Hexadecyl palmitate

Sediment toxicity

Description of key information

Taking all available information into account, the substance is unlikely to pose a risk to sediment organisms.

Additional information

Hexadecyl palmitate (CAS 540-10-3) has a high log Koc (> 5.0, EPI Suite v4.11), which may raise concern with respect to its potential sediment toxicity. There is no experimental data evaluating the toxicity of the substance to sediment organisms. Nonetheless, toxicity to sediment organisms is not expected to be of primary concern based on the available data presented below.

Based on the physico-chemical substance properties, only negligible releases from sewage treatment plants (STPs) into surface waters are expected to take place due to: a) the ready biodegradability and b) the high adsorption properties, 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 (< 0.846 µg/L). In addition, available data indicates that the substance is not bioaccumulative.

Intrinsic properties and fate

The substance is readily biodegradable according to the OECD criteria. 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 STPs (ECHA, 2016). Therefore, after passing through conventional STPs, only low concentrations of the substance are likely to be released into the environment (if at all).

Furthermore, the substance has a high estimated log Koc > 5.0(25 °C, EPI Suite v4.11, KOCWIN v2.00, MCI method)and is poorly water soluble (< 0.846 µg/L, OECD 105). The Guidance on information requirements and chemical safety assessment, Chapter R.7b (ECHA, 2016) states that once insoluble chemicals enter a standard STP, they will be extensively removed in the primary settling tank and fat trap and therefore only limited amounts will come into contact with activated sludge organisms. In the case where 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, 2016) and the remaining fraction will be extensively biodegraded (due to ready biodegradability). Thus, discharged concentrations of these substances into the aqueous/sediment compartment are likely to be negligible.

In conclusion, the bioavailability of the substance in the sediment environment is presumably very low, which reduces the probability of chronic exposure of sediment organisms in general.

Aquatic ecotoxicity data

Available data for acute toxicity to fish, aquatic invertebrates and algae as well as for chronic toxicity to aquatic invertebrates and algae show that no adverse effects occur within the range of the water solubility. Moreover, no toxic effects on activated sludge microorganisms were observed. The obtained results provide evidence that the substance is not likely to exert toxic effects on sediment organisms. It is highly unlikely that results from additional studies with sediment organisms will significantly deviate from the overall ecotoxicological trends.

Metabolism/Bioaccumulation

After absorption, long chain aliphatic esters are expected to be enzymatically hydrolyzed by carboxylesterases, yielding the corresponding alcohol and fatty acid. Both hydrolysis products are expected to be metabolized in aquatic organisms. The metabolization of the hydrolysis products is well established and not of concern in terms of bioaccumulation (for further information see chapter 5.3 of the technical dossier).

In summary, the substance is expected to be rapidly hydrolyzed to the respective fatty acid and fatty alcohol and the potential for bioaccumulation of both the substance as well as its metabolites is low.

Conclusion

Due to the ready biodegradability, extensive degradation of the substance will take place in conventional STPs and only low concentrations are expected to be released into the environment (if at all).Once present in the aquatic compartment, further biodegradation will occur and, due to the high log Kow, low water solubility and high adsorption potential, the remaining fraction will preferentially distribute into the sediment compartment where uptake by sediment organisms will predominantly occur via feed and contact with suspended organic particles. However, based on the physico-chemical properties, the bioavailability of the substance is expected to be low due to strong adsorption. Moreover, if uptake by sediment species occurs, extensive and fast biotransformation into the corresponding fatty acid and alcohol is expected through the action of ubiquitous carboxylesterases.

Furthermore, the available aquatic toxicity data show that no effects occur up to the limit of water solubility. Therefore, the substance is unlikely to pose a risk for sediment organisms and testing was not considered necessary.