Photochemical Transformation of Free Chlorine Induced by Triplet State Dissolved Organic Matter

Photolysis of free chlorine is an increasingly recognized approach for effectively inactivating microorganisms and eliminating trace organic contaminants. However, the impact of dissolved organic matter (DOM), which is ubiquitous in engineered water systems, on free chlorine photolysis is not yet well understood. In this study, triplet state DOM (³DOM*) was found to cause the decay of free chlorine for the first time. By using laser flash photolysis, the scavenging rate constants of triplet state model photosensitizers by free chlorine at pH 7.0 were determined to be in the range of (0.26-3.33) × 10⁹ M^-1 s^-1. ³DOM*, acting as a reductant, reacted with free chlorine at an estimated reaction rate constant of 1.22(±0.22) × 10⁹ M^-1 s^-1 at pH 7.0. This study revealed an overlooked pathway of free chlorine decay during UV irradiation in the presence of DOM. Besides the DOM's light screening ability and scavenging of radicals or free chlorine, ³DOM* played an important role in the decay of free chlorine. This reaction pathway accounted for a significant proportion of the decay of free chlorine, ranging from 23 to 45%, even when DOM concentrations were below 3 mg_C L^-1 and a free chlorine dose of 70 μM was present during UV irradiation at 254 nm. The generation of HO^• and Cl^• from the oxidation of ³DOM* by free chlorine was confirmed by electron paramagnetic resonance and quantified by chemical probes. By inputting the newly observed pathway in the kinetics model, the decay of free chlorine in UV₂₅₄-irradiated DOM solution can be well predicted.