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

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 (E_H) values for oxide-bound Fe²⁺ species. Recently, our group demonstrated that E_H values for hematite- and goethite-bound Fe²⁺ can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated E_H 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 (k_SA) values and E_H and pH values [log(k_SA) = -E_H/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.