Kinetics, radical chemistry and machine-learning-optimized abatement of capecitabine by UV/peroxymonosulfate: simultaneous micropollutant removal and attenuation of disinfection by-products formation potential.

As a widely used anticancer prodrug, capecitabine (CAPT) is poorly removed by conventional treatment and may generate toxic transformation products and disinfection by-products (DBPs). This study systematically evaluated CAPT abatement by UV/peroxymonosulfate (UV/PMS), integrating reaction kinetics, energy analysis, reactive species quantification, machine learning, transformation products identification and subsequent disinfection assessment. UV/PMS reduced CAPT below the quantification limit within 4 min, with the pseudo-first-order rate constant (kobs) of 0.7854 min-1 and the lowest electrical energy per order (EE/O) among the tested processes. SO4•- dominated CAPT oxidation, contributing 65.20% of the overall degradation. HO• contributed 23.92%, while direct UV photolysis and PMS oxidation accounted for 9.94% and 0.94%. The contribution of singlet oxygen was negligible. The second-order rate constants of CAPT with SO4•- and HO• were 4.02 × 109 and 7.48 × 109 M-1 s-1, and UV/PMS exhibited strong process synergy effect. The PMS dose of approximately 200 μM under mildly alkaline conditions provided a favorable balance between degradation kinetics and energy consumption, whereas natural organic matter exerted the strongest matrix inhibition. XGBoost accurately predicted kobs and EE/O, and Pareto optimization identified a balanced operating point with kobs≈5.5 min-1 and EE/O≈1.88 kWh m-3 order-1. Transformation products were tentatively identified and evaluated using DFT calculations, Fukui functions, frontier molecular orbital analysis and elemental ratio analysis. The results suggested that oxidation likely involved the fluoropyrimidine moiety and the aliphatic side chain, followed by hydroxylation, defluorination, deamination, hydrolysis and C-N bond cleavage. UV photodegradation alone increased the halogen reactivity of residual precursors, whereas UV/PMS minimized both total and toxicity-weighted levels of the four targeted DBPs during chlorination and chloramination. In conclusion, UV/PMS combined rapid, energy-efficient CAPT abatement with reduced DBPs formation and the related-toxicity burden.