Spread out: organic matter scatters carbon nanotubes in water.

Although carbon nanotubes usually clump in water, they readily disperse when the water contains natural organic matter, researchers report. Their study provides a glimpse of how the nanotubes might behave if released into a waterway.

Carbon nanotubes are prized for their strength and electrical properties (SN: 6/14/03, p. 372), and their production and market continue to grow. Researchers have reported that in the laboratory, these nanoparticles are strongly repulsed by water and attracted to each other. But scant information exists about how the tubes would behave in the environment, notes Jae-Hong Kim, an environmental engineer at the Georgia Institute of Technology in Atlanta.

Kim and his colleagues set out to see whether the decayed plant and animal material found in waterways would affect carbon nanotubes. The scientists used multiwalled nanotubes, each of which is a collection of concentric tubes.

The organic matter came from the Suwannee River in Georgia. The researchers used water that they collected from the river and a prepared powder that they purchased and diluted.

Team members added the carbon nanotubes to flasks containing either the river water or a solution of the powdered organic matter. They also added the nanotubes to two other flasks containing either distilled water or a surfactant solution that scientists routinely use to keep carbon nanotubes from clumping during experiments. The team shook all the flasks for an hour, let the contents settle, and then observed them for over a month.

As expected, all the nanotubes in the distilled water rapidly clumped and most settled at the bottom of the flask, whereas in the surfactant solution, some nanotubes dispersed. To the researchers' surprise, says Kim, the nanotubes dispersed most readily in the presence of either type of natural organic matter. The team reports its findings online in Environmental Science & Technology.

The researchers suspect that, like a surfactant, the organic material coats the carbon nanotubes. Surtitctants change nanotubes' surface chemistry, making it easier for the particles to disperse individually.

Although the work is preliminary, says Kim, it indicates that if nanotubes were to contaminate a waterway, "you can easily imagine that they [could] transport very. well in the water."

"The surprising thing about this work is that the dispersing agent can simply be the natural organic matter in the water," comments Mason B. Tomson, an environmental chemist at Rice University in Houston.

Tomson adds that the next step is to study how other natural factors, such as water flow, microorganisms, and sediments, might affect the distribution of nanotubes. Then, researchers could examine the toxic effects of the suspended tubes.

"This is just one of a number of studies that would need to be done to understand the fate of these materials in practical environmental systems," Tomson says.