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

• Published in: Environmental science & technology Vol. 52; no. 5; pp. 2751 - 2759
• Main Authors: Notini, Luiza, Latta, Drew E, Neumann, Anke, Pearce, Carolyn I, Sassi, Michel, N’Diaye, Alpha T, Rosso, Kevin M, Scherer, Michelle M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.03.2018; American Chemical Society (ACS)
• Subjects: absorption; Circular dichroism; circular dichroism spectroscopy; Computational chemistry; Computer applications; Defects; Dichroism; Electron transfer; Electrons; Environmental Molecular Sciences Laboratory; environmental science; Goethite; hot water treatment; hydrolysis; Hydrothermal treatment; Iron; Iron 57; Iron deficiency; Low temperature; magnetism; Mossbauer spectroscopy; nutrient deficiencies; Oxidation; oxygen; Regeneration; seasonal variation; Seasonal variations; Surface defects; temperature; X-radiation; X-ray absorption spectroscopy
• ISSN: 0013-936X; 1520-5851; 1520-5851
• DOI: 10.1021/acs.est.7b05772

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. We used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies 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, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe­(II)–Fe­(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe­(II) oxidation, and change 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. 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 will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.