Bioavailability of engineered nanoparticles
Creators: name: Al-Ejji, Maryam
Publisher: name: Imperial College London
Year: 2021

description: Engineered nanomaterials (ENMs) are finding increasing applications because of their unique physicochemical properties. The increasing usage of these nanomaterials, however, raises concerns about their potential toxicity and of lack of control or even understanding over the life cycle from production, to use, and finally disposal. ENMs used in consumer products are highly likely to reach the environment during use and disposal, so it is critical to investigate their potential impact. Detailed mechanistic information about the particles and their state at the point of exposure to organisms must be developed to understand this risk. In this thesis, the possible transformation of ENMs in synthetic and realistic environments is explored, and the impact on marine organisms investigated. The work encompasses comparisons of transformations under inorganic versus organic sulfides and anaerobic digestion and correlative experiments on the effect of these particles on marine algae. Natural organic matter (NOM) is the main content of the natural system, and in particular, humic acid (HA) is an organic matter that is known to influence the transformation of silver nanoparticles (AgNPs). An in situ sulfidation process was used to follow the transformation of citrate-capped silver nanoparticles (Cit-AgNPs) in environments containing organic versus inorganic sulfide. In both cases, sulfidation was observed, with a core-shell structure being more stabilised in organic components, and humic acid capped silver sulfide nanoparticles (HA-Ag2SNPs) displayed evidence of a hollow sphere structure. Anaerobic digestion is a wastewater treatment plant process, so ENMs polluting consumer waste streams will likely be exposed to this process. Its impact on particle transformation is essential to understand, as well as any impact that the particle may have on the digestion processes. Thus, ENMs were tested within a lab-scale anaerobic digester: 10 mg/l of AgNPs, titanium oxide nanoparticles (TiO2NPs), cerium oxide nanoparticles (CeO2NPs) and silver sulfide nanoparticles (Ag2SNPs) were shown to have no significant effect on biogas production, indicating that at low concentrations, the ENMs do not interfere with the reactors. Also, there was no differences between the impacts of AgNPs and Ag2SNPs on the performance of the reactor, and both structures aggregated and became fully sulfided). The speciation of zinc oxide nanoparticles (ZnONPs) and zinc sulfide nanoparticles (ZnSNPs) were observed after 35 days: 54% of ZnONPs had absorbed iron oxyhydroxides (Zn-Fe-Ox), and the rest had a different ratio of zinc oxide (ZnO), zinc phosphate Zn3PO4, and ZnS. ZnSNPs had mostly transformed into zinc phosphate (Zn3PO4). Both ZnONPs and ZnSNPs had a ratio of ZnO, which was not less than 15%. Finally, to achieve the objective of mimicking the environmental conditions, AgNPs were aged in Economic Co-operation and Development-((3-(N-morpholino) propanesulfonic acid) medium (OECD-MOPs). The medium was prepared according to organisation guidance 201 before AgNPs incubation. An algal growth inhibition test with MOPS buffer was carried out by spiking the freshwater green algae Raphidocelis subcapitata (R. subcapitata) with the particle-containing medium. The morphology of algal cells after treatment showed extensive deformation and disorganised cell walls. Some cells had evident changes on the cell wall from a rigid structure to a ‘hairy’ exterior and, in more extreme cases, had released extracellular polymeric substances (EPS). This EPS trapped the particles outside the algae and resulted in nanoparticle aggregation. Cytotoxicity was observed when Algae were exposed to AgNPs as well as some sulfidation, which correlated with intracellular uptake, dissolution, and precipitation of secondary Ag2SNPs. For the AgNPs, the release of ions was directly linked to toxicity, and this changed in the presence of light, oxygen, and EPS.