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This work utilizes nitrogen-doped porous carbon (NPC) and carbon black (CB) to fabricate a novel rolled carbonaceous gas-diffusion cathode by polytetrafluoroethylene (PTFE) as adhesive, which greatly improves the production of H2O2 to 7.27 mg h−1 cm−2 by optimization in near neutral pH condition. Further application of NPC/CB cathode in heterogeneous electro-Fenton (EF) experiments with synthetic pyrite (FeS2) as catalyst, metal sulfides (MoS2, WS2) as co-catalysts, the removal efficiency of the tetracycline (50 mg L−1) could reach 100% and the total organic carbon (TOC) removal efficiency of counterpart could reach 81.3%, respectively. Because of the FeS2 produces H+ and Fe2+ in the presence of H2O2, which provides pH value and catalytic ion that suitable for EF. Meanwhile, the metal sulfides can accelerate the conversion of Fe3+ to Fe2+ by exposing reducible metal active sites on the surface of the catalysts, thus drastically increased the efficiency of H2O2 decomposition and pollutant degradation, also optimized the disadvantage of little Fe2+ leaching from FeS2 in heterogeneous EF. Further tests of this system indicated its potential for the treatment of tannery effluents. It provided an effective and feasible scheme for promoting the improvement of the heterogeneous EF system and the development of practical application.

In this study, we demonstrated that hydrothermal synthesized FeS2 was highly efficient to catalyze the H2O2 decomposition for diclofenac sodium (DCF) degradation in a wide range of initial pH (3–9) by heterogeneous electro-Fenton (EF) process, and this “Pyrite-EF” showed a better performance for the mineralization of DCF in comparison with the classic EF process. Effect of pyrite content on the hydroxyl radicals generation and iron dissolution produced by the decomposition of hydrogen peroxide (H2O2), and applied current density on the degradation kinetics and mineralization efficiency were studied. Moreover, toxicity assessment by means of microtox method showed the solution toxicity was removed after treatment. Experimental results revealed that the enhancement of DCF degradation rate in the Pyrite-EF process was attributed to the molecular oxygen activation induced by more surface bound ferrous ions on FeS2, generating superoxide anions to accelerate the Fe(II)/Fe(III) cycle on the FeS2 surface, which favored the H2O2 decomposition to generate more hydroxyl radicals for the DCF degradation via a heterogeneous EF-like process. These findings could provide some new insights into the molecular oxygen activation induced by FeS2 and the subsequent heterogeneous Pyrite-EF degradation of organic pollutants from wastewater.

This study has investigated a novel jet-type reactor (vertical flow reactor) in order to improve the oxygen utilization for enhanced production of hydrogen peroxide (H2O2). In vertical flow electro-Fenton (EF) system, the effects of current density, water flow rate, Fe2+ concentration and initial pH were investigated for the removal of tetracycline hydrochloride. The efficient removal of tetracycline hydrochloride could reach 100% within 60 min under the current density of 24 mA cm−2, water flow rate of 10 L h−1 and pH of 3. The vertical flow reactor was relatively stable and reusable for the removal of pollutants. The removal rates of tetracycline hydrochloride and total organic carbon (TOC) were maintained at about 95% and 80%, respectively in a 5-time continuous runs. Moreover, the vertical flow reactor could efficiently remove a wide type of organic pollutants, and the TOC removals of acid magenta, rhodamine B, alizarin yellow R, orange Ⅳ and diclofenacdium were 75.96, 77.90, 61.27, 72.11 and 82.89%, respectively after 3 h of reaction time. Cathodic adsorption, anodization, and H2O2 oxidation played minor roles in the removal of tetracycline hydrochloride, while the OH radicals are the most pivotal oxidizing substance in the system. As compared with the traditional aeration, the vertical flow reactor has a high-speed jet flow state, which makes the liquid-gas mixture homogeneous, and renders it high oxygen absorption rate, cost effectiveness and high power efficiency for the degradation of pollutants.

In this work, the active carbon fibers (ACFs) was modified by zeolitic imidazolate framework-8 (ZIF-8) to prepare a novel modified electrode as cathode. The effect of different amount of ZIF-8 modified ACFs (0–0.30 g) on H2O2 production and electric energy consumption (EEC) were investigated and the optimal ZIF-8 amount was 0.15 g. Characterized by linear sweeping voltammetry (LSV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), contact angle, and X-ray photoelectron spectroscopy (XPS) after modification were observed considerably changed. Some nanoparticles and oxygen-contained functional groups appeared on the ZIF-8/ACF cathode surface, which significantly enhanced the hydrophilic property and the electrochemical performance. It was observed that H2O2 production with 0.15 g-ZIF-8/ACF as cathode (1545.1 mg L−1) was much larger than with raw ACF as cathode (64.1 mg L−1). Meanwhile, H2O2 production could also achieve a balance in a high state of H2O2 production (around 1500 mg L−1) with low EEC ( OH radicals could be generated in the EF system with 0.15 g-ZIF-8/ACF. Therefore, EF system with ZIF-8/ACF cathode is a good alternative process for pollutant removal.