Case Study The Lifecycle of Conductive Plastics Based on Carbon Nanotubes - Safety of Nanomaterials along Their Lifecycle - page 406

For example, the matrix degradation may occur due to weathering or heat stress dur-
ing the use and transport in water or storage in soils and sediments.
Although the transport of CNTs in different matrices can be simulated like any
other particle in the environment, transport of CNTs either as single fibers or in
bundles may behave differently due to their chemical and physical nature. One deter-
mining factor for the transport of CNTs is their chemical composition. Basically, a
CNT consists of one or several rolled-up layers of graphene and behaves chemically
very similar to carbon black and particles of elemental carbon. The CNT is hydro-
phobic and hence is not easy to disperse in water. It readily quenches radicals and is
black in color.
This behavior can significantly change when groups such as -OH, -COOH, -NO,
and others are added to the surface of CNTs (Balasubramanian and Burghard 2005).
These groups may be introduced in the CNTs directly during production to give
them specific features needed for dispersion or embedding into a matrix or may
occur during the lifecycle through, for example, thermal stress or UV weathering.
Such surface modifications may also render the CNT readily dispersible in aquatic
media. Another mechanism leading to higher dispersion of CNT in the environment
is concentration-dependent adsorption to particular types of organic matter (Hyung
and Kim 2008).
The physical parameters of CNTs (i.e., their specific morphology and state of
agglomeration) may be of lesser importance for the particle mobility in the envi-
ronment, whereas the chemical nature of the CNT significantly determines its
dispersibility in water and hence its mobility. Fibers and tubes often align them-
selves according to the flow, with the tube diameter as the determining particle
diameter. The morphology, that is, the length of the fibers increases this mobility
diameter slightly but is of some importance when the flow direction, in liquids
or gases, changes such as when the long fibers “attach” more rapidly to surfaces.
Therefore, the transport of CNTs in air can be described in first approximation
being similar to spherical particles with a diameter as that of the CNT-tube. Free
CNTs will agglomerate readily according to the particle number concentration
with other particles and form agglomerates. Depending on the size and agglom-
eration, some CNT fibers can stay airborne up to a few days in outdoor air and can
be transported over long distances in analogy to spherical particles (Pruppacher
and Klett 2010).
If released and transported in water, agglomeration can happen quite rapidly.
Sedimentation tests using radiolabeled 14 C-CNTs showed high sediment partitioning
coefficients (>95%) for both the addition of CNTs to the systems with the sediment as
well as CNTs dispersed in water (A. Schaeffer, personal communication). Significant
stabilization was observed in some cases, such as the addition of natural organic
matter (Schwyzer et al. 2013).
Once the CNTs have been transported in air or water, they will ultimately deposit
in either sediments or soils. The large aspect ratio of CNTs can lead to relatively
higher deposition rates compared to colloids or other nanomaterials (Jaisi and
Elimelech 2009; Wang et al. 2012), possibly because the CNTs can coil around soil
and sediment particles (Sedlmair et al. 2012). This may explain the observed relative
low mobility of CNTs in soil column tests and the possible accumulation in the upper