Triplet-Excited Benzoates Significantly Enhance HO Formation and Free Chlorine Decay in a UV/Chlorine Advanced Oxidation Process.

Far UV light (UV) is a promising alternative to traditional 254 nm UV light (UV) in water treatment, raising great interest in UV-based advanced oxidation processes, particularly UV/chlorine. This study uncovers a novel mechanism for radical generation in UV/chlorine mediated by benzoate (BA), a model compound for benzoate pollutants and moieties of natural organic matter. 10 μM BA significantly enhanced HO formation and free chlorine decay by 77% and 530% in UV/chlorine, respectively, but showed less effect on UV/chlorine. This enhancement was attributable to an excited-state transformation pathway of BA in UV/chlorine, contributing 64% to BA degradation due to the strong absorption of BA at 222 nm and the high photon energy of UV. The triplet excited state (BA*) was a key intermediate from BA photoexcitation, followed by the generation of hydrated electrons (e) via BA* photoionization. Both BA* and e reacted with free chlorine to form HOCl at determined rate constants of 1.33 × 10 M s and 1.50 × 10 M s at pH 7, respectively, and then HOCl rapidly decomposed to HO. Finally, monohydroxylated and dihydroxylated benzoates were produced via the hydrolysis of BA intermediates. This study highlights the critical role of excited benzoates in radical formation and discloses a new abatement mechanism of benzoates in UV/chlorine.