Factor affecting the role of radicals contribution at different wavelengths, degradation pathways and toxicity during UV-LED/chlorine process

Abstract

Abstract A light-emitting diode (LED)-driven UV/chlorine process (UV-LED/chlorine) was adopted to eliminate acetaminophen (AAP) at three wavelengths (278, 308 and 365 nm). Our results showed that AAP was eliminated efficiently during UV-LED/chlorine process at three wavelengths, and the AAP degradation followed pseudo-first-order kinetics. The pseudo-first-order rate constants (Kobs) exhibited significant wavelength-dependence. Shorter wavelengths of UV-LED were demonstrated to be more effective in AAP degradation (Kobs,278 nm > Kobs,308 nm > Kobs,365 nm). The chlorination and reactive chlorine species (RCS) oxidation were ascribed for more than 94% of AAP removal and the Kobs, Kobs, chlorination and Kobs,RCS reached the maximum value at pH 7.0. The Kobs, Kobs, chlorination, Kobs, OH and Kobs, RCS increased linearly with the increase of chlorine dosage. The presence of NOM decreased the contributions of chlorination, OH and RCS oxidation. The Kobs increased slightly as the concentration of HCO3− and NO3− increased. Twelve intermediates were identified by GC/MS and LC/MS, and the major intermediates identified were 1,4-benzoquinone, phenol, 1,2,4-trihydroxybenzene, 2-hydroxy-4-(N-acetyl)-aminophenol, chloro-4-acetamidophen- ol and dichloro-4-acetamidophenol. The proposed mechanism for AAP oxidation involves chlorine substitution on the benzene ring or an attack at the ortho and para positions of the aromatic ring by OH and Cl . The degradation of AAP was not a simultaneous detoxification process and the LED 278 nm has an advantage over LED 308 nm and LED 365 nm during UV/chlorine processes in acute toxicity controlling.