Reductive transformation of nitrobenzenes and their reaction intermediates by aqueous and mineral-associated iron species
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Nitrobenzenes are ubiquitous environmental contaminants because of their use as munitions, pesticides, pharmaceuticals, and industrial feed stock chemicals. Reduction is their main transformation pathway in anoxic environments. In order to predict the fate of nitrobenzenes and their reaction intermediates in the environment, it is important to know how their degradation is affected in diverse environmental scenarios. This study examined the influence of aqueous and mineral surface-associated Fe(II) on the reductive transformation of nitrobenzenes and their reaction intermediates, namely, nitrosobenzenes and N-hydroxylanilines. Compounds within each class of chemicals were selected to contain either electron withdrawing- or donating- functional groups to increase or decrease, respectively, their willingness to accept electrons. The degradation of these three classes of compounds was monitored in well-defined aqueous batch reactors. The naturally occurring oxides used in this study were hematite, goethite, lepidocrocite and ferrihydrite. An assessment of the effect of systematically changed solution composition variables on the reaction kinetics was performed. The extent of reduction of nitrobenzenes and N-hydroxylanilines was dependent on the compound structure, the ferric oxide and solution composition. The predictability of the rate of reduction of nitrobenzenes in Fe(II)-treated oxides systems was improved, over what is currently available in the literature, by using the energy of the lowest-unoccupied molecular orbital as the chemical predictor. The presence of natural organic matter significantly slowed down the reduction of a nitrobenzene by surface-associated Fe(II). In such systems, disappearance of Fe(II) from the aqueous phase was attributed to oxidation and or complex formation with the natural organic matter. Our results suggest that the rate determining step for the reduction of N-hydroxylanilines in the presence of Fe(II)-treated oxides involves the breakage of the N-O bond. The reduction kinetics of nitrosobenzenes was extremely fast in the presence of aqueous Fe(II). It showed a dependence on the electron withdrawing-/donating- capability of substituent functional groups in the compound. This dissertation improves our understanding of how the studied compounds interact with aqueous and oxide-associated Fe(II) and how environmentally relevant variables affect their reduction rates.