This study demonstrates UV222 significantly improves HO• production and chlorine decay in benzoate-treated water, indicating a novel oxidation mechanism.
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 (eaq-) via 3BA* photoionization. Both 3BA* and eaq- 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.
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Guo et al. (2025) studied this question.
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