Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2+

The reduction of environmental pollutants by Fe2+ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, EH, of the interfacial Fe3+/Fe2+ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe2+ bound to goethite (alpha-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe3+/Fe2+ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the E-H-dependence of reaction rates involving oxide-bound Fe2+ as a reductant.