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Highly Mobile Iron Pool from a Dissolution-Readsorption Process
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.
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. Vol. 24, no 14, 7054-7057 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-10119DOI: doi:10.1021/la800785uPubMedID: 18549293OAI: oai:DiVA.org:umu-10119DiVA: diva2:149790
Note
Web Release Date June 13, 2008Available from: 2008-08-05 Created: 2008-08-05 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Molecular perspectives on goethite dissolution in the presence of oxalate and desferrioxamine-B
Open this publication in new window or tab >>Molecular perspectives on goethite dissolution in the presence of oxalate and desferrioxamine-B
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:nbn:se:umu:diva-46605 (URN)978-91-7459-208-5 (ISBN)
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

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