Nitrogen Incorporation during Breakpoint Chlorination and the Formation of Dichloroacetonitrile and Higher-Carbon Nitrogenous Disinfection Byproducts

Nitrogenous disinfection byproducts (N-DBPs) such as dichloroacetonitrile (DCAN) and higher-carbon N-DBPs (with ≥3 carbon atoms) have raised concerns due to their high toxicity and prevalence in disinfected waters. Breakpoint chlorination is widely applied to ammonia-containing source waters to achieve free chlorine residual, but its role in the formation of DCAN and higher-carbon N-DBPs remains poorly understood. Using model compounds with systematically varied structures, this study showed that under breakpoint chlorination conditions (pH 7, Cl₂:NH₃ molar ratios 0.5 above the breakpoint), the molar yields of DCAN from phenols and anilines were 0.1-7.4% and 0.4-2.6%, respectively. Anilines' DCAN yields from breakpoint chlorination were higher than those under free chlorination (pH 7, Cl₂:precursor = 5:1 mol:mol). Liquid chromatography-high-resolution mass spectrometry analyses identified higher-carbon N-DBPs from the breakpoint chlorination of aniline and phenol, such as chloro(hydroxy)anilines and (chloro)benzoquinone (di)imines. Specifically, ammonia-nitrogen was incorporated into the aromatic moieties as amine and benzoquinone (di)imine groups, which can be attributed to trichloramine reactions based on kinetic modeling and experimental evidence. Some of the higher-carbon N-DBPs may also serve as important intermediates in DCAN formation. These findings advance the understanding of N-DBP formation mechanisms in breakpoint chlorination and highlight new classes of higher-carbon N-DBPs.