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Monitoring the behaviour of silver nanoparticles as emerging contaminants in urban wastewaters
Author(s)
Bisi-Johnson, Moses Ajibade
Date Issued
2023
Type
Thesis
Publisher
Cape Peninsula University of Technology
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.
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.
Additional information
Thesis (MEng (Chemical Engineering))--Cape Peninsula University of Technology, 2023
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