Carbon nanotubes have made a meteoric career in the past 15 years, even if
their applications are still limited. Recent research results show that - apart
from their favorable mechanical and electrical properties - they also have disadvantageous
characteristics. One aspect which has rarely been considered so far is now addressed
by researchers of the research center Forschungszentrum
Dresden-Rossendorf. "If the application of products and commodities
containing carbon nanotubes will increase in the future, then there will be
a higher probability for the tubes to get into the environment during their
production, usage or disposal, to be distributed there, and to bind pollutants
such as heavy metals on their way trough the environment", says Harald
Zaenker, scientist at the FZD.
Via water into the environment
An important way for carbon nanotubes of getting into the environment is the
way via the water. In their original state, the flimsy carbon fibers with a
diameter of less than 50 nanometers (1 nanometer = 1 millionth of a millimeter)
are hardly water-soluble. At first glance, they should therefore not be mobile
in groundwater, lakes etc., i.e. they should rapidly settle or deposit. However,
carbon nanotubes are able to form colloidal solutions if their surface structure
is changed. Changes in the surface structure can be brought about deliberately
during the production of the tubes or can be induced by natural processes if
the tubes are released into the environment.
A colloidal solution, unlike a true solution of water-soluble substances, is
a solution in which the apparently dissolved substance is finely dispersed in
the solvent forming tiny particles. These particles are still much bigger than
the molecules of a dissolved substance in a true solution. As colloids, carbon
nanotubes might be transported anywhere in environmental waters. It is known
meanwhile that the tubes can even penetrate cell walls and, thus, might theoretically
be able to enter also animal or human cells. In addition, changes in the surface
structure of carbon nanotubes cause another effect: their capability to bind
heavy metals is increased.
Tubes with changed surface
The scientists investigated carbon nanotubes both in their original state and
in a state changed by oxidizing acids (such as a mixture of nitric and sulfuric
acid). They found out that solutions of treated carbon nanotubes scatter light
more strongly. “This is an indication that colloids have formed which
do not settle”, Harald Zaenker says. The researchers provided evidence
for the first time that the heavy metal uranium, which is ubiquitous in the
environment and, hence, also in the water, is particularly attached to the surface
of treated carbon nanotubes. The scientists found out that the uranium uptake
capacity is increased by an order of magnitude in comparison to untreated carbon
nanotubes. “Therefore, it is plausible to assume that carbon nanotubes,
if released to the environment, influence the transport of uranium in environmental
waters and even in biological systems. The possible impact on the environment
and on human health has in general been considered too little”, Harald
Zaenker says.
On the other hand, the high bonding capacity of carbon nanotubes for uranium
and other heavy metals also suggests using them for the removal of heavy metals
from waters. However, they are not yet a cost-efficient alternative to classic
water purifiers, Zaenker says. “Eventually, it is important to further
study the behavior of carbon nanotubes in waters”, the scientist says.
“Only then can the positive and negative aspects of carbon nanotubes be
better assessed.”
Posted May 4th, 2009