Establishment of a reagent-free three-dimensional electro-Fenton system for high H2O2 production and efficient degradation of Roxarsone

Urgent attention is needed to address the potential threat of organic arsenic agents causing water pollution. A novel approach was developed for the removal of Roxarsone (ROX) employing a reagent-free three-dimensional electro-Fenton (TD-EF) system. This system utilized gas diffusion electrode (GDE) as the cathode, which consisted of ZIF-67 carbonized in a N2 atmosphere (ZIF-67-NC) and carbon black (CB) in a specific ratio (ZIF-67-NC/CB-GDE), graphite as the anode and granular activated carbon (GAC) loaded with FeSO4·7H2O (FeSO4-GAC) as the particle electrode. The characterization and electrochemical measurements indicated that ZIF-67-NC contained abundant active sites and had excellent catalytic properties for the oxygen reduction reaction (ORR). It was observed that under specific conditions of aeration, 100 mM Na2SO4 electrolyte solution, electrode distance of 4 cm and current of 200 mA, 170 mM of H2O2 could be produced in 60 min, and the removal rate of ROX could reach 98.1 % within 100 min. Reactive oxygen species (ROSs) trapping experiments confirmed that O2− and OH were the major ROSs for ROX degradation. The possible degradation pathways were further investigated, and the results were consistent with Density Functional Theory (DFT) calculations. The evolution of inorganic arsenic during the reaction was explored using liquid chromatography-atomic fluorescence coupled with atomic fluorescence (LC-AFS). These studies revealed that the toxicity of the degraded intermediates was mitigated by the degradation reaction. Furthermore, the degradation rate of ROX in the system was at least 90 % over a relatively wide pH range (3.0–8.9), and the removal efficiency of the system remained almost constant after five cycles. This work provides new ideas for the construction of a reagent-free TD-EF system offering high H2O2 yield and excellent durability for efficient degradation of organic pollutants.