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

This study compares the electrochemical oxidation of acid orange 74 (AO 74), a typical metal complex azo dye, on four anode materials (Ru, IrO2, PbO2, 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 (center dot OH and O-center dot(2)- for the BDD anode and center dot OH for the PbO2 anode), whereas active electrodes (Ru and IrO2) primarily depend on direct oxidation. The UV-vis spectrum demonstrated that Cr3+ 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 CO2 and H2O were formed.