Reactivity of Chlorine Radicals (Cl• and Cl₂•–) with Dissolved Organic Matter and the Formation of Chlorinated Byproducts

Chlorine radicals, including Cl• and Cl₂•-, can be produced in sunlight waters (rivers, oceans, and lakes) or water treatment processes (e.g., electrochemical and advanced oxidation processes). Dissolved organic matter (DOM) is a major reactant with, or a scavenger of, Cl• and Cl₂•- in water, but limited quantitative information exists regarding the influence of DOM structure on its reactivity with Cl• and Cl₂•-. This study aimed at quantifying the reaction rates and the formation of chlorinated organic byproducts produced from Cl• and Cl₂•- reactions with DOM. Laser flash photolysis experiments were conducted to quantify the second-order reaction rate constants of 19 DOM isolates with Cl• (kDOM-Cl•) and Cl₂•- (kDOM-Cl2•-), and compare those with the hydroxyl radical rate constants (kDOM-•OH). The values for kDOM-Cl• ((3.71 ± 0.34) × 10⁸ to (1.52 ± 1.56) × 10⁹ MC⁻¹ s⁻¹) were orders of magnitude greater than the kDOM-Cl2•- values ((4.60 ± 0.90) × 10⁶ to (3.57 ± 0.53) × 10⁷ MC⁻¹ s⁻¹). kDOM-Cl• negatively correlated with the weight-averaged molecular weight (MW) due to the diffusion-controlled reactions. DOM with high aromaticity and total antioxidant capacity tended to react faster with Cl₂•-. During the same experiments, we also monitored the formation of chlorinated byproducts through the evolution of total organic chlorine (TOCl) as a function of chlorine radical oxidant exposure (CT value). Maximum TOCl occurred at a CT of 4-8 × 10⁻¹² M·s for Cl• and 1.1-2.2 × 10⁻¹⁰ M·s for Cl₂•-. These results signify the importance of DOM in scavenging chlorine radicals and the potential risks of producing chlorinated byproducts of unknown toxicity.