Carbon Nanotubes: Carbon-14 Labeling and Ecological Availability.

Abstract

Carbon nanotubes comprise a class of nanomaterials having demonstrated promise for broad ranges of potential applications. The ecological and human health risks these nanomaterials may pose after release into environmental systems, however, are yet largely unknown. The lack of an adequate method for quantifying carbon nanotubes in environmental media has been a principal challenge associated with determining their environmental behaviors. To address this problem, a modified chemical vapor deposition process employing carbon-14 labeled methane was used to synthesize radioactively labeled single- and multi-walled carbon nanotubes (SWNTs and MWNTs). These nanotubes were used to show that HeLa cells rapidly and apparently irreversibly assimilated unmodified MWNTs. Given these results, previous qualitative detections of nanotubes in ecological receptors, and shared chemical properties with polycyclic aromatic hydrocarbons (PAHs), the bioaccumulation potential of nanotubes was investigated. The labeled nanotubes and a representative PAH, pyrene, were individually spiked to identical sediment and soil samples. The uptake and depuration behaviors of these compounds by the earthworm Eisenia foetida and the oligochaete Lumbriculus variegatus, potential entry points to terrestrial and aquatic food chains, were then assessed. Bioaccumulation values determined for the nanotubes were almost two orders of magnitude smaller than those measured for pyrene, indicating that purified nanotubes, unlike pyrene, are not readily absorbed into organisms. Carbon nanotubes are also commonly physically and chemically altered, and these modifications can change their physicochemical properties and possibly also their environmental behaviors. Purified MWNTs were treated with a 3:1 mixture of sulfuric to nitric acid, a process that made the nanotubes more hydrophilic. These nanotubes were similarly spiked to soils and sediments, but their ecological uptake was determined to be the same as that for the unmodified nanotubes. The octanol-water distribution coefficient, kow, represents a chemical property known to relate to bioaccumulation and is frequently employed for predictions thereof. A modified shake-flask method was used to measure the distributions of purified and 3:1 acid modified MWNTs between water and octanol. While their bioaccumulation behaviors were similar, different distribution coefficients were found for these nanotubes thus suggesting that, unlike typical hydrophobic organic chemicals, kow coefficients may not predict such behaviors for nanotubes.