Electrochemical removal of thiamethoxam using three-dimensional porous PbO2-CeO2 composite electrode: Electrode characterization, operational parameters optimization and degradation pathways

Abstract

Highly performed electrode is one of the key factors affecting the efficiency of electrochemical oxidation toward organic pollutants. In the present work, three-dimensional porous PbO2-CeO2 composite electrodes (3D/PbO2-CeO2) were fabricated by composite electrodeposition method using oxygen bubbles as template, to eliminate neonicotinoid thiamethoxam in aqueous solution. The surface morphology, structure, and composition of the 3D/PbO2-CeO2 electrodes were well characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Then the influence of the applied current density, the initial concentration of thiamethoxam, the concentration of supporting electrolyte, and initial pH value on the thiamethoxam removal ratio were also optimized. Amount to twenty deductive intermediates of thiamethoxam degradation were identified by HPLC-MS analysis, and dechlorination, hydrogen abstraction, denitration, and hydroxylation were observed in electrochemical oxidation process of thiamethoxam. Based on these identified byproducts, a hypothetical degradation mechanism has been proposed, which was divided into two parallel pathways according to the different initial oxidation position on thiamethoxam, such as chlorine atom on thiazole ring and nitro group on 1,3,5-oxadiazinane ring. All the intermediates were ultimately mineralized into H2O and CO2. The electrochemical oxidation based on 3D/PbO2-CeO2 electrodes in this work is a promising approach to effective removal of thiamethoxam.