1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters

Bioaccumulation: aquatic / sediment

Description of key information

The bioaccumulation potential is expected to be low.

Additional information

No experimental data is available concerning the bioaccumulation potential of 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters (CAS No. 90193-76-3). 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.
1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters exhibits a log Koc value of > 5 and is poorly water soluble (< 0.05 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.
Therefore, the relevant uptake route of 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters in organisms is considered predominately to be by ingestion of particle bound substance.

If the substance is taken up by ingestion, absorption of 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters 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, 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters has a high molecular weight of 615 to 671 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 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters 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 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters, 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 (Arnot-Gobas estimate, including biotransformation, upper trophic). The BAF values calculated for the main component of the substance ranged from 0.89 - 0.93 L/kg. Whereas BCF values of 0.89 L/kg were calculated for the main components. Even though the 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl 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, 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters will be bioavailable to aquatic organisms mainly via feed and contact with suspended organic particles. Nevertheless, this substance has a high molecular weight and a log Kow value > 10; both factors hinder a crossing of biological membranes. Taking all this information into account, it can be concluded that the bioaccumulation potential of 1,2-Benzenedicarboxylic acid, di-C16-18-alkyl esters (CAS No. 90193-76-3) is low.