Are there overlooked precursors? Unveiling the formation of dichloroacetonitrile from ammonia and nitrogen-free organic matter during UV/chlorine treatment

While dissolved organic nitrogen (DON) is traditionally considered a crucial precursor of nitrogenous disinfection byproducts (N-DBPs) like dichloroacetonitrile (DCAN), the present study hypothesizes that dissolved inorganic nitrogen (DIN) and nitrogen-free organic matter (NFOM) can also serve as important precursors of DCAN. To validate this hypothesis, this study compared the DCAN yields from DIN coupled with NFOM versus those from DON during the ultraviolet (UV)/chlorine process. Under identical carbon, nitrogen, and chlorine dosage conditions, the levels of DCAN formation followed the order: NFOM + ammonia (NH₄⁺) > DON ≫ NFOM + NO_x⁻ (nitrate and nitrite), highlighting the significance of DIN and NFOM in DCAN formation. Among NFOMs, compounds with benzene rings generated more DCAN than those without benzene structures, indicating that the former should be prioritized for control. Among model DON compounds, the DCAN yields decreased with increasing number of carbon, suggesting a dilemma in regulation compliance and control efficiency. Hydroxyl radicals together with reactive chlorine species, reactive nitrogen species, and monochloramine contributed 57%, 37%, and 6% to DCAN formation, respectively, whereas UV alone had negligible effects. The evolutions of parent compounds and intermediate products showed various relations with DCAN formation, suggesting that the process might involve multiple pathways for N-DBPs. Overall, this study provides new insights into the formation mechanisms of DCAN and highlights the necessity to re-evaluate precursor control strategies by controlling DIN and aromatic NFOM alongside DON.