Rethinking arsenate coordination at the surface of goethite
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
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.
arsenate, coordination modes, geochemistry, goethite, surface chemistry
IdentifiersURN: urn:nbn:se:umu:diva-21135DOI: 10.1002/chem.200900284PubMedID: 19334024OAI: oai:DiVA.org:umu-21135DiVA: diva2:210645