Iron (Fe) (oxyhydr)oxides participate in redox reactions with numerous environmentally-relevant phases, including nutrients, pollutants, and microorganisms. Despite much work advancing the mechanistic understanding of these processes, it is still difficult to predict reaction extents in quantitative terms. This is due in large part to the lack of accurate reduction potential (Eh) values for Fe oxide - aqueous Fe(II) redox couples. Calculating such values from thermodynamic terms is challenging in that Fe oxide interactions with aqueous Fe(II) are complex and can involve irreversible steps that lead to metastable states. Experimentally determining values is rarely possible using conventional measurement techniques because Fe oxides do not reach redox equilibrium with electrodes during measurement times. Here, we demonstrate that electron transfer mediators can be used to overcome this experimental challenge, allowing for rapid and accurate reduction potential measurements.<br/>We measured reduction potential values for goethite, hematite, and magnetite as a function of pH and aqueous Fe(II) concentration. We performed these measurements using batch reactor experiments that contained a small amount of an electron transfer mediator, which rapidly reached redox equilibrium with the Fe oxide suspension and the measuring electrode. For each oxide, we collected standard reduction potential values, pH dependencies, and aqueous Fe(II) dependencies, which were in good agreement with predicted reaction stoichiometries. For goethite, we also examined the reversibility of electron transfer to and from the oxide using nitrobenzene and oxidized electron transfer mediators as oxidant probes. This work provides two key results: (1) reduction potential values for synthetic Fe oxides exposed to Fe(II) that can be used in future studies and (2) a relatively simple approach to directly measure reduction potential values in complex environmental samples.

Iron oxide – aqueous Fe(II) standard reduction potential measurements: A mediated electrochemical study

249th National Meeting (2015)

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