Book Description

Redox reactions have important implications for contaminant fate and transport in the environment, as they can lead to transformations that affect chemical mobility, toxicity, and bioavailability. Many classes of contaminants can be reduced by ferrous iron (Fe2+) associated with iron oxides in subsurface anoxic environments. These include contaminants containing nitroaromatic functional groups, which are common due to their widespread release into the environment as pesticides and explosives. Understanding the transformations of these chemicals is essential for determining how to remediate contaminated sites. Nitrobenzene is a useful proxy for nitroaromatic contaminants because it provides a relatively simple model system that can give insight into how more complex or less reactive chemicals are transformed in the environment.While the reduced products that form as a result of these reactions are often well characterized, the rates at which they occur are typically difficult to predict. Measured values from well-controlled laboratory studies frequently vary by orders of magnitude. In principle, these rates can be described and possibly estimated by the thermodynamic driving force of the reaction (e.g., reduction potential values for the reductant and oxidant), but demonstrating this has been difficult to test due to the difficulty in obtaining meaningful and reliable reduction potential values for iron redox couples. Recently, that limitation was overcome using mediated potentiometry, a technique that has led to a quantitative understanding of the thermodynamics of Fe2+-Fe oxy(hydr)oxide redox couples. Using mediated potentiometry, reduction potentials for these redox couples can be measured, predicted, and controlled.The ability to determine reduction potential of the Fe2+-Fe oxide couple introduces the opportunity to evaluate the relationship between reaction rates and thermodynamic parameters. In this study, I hypothesized that reduction potentials could be used to explain redox reaction rates between nitrobenzene and the Fe2+-goethite (-FeOOH) couple. This was tested by measuring nitrobenzene reduction rates as a function of solution pH, Fe2+ concentration, and goethite loading. With these results, the reduction potential of the Fe2+-goethite couple was correlated with the reaction rate constant over all solution conditions using a linear free energy relationship (LFER). The reduction of nitrobenzene was rate-limited by the first electron transfer and the first proton transfer steps, which appeared to be coupled. The best correlation for the data was achieved by normalizing the reaction rate constant to surface area of the oxide, implying nitrobenzene was reduced at the oxide surface by delocalized electrons within the solid, rather than directly by discrete oxide-associated Fe2+ sites.This LFER was further used to determine how changing the goethite particle size, and hence its thermodynamic properties, influence nitrobenzene reduction rates. From experiments with nanogoethite-associated Fe2+, it was found that the surface area normalized reaction rates for nitrobenzene with the Fe2+-nanogoethite couple were better described by the reduction potential of micron-sized goethite than that of nanogoethite. This data suggests that reduction rates by goethite-associated Fe2+ kinetically depend on the surface area of the oxide, but the thermodynamic driving force of the reaction only depends on the standard reduction potential of bulk goethite. These conclusions were further supported by comparisons to data in the literature for goethite and hematite-catalyzed reactions with substituted nitrobenzenes. Previously reported reaction rates correlated well with the LFER developed in this study.These results corroborate the hypothesis that redox reactions involving Fe2+-Fe oxy(hydr)oxide couples could be explained by growth of the oxy(hydr)oxide crystals. The reduction potential of this reaction describes the thermodynamic driving force of nitroaromatic reduction, and the reaction rate is related to the surface area of the oxides. Ultimately, this work provides insight into the mechanisms of important environmental transformations, and can lead to improved predictive models for contaminant reduction rates as a function of geochemical conditions.