Synergistic mechanism and toxicity reduction potential in medium-pressure UV/chlorine/TiO2 system for metronidazole removal

This study systematically investigated the degradation of metronidazole (MNZ) using a medium-pressure UV (MPUV)/chlorine/TiO 2 process. Under the optimal concentrations of 150 μM chlorine and 100 mg/L TiO 2 , the MNZ degradation efficiency reached 90.7 % within 6 min with a pseudo-first-order rate constant of 0.3661 min −1 , along with a synergistic factor of 1.99. The contribution rate of •OH ranges from 54.6 % to 60.0 % under various chlorine concentrations, indicating its primary role in the degradation of MNZ. Additionally, h vb + and O 2 ●– also contributed due to their promoting effect on •OH formation. Typically, acidic conditions favored MNZ degradation, with Cl − having a minor effect, NO 3 − showing a slight promotional effect, and HCO 3 − and HA inhibiting MNZ degradation. The potential degradation pathways of MNZ were proposed based on LC/TOF/MS analysis. Compared to MPUV/chlorine, MPUV/chlorine/TiO 2 was superior in controlling the formation of disinfection by-products. Acute toxic changes in Escherichia coli , in conjunction with Toxicity Estimation Software Tool, revealed that the bioconcentration factor, mutagenicity and acute toxicity of most intermediate products were lower than that of MNZ, although some intermediates showed an elevated risk for developmental toxicity. These findings indicated that the MPUV/chlorine/TiO 2 process is an efficient and safe treatment method for MNZ-contaminated water remediation.