Oxidation Kinetics And Mechanism Of Typical Secondary Aromatic Amines By Permanganate In Water

In recent years, diclofenac （DCF） and diphenylamine （DPA） as therepresentative of secondary aromatic amines are frequently detected in surface water,groundwater and sewage over many countries. DCF and DPA have strong biologicaltoxicity, and after disinfection they are easily converted into nitrogenous byproductswith strong carcinogenic. Chemical oxidation processes and advanced oxidationprocesses can effectively degrade such secondary aromatic amines, including ozone,chlorine dioxide, hydrogen peroxide, ferrate and photocatalytic oxidation, etc.However, little attention has been paid to permanganate （KMnO₄）, which has arelatively strong oxidizing ability and is not easy to produce toxic byproducts.KMnO₄also has the characteristics of ease of handling, cost-effective, andconvenience in storage and transportation. So, the oxidation kinetics and mechanismof DCF and DPA by permanganate were investigated in this study, with the focus onthe effects of the common cation ions and ligands.Permanganate exhibited appreciable reactivity to DCF and DPA, as well astheir structural models （i.e.,2,6-Dichlorodiphenylamine and N-phenylanthranilieacid）. The reactions followed a generalized second-order rate law. The second-orderrate constants k2for them were1.5,4655.2M-1s-1,30.4M-1s-1,37.0M-1s-1at T=250.2℃and pH=7.0, respectively. The degradation rate of DPA by permanganate wassignificantly higher than others. The electron withdrawing effect of the chlorineatom and the carboxy substituent reduce the electron density on the phenyl ring,leading to the oxidation of organic compounds by permanganate difficultly.Moreover, the oxidation of all selected secondary aromatic amines displayedautocatalysis, suggesting the catalytic role of in situ formed manganese oxides（MnOx）.The presence of common cation ions Ca²⁺, Mg²⁺ and ligands （pyrophosphoricacid, nitrilotriacetic acid, ethylenediaminetetraacetic acid） can significantly inhibitthe oxidation of secondary aromatic amines by permanganate. Ca²⁺and Mg²⁺canmake MnOx cohesion in the reaction system, weakening the adsorption ability ofMnOx to organic compounds and therefore inhibit the efficiency of oxidativedegradation of organic compounds by permanganate. Ligands can ligand withintermediate valence Mn species in situ formed and therefore inhibit intermediatevalence Mn converted into manganese oxides, thus ligands also showed inhibition.The effect of inhibition was influenced by pH in the reaction solution, type andconcentration of the ligands, etc. Seven major products of DCF were found in negative ESI source byLC-MS/MS, whereas2,6-dichloroaniline was found as the major product in positivemode ESI source. There are many differences between these8oxidation products ofDCF by permanganate and the products from several other oxidation means, but2,6-dichloroaniline is also frequently detected when oxidize DCF by ozonation. So,oxidation process and mechanism of DCF by permanganate is different via fromozonation, photo-Fenton, chlorination, etc.