A novel induced zero-valent iron electrode for in-situ slow release of Fe2+ to effectively trigger electro-Fenton oxidation under neutral pH condition: Advantages and mechanisms

Abstract In-situ generation of iron catalysts in electro-Fenton (EF) process deserves more attention like in-situ generation of H2O2. In this work, a novel induced zero-valent iron (ZVI) electrode, which was composed of a graphite plate with good conductivity, ZVI powders and a small magnet, was for the first time designed through magnetic assembly. Fe2+ could be slowly generated in-situ from induced ZVI electrode and effectively trigger EF oxidation of chlorobenzenes (CB) under neutral pH. Compared to the in-situ rapid release of Fe2+ from sacrificial anodes and traditional one-time external dosing of Fe2+ in EF, in-situ slow release of Fe2+ from induced ZVI electrode was the most effective way to activate electrogenerated H2O2 under neutral pH. Induced ZVI electrode had obvious advantages in terms of effective active areas, corrosion potentials and current responses. The optimum conditions including pH of 7.0, current of 29.9 mA, ZVI of 0.10 g and electrolysis time of 35.25 min were obtained by the response surface methodology (RSM) model with the maximum removal efficiency of 99.12%. The uniformed weak positive charge on the ZVI powders induced by the electric field was the dominant contribution to the in-situ slow release of Fe2+ from the induced electrode. And, the impact of magnetic field on the CB removal and the release of iron ions was negligible. The degradation mechanism was revealed through the Fukui function based on the density functional theory (DFT) and by-products analysis. The evolution of acute toxicity to zebrafish in EF was further clarified. Besides, this induced ZVI electrode could be easily reused and has satisfactory stability. In briefly, this work provided a novel idea for the in-situ slow release of Fe2+ in EF process.