title: Transformation of PVP coated silver nanoparticles in a simulated wastewater treatment process and the effect on microbial communities.
abstract: BACKGROUND
  Manufactured silver nanoparticles (AgNPs) are one of the most commonly used nanomaterials in consumer goods and consequently their concentrations in wastewater and hence wastewater treatment plants are predicted to increase. We investigated the fate of AgNPs in sludge that was subjected to aerobic and anaerobic treatment and the impact of AgNPs on microbial processes and communities. The initial identification of AgNPs in sludge was carried out using transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) analysis. The solid phase speciation of silver in sludge and wastewater influent was then examined using X-ray absorption spectroscopy (XAS). The effects of transformed AgNPs (mainly Ag-S phases) on nitrification, wastewater microbial populations and, for the first time, methanogenesis was investigated.
  
  
  RESULTS
  Sequencing batch reactor experiments and anaerobic batch tests, both demonstrated that nitrification rate and methane production were not affected by the addition of AgNPs [at 2.5 mg Ag L(-1) (4.9 g L(-1) total suspended solids, TSS) and 183.6 mg Ag kg (-1) (2.9 g kg(-1) total solids, TS), respectively]. The low toxicity is most likely due to AgNP sulfidation. XAS analysis showed that sulfur bonded Ag was the dominant Ag species in both aerobic (activated sludge) and anaerobic sludge. In AgNP and AgNO3 spiked aerobic sludge, metallic Ag was detected (~15%). However, after anaerobic digestion, Ag(0) was not detected by XAS analysis. Dominant wastewater microbial populations were not affected by AgNPs as determined by DNA extraction and pyrotag sequencing. However, there was a shift in niche populations in both aerobic and anaerobic sludge, with a shift in AgNP treated sludge compared with controls. This is the first time that the impact of transformed AgNPs (mainly Ag-S phases) on anaerobic digestion has been reported.
  
  
  CONCLUSIONS
  Silver NPs were transformed to Ag-S phases during activated sludge treatment (prior to anaerobic digestion). Transformed AgNPs, at predicted future Ag wastewater concentrations, did not affect nitrification or methanogenesis. Consequently, AgNPs are very unlikely to affect the efficient functioning of wastewater treatment plants. However, AgNPs may negatively affect sub-dominant wastewater microbial communities.
authors.affiliation: School of Agriculture Food & Wine, The University of Adelaide, PMB 1, Glen Osmond, SA, 5064, Australia.
authors.forename: Casey L
authors.initials: CL
authors.lastname: Doolette
authors.affiliation: School of Agriculture Food & Wine, The University of Adelaide, PMB 1, Glen Osmond, SA, 5064, Australia ; CSIRO Land and Water, Environmental Biogeochemistry Program, Advanced Materials Transformational Capability Platform-Nanosafety, Waite Campus, Waite Road, Urrbrae, SA, 5064, Australia.
authors.forename: Mike J
authors.initials: MJ
authors.lastname: McLaughlin
authors.affiliation: CSIRO Land and Water, Environmental Biogeochemistry Program, Advanced Materials Transformational Capability Platform-Nanosafety, Waite Campus, Waite Road, Urrbrae, SA, 5064, Australia.
authors.forename: Jason K
authors.initials: JK
authors.lastname: Kirby
authors.affiliation: Advanced Water Management Centre, The University of Queensland, St Lucia, Queensland, 4072, Australia.
authors.forename: Damien J
authors.initials: DJ
authors.lastname: Batstone
authors.affiliation: School of Chemistry and Physics, The University of Adelaide, Adelaide, SA, 5005, Australia.
authors.forename: Hugh H
authors.initials: HH
authors.lastname: Harris
authors.affiliation: Advanced Water Management Centre, The University of Queensland, St Lucia, Queensland, 4072, Australia.
authors.forename: Huoqing
authors.initials: H
authors.lastname: Ge
authors.affiliation: Department of Chemistry, The University of Gothenburg, Kemivägen 10, Göteborg, 41296, Sweden.
authors.forename: Geert
authors.initials: G
authors.lastname: Cornelis
keywords: Biosolids; Microbial communities; Nitrification; Pyrotag sequencing; STEM HAADF; Sequencing batch reactor; Silver nanoparticles; Silver speciation; Silver sulfide; Synchrotron; Wastewater treatment; XAS