Temperature Dependence of Halogen Radical Reactivity with Dissolved Organic Matter

Reactions with dissolved organic matter (DOM) govern the sinks and concentrations of halogen radicals in aquatic environments, yet the impact of seasonal and regional temperature variations remains unclear. Here, second-order rate constants (<i>k</i>) for four halogen radicals (Cl^•, Br^•, Cl₂^•-, and Br₂^•-) reacting with six DOM isolates were determined from 9 to 39 °C. At 25 °C, <i>k</i> values (M_C^-1 s^-1) were approximately 10⁵-10⁶ for Br₂^•-, 10⁶-10⁷ for Cl₂^•-, and 10⁸ for Cl^•/Br^•. Their temperature dependence followed the Arrhenius equation, yielding apparent activation energies (<i>E</i>_a) of 5.8-34.1 kJ mol^-1. DOM reactions with Cl^• and Br^• were nearly barrierless, with average <i>E</i>_a of 9.7 (Cl^•) and 13.2 kJ mol^-1 (Br^•), while Cl₂^•- and Br₂^•- displayed stronger temperature dependence, with average <i>E</i>_a of >20 kJ mol^-1. Empirical models were developed to predict both <i>E</i>_a and the pre-exponential factor (<i>A</i>) from bulk DOM properties. Thermodynamic analysis reveals entropy-driven control of Cl^•/Br^• reactions with DOM, whereas both enthalpy and entropy shaped Cl₂^•-/Br₂^•- reactivity. Incorporating temperature dependence into kinetic modeling quantitatively explained the micropollutant degradation during advanced oxidation. This study establishes a quantitative framework linking water temperature and DOM properties with halogen radical fate in aquatic systems.