Differences in degradation and transformation product profile during UV/chlorine at 254, 275 and 295 nm

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This study compares the degradation rates, mechanistic pathways and transformation product (TP) profiles during UV/chlorine at 254, 275 and 295 nm and benchmarked results against UV/H 2 O 2 at 254 nm. Fluence-based degradation rates of pesticides metazachlor, propyzamide, quinmerac, flufenacet and clopyralid were compared under all wavelengths. An analysis of the electrical energy per order (EEO) showed that from an energy perspective UV/chlorine at pH 6 and 295 nm was more efficient than UV/chlorine at 254 nm, despite lower kinetic rates. Metazachlor and quinmerac were selected to investigate the influence of the wavelength on degradation mechanisms and TP fingerprint, with experimentally determined second order k OH of 8.05×10 9 M −1 s −1 and 5.45×10 9 M −1 s −1 , respectively. TP fingerprint and atomic ratio analysis identified 12 metazachlor and 8 quinmerac TPs. Results showed that UV/chlorine at 254 nm promoted analogous TP profile as UV/H 2 O 2 , whereas higher wavelengths favour selective RCS chemistry. Namely, at 275 nm degradation shifted toward larger oxygenated and unsaturated TPs. Results indicated that the wavelength influenced not only halogenation but also nitrogen incorporation pathways. This study indicated that wavelength fundamentally impacts TP profiles during UV/chlorine treatment, not merely degradation kinetics, providing a mechanistic insight into wavelength-dependent by-product formation.