The Role of Reactive Nitrogen Species in Sensitized Photolysis of Wastewater-Derived Trace Organic Contaminants

Under conditions typically encountered in the aquatic environment, the absorption of sunlight by nitrite and nitrate leads to the transformation of trace organic contaminants. In addition to the well understood mechanism through which hydroxyl radical (·OH) produced by nitrate and nitrite photolysis oxidizes contaminants, absorption of light also results in the formation of reactive nitrogen species that transform organic contaminants. To assess the importance of this process on the fate of trace organic contaminants, radical quenchers and transformation product analysis were used to discriminate among potential reaction pathways. For sulfamethoxazole, an antibiotic that is frequently detected in municipal wastewater effluent, nitrate and nitrite-sensitized photolysis pathways resulted in production of transformation products that were not detected during direct photolysis or reaction with ·OH. The reactivity of sulfamethoxazole with the reactive species produced when nitrite absorbed sunlight was affected by the presence of hydroxyl radical scavengers, indicating the likely involvement of nitrogen dioxide, which forms when nitrite reacts with hydroxyl radical. Reactive nitrogen species also reacted with emtricitabine, propranolol, and other trace organic contaminants commonly detected in wastewater effluent, indicating the potential importance of this process to the fate of other trace organic contaminants. A kinetic model indicated that reactive nitrogen species could be important to the phototransformation of trace organic contaminants when relatively high concentrations of nitrite are present (e.g., in surface waters receiving reverse osmosis concentrate from potable water reuse projects or in agricultural runoff).