The release of nitrogen-oxygen containing contaminants (NOCs) into the environment is an important emerging water issue. The nitrogen-oxygen single bond is commonly found in organic contaminants ranging from aromatic N-oxides, hydroxylamines, to isoxazoles. Fe<sup>II</sup> associated with either iron oxides or organic ligands (e.g., tiron) has been widely examined as environmental reductants in reducing organic contaminants. This research combined both experimental and computational approaches to examine the poorly understood reduction kinetics and mechanisms of various NOCs by soluble and surface-associated Fe(II) complexes.  Results showed that the NOCs were reduced by both reductants at different rates. Reduction products of NOCs were mainly the deoxygenated or ring-opening analogs. Density functional theory calculations suggested that the elementary reactions, including protonation and N–O bond cleavage, were barrierless or having small energy barriers, indicating that the electron transfer was rate-limiting. Consistent with the theoretical results, the experimental kinetic isotope effects of the reduction of NOCs suggested that protonation was spontaneous, while the electron transfer was rate-limiting. Electrochemical cell experiments that separated complexation from electron transfer revealed that various types of complexes formed between NOCs and the reductant, leading to different reactivity and mechanisms. The electron transfer process was facilitated to different extents depending on the types of complexation. Overall, this project provided a wealth of new mechanistic information on the transformation of NOCs in model aquatic environments.

Reduction of N–O containing compounds (NOCs) by soluble and surface complexed Fe(II)

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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