Assuming that partitioning processes leading to an eventual bioaccumulation of nanotubes by organisms depend on establishment of a thermodynamic equilibrium condition between sediment organic carbon and organism lipid phases, the fraction of organic carbon in the sediment would not be expected to affect BSAF values for nonionic organic chemicals (Di Toro et al. 1991
). This is in contrast to the significant differences in BSAF values between nanotubes in unamended sediments and those in sediments containing 10% MI Peat observed here. These findings suggest that the nanotubes detected have not been absorbed into organism tissues but rather are associated with sediment matter remaining in the gut of the organism.
Interestingly, standard deviations of BSAF values for carbon nanotubes are significantly larger than those for pyrene. This result may support the notion that a significant fraction of the radioactivity detected in the aquatic worms was from sediment-associated nanotubes not purged from the organisms after 6 hr of depuration, a parameter that would reasonably vary over a greater range than that of absorption by tissues. This variability may also stem from greater heterogeneities of carbon nanotube distributions in the sediment. Although all pyrene was dissolved in acetone prior to spiking, some carbon nanotubes may not have been fully dispersed by sonication. Larger aggregates of carbon nanotubes may then have caused small regions of elevated nanotube mass concentration.
The depuration behaviors of nanotubes and pyrene also suggest different distribution patterns in the organisms. The relatively slow depuration of pyrene is attributed to slow rates of clearance from organism tissues compared with rates of sediment gut purging. Conversely, rapid elimination of carbon nanotubes suggests that the major fraction of carbon nanotubes present in the worms after an initial 6 hr of depuration consisted of nanotubes associated with residual gut sediment, an explanation in accord with results from the bioaccumulation experiments. One possible approach in estimating the concentration of nanotubes in the guts of oligochaetes is the work of Mount and co-workers (1999)
, who determined that the concentration of sediment in the guts of the organisms decreased from approximately 0.14% after 6 hr of depuration to 4.3 × 10–7% after 24 hr. As such, the BSAF values in the worms after 24 hr of depuration may be taken as an estimate of the concentration in the worms not associated with the gut contents.
Differences between the uptake and depuration behaviors of carbon nanotubes and pyrene might be attributable to several factors. L. variegatus can uptake sediment via three routes: pore water, overlying water, and ingestion of sediments. One possible explanation is that, unlike pyrene, carbon nanotubes are not present at significant concentrations in either pore or overlying waters. Radioactivity was not detected in overlying waters during any of the exposure experiments, suggesting that uptake of carbon nanotubes through overlying and pore water routes was minimal. This behavior could be a result of nanotube insolubilities or their strong sorption to sediment organic matter. Negligible increases in BSAF values for the nanotubes after the first day, however, suggests that an apparent equilibrium (or steady state) was rapidly reached because of lack of absorption in the lipids of the organisms.
The sizes of carbon nanotubes may be another factor in the lack of absorption by organisms. Size-dependent toxicities of SWNTs have been shown previously for the copepod A. tenuiremis
(Templeton et al. 2006
), and the uptake of the polybrominated diphenyl ether congener BDE 209 is significantly lower than that for other smaller and less hydrophobic congeners (Ciparis and Hale 2005
). The low uptake of this congener was speculated to be a result either of its minimal desorption from sediment particles into pore waters or, after ingestion, into organism intestinal fluids, or from the large molecular size of the compound hindering cellular uptake. Cellular uptake of single- and multiwalled carbon nanotubes has been found in numerous studies (Cherukuri et al. 2004
; Heller et al. 2005
; Kam et al. 2004
; Kostarelos et al. 2007
; Monteiro-Riviere et al. 2005
), but the extent to which nanotubes can travel across organisms tissues is unknown. Nanotubes may simply accumulate on the outermost cells of organisms and then be removed periodically when those cells are sloughed, or they may be able to pass through these cells and eventually enter systemic circulation in organisms. Furthermore, different nanotube synthesis techniques and physical or chemical modifications to the nanotubes may profoundly influence their biodistribution in organisms. Additional research to understand such nanotube interactions in biological systems and to link toxicologic impacts to nanotube concentrations in organisms is essential, and the 14
C nanotubes developed here are ideally suited for such investigations.
Beyond the risks posed by nanotubes themselves, it is entirely possible that such materials may influence the bioaccumulation and fate of other pollutants in environmental systems. Carbon nanotubes possess strong sorptive capacities for such metals as lead, cadmium, and copper and various hydrophobic organic chemicals (Li et al. 2003
; Yang et al. 2006
). Hypothetically, carbon nanotubes may act in a manner similar to charcoal and other forms of black carbon by sequestering such compounds and limiting their bioavailability and mobility. It is also possible that nanotubes could serve as concentrators, durable sources, and transporters of such chemicals into organisms, thus exacerbating bioaccumulation and food chain transfer. Although nanotubes were not shown here to accumulate within oligochaetes, the passage of materials loaded with elevated concentrations of toxic chemicals through organisms could pose serious environmental and human health risks. Elucidating these effects represents another critical future research direction.