Triplet-Excited Benzoates Significantly Enhance HO • Formation and Free Chlorine Decay in a UV 222 /Chlorine Advanced Oxidation Process

Far UV light (UV 222 ) is a promising alternative to traditional 254 nm UV light (UV 254 ) in water treatment, raising great interest in UV 222 -based advanced oxidation processes, particularly UV 222 /chlorine. This study uncovers a novel mechanism for radical generation in UV 222 /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 222 /chlorine, respectively, but showed less effect on UV 254 /chlorine. This enhancement was attributable to an excited-state transformation pathway of BA in UV 222 /chlorine, contributing 64% to BA degradation due to the strong absorption of BA at 222 nm and the high photon energy of UV 222 . The triplet excited state ( 3 BA*) was a key intermediate from BA photoexcitation, followed by the generation of hydrated electrons (e aq – ) via 3 BA* photoionization. Both 3 BA* and e aq – reacted with free chlorine to form HOCl •– at determined rate constants of 1.33 × 10 9 M –1 s –1 and 1.50 × 10 10 M –1 s –1 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 222 /chlorine.