Fate of Zinc and Silver Engineered Nanoparticles in Sewerage Networks
Brunetti, G., E. Donner, G. Laera, R. Sekine, K. Scheckel, M. Khaksar, K. Vasilev, G. De Mastro, AND E. Lombi. Fate of Zinc and Silver Engineered Nanoparticles in Sewerage Networks. Mark van Loosdrecht (ed.), WATER RESEARCH. Elsevier Science Ltd, New York, NY, 77:72-84, (2015).
The rapid commercialisation of nanotechnology over the last decade has raised concerns about the potential environmental consequences of engineered nanoparticles used in consumer products. Silver (Ag) and zinc oxide (ZnO) are among the most common engineered nanoparticles used in this manner. For instance, nearly a quarter of the more than 1600 consumer products containing nanomaterials listed by the Project on Emerging Nanotechnologies (2013) claim to contain Ag nanomaterials, and it is reported that up to 500 tons/year of Ag nanoparticles (Ag-NPs) are produced and used worldwide (Gottschalk et al., 2009; Sun et al. 2014). Silver nanomaterials are widely used as antibacterial and antifungal agents in a large variety of consumer products, as well as in food technology, textiles/fabrics, and medical products and devices. In the case of nanoscale ZnO, the estimated worldwide production in 2011 was greater than 33,400 tons/year (Future Markets Inc, 2012). Zinc oxide nanoparticles (ZnO-NPs) are mainly used as active ingredients in sunscreens and as industrial coatings to protect plastics, wood and textiles from UV exposure and microbial degradation, and they are also used in photoprinting and as thin conductive films in household appliances and solar cells (Lamb et al. 2004; Mu and Sprando, 2010). The aim of this study was to investigate the transformations that can occur in the sewer before Ag- and ZnO-NPs reach WWTPs. To this end, a lab-scale sewerage system was set up and operated for 3 months (to establish biofilm) prior to spiking with Ag-NPs and ZnO-NPs for 3 consecutive days. A control system was run in parallel using equivalent amounts of Ag and Zn spiked in dissolved forms instead of as NPs.
Engineered zinc oxide (ZnO) and silver (Ag) nanoparticles (NPs) used in consumer products are largely released into the environment through the wastewater stream. Limited information is available regarding the transformations they undergo during their transit through sewerage systems before reaching wastewater treatment plants. To address this knowledge gap, laboratory-scale systems fed with raw wastewater were used to evaluate the transformation of ZnO- and Ag-NPs within sewerage transfer networks. Two experimental systems were established and spiked with either Ag- and ZnO-NPs or with their dissolved salts, and the wastewater influent and effluent samples from both systems were thoroughly characterised. X-ray absorption spectroscopy (XAS) was used to assess the extent of the chemical transformation of both forms of Zn and Ag during transport through the model systems. The results indicated that both ZnO- and Ag-NPs underwent significant transformation during their transport through the sewerage network. Reduced sulphur species represented the most important endpoint for these NPs in the sewer with slight differences in terms of speciation; ZnO converted largely to Zn sulfide, while Ag was also sorbed to cysteine and histidine. Importantly, both ionic Ag and Ag-NPs formed secondary Ag sulfide nanoparticles in the sewerage network as revealed by TEM analysis. Ag-cysteine was also shown to be a major species in biofilms. These results were verified in the field using recently developed nanoparticle in situ deployment devices (nIDDs) which were exposed directly to sewerage network conditions by immersing them into a municipal wastewater network trunk sewer and then retrieving them for XAS analysis.