Thermodynamic Characterization of Iron Oxide-Aqueous Fe²⁺ Redox Couples

Iron is present in virtually all terrestrial and aquatic environments, where it participates in redox reactions with surrounding metals, organic compounds, contaminants, and microorganisms. The rates and extent of these redox reactions strongly depend on the speciation of the Fe²⁺ and Fe³⁺ phases, although the underlying reasons remain unclear. In particular, numerous studies have observed that Fe²⁺ associated with iron oxide surfaces (i.e., oxide-associated Fe²⁺) often reduces oxidized contaminants much faster than aqueous Fe²⁺ alone. Here, we tested two hypotheses related to this observation by determining if solutions containing two commonly studied iron oxides - hematite and goethite - and aqueous Fe²⁺ reached thermodynamic equilibrium over the course of a day. We measured reduction potential (E_H) values in solutions containing these oxides at different pH values and aqueous Fe²⁺ concentrations using mediated potentiometry. This analysis yielded standard reduction potential (EH0) values of 768 ± 1 mV for the aqueous Fe²⁺-goethite redox couple and 769 ± 2 mV for the aqueous Fe²⁺-hematite redox couple. These values were in excellent agreement with those calculated from existing thermodynamic data, and the data could be explained by the presence of an iron oxide lowering E_H values of aqueous Fe³⁺ Fe²⁺ redox couples.