Mechanistic insights into paracetamol transformation in UV/NH2Cl process: Experimental and theoretical study

Abstract

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 NH2Cl dose, pH, natural organic matter, HCO3−, and NO3− 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 Cl2·− play a certain role through single electron transfer (SET). ·NH2 and Cl· destructed AAP mainly through HAT. Based on the mechanism analysis, the second-order rate constants of AAP reacts with HO·, Cl·, ·NH2, ClO·, Cl2·− and ·NO2 were calculated through transition state theory as 2.66×109 M−1 s−1, 2.61×109 M−1 s−1, 1.02×107 M−1 s−1, 7.74×106 M−1 s−1, 1.32×106 M−1 s−1, 1.48×103 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.