The influence of natural organic matter on the toxicity of multiwalled carbon nanotubes

Engineered carbon nanostructures, such as multiwalled carbon nanotubes (MWNTs), are inherently hydrophobic and are not readily stable in aqueous media. However, the aqueous stability and bioavailability of these nanotubes may be influenced by the water quality parameters such as ionic strength, pH, and natural organic matter (NOM). Natural organic matter adsorbs onto the surface of MWNTs, effectively covering the hydrophobic surface and resulting in increased aqueous stability. This enhanced stability is likely to lead to an increased residence time in the water column and increased exposure times for pelagic organisms. In the current study, NOM from three different river systems in the southeast United States increased the stability of MWNT suspensions. The effects of these suspensions were evaluated using acute and chronic bioassays with Daphnia magna and Ceriodaphnia dubia. The 96-h LC50 for D. magna exposed to MWNTs suspended in Suwannee River (USA) NOM was approximately 2.0 mg/L and was not significantly influenced by NOM concentrations ranging from 1.79 to 18.5 mg/L DOC. However, there were differences in 96-h LC50 values among different sources of NOM (Suwannee, Black, and Edisto Rivers, USA). Daphnid growth was reduced in both D. magna and C. dubia, whereas reproduction was reduced in C. dubia. Characterization of the different NOM sources and MWNT suspensions was conducted. Visual inspection using transmission electron microscopy (TEM) and gut elimination observations suggested that the toxicity was attributable to ingested MWNTs clogging the gut tract of D. magna. The TEM micrographs indicated that MWNTs can disaggregate within the gut tract, but single MWNTs are unable to absorb across the gut lumen.