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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 on the bioaccumulation potential of the PFAE aromatic category members. Therefore, all available related data are combined in a Weight of Evidence approach (WoE), which is in accordance to the REACh 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 Regulation (EC) No. 1907/2007 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.

In the case of the PFAE aromatic category members, uptake of dissolved substance via water is expected to be low, since the substances can be effectively removed in conventional STPs by biodegradation and sorption to biomass due to their enhanced ultimate biodegradability and high potential for adsorption (calculated log Koc > 5, KOCWIN v2.00).

Furthermore, the low water solubility (< 0.05 mg/L) and high estimated log Kow values (calculated log Kow > 10, KOWWIN v1.68) indicate that the members of the PFAE aromatic category are highly lipophilic. If released into the aquatic environment, the category members undergo extensive biodegradation and sorption on organic matter, as well as sedimentation. Thus, the bioavailability of these substances in the water column is reduced rapidly, which reduces the probability of uptake by aquatic organisms (e.g., see McKim et al, 1984; Björk, 1995; Haitzer et al., 1998).

Therefore, the relevant uptake route of PFAE aromatic category members in organisms is considered predominately to be by ingestion of particle bound substance.

If the substance is taken up by ingestion, absorption of the members of the PFAE aromatic category is expected to be low based on the molecular weight, size and structural complexity of the substances. 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.

Furthermore, the members of the PFAE aromatic category have high molecular weights of 546 to 757 g/mol. Thus, it is unlikely that they are readily absorbed, due to the steric hindrance of crossing biological membranes. 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 substances are considered to be poorly absorbed after oral uptake (also see Hsieh & Perkins, 1976).

However, ingested molecules of the PFAE aromatic category members can undergo a stepwise hydrolysis into the respective alcohol as well as the di- or monoester and the aromatic acid by gastrointestinal enzymes ((Lehninger, 1970; Mattson and Volpenhein, 1972) Further metabolism is expected for that part of the substance that has been absorbed. During the first step of alcohol metabolism, the alcohols are oxidised to the corresponding carboxylic acids, followed by a stepwise elimination of C2-units in the mitochondrial beta-oxidation process (OECD SIDS, 2006). 1,2,4-Benzene tricarboxylic acid might be further metabolized by beta-oxidation, the degradation pathway of fatty acids after cleavage of the aromatic ring. Excretion of the metabolites is expected via urine or via expired air as CO2 after metabolic degradation.

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 members of the PFAE aromatic category, are considered to have a low bioaccumulation potential (e.g., Nendza & Müller, 2007; 2010).

Furthermore, for those substances with a log Kow value > 10 it is unlikely that they reach the pass level of being bioaccumulative according to OECD criteria for the PBT assessment (BCF > 2000 L/kg; ECHA, 2012). This assumption is supported by QSAR calculations using BCFBAF v3.01 performed for the PFAE aromatic category members. BCF/BAF values of 0.89 L/kg and 0.89-10.92 L/kg were obtained (Arnot-Gobas estimate, including biotransformation, upper trophic). Even though the members of the PFAE aromatic category are 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

The members of the PFAE aromatic category are characterized by a low water solubility (< 0.05 mg/L), high log Kow values (> 10) and high molecular weights (546-757 g/mol). Based on the physico/chemical properties such as low water solubility and high potential for adsorption a reduced availability in water is expected. The high molecular weight of the substance significantly reduces the absorption due to steric hindrance to cross biological membranes. Absorbed molecules will be metabolized and the metabolites will be excreted. It can be concluded that the bioaccumulation potential of the PFAE aromatic category members is negligible. BCF/BAF values estimated by QSAR (BCFBAF v3.01) also support this assumption (BCF values all well below 2000 L/kg). Taking all these information into account, it can be concluded that bioaccumulation of the PFAE aromatic category members is unlikely to occur.

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within CSR.