Generation of active Co(III) and peroxodiphosphate by synergistic electrocatalytic system with phosphate and the mediator cobalt(II) and its degradation performance

The promising synergistic electrocatalytic system of phosphate (PO 4 3− ) with the mediator cobalt(II) (for short E-Co(II)-PO 4 3− ) was employed to degrade cationic dye methylene blue (MB). The exploration in the electrocatalytic process revealed that the main intermediate active oxidation products were Co(III), accompanied with hydroxyl radicals and peroxodiphosphates (P 2 O 8 4− ). Their synergistic electrocatalytic degradation rate to MB and total organic carbon (TOC) was up to 100 and 60% in 40 min, respectively, which was 5 times and 2.6 times that in a direct electrocatalytic system, correspondingly. The degradation process of the E-Co(II)-PO 4 3− system on MB started with the bond being broken at the N-C junction of the MB molecule and intermediate active oxidation substances being generated, such as phenothiazine, 2-amino-5-(N-methylformamide) benzene sulfonic acid and N 1 ,N 1 -dimethyl-1,4 diaminobenzene. Then, the intermediates were degraded into aniline, phenol and benzene sulfonic acid, and eventually decomposed into inorganic substances like CO 2 and water. The electrocatalytic degradation mechanism of E-Co(II)-PO 4 3− system on MB was the combination of indirect oxidation of the intermediate oxidants like Co(III), P 2 O 8 4− and the hydroxyl radical with direct electrocatalysis on the platinum titanium electrode, where the electrocatalytic oxidation of Co(III) was dominant. Active Co(III) and peroxodiphosphate generated in a novel electrocatalytic system have synergistic electrocatalytic effect. MB removal rate reached 100% in 40 min in the novel electrocatalytic system with phosphate and the mediator cobalt(II). Co(III) is the dominant active oxidation substance in MB oxidation and mineralization.