Single-walled carbon nanotubes (SWNTs) are filamentous manifestations of a repeating aromatic carbon structure formed into an open cylinder. Because of their unique physicochemical properties and potential for large-scale commercialization, concerns have emerged over potential adverse effects of nanomaterials such as SWNTs in the aquatic environment. These concerns include direct toxicity to aquatic organisms as well as potential effects on distribution of hydrophobic organic contaminants (HOC) through adsorptive sequestration.

In order to address the potential for SWNTs to enter, persist, and be transported within estuaries, we have utilized dynamic light-scattering analysis to examine the agglomeration of colloidal SWNT suspensions under simulated estuarine conditions. In addition, we have assessed the sorptive capacity of SWNTs for model hydrophobic organic contaminants. Results indicate that SWNTs exist as stable colloidal suspensions (average particle size < 250 nm) in buffered (pH 7) solutions of low ionic strength (10 mM) but that relatively large (average particle size > 2 ?m) SWNT flocs formed over rapid (minute) timescales as salinity increased over 5 ppt. SWNT aggregation was accelerated and occurred at lower ionic strength in electrolyte solutions containing divalent cations; however, the presence of dissolved organic matter (DOM) inhibited the aggregation of SWNTs in saline solutions. These results indicate that estuarine mixing may play an important role in determining the fate of SWNTs after entry into the aquatic environment (e.g. settling or association with suspended particulates). Adsorption experiments with naphthalene and hexachlorobenzene indicate that SWNTs are more sorptive to these model HOCs than is diesel particulate soot and that the sorption is highly nonlinear. If released in significant quantity, these carbonaceous nanomaterials thus have the potential to significantly impact the disposition of HOCs in the aquatic environment.

In light of these results, we have performed experiments to elucidate the impacts of sequestration through sorption to SWNTs on HOC bioavailability to sediment-ingesting benthic invertebrates. Two model benthic invertebrates (Amphiascus tenuiremis and Strebelospio benedicti) were utilized in a sediment-phase bioaccumulation assay in the presence of three persistent HOC classes (i.e. polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and polybrominated diphenyl ethers) to test the effect of SWNTs on HOC bioavailability to sediment-ingesting organisms relative to a reference sorbent (e.g. soot carbon) and non carbon-amended sediments. Results indicated a significant decrease in the bioavailability of many HOCs to the polychaete worm S. benedicti in the presence of SWNTs (7 of 9 detectable compounds), while soot amended sediments substantially increased S. benedicti bioaccumulation of HOCs when compared to non carbon-amended sediment. The harpacticoid copepod A. tenuiremis did not show the same carbon-dependent bioaccumulation results with most (13 of 17) compounds giving comparable lipid- and sediment organic carbon-normalized bioaccumulation factors in the presence of SWNT amended, soot carbon amended, or noncarbon amended sediments. These results indicate that SWNTs have the potential to reduce the bioavailability of HOCs to deposit feeding estuarine invertebrates; however, the results are dependent on the test organism and may vary based on feeding strategies.

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