Preparation Of Heterogeneous Photo-fenton Catalyst NCDs/Fe₂O₃/g-C₃N₄ And Degradation Of Typical Pollutants In Coal Chemical Wastewater

The control and treatment of a large amount of high concentration organic wastewater produced in coal conversion process has been a difficult problem in the field of coal chemical industry.The resources of coal and water inverse distribution in China,large-scale development of coal chemical industry will be limited by the"quality"?water quality?and"quantity"?water quantity?.of water resources.The homogeneous Fenton reagent(Fe²⁺/H₂O₂)generates hydroxyl radicals??OH?,which non-selectively destroy the organic pollutants and has obvious advantages for the removal of aromatics,phenols and aromatic amines from coal chemical wastewater.But there exists some key problems such as strong acid treatment environment,high sludge production and difficult separation of catalyst.The heterogeneous Fenton catalyst solidifies iron ions to form iron,iron ion doped type,iron oxides and so on,which promotes H₂O₂decomposition at high initial pH and inhibits iron mud production,but the interaction of atoms during solid state process makes the Fe?III?/Fe?II?cycle rate slower,leading to the problem of decreasing catalyst activity.The key issue to improve the activity and efficiency of heterogeneous Fenton catalytic reaction is by promoting the Fe?III?/Fe?II?cycle.Therefore,It proposesd to construct heterogeneous photo-Fenton catalysts with full visible-light range response using carbon dots,Fe₂O₃ and g-C₃N₄ to excite more photoelectrons using wider spectra to accelerate the Fe?III?/Fe?II?cycle.N-doped carbon dots?NCDs?prepared by hydrothermal method using urea and citric acid as raw materials were synthesized by hydrothermal reaction and calcination with Fe₂O₃ and g-C₃N₄ to prepare heterogeneous photo-Fenton catalyst NCDs/Fe₂O₃/g-C₃N₄ and characterized related performance.The results of surface morphology,crystal structure and material composition indicated that the composite of NCDs and Fe₂O₃have no affect on g-C₃N₄ structure,and the formation of a tight interface after the composite is beneficial to broaden the spectral absorption range of the material.Studying the loading of Fe₂O₃,NCDs,g-C₃N₄ on catalyst activity,due to NCDs can enhance the light absorption capacity and inhibit hole-electron recombination,prolong the photoelectron lifetime and increase the chance of electron capture by the active site,thus improved the catalytic degradation efficiency.While further raised the NCDs amount,the photo degradation rate was suppressed,as the“shield effect”of NCDs.As iron source,Fe₂O₃ enhanced the light absorption and promoted the decomposition of H₂O₂ to produce active radicals,but excess Fe₂O₃ served as the recombination center of the photo-generated electron-hole pairs in the photocatalytic process,and sequentially covered the active center on the surface of g-C₃N₄,which reduced the degradation efficiency.When the mass of Fe₂O₃ and NCDs is fixed,the composite material prepared with FeCl₃:6H₂O:NCDs:g-C₃N₄ mass ratio of 3:1:1,due to g-C₃N₄ can match the energy band of Fe₂O₃,the degradation rate was highest.The typical pollutants of coal chemical wastewater,phenol and indole,were selected as degradation targets,and simulated wastewater was prepared to study the degradation effects of composite materials,degradation system,initial pH,H₂O₂ dosage,catalyst dosage,and initial concentration of pollutants.The catalytic activity of NCDs/Fe₂O₃/g-C₃N₄ composite was significantly higher than that of Fe₂O₃/g-C₃N₄ and Fe₂O₃,indicatied favorable cooperation between NCDs,g-C₃N₄ and Fe₂O₃.NCDs/Fe₂O₃/g-C₃N₄ composite can broaden the range of the pH in homogeneous photo-Fenton catalysis.With the dosage of H₂O₂ or catalyst increasing,the degradation efficiency showed the trend of first rising and then decreasing,and gradually decreased with the increase of the initial concentration of pollutants.The photocurrent response curves and active species were characterized to elucidate the degradation mechanism,which discovered the transient photoresponse current of NCDs/Fe₂O₃/g-C₃N₄ composite was higher than Fe₂O₃/g-C₃N₄ and Fe₂O₃,the value was 5.86?A/cm^-2,2.4 and 9.7 times of Fe₂O₃/g-C₃N₄ and g-C₃N₄ transient photoresponse currents.The photoresist of the NCDs/Fe₂O₃/g-C₃N₄ composite was significantly reduced,indicating that NCDs in the Fe₂O₃/g-C₃N₄ system can further improve the photoelectrochemical performance,promote the charge transfer between the Fe₂O₃ and the g-C₃N₄,which improved the photo-induced charge separation and transfer efficiency.The radical scavenging experiment and the EPR/DMPO experiment indicated that hydroxyl radicals??OH?and holes?h⁺?were the main active species in the photocatalytic process.The NCDs and g-C₃N₄ photogenerated electrons transfered to the Fe₂O₃ under light irradiation,which impeled the Fe?III?on the Fe₂O₃ surface to be reduced to Fe?II?,and the photogenerated h⁺can react with the H₂O to produce and then improved the reaction efficiency.By conducting a cycle test and combining the results of XRD and FTIR of the composite material,found NCDs/Fe₂O₃/g-C₃N₄ composite had better stability in the presence of H₂O₂ and light irradiation.In this paper,the goal of promoting Fe?III?/Fe?II?recycling in UV-Vis region by heterogeneous photo-Fenton catalyst was realized as a whole,which had high degradation efficiency and wide application prospect.in coal chemical wastewater.This paper contains 41 figures,6 tables and 131references.