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

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. We used a combination of Fe 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.

Oxidation induced strain and defects in magnetite crystals

“…Since there is a continuous solid solution between magnetite and maghemite, removal of Fe(II) from magnetite leads to the formation of off-stoichiometric magnetite (Fe(II)/Fe(III) ratio between 0.5 and 0) and lattice contraction 19,20 , but three-dimensional (3D) observations of the lattice spacing variation with associated strain and defect structures remain limited 16,17 . Strain and defects in minerals are important factors in controlling kinetics of geochemical reactions, such as mineral dissolution/growth and isotope exchange reactions 21–23 . In particular, off-stoichiometric magnetite can undergo a reverse reaction, i.e., reductive growth in the presence of aqueous Fe(II) 24 .…”

<strong>Cr release from Cr-substituted goethite during the aqueous Fe(II)-induced recrystallization</strong>

“…(Figure 6b), suggesting that some of the 57 Fe(II) aq was oxidized and entered into the structure of goethite resulting in the formation of 57 goethite. The results clearly suggest the occurrence of electron transfer and Fe atom exchange between Fe(II) aq and Cr-goethite [30]. From the Mössbauer characterization results, we can also find that new goethite consisting of 57 Fe was formed in the Cr-56 goethite following reaction with 57 Fe(II) aq .…”

“…For the characterization of Fe(II) aq -induced recrystallization of Cr-goethite with Mössbauer spectroscopy, the 9.03% Cr-substituted 56 Fe-goethite samples (9.03% Cr-56 goethite) were prepared from 56 Fe (0) powder (Isoflex, 99.94%) using a modification of previously described methods [3,30]. In brief, 56 Fe (0) was dissolved in 5 M HCl to obtain an Fe(II) stock solution, and the solution was then oxidized using excess 30% H 2 O 2 to produce Fe(III).…”

“…The Cr-goethite was recrystallized through induction by Fe(II)aq, while the recrystallized products had a crystalline phase similar to that of the initial mineral. The rate of Fe atom exchange between the Fe(II) aq and Fe(III) oxide of iron (hydr)oxides were reported to be affected by various structural properties, for example, structural defects, particle sizes, and solid electron conductivity [6,30,32,37,38]. Notini et al [28] reported that surface defects of the goethite played an important role in Fe(II)-goethite redox interactions, and that the oxidation of Fe(II) aq by goethite was kinetically inhibited on structurally perfect surfaces.…”

Fe(II)-mediated activation of oxygen by goethite for the As(III) oxidation and the mechanisms

“…进一步的BET表征发现, 针铁矿的比表面积为101.2 m 2 /g [30] . As(III)含量在5 min内迅速下降约15%, 并在120 min内 持续缓慢降低剩约80%, 这可能是溶液中Fe(II)被氧化, 并水解形成水合铁氧化物次级矿物吸附溶液中As(III) 所致 [32,33] . 当针铁矿(1.…”