Determination of specific kinetic parameters for different ionization states of enrofloxacin during UV/chlorine treatment: A pH-independent kinetic modelling method

Specific kinetic parameters were determined for each enrofloxacin species (ENR + , ENR 0 , ENR – ) in UV/chlorine treatment. • A pH-independent kinetic modelling method was proposed to decouple the pH-dependent complexity of ENR degradation. • The specific direct photolysis rate constants were calculated to be (1.51–15.09) × 10 –3 cm 2 Ein –1 for each ENR species. • The specific second-order rate constants of ENR towards reactive free radicals were calculated to be (1.11–41.5) × 10 9 L mol –1 s –1 for each ENR species. • ClO • was identified as the dominant reactive species, contributing to over 90% of ENR removal collectively with direct photolysis. The UV/chlorine process has demonstrated superior efficacy in micropollutant degradation. However, the strong pH dependence of second-order rate constants between contaminants and reactive species poses a fundamental challenge to the accuracy and generalizability of kinetic models. In this study, a comprehensive kinetic investigation was conducted on the degradation of enrofloxacin (ENR) by UV/chlorine across its three ionic speciation states (ENR + , ENR 0 , and ENR – ). Key mechanistic insights were obtained through systematic differentiation of degradation pathways, including direct UV photolysis, oxidation by free chlorine, HO • and reactive chlorine species. The specific photolysis rate constants were calculated to be 1.51 × 10 –3 , 15.09 × 10 –3 , and 9.57 × 10 –3 cm 2 Ein –1 for ENR + , ENR 0 , and ENR – , respectively. The specific second-order rate constants for the reaction toward ClO • for ENR + , ENR 0 , and ENR – were calculated to be 5.46 × 10 9 , 1.11 × 10 9 and 4.66 × 10 9 L mol –1 s –1 , respectively. Based on species-specific kinetic parameters, a pH-independent kinetic model for ENR degradation by UV/chlorine were construct, enabling reliable prediction of degradation kinetics at any pH within the selected range. This model provides a robust framework for predicting and optimizing treatment performance, thereby offering critical guidance for the design and engineering application of UV/chlorine processes. Download: Download high-res image (291KB) Download: Download full-size image