title: Fate and transformation of silver nanoparticles in urban wastewater systems.
abstract: Discharge of silver nanoparticles (Ag-NP) from textiles and cosmetics, todays major application areas for metallic Ag-NP, into wastewater is inevitable. Transformation and removal processes in sewers and wastewater treatment plants (WWTP) will determine the impact of Ag-NP on aquatic and terrestrial environments, via the effluents of the WWTP and via the use of digested sludge as fertilizer. We thus conducted experiments addressing the behavior of Ag-NP in sewers and in WWTP. We spiked Ag-NP to a 5 km long main trunk sewer and collected 40 wastewater samples after 500 m, 2400 m and 5000 m each according to the expected travel times of the Ag-NP. Excellent mass closure of the Ag derived by multiplying the measured Ag concentrations times the volumetric flow rates indicate an efficient transport of the Ag-NP without substantial losses to the sewer biofilm. Ag-NP reacted with raw wastewater in batch experiments were sulfidized to roughly 15% after 5 h reaction time as revealed by X-ray absorption spectroscopy (XAS). However, acid volatile sulfide (AVS) concentrations were substantially higher in the sewer channel (100 μM) compared to the batch experiments (3 μM; still sufficient to sulfidize spiked 2 μM Ag) possibly resulting in a higher degree of sulfidation in the sewer channel. We further investigated the removal efficiency of 10 nm and 100 nm Ag- and gold (Au)-NP coated with citrate or polyvinylpyrrolidone in activated sludge batch experiments. We obtained very high removal efficiencies (≈ 99%) irrespective of size and coating for Ag- and Au-NP, the latter confirming that the particle type was of minor importance with respect to the degree of NP removal. We observed a strong size dependence of the sulfidation kinetics. We conclude that Ag-NP discharged to the wastewater stream will become sulfidized to various degrees in the sewer system and are efficiently transported to the WWTP. The sulfidation of the Ag-NP will continue in the WWTP, but primarily depending on the size the Ag-NP, may not be complete. Very high removal efficiencies in the WWTP will divert most of the Ag-NP mass flow to the digester and only a small fraction of the Ag will be released to surface waters.
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland. ralf.kaegi@eawag.ch
authors.forename: Ralf
authors.initials: R
authors.lastname: Kaegi
authors.forename: Andreas
authors.initials: A
authors.lastname: Voegelin
authors.forename: Christoph
authors.initials: C
authors.lastname: Ort
authors.forename: Brian
authors.initials: B
authors.lastname: Sinnet
authors.forename: Basilius
authors.initials: B
authors.lastname: Thalmann
authors.forename: Jasmin
authors.initials: J
authors.lastname: Krismer
authors.forename: Harald
authors.initials: H
authors.lastname: Hagendorfer
authors.forename: Maline
authors.initials: M
authors.lastname: Elumelu
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Mueller
mesh_terms: D002947:Cities; D053768:Metal Nanoparticles / Q000032:analysis* / Q000648:ultrastructure; D017348:Microscopy, Electron, Scanning Transmission; D008858:Microscopy, Phase-Contrast; D012722:Sewage / Q000737:chemistry; D012834:Silver / Q000032:analysis*; D062065:Wastewater / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D012722:Sewage; D012834:Silver; D062065:Wastewater