Construction Of Heterogeneous Fenton System Enhanced By Electric Field And Study On Mechanism Of Catalyst Reactivation

During the rapid development of China’s chemical industry,a large amount of refractory organic wastewater has been produced.Because traditional physicochemical methods and biological methods are difficult to treat such wastewater,advanced oxidation processes（AOPs）,represented by Fenton oxidation,were born.However,the Fenton oxidation method has exposed the problems of narrow p H range and large iron sludge production in the actual application process while the heterogeneous Fenton oxidation method overcomes these problems to a certain extent.Traditional Fe-based heterogeneous Fenton catalysts,represented by Fe₃O₄,have the problem of low generation rate of Fe（II）from Fe（III）,which brings poor reaction efficiency and causes catalysts deactivation.In this work,Fe₃O₄@N-MWCNT catalyst with both good magnetism and high electrical conductivity was synthesized.Nitrogen-doped oxidized-MWCNT（N-MWCNT）was selected as the growth carrier of Fe₃O₄ particles due to its powerful ability to store electrons and abundant defective structure.The XRD patterns and SEM images revealed the great dispersivity of Fe₃O₄ attached on oxidized-MWCNT and the average crystallite size of Fe₃O₄ particles was64nm.XPS was used to further investigate the component and its chemical state of the catalysts.There were many oxygen-containing functional groups on the surface of MWCNT,nitrogen is mainly combined with carbon and introduces defective structures into MWCNT.Chemical bond was formed between Fe₃O₄ and MWCNT,the Fe（II）/Fe（III）ratio in the fresh Fe₃O₄@N-MWCNT catalyst is 0.778.The magnetic properties of Fe₃O₄ retained in the synthesized catalyst and the saturation magnetization was 29.13 emu·g^-1.Electrochemical characterizations showed that the synthesized catalyst has better redox performance and capacitance performance than pure Fe₃O₄.The catalyst showed obvious redox peaks in CV test and the specific capacitance was 124.5 F·g^-1.The catalyst shown positive Zeta potential under acidic environment.Synthesized catalyst was used in the heterogeneous Fenton degradation of phenol,and the optimal conditions of the degradation experiment were determined.The combination of Fe₃O₄and MWCNT was able to significantly improve the heterogeneous Fenton catalytic activity of phenol and the apparent degradation rate constant K_app of the catalyst was 13 times as high as that of Fe₃O₄.Under the optimal condition(catalyst Fe₃O₄ content 46.1%,catalyst dosage 400mg·L^-1,initial p H 4.0,initial H₂O₂ concentration 625 mg·L^-1),phenol could be totally removed in 60 min.However,the Fe（II）/Fe（III）ratio of the catalyst declined to 0.547 after 5 cycles and caused the decrease of cycling performance.On this basis,a novel unibody‘pollutant degradation-catalyst reactivation’reactor was designed for electric field enhanced heterogeneous Fenton degradation.The concept of dynamic membrane heterogeneous Fenton（DMHF）was introduced and the benefit of catalyst reactivation on the cathode was verified.The Fe（II）/Fe（III）ratio from XPS characterization was used to confirm the Fe（II）recovery during the reactivation experiment,the Fe（II）/Fe（III）ratio of the catalyst was 0.755 after 5 cycles.Heterogeneous Fenton reaction,electrochemical oxidation and catalyst reactivation were combined for a first time,these three processes could be carried out simultaneously within a certain period.It is demonstrated that the catalyst can replenish the sacrificed electrons during the reactivation process.Comparing the cycling test,electric field enhancement could improve the cycle performance of the catalyst,extend the life of the catalyst,and provide a new idea for heterogeneous Fenton degradation.