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Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Aeppli Meret, Voegelin Andreas, Gorski Christopher A., Hofstetter Thomas B., Sander Michael,
Project Characterization of the redox properties of iron minerals by combined electrochemical and spectroscopic analyses
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Original article (peer-reviewed)

Journal Environmental Science & Technology
Volume (Issue) 52(2)
Page(s) 560 - 570
Title of proceedings Environmental Science & Technology
DOI 10.1021/acs.est.7b04411

Open Access

URL http://dx.doi.org/10.1021/acs.est.7b04411
Type of Open Access Publisher (Gold Open Access)

Abstract

Iron (oxyhydr-)oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, ΔrG, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (−0.53 to −0.17 V vs SHE), and Fe2+ concentrations (up to 40 μM). Ferrihydrite reduction was complete and fast at all tested ΔrG values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as ΔrG values became less negative. Reductions at intermediate ΔrG values showed negative linear correlations between the natural logarithm of the reduction rate constants and ΔrG. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe2+-facilitated transformation of ferrihydrite to goethite.
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