Preparation Of Carbon-doped Bismuth-based Modified Photocatalysts And The Mechanism For Antibiotic Degradation

In recent years,antibiotics have been frequently detected in water due to the abuse of medication and inadequate treatment.Residual antibiotics in the aquatic environment can induce the formation of antibiotic-resistant bacteria and the evolution of antibiotic-resistant genes,resulting in a decline in antibiotic efficacy and endangering public health.As a new advanced oxidation technology,photocatalytic oxidation has the advantages of low cost,complete degradation and simple reaction compared with traditional physical,biological and chemical treatments.As a new class of visible light response photocatalysts,bismuth-based semiconductors have attracted extensive attention from researchers,but there are still problems such as poor visible light absorption and rapid recombination of photogenerated electron-holes.In this study,three novel carbon-doped bismuth-based modified photocatalysts were synthesized by three modification methods,i.e.elemental doping,bismuth-rich strategy and surface modification.Based on material characterization,the physicochemical structure and photoelectric properties of catalysts were systematically studied.Three typical antibiotics were selected and the photocatalytic activity,catalytic mechanism and practical application potential of the synthesized materials were systematically investigated through pollutant degradation experiments,free radical quenching experiments,recycling experiments and influencing factor analysis experiments.The main research contents and conclusions are as follows:(1)A novel carbon quantum dot(CQDs)modified carbon-doped α-Bi2O3 photocatalyst(CBO/CQDs)was synthesized by secondary solvothermal reduction of partially doped carbon.Under simulated sunlight,the first-order reaction rate constant k of CBO/CQDs photocatalytic degradation of ceftriaxone sodium is 11.4 times and 3.2 times higher than that of pure α-Bi2O3 and CBO-3,respectively.Material characterizations show that the enhanced photocatalytic activity is due to a combination of carbon doping,CQDs modification and reduced particle size:(Ⅰ)interstitial carbon doping introduces a local state above the valence band(VB),which enhances the utilization of visible light and promotes the separation of photogenerated electrons and holes;(Ⅱ)surface-modified CQDs improve photo generated carrier separation and prolong visible light response;(Ⅲ)the reduced particle size of CBO/CQDs accelerates the migration of photo generated carriers.Free radical quenching experiments show that superoxide radical(·O2-)was the most critical active species in the degradation of ceftriaxone sodium,and the conversion experiments of nitrotetrazolium blue chloride(NBT)also confirm that the most·O2-was produced in the photocatalytic system of CBO/CQDs.Cyclic experiments show that CBO/CQDs have good photochemical stability,indicating that the material has good environmental application potential.Three-dimensional excitation emission fluorescence spectroscopy(3D EEMs)and total organic carbon(TOC)analysis show that longer reaction times or higher photocatalyst dosages are required to obtain sufficient mineralization rate.Based on intermediate products analysis and density functional theory(DFT)calculation,the photocatalytic degradation pathways of ceftriaxone sodium were proposed.According to the Quantitative Structure-Activity Relationship(QSAR)analysis,the partially degraded intermediate products are highly toxic and require sufficient mineralization to ensure safe discharge.(2)New carbon-doped Bi24O31Br10 photocatalytic materials(Cx-BOB)were synthesized by hydrothermal calcination method by doping exogenous carbon into the layered structure of Bi24O31Br10.The photodegradation experiments of sulfamonomethoxine(SMM)show that C0.4-BOB has the strongest photocatalytic degradation activity under simulated sunlight and visible light,and the k value of simulated sunlight degrading SMM is 2.4 times that of pure Bi24O31Br10(BOB).Free radical quenching experiments show that holes(h+)and ·O2-are the most active species in photodegradation experiments.Four cycles of the material show that C0.4-BOB has good photochemical stability.In addition,C0.4-BOB has good photocatalytic degradation effect on a variety of antibiotics,indicating that it has wide applicability.The intermediate products in the degradation process were analyzed by LC-MS,and the photocatalytic degradation paths of SMM were proposed.The effects of water quality parameters(different pH,HA and SO42-concentrations)on SMM photodegradation were studied by the Box-Behnken experimental design,and the experimental results show that the photodegradation rate of SMM increases with the increase of pH,but decreases with the increase of HA concentration.(3)Different proportions of plasma photocatalysts C0.4-BOB/x%Ag were synthesized by simple photoreduction AgNO3 method on the surface of C0.4-BOB.Material characterizations demonstrate the successful deposition of silver nanoparticles(Ag NPs)on the surface of the C0.4-BOB sheet-like structure.The surface plasmon resonance(SPR)effect of Ag NPs greatly improves the light absorption in the visible light segment of the material,and can effectively inhibit the carrier recombination at the C0.4-BOB interface and accelerate the interfacial charge transfer,which are conducive to the improvement of photocatalytic activity.The optimal Ag deposition ratio was determined by photo degradation levofloxacin(LOF)experiments,and the k value of C0.4-BOB/10%Ag in simulated sunlight degradation LOF is 4.2 times and 2.0 times higher than that of BOB and C0.4-BOB,respectively.Through free radical quenching experiments,it is concluded that·O2-is the main active species for LOF degradation.The four-cycle experiments show that C0.4-BOB/10%Ag has good stability in the application of photocatalytic degradation of LOF.The influencing factors experiments show that neutral pH is not conducive to the degradation of LOF by materials,and high concentrations of HA will also inhibit the degradation of LOF.(4)In this study,three new carbon-doped bismuth-based modified photocatalytic materials were synthesized on the basis of carbon doping combined with different modification methods.The catalytic activities of the three materials are compared by simulate sunlight degradation LOF experiments,among which C0.4-BOB/10%Ag has the best photocatalytic effect,and the k value of degrading LOF is 2.23 times and 2.03 times that of CBO/CQDs and C0.4-BOB,respectively.The higher photocatalytic activity is due to the strong built-in electric field(IEF)induced by carbon doping and the SPR of Ag NPs.