Adsorption of phenanthrene to carbon nanotubes and its influence on phenanthrene bioavailability/toxicity in aquatic organisms
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- Institutt for biologi 
Carbon nanotubes (CNTs) are engineered nanomaterials (ENMs) which due to their unique properties are being incorporated into a growing number of applications. The increased production and use of CNTs increases the risk of their release into the environment. Consequently, there is a need to investigate the possible environmental impacts of CNTs. An ecotoxicological relevant aspect of CNTs is their interaction with other environmental pollutants. Due to their high specific surface area (SSA) and hydrophobicity they are potent sorbents of hydrophobic organic compounds, including polycyclic aromatic hydrocarbons (PAHs). In aquatic ecosystems, the adsorption of PAHs to CNTs can possibly decrease the bioavailability of PAHs to aquatic organisms as the concentration of freely dissolved PAHs is reduced. However, it is also possible that the PAH bioavailability increases if PAHs concentrated on the CNTs are released from the particles when they are ingested by or come in contact with aquatic organisms. A range of CNTs with different physicochemical properties (e.g. diameter, SSA and surface chemistry) are being produced. The physicochemical properties of the CNTs can possibly influence both the dispersion behavior of the CNTs in aqueous media, and their adsorption and effect on PAH bioavailability and toxicity to aquatic organisms. The aim of the current thesis was to investigate the interaction between phenanthrene, a model PAH, and a suite of CNTs, and furthermore the influence of phenanthrene adsorption to CNTs on phenanthrene bioavailability and toxicity to aquatic organisms. The CNTs exhibited different physicochemical properties and included one single-walled CNT (SWCNT), two non-functionalized multi-walled CNTs (MWCNT-15 and MWCNT-30; numbers are referring to the maximum outer diameter [nm]) and two functionalized MWCNTs (MWCNT-OH and MWCNT-COOH). Throughout the studies, the CNTs were dispersed in synthetic growth media, commonly used in aquatic ecotoxicological tests, and in the presence of natural organic matter (NOM). A method to determine phenanthrene adsorption to the suite of dispersed CNTs was developed. Furthermore, their dispersibility (dispersed CNT concentration) and dispersion stability (CNT concentration remaining in the water phase over time) in the different media was examined. Phenanthrene adsorption to the CNTs dispersed in two different growth media was determined, and the subsequent influence on phenanthrene toxicity was investigated using two freshwater species: the algae Pseudokirchneriella subcapitata and the crustacean Daphnia magna. The studies presented in the current thesis showed that phenanthrene adsorption to the suite of dispersed CNTs could be investigated using a filtration method. The dispersed CNTs were separated from the water phase using a 0.1 µm hydrophilic PTFE filter membrane, allowing determination of the freely dissolved phenanthrene concentration remaining in the water phase and subsequent calculation of the phenanthrene concentration adsorbed to the CNTs. CNT dispersibility and dispersion stability depended on both CNT physicochemical properties and media properties. The influence of CNT surface chemistry on CNT dispersibility was highest in the synthetic media exhibiting the lowest ionic strength and concentration of divalent cations. The dispersion behavior of the functionalized MWCNTs was more sensitive to changes in media properties than that of the non-functionalized CNTs. The SWCNTs exhibited a higher adsorption capacity of phenanthrene than the MWCNTs, most likely related to the higher SSA of the SWCNTs compared to the MWCNTs. The presence of oxygen-containing surface functional groups on the functionalized MWCNTs significantly suppressed phenanthrene adsorption in the media with low ionic strength and divalent cation concentration. In contrast, there was no relation between surface chemistry and phenanthrene adsorption to the CNTs in the media exhibiting higher ionic strength and divalent cation concentration. The presence of dispersed CNTs reduced or had no effect (depending on CNT type) on the overall phenanthrene toxicity to P. subcapitata and D. magna. However, the studies presented in the current thesis suggest that phenanthrene adsorbed to all CNT types remains bioavailable and contributes to exert toxicity in both species. No relation between specific CNT properties and bioavailability/toxicity of CNT-bound phenanthrene was observed.