Redox behavior of magnetite in the environment: moving towards a semiconductor model

Magnetite (Fe3O4) is a commonly found in the environment and can form via several pathways, including biotic and abiotic reduction of Fe oxides and the oxidation of Fe and Fe. 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/Fe) 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 (ArNO2) were measured. The kinetic rates varied over five orders of magnitude as the particle stoichiometry increased from x = 0.31 to 0.50. Apparent N kinetic isotope effects (N-AKIE) values for ArNO2 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 (EOCP) were measured. EOCP 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 and magnetite was investigated. Similar to previous findings for other Fe oxides, the formation of a stable sorbed Fe species was not observed; instead, the sorbed Fe underwent interfacial electron transfer to form a partially oxidized magnetite phase, which was accompanied by reduction of the