Preparation Of G-C₃N₄ Based Photocatalyst And Combined Removal Of Norfloxacin By G-C₃N₄ Photocatalytic Oxidation And Microalgal Degradation

Quinolones have been widely used in mariculture due to their wide antibacterial spectrum,strong bactericidal ability and no cross-resistance with other antibiotics.Improper disposal of antibiotics in mariculture will inevitably affect the Marine ecosystem.How to effectively control the pollution of quinolone antibiotics in seawater environment has become an urgent problem.g-C₃N₄ photocatalytic oxidation is a new efficient treatment technology for pollutants in recent years.It has the advantages of low cost,green non-toxic,suitable gap width and good biocompatibility,which can be applied to the removal of quinolones in mariculture water bodies.However,g-C₃N₄ material has the following problems to be solved,such as low specific surface area,poor photon response and high recombination rate of photogenerated electron/hole pair.Therefore,single g-C₃N₄ material cannot meet the actual demand of photocatalytic oxidation of quinolones,and the preparation of multiple composite photocatalyst based on g-C₃N₄ can solve this problem.Although studies have shown that microalgae can degrade and remove a variety of antibiotics,the direct use of microalgae degradation to remove antibiotics is not high activity and takes a long time due to the high toxicity of antibiotics.Therefore,in this paper,norfloxacin（NFX）,a typical quinolone antibiotic,was taken as the degradation and removal object.Based on the preparation of g-C₃N₄ composite photocatalyst,the feasibility of combining the photochemical oxidation of g-C₃N₄ photocatalyst with the degradation of microalgae was explored in order to achieve the purpose of efficient removal of Norfloxacin.In this paper,two novel and highly efficient photocatalysts were prepared,one of which used Zn O as substrate and loaded magnetic Fe₃O₄ and g-C₃N₄ on its surface as modification.Zn O/g-C₃N₄/Fe₃O₄（ZGF）composite lamellar photocatalyst was prepared by hydrothermal method and high-temperature calcination method.The other is the synthesis of petal-like Bi₂WO₆-Bi₂S₃ photocatalytic material by one-step hydrothermal method with the help of surfactant CTAB,and the formation of Bi₂WO₆/Bi₂S₃/g-C₃N₄（Bi SW-GCN）by physical doping with tubular g-C₃N₄.The prepared materials were characterized in a variety of ways.The results show that g-C₃N₄ and Fe₃O₄ of the first prepared photocatalytic material can be successfully loaded on the flake Zn O.The specific surface area and pore volume of the material are improved,and the material has good magnetic properties,and its absorption of visible wavelengths is widened.In the second photocatalytic material prepared,g-C₃N₄ and Bi₂S₃ were well loaded on the petal-like Bi₂WO₆,which improved the wavelength absorption in the visible light range,decreased the fluorescence intensity,and reduced the aperture size distribution,which was conducive to the effective separation of photogenerated electron hole pairs.The photocatalytic activity of the two materials was significantly improved after the heterojunction was formed.The photocatalytic degradation of magnetic composite photocatalyst ZGF and petal-like photocatalyst Bi SW-GCN under visible light irradiation was studied with NFX as the target pollutant.The experimental results show that the kinetic constant of ZGF is 0.01446(min^-1).When the catalyst dosage was 1.43 g/L,the solution p H value was 7.12,and the NFX concentration was less than 8.61 mg/L,the NFX removal rate was higher than 90%.After 5cycles,the degradation rate of the magnetic material to NFX decreased to 92.8%of the first one,and the TOC removal rate was 30%within 2 h.The capture experiment showed that the toxicity of the photocatalytic machine was mainly caused by holes.Bi SW-GCN material has the best effect in neutral environment.When 1 g/L bisw-gcn is added,the degradation rate of8 mg/L NFX solution can reach 92.4%within 120 min,and the TOC removal rate can reach38.8%.According to the inhibitory effect of NFX on Escherichia coli,the results showed that NFX degradation products had no toxic effect on Escherichia coli by Bauer Kirby toxicity evaluation method.LC-Q-TOF method was used to detect and analyze the intermediate products transformed after NFX photocatalytic degradation,and the NFX photocatalytic degradation pathway was proposed.The oxidative degradation of NFX was mainly divided into three pathways:heterocyclic destruction,benzene ring defluorination and decarboxylation reaction,and piperazine ring opening process existed in all three pathways.The feasibility of the combination of photocatalytic oxidation and microalgal degradation was also studied.With tubular g-C₃N₄ and magnetic ZGF as the main body,the combination of photocatalytic oxidation and chlorella degradation of Norfloxacin（NFX）was studied.The results showed that the degradation intermediates generated after NFX photocatalytic degradation could be reused by Chlorella vulgaris（C.vulgaris）.The degradation rates of residual NFX after photocatalytic degradation in g-C₃N₄-C.vulgaris combined group and ZGF-C.vulgaris combined group could reach 72.5%and82.1%,which were 3.1 times and 1.7 times higher than that of pure photocatalytic degradation.During the degradation process,the biomass of C.vulgaris began to increase significantly after 4 and 3 days.The transcriptome analysis of C.vulgaris showed that,compared with before NFX degradation concentrate on inhibition of C.vulgaris,NFX photocatalytic degradation products for the degradation of Chlorella,its catalytic activity,biological detoxification activity,hydrolytic enzymes,REDOX enzyme activity,biomineralization of active gene enrichment degree is high,The expression of genes directly related to NFX degradation in C.vulgaris,such as P450 drug metabolism gene,also increased significantly.The photocatalytic degradation products can be well degraded and utilized by C.vulgaris.The combination of the two can solve the problem that the catalytic activity of the photocatalytic reaction is reduced when the intermediates are complex,and also reduce the possibility of secondary environmental pollution of the intermediates.