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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

The bioaccumulation potential is low

Key value for chemical safety assessment

Additional information

No experimental data is available concerning the bioaccumulation potential of Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters (CAS No. 173832-46-7). Therefore, all available related data are combined in a Weight of Evidence approach (WoE), which is in accordance to Regulation (EC) No. 1907/2006, Annex XI General rules for adaptation of the standard testing regime set out in Annexes VII to X, 1.2, to cover the data requirements of Annex IX and X.

Bioaccumulation refers to the uptake of a substance from all environmental sources including water, food and sediment. However, the accumulation of a substance in an organism is determined, not only by uptake, but also by distribution, metabolism and excretion. Accumulation takes place if the substance is taken up faster than it can be metabolised and/or excreted.

 

Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters exhibits a log Koc value of > 5 and is poorly water soluble (< 1 mg/L). The Guidance on information requirements and chemical safety assessment, Chapter R7.b (ECHA, 2012) states that once insoluble chemicals enter a standard STP, they will be extensively removed in the primary settling tank and fat trap and thus, only limited amounts will get in contact with activated sludge organisms. Nevertheless, once this contact takes place, these substances are expected to be removed from the water column to a significant degree by adsorption to sewage sludge (Guidance on information requirements and chemical safety assessment, Chapter R.7a, (ECHA, 2012). Thus, discharged concentrations of this substance (if at all) into the aqueous compartment are likely to be low.

 

If the substance is taken up by ingestion, absorption of Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters is expected to be low based on the molecular weight, size and structural complexity of the substance. Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters has high molecular weights of 973.62 to 11184 g/mol. Thus, it is unlikely that it is readily absorbed, due to the steric hindrance of crossing biological membranes. According to Rekker and Mannhold (1992) a molecular weight of > 700 in combination with a calculated log Kow of > 8 can be used as enough evidence to conclude that a substance is unlikely to bioaccumulate (as stated in Guidance on information requirements and chemical safety assessment; Chapter R.11: PBT Assessment; ECHA, 2012). Following the ‘rule of 5’ (Lipinski et al., 2001), developed to identify drug candidates with poor oral absorption based on criteria regarding partitioning (log Kow > 5) and molecular weight (> 500 g/mol), the substance is considered to be poorly absorbed after oral uptake (also see Hsieh & Perkins, 1976).

Furthermore, these large and complex structures assume a high degree of conformational flexibility. Dimitrov et al. (2002) revealed a tendency of decreasing log BCF with an increase in conformational flexibility of molecules. They suggest that this effect is related to the enhancement of the entropy factor on membrane permeability of chemicals. This concludes a high probability that the substance may encounter the membrane in a conformation which does not enable the substance to permeate. The interaction between lipophilicity, bioavailability and membrane permeability is considered to be the main reason why the relationship between the bioaccumulation potential of a substance and its hydrophobicity is commonly described by a relatively steep Gaussian curve with the bioaccumulation peak approximately at log Kow of 6-7 (e.g., see Dimitrov et al., 2002; Nendza & Müller, 2007; Arnot and Gobas 2003). Substances with log Kow values above 10, which have been calculated for the test substance, are considered to have a low bioaccumulation potential (e.g., Nendza & Müller, 2007; 2010). According to the Guidance on information requirements and chemical safety assessment, Chapter R7.c (ECHA, 2012), the potential for bioaccumulation can be estimated from the log Kow value of the substance as a screening approach. Generally, at log Kow values > 6 a decrease in BCF values is observed, probably caused by the reduced uptake with the expected increasing molecular size of such substances. Even though experimental data evaluating bioaccumulation for substances with log Kow > 10 (such as Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters) is not known, these substances are expected to have BCF values < 2000 L/kg (criterion used to consider a substance Bioaccumulative)(Guidance on information requirements and chemical safety assessment, Chapter 11 (ECHA, 2012).

This assumption is supported by QSAR calculations using BCFBAF v3.01 performed for of Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters. BCF/BAF values of 0.89 L/kg were obtained (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters is outside the applicability domain of the model, the estimation can be used as supporting indication of low bioaccumulation potential. The model training set is only consisting of substances with log Kow values of 0.31 - 8.70. But it supports the statement that substances with high log Kow values (> 10) have a lower potential for bioconcentration as summarized in the ECHA Guidance R.11 and they are not expected to meet the B/vB criterion (ECHA, 2012).

 

Conclusion

Due to its log Koc value of > 5, significant adsorption of this substance to activated sludge in conventional STPs will take place and only low concentrations are expected to be released (if at all) into the environment. Once present in the aquatic compartment, due to the high log Kow, low water solubility and high adsorption potential, Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters will be bioavailable to aquatic organisms mainly via feed and contact with suspended organic particles. . Nevertheless, this substance has a high molecular weight (973.62 to 11184 g/mol) and a log Kow value > 10, factors that hinder thecrossing of biological membranes.. The bioaccumulation potential of this substance is thus expected to be low. BCF/BAF values estimated by QSAR (BCFBAF v3.01) also support this assumption (BCF values0.89 L/kg).

Taking all these information into account, it can be concluded that bioaccumulation of Fatty acids, C18 unsaturated, trimers, 2-ethylhexyl esters (CAS No. 173832-46-7) is expected to be low.