Hydraulic flow direction alters impacts of AgNPs on pollutant removal and silver spatial distribution in vertical flow constructed wetlands

Abstract

This study investigated the effects of AgNPs on pollutant removals in constructed wetlands (CWs) with different flow patterns and spatial distributions of silver. Before exposure to AgNPs, upward flow constructed wetland (UCW) had better nitrogen removal than down-flow CW (DCW). And 0.5 mg/L AgNPs evidently inhibited nitrogen and phosphorus removal, including ammonia, nitrate, and TP (total phosphorus), with average effluent concentrations increasing by 70.83% of NH4+-N in UCW, 18.75% of TP in UCW, and 28.33% and 25.06% of NO3--N in DCW and UCW, respectively, while COD (chemical oxygen demand) was not affected. Moreover, presence of 2 mg/L AgNPs slightly inhibited organic compounds and NH4+-N removal in two systems during stage 4 (dosing 2 mg/L AgNPs). However, the response of NO3--N and TN removal to 2 mg/L AgNPs in two systems were different, and nitrogen concentrations in effluent at the end of stage 4 significantly increased in DCW. Addition of 2 mg/L AgNPs significantly affected TP removal in two systems. Two wetlands showed high removal efficiencies of about 98% on AgNPs, indicating that CWs could provide a feasible approach for ecological restoration of nanoparticles pollution. This study also found that AgNPs mainly accumulated in the upper layer with the Ag content of 17.55–20.26 mg/kg dry weight in sand layer and 7.25–10.85 mg/kg dry weight in gravel layer. Plant roots absorbed AgNPs, with Ag content at 50.80–101.40 mg/kg and bioconcentration factors 2.80–5.00. The obtained results showed that up-flow CWs had better performance and higher resistance to the exposure of AgNPs pollution, compared with down-flow CWs.