MnOx valence cycle-mediated non-radical electron transfer: Dual regulatory role of MnOx in the chlorination degradation of NOR in drinking water distribution systems

Drinking water distribution systems(DWDS) are the ultimate line of defense for ensuring water quality safety. However, particulate matter in DWDS has received limited research attention, and the mechanism by which manganese oxides (MnOx) affect the chlorination degradation of emerging contaminants remains poorly understood. This study uncovers that MnOx mediates non-radical electron transfer through its valence cycle, thereby realizing its dual regulatory role in the chlorination of norfloxacin (NOR). The MnOx-mediated chlorination of NOR follows the Langmuir–Hinshelwood model: MnOx adsorbs NOR via surface Lewis acid sites and binds to sodium hypochlorite (NaClO) through oxygen vacancies, where the Mn(II)/Mn(III)/Mn(IV) valence cycle accelerates electron transfer between NOR and NaClO. Notably, the effect of MnOx varies with free chlorine levels. At sufficient free chlorine concentrations (≥0.1 mg/L), it promotes NOR degradation and induces quinolone ring-opening reactions that are exclusively detected in the MnOx/NaClO system, enhancing the thoroughness of NOR degradation, reducing the formation of disinfection byproducts (DBPs), and mitigating water toxicity. At insufficient concentrations (<0.1 mg/L), the presence of MnOx inhibits NOR degradation and increases water toxicity. This study clarifies the dual active regulatory role of MnOx in the chlorination degradation of NOR, and this dual regulatory role is primarily dependent on free chlorine concentration (≥0.1 mg/L vs. <0.1 mg/L), which correlates with the distance from water treatment plants. From water chemistry perspective, this work provides novel insights into the transport and transformation of micropollutants in DWDS and offers a valuable theoretical basis for the targeted control of emerging contaminants in DWDS.