title: Permanganate oxidation of diclofenac: The pH-dependent reaction kinetics and a ring-opening mechanism.
abstract: In this work, the fate of diclofenac (DCF) during permanganate (Mn(VII)) oxidation was investigated at environmentally relevant pH conditions (from 5 to 9). The batch experiments showed that the kinetics of the Mn(VII)/DCF reaction follows a second-order rate law with an apparent rate constant of 1.57±0.02 M(-1) s(-1) at pH 7 and 20 °C. The half-value of DCF was calculated to be 37.5 min, when the concentration of Mn(VII) (0.4 mM) was 20-fold excess of DCF. The pH-dependence of the reaction kinetics was investigated, and the DCF reactivity with Mn(VII) was found to decrease with increasing pH. The second-order rate constants were then quantitatively described by incorporating the species distribution of DCF. A lower reactivity of the anionic DCF (DCF(-)) in comparison with its neutral counterpart (DCF(0)) was most likely attributable to the interaction between the ionized carboxylate group and amine nitrogen position, which can reduce the nucleophilicity of amine nitrogen by inductive and resonance effects. Moreover, a range of degradation products and the corresponding structures were proposed on the basis of the LC-Q-TOF-MS analysis. A detailed ring-opening reaction mechanism was proposed as follows: Mn(VII) acts as an electrophile to attack the amine moiety, leading to the formation of the primary intermediate products 2,6-dichloroaniline and 5-hydroxy-diclofenac, which can be further transformed. The further degradation proceeded through a multistep process including ring-opening, decarboxylation, hydroxylation, and cyclation reactions.
authors.affiliation: Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100039, China. Electronic address: owenchy@163.com.
authors.forename: Hanyang
authors.initials: H
authors.lastname: Cheng
authors.affiliation: Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; University of Chinese Academy of Sciences, Beijing 100039, China. Electronic address: songdean07@mails.ucas.ac.cn.
authors.forename: Dean
authors.initials: D
authors.lastname: Song
authors.affiliation: Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. Electronic address: hjliu@rcees.ac.cn.
authors.forename: Huijuan
authors.initials: H
authors.lastname: Liu
authors.affiliation: Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. Electronic address: jhqu@rcees.ac.cn.
authors.forename: Jiuhui
authors.initials: J
authors.lastname: Qu
mesh_terms: D002853:Chromatography, Liquid; D004008:Diclofenac / Q000031:analogs & derivatives* / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D017895:Manganese Compounds / Q000737:chemistry*; D013058:Mass Spectrometry; D008956:Models, Chemical; D009584:Nitrogen; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D014865:Waste Disposal, Fluid / Q000379:methods*; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D017895:Manganese Compounds; D010087:Oxides; D014874:Water Pollutants, Chemical; C048856:permanganic acid; D004008:Diclofenac; C117715:5-hydroxydiclofenac; D009584:Nitrogen
keywords: Diclofenac; Permanganate; Ring-opening mechanism; pH-dependent kinetics