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Rethinking Arsenate Coordination at the Surface of Goethite

På svenska

Author

  • John S. Loring
  • Malin H. Sandstrom
  • Katarina Norén
  • Per Persson

Summary, in English

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 (alpha-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 Co-As distances of only 3.25 angstrom. 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(ill) 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 k(3)-weighted EXAFS spectra of arsenate adsorbed on goethite results in values of CNAs-Fe 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 (alpha-FeOOD) in D2O, and we show for the first time the As-O 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 As-O 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.

Publishing year

2009

Language

English

Pages

5063-5072

Publication/Series

Chemistry: A European Journal

Volume

15

Issue

20

Document type

Journal article

Publisher

Wiley-Blackwell

Topic

  • Earth and Related Environmental Sciences
  • Natural Sciences
  • Physical Sciences

Keywords

  • surface chemistry
  • goethite
  • geochemistry
  • arsenate
  • coordination modes

Status

Published

ISBN/ISSN/Other

  • ISSN: 1521-3765