Kinetic modelling of ozonation and photolytic ozonation of metronidazole removal from water

• HO radicals are present in all processes. • 300–400 nm MTZ photolysis generates HO but no 1 O 2 . • Quantum yields MTZ=1.2 × 10 −3 , O 3 = 0.72 mol Einstein −1 . • Kinetic models require intermediates consideration. This work addresses the ozonation, photolysis, and photolytic ozonation of metronidazole (MTZ), a widely used antibiotic frequently detected in urban wastewater. A UVA source emitting radiation between 300 and 400 nm was utilized. The presence of scavengers such as t-butanol and sodium azide confirmed the role of hydroxyl radicals in both ozonation and photolysis processes. Conversely, singlet oxygen did not influence the photolytic processes. The average quantum yields of MTZ and ozone were determined to be 1.2 × 10 −3 and 0.72 mol/Einstein, respectively, for the 300–400 nm wavelength range. In direct MTZ photolysis, a stoichiometric ratio of 0.34 mol of hydroxyl radicals formed per mol of photolyzed MTZ was observed. However, the effects of hydroxyl radicals on the MTZ photolytic rate were only significant after 45 min of reaction time. Using rate constant data from literature and quantum yields calculated in this study, a kinetic model for both MTZ ozonation and photolytic ozonation was proposed, enabling the prediction of MTZ conversion and degradation rates. This model includes three intermediate compounds that also consume ozone and hydroxyl radicals. The results show that experimental and calculated concentrations of MTZ are within an error margin of less than 14 % in all cases.