Redox behavior of magnetite: Implications for contaminant reduction

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 ( ¹⁵N-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 measurementsshowedthat theE_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.