Enhanced electro-catalytic generation of hydrogen peroxide and hydroxyl radical for degradation of phenol wastewater using MnO₂/Nano-G|Foam-Ni/Pd composite cathode

To improve the electro-catalytic degradation efficiency of the cathode in electrochemical advanced oxidation processes, MnO₂ modified Nano-graphite (MnO₂/Nano-G) and Pd loaded Foam-Ni (Foam-Ni/Pd) composites were prepared by the chemical redox and electro-deposition methods, respectively, and a two-layer type MnO₂/Nano-G|Foam-Ni/Pd composite cathode was prepared with the chitosan as a binder. The composites were characterized by X-ray diffraction, scanning electrons microscopy and X-ray photoelectron spectroscopy. Results showed that the mix-crystal structural MnO₂ (α-MnO₂ and γ-MnO₂) nanorods with length of 80–200 nm and width of 20–50 nm were uniformly loaded on the Nano-G surface, and the three-dimensional Pd° metal trees/crystals were tightly deposited on the Foam-Ni substrate. The as-prepared cathode was applied to the electro-catalytic degradation of phenol wastewater, and the reaction parameters were optimized. Compared with MnO₂/Nano-G|Foam-Ni and Nano-G|Foam-Ni cathodes, the degradation efficiency of phenol by MnO₂/Nano-G|Foam-Ni/Pd cathode was significantly improved. The removal efficiencies of phenol and total organic carbon (TOC) by MnO₂/Nano-G|Foam-Ni/Pd cathode reached 98.7% and 85.3% after 120 min electrolysis under oxygen aeration condition, respectively. The optimal reaction parameters were current density of 39 mA cm⁻², electrolyte (Na₂SO₄) concentration of 0.1 mol L⁻¹, electrode distance of 4 cm and initial pH of 7. By determining the variations of H₂O₂ and [rad]OH content in the cathode chamber, it was found that Pd metal crystals improved the reduction of O₂ to H₂O₂ and MnO₂ nanorods accelerated the dissociation of H₂O₂ to [rad]OH, producing more H₂O₂ and [rad]OH to oxidize phenol and eliminate TOC in wastewater. The results in this study provide useful information for the control of refractory organic pollutants with electrochemical advanced oxidation in wastewater treatment.