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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Reference
Endpoint:
bioaccumulation in aquatic species: fish
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance has a low potential to cross biological membranes
Justification for type of information:
JUSTIFICATION FOR DATA WAIVING
When bulk and nanoforms of carbon black are suspended in water or aqueous media, they will behave in accordance with the laws of colloidal chemistry. In water, at pH values near the isoelectric point (IEP), particle aggregation occurs which is testament to the hydrophobic character of elemental carbon, the main component of the respective forms (Ridaoui et al. 2006). Surface energy measurements of not treated nanoforms and their treated nanoform counterparts were performed as part of a study designed to evaluate and compare their in vitro effects (IUCLID 5.6). In this study, the information shows that the treated nanoforms are less hydrophobic as evidenced by their higher surface energies which ranged from 49.7 mJ/m2 to 58.7 mJ/m2 in comparison to 5.9 mJ/m2 to 16.2 mJ/m2 for the not treated source nanoforms. Nonetheless, the surface treated forms are still hydrophobic as their surface energies are well below the threshold of 113 mJ/m2, which is considered the surface energy at which hydrophilic behaviour commences. The bulk form is expected to also be similarly as hydrophobic as the not treated nanoforms seeing as it too has not been treated and is also mainly composed of carbon.
This hydrophobic nature of the (nano)forms increases their propensity to aggregate and agglomerate in aqueous and environmental media (Ridaoui et al. 2006). Aggregation and agglomeration lead to particles that are in the micrometre size range. For example, aggregate particle sizes of about 6 µm to 10 µm were measured for treated and untreated nanoforms of carbon black in cell culture media (Dulbecco’s Modified Eagle Medium (DMEM)) (IUCLID 5.6. In another study ((Degussa et al 2009), IUCLID 4.28.1) aggregate sizes (D50) of four not treated carbon black (D50 14 nm – 51 nm; and BET 30 m2/g – 350 m2/g) were ca. 13 µm. (Ridaoui et al. 2006) also reports micron-sized aggregates for the not-treated carbon black grade N234 (D50 ca. 21 nm and BET 118 m2/g) when dispersed in water. Given the larger constituent particle sizes of the bulk form in comparison to the nanoforms, it is expected that they too will behave similarly as the nanoforms and form aggregate particles of sizes equal to or even larger than the nanoforms when dispersed in aqueous and environmental media. Wiemann et al 2020 showed agglomeration of N990, a bulk form of carbon black when it was dispersed in water (IUCLID 7.9.4).
Agglomerates of the bulk and nanoforms of carbon black can be broken down into aggregates, if adequate force is applied (as often done to prepare samples in toxicological studies using sonification). Nonetheless, the constituent particles are not released as they have been fused together (i.e., they don’t have physical boundaries anymore) during the enclosed manufacturing process to form aggregates (Levy et al. 2012). The fact that constituent particles cannot be released from aggregates or agglomerates under normal environmental and physiological conditions has important implications, in that the aggregate size is the smallest size potentially available and hence will be the size that determines penetration potential through biological barriers and consequentially biological response. Accepting this and considering the large micrometre sizes of the aggregates when dispersed in aqueous solution, it is not expected that that bulk and nanoforms of carbon black will cross biological barriers in quantities that are biologically relevant, and they will display similarly low absorption rates and similar toxicokinetics. Insignificantly low levels absorption has been shown in vivo in two toxicokinetic studies evaluating the uptake of two nanoforms via the pulmonary (not treated nanoform) and the oral (treated nanoform) route of exposure (see section 7.1.). Lack of systemic availability is substantiated by the general lack of systemic effects in the available (eco)toxicity studies for the bulk and nanoforms (see IUCLID sections 6 and 7). Based on these results and given the propensity of the different forms of carbon black to form micrometre sized aggregates and agglomerates, it is expected that similarly low levels of absorption will be seen also in aquatic/sediment and terrestrial organisms.
The weight of evidence shows that bulk and nanoforms of carbon black have low potential to cross biological membranes. Consequentially, the accumulation potential in aquatic and terrestrial organisms is low.

Levy, L., I. S. Chaudhuri, N. Krueger, and R. J. McCunney. 2012. "Does carbon black disaggregate in lung fluid? A critical assessment." Chem Res Toxicol 25 (10):2001-6. doi: 10.1021/tx300160z.

Ridaoui, H., A. Jada, L. Vidal, and J. B. Donnet. 2006. "Effect of cationic surfactant and block copolymer on carbon black particle surface charge and size." Colloids and Surfaces A: Physicochemical and Engineering Aspects 278 (1):149-159. doi: https://doi.org/10.1016/j.colsurfa.2005.12.013.

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Description of key information

Based on the physico-chemical properties of bulk forms of carbon black as an inert solid, its insolubility and stability in water and organic solvents and because of its hydrophobic character and tendency to form aggregates and agglomerates, a diffusion through the gills or through the membranes of the body of aquatic organisms is not to be expected. Carbon is a natural element and widely distributed in nature. It is an element of the composition of all living organisms.

Key value for chemical safety assessment

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