Aromatic Structures Govern the Formation of Chlorinated Byproducts in Dichlorine Radical Reactions

Radical-induced disinfection byproduct (DBP) formation is drawing attention with increasing applications of advanced oxidation processes (AOPs). Cl₂^•- represents one of the extensively generated radicals in AOPs, whose behavior in DBP formation remains unknown. In this study, we found that aromatic structures serve as the main DBP precursors in Cl₂^•- reactions by employing diverse groups of model compounds. At a typical Cl₂^•- exposure of 1.2 × 10^-9 M·s, the sum concentrations of 7 regulated aliphatic DBPs (e.g., trichloromethane, chloroacetic acids) are ∼0.10 to 0.48 μM for aromatic precursors and <0.05 μM for aliphatic ones. The DBP formation mechanisms from Cl₂^•- reactions involved the formation of chlorinated aromatics, radical-induced oxygen incorporation followed by ring cleavage, and the interactions of Cl₂^•- with ring-cleavage intermediates. In reacting with DOM, Cl₂^•- reactions produced much fewer aliphatic DBPs (5% of the total organochlorine vs 40% for chlorination) and chloroacetic acids dominated the aliphatic DBPs (usually trihalomethane for chlorination), which can be well interpreted by the precursors and mechanisms proposed. This work comprehensively reveals the precursors, formation patterns, and mechanisms of DBPs during the less-studied Cl₂^•- reactions, highlighting the importance of eliminating the aromatic structures of DOM before the AOPs.