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Sediment toxicity

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Description of key information

Key value for chemical safety assessment

Additional information

No experimental data evaluating the toxicity to sediment organisms is available for Hexanoic acid, 2-ethyl-, C16-18 alkyl esters (CAS No. 90411-68-0). Only negligible releases into surface waters from sewage treatment plants are expected to take place due to: a) the ready biodegradability and b) the high adsorption properties of this substance, resulting in an effective removal in sewage treatment plants. Therefore chronic exposure of sediment organisms is unlikely. Furthermore, the substance is not toxic to aquatic organisms up to the limit of water solubility. In addition, available data indicate, that Hexanoic acid, 2-ethyl-, C16-18 alkyl esters is not bioaccumulative. Based on the available information, toxicity to sediment organisms is not expected to be of concern.

Intrinsic properties and fate

Hexanoic acid, 2-ethyl-, C16-18 alkyl esters (CAS No. 90411-68-0) is readily biodegradable (90% CO2 evolution in 28 days; Bouillon, 2012). According to the Guidance on information requirements and chemical safety assessment, Chapter R.7b, readily biodegradable substances can be expected to undergo rapid and ultimate degradation in most environments, including biological Sewage Treatment Plants (STPs) (ECHA, 2012b). Therefore, after passing through conventional STPs, only low concentrations of these substances are likely to be (if at all) released into the environment.

Furthermore, the substance exhibits a log Koc value of > 5 and is poorly water soluble (< 0.15 mg/L measured in aqua dest). The Guidance on information requirements and chemical safety assessment, Chapter R.7b (ECHA, 20012b) 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, 2012a) and the rest will be extensively biodegraded (due to ready biodegradability). Thus, discharged concentrations of these substances into the aqueous/sediment compartment are likely to be negligible.

Considering this one can assume that the availability of Hexanoic acid, 2-ethyl-, C16-18 alkyl esters in the sediment environment is generally very low, which reduces the probability of chronic exposure of sediment organisms in general.

Aquatic ecotoxicity data

Acute and chronic aquatic toxicity tests of the substance Hexanoic acid, 2-ethyl-, C16-18 alkyl esters (CAS No. 90411-68-0) and the analogue substances C8-16-Fatty acid-2-ethylhexyl ester (CAS No. 135800-37-2) and 2-Ethylhexyloleat (CAS No. 26399-02-0) to fish, invertebrates, algae and microorganisms showed no adverse effects occurred. The obtained results indicate that Hexanoic acid, 2-ethyl-, C16-18 alkyl esters is likely to show no toxicity to sediment organisms as well.

Metabolisms/Bioaccumulation

After absorption, Hexanoic acid, 2-ethyl-, C16-18 alkyl esters is expected to be enzymatically hydrolysed in vivo by the ubiquitary enzyme carboxylesterase, yielding the corresponding alcohols and 2-ethylhexanoic acid. QSAR estimations using BCFBAF v3.0 support the expected rapid biotransformation of this substance with BCF/BAF values < 130 L/kg, respectively (Arnot-Gobas estimation including biotransformation, upper trophic level). 

The metabolism of the hydrolysis products: alcohol (i.e. C16/C18 fatty alcohols) and 2-ethylhexanoic acid is well established and not of concern in terms of bioaccumulation (for further information see chapter 5.3 of the technical dossier).

Summarizing, Hexanoic acid, 2-ethyl-, C16-18 alkyl esters is expected to be rapidly hydrolyzed to 2-ethylhexanoic acid and the corresponding alcohols. The hydrolysis products are supposed to be satisfactory metabolized in aquatic organisms and are not bioaccumulative. Therefore, no potential for bioaccumulation is to be expected.

Conclusion

Due to its readily biodegradable nature, extensive degradation of this substance 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, further biodegradation will occur and, due to the high log Kow, low water solubility and high adsorption potential, Hexanoic acid, 2-ethyl-, C16-18 alkyl esters will be bioavailable to sediment organisms mainly via feed and contact with suspended organic particles. After uptake by sediment species, extensive and fast biotransformation of the substance by carboxylesterases into 2-ethylhexanoic acid and C16/C18 fatty alcohols is expected. The supporting BCF/BAF values estimated with the BCFBAF v3.01 program, Arnot-Gobas model including biotransformation, also indicate that this substance will not be bioaccumulative (all well below 2000 L/kg). Furthermore, aquatic toxicity data show that no effects occur up to the limit of water solubility. Therefore, Hexanoic acid, 2-ethyl-, C16-18 alkyl esters is unlikely to pose a risk for sediment organisms in general and testing is thus omitted.

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