Cu ²⁺ -Mediated Promotion on Transformation of Neonicotinoid Pesticides during Drinking Water Transportation: Catalytically Enhanced Chlorination and Radical-Driven Oxidation

Copper ions (Cu2+) leach from drinking water transport pipes, affecting micropollutant conversion; neonicotinoid pesticides (NPs) are emerging micropollutants incompletely removed at waterworks, and residual chlorine (mainly hypochlorous acid, HOCl) is essential for maintaining chemical and biological stability in water distribution systems. This study investigated how Cu2+ affects the reactions between NPs and HOCl. Among 9 NPs, Cu2+ most significantly promoted the conversion of clothianidin (CLO) and thiacloprid (THI), increasing their observed rate constants (kobs) by 1.0-fold and 0.6-fold, respectively. Cu2+ coordinated with nitrogen and sulfur-containing groups in the two NPs, acting as an electron acceptor to form low-valent copper (Cu+), which reacted with HOCl to generate reactive radicals. Hydroxyl radical (HO•) was the most readily generated product, as indicated by its lower standard Gibbs free energy (ΔG° = −11.6 kJ mol–1). The Cu2+-mediated promotion can be attributed to two concurrent mechanisms: catalytically enhanced chlorination and radical-driven oxidation. Plausible conversion pathways of CLO and THI perturbed by Cu2+ were proposed based on reaction products identified tentatively using UPLC-QTOF-MS/MS. This study deepens mechanistic understanding of micropollutant fate in drinking water transportation, highlighting overlooked roles of transition metal ions in perturbing chemical processes.