Effect of ZnFe-LDHs modified oyster shell on the removal of tetracyclines antibiotics and variation of tet genes in vertical flow constructed wetlands

The emission of residual antibiotics has already caused environmental hazards. The removal performance of typical tetracyclines antibiotics (oxytetracycline and tetracycline) and the effect of 4 corresponding antibiotic resistance genes (ARGs) were investigated in vertical flow constructed wetlands (VFCWs) with ZnFe-layered double hydroxides (ZnFe-LDHs) modified substrates. The characterizations tests exhibited ZnFe-LDHs were successfully coated to the oyster shell, and increased the specific surface area and atomic species of the modified oyster shell. Adsorption experiments showed that the highest saturated adsorption capacity of oxytetracycline and tetracycline in ZnFe-LDHs respectively reached 1171.51 and 2035.25 mg/kg, which was higher than those of the gravel and oyster shell (264.37 and 353.74 mg/kg, respectively). The CWs experiment indicated that the average removal rates of oxytetracycline and tetracycline in modified oyster shell and artificial sphere systems were respectively 87.27%, 86.55% and 71.94%, 74.09%, which was significantly superior to the gravel (45.57% and 39.3%) and oyster shell systems (59.79% and 54.86%). Moreover, the microbial ASVs/OTUs numbers and the total relative abundance of four resistance genes in the modified systems (0.315 and 0.228) were lower than those in natural systems (0.237 and 0.427). These results suggested that ZnFe-LDHs coating significantly affected microbial activity and reduced the accumulation of ARGs. Correlation analysis between ARGs and conventional pollutants exhibited that nutrient might affect the spread of ARGs in CWs. In summary, ZnFe-LDHs modified substrates could significantly improve the removal efficiency of tetracycline and oxytetracycline, and affected the N biodegradation, while ARGs might still accumulate in the systems due to the presence of nutrients.