Triplet-Excited Benzoates Significantly Enhance HO^• Formation and Free Chlorine Decay in a UV₂₂₂/Chlorine Advanced...

Far UV light (UV₂₂₂) is a promising alternative to traditional 254 nm UV light (UV₂₅₄) in water treatment, raising great interest in UV₂₂₂-based advanced oxidation processes, particularly UV₂₂₂/chlorine. This study uncovers a novel mechanism for radical generation in UV₂₂₂/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 UV₂₂₂/chlorine, respectively, but showed less effect on UV₂₅₄/chlorine. This enhancement was attributable to an excited-state transformation pathway of BA in UV₂₂₂/chlorine, contributing 64% to BA degradation due to the strong absorption of BA at 222 nm and the high photon energy of UV₂₂₂. The triplet excited state (³BA*) was a key intermediate from BA photoexcitation, followed by the generation of hydrated electrons (e_aq^-) via ³BA* photoionization. Both ³BA* and e_aq^- reacted with free chlorine to form HOCl^•- at determined rate constants of 1.33 × 10⁹ M^-1 s^-1 and 1.50 × 10¹⁰ 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 UV₂₂₂/chlorine.