Reevaluation of radical-induced differentiation in UV-based advanced oxidation processes (UV/hydrogen peroxide, UV/peroxydisulfate, and UV/chlorine) for metronidazole removal: Kinetics, mechanism, toxicity variation, and DFT studies

Abstract

Metronidazole (MNZ) is widely employed as an antibiotic, but it is recalcitrant in the water environment. The residual of MNZ has sparked significant concern because to its detrimental impacts on environmental safety and human health. In this study, the elimination of metronidazole (MNZ) was comparatively examined by UV/hydrogen peroxide (UV/H2O2), UV/peroxydisulfate (UV/PDS), and UV/chlorine processes. The best degradation rate constant was demonstrated by the UV/PDS (2.4 × 10−3 s−1), followed by UV/H2O2 AOP (1.0 × 10−3 s−1) and UV/chlorine AOP (3.7 × 10−4 s−1). The hydroxyl radicals (HO) played significant roles in UV-AOPs eliminating MNZ by 87.7 %, 30.2 %, and 43.0 % in UV/H2O2, UV/PDS, and UV/chlorine treatments, respectively. Additionally, the sulfate radical (SO4−) and chlorine radical (Cl) were essential for the elimination of MNZ, contributing 63.0 % and 36.2 %, respectively, in UV/PDS and UV/chlorine. The first-order rate constants (kobs) increase linearly with the increasing oxidant dosage in various UV-AOPs. The higher degradation efficiency of MNZ was observed in acid conditions due to the transition of oxygen active species. The presence of chloride (Cl−), bicarbonate (HCO3–), nitrate (NO3–), and humid acid (HA) remarkably inhabited the degradation of MNZ in UV/H2O2 and UV/PDS, whereas NO3– showed a promotion effect in UV/chlorine. In diverse water matrixes, UV/PDS usually consumed less electrical energy than UV/H2O2 and UV/chlorine. Combining density functional theory calculations and transformation products identification, it was revealed that HO was conducive to attack the nitrogenous heterocycle of MNZ via hydrogen atom transfer (HAT) and radical adduct formation (RAF) pathways, whereas SO4− and Cl oxidation favored RAF route. Some chlorine- and hydroxylation-intermediates, notably those produced in UV/chlorine AOP, displayed higher toxicity than the parent compounds. Overall, the findings unequivocally prove that UV-AOPs are effective, economical, and practical for the remediation of antibiotics-contaminated water.