Monitoring the behaviour of silver nanoparticles as emerging contaminants in urban wastewaters

Abstract

Nanomaterials (NMs) have become a distinctive component in several manufactured commercial, industrial, and domestic products due to the extensive development of nanotechnology over the last decade. As a result, there are some concerns over the intentional and incidental discharges of NMs into the environment due to widespread uncertainty of their roles as emerging contaminants (ECs). As an EC, NMs will find their way into municipal wastewater treatment plants at various stages of their life cycles. If not effectively treated, they will eventually be discharged into the aquatic environment, which serves as the final sink. Despite advances in the applications of NMs, there is still a lack of knowledge in NM quantification, as well as in their fate, transport and behaviour in the environment. Of all NMs, silver nanoparticles (AgNPs) are the most common and extensively produced and utilized nanoparticles (NPs), found regularly in various nano-products (including personal care products, home appliances, laundry additives, cosmetics, food preservatives, paints, and textiles) mainly due to their unique antifungal, antiviral, and antibacterial attributes. As a result, AgNPs are likely to be released into the aquatic environment as NPs, aggregates or agglomerates, and soluble or insoluble ions, which will be a source of contaminants and could have toxic effects on aquatic organisms. The fate and behaviour of AgNPs are influenced by intrinsic NP characteristics such as shape, size, structure, coating, morphology, surface area; as well as environmental conditions of the media such as pH, salinity, ionic strength, total dissolved solids (TDS), natural organic matter (NOM), and dissolved organic matter (DOM). In this study, a simulated wastewater treatment plant (SWWTP) was constructed according to the Organization for Economic Cooperation and Development (OECD) guidelines (OECD 303A) to mimic a typical municipal sewage treatment plant. The SWWTP consisted of three units (a control containing no NPs, and two test units (containing 5 mg/L and 10 mg/L, respectively)) run simultaneously. Each unit consisted of an influent holding tank (5 L), aeration chamber (3 L), a settling vessel (1.5 L), and an effluent tank (5 L). The influent and aeration tanks were constantly stirred to keep the wastewater in suspension. The aeration chamber was fed by pumping influent at a rate of 6.33 L/min using a peristaltic pump, with air being constantly supplied using a glass frit at a flow rate of 290 L/min to keep the dissolved oxygen (DO) above 2 mg/L and hydraulic retention time maintained at 6 hours. The SWWTP was stabilized and optimized for seven days before the introduction of the AgNPs, after which the wastewater characteristics were tested at various stages of the treatment process before and after the exposure to AgNPs. The AgNPs were also characterized pre- and post-exposure. The AgNP were characterized using different techniques including transmission electron microscopy (TEM), X-ray Diffraction (XRD) to establish NP morphology and chemical state, and an energy dispersive X-ray spectroscopy analysis (EDS) to determine the elemental composition. The specific surface area of the AgNP nano-powder was analysed by BET. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine the metal (Ag) concentration within the each sample. The results obtained indicate that the physicochemical properties of the aqueous media are a critical governing factor that influences the transformation of AgNPs in the wastewater system. A comparison analysis into the result obtained from the two test units provides a distinct correlation between particle concentration and transformation process of AgNP in wastewater media, with the 10 mg/L AgNPs test unit having formed larger aggregates and a fast dissolution rate in the influent, sludge and effluent ICPOES confirm that approximately 70% of Ag was collectively retained in sludge and effluent in both treatments, thus potentially posing a threat to both environmental and human health.