Weathered multi-walled carbon nanotubes in the aquatic environment : fate, bioaccumulation, effects, and mixture toxicity

The production of carbon nanotubes (CNT) has increased significantly in recent years and has reached an industrial scale. Not only at production sites, but also due to the increased use of CNT in various application areas, an increased release of the nanomaterials into ecosystems of our earth can be assumed. In the present study, the distribution of radiolabeled and weathered multi-walled CNT (14C-wMWCNT) in different aquatic compartments was investigated by means of a water sediment study. Over time (180 days), the applied wMWCNT exponentially sedimented from the water phase and simultaneously accumulated in the sediment phase. Furthermore, very little complete degradation of 14C-wMWCNT to 14CO2 was observed. Consequently, the uptake of 14C-wMWCNT in green algae, water fleas and fish was quantified. It was shown that wMWCNT interact and associate with the green algae Chlamydomonas reinhardtii and Raphidocelis subcapitata. To investigate food web transfer, R. subcapitata was subsequently loaded with 14C-wMWCNT and consequently fed to the primary consumer Daphnia magna. The accumulation of 14C-wMWCNT in daphnids via food transfer was compared with 14C-wMWCNT uptake by water fleas from the water phase without algae. The results indicate that short-term CNT exposure via food transfer, in contrast to water exposure, does not lead to nanomaterial accumulation. A further study on the accumulation of 14C wMWCNT in a growing population of D. magna further revealed exposure to CNT, which is of concern under the EU REACH regulation. The uptake of wMWCNT via the water phase and food was also observed in the zebrafish Danio rerio. Examination of various parts of the fish's body have shown that the majority of nanomaterials are accumulated in the gastrointestinal tract of secondary consumers. Furthermore, CNT are known to adsorb organic compounds and thus can change the fate of environmental pollutants. In an adsorption study, the interaction of unlabeled wMWCNT and the labeled biocide triclocarban (14C-TCC) was considered. Effects of the individual substances as well as the substance mixtures, so-called "Trojan horse" effects, were investigated using green algae and water fleas. An influence of the nanomaterials on the toxicity of TCC could only be observed from very high, non-environmentally relevant concentrations (estimated environmental concentration is in the ng/L range for surface waters). In summary, a long residence time of CNT in the aqueous phase and thus an exposure for pelagic and benthic organisms is shown. In addition, a food web transfer was observed, which is why, due to the increasing CNT production, research efforts regarding the effects of the single substance CNT and mixing effects in combination with chemicals should be further considered