Dissolved oxygen prompts low-oxygen aliphatic disinfection byproduct formation by reshaping natural organic matter...

Dissolved oxygen (DO) is routinely maintained to suppress anaerobic microbial growth and enhance disinfection efficiency in drinking water treatment systems. Despite its widespread presence, the roles of DO in regulating disinfection byproduct (DBP) formation during natural organic matter (NOM) chlorination remain poorly understood, particularly at the molecular level. This study examined the influence of DO on chlorinated DBP (Cl-DBP) formation, acute toxicity, and NOM transformation pathways during the chlorination of three NOM matrices. Under DO-saturated conditions, total organic chlorine levels increased by 39-85%, accompanied by 78-401% increases in acute toxicity compared to chlorination without DO. Fourier transform ion cyclotron resonance mass spectrometry revealed that DO promoted previously unrecognized, low-oxygen, aliphatic Cl-DBP formation while substantially disrupting NOM aromaticity. Molecular linkage analysis indicated that chlorine substitution (+2Cl-2H) and sequential substitution/addition (+Cl₂O) dominated unknown Cl-DBP formation. DO facilitated chlorination of lignin-like precursors with low O/C (< 0.4) and high H/C ratios, enhanced decarboxylation reactions, and suppressed oxygenation pathways. Radical-quenching experiments confirmed that DO-induced radicals mediated these transformations and promoted aliphatic Cl-DBP generation. Two-dimensional Fourier transform infrared correlation spectroscopy further demonstrated additional activation of aliphatic C-OH structures during DO-involved chlorination, with perturbations of aromatic CC and quinone CO functionalities preceding C-Cl bond formation. Collectively, these findings identified DO as an overlooked regulator of NOM reactivity and Cl-DBP formation during chlorination, underscoring the need to manage DO and/or aliphatic Cl-DBPs in oxygen-rich drinking water treatment systems.