Mechanistic insights into paracetamol transformation in UV/NHCl process: Experimental and theoretical study.

The UV/monochloramine (NHCl) process is an advanced oxidation process that can effectively remove emerging contaminants (ECs). However, the degradation mechanisms of reactive radicals with ECs are not clear. In this work, we combined theoretical calculations with experimental studies to investigate the kinetics and mechanism of radical-mediated degradation of paracetamol (AAP) in UV/NHCl process. The degradation of AAP in UV/NHCl process accords with the pseudo first-order kinetics. Impact factors including NHCl dose, pH, natural organic matter, HCO, and NO were evaluated. The reaction mechanisms of AAP with hydroxyl radical (HO), reactive chlorine species (RCS), and reactive nitrogen species (RNS) were discussed in detail. Specifically, HO attacked AAP mainly through hydrogen atom transfer (HAT) and radical adduct formation (RAF), while Cl play a certain role through single electron transfer (SET). NH and Cl destructed AAP mainly through HAT. Based on the mechanism analysis, the second-order rate constants of AAP reacts with HO, Cl, NH, ClO, Cl and NO were calculated through transition state theory as 2.66×10 M s, 2.61×10 M s, 1.02×10 M s, 7.74×10 M s, 1.32×10 M s, 1.48×10 M s respectively. The second-order rate constants were then used to distinguish the contribution of radicals to the degradation of AAP. Thirteen transformation products were identified by high-resolution mass spectrometry. Combined active sites with potential energy surface, the detailed reaction pathways were proposed. Overall, this study provides deep insights into the mechanism of radical-mediated degradation of AAP.