Effects And Mechanism Of Sulfamethoxazole On Nitrogen Conversion In Constructed Wetlands

The widespread use of antibiotics inevitably leads their increasing discharge into wastewater,which may pose a potential threat to biological denitrification.Constructed wetlands（CWs）is an efficient low C/N wastewater treatment system,which also an important gathering place of antibiotics.However,the influence mechanism of antibiotics on nitrogen removal in CWs is not clear.In this study,the effects and mechanism of different concentrations(0,1,10 mg·L^-1)of sulfamethoxazole（SMX）on nitrogen transformation in vertical flow constructed wetlands（VFCWs）were analyzed.Besides,the effects of different substrates（zeolite,ceramsite and volcanic rock）on nitrogen metabolism in VFCWs under high concentration SMX stress were investigated.The main results are as follows::（1）The removal efficiencies of 1 and 10 mg·L^-1 SMX in VFCWs were 98.92±2.74%and 58.63±12.86%,respectively.Exposure to 1 and 10 mg·L^-1SMX suppressed the removal of NH₄⁺-N,but significantly increased NO₃^--N removal（p<0.05）by 28.01%and 22.77%（p<0.05）,respectively.Metagenomic analysis certified that 1 mg·L^-1SMX had a selective inhibitory effect on the abundance of nitrifying genes,while 10 mg·L^-1SMX restrained the abundance of nitrifying genes and key enzyme.In addition,as a signal molecule,SMX significantly increased the abundance of denitrifying genes and key enzymes,which was responsible for the improvement of NO₃^--N removal.Correlation network analysis and nitrogen metabolic pathway analysis showed that low concentration of SMX inhibited the ammonia oxidation process of nitrifying bacteria,while high concentration of SMX promoted the assimilation of NO₃^--N by autotrophic denitrifying bacteria.In addition,SMX changed the microbial community structure in VFCWs,and the structural stability was proportional to the concentration of SMX.（2）The removal efficiency of SMX by CW-C（ceramsite）and CW-D（volcanic rock）was much lower than that of CW-B（zeolite system）（p<0.05）under 10 mg·L^-1SMX stress.Besides,different substrates had no significant effect on the removal of NH₄⁺-N and NO₃^--N（p>0.05）.Metagenomic analysis showed that the physicochemical properties of substrates in VFCWs affected the structure of microbial community and the composition of nitrogen-functional bacteria attached to them to a certain extent,but did not affect the expression of nitrogen-functional genes under high SMX stress.Overall,the removal of nitrogen and antibiotics by CW-B is slightly better than that of CW-C and CW-D,because the number of functional groups and absorption peak intensity of zeolite surface after adding antibiotics are higher than that of ceramite and volcanic rock.Moreover,the analysis of nitrogen metabolism pathway also showed that the abundance of key enzymes in CW-D is inhibited by SMX than that of CW-B and CW-C.