Thermodynamic Characterization of Iron Oxide–Aqueous Fe²⁺ Redox Couples

Abstract

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 (EH) 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 EH values of aqueous Fe3+/Fe2+ redox couples.

Rapid electron transfer by the carbon matrix in natural pyrogenic carbon

“…4 for better demonstration of iron mineral reductions as their reduction potentials lay in between. Magnetite and hematite minerals showed a very similar reduction potential that ranged from −150 to −100 mV versus SHE from a high to low scan rate, which was consistent with previously reported potentials (about −200 to −120 mV versus SHE) obtained by thermodynamic calculations and microbial mineral reduction414243. The observed reduction rate (the rate was normalized by the mineral surface area, Fig.…”

Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

“…56,57 Alternatively, mediated electrochemical techniques make use of dissolved redox active compounds, which facilitate electron transport between mineral and electrode. 58 Consequently, through various theoretical and measurement based techniques, 58,59 the standard redox potentials of a number of different Fe(III) minerals have been determined for their reduction to Fe(II) ( Table 3). reduction" refers to experimental quantification of ferrous iron after reduction of minerals with H2; 59 "Mediated potentiometry" refers to values obtained by measuring redox potential of mineral suspensions using a Pt redox electrode in the presence of a soluble electron transfer mediator.…”

Fe2+ adsorption on iron oxide: the importance of the redox potential of the adsorption system

“…3 (red straight line) and we found that the parameter E 0 = 1120 mV. This value is higher than the theoretical equilibrium potential for the reaction (3): E 0 = 937 mV (Gorski et al 2016). Probably, the determined value of E 0 is a mixed potential of two or more redox processes like the oxygen reduction to water with equilibrium potential of E 0 = 1230 mV (compare with Figure 8.28b in the reference (Stumm and Morgan 1996)).…”

“…In natural waters dissolved iron can exist as Fe 2+ ion with combination with Fe 3+ oxides and hydroxides (Stumm and Morgan 1996). Therefore, the reactions between dissolved Fe 2+ ions and iron(III) oxides were thoroughly studied both experimentally and theoretically (Dixit and Hering 2006;Gorski and Scherer 2011;Gorski et al 2016;Kerisit et al 2015;Larese-Casanova et al 2012;Larese-Casanova and Scherer 2007;Tanwar et al 2008;Rosso 2017, 2018).…”

“…So far only a few experimental works have tried to keep these standards [e.g. (Gorski et al 2016;Nano and Strathmann 2006)].…”