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Linking Thermodynamics to Pollutant Reduction Kinetics by Fe2+ Bound to Iron Oxides

Stewart, Sydney M., Hofstetter, Thomas B., Joshi, Prachi, Gorski, Christopher A.
Environmental science & technology 2018 v.52 no.10 pp. 5600-5609
Gibbs free energy, data collection, electron transfer, iron, iron oxides, models, nitrobenzenes, pH, pollutants, reaction kinetics
Numerous studies have reported that pollutant reduction rates by ferrous iron (Fe²⁺) are substantially enhanced in the presence of an iron (oxyhydr)oxide mineral. Developing a thermodynamic framework to explain this phenomenon has been historically difficult due to challenges in quantifying reduction potential (EH) values for oxide-bound Fe²⁺ species. Recently, our group demonstrated that EH values for hematite- and goethite-bound Fe²⁺ can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated EH values for oxide-bound Fe²⁺ could be used to develop a free energy relationship capable of describing variations in reduction rate constants of substituted nitrobenzenes, a class of model pollutants that contain reducible aromatic nitro groups, using data collected here and compiled from the literature. All the data could be described by a single linear relationship between the logarithms of the surface-area-normalized rate constant (kSA) values and EH and pH values [log(kSA) = −EH/0.059 V – pH + 3.42]. This framework provides mechanistic insights into how the thermodynamic favorability of electron transfer from oxide-bound Fe²⁺ relates to redox reaction kinetics.