Mechanistic insights into paracetamol transformation in UV/NH2 Cl process: Experimental and theoretical study

The UV/monochloramine (NH2Cl) 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/NH2Cl process. The degradation of AAP in UV/NH2Cl process accords with the pseudo first-order kinetics. Impact factors including NH2 Cl dose, pH, natural organic matter, HCO3-, and NO3- were evaluated. The reaction mechanisms of AAP with hydroxyl radical (HO center dot), reactive chlorine species (RCS), and reactive nitrogen species (RNS) were discussed in detail. Specifically, HO center dot attacked AAP mainly through hydrogen atom transfer (HAT) and radical adduct formation (RAF), while Cl-2(center dot-) play a certain role through single electron transfer (SET). (NH2)-N-center dot and Cl-center dot destructed AAP mainly through HAT. Based on the mechanism analysis, the second-order rate constants of AAP reacts with HO center dot, Cl-center dot, (NH2)-N-center dot, ClO center dot, Cl2(center dot-) and (NO2)-N-center dot were calculated through transition state theory as 2.66x10(9) M-1 s(-1), 2.61x10(9) M-1 s(-1), 1.02x10(7) M-1 s(-1), 7.74x10(6) M-1 s(-1), 1.32x10(6) M-1 s(-1), 1.48x10(3) M-1 s(-1) 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. (C) 2021 Elsevier Ltd. All rights reserved.