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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

Key value for chemical safety assessment

BCF (aquatic species):
35 L/kg ww

Additional information

Benzene, mono-C10-13-alkyl derivs., distn. residues (Heavy Alkylate Bottoms – HAB) is an extremely complex hydrocarbon UVCB. The substance is comprised of 21 different categories of components. The total number of individual components is estimated to be nearly 1000. It has been estimated that there are 210 possible isomers in the category of dialkyl benzenes alone. Standard tests for this endpoint are intended for single substances and are not appropriate for this complex substance. Further complicating the potential bioaccumulation testing of HAB is the extremely low water solubility (WATERNT estimates from 2.61e-03 to 5.83e-07 mg/L) and extremely high log Kow’s (KOWWIN estimates from 7.87 to 19.62) of the individual components. These properties indicate that the HAB components will have little, if any, bioavailability.

As a result, the potential bioaccumulation of representative structures from each category of components identified in HAB has been calculated using the BCFBAF model (v3.01) found in the US EPA’s EPISuite (v4.11) group of QSAR models. The calculated BCF values ranged from 3 to 880 (regression based model) and 1 to 151 (Arnot-Gobas Upper Trophic – including biotransformation rate estimates).

In addition, the bioaccumulation factors for three representative compounds, p-bis (1-methylundecyl) benzene, p-bis 2,9-diphenyldodecane, and 1-methyl-5-(1-methylundecyl)-2-(1-pentyl) benzene, were calculated using EPI Suite v3.20 software during the OECD HPV review for the category Linear alkylbenzene alkylate bottoms. The log BCF determined for all three compounds was 0.5 (i.e., BCF = 3.12). This is the minimum log BCF calculated from the estimation software when the log Kow is greater than 7.

The QSAR predictions for the component categories of HAB indicate there are no concerns for bioaccumulation. The category which has the highest predicted BCF value is the monoalkylbenzenes. This category is represented by Linear Alkylbenzene (LAB) for which there is a measured BCF value in Bluegill Sunfish. The measured value of 35 confirms that there are no bioaccumulation concerns for this category. Additionally, the calculated log Kow’s for the remaining categories all exceed 10. As seen in Figure R.11-6 of the Guidance on Information Requirements and Chemical Safety Assessment Chapter R.11, once the log Kow exceeds 10, there are no known substances that have a BCF greater than 2000.

In addition, HAB is expected to undergo biotransformation and elimination based on a bioaccumulation study conducted with bluegill sunfish on LAB. In that study LAB reached 90% of steady state within a week and was rapidly eliminated (50% of steady state concentration was cleared in 2 days). The resulting measured BCF value is much lower than the value (880) calculated by the BCFBAF regression based model which doesn’t take into account biotransformation. However, it is very similar to the upper trophic value (151) calculated by the Arnot-Gobas model which does adjust for biotransformation rate.

It is not clear whether similar biotransformation results would be observed for HAB, since these mixtures are generally more complex. However, data on linear alkyl chains suggests a mechanism for LAB elimination, namely conversion of the terminal carbons of linear alkyl chains (alkanes) to carboxylic acids followed by rapid metabolism of the resulting fatty acids. This mechanism is consistent with the rapid elimination rate observed for LAB in bluegill sunfish. Further, this mechanism has been demonstrated as the biotransformation mechanism for linear alkylbenzene sulfonate (LAS), the sulfonated derivative of LAB, which also demonstrates lower bioaccumulation potential than predicted from its octanol-water partition coefficient (OECD 2005). HABs would be subject to similar metabolism due to the presence on all HAB components of C10-C14 linear alkyl chains with unhindered terminal carbons similar to those in LAB and LAS. In addition, the calculated BCF values for the components of HAB are lower using the Arnot-Gobas model than those calculated by the regression based model. This indicates that biotransformation is predicted to occur for each category of components.

Based on a weight of evidence, the substance (Benzene, mono-C10-13-alkyl derivs., distn. residues) will not meet the criteria for a bioaccumulative substance.