Electrochemical oxidation of acid orange 74 using Ru, IrO2, PbO2, and boron doped diamond anodes: Direct and indirect oxidation

Abstract

• The BDD anode was most efficient in electrochemical oxidation of AO 74. • Mechanism of direct and indirect oxidation on different anodes were investigated. • Active radicals of indirect oxidation on PbO 2 and BDD anodes were examined. • The degradation pathways of AO 74 on different anodes were established. This study compares the electrochemical oxidation of acid orange 74 (AO 74), a typical metal complex azo dye, on four anode materials (Ru, IrO 2 , PbO 2 , and boron doped diamond (BDD)). The results show that the BDD electrode was quite efficient in the anodic oxidation of AO 74 with a 100% removal of AO 74 and an 84.3% removal of COD. Analyzing the active species at different electrodes, we reported that oxidation on an inert electrode primarily depends on direct oxidation and indirect oxidation of active radicals ( OH and O 2 − for the BDD anode and OH for the PbO 2 anode), whereas active electrodes (Ru and IrO 2 ) primarily depend on direct oxidation. The UV–vis spectrum demonstrated that Cr 3+ could be oxidized only on the BDD anode, proving that the BDD electrode has the highest oxidation potential. Similar organic intermediates species were observed on different electrodes by GC–MS. Furthermore, the different organic products are attributed to the difference in the oxidation ability of the electrodes. From the UV–vis spectrum and GC–MS analysis, we speculate that AO 74 was first degraded to aromatic compounds and pyrazole because of the cleavage of the azo-bond. Then it formed amides and organic acid because of the cleavage of the benzene ring, and finally CO 2 and H 2 O were formed.