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Sediment toxicity

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No sediment toxicity data are available for the registration substance. Testing for toxicity to sediment organisms is not considered necessary because:

The hydrolysis half-life of the registration substance is approximately 43 hours at neutral pH. The sediment is exposed to the substance in the environment via the effluent water released from the wastewater treatment plant (WWTP). The sediment will therefore be exposed to the same species as the receiving waters initially. As the substance resides and settles in the sediment compartment, it will be susceptible to further hydrolysis. Although significant hydrolysis is expected before the sediment is exposed to the substance, the parent substance has a much higher log Kow than the silanol hydrolysis product and is therefore more likely to partition to the sediment. In addition, the parent substance is expected to be more toxic than the silanol hydrolysis product. However, as the sediment can act as a sink for chemicals, and the silanol hydrolysis product, (2,4,4-trimethylpentyl)silanetriol, is not readily biodegradable, it may persist in the sediment compartment for longer than the parent substance. Accordingly, exposure and chemical safety of the sediment should be based on both the parent substance and the hydrolysis products.

Currently, the available data assess the toxicity of the silanol hydrolysis product.

In accordance with Column 2 of REACH Annex X, there is no need to further investigate the effects of the substance in a long-term sediment study because, as indicated in guidance R.7.11.6 (ECHA 2016), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1, and therefore the risk is already adequately controlled and further testing is not justifiable.

The silanol hydrolysis product is not readily biodegradable but has low potential for bioaccumulation and low bioavailability (based on log Kow <3 (0.9)), and there is no reason to expect any specific mechanism of toxicity beyond narcosis.

Long-term toxicity testing to aquatic invertebrates has been carried out with the registered substance as well as a structural analogue. During these tests, it is likely that organisms were exposed to the hydrolysis products of the test substance. The occurrence of more severe toxic effects in the sediment compartment that were not expressed in these aquatic studies (conducted at concentrations up to 100 mg/l) would be considered unlikely.

The PNEC calculated by Equilibrium Partitioning has been derived for the purpose of chemical safety assessment and the risk characterisation ratios are below 1.

Overall it is concluded that the risk characterisation conclusion is sufficiently conservative in respect of any uncertainties and therefore further testing is not considered necessary.

Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0 and Chapters 9 and 10 of the Chemical Safety Report, respectively.

Long-term aquatic toxicity tests are being conducted in accordance with ECHA Final Decision TPE-D-2114455990-41-01/F. The OECD TG 210 Fish, Early Life-Stage Toxicity test and the OECD TG 211 Daphnia Reproduction test are currently ongoing. The substance dataset and risk assessment will be updated once results of the chronic aquatic studies are available and the studies are finalised. An update is planned to be completed within three months from when the last final report is received. The draft reports are expected in January/February 2021, with the final reports following in March/April 2021.