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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

If aquatic exposure occurs, Sorbitan esters category members will be mainly taken up by ingestion and digested through common metabolic pathways, providing a valuable energy source for the organism, as dietary fats and sugars. These substances are thus not expected to bioaccumulate in aquatic or sediment organisms and secondary poisoning does not pose a risk.

Key value for chemical safety assessment

Additional information

No experimental data is available on the bioaccumulation potential of Sorbitan ester category members. Therefore, all available related data is combined in a Weight of Evidence (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 (ECHA, 2012).

Bioaccumulation refers to 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 uptake rate is faster than the subsequent metabolism and/or excretion. 

In the case of Sorbitan esters, uptake of dissolved substance via water is expected to be low. The water solubility is generally low, with the exception of the smallest substance Sorbitan, octanoate (2:3), which is highly soluble. All substances are regarded as readily biodegradable and are thus assumed to be eliminated in sewage treatment plants to a high extent. For the larger category members, high adsorption potential further promotes rapid removal from waste water. If fractions of a sobitan ester were to be released in the aquatic environment, the concentration in the water phase will be reduced by rapid biodegradation and adsorption to solid particles and to sediment. Additionally, the substances in the Sorbitan esters category have estimated low potential for dermal absorption. QSAR estimations, taking molecular weight, log Pow and water solubility into account, resulted in a dermal absorption of 4.2*10-11 to 0.006 mg / cm h for the Sorbitan esters category members (DERMWIN v2.01, 2012). Due to low exposure concentrations through water and low dermal absorption potential, no significant uptake through the water phase can therefore be expected.

Food ingestion is likely to be the main uptake route for Sorbitan esters category members in fish, since the substance will be adsorbed to solid particles potentially ingested by fish. Also for sediment-dwelling organisms the main uptake route will be ingestion of contaminated sediment. In the case of ingestion, Sorbitan esters category members are predicted to undergo metabolism or excretion. Esters are known to hydrolyse into carboxylic acids and alcohols by esterases (Fukami and Yokoi, 2012). Carboxylesterase activity has been noted in a wide variety of tissues in invertebrates as well as in fish (Leinweber, 1987; Soldano et al, 1992; Barron et al., 1999, Wheelock et al., 2008). Therefore, it is expected that under physiological conditions, members of the Sorbitan esters category will hydrolyse to D-glucitol and the respective fatty acids. The hydrolysis of Sorbitan fatty acid esters occurs within a maximum of 48 h for mono-, di- and tri-ester, but decreases with the number of esterified fatty acid so that no hydrolysis of hexa-ester occurs (Croda 1951, Mattson and Nolen 1972, Treon 1967, Wick and Joseph 1953). The resulting fatty acids are either metabolised via the β-oxidation pathway in order to generate energy for the cell or reconstituted into glyceride esters and stored in the fat depots in the body (Berg, 2002). D-glucitol is primarily metabolised in the liver. The first step of its metabolism involves oxidation by L-iditol dehydrogenase to fructose, which is metabolised by the fructose metabolic pathway (Senti, 1986). D-glucitol is naturally found in several berries and fruits as well as in seaweed and algae (FDA, 1972). Larger Sorbitan fatty acid esters that will not be hydrolysed, such as hexaesters, are unlikely to cross biological membranes due to their high molecular weight. Metabolic pathways in fish are generally similar to those in mammals. Lipids and their constituents, fatty acids, are in particularly a major organic constituent of fish and play a major role as source of metabolic energy in fish, for growth, reproduction and mobility, including migration (Tocher, 2003).

QSAR calculations support the assumption of rapid metabolism within fish. Using the Arnot-Gobas method, including biotransformation, BCF values of 0.9 - 36.5 were obtained (BCFBAF v3.01, Kampara 2011). Although, the triesters are generally outside the log Kow and/or molecular weight range of the training set, the results can be taken as an indication of low bioaccumulation potential.

In conclusion, Sorbitan esters category members will be mainly taken up by ingestion and are digested through common metabolic pathways, providing a valuable energy source for the organism, as dietary fats and sugars. These substances are thus not expected to bioaccumulate in aquatic or sediment organisms.

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