Comparison of acetaminophen degradation in UV-LED-based advance oxidation processes: Reaction kinetics, radicals contribution, degradation pathways and acute toxicity assessment.

Ultraviolet light emitting diode (UV-LED)-based advanced oxidation processes (AOPs) including UV-LED/chloramine (UV-LED/NHCl), UV-LED/hydrogen peroxide (UV-LED/HO) and UV-LED/persulfate (UV-LED/PS), were adopted for acetaminophen (AAP) removal. Results showed that AAP could be effectively degraded by the hybrid processes compared to solely using with UV irradiation and oxidants. The AAP degradation in the three UV-LED-based AOPs were in the order of UV-LED/PS > UV-LED/HO > UV-LED/NHCl and followed a pseudo-first-order kinetics. The degradation rate constant (k) increased with increasing oxidant dosage, whereas overdosing lowered the AAP degradation. The second-order rate constants of HO, SO, and Cl with AAP were calculated as 5.15 × 10, 7.66 × 10 and 1.08 × 10 M s, respectively. Under neutral conditions, the contributions of UV-LED, HO, and Cl to AAP degradation were 4.21%, 60.15% and 35.64% in the UV-LED/NHCl system, whereas the respective contributions of UV-LED, HO and SO to AAP degradation were 2.09%, 22.84% and 75.07% in UV-LED/PS system, respectively. Meanwhile, the corresponding contributions of the involved reactive species were found to be pH-dependence. The natural organic materials (NOM) inhibited the AAP degradation, and the presence of Cl, HCO, and NO had different effects on AAP degradation in the three hybrid processes. The AAP degradation was significantly inhibited in the three UV-LED-based AOPs in real water. In addition, the intermediate products were also identified, and possible degradation pathways were proposed in the three UV-LED-based AOPs. The acute toxicity bioassay using bacterium Vibrio fischeri suggested that the UV-LED/PS process was more effective than the UV-LED/HO and UV-LED/NHCl processes in reducing the acute toxicity of the reacted AAP solution. Among the three UV-LED-based AOPs, the UV-LED/PS was found to be the most efficient process for AAP degradation.