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Lookup NU author(s): Dr Anke Neumann
Despite substantial experimental evidence for Fe(II)-Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. Here we used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption, and magnetic circular dichroism (XAS/XMCD) spectroscopies supported by density functional theory calculations to investigate whether Fe(II)-goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen. Corresponding XMCD shows that this non-stoichiometric surface displays uncompensated octahedral Fe3+ that is weakly ferrimagnetic. This non-stoichiometric goethite undergoes facile Fe(II)-Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite at 150 oC, however, imparts bulk stoichiometry and antiferromagnetism at the surface. Hydrothermal treatment decreases the amount of Fe(II) oxidation, and changes the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. We propose that Fe vacancies comprise the defects that enable electron transfer by providing sites into which Fe(II) can strongly bind and be oxidized by the lattice, depositing Fe(III) that propagates the goethite structure. Our findings suggest that surface defects play a commanding role in Fe(II)-goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction, which also likely underlies Fe(II)-catalyzed recrystallization and trace element release and incorporation, will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.
Author(s): Notini L, Latta DE, Neumann A, Pearce CI, Sassi M, N'Diaye AT, Rosso KM, Scherer MM
Publication type: Article
Publication status: Published
Journal: Environmental Science & Technology
Year: 2018
Volume: 52
Issue: 5
Pages: 2751–2759
Print publication date: 06/03/2018
Online publication date: 06/02/2018
Acceptance date: 06/02/2018
Date deposited: 17/03/2018
ISSN (print): 0013-936X
ISSN (electronic): 1520-5851
Publisher: American Chemical Society
URL: https://doi.org/10.1021/acs.est.7b05772
DOI: 10.1021/acs.est.7b05772
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