Carbon nanotubes, which can be single- or multiwalled, are rolled up sheets of graphite just nanometres in diameter with excellent electronic and mechanical properties. They could find use in a variety of potential applications, including electronic devices, composites and optics. However, how these materials behave in the natural environment is still largely unknown.
Kim and co-workers looked at how multiwalled carbon nanotubes interacted with a sample of water from the Suwannee River -- a relatively unpolluted river in Southern Georgia. The researchers also looked at how the nanotubes behaved in samples of organic-free water, water containing a 1% solution of the surfactant sodium dodecyl sulphate (SDS) and water containing a commercially available sample of Suwannee River water. They shook the nanotubes together with each sample and then left them to stand for a month.
Using transmission electron microscopy and measurements of opacity and turbidity, among other techniques, Kim's team observed that the nanotubes immediately made all of the water samples dark and cloudy. The difference was in how long the nanotubes took to settle in the water so that the solution became clear again. The nanotubes in the organic-free sample settled quickly and the water became completely transparent in less than an hour, while the nanotubes in the SDS solution took around a day to settle. However, both the commercially available and real samples of Suwannee River water lightened only gradually after around four days of settling, with some nanotubes remaining suspended in solution for more than a month.
The Georgia Tech team also found that most multiwalled carbon nanotubes in the river samples were individually dispersed rather than clustered together, like some other nanomaterials are in water. "This might make them more likely to be transported in the natural environment," said Kim.
The researchers say that it is the natural organic matter in river water that is responsible for suspending the nanotubes. And although they are unsure as to the exact mechanism, they believe that the organic matter has chemical properties that encourage the nanotubes to adhere to it more than they would to some other surfactants.
The team has now started investigating the behaviour of other nanomaterials, such as carbon-60 ("buckyballs"), in natural aqueous environments. It hopes that, in the long term, the research will help us to understand better the impact of nanotechnology on the environment.
The work is published in Environ. Sci. Techol..