Laboratory study of spectral induced polarization responses of magnetite — Fe2+ redox reactions in porous media

Abstract

Spectral induced polarization (SIP) phase anomalies in field surveys at contaminated sites have previously been shown to correlate with the occurrence of chemically reducing conditions and/or semiconductive minerals, but the reasons for this are not fully understood. We report a systematic laboratory investigation of the role of the semiconductive mineral magnetite and its interaction with redox-active versus redox-inactive ions in producing such phase anomalies. The SIP responses of quartz sand with 5% magnetite in solutions containing redox-inactive [Formula: see text] and [Formula: see text] versus redox-active [Formula: see text] were measured across the pH ranges corresponding to adsorption of these metals to magnetite. With redox inactive ions [Formula: see text] and [Formula: see text], SIP phase response showed no changes across the pH range 4–10, corresponding to their adsorption, showing [Formula: see text] anomalies peaking at [Formula: see text]–74 Hz. These large phase anomalies are probably caused by polarization of the magnetite-solution interfaces. With the redox-active ion [Formula: see text], frequency of peak phase response decreased progressively from [Formula: see text] to [Formula: see text] as effluent pH increased from four to seven, corresponding to progressive adsorption of [Formula: see text] to the magnetite surface. The latter frequency (3 Hz) corresponds approximately with those of phase anomalies detected in field surveys reported elsewhere. We conclude that pH sensitivity arises from redox reactions between [Formula: see text] and magnetite surfaces, with transfer of electrical charge through the bulk mineral, as reported in other laboratory investigations. Our results confirm that SIP measurements are sensitive to redox reactions involving charge transfers between adsorbed ions and semiconductive minerals. Phase anomalies seen in field surveys of groundwater contamination and biostimulation may therefore be indicative of iron-reducing conditions, when semiconductive iron minerals such as magnetite are present.