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EC number: 284-660-7
CAS number: 84961-70-6
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
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