THE ROLE OF DEFECTS IN FE(II) - GOETHITE ELECTRON TRANSFER

Despite substantial experimental evidence for Fe(II)-Fe(III) oxide electron transfer (ET), computational chemical calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. Here we used a combination of isotope specific ⁵⁷Fe Mössbauer spectroscopy and synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)-goethite electron transfer is influenced by defects and to determine the nature of the defects in control. Specifically, Fe L-edge and O K-edge XAS confirms that the upper 5 nm of goethite synthesized by Fe(III) hydrolysis at 70^oC is iron deficient relative to oxygen, and corresponding XMCD shows that this non-stoichiometric surface displays uncompensated octahedral Fe³⁺ that is weakly ferrimagnetic. This goethite undergoes facile Fe(II)-Fe(III) oxide ET consistent with experimental precedent. Hydrothermal treatment of this goethite at 150^oC, however, imparts bulk stoichiometry and antiferromagnetism at the surface, decreasing Fe(II) oxidation. When hydrothermally treated goethite was ground, surface defect characteristics as well as ET were largely restored. We propose that Fe vacancies enable ET by providing sites into which Fe(II) can strongly bind and transfer electrons to lattice Fe(III). Our findings suggest that surface defects play a commanding role in Fe(II)-goethite redox interaction, as predicted by computational chemical modeling. Moreover it suggests that in the environment the extent of this interaction, which also underlies Fe(II)-catalyzed recrystallization and trace element release and incorporation, will vary depending on diagenetic history, local redox conditions, and be subject to regeneration via seasonal fluctuations.