Numerous organic compounds that are used as explosives, pesticides, and pharmaceuticals involve the use of nitrogen substitute aromatic compounds. These compounds can enter into aquatic environments after use in agriculture and firing ranges or from industrial and household discharges. It is known that these compounds may form polymers in waters reactive in the presence of oxygen. The current paradigm is that the polymerization of these compounds only occurs under conditions with oxygen dissolved in the aqueous waters. However, preliminary evidence has shown coupling reactions can also occur in the absence of oxygen which may have important implications to their toxicity in ground waters and other waters with low oxygen concentrations. The main focus of this project is to provide a mechanistic basis for the anaerobic coupling reactions between reactive intermediates derived from the microbial reduction of nitroaromatic pollutants.
In anaerobic environments, the nitroaromatic compounds will undergo reductive (bio)transformation to aromatic amines via nitroso- and hydroxylamine-intermediates. The research will explore whether certain taxonomic groups of anaerobic microorganisms with nitroreductase genes become enriched as a consequence of the bioreduction of nitroaromatic compounds. Next the project will test three hypothesized mechanisms of coupling. The first two involve nucleophilic attacks on nitrosobenzene intermediates by either aromatic hydroxylamines or amines, forming azoxy (Ar-N+(O-)=N-Ar) or azo bonds (Ar-N=N-Ar), respectively. The third hypothesis is the coupling of radicals formed from the oxidation of aromatic amines by metals. The extent of the coupling reactions is expected to depend on the nitroaromatic structure. Coupling of compounds with just one nitro group may be limited to dimers; whereas, structures with multiple nitro groups can continue coupling to form large polymers. Extensive coupling reactions are also expected with a continuous supply of nitroaromatic pollutants which would allow for a continuous production of reactive intermediates. Isotopically labeled model compounds (15N, 13C and 14C) will be used to help elucidate the structure of humic-like substances formed in the absence of O2. Preliminary data indicate that dimers have an increased toxicity compared to the original nitroaromatic compounds justifying a detailed evaluation of their microbial and aquatic toxicity in this project. The project will have impacts on risk assessment, green synthesis, high school STEM education, inclusion of minorities in the scientific and educational enterprises, and the long term goals of the research group. Azo dyes lend themselves nicely to STEM projects because concentration is visually observable.
