Redox Potentials of Magnetite Suspensions under Reducing Conditions

Predicting the redox behavior of magnetite in reducing soils and sediments is challenging because there is neither agreement among measured potentials nor consensus on which Fe(III) | Fe(II) equilibria are most relevant. Here, we measured open-circuit potentials of stoichiometric magnetite equilibrated over a range of solution conditions. Notably, electron transfer mediators were not necessary to reach equilibrium. For conditions where ferrous hydroxide precipitation was limited, Nernstian behavior was observed with an E_H vs pH slope of -179 ± 4 mV and an E_H vs Fe(II)_aq slope of -54 ± 4 mV. Our estimated E_H^o of 857 ± 8 mV closely matches a maghemite|aqueous Fe(II) E_H^o of 855 mV, suggesting that it plays a dominant role in poising the solution potential and that it's theoretical Nernst equation of E_H[mV] = 855 - 177 pH - 59 log [Fe²⁺] may be useful in predicting magnetite redox behavior under these conditions. At higher pH values and without added Fe(II), a distinct shift in potentials was observed, indicating that the dominant Fe(III)|Fe(II) couple(s) poising the potential changed. Our findings, coupled with previous Mössbauer spectroscopy and kinetic data, provide compelling evidence that the maghemite/Fe(II)_aq couple accurately predicts the redox behavior of stoichiometric magnetite suspensions in the presence of aqueous Fe(II) between pH values of 6.5 and 8.5.