Reduction Of Antibiotics And Secondary Pollutants And Toxicity Of Multi-walled Carbon Nanotube-bismuth Vanadate Composite Photocatalytic Material

As a country with a large population,China needs to produce a lot of food and meat.In recent year,in order to meet the increasing domestic material demand,the livestock and poultry breeding industry has developed rapidly through intensive production and large-scale operation,but the large amount of wastewater generated during the breeding production process has become a major problem in environmental pollution control.Livestock and poultry breeding wastewater contains a large amount of antibiotics,and the long-term existence of multiple antibiotics in the water body stimulates the reproduction of a large number of antibiotic resistant bacteria（ARB）in the water body,which leads to ARB show multiple and stronger drug resistance.In the process of ARB reproduction and apoptosis,antibiotic reistance genes（ARGs）,a new type of pollutants,will increase the abundance in the water body through vertical transmission and horizontal gene migration（HGM）.The large number of antibiotics,ARB and ARGs in the environment is undoubtedly a potential threat to human health and environmental ecology,but the current traditional water treatment technology is difficult to effectively remove the pollutants with low concerntration in the treatment of aquaculture wastewater.Therefore,it is obviously necessary to develop corresponding new water treatment technologies to control the pollution of antibiotics,ARB and ARGs in livestock and poultry breeding wastewater.This study focused on oxytetracycline（a broad-spectrum veterinary antibiotic）,multi-antibiotic resistant Shigella（multi-antibiotic resistant pathogens extracted from aquaculture water）,and ARGs in the livestock and poultry breeding wastewater.The photocatalytic oxidation technology was used to solve the problem of antibiotic removal and the inactivation of ARB,and to study the impact on ARGs.In this paper,multi-walled carbon nanotubes（MWCNT）were used to modify bismuth vanadate（BiVO₄）to prepare MWCNT/BiVO₄photocatalytic materials with visible light response.By analyzing the photocatalytic activity of the material through related characterization methods,the research on the photocatalytic performance and degradation mechanism of the photocatalytic material on oxytetracycline were carried out.At the same time,combining the relevant characteristics of the material and adjusting the material preparation plan,the photocatalytic inactivation of multi-antibiotic resistant Shigella flexneri HL was studied,and the inactivation mechanism was explained.In the process of photocatalytic inactivation of multi-antibiotic resistant Shigella flexneri,the abundance changes of the supernatant and precipitated ARGs were analyzed by DNA extraction and q PCR determination after low-speed centrifugation for exploring the impact of photocatalysis on ARGs in water bodies and assessing environmental risks.Combining the acute toxicity experiment of the photocatalytic materials on tropical Xenopus tadpoles and stereo microscope observation,the toxicity of the photocatalytic materials were analyzed to determine whether the prepared materials were low-toxic and environmentally friendly.Finally,combined with the removal of antibiotics,the inactivation of ARB,the influence of ARGs and the toxicity of photocatalytic materials,the possibility of photocatalytic materials used in actual livestock and poultry breeding wastewater treatment applications was comprehensively evaluated.The main conclusions were as follows:（1）The modification of BiVO₄ by MWCNT could effectively improve the light energy utilization rate of photocatalytic materials,reduce the band gap of photocatalytic materials,and inhibit the recombination rate of photo-generated electron-hole pairs of photocatalytic materials.At the same time,the modification of MWCNT improved the photocatalytic performance of BiVO₄ to produce hydroxyl radicals（·OH）and superoxide radicals（·O₂^-）,and thereby improving the photocatalytic activity of BiVO₄.（2）In the process of photocatalytic degradation of oxytetracycline by MWCNT/BiVO₄nanofibers,the degradation rule of oxytetracycline conformed to the first-order reaction kinetic model.Combined with free radical analysis,the main active species for oxidative degradation of oxytetracycline was·OH and·O₂^-.（3）The photocatalytic degradation system of oxytetracycline was analyzed by liquid chromatography-mass spectrometry（UPLC-MS/MS）.22 kinds of oxytetracycline degradation intermediate products were detected,and 6 degradation processes were deduced.The main reaction pathways were demethylation（-14 Da）,secondary alcohol oxidation（-2 Da）,dehydration（-18 Da）,hydroxylation（+16 Da）,decarbonylation（-28 Da）and deamination（-15 Da）.（4）The MWCNT/BiVO₄ powder material prepared under the hydrothermal condition of pH=1 had the most excellent photocatalytic bactericidal performance.·O₂^-,photo-generated holes（h⁺）and·OH produced by MWCNT/BiVO₄ could oxidize the cell wall and cell membrane of multi-antibiotic resistant Shigella flexneri cells.They destroyed the surface physical structure of the bacteria and changed the permeability of the cell membrane,resulting in the inactivation of the bacteria.（5）Through the ARGs identification of multi-antibiotic resistant Shigella flexneri,it was known that multi-antibiotic resistant Shigella flexneri contains 6 ARGs（tet A,tet X,sul1,sul2,FOX and TEM）and an integron（Int I1）.q PCR analysis showed that most of the ARGs freed to the outside of the bacteria during the photocatalytic inactivation process were 1 to 2 orders of magnitude lower than the ARGs remaining inside the bacteria.But tet X was a kind of ARGs loaded on the transposon that its migration and diffusion were significantly different from most ARGs diffusion and migration.In the process of photocatalytic inactivation on ARB,ARGs did not accumulate further,indicating that the photocatalytic system had a certain inhibitory effect on the accumulation of ARGs.（6）Through the acute toxicity experiment,comprehensive analysis of the photocatalytic performance and toxicity of the photocatalytic material showed that the MWCNT/BiVO₄composite photocatalytic material is a low-toxicity and low-cost environmentally friendly photocatalytic material,which is expected to be further developed and applied to the actual livestock and poultry breeding treatment.