Impacts Of Silver Nanoparticles On Nitrogen Removal Performance Of Constructed Wetland

With the rapid development of nanotechnology,silver nanoparticles?AgNPs?are widely used in various fields due to their excellent properties.However,a large number of AgNPs contained in daily necessities and industrial products are inevitably discharged into the sewage treatment system,thus posing unknown risks to ecological environment.In this study,the effects of long-term exposure?450d?of AgNPs on the nitrogen removal in constructed wetlands?CWs?were systematically investigated along with their removal and fate.The study provides valuable information and reference for CWs treating wastewater containing this kinds of emergent contanminants.In three lab-scale CWs with I.pseudacorus,after dosing different concentrations of AgNPs?0,50 and 200?g/L?,the removal of TN and NH₄⁺-N at the early stage significantly decreased,and the inhibitory effects were positively correlated with the AgNPs concentrations.Then the removal efficiencies of nitrogen were gradually recovered with time,and the differences of inhibitory effects on two CWs exposed to 50 and 200?g/L of AgNPs were also decreased,respectively.Compared with the wetland without AgNPs,both of 50 and 200?g/L of AgNPs had no inhibitory effects on the removal efficiencies of TN and NH₄⁺-N and the nitrogen removal kept stable in the later experiment.The variations of effluent along the way showed that TN and NH₄⁺-N were mainly removed by the top soil layer,correspondingly AgNPs mainly affected the degradation of soil layer.The results of quantitative polymerase chain reaction?qPCR?showed that the long-term exposure of AgNPs promoted the expression of nitrification functional genes?amoA,nxrA?in the soil layer,while having no effects on the denitrification functional genes?nirK;nirS;nosZ?and anammox bacterial 16S rRNA.The pH value of CWs was not affected.In addition,COD removal was not affected at 50?g/L of AgNPs,while 200?g/L of AgNPs significantly inhibited the COD removal in the early exposure time,then COD removal returned to normal levels and had no statistical difference with control group.Finally,more than 90%of AgNPs in wastewater were removed by CWs.In four lab-scale CWs,the effects of AgNPs on the performances of CWs with different plant species?I.pseudacorus,A.donax and C.alternifolius?were investigated.The results showed that 100?g/L of AgNPs significantly inhibited nitrogen removal in all CWs in the initial exposure stage.Compared with unplanted wetlands,the wetland with plants showed higher nitrogen removal,revealing that the prensence of macrophytes could significantly reduce the toxic effects of AgNPs.It was found that I.pseudacorus had strong resistance to the acute toxicity of AgNPs,while C.alternifolius showed the higher resistance to AgNPs at low temperature conditions.The variations of effluent concentrations of pollutants along the way showed that AgNPs obviously affected the removal of TN and NH₄⁺-N in soil layer.During the whole experiment period,the removal efficiency of COD and pH value in all wetlands were not obviously affected.In addition,the wetlands could remove above 90%of AgNPs from wastewater.The silver concentrations in the overground part of wetland plants were lower than those of the underground parts.C.alternifolius had the higher abilities to absorb,enrich,and transfer AgNPs than the other plants.The effects of 20?g/L AgNPs on the nitrogen removal in CW for the advanced wastewater treatment system were also investigated.The results showed that 20?g/L of AgNPs significantly inhibited the removal of TN and NH₄⁺-N,and the inhibition increased with time.After CW was exposed to AgNPs for 330 days,the removal efficiencies of TN and NH₄⁺-N gradually kept stable.The 20?g/L of AgNPs had no significant effects on the COD removal and pH value.In addition,more than 90%of AgNPs could be removed from wastewater by advanced wetlands.