Overlooked Dichloroacetonitrile Formation from Ammonia and Phenolic Compounds during UV-Activated Mixed Chlorine/Chloramine Treatment

To evaluate the role of inorganic nitrogen and nitrogen-free organic matter in forming nitrogenous disinfection byproducts (N-DBPs), this study investigated dichloroacetonitrile (DCAN) formation during mixed chlorine/chloramine (MCC) processes with and without ultraviolet (UV) irradiation. Results showed that ammonia and phenol yielded higher DCAN (0.036%) than representative dissolved organic nitrogen precursors, including nitrobenzene (0.012%) and aminobenzene (0.016%), under identical nitrogen, carbon, and chlorine dosages, indicating that inorganic nitrogen can be a dominant contributor to N-DBPs formation. Among UV-based processes, DCAN yields followed the order UV/chlorine (0.256%) > UV-activated MCC (0.099%) ≫ UV/monochloramine (0.011%), while MCC without UV generated less DCAN. For phenolic compounds, DCAN formation decreased with increasing number of hydroxyl groups (−OH) and varied by substitution position. A quantitative structure–activity relationship model (R2 = 0.842) identified acid dissociation coefficient (pKa) and chemical softness (S, a measure of a molecule’s susceptibility to electrophilic attack) as dominant descriptors. Transformation products analysis revealed two parallel DCAN formation pathways in the UV-activated MCC process: (i) nitrogen incorporation after oxidative cleavage and (ii) amination before aromatic ring opening. These results demonstrate that ammonia can be converted into DCAN via multiple pathways, highlighting an overlooked route for N-DBPs formation during the water disinfection process.