TY - JOUR
T1 - Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions
AU - Aeppli, Meret
AU - Voegelin, Andreas
AU - Gorski, Christopher A.
AU - Hofstetter, Thomas B.
AU - Sander, Michael
N1 - Publisher Copyright:
© 2017 American Chemical Society.
PY - 2018/1/16
Y1 - 2018/1/16
N2 - 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 (Δ0.53 to Δ0.17 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.
AB - 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 (Δ0.53 to Δ0.17 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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U2 - 10.1021/acs.est.7b04411
DO - 10.1021/acs.est.7b04411
M3 - Article
C2 - 29200267
AN - SCOPUS:85041435443
SN - 0013-936X
VL - 52
SP - 560
EP - 570
JO - Environmental Science and Technology
JF - Environmental Science and Technology
IS - 2
ER -