Study On The Decontamination Performance Of Constructed Wetlands For Advanced Treatment Under The Stress Of Silver Nanoparticles Exposure

In recent years,nanotechnology has developed rapidly,and the application of nanoparticles has been generalized to daily life,which has led to the inevitable entry of nanoparticles into the water environment.Silver nanoparticles?AgNPs?,as the most widely used nanoparticles,has strong antibacterial properties.AgNPs exposure may have potentially negative effects on the environment,and the toxicity mechanism of AgNPs and the corresponding environmental effects are worthy of study further.In four constructed wetlands?CWs?with I.pseudacorus exposure to AgNPs,Ag⁺and Ag₂S-NPs?low concentration?0.5mg/L?and high concentration?2mg/L??for advanced wastewater treatment.The results showed that the exposure of AgNPs,Ag⁺and Ag₂S-NPs in low concentration?0.5mg/L?had no effect on the removal of COD.The AgNPs affected NO₃^--N removal,and Ag₂S-NPs affected the removal of NH₄⁺-N.Besides the Ag⁺had effect on the removal of NO₃^--N and NH₄⁺-N,and Ag₂S-NPs could affect TP removal.The AgNPs,Ag⁺and Ag₂S-NPs exposure of high concentration?2mg/L?affected the removal of COD,and the rank of inhibition effect at the most toxic moment was:AgNPs>Ag⁺>Ag₂S-NPs.Three kinds of silver existing forms inhibited the removal of NH₄⁺-N.AgNPs and Ag⁺inhibited NO₃^--N removal,and Ag₂S-NPs had no effect on the removal of NO₃^--N.The rank of inhibition effect to phosphorus removal:Ag⁺>AgNPs?Ag₂S-NPs.In two different flow of CWs with I.pseudacorus exposed to AgNPs?0.5mg/L and 2mg/L?.The results showed that under low concentration?0.5mg/L?AgNPs exposure,there was no difference in the removal of COD and TP in these two different flows of CWs.But the upflow CWs in denitrification was better than the downflow CWs.Under the AgNPs exposure of high concentration?2mg/L?,the removal of COD and the denitrification effect were inhibited obviously in the downflow CWs.However,there was little inhibition phenomenon in the nitrogen removal in the upflow CWs.In these two different flows of CWs,the phosphorus removal was affected by AgNPs exposure,but the recovery ability of the upflow CWs was obviously better.In a word,the upflow CWs is superior to the downflow CWsIn the CWs with 0.5mg/L?50 days?and 2mg/L?100 days?continuous silver exposure,the migration and regression of silver were investigated.The results showed that the CWs with I.pseudacorus were good for the removal of three kinds of silver.The silver had no effect on the biomass of I.pseudacorus in the downflow CWs,but AgNPs exposure promoted the increase of I.pseudacorus biomass in the upflow CWs.In three kinds of silver existing forms,AgNPs had the strongest migration ability in the matrix,Ag⁺had the strongest ability of transfer and enrichment in I.pseudacorus,and Ag₂S-NPs had a weak ability to transfer.Compared with plants,the matrix was more significant to silver removal.In the downflow CWs,95%silver was adsorbed by the sand layer,but zeolite and gravel adsorbed AgNPs mainly in the upflow wetland system.