Characterization of the Degradation and Nitrogenous Disinfection Byproduct Formation of Nitrogen-Containing Benzoheterocyclic Compounds during the UV-LED (275 nm)/Chlorine Process: Roles of Reactive Species and Pathway of Dichloroacetonitrile Formati

Nitrogen-containing benzoheterocyclic compounds (NBHCs) are important precursors of nitrogenous disinfection byproducts (N-DBPs). The degradation of three NBHCs with different nitrogen atoms, benzotriazole (BTA), indazole (IDZ), and indole (IDO), and N-DBP formation during the ultraviolet light-emitting diode (UV-LED)/chlorine process deserve to be investigated. The extremely rapid IDO degradation was attributed to a fast electrophilic substitution of chlorine at the 3-position of the IDO molecule. Dichloroacetonitrile (DCAN) was the most dominant N-DBP during the UV-LED/chlorine treatment of the three NBHCs. DCAN yields from IDO, containing one nitrogen atom, were 3.5 and 36.4 times higher than those from BTA, with three nitrogen atoms, and IDZ, with two nitrogen atoms, respectively. The possible important roles of HO^• and Cl^• were identified in the DCAN formation for the three NBHCs. Especially, more elaborate possible pathways for DCAN formation were proposed: the pyrrole ring of IDO opened, or the triazole ring of BTA underwent N₂ elimination to form chlorinated (hydroxyimino)cyclohex-ene-dione, and then ring-opening took place to gradually form DCAN. Particularly, the pyrrole ring of IDO can form DCAN directly. These can provide useful references and open ideas for research on DCAN formation from other NBHCs and their derivatives by free radicals.