Linking Thermodynamics to Pollutant Reduction Kinetics by Fe Bound to Iron Oxides.

Numerous studies have reported that pollutant reduction rates by ferrous iron (Fe) 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) values for oxide-bound Fe species. Recently, our group demonstrated that E values for hematite- and goethite-bound Fe can be accurately calculated using Gibbs free energy of formation values. Here, we tested if calculated E values for oxide-bound Fe 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) values and E and pH values [log( k) = - E/0.059 V - pH + 3.42]. This framework provides mechanistic insights into how the thermodynamic favorability of electron transfer from oxide-bound Fe relates to redox reaction kinetics.