Generation of Reactive Nitrogen Species in UV Photolysis of Dichloramine and Their Incorporation into Nitrogenous Byproducts

Dichloramine (NHCl₂) often coexists with monochloramine (NH₂Cl) in reverse osmosis (RO) permeate in potable reuse scenarios when NH₂Cl is added upstream of RO for membrane fouling control such that UV photolysis of NHCl₂ occurs during the downstream UV/chloramine process. However, the formation of reactive nitrogen species (RNS) and their incorporation into byproducts during the UV/NHCl₂ process are largely unknown. This study quantitatively evaluated the generation of RNS in the UV/NHCl₂ process and investigated the role of RNS in micropollutant transformation. UV photolysis of NHCl₂ produced comparable RNS concentration to that of NH₂Cl at the same oxidant dosage (100 μM) at pH 5.5. Under the experimental conditions, the RNS contributed greatly (40.6%) to N,N-diethyl-3-methylbenzamide (DEET) degradation. By using ¹⁵N-labeling and mass spectrometry methods, seven nitrogenous byproducts of DEET degradation with the incorporation of nitrogen originating from the RNS were detected. Among these seven byproducts, six were identified to contain a nitro group (−NO₂). While the UV/NHCl₂ process formed comparable intensities of −NO-containing products to those in the UV/NH₂Cl process, the later process formed 3–91% higher intensities of −NO₂-containing products. These findings are essential in furthering our understanding of the contribution of the UV/NHCl₂ process in potable reuse scenarios.