Electrochemical Characterization of Magnetite Samples from Natural, Commercial, and Laboratory Sources and Implications for Remediation Research
Authors: Bradley Miranda J , Tratnyek Paul G 
Published: February 2020

Magnetite (Fe3O4) is a naturally occurring iron oxide found throughout the Earth’s crust. Due to the mixed valence state of the iron in magnetite (in stoichiometric magnetite the Fe2+:Fe3+ ratio is 1:2, or x = 0.5) it is one of the more redox active minerals in the environment. Magnetite can influence the fate of contaminants, both as a part of the existing substrate for natural attenuation and as a material that is added to enhance contaminant reduction. Contaminant reduction by magnetite is increased by doping with Fe(II) (i.e., x > 0.5) and decreased by the formation of a passivating layer on its outer surface (x< 0.5). To study these effects, we have developed a protocol for characterizing magnetite samples from various sources, with the ultimate aim of developing a quantitative model for predicting the redox activity of these materials.

Characterizing environmentally sourced samples requires that they be protected from oxidation during sample preparation. For this, we have developed a packed powder disk electrode (PDE), using agarose-stabilized electrolyte in the pore space, for direct electrochemical characterization in environmentally relevant conditions. With it, we tested magnetite separated from sediment in the Columbia River Estuary, the Willamette River, the Hanford decommissioned nuclear production site, and the Pena Colorada mine in Colima, Mexico. For comparison, magnetite samples from three commercial sources and magnetite synthesized by collaborators was also characterized.

Samples were allowed to equilibrate for 14+ hours while the open circuit potential was monitored, and the final value was recorded as the passive measurement of the sample’s redox potential. Small perturbation (+/- 10 mV) dynamic polarization measurements were performed multiple times throughout the experiment to monitor the sample stability and determine polarization resistance and an active measurement of the redox potential (the applied potential resulting in zero current). Impedance measurements were also repeated to ensure stability, and the resulting spectra were fit to an equivalent circuit model. Finally, dynamic polarization measurements with greater overpotentials (+/- 250 mV) were performed and interpreted by Tafel slope analysis.

The parameters determined through these measurements include the passively and actively obtained redox potential (open circuit potential vs corrosion potential), polarization resistance, corrosion current, electron transfer coefficient, double-layer capacitance, and charge transfer resistance. Open circuit potentials varied from 149 to 263 mV vs SHE for naturally sourced magnetite, -10 to 260 mV vs SHE for commercial magnetite, and -257 to 169 mV vs SHE for laboratory-synthesized magnetite (prepared with stoichiometric ratios varying from 0.5 to 0.28). Corrosion potentials determined by the small perturbation experiments were consistent with the passive measurement, but the larger perturbation experiments gave potentials that were around 5 to 150 mV more negative. Measurements of polarization resistance and impedance also correlate with each other, as expected, and reflect the oxidation state of the sample.

We are currently measuring the reactivity of these samples in batch reactors using a variety of probe/model compounds, such as resazurin, indigo disulfonate, and 4-chloronitrobenzene. When these data are modelled, the rate constants will be correlated to the electrochemical properties of the magnetite samples. Based on preliminary work some correlation to redox potential is expected, but correlations to the impedance properties would provide more insight into the effects of weathering on magnetite activity, and so will be discussed.