A Theoretical Study on the Degradation Mechanism, Kinetics, and Ecotoxicity of Metronidazole (MNZ) in •OH- and SO4•−-Assisted Advanced Oxidation Processes

Metronidazole (MNZ), a typical example of nitroimidazole antibiotics, is widely used in the treatment of infectious diseases caused by anaerobic bacteria. The degradation mechanism and kinetics of MNZ in the presence of HO• and SO4•− were studied using density functional theory (DFT). It was confirmed that both HO• and SO4•− easily added to the carbon atom bonded to the NO2 group in the MNZ molecule as the most feasible reaction channel. This study shows that subsequent reactions of the most important product (M-P) include the O2 addition, hydrogen abstraction and bond breakage mechanisms. The rate constants of HO• and SO4•−-initiated MNZ in the aqueous phase were calculated in the temperature range of 278–318 K. The total rate constants of MNZ with HO• and SO4•− were determined to be 8.52 × 109 and 1.69 × 109 M−1s−1 at 298 K, which were consistent with experimental values of (3.54 ± 0.42) × 109 and (2.74 ± 0.13) × 109 M−1s−1, respectively. The toxicity of MNZ and its degradation products to aquatic organisms has been predicted. The results proposed that the toxicity of the initial degradation product (M-P) was higher than that of MNZ. However, further degradation products of MNZ induced by HO• were not harmful to three aquatic organisms (fish, daphnia, and green algae). This study provides a comprehensive theoretical basis for understanding the degradation behavior of MNZ.