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

Abstract

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 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 OH, producing more H2O2 and 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.