Dispersion Characterization of Carbon Nanotubes in Freshwater Media and Implications of their Adsorption Behavior for the Bioavailability and acute Toxicity of Phenanthrene in Daphnia magna.
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- Institutt for biologi 
To ensure the reproducibility of studies with engineered nanomaterials (ENMs) and to be able to evaluate the study outcomes properly, the state of the investigated ENMs dispersions needs to be characterized. In the course of this study, the dispersions of five different carbon nanotubes (CNTs) one single-walled CNT (SWCNT), two pristine multi-walled CNTs (MWCNT-2 and MWCNT-3), and two functionalized multi-walled CNTs (MWCNT-OH and MWCNT-COOH) were analyzed in two common freshwater media (Elendt M7 and TG201) over a period of more than 2 months. Dispersion stability and the settling behavior of the dispersed CNTs were assessed based on water chemistry parameters, zeta potential, and optical density. It was shown that the dispersion of each CNT type was unstable in both media, as zeta potential values were measured between 0 and -30 mV and the CNT concentration in the water phase decreased over time. However, differences in CNT settling behavior were observed between media types and between CNT types. The observed differences were in accordance with the literature and imply difficulties in maintaining comparable exposure conditions in studies with different CNT types or freshwater media. As CNTs are increasingly released into the aquatic environment, their interaction with other organic pollutants should be taken into account when evaluating environmental risks. This study investigated the implications of the adsorption of a model polycyclic aromatic hydrocarbon, phenanthrene, to the five aforementioned CNT types. A series of acute immobilisation tests with Daphnia magna exposed to phenanthrene only and to phenanthrene in the presence of CNTs was performed. When EC50s were based on the total phenanthrene concentration, the presence of each CNT type caused higher EC50 values (between 347.2 423.2 µg phenanthrene L-1 media for MWCNT-COOH and SWCNT, respectively) than it was observed for exposure with phenanthrene only (321.3 µg/L). It was suggested that CNT presence decreases the bioavailability of phenanthrene in the water phase due to sorption. However, only the presence of three CNT types (SWCNT, MWCNT-3, and MWCNT-COOH) caused a significant increase of the EC50s. When EC50s were calculated based on the phenanthrene concentration available in the water phase, each EC50 determined in the presence of CNTs was significantly lower than for the exposure with phenanthrene only. This showed that some of the phenanthrene which was adsorbed to the CNTs still was available to D. magna and a shift in the exposure routes was suggested. The results were assessed with regard to the estimated quantity of the phenanthrene which was adsorbed to the CNTs (between 25% and 51% of the total phenanthrene); the SWCNT and the MWCNT-3 showed the highest adsorption capacities, which underpinned the suggested effects of the CNT presence on the phenanthrene bioavailability as the SWCNT and the MWCNT-3 caused also the highest EC50 values for phenanthrene. In this light, a positive relation between the decrease in phenanthrene bioavailability due to CNT presence and the CNT type specific surface area (SSA) was discussed. The outcomes of this study underline that interactions between CNTs and organic pollutants have to be taken into account when it comes to the evaluation of the environmental risk of CNTs in aquatic systems.