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.