Oxidation of diclofenac by permanganate: Kinetics, products and effect of inorganic reductants

Abstract

The large consumption and discharge of diclofenac (DCF) lead to its frequent detection in surface water and groundwater, posing great threats to humans and ecosystems. This study explored the oxidation kinetics of DCF by permanganate (Mn(VII)), and expounded the underlying reason for the unusual pH-dependency that was unclear in previous studies. The kinetics of DCF analogues (i.e., aromatic secondary amines) by Mn(VII) oxidation were comparatively investigated. Then, a tentative kinetic model involving the formation of an intermediate between Mn(VII) and DCF or its analogues was proposed to fit the pH-rate profile. Since DCF contained two chloro groups, and a carboxyl group which could be ionized by negative electrospray ionization, a precursor ionization scanning approach was used for the first time for detection of N-containing chlorinated oxidation products. New degradation pathways of DCF containing ring opening, carboxylation, carbonylation, electrophilic addition, hydroxylation and dehydrogenation were proposed based on the identified oxidation products. Moreover, it was demonstrated that the introduction of various reducing agents such as Mn(II), Fe(II) and bisulfite significantly improved the oxidation kinetics of DCF by Mn(VII). The positive effects of Mn(II) and Fe(II) were mainly attributed to the accelerated formation of MnO2 that acted as a catalyst or co-oxidizer contributing to DCF degradation. The presence of bisulfite caused two-stage kinetics, where a sharp drop of DCF concentration followed by a slowdown of DCF removal. In the first stage, potent reactive manganese species (e.g., Mn(III), Mn(V), and Mn(VI)) and sulfate radical were generated during reaction of bisulfite with Mn(VII), whereas bisulfite was depleted fast due to excess Mn(VII) concentrations and the system became the Mn(VII)/MnO2 system in the second stage. These results provide new insight into reaction mechanism of DCF with Mn(VII) as well as propose a feasible strategy for enhancing the treatment of DCF contaminated water by Mn(VII).