Formation and Decomposition of <i>N</i>-Chloroaldimines in Drinking Water Disinfection: An Overlooked Subclass of Organic Chloramines

<i>N</i>-chloroaldimines are a crucial subclass of organic chloramines and overlooked intermediates of various disinfection byproducts. Although much effort has been made on organic chloramines in chlorination, great discrepancies exist in occurrence, decomposition kinetics, and molecular-level mechanisms of <i>N</i>-chloroaldimines. Herein, we propose a high-throughput method integrating directional reduction to distinguish organic chloramines from the nontarget-analyzed CHNOCl compounds and identify 86.6% of the CHNOCl compounds as <i>N</i>,<i>N</i>-dichloramines, <i>N</i>-monochloramines, and <i>N</i>-chloroaldimines. <i>N</i>,<i>N</i>-dichloramines show higher instability than <i>N</i>-monochloramines and can decompose to <i>N</i>-chloroaldimines. Furthermore, we quantitatively demonstrate the slower decomposition of <i>N</i>-chloroaldimines (10^-6 s^-1) than <i>N</i>,<i>N</i>-dichloramines (10^-5 s^-1) with 1 order of magnitude, and both of them are sensitive to engineering conditions, e.g., pH and residual disinfectants. We find that residual disinfectants can accelerate the decomposition of <i>N</i>-chloroaldimines. Meanwhile, we unravel the significant roles of NH₂-linked R groups on the decomposition pathways of <i>N</i>-chloroaldimines through experimental and theoretical perspectives. Alkyl chains linked to NH₂ groups lead to only the formation of nitriles in <i>N</i>-chloroaldimine decomposition, whereas carbocycles or benzene rings could result in the formation of aldehydes. These key insights will enable the effective control of <i>N</i>-chloroaldimine-derived disinfection byproducts and future optimization of the disinfection process.