When mixed with natural organic matter in water from the Suwannee River – a relatively unpolluted waterway that originates in southern Georgia – multi-walled carbon nanotubes remain suspended for more than a month, making them more likely to be transported in the environment, according to Georgia Tech researchers.
Carbon nanotubes, which can be single- or multi-walled, are cylindrical carbon structures with novel properties that make them potentially useful in a wide variety of applications including electronics, composites, optics and pharmaceuticals.
"We found that natural organic matter, or NOM as we call it, was efficient at suspending the nanotubes in water," said Jaehong Kim, an assistant professor in the Georgia Tech School of Civil and Environmental Engineering.
The research will be published in the January issue of the American Chemical Society journal Environmental Science and Technology. Kim is the senior author and conducted the research with professor Joseph Hughes, graduate student Hoon Hyung, both at Georgia Tech, and postdoctoral researcher John Fortner from Georgia Tech and Rice University.
"We don't know for certain why NOM is so efficient at suspending these nanotubes in the laboratory," Kim said. "We think NOM has some chemical characteristics that promote adhesion to the nanotubes more than to some surfactants. We are now studying this further."
Carbon Nanotubes in Natural Organic Matter are Vulnerable
In the lab, Kim and his colleagues compared the interactions of various concentrations of multi-walled carbon nanotubes with different aqueous environments – organic-free water, water containing a 1 percent solution of the surfactant sodium dodecyl sulfate (SDS), water containing a commercially available sample of Suwannee River natural organic matter and an actual sample of Suwannee River water from the same location as the commercially available preparation. They agitated each sample for 1 hour and then let it sit for up to 1 month.
The researchers then used transmission electron microscopy, measurements of opacity and turbidity and other analyses to determine the behavior of multi-walled carbon nanotubes in these environments. The results were:
- Multi-walled carbon nanotubes added to organic-free water settled quickly, and the water became completely transparent in less than an hour.
- When added to the SDS solution, the nanotubes immediately made the water dark and cloudy. After one day of settling, some nanotubes remained suspended, and the water was a light gray color.
- Water containing the commercially available sample of Suwannee River natural organic matter originally appeared dark and cloudy, then gradually lightened after four days of settling. Some multi-walled carbon nanotubes remained suspended for more than a month.
- The results with an actual Suwannee River sample were similar to those with the commercially available preparation.
In addition, Kim and his colleagues used transmission electron microscopy to find that most multi-walled carbon nanotubes in both samples of natural organic matter were suspended as individually dispersed nanotubes, rather than being clustered together as some other nanomaterials do in water.
"This individual dispersion might make them more likely to be transported in a natural environment," Kim explained.
Research Expanded to Include Other Nanomaterials
In light of these findings, Kim and his colleagues have expanded their research to other nanomaterials, including single-walled carbon nanotubes and C60, the so-called "buckyball" molecules in the same family as carbon nanotubes. They also are experimenting with other natural organic matter sources and studying different mixing conditions.
"We are getting some interesting results, though our findings are still preliminary," Kim said.
With nanomaterials already being used in an estimated 300 commercial products, Kim asserted that it's essential for scientists and engineers to study the materials' potential environmental impact.
"Natural organic matter is heterogeneous," Kim explained. "It's a complex mixture made from plants and microorganisms, and it's largely undefined and variable depending on the source. So we have to continue to study nanomaterial transport in the lab using various natural organic matter sources to try to better understand their potential interaction in the natural environment."
In related research, Kim's research team is studying various other aspects of the fate of nanomaterials in water – including photochemical and chemical reactions of C60 colloidal aggregates – with the ultimate goal of understanding the environmental implications of nanotechnology.