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The Role of Defects in Fe(II)-Goethite Electron Transfer

Lookup NU author(s): Dr Anke Neumann

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Abstract

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.


Publication metadata

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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