Multi-Walled Carbon Nanotubes (MWCNT), synthetic graphite in tubular shape

Endpoint summary

Description of key information

MWCNT does not seem to exhibit a concern for bioaccumulation neither for the aquatic nor the terrestrial environment. The conclusion is based on physical-chemical considerations and experimental findings of studies performed with Jenotubes (i.e. dispersion stability and dissolution rate), as well as experimental results from publications that investigated MWCNT bioaccumulation potential in aquatic and terrestrial organisms.

Additional information

The bioaccumulation potential of MWCNT was assessed with regard to both, the aquatic and terrestrial environment, considering the knowledge on physico-chemical properties and environmental fate characteristics. In conclusion, the terrestrial compartment was considered the most sensitive environmental compartment for potential bioaccumulation of MWCNT. Both, the potential of aquatic and terrestrial bioaccumulation of MWCNT were thoroughly evaluated and is reported in the following paragraphs. The conclusion is an overall low potential of bioaccumulation for the submission substance.

 

Bioaccumulation in the aquatic environment

According to column 2 of REACH Annex IX, any physico-chemical property for nanomaterials (e.g., octanol water partition coefficient, dissolution rate, dispersion stability) must include an adequate justification of its relevance to low potential for bioaccumulation.

In reliable tests according to OECD 105 and OECD GD 29 it was impossible to detect any dissolution of the test item. MWCNT are an allotropic form of carbon very similar to graphite, which is known to be insoluble in water due to its chemical structure. The covalently bonded MWCNT lack polarity and thus will not dissolve in water (covalent bonds of high binding enthalpy of 348 kJ/mol for each C-C bond; no hydrolysable functional groups, no enthalpy gain by hydration - no compensation of the binding enthalpy of covalent C-C bonds; sp2-hypridisation of C, each carbon covalently bonded to three other carbons). Accordingly, any dissolved fraction potentially causing any bio-concentration from water can be excluded with absolute certainty. This fully agrees with ECHA Appendix R7-2 (2021) stating that the aqueous exposure route resulting in a concentration factor (BCF) is applicable for most nanomaterials if they remained as nanoparticles (i.e. insoluble).

MWCNT are large molecules with typically several million carbon atoms and consequently high molecular weight. MWCNT are "entangled", forming assemblies (bundles, i.e. aggregates) of a bundle diameter in the low µm-range and a bundle length of around 30 - 80 µm. Bundles are prone to agglomerate, agglomerates are very difficult to disperse in the first place, and due to re-agglomeration any dispersions are highly instable. This is based on reliable testing data on dispersion stability (OECD TG 318) demonstrating that the set of MWCNT is very difficult to disperse, re-agglomerating and adsorbing instantly to glass walls and the sonotrode. A very low dispersion stability under all conditions with NOM, electrolyte concentrations and pH was found from the OECD TG 318 data. It is interesting to note that the experimental results from the scoping study on the feasibility of OECD 303A STP simulation testing independently confirmed the OECD TG 318 work: at different nominal concentrations (3, 30 and 300 mg/L) agglomerates were clearly formed, also at the lowest MWCNT concentration applied. While ultrasonic dispersion did reduce the number of large agglomerates, this partial dispersion was not stable and larger agglomerates were formed back quickly.
Their extremely high molecular weight combined with their occurrence exclusively in the aggregated state (entangled, forming bundles) and furthermore their propensity to agglomerate, positions the set of MWCNT rather in the (large) micron than nano world (clearly visible by eye and light microscopy at 20x magnification). According to experimental results from the OECD 303A scoping study, the agglomerate size of MWCNT is up to hundreds of micrometer (SEM image analysis) large. Accordingly, even any relevant active uptake of undissolved MWCNT by organisms as the prerequisite for bioaccumulation is highly improbable, not to mention membrane permeation. This conclusion agrees with ECHA guidance chapter R11 (2017), characterizing a substance as unlikely to bioaccumulate or bioconcentrate when the molecular weight is > 1100 g/mol and the diameter Dmax is > 1.7 nm, and/or the maximum molecular length (MML) is > 4.3 nm.

 

Supporting evidence is given by Maes et al. (2014)[1]. The authors investigated the accumulation and distribution of MWCNT in Zebrafish (Danio rerio). Results showed that MWCNT quickly associated with the fish after exposure and a steady state was reached within 1 day. MWCNTs were quickly released to the water phase after transfer to clear medium; however, on average of 5 ng MWCNT/kg fish dry weight remained in the fish. MWCNT mainly accumulated in the gut of the fish and little fractions of MWCNT were detected in the blood and muscle tissue. A bioconcentration factor of 16 L/kg fish wet weight was derived indicating very low potential for bioaccumulation.

In conclusion, the bioaccumulation potential for MWCNT was considered low for the aquatic environment.

 

Bioaccumulation in the terrestrial environment

According to ECHA Appendix R7-2 (2021), soil and sediment compartments are considered potential sinks for nanomaterials and therefore considered relevant when considering nanomaterial fate in the environment. This is consistent with our evaluation of MWCNT and its environmental fate: MWCNT was found to adsorb to sludge and not remain in the water phase in the STP feasability study included in this dossier. Therefore, we consider the terrestrial compartment as vulnerable to potential concern of bioaccumulation of MWCNT rather than the aquatic environment.   

Also, ECHA appendix R7-2 (2021) for the endpoint specific assessment of nanomaterial considers the soil compartment as a potential sink for nanomaterials and therefore relevant when considering nanomaterial fate in the environment. Since the terrestrial environment was considered the relevant route of exposure to MWCNT in terms of bioaccumulation, two relevant experimental studies on bioaccumulation in earthworm after 28 days of exposure are included in this dossier.

Peterson et al. (2008)[2] investigated bioaccumulation of MWCNT by Eisenia foetida, specifically the uptake and depuration behaviors. Bioaccumulation factors (i.e., biota-to-soil accumulation factor (BSAF)) were determined in the range of 0.014 to 0.023 (BSAF average 0.02 ± 0.006) covering two different types of soil. The result indicated that purified carbon nanotubes were neither readily absorbed into organism tissues nor manifested equilibrium partitioning.

Also, Li et al. (2013)[3] investigated the bioaccumulation of MWCNT in earthworms after 28 days of exposure. Results of a biota-to-soil accumulation factor (BSAF) of 0.015±0.004 covering one soil indicated a low bioaccumulation potential for MWCNT. The results were in line with earlier findings by Peterson et al. (2008).

In conclusion, the bioaccumulation potential for MWCNT was considered low for the terrestrial environment.

 

Overall conclusion

MWCNT does not seem to exhibit a concern for bioaccumulation, neither for the aquatic nor the terrestrial environment. The conclusion is based on physico-chemical considerations and experimental findings of studies performed with the submission substance (e.g., dispersion stability and dissolution rate), as well as experimental results from publications that investigated MWNCT bioaccumulation potential in aquatic and terrestrial organisms.

Also, authors Bjorkland et al. (2017)[4] from the US EPA concluded in their exhaustive literature review of bioaccumulation of carbon nanotubes (both single and multi-walled) in terrestrial and aquatic organisms that carbon nanotubes appeared to form a class that should be designated as a low concern for bioaccumulation. In particular, because of low levels of absorption across epithelial surfaces.

In detail, Bjorkland et al. (2017) reviewed bioaccumulation of carbon nanotubes (CNT) in plants, invertebrates and non-mammalian vertebrates by summarizing a total of 42 studies to improve the assessment and understanding of the potential for bioaccumulation concern for CNT. In the assessed studies, absorption into tissues and elimination behaviours across species were investigated. All the invertebrate and non-mammalian vertebrate studies showed little to no absorption of the material from the gut tract to other tissues. The findings on bioaccumulation in combination with the lack of biomagnification reported in the assessed trophic transfer studies suggested that the overall risk of trophic transfer of CNT was low according to the authors.

 

[1] Maes, H.M., Stibany, F., Giefers, S., Daniels, B., Deutschmann, B., Baumgartner, W. and Schäffer, A., 2014. Accumulation and distribution of multiwalled carbon nanotubes in zebrafish (Danio rerio). Environmental science & technology, 48(20), pp.12256-12264.

[2] Petersen, E.J., Huang, Q. and Weber, Jr, W.J., 2008. Bioaccumulation of radio-labeled carbon nanotubes by Eisenia foetida. Environmental science & technology, 42(8), pp.3090-3095.

[3] Li, S., Irin, F., Atore, F.O., Green, M.J. and Cañas-Carrell, J.E., 2013. Determination of multi-walled carbon nanotube bioaccumulation in earthworms measured by a microwave-based detection technique. Science of the total environment, 445, pp.9-13.

[4] Bjorkland, R., Tobias, D.A. and Petersen, E.J., 2017. Increasing evidence indicates low bioaccumulation of carbon nanotubes. Environmental Science: Nano, 4(4), pp.747-766.