From Micropollutant to DBP Driver: The Unexpected Reactivity of 1-Chlorobenzotriazole in Water Disinfection

The increasing adoption of potable wastewater reuse is challenged by the persistence of micropollutants—such as benzotriazole (BTR)—which are poorly removed by conventional water/wastewater treatment and may produce toxic disinfection by-products (DBPs) with potential environmental and human health risks. This study investigated the chlorination of BTR and identified 1-chlorobenzotriazole (1-Cl BTR) as the primary chlorination product, formed preferentially under near-neutral to acidic pH and excess chlorine—conditions typically found in water disinfection. The DBP formation potential of 1-Cl BTR was evaluated using Suwannee River humic acid (SRHA), natural organic matter (SRNOM), and secondary wastewater effluents (SWE), and compared against hypochlorous acid (HOCl) and monochloramine (NH2Cl). Surprisingly, 1-Cl BTR formed DBP levels comparable or greater than those from HOCl, including trihalomethanes, haloaldehydes, haloketones, haloacetonitriles and halonitromethanes. This unexpected reactivity is attributed to 1-Cl BTR’s unique ability to function as a free chlorine reservoir, sustaining chlorination reactions over time and promoting elevated DBP formation—unlike typical N-halamines. DBP speciation trends with 1-Cl BTR were strongly pH-dependent and consistent with those observed for HOCl, further supporting its role as a free chlorine reservoir. Additionally, 1-Cl BTR exhibited precursor- and matrix-dependent reactivity, especially with complex matrices like SWE, where it acted both as a chlorine source and direct DBP precursor. This work presents the first detailed evaluation of 1-Cl BTR’s DBP formation potential, revealing an overlooked pathway for halogenated DBP production in water disinfection. These findings emphasize the importance of considering BTR transformation products in water treatment and highlight the need for improved strategies to mitigate DBP risks in advanced reuse systems.