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Rethinking Arsenate Coordination at the Surface of Goethite

Loring, John S.; Sandstrom, Malin H.; Norén, Katarina LU and Persson, Per LU (2009) In Chemistry: A European Journal 15(20). p.5063-5072
Abstract
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... (More)
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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
surface chemistry, goethite, geochemistry, arsenate, coordination modes
in
Chemistry: A European Journal
volume
15
issue
20
pages
5063 - 5072
publisher
John Wiley & Sons
external identifiers
  • wos:000266292200014
  • scopus:66149091115
ISSN
1521-3765
DOI
10.1002/chem.200900284
language
English
LU publication?
yes
id
685b74a7-af4d-453f-9e26-a07e0590e30e (old id 1425334)
date added to LUP
2009-07-02 16:20:17
date last changed
2017-09-24 03:55:03
@article{685b74a7-af4d-453f-9e26-a07e0590e30e,
  abstract     = {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.},
  author       = {Loring, John S. and Sandstrom, Malin H. and Norén, Katarina and Persson, Per},
  issn         = {1521-3765},
  keyword      = {surface chemistry,goethite,geochemistry,arsenate,coordination modes},
  language     = {eng},
  number       = {20},
  pages        = {5063--5072},
  publisher    = {John Wiley & Sons},
  series       = {Chemistry: A European Journal},
  title        = {Rethinking Arsenate Coordination at the Surface of Goethite},
  url          = {http://dx.doi.org/10.1002/chem.200900284},
  volume       = {15},
  year         = {2009},
}