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Molecular structures of citrate and tricarballylate adsorbed on α-FeOOH particles in aqueous suspensions
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.
2009 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, no 18, 10639-10647 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the adsorption of citric (2-hydroxypropane-1,2,3-tricarboxylic acid) and tricarballylic (propane-1,2,3-tricarboxylic acid) acids onto α-FeOOH (goethite) in aqueous suspensions was studied as a function of pH and total ligand concentration in 0.1 M NaCl at 25.0 °C, and the molecular structures of the surface complexes formed were analyzed by means of ATR-FTIR spectroscopy. The adsorption experiments were carried out as a series of batch experiments, and a newly developed simultaneous infrared and potentiometric titration technique was used to collect in situ infrared spectra with high signal-to-noise ratios. The high quality of the infrared spectra allowed analysis by means of two-dimensional correlation spectroscopy formalism that aided the resolution of pH-dependent spectral features. This has enabled the detection of two previously unidentified citrate-goethite surface complexes: one protonated species at low pH, and one inner sphere complex prevailing at high pH and coordinated via a combination of hydroxyl and carboxylate groups. In addition, an inner sphere complex involving only carboxylate coordination predominating at low pH and an outer sphere complex existing in the circumneutral pH region were identified. The behavior of tricarballylate parallels that of citrate, except no inner sphere surface complex is formed at high pH values, which is in accordance with the lack of an α-hydroxyl group. The comparison between citrate and tricarballylate reinforces previous observations showing that inner sphere surface complexes of pure carboxylates at water-iron oxide interfaces are suppressed at high pH values, where outer sphere species are relatively more predominant. It also shows that significant amounts of inner sphere surface complexes of carboxylates only seem to form in the basic pH region when the ligands contain complementary functional groups, such as the hydroxyl or amine groups.

Place, publisher, year, edition, pages
American Chemical Society , 2009. Vol. 25, no 18, 10639-10647 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:umu:diva-25718DOI: 10.1021/la900852pOAI: oai:DiVA.org:umu-25718DiVA: diva2:233401
Available from: 2009-09-01 Created: 2009-09-01 Last updated: 2017-12-13Bibliographically 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

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