Modification Of BiPO₄ And G-C₃N₄ And Study On Photocatalytic Degradation Performance And Mechanism Of Typical PPCPs

In recent years,with the continuous improvement of people’s living standards,a large number of drugs and personal care products（PPCPs）have been used,making them frequently detected in the effluent of sewage treatment plants,river water,Lake water,drinking water,groundwaterand other environmental waters.However,PPCPs exposed to water environment for a long time may cause potential risks to ecosystem and human health.Currently,advanced photocatalytic oxidation technology has been widely used for the removal of organic pollutants in water with the advantages of cleanliness,environmental protection and solar energy utilization.BiPO₄ and g-C₃N₄,as the photocatalysts with excelent properties,have been extensively studied.However,BiPO₄ only has deep ultraviolet response and cannot separate photogenerated carriers wel,and the g-C₃N₄ has disadvantages of low visible light utilization and high photogenerated carrier recombination efficiency,these disadvantages limit their application in the degradation of environmental pollutants.In this thesis,carbon dots（CDs）composite BiPO₄,g-C₃N₄ combined persulfate（PDS）,oxygen doped g-C₃N₄ and CDs composite OCN were used to modify these two semiconductor photocatalysts to enhance their photocatalytic activity,and further applied to the degradation of several typical PPCPs in water.The main research results are as follows:（1）In this study,novel CDs/BiPO₄（CDBP）photocatalytic complexes were successfully synthesized via a facile hydrothermal-calcination synthesis strategy.The physicochemical properties of the synthesized samples were studied by XRD,UV–vis DRS,FT-IR,etc.The activities of the CDBP were evaluated through the photocatalytic degradation of Indometacin（IDM）in an aqueous solution under simulated sunlight irradiation.With increasing concentrations of CDs,the photocatalytic activity of the CDBP initially increased,and then decreased.A CDs content of 3.0wt%shows 12 times higher photocatalytic activity than that of pristine BiPO₄.Reactive oxidative species,particularly·O₂^-and h⁺,were the two critical ones to mediate the photocatalytic degradation of IDM immediately.A notable sign of 5,5-dimethyl-1-pyrrolidone-N-oxyl（DMPOX）was observed through EPR with CDBP as the photocatalyst,which indicated a higher oxidability than pristine BiPO₄ under simulated sunlight irradiation.This enhanced photocatalytic activity might be due to high-efficiency charge separation,unique up-converted PL properties as well as the bandgap narrowing of the CDs.Moreover,the byproducts of IDM were detected by HPLC-MS/MS and GC-MS,and the probable pathways were deduced.（2）g-C₃N₄ was synthesized using dicyandiamide as the precursor.The morphology and optical properties of g-C₃N₄ were characterized by TEM,XRD and UV-vis DRS.In this study,the effect of the addition of PDS on the degradation of sulfamethazine（SMZ）by g-C₃N₄photocatalytic degradation was studied.The results show that PDS can improve the photocatalytic degradation of SMZ by g-C₃N₄.The fluorescence test showed that PDS could effectively separate the photogenerated h⁺and photogenerated e^-from g-C₃N₄ to enhance its photocatalytic performance.The different factors of PDS/g-C₃N₄ system on the photocatalytic degradation of SMZ were also investigated.The degradation of SMZ followed pseudo-first-order kinetics.Acidic condition is conductive to the degradation of SMZ.Quenching experiment demonstrated that h⁺was present in the PDS/g-C₃N₄ photocatalytic system when using sodium oxalate as photogenerated scavenger,and the contribution of h⁺was calculated to be 65.9%.This result indicated the major role of h⁺in degradation of SMZ.The detection of TOC indicated that the addition of PDS enhanced the mineralization of SMZ.（3）Polymeric photocatalysts have desirable application prospects due to their low cost and controllable structures.Here,a novel photocatalyst,carbon nitride organic polymer（OCN）with oxygen and nitrogen-linked heptazine,was synthesized by using a single-step approach.OCN is a metal-free material and has properties of porous and low crystal inity and nitrogen vacancies.In the photocatalytic degradation of several typical PPCPs,the OCN exhibited excelent photocatalytic performance compared to g-C₃N₄.The mechanisms of improved photocatalytic performance were further investigated.The conduction band,crystallinity and the recombination rate of photogenerated carriers reduced simultaneously due to the introduction of oxygen atoms.Through these processes,the PPCPs,at the first time,are degraded due to reactive species.The mechanism of hydroxyl radical generation in the OCN system was first proposed under this system.The O linkages can form hydrogen bonds with H₂O₂ to promote the catalytic generation of·OH.The presence of·OH,·O₂^-and ¹O₂ indicated that the OCN polymer could be used to degrade organic pollutants.（4）Here,we report on metal-free co-catalysts with the in-situ incorporation of CDs into a polymeric O and N co-linked carbon nitride（OCN）framework,which could significantly enhance the yield of H₂O₂ and·O₂^-due to double reactive sites.In the photocatalytic degradation of diclofenac（DCF）by using the CDs/OCN under the visible light irradiation,the CDs/OCN shows a stonger catalytic activity,which the degradation rate was 11.6 times higher than that of pure g-C₃N₄ and OCN.The CDs/OCN exhibited higher electron–hole separation efficiencies than g-C₃N₄ and OCN,which was due to the CDs has an excellent charaterstic of electron transfer.The yield of H₂O₂ generation by CDs/OCN was higher than that of g-C₃N₄and OCN.Meanwhile,the EPR revealed that more·O₂-and·OH could be generated via the CDs/OCN system.Accroding to the results of Density functional theory（DFT）and Raman spectrum analyses,double reactive sites were present in the CDs/OCN.There are two reasons could be explained this phenomenon,one is that O₂ was more prone to 2-electron reduction on the OCN,the other is O₂ was more easily reduced to·O₂-on the surface of CDs through the additional e-provided by the OCN.The byproducts of DCF were detected by HRLC-MS/MS,and the probable pathways were further deduced.