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Posted: Dec 28, 2006
Nanotubes in the environment
(Nanowerk News) The explosion in the creation of better and more complicated nanomaterials is moving far ahead of research on such materials’ environmental safety, leaving many to wonder about their potential effects on human health and the environment. New research published in Environmental Science & Technology ("Natural Organic Matter Stabilizes Carbon Nanotubes in the Aqueous Phase") underscores that these anthropogenic nanomaterials can readily interact with natural materials.
The data are the first to show how multiwalled carbon nanotubes (MWNTs) might behave in natural aquatic environments. Researchers say that because organic material stabilizes the nanotubes, their potential for dispersal increases dramatically, but the toxicity of the new materials in natural environments remains relatively unknown.
Industrial manufacturers have created many types of nanomaterials for use in a broad spectrum of applications. In addition to “buckyballs” – the C60 molecules that are also called fullerenes—nanotubes can be tiny straw-shaped structures made with single or multiple layers of carbon atoms. Both the particles and the tubes tend to clump together, but surfactants and added polymers can keep them from cohering in laboratory settings. Even so, previous experiments demonstrate that C60 aggregates in natural settings.
A team led by Jae-Hong Kim of the Georgia Institute of Technology examined how MWNTs interact with Suwannee River natural organic matter suspended in a 1% solution of sodium dodecyl sulfate (a surfactant known to keep nanotubes and nanoparticles discrete). Electron microscopy, measurements of opacity and turbidity, and other analyses were used to determine the behavior of the MWNTs in various mixtures left to settle over hours and days.
In clean water, the nanotubes settled to the bottom within an hour. In contrast, the surfactant-only solution kept the tubes suspended for a day or more. But the water containing organic matter remained cloudy for more than 4 days, and a month later the mixture retained a grayish background color.
Natural organic matter “suspends [MWNTs] in water and becomes a vehicle for transport,” says a coauthor of the paper, Joseph Hughes of the Georgia Institute of Technology. The team’s finding “clearly demonstrates the fact that as we produce these anthropogenic nanomaterials, they’re going to interact with natural materials,” he says. Using electron microscopy, the researchers showed that natural organic matter not only acts to stabilize individual nanotubes in solution but also sometimes even creates bridges between two tubes.
Corresponding author Kim calls the results intriguing. “The fact that natural organic matter stabilized the model carbon nanotube (MWNT) in the aqueous phase more efficiently than surfactant was pretty surprising to us,” Kim said, “since similar behavior was not observed with another widely investigated carbon nanomaterial – C60.”
“It’s interesting that the nanotubes remain as discrete units,” says Ron Turco of Purdue University. “I would have said they would cluster up.” This experiment is the first basic research to address carbon nanotubes in the environment, and even then only the most basic form of such nanomaterials, he continues. “It is going to have to be repeated,” he adds, in flowing systems with sediments. Researchers will also need to track whether the nanotubes degrade, he says.
A question for future research, Turco notes, is whether single tubes are more toxic than an aggregate of MWNTs. The natural organic matter “may allow them to transfer farther and [eventually] interact with cells, bacteria, or fungi, or higher up the food chain, differently than they would as a cluster.” Research so far shows that single-walled carbon nanotubes can behave much like silica fibers in pulmonary tissues, causing inflammation associated with lacerations; this effect makes them an occupational health hazard. Cell cultures and other experiments indicate varying levels of toxic effects.
Turco says that the field is “wide open” and constantly changing, as industrial manufacturers introduce ever more “flavors” of nanotubes and particles with various components attached to their outer coatings. “Everyone’s looking for a material with a substitution that will do something new and unique to replace something else. It’s great chemistry,” he says. “But in terms of environmental effects, we’re not able to keep up right now. We’re still trying to figure out plain carbon nanotubes.”