Performance And Mechanism Of Photocatalytic Oxidation Of Organic Pollutants By Graphitic Carbon Nitride-based Composites

Photocatalysis,as an advanced oxidation technology(AOPs),is a green technology with high efficiency and no secondary pollution,which has become a research hotspot in the field of refractory organic wastewater treatment.Graphitic carbon nitride(g-C₃N₄)is a very promising photocatalytic material for the degradation of organic wastewater due to its high chemical stability and has visible light harvesting ability.However,its high photo-generated electron-hole recombination rate and low specific surface area inhibit the photocatalytic performance of g-C₃N₄ on organics.Chemical modification and composition are effective methods to improve the photocatalytic performance of g-C₃N₄.In this paper,organic dyes and pharmaceuticals and personal care products(PPCPs)were chosen as target contaminants,and visible light-responsive g-C₃N₄ composite semiconductor materials were prepared.A series of equipments were used to characterize the micro-morphology,crystal phase and elemental composition,functional groups,optical absorption and other characteristics of the composite materials.Composite materials could improve the utilization of visible light and promote the transfer and separation of photo-generated electrons and holes.At the same time,hydrogen peroxide(H₂O₂),persulfate(PS),and peroxymonosulfate(PMS)were introduced into the photocatalytic system as electron acceptors.They have the dual functions of separating photo-generated electrons from holes and forming active radicals so that a heterogeneous oxidation system of multiple active species can be built to enhance the photocatalytic activity.The main research findings are as follows:g-C₃N₄ porous nanosheet was prepared by thermal polymerization with urea as raw material.Its specific surface area was higher than the bulk state and it had mesoporous structure.Then the?-Bi₂O₃/g-C₃N₄ composites were prepared,and methylene blue(MB)was photocatalytically degraded under visible light.The results showed that the composite sample had enhanced photocatalytic activity,which could degrade 88.2%of MB in 120 min.The direct Z-Scheme heterostructure suppressed the recombination of photogenerated carriers of the composite material and improved the photocatalytic performance.Under visible light,this system was able to degrade 96.2%of rhodamine B(Rh B)in 40 min,92.1%of tetracycline(TC)and 86.6%of doxycycline(DOX)in 120 min,respectively.?-Bi₂O₃nanosheet was prepared by a reduction-calcination method.Because the prepared?-Bi₂O₃ showed low specific surface area and narrower visible light absorption range,multi-walled carbon nanotubes(MWCNTs)with high specific surface area and good electrical conductivity were used to modify?-Bi₂O₃ to improve its photocatalytic performance.The results showed that after the introduction of MWCNTs,the specific surface area of the composite was 9.2 times of?-Bi₂O₃,and it showed enhanced visible light absorption range and reduced recombination rate of photogenerated carriers.The degradation efficiency of DOX under visible light reached91.0%in 120 min.It has shown that the formation of composite material and the increase in specific surface area contribute to the photoelectron transfer and the increase of the catalytic sites,which help to improve the photocatalytic degradation efficiency.This study provides ideas for the modification strategy of g-C₃N₄ by porous and conductive materials.Since the increase in specific surface area had a positive impact on the performance of the photocatalyst,in order to further improve the DOX degradation efficiency,g-C₃N₄@Ce O₂ composite photocatalyst with high specific surface area(82.4m²/g)was synthesized.H₂O₂ as an electron acceptor was added in the system,it could not only improve the photogenerated electrons transfer rate,but also react with the photogenerated electrons and Ce³⁺in the system to generate more hydroxyl radicals,which made 84.4%DOX degraded within 60 min under visible light,the degradation rate of this system was 1.8 times of MWCNTs/?-Bi₂O₃ system.Liquid chromatography(LC-MS)results showed that DOX was decomposed into small molecule products by C-N breaking and opening of benzene ring.The degradation efficiency of TC and norfloxacin(NOR)by this system was 91.9%and 72.4%in 60 min,respectively.The degradation rate of TC was 2.0 times that of NOR,which indicated that the introduction of H₂O₂ could significantly improve the degradation performance of PPCPs.The photocatalytic system with the participation of H₂O₂ showed lower degradation efficiency for NOR than for TC.In order to improve the degradation efficiency of NOR under visible light,the effect of adding PS in the catalyst system was investigated.Shuttle-like Ce O₂ modified g-C₃N₄ composite was prepared.Compared with H₂O₂,the addition of PS not only effectively used photogenerated electrons,but also promoted the generation of sulfate radicals and singlet oxygen,these active oxygen components have stronger oxidation performance than hydroxyl radicals,and they synergistically worked with the photocatalytic system,which caused 88.6%of NOR degraded within 60 min.It has shown that photocatalytic systems based on sulfate radicals have an enhanced degradation activity against refractory organics.LC-MS proved that NOR was decomposed by hydroxyl substitution,benzene ring opening and C-N bond breaking.Inspired by the improvement of catalyst performance with porous and conductive materials,Ag/AgCl@ZIF-8/g-C₃N₄ composites were prepared.And the photocatalytic degradation of levofloxacin(LVFX)by g-C₃N₄ modified by MOFs was studied in the presence of PMS.Studies have shown that after adding PMS,the degradation efficiency of levofloxacin(LVFX)reached 87.3%in 60 min under visible light.This is because ZIF-8 increased the specific surface area of the composite(47.5 m²/g)and the reaction sites of the system.And the photogenerated electrons of the system could be transferred to the electron acceptor PMS,which not only inhibited the recombination of photogenerated carriers,but also promoted the generation of sulfate radicals and singlet oxygen with strongly oxidation properties,which could form an oxidation system of multiple active species with the catalytic system,and LVFX was then degraded by the removal of substituents and the opening of benzene ring by nucleophilic substitution.The g-C₃N₄@Bi₂MoO₆ composite without noble metals and with narrow band gap and high specific surface area was prepared.After adding PMS,the degradation efficiency of nimesulide(NIM)under visible light reached 97.4%in 12 min.This is because the enhanced specific surface area of the composite provided more active sites for the reaction and the addition of PMS enhanced the transfer of photogenerated electrons and formed sulfate radicals and singlet oxygen with strongly oxidation properties.Multiple active species in the system promoted the efficient degradation of NIM.LC-MS analysis showed that NIM was gradually decomposed by gradually removing the substituents on the benzene ring and the cleavage of the ether bond.