Effect Of ZnFe-LDHs Modified Oyster Shell On The Removal Of Tetracyclines Antibiotics And Variation Of Tet Genes In Vertical Flow Constructed Wetlands

While Antibiotics play an important role in treating and preventing diseases,they also bring potential harm to the environment and human health.Antibiotics are difficult to fully absorb and degrade in living organisms,and the discharge of residual antibiotics into water will make microorganisms produce horizontal migrating resistance genes（ARGs）,which will further harm the natural environment.In this study,in order to remove tetracyclines antibiotics（TCs）in sewage,the waste oyster shell was used as a substrate for constructed wetlands.The Zinc-iron-Layered double hydroxides（Zn Fe-LDHs）were coated on the oyster shell surface by the hydrothermal-co-precipitation method,and the characterization of the modified oyster shells and the adsorption experiments were performed to verify its adsorption properties.The removal of oxytetracycline（OTC）and tetracycline（TC）common in tetracyclines antibiotics were researched by designing of the vertical flow constructed wetlands（VFCWs）within 126 days,and the strengthening effect of modified substrates on these systems was clarified.The microbial communities within the constructed wetland were analyzed to explore the effect of the modified substrates on microorganisms and the four target tetracycline resistance genes（tet A,tet W,tet M and tet O）,and the correlations between the contaminants and the ARGs were also analyzed.The main study conclusions are as follows:（1）Characterization tests of Scanning Electron Microscopy（SEM）,Brunauer Emmett Teller（BET）and X-ray diffraction（XRD）showed that Zn Fe-LDHs were successfully coated on oyster shells,and increased the specific surface area and atomic species of modified oyster shells.The adsorption experiments showed that the maximum adsorption capacity of OTC and TC by Zn Fe-LDHs modified oyster shells were 438.99and 812.33mg/kg,respectively,which was significantly higher than gravel and natural oyster shells（164.37 and 232.17 mg/kg,264.37 and 353.74 mg/kg,respectively）.The adsorption kinetic model showed that the adsorption processed of two antibiotics mainly occurred on the surface of the matrix and was mainly chemisorption.The introduction of zinc and iron improved the removal effect of material through the chelation between metal ions and antibiotics.（2）Constructed wetlands experiment showed that all systems can high-effectively remove COD,and without significant difference.Modified oyster shell affected the diversity and richness of microorganisms associated NH₄⁺-N removal in the system,and decreased the nitrogen removal rates.Modified oyster shell and modified artificial oyster sphere systems significantly improved the removal rates of OTC,TC and TP,reached to85.67%,87.27%,86.55%and 91.45%,71.94%and 74.09%,respectively,which were significantly higher than gravel（51.32%,45.57%and 39.3%）and oyster shell system（48.76%,59.79%and 54.86%）.Micro devices experiments suggested possible competitive adsorption between TP,OTC and TC.（3）The number of microbial ASVs/OTUs and the total relative abundances of four tet genes（0.315 and 0.228）in the modified oyster shell and the modified artificial oyster sphere systems were lower than those in the gravel and oyster shell systems（0.237 and0.427）.These results revealed that the Zn Fe-LDHs significantly affected the microbial activity and reduced the accumulation of ARGs.Then,the correlation analysis of ARGs and conventional contaminants suggested that nutrients may promote the spread of ARGs in constructed wetlands,and intl1 can also contribute to the production of ARGs.To sum up,oyster shell modified by ZnFe-LDHs can significantly improve the removal efficiency of OTC and TC,and affected the nitrogen biodegradation.However,ARGs may still exist in the system and be difficult to completely remove due to the presence of nutrients.