Redox behavior of magnetite in the environment

Abstract

Magnetite (Fe 3 O 4) is a commonly found in the environment and can form via several pathways, including biotic and abiotic reduction of Fe 3+ oxides and the oxidation of Fe 2+ and Fe 0. Despite extensive research, the redox behavior of magnetite is poorly understood. In previous work, the extent and kinetics of contaminant reduction by magnetite varied by several orders of magnitude between studies, two fundamentally different models are used to explain magnetite oxidation (i.e., core-shell diffusion and redox-driven), and reported reduction potentials vary by almost 1 V. In other fields of science (e.g., physics), magnetite stoichiometry (x = Fe 2+ /Fe 3+) is a commonly measured property, however, in environmental studies, the stoichiometry is rarely measured. The stoichiometry of magnetite can range from 0.5 (stoichiometric) to 0 (completely oxidized), with intermediate values (0 < x < 0.5) referred to as nonstoichiometric or partially oxidized magnetite. To determine the relationship between magnetite stoichiometry and contaminant fate, the reduction rates of three substituted nitrobenzenes (ArNO 2) were measured. The kinetic rates varied over five orders of magnitude as the particle stoichiometry increased from x = 0.31 to 0.50. Apparent 15 N kinetic isotope effects (15 N-AKIE) values for ArNO 2 were greater than unity for all magnetite stoichiometries investigated, and indicated that mass transfer processes are not controlling the reaction rate. To determine if the reaction kinetics were redox-driven, magnetite open circuit potentials (E OCP) were measured. E OCP values were linearly related to the stoichiometry, with more stoichiometric magnetite having a lower potential, in good agreement with redox-driven models. The reaction of aqueous Fe 2+ and magnetite was investigated. Similar to previous findings for other Fe 3+ oxides, the formation of a stable sorbed Fe 2+ species was not observed; instead, the sorbed Fe 2+ underwent interfacial electron transfer to form a partially oxidized magnetite phase, which was accompanied by reduction of the viii TABLE OF CONTENTS LIST OF TABLES .

Chemical Reactivity Probes for Assessing Abiotic Natural Attenuation by Reducing Iron Minerals

“…In contrast, the synthetic hematite used by Gorski showed a much lower reduction potential than the Bayferrox hematite used in this study. 34 The reason for this discrepancy is unclear, but it is consistent with other work showing that synthetic hematite is generally more reactive with contaminants than the Bayferrox hematite. 31 Comparison with Electrochemically Measured Potentials.…”

“…16,22 The calculated mineral reduction potentials were further compared to previously measured or tabulated values for Fe(II) containing minerals, which are summarized in the far right column of Figure 2. The measured potentials for Fe(II) sorbed onto goethite reported by Orsetti et al 33 and for Fe(II) sorbed to synthetic hematite and magnetite reported by Gorski 34 are of particular relevance because (i) both studies used thermodynamic CRPs (i.e., redox indicators) to calculate the mineral reduction potentials and (ii) the origins of minerals used in those studies are the same as some of the minerals used here.…”

“…32 Alternatively, if the speciation of the mediator can be determined spectrophotometrically (provided negligible sorption of the mediator to mineral surfaces), these data can be used in the Nernst equation to calculate reduction potentials of the medium. 33,34 Various redox indicators have been used to characterize the redox processes of biogeochemical systems, including iron oxides, 35−37 soil suspensions, 38 and anaerobic sediments. 39,40 However, the power of this approach for quantification of specific mineral redox properties (potentials, kinetics, and capacities) has not been systematically developed.…”

Electrochemical Characterization of Magnetite with Agarose-Stabilized Powder Disk Electrodes and Potentiometric Methods

“…A preliminary test of this approach using reduction of nitrobenzene as the contaminant reaction yielded promising correlations, 21 but subsequent efforts raised fundamental questions about how the PDE electrode response relates to material structure (e.g., E OC vs x), and what electrochemical properties might be most predictive of contaminant reduction rates (e.g., E OC or i COR ). Other studies have shown good correlations between (i) magnetite stoichiometry (x) and the kinetics of nitrobenzene reduction 89,110 or U VI reduction, 111 (ii) between redox potentials of mineral suspensions determined potentiometrically with electron-transfer mediators and nitrobenzene reduction rates, 37 and (iii) between redox potentials of various iron mineral suspensions determined spectrophotometrically with chemical redox probes (CRPs) and the rates of contaminant nitro reduction and dechlorination. 39 However, this study shows that freshly synthesized magnetite can have lower E and R P with higher i COR (all of which suggest faster reduction of contaminants), even when prepared with low stoichiometry (which would suggest slower contaminant reduction rates).…”

Thermodynamic Characterization of Iron Oxide–Aqueous Fe2+ Redox Couples

“…21,63,65 In our previous work, we tested the latter approach, but found that it did not provide sufficient accuracies due to the need for high concentrations of the compounds to achieve adequate absorbances, which altered E H values of the suspensions. 66 Here, we performed E H measurements using mediated potentiometry, a technique that combines elements of the two aforementioned approaches. In mediated potentiometry, soluble electron transfer mediators are used to facilitate redox equilibration between an aqueous system and a redox electrode performing a potentiometric (i.e., open-circuit potential) measurement.…”