Temperature Dependence of Halogen Radical Reactivity with Dissolved Organic Matter

Area of Science:

• Environmental Chemistry
• Aquatic Chemistry
• Chemical Kinetics

Background:

• Dissolved organic matter (DOM) reactions control halogen radical sinks in aquatic environments.
• The influence of seasonal and regional temperature variations on these reactions is poorly understood.
• Halogen radicals (Cl•, Br•, Cl2•-, Br2•-) are important reactive species in water treatment and natural systems.

Purpose of the Study:

• To quantify the temperature dependence of reactions between halogen radicals and DOM.
• To develop models predicting reaction rates and activation energies based on DOM properties.
• To integrate temperature effects into kinetic models for improved micropollutant degradation prediction.

Main Methods:

• Second-order rate constants (k) were measured for four halogen radicals reacting with six DOM isolates across a temperature range of 9-39 °C.
• Arrhenius equation was used to determine apparent activation energies (Ea).
• Empirical models were developed relating Ea and pre-exponential factors (A) to bulk DOM properties.

Main Results:

• Rate constants varied significantly with radical type and temperature, ranging from 10^5 to 10^8 M^-1 s^-1 at 25 °C.
• Apparent activation energies (Ea) ranged from 5.8 to 34.1 kJ mol^-1, indicating varying temperature sensitivity.
• Cl• and Br• reactions were nearly barrierless (average Ea ~10-13 kJ mol^-1), while Cl2•- and Br2•- showed stronger temperature dependence (average Ea >20 kJ mol^-1).

Conclusions:

• Temperature significantly modulates halogen radical reactivity with DOM, impacting their environmental fate.
• Developed models provide a quantitative link between water temperature, DOM characteristics, and radical kinetics.
• Incorporating temperature dependence enhances the accuracy of kinetic models for predicting micropollutant degradation in advanced oxidation processes.