Study On The Photocatalytic Activation Of PMS For Degradation Of Pharmaceutical Organic Pollutants By Transition Metal Modified Carbon Nitride

With the increase of population and the improvement of the level of light industry,the society is faced with the increasingly severe problem of water environment pollution.At present,trace amounts of drugs such as carbamazepine（CBZ）have been detected in industrial wastewater,urban sewage,surface water and groundwater.This kind of organic pollutants have a huge impact on water ecological environment and biodiversity of biological population,and will continue to threaten human health and safety.Photocatalytic oxidation is an effective method to remove low concentration organic pollutants from water due to its advantages of environmental protection,simple operation and mild reaction conditions.However,the low quantum efficiency of photocatalysis and low solar energy utilization rate are common problems of this technology.Therefore,the development of highly efficient photocatalytic materials with visible light response is the key to realize the industrial application of photocatalytic technology.Graphite-phase carbon nitride（g-C₃N₄）,as a two-dimensional metal-free polymer semiconductor material with good thermal and chemical stability and visible light response,has been widely used in the field of photocatalysis.However,the high recombination rate of photogenerated electron-hole pair（e^-/h⁺）and the low utilization rate of visible light absorption limit the development of its practical application.In view of the existing defects of g-C₃N₄photocatalyst in visible light utilization efficiency and quantum efficiency,this paper designed and prepared an efficient photocatalyst by modifying g-C₃N₄with metal modification,and combined with persulfate oxidation technology to improve its photocatalytic degradation efficiency of organic pollutants in wastewater.Ultrafine VO_x nanocluster supported g-C₃N₄photocatalyst（VO_x@g-C₃N₄）was prepared by a simple thermal polymerization combined with vanadate thermal decomposition at low temperature.The photocatalytic degradation mechanism of the new material on target organic pollutants（CBZ）in wastewater was discussed.Through TEM,XPS and FT-IR characterization,it was found that vanadium species may exist in the form of ultra-small VO_x nanoclusters and be fixed to g-C₃N₄ by N-V=O bond coordination.DRS,TPC and PL analyses confirm that VO_x@g-C₃N₄composite photocatalyst broadens its absorption range in the visible region,improves the utilization efficiency of sunlight and the photoinduced separation and transfer efficiency of e^-/h⁺,thus accelerating the photocatalytic reaction rate.Further investigation of the catalytic performance of VO_x@g-C₃N₄and g-C₃N₄ activated PMS to degrade CBZ showed that 8%VO_x@g-C₃N₄/PMS photocatalytic system could completely degrade CBZ within 15 min,which was much higher than g-C₃N₄/PMS system（33%）.And the photocatalytic performance was still good after repeated use for 5times.The TOC removal rate of 8%VO_x@g-C₃N₄for CBZ reached 65.3%in 1 h,indicating that the new catalyst has excellent photocatalytic activity and mineralization ability for organic pollutants in water.The trapping agent quenching experiment and EPR characterization results showed that,compared with pure g-C₃N₄,VO_x@g-C₃N₄was more effective in generating active species such as SO₄·^-、·OH、O₂·^-and ¹O₂in PMS under light induction.In addition,The continuous formation of SO₅·^-and SO₄·^-was ensured by the oxidation reduction reaction（REDOX）cycles of V（Ⅳ）and V（Ⅴ）.These free radicals oxidized and degraded CBZ in water by synergistic action,in which SO₄·^-、O₂·^-and ¹O₂played major roles in the 8%VO_x@g-C₃N₄/PMS photocatalytic system.In view of the shortcomings of the current g-C₃N₄ photocatalyst,such as low specific surface area,low utilization efficiency of visible light and high recombination rate of e^-/h⁺.In this paper,metal-doped carbon g-C₃N₄was prepared by one-step thermal polymerization of transition metal and carbon nitride precursor,and was applied for degradation of CBZ coupling photocatalysis with PMS-based oxidation.The effects of Fe,Cu,Mn and other transition metal doping and different morphologic carbon nitride doping on the photocatalytic properties of materials were systematically studied.It was found that Fe doped porous g-C₃N₄（Fe N_x-CN）prepared with urea as the precursor can effectively improve the PMS activation and photocatalytic properties of carbon nitride.The effect of thermal polymerization on the structure and photocatalytic properties of Fe-doped porous g-C₃N₄ under different atmospheres（O₂ and N₂）was investigated for the first time,and the relationship structure and properties was also investigated.The results showed that with PMS as oxidant and simulated sunlight as light source,the removal rate of CBZ by 1%Fe N_x-CNO prepared in O₂ could reach 96%within 20 min,which was much higher than 1%Fe N_x-CNN prepared in N₂（63%）.The degradation rate of 1%Fe N_x-CNO was 3.1 times of that of 1%Fe N_x-CNN.Otherwise,Fe doping within g-C₃N₄structure could not affect the main structure of the carbon nitride framework under the condition of hot polymerization of O₂ and N₂shown in XRD and FT-IR.Meanwhile,TEM and XPS results show that Fe elements are embedded in the carbon nitride structure in the form of Fe-N_x coordination,and Fe is uniformly dispersed in composites with very small nanometer size.The specific surface areas of g-C₃N₄,Fe N_x-CNO and Fe N_x-CNN were 66.3,136.2 and 74.2 m² g^-1,respectively,indicating that thermal polymerization in air can not only in-situ doping Fe into the g-C₃N₄structure,but also produce higher specific surface areas and more pores.More active sites can be provided to participate in the photocatalytic oxidation-reduction reaction,so as to improve the photocatalytic performance.According to DRS studies,compared with CNO（480 nm）and CNN（460nm）,the optical absorption edges of Fe N_x-CNO and Fe N_x-CNN can be extended to 520nm and 480 nm,respectively.These results indicate that Fe doping can regulate the band structure of carbon nitride.And Fe N_x-CNO has stronger absorption capacity to visible light.Moreover,TPC and PL analysis further proved that compared with Fe N_x-CNN,Fe N_x-CNO has higher photoinduced e^-/h⁺separation-transfer efficiency,thus speeding up the photocatalytic reaction rate.In addition,the photocatalytic system of Fe N_x-CNO/PMS and Fe N_x-CNN/PMS is major for production of O₂·^-and ¹O₂,and contains a small amount of SO₄·^-and·OH.These free radicals act synergically to oxidize and degrade CBZ in water.However,compared with the Fe N_x-CNN/PMS photocatalytic system,more O₂·^-,¹O₂,SO₄·^-and·OH reactive species can be produced in the Fe N_x-CNO/PMS system,which explains the mechanism why Fe N_x-CNO photocatalyst has better photocatalytic degradation performance of organic pollutants.And this paper provides a reference for the design and preparation of high efficiency transition metal-based photocatalysts and the application research of degradation of organic pollutants in actual wastewater.