Preparation Of Metal And Metal-free Modified Carbon Nitride Composite And Their Photocatalytic Degradation Performance And Mechanism Toward Typical PPCPs Under Visible Light Irradiation

Recently,PPCPs are commonly detected over a wide range of aquatic environments including river water,groundwater,drinking water,and wastewater treatment plants（WWTPs）effluent,due to their extensive use.Exposure to PPCPs will lead to negative impacts in human health as well as for ecosystems.Photocatalysis technology can directly use solar energy to generate strong oxidizing reactive species for the treatment of pollutants in water.Graphitic carbon nitride（g-C₃N₄）with a suitable visible band structure and superior chemical stability is a metal-free polymeric photocatalyst,which has been used for the photocatalytic degradation of environmental pollutants.However,the low specific surface area,high recombination rates of photo-generated electron-hole pairs and low visible-light absorption efficiencies of pristine g-C₃N₄ greatly hinder its practical applications.To solve these problem,g-C₃N₄ was modified by morphological control,metal and metal-free loading method.The composite was then used for the photocatalytic degradation of PPCPs.The main research contents and results are as follows:（1）N-doped carbon dot（NCDs）with excellent optical and electrical property was prepared by solvothermal method.Subsequently,NCDs/g-C₃N₄ composite was successfully synthesized by loading NCDs nanoparticles onto the surface of g-C₃N₄ via a facile polymerized method.The photocatalytic performance of the NCDs/g-C₃N₄ was significantly higher than that of g-C₃N₄ toward the degradation of indomethacin（IDM）under visible light irradiation.In addition,the photocatalytic activity of composite was significantly enhanced after doping with N atom into the CDs.With increasing NCDs loading volumes,the photocatalytic activity of NCDs/g-C₃N₄ initially increased,and then decreased.A NCDs content of 1.0 wt%resulted in a 13.6 fold higher photocatalytic activity than that of g-C₃N₄.This enhanced photocatalytic activity might have been ascribed to the unique up-converted PL properties and efficient charge separation of the NCDs.Reactive species（RS）scavenging experiments revealed that superoxide radical anions（O₂.^-）and photogenerated holes（h⁺）played key roles during the photocatalytic degradation of IDM.Potential photocatalytic pathways of IDM were proposed through the identification of intermediates and the prediction of reaction sites,which involved decarboxylation,hydroxylation,as well as the addition and cleavage of indole rings.（2）Single-atom dispersed Ag loaded ultrathin g-C₃N₄ hybrid（AgTCM/UCN）was synthesized via a facile one-step co-polymerization of dicyandiamide with silver tricyanomethanide（AgTCM）and NH₄Cl.The morphology,composition and photoelectrical properties of composite were studied by a series of characterization.Subsequently,AgTCM/UCN used for the degradation of sulfamethazine（SMT）in the presence of an electron acceptor peroxymonosulfate（PMS）.Under visible light irradiation,the AgTCM/UCN/PMS process showed higher efficiency for SMT degradation than AgTCM/UCN,UCN/PMS,and g-C₃N₄/PMS systems.With increasing AgTCM loading amounts,the photocatalytic activity of AgTCM/UCN/PMS initially increased,and then decreased.The excessive AgTCM loading amounts may develop new charge recombination centers.An increase in the PMS concentration resulted in the enhancement of the degradation efficiency.This enhanced photocatalytic activity might have been attributed to the synergistic effects encompassing the surface plasmon resonance（SPR）of Ag,high surface area of UCN,and efficient charge separation of PMS.Electron-spin resonance（ESR）and reactive species（RSs）scavenger-quenching experiments revealed that SO₄^·-was generated following the addition of PMS,whereas O₂^·-and h⁺were predominantly responsible for the degradation of SMT.Three degradation pathways of SMT were deduced,including the cleavage of sulfonamide bonds,SO₂ extrusion,and the oxidation of the aniline moiety,based on LC-MS results and theoretical FEDs calculations.The degradation of SMT in ambient water revealed that the AgTCM/UCN/PMS photocatalytic process can be efficaciously applied for the remediation of SMT contaminated natural waters,particularly sea water.（3）A novel ternary photocatalyst comprised of single atom-dispersed silver and carbon quantum dots,co-loaded with ultrathin g-C₃N₄（AgTCM-CDs/UCN）was synthesized via a facile co-polymerization of dicyandiamide with AgTCM,CDs and NH₄Cl,and used for the photocatalytic degradation of naproxen（NPX）under broad-spectrum irradiation.AgTCM-CDs/UCN exhibited a highly enhanced photocatalytic activity than that of UCN,CDs/UCN,and AgTCM/UCN under UV,visible,and near-infrared light irradiation.The content of 1.0 wt%of CDs and 3.0 wt%of Ag resulted in 2.4-fold and 10-fold higher photocatalytic activity than that of CDs/UCN and UCN,respectively under visible light irradiation.On the one hands,the introduction of Ag and CDs could narrow the band gap of UCN.On the other hands,This improved broad-spectrum photocatalytic performance may be attributed to the surface plasmon resonance effect of Ag,up-converted fluorescent properties of CDs,as well as the electron separation and transfer capacity of both the Ag and CDs.ESR and RS scavenging experiments indicated that ¹O₂ and O₂.^-were the dominant active species involved in the degradation of NPX.Product identification and reaction site prediction revealed that the photocatalytic degradation of NPX included decarboxylation,hydroxylation,as well as the opening of the naphthalene ring.Reactions in different water matrices indicated that AgTCM-CDs/UCN can be effectively employed for the degradation of NPX under ambient water conditions.