Molecular Insights into the Transformation of Dissolved Organic Matter during Chlorine Photolysis: The Importance of Oxygen Addition Reactions in the Disinfection Byproducts Formation

When wastewater disinfected with excessive chlorine disinfectant is discharged into surface water, the dissolved organic matter (DOM) may undergo transformation due to solar photolysis of chlorine. In addition, solar photolysis of chlorine also presents a promising disinfection strategy. However, the changes in molecular composition and the formation of chlorinated disinfection byproducts (Cl-DBPs) of DOM remain unclear, and further investigation into the detailed reaction mechanisms is necessary. This study systematically examined the impact of solar photolysis of chlorine on the molecular composition and formation potential of Cl-DBPs (Cl-DBPsFP) of DOM. The solar photolysis of chlorine significantly increased the Cl-DBPsFP compared to dark chlorination. Notably, Cl-DBP precursors were predominantly concentrated within the specific area in Van Krevelen diagrams (0.3 < O/C < 0.75, 0.5 < H/C < 1.3). Compared with dark chlorination, solar photolysis of chlorine generated more Cl-DBP precursors from the original DOM primarily through oxygen addition reactions, thereby increasing subsequent Cl-DBPsFP. The presence of hydroxyl radicals and ozone were both essential for facilitating these oxygen addition reactions and the formation of Cl-DBP precursors. This study reveals the underlying reaction mechanisms at the molecular level through which solar photolysis of chlorine affects the Cl-DBPsFP of DOM, subsequently posing potential risks to downstream water treatment plants.