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

The mixed and variable valence of iron in magnetite (Fe(III)_tet[Fe(II),Fe(III)]_octO₄^2-) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E_OC) and the active method LPR gave corrosion potentials (E_0,LPR) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from -267 to +253 mV vs SHE). The active method LSV gave values of E_0,LSV that become increasingly more negative than E_OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R_p) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.