexes on alpha-FeOOH particles
Open this publication in new window or tab >>Formation of ternary metal-oxalate surface complexes on alpha-FeOOH particles
(English)Manuscript (preprint) (Other academic)
Keyword
infrared spectroscopy, EXAFS, goethite, dissolution, readsorption
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:umu:diva-46553 (URN)
Available from: 2011-09-06 Created: 2011-09-05 Last updated: 2011-09-07Bibliographically approved
2. Evidence for ligand hydrolysis and Fe(III) reduction in the dissolution of Goethite by desferrioxamine-B
Open this publication in new window or tab >>Evidence for ligand hydrolysis and Fe(III) reduction in the dissolution of Goethite by desferrioxamine-B
2010 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 74, no 23, 6704-6720 p.Article in journal (Refereed) Published
Abstract [en]

Desferrioxamine-B (DFOB) is a bacterial trihydroxamate siderophore and probably the most studied to date. However, the manner in which DFOB adsorbs at mineral surfaces and promotes dissolution is still under discussion. Here we investigated the adsorption and dissolution reactions in the goethite–DFOB system using both in situ infrared spectroscopic and quantitative analytical methods. Experiments were carried out at a total DFOB concentration of 1 μmol/m2, at pH 6, and in the absence of visible light. Our infrared spectroscopic results indicated that the adsorption of DFOB was nearly complete after a 4-h reaction time. In an attempt to determine the coordination mode at the goethite surface, we compared the spectrum of adsorbed DFOB after a 4-h reaction time to the spectra of model aqueous species. However, this approach proved too simplistic in the case of such a complex ligand as DFOB, and we suggest that a more detailed investigation (IR in D2O, EXAFS of adsorbed model complexes) is needed to elucidate the structure of the adsorbed siderophore. Between a 4-hand 4-day reaction time, we observed the growth of carboxylate stretching bands at 1548 and 1404 cm-1, which are indicators of DFOB hydrolysis. Acetate, a product of DFOB hydrolysis at its terminal hydroxamate group, was quantified by ion chromatography. Its rate of formation was linear and nearly the same as the rate of Fe(III) dissolution. The larger hydrolysis product, a hydroxylamine fragment, was not detected by LC–MS. However, a signal due to the oxidized form of this fragment, a nitroso compound, was found to increase linearly with time, which is an indirect indication for Fe(III) reduction. Based on these findings, we propose that DFOB undergoes metal-enhanced hydrolysis at the mineral surface followed by the reduction of surface Fe(III). While Fe(II) was not detected in solution, this is likely because it remains adsorbed at the goethite surface or becomes buried in the goethite crystal by electron conduction. Taking into account the extent and similarity between the rates of hydrolysis and dissolution, we suggest that a reductive mechanism could play an important part in the dissolution of goethite by DFOB. This possibility has not been considered previously in the absence of light and at circumneutral pH.

Place, publisher, year, edition, pages
Elsevier Ltd, 2010
Identifiers
urn:nbn:se:umu:diva-46551 (URN)10.1016/j.gca.2010.08.037 (DOI)
Available from: 2011-09-06 Created: 2011-09-05 Last updated: 2017-12-08Bibliographically approved
3. Surface reactions controlling the decomposition of DFOB in the presence of goethite
Open this publication in new window or tab >>Surface reactions controlling the decomposition of DFOB in the presence of goethite
(English)Manuscript (preprint) (Other academic)
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:umu:diva-46555 (URN)
Available from: 2011-09-06 Created: 2011-09-05 Last updated: 2011-09-07Bibliographically approved
4. Highly Mobile Iron Pool from a Dissolution-Readsorption Process
Open this publication in new window or tab >>Highly Mobile Iron Pool from a Dissolution-Readsorption Process
2008 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 24, no 14, 7054-7057 p.Article in journal (Refereed) Published
Abstract [en]

Oxalate (C2O42−) acts synergistically on the dissolution of goethite (α-FeOOH) in the presence of siderophores that are secreted by plants and microorganisms to sequester iron. We report here the first in situ molecular-scale observations of synergistic ligand-promoted dissolution processes. We show that there are conditions under which oxalate promotes goethite dissolution, but dissolved Fe(III) concentrations do not increase because Fe(III)-oxalate complexes readsorb to the mineral surface. We demonstrate that these readsorbed Fe(III)-oxalate complexes are highly mobile, extremely reactive in the presence of uncomplexed siderophores, and responsible for the synergistic effects on the dissolution of goethite.

Place, publisher, year, edition, pages
American Chemical Society, 2008
National Category
Chemical Sciences
Identifiers
urn:nbn:se:umu:diva-10119 (URN)doi:10.1021/la800785u (DOI)18549293 (PubMedID)
Note
Web Release Date June 13, 2008Available from: 2008-08-05 Created: 2008-08-05 Last updated: 2011-09-12Bibliographically approved

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