Kinetic and Mechanistic Considerations of the Photosensitized Transformation of Chlorine in Chromophoric Dissolved Organic Matter Solutions under Simulated Solar Irradiation.

Chlorination is one of the most common disinfection methods for water treatments. Although the direct photolysis of free available chlorine (FAC) induced by solar irradiation has been extensively investigated, the photosensitized transformation of FAC caused by chromophoric dissolved organic matter (CDOM) has not previously been examined. Our results suggest that the photosensitized transformation of FAC can occur in sunlit CDOM-enriched solutions. Interestingly, the photosensitized decay of FAC can be fitted using a combined zero- and first-order kinetic model. The photogenerated O 2 •- from CDOM contributes to the zero-order kinetic component. The reductive triplet CDOM ( 3 CDOM*) contributes to the pseudo-first-order decay kinetic component. The bimolecular reaction rate constants of the model triplet (3-methoxyacetophenone) with HOCl and OCl - were (3.6 ± 0.2) × 10 9 and (2.7 ± 0.3) × 10 9 M -1 s -1 , respectively. Under simulated solar irradiation, the quantum yield coefficient of the reductive 3 CDOM* toward FAC attenuation ( f FAC = 840 ± 40 M -1 ) was 13 times greater than that of the oxidative 3 CDOM* toward trimethylphenol (TMP) attenuation ( f TMP = 64 ± 4 M -1 ). This study provides new insights into the photochemical transformation of FAC in sunlit surface waters, and the results are applicable when sunlight/FAC system is employed as an advanced oxidation process.