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Rethinking arsenate coordination at the surface of goethite
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: Chemistry - A European Journal (Weinheim an der Bergstrasse, Germany), ISSN 1521-3765, Vol. 15, no 20, 5063-5072 p.Article in journal (Refereed) Published
Abstract [en]

A fundamental precept of geochemistry is that arsenate coordinates at mineral surfaces in a predominately bridging-bidentate fashion. We show that this is incorrect for the model system, arsenate adsorbed at the surface of goethite (-FeOOH), using a combination of XRD, EXAFS, and IR spectroscopic results. We report the crystal structure of pentaamminecobalt(III) arsenate, which consists of monodentate-coordinated metal-arsenato complexes that have CoAs distances of only 3.25 Å. This result implies that metal-arsenic distances are not diagnostic for the coordination mode of arsenate. We show that the K-edge EXAFS spectra of pentaamminecobalt(III) arsenate and arsenate-goethite surface complexes are strikingly similar, which suggests that arsenate could be coordinated at the goethite surface in a monodentate fashion. Refinements of the k3-weighted EXAFS spectra of arsenate adsorbed on goethite results in values of CNAsFe between 0.8-1.1 (±0.7), and there is no evidence that the coordination mode of arsenate changes as a function of pH or arsenate surface coverage. We report IR spectra from the first simultaneous IR and potentiometric titration of arsenate adsorbed on deuterated goethite (-FeOOD) in D2O, and we show for the first time the AsO stretching bands of arsenate-goethite surface complexes. We deduce that arsenate-goethite surface complexes are un-, singly , or doubly protonated, depending on pH, from a principal component analysis of the AsO stretching region and an interpretation of the Type-B OH stretching region. In summary, our cumulative results show that arsenate coordinates at the water-goethite interface in a predominately monodentate fashion. Furthermore, we find no evidence for bridging-bidentate coordination, which is a finding that impacts oxoanion bioavailability and challenges theories of mineral dissolution and surface complexation.

Place, publisher, year, edition, pages
Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA , 2009. Vol. 15, no 20, 5063-5072 p.
Keyword [en]
arsenate, coordination modes, geochemistry, goethite, surface chemistry
URN: urn:nbn:se:umu:diva-21135DOI: 10.1002/chem.200900284PubMedID: 19334024OAI: diva2:210645
Available from: 2009-04-03 Created: 2009-04-03 Last updated: 2012-06-14Bibliographically approved

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Loring, John SSandström, Malin H
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