Impacts of Ag and Ag2S nanoparticles on the nitrogen removal within vertical flow constructed wetlands treating secondary effluent

Abstract In this study, the effects of silver (Ag NPs) and sliver sulfide nanoparticles (Ag2S NPs) on nitrogen removal and nitrogen functional microbes in constructed wetlands were investigated. The obtained results demonstrated that inhibition extent on nitrogen removal relied on NPs types and high concentrations NPs showed higher negative effects. 0.5 mg/L Ag NPs had no influence on NH4+-N removal, amoA and nxrA gene copies, whereas Ag2S NPs and Ag+ decreased NH4+-N removal by reducing abundances of nitrifying genes. The concentrations of NO3−-N and TN in all 0.5 mg/L obviously increased compared with control, resulting from decreasing functional genes and denitrifying bacteria. And 0.5 mg/L Ag NPs exhibited largest inhibitory effects, with the highest NO3−-N effluent concentrations. 2 mg/L Ag NPs decreased NH4+-N removal, but adverse effects gradually vanished with extension of time, whereas both Ag2S NPs and Ag+ at 2 mg/L influenced NH4+-N transformation and decreased the abundance of amoA and nxrA genes and the AOB Nitrosomonas in CWs. Moreover, 2 mg/L of Ag NPs reduced NO3−-N removal by decreasing abundance of nirS and key denitrifying bacteria. To sum up, the inhibition mechanisms concluded from current results were possibly in that Ag NPs exhibited nanotoxicity rather than ionic toxicity.