Mechanistic shift of Chlorine/Trimethylamine system by halogenated phenolics: beyond electrophilic substitution of
Kaiting Zhang1, Zhantu Ye1, Linke Jiang1
1Fujian Key Laboratory of Coastal Pollution Prevention and Control, College of the Environment & Ecology, Xiamen University, Xiamen, 361102, China.
Abstract:
Chlorine, widely used for water decontamination, readily reacts with various water components such as tertiary amines to form a reactive quaternary ammonium intermediate (R3N+-Cl). The species are known to enhance the degradation of electron-rich organic contaminants via electrophilic substitution. However, the transformation mechanism by R3N+-Cl for toxic halogenated phenolics (HPs) in natural water remains unexplored. This study reveals a substrate-dependent shift in mechanism in the chlorine/trimethylamine (TMA) system. Beyond electrophilic substitution by R3N+-Cl alone, Cl2 and Cl2O may potentially serve as auxiliary oxidants in HPs degradation. This variation was evidenced by anomalous degradation kinetics for 32 HPs compared to classical electrophilic patterns, especially the slow removal of para-halogenated phenol. The different transformation products (e.g., hydroxyl substitution for Cl), which are predicted to have lower ecotoxicity, were also identified in the chlorine/TMA system compared with chlorination. Kinetic modeling and validation experiments (including Cl- addition, reaction-order determination, and pH-dependent experiments) indicated the primary reactive species of R3N+-Cl and the auxiliary contributors of Cl2 and Cl2O in degradation. Furthermore, the QSAR model provided theoretical support, revealing a positive correlation with local electrophilic softness (s+) and a negative correlation with pKa, contradicting the single electrophilic substitution pathway. This work provides new insights into the reaction mechanism of the chlorine/TMA system in HP-containing water matrices and may inform more selective oxidation strategies in water treatment.
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