Computational insights into chlorine-dioxide-mediated advanced oxidation process of halogenated phenols

The increasing presence of phenolic micro-pollutants in aquatic systems poses a significant challenge for water quality management. Among the oxidants used in advanced oxidation processes (AOPs), chlorine dioxide (ClO₂) has emerged as an effective oxidant due to its selective reactivity and reduced formation of harmful disinfection by-products (DBPs). However, the formation of free available chlorine (FAC) during oxidation remains poorly understood with respect to molecular structure and degradation pathways. In this study, density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) analyses have been employed to investigate the effect of halogen substituents on the oxidation of phenols by ClO₂ and their influence on FAC formation. Two reaction pathways (P1 and P2) were proposed, with P1 being thermodynamically favourable, while P2 is kinetically favoured for para-substituted derivatives. The position and nature of substituents were found to influence the stabilization of the transition state, particularly through hydrogen bonding in ortho- and meta-substituted systems and weaker halogen bonding in para-substituted systems. This reduced stabilization increases the activation barrier and thereby promotes an alternative pathway, leading to lower FAC yields in para-substituted compounds. Further results reveal that electron-withdrawing substituents increase activation barriers and reduce reaction rates, indicating that the transition state is stabilized by electron-rich environments. Overall, upon comparing the theoretical and experimental results, it is evident that substituent position governs pathway selection, which in turn influences both reaction kinetics and FAC formation. These findings provide a mechanistic framework for predicting by-product formation in ClO₂-mediated oxidation processes.