Degradation kinetics, byproducts formation and estimated toxicity of metronidazole (MNZ) during chlor(am)ination

Abstract

The residues of pharmaceuticals and personal care products (PPCPs) in environmental waters have been widespread concerned. Metronidazole (MNZ), normally employed to treat inflammation and infection, was chosen as one model PPCP. The degradation of MNZ by chlorination could be fitted by pseudo-first-order kinetics as the observed pseudo-first-order rate constants increasing from 0.0302 min^-1 to 0.2872 min^-1. However, the kinetics during chloramination of MNZ followed pseudo-second-order reaction, whose estimated half-live was approximately 6-8 times longer than chlorination. The chlor(am)ination of MNZ especially formed chloroform (CF), dicholoacetamide (DCAcAm), tricholoacetamide (TCAcAm) and dichloroacetonitrile (DCAN), and their yields were overall lower under chloramination than chlorination. During chlorination, the yield of CF was increased from 0.35 ± 0.02% to 2.06 ± 0.12% with 1-20 chlorine/MNZ molar ratio, whereas the formations of DCAcAm, TCAcAm and DCAN increased firstly and then decreased. Increasing chloramine dosage promoted the concentrations of scheduled disinfection byproducts (DBPs). CF and TCAcAm kept continuous generation in chlor(am)ination versus reaction time. Compared with the chlorination, the chloramination of MNZ was more dependent on pH value due to the self-degradation of chloramine. Faintly acidic condition favored N-DBPs' formation in MNZ when it was subjected to chlor(am)ination. The chloramination of MNZ produced cytotoxicity and genotoxicity by 10-15 folds lower than chlorination, and DCAN formed during chloramination dominated both DBPs' yields and toxicity contribution. Opposite to chlorination, the integrated toxicity of MNZ during chloramination varied linearly versus N-DBPs' yields.

Keywords: Chlor(am)Ination; Disinfection byproducts; Formation pathways; Integrated toxicity; Kinetics; Metronidazole.