Sorption to engineered nanomaterials and its impact on the bioavailability/toxicity of fossil fuel-derived hydrocarbons to aquatic organisms

The project aims to understand how carbon nanomaterials (e.g. carbon nanotubes and fullerenes) behave in freshwater environments and how they interact with common organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). Carbon nanotubes (CNTs) with different physical (walls, lengths and diameters) and chemical (surface chemistry) properties and fullerenes (nC60) have been studied in relation to their environmental behaviour (dispersion concentration and stability). The influence of natural organic matter (NOM) on the behaviour and transport of CNMs in environmental relevant conditions is also investigated. Novel ecotoxicity exposure systems, based upon standard OECD test guidelines, are being developed for freshwater ecosystems containing organic pollutants, NOM and CNTs. The adsorption of PAHs to CNTs and the subsequent influence on ecotoxicity is studied. 5 CNTs with different physicochemical properties were characterised in detail: shape, size, purity, surface chemistry, specific surface area (SSA) and elemental composition. Studies investigated CNT aquatic fate (dispersion concentration and stability) in three different ecotoxicity media in the presence and absence Suwannee River NOM (SR-NOM). Large CNT SSA correlated to low dispersibility and stability, whilst polar functional groups increased the dispersibility and stability of CNTs. SR-NOM increased both dispersibility and stability of all CNTs, but behaviour varied in different media types. Adsorption of phenanthrene (Phe) to CNTs dispersed in freshwater algae media with SR-NOM was determined at different Phe concentrations. Larger CNT SSA correlated to higher adsorption, whilst polar functional groups reduced CNT adsorption capacity for Phe. The Dubinin-Ashtakhov model has been successfully used to describe Phe adsorption to the CNTs. The influence of environmental factors (NOM, salinity and pH) on the adsorption of Phe to carbon nanomaterials (CNMs; including CNTs and nC60) were investigated. The influence of SR-NOM, including co-occurring and preloaded, on the CNM-adsorbed and freely dissolved concentrations of Phe were investigated. As the initial concentration of SR-NOM preloaded to CNMs increased, the SSA of CNMs decreased, and the adsorption of Phe decreased accordingly. As the concentration of SR-NOM and CNMs was increased, the freely dissolved concentration of Phe continuously decreased. In contrast, salinity and pH over a wide range had only minor effects on the sorption of Phe to CNMs. To understand the effects of molecular weight fractions of NOM on aggregation kinetics of nC60, pristine SR-NOM and molecular weight fractions of SR-NOM (Mf-SRNOMs) were comprehensively characterized and their impacts on nC60 aggregation investigated in monovalent and divalent electrolyte solutions at varying concentrations. Aromatic structures were abundant in high Mf-SRNOMs and carboxylic functional groups were more abundant in low Mf-SRNOMs. At low electrolyte concentrations (monovalent and divalent) 1 mg L-1 (as carbon) SRNOM>100 kD provided significantly better stability of nC60 than other Mf-SRNOMs. In high electrolyte concentrations, the stability of nC60 positively correlated to the MW of Mf-SRNOMs in NaCl. However, due to the cation-bridging effect of divalent CaCl2 and MgCl2 with Mf-SRNOMs, an enhancement or acceleration of aggregation kinetics of nC60 were observed in some solutions, especially in high Mf-SRNOMs. Overall, high Mf-SRNOMs provided significantly different influences on nC60 aggregation among monovalent and divalent electrolyte solutions, compared with their low MW counterparts. The bioavailability and toxicity of Phe in aqueous CNT dispersions containing NOM was evaluated using the freshwater algae Pseudokirchneriella subcapitata and the water flea Daphnia magna. None of the CNTs were acutely toxic to either species. Adsorption of Phe to CNTs does not reduce the bioavailability to P. subcapitata for MWCNTs, with most adsorbed Phe remaining bioavailable. CNT SSA and surface chemistry influence Phe toxicity to algae, as seen by a higher effect from SWCNT at one 10x lower concentration than MWCNT-COOH. Adsorption to CNTs slightly reduces the overall bioavailability of Phe to D. magna for some CNT types, but a significant proportion of adsorbed Phe remains bioavailable. As demonstrated by microscopy images, attachment (P. subcapitata) and ingestion (D. magna) offer alternative routes for Phe update and toxicity when adsorbed to CNTs. The bioavailability and toxicity of Phe in aqueous nC60 dispersions was also evaluated using junior carp (Cyprinus carpio). Particles of nC60 were mostly distributed in fish liver and intestines, indicating uptake was through ingestion. Between 22-100% of the Phe-nC60 complex contributed to Phe bioaccumulation, but the complex did not contribute to toxicity.

The research project is focused on generating new knowledge about the interaction and potential impacts of engineered nanoparticles (ENPs) with pollutant petroleum-derived compounds present in aqueous environments. This project will study the commercially and scientifically important silver and fullerene ENPs. As part of this project a physico-chemical characterisation scheme will be implemented that is relevant for ENPs released into aquatic environments. Importantly, the project will investigate how am bient aqueous environmental conditions (e.g. natural organic matter, salinity/ionic strength, pH and temperature) affect both the physico-chemical properties of the nanoparticles (e.g. aggregation) and their adsorption of crude oil compounds. An assessmen t of how these parameters affect the concentration and distribution of crude oil compounds adsorbed to the tested ENPs will also be conducted. The studies will use zooplankton and fish species to study the bioavailability and ecotoxicity of crude oil com ponents adsorbed to ENPs. ENPs with crude oil compounds adsorbed to them will be exposed to organisms and acute toxicity endpoints determined. In addition, the zooplankton and fish species will be used to study uptake and trophic transfer of both petrole um compounds and ENPs by aquatic organisms. Zooplankton will be exposed to a mixture of nanoparticles and dissolved petroleum compounds and then subsequently exposed to fish in order to assess transfer potential. The findings of these studies will be dire ctly relevant to the environmental fate and effects other types of engineered nanoparticles and organic aquatic pollutants.