title: Thermodynamic Characterization of Iron Oxide-Aqueous Fe(2+) 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.
authors.affiliation: Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States.
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Gorski
authors.affiliation: Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Edwards
authors.affiliation: Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich , 8092 Zürich, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Sander
authors.affiliation: Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich , 8092 Zürich, Switzerland.|Eawag, Swiss Federal Institute of Aquatic Science and Technology , CH-8600 Dübendorf, Switzerland.
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Hofstetter
authors.affiliation: Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States.
authors.forename: Sydney M
authors.initials: SM
authors.lastname: Stewart
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry; D008903:Minerals / Q000737:chemistry; D009930:Organic Chemicals; D010084:Oxidation-Reduction; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; D009930:Organic Chemicals; D014867:Water; C094886:goethite; C000499:ferric oxide; D007501:Iron