Synthesis Of BPC/g-C₃N₄ Composite And Its Adsorption-photocatalytic Properties For Oxytetracycline

Oxytetracycline（OTC）,as a typical tetracycline antibiotic has been widely used in clinical treatment and animal husbandry.However,human health and the ecosystem are being threaten severely by the residue of OTC in the environment.Therefore,it is necessary to develop new materials and technologies to effectively eliminate OTC contamination.Photocatalysis is considered to be one of the most effective methods to degrade antibiotics because of its ability to produce superoxide free radicals（·O₂^-）and hydroxyl free radicals（·OH）.Among many semiconductor photocatalyst materials,g-C₃N₄ has been attracted much attention due to its suitable band gap,high thermal and chemical stability,and excellent photoelectric properties.Whereas,the photocatalytic activity of pure g-C₃N₄ is usually limited by poor visible light availability and low quantum efficiency,which is mainly due to the small specific surface area and rapid recombination of photogenic electron hole pairs.In order to solve the above problems,this paper proposed the synthesis of biomass porous carbon and carbon nitrode composites（BPC/g-C₃N₄）with high specific surface area and low electron hole recombination rate through thermal polycondensation.In addition,the photocatalytic degradation performance of BPC/g-C₃N₄ on OTC was investigated,and the mechanism of OTC adsorption-photocatalytic degradation by BPC/g-C₃N₄ was elucidated.The main research contents are as follows:（1）Using waste eucalyptus bark as a raw material,eucalyptus bark based porous carbon（BPC）was prepared by hydrothermal and activation.Then,melamine was used as the precursor,and BPC/g-C₃N₄ composite was synthesized by molten salt-assisted thermal polycondensation method,which made the BPC loading on g-C₃N₄.Taking oxytetracycline（OTC）as the target pollutant,the effects of structure,physical and chemical properties of the material and the doping amount of BPC on the adsorption and photocatalytic performance of BPC/g-C₃N₄ were investigated.The results showed that BPC was successfully loaded on g-C₃N₄.The morphology of g-C₃N₄ also changed significantly from massive to hollow tubular structure,and the specific surface area increased from5.23 m²/g to 54.72 m²/g,which resulted in more active sites.The results of the adsorption and photocatalytic test showed that the BPC/g-C₃N₄ exhibited optimal removal efficiency for OTC when the BPC loading content was 5%,and the adsorption capacity and photocatalytic reaction rate were 3.67 times and 5.63times higher than that of the g-C₃N₄,respectively.The photocatalytic mechanism of BPC/g-C₃N₄ was analyzed by using active group capture experiment and photoelectrochemical characterization.The BPC/g-C₃N₄ with hollow tubular structure provided more active sites for the adsorption and reaction process of pollutants.In addition,BPC with strong adsorption properties eniched the OTC molecules around the material,and then accelerated the photocatalytic reaction rate.Moreover,the separation of photoinduced carriers were significantly improved by the excellent conduction band position of BPC,thus enhanced the photodegradation efficiency.（2）Firstly,the precursor of g-C₃N₄ with network structure was prepared by copolymerization of 2,4,6-triaminopyrimidine（TAP）and melamine,and then by further doping porous carbon（BPC）,the TAP-BPC/CN composite was synthesized by thermal polycondensation method.Taking oxytetracycline（OTC）as the target pollutant,the effects of structure,physical and chemical properties of the material and the doping amount of TAP on the adsorption and photocatalytic performance of TAP-BPC/CN were investigated.The mechanism of OTC photocatalytic on TAP-BPC/CN composite was discussed,and the possible pathway of OTC degradation by composite material was further proposed by the liquid chromatography-mass spectrometry（LC-MS）experiments.The results showed that carbon atoms in TAP skeleton partially replaced nitrogen in tri-s-triazine skeleton,and introduced into g-C₃N₄ network structure successfully.Because the electronegativity of carbon is lower than that of nitrogen,the conductivity of g-C₃N₄ was improved,and the recombination of photogenerated carriers was inhibited.In addition,the interlayer distance of graphite lamellar structure was reduced and the structure characteristics of g-C₃N₄ were retained.The results of the adsorption and photocatalytic test showed that the sample of1TAP-BPC/CN exhibited the most optimal removal efficiency for OTC,and the adsorption capacity and photocatalytic reaction rate was 4.39 times and 5.71 times higher than that of the g-C₃N₄,respectively.The mechanism of OTC photocatalytic on TAP-BPC/CN composite was explored by active group capture experiments and photoelectrochemical characterization analyses.BPC loaded on g-C₃N₄ serviced as the active site,and enriched more OTC molecules around the material,then the photocatalytic reaction rate was accelerated.In addition,the introduction of TAP reduced the band gap value of the composite,and then improved the photocarrier separation efficiency,thus the photocatalytic performance of the composite was enhanced.Furthermore,two major OTC degradation pathways were presented by the LC-MS experiments.OTC had two possible degradation pathways.OTC has two possible degradation pathways.One is that the methyl group in the dimethylamine group（-N（CH₃）₂）of OTC molecule was continuously attacked by H⁺and then degraded into a small molecular structure after a series of demethylation,dehydration,dehydroxylation and hydroxylation reactions.Another is that the OTC molecule is oxidized by·O₂^-to remove the amide group,and then decomposed by·OH and·O₂^-groups.Finally,the carbon ring was destroyed and gradually degraded into small molecules.Based on this thesis,the received findings will not only slove effectively the environmental pollution and resource utilization problems caused by a great amount of waste eucalyptus bark,but also provide a reasonable scientific basis for improving the photocatalytic degradation performance of g-C₃N₄ and the application of antibiotics pollution treatment.