Although they consist only of a carbon skeleton, carbon nanotubes (CNTs) have a plurality of different structures: single-walled and multi-walled tubes, different lengths, and surface coatings. These variations make it difficult to directly compare the environmental behaviour of carbon nanotubes. They also are often not present as single tube, but are aggregated to bundles. Therefore it is difficult to generalise about the environmental fate of this great diversity of CNTs.

 

The environmental behaviour of carbon nanotube strongly depends on the shape and properties of the tubes. They may e.g. be water soluble or insoluble in water, which essentially affects their behaviour in surface waters.Overview on different carbon nanotube structures: single-walled (SWCNT), double-walled (DWCNT), multi-walled (MWCNT) and possible surface modifications.Overview on different carbon nanotube structures: single-walled (SWCNT), double-walled (DWCNT), multi-walled (MWCNT) and possible surface modifications.

 

Uncoated carbon nanotubes hardly dissolve in water and therefore deposit quickly (sediment). An improved water solubility and hence a longer residence time in water can be achieved by modification of the CNT surface, for example by various coatings [3]. Dissolved organic substances (e.g. decomposition products of dead organisms) stabilise multi-walled carbon nanotubes in surface waters to prevent sedimentation to the bottom of the water [3]. An opposite effect is caused by calcium ions. They lead to an agglomeration of carbon nanotubes into larger bundles resulting in an accelerated sedimentation [4].

 

Even in soil the surface properties of the CNTs crucially influence their fate. For example, single-walled carbon nanotubes have a low mobility if their solubility in water has been improved by a specific coating. Due to their fibre-like form the CNTs quickly settle and hence have limited transport upon release [1,2].

 

Similar to other well-known carbon-based materials (e.g. activated carbon), carbon nanotubes can be very effective in binding other substances and are thus applied as filters. Based on this property, carbon nanotubes can also bind and accumulate numerous environmental pollutants [5,6,7]. Influence on the pollutant binding has the pollutant itself [7,8], but also the available free surface area [6,8] and the extent of surface modification of the CNTs [9]. Likewise, binding of the contaminants to the carbon nanotubes can be influenced by the presence of copper ions or surfactant [7,10]. Copper ions increase the pollutant binding by connecting the CNTs with the pollutant like a "bridge". In contrast, the components of the surfactant occupy the surface of the carbon nanotubes, so that hardly any space for pollutants remains. Compared to carbon black, however, the environmental concentrations of the CNTs are considered to be very low, hence the pollutant binding to CNTs is expected to be minimal [11].

There is preliminary evidence that carbon nanotubes can be degraded by certain plant enzymes [12-14]. Unmodified CNTs are protected from these enzymes whilst carbon nanotubes with certain surface modifications (e.g. carboxyl groups) can be well degraded.

 

In summary, the behaviour of carbon nanotubes in the environment is dependent on their properties, such as length and surface state. In addition, the CNTs interact in the environment with numerous other substances. The differences in behaviour also influence the type or degree of toxicity towards environmental organisms.

 

 

Literature arrow down

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