Redox Behavior of Magnetite: Implications for Contaminant Reduction

Abstract

The factors controlling rates of contaminant reduction by magnetite (Fe3O4) are poorly understood. Here, we measured the reduction rates of three ArNO2 compounds by magnetite particles ranging from highly oxidized (x = Fe2+/Fe3+ = 0.31) to fully stoichiometric (x = 0.50). Rates of ArNO2 reduction became almost 5 orders of magnitude faster as the particle stoichiometry increased from x = 0.31 to 0.50. To evaluate what was controlling the rate of ArNO2 reduction, we measured apparent 15N kinetic isotope effects ((15)N-AKIE) values for nitrobenzene and magnetite open-circuit potentials (E(OCP)). 15N-AKIE values were greater than unity for all magnetite stoichiometries investigated, indicating that mass transfer processes are not controlling the rate of ArNO2 reduction by magnetite. E(OCP) measurements showed that the E(OCP) for magnetite was linearly related to the stoichiometry, with more stoichiometric magnetite having a lower potential. Based on these results, we propose that conceptual models that incorporate both redox and Fe2+ diffusion processes, rather than those that rely solely on diffusion of Fe2+, are more appropriate for understanding contaminant reduction by magnetite. Our work indicates that particle stoichiometry should be considered when evaluating rates of contaminant reduction by magnetite.

Electron uptake by iron-oxidizing phototrophic bacteria

“…FeOBs often contend with the precipitation of insoluble iron oxides outside the cell that are a byproduct of their metabolic activity and potentially limit Fe(II) availability 21,28 . TIE-1 produces poorly crystalline Fe(III) hydroxides, which over time are abiotically transformed to the (semi)conductive minerals goethite and magnetite 16,29 . Conduction of electrons through this matrix would allow TIE-1 (and potentially other FeOBs) access to electrons from remote electron donors, including Fe(II) (Supplementary Fig.…”

Soft X-ray Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles

“…51 Our conclusion that incompletely oxidized magnetite nanoparticles exhibit modified band-edge electronic structure is consistent with a recent study that correlated the ferrous iron content of ~10-nm magnetite nanoparticles subjected to varying degrees of oxidation with measurements of their redox potential and the kinetics of interfacial reduction processes. 20 A consistent description of the reduction kinetics was possible only if the ferrous iron content affected the thermodynamics for interfacial electron transferi.e., the effective energy positions of band-edge states. Moreover, our findings resemble the Burstein shift observed in TiO 2 nanocrystalline electrodes in which potentiostat-controlled electron transfer to the TiO 2 nanoparticles causes a reversible increase in the optical band gap.…”

“…16,20 Despite the many studies of magnetite oxidation, obtaining an accurate description of the structure and chemical properties of the oxidized products remains a considerable challenge. In related research,…”

Compound-specific isotope analysis of benzotriazole and its derivatives

“…2-Chloronitrobenzene (compound 1) is reacted with ammonia to 2-nitroaniline (2), which is then reduced to the benzotriazole precursor o-phenylenediamine (3) [41]. A ring closing diazotization reaction of o-phenylenediamine with nitrous acid in dilute sulfuric acid or with sodium nitrite and acetic acid leads to the formation of 1H-BT [41,42] and −46 reported for the reduction of nitrobenzene to aniline [43][44][45], however, indicate singlereaction steps during 1H-BT production to be strongly fractionating. The same reason is valid for 4-CH 3 -BT and 5-CH 3 -BT that are produced via the same pathway as 1H-BT (Fig.…”