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Reliable short-term aquatic toxicity tests are available for freshwater fish (Brachydanio rerio, new name Danio rerio), invertebrates (Daphnia magna) and algae (Scenedesmus subspicatus, new name Desmodesmus subspicatus) for the registered substance, 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0, EC No. 224-588-5).

 

The relevant short-term values are:

Fish LC50 (96 h): 439 mg/l, equivalent to 345 mg/l when expressed in terms of concentration of the hydrolysis product, 3-(trihydroxysilyl)propanethiol.

Daphnia EC50 (48 h): 6.7 mg/l, equivalent to 5.3 mg/l when expressed in terms of concentration of the hydrolysis product, 3-(trihydroxysilyl)propanethiol.

Algae ErC50 (72 h): 931 mg/l, ErC10: 92 mg/l and NOErC: 40 mg/l, equivalent to 731, 72 and 31 mg/l, respectively, when expressed in terms of concentration of the hydrolysis product, 3-(trihydroxysilyl)propanethiol.

 

The rapid hydrolysis of 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0) (half-life 2.6 hours at pH 7 and 20-25°C) means it is likely that the test organisms in each test were exposed primarily to the hydrolysis products, 3-(trihydroxysilyl)propanethiol and methanol, and the tests have been interpreted accordingly.

 

An activated sludge respiration inhibition 3-hour EC50 value of 463 mg/l and EC10 value of 32.6 mg/l (nominal in terms of the parent substance) were determined in a reliable study.

 

No long-term toxicity data are available with the registered substance. A long-term Daphnia reproduction test in accordance with OECD TG 211 is being conducted with the analogue substance 3-(triethoxysilyl)propanethiol (CAS 14814-09-6, EC No. 238-883-1). Depending on the results of the long-term Daphnia test, a long-term fish toxicity test may also be conducted. These tests with 3-(triethoxysilyl)propanethiol (CAS 14814-09-6) will be read across to the registered substance to fulfil the REACH Annex IX requirements. Justification for read-across is discussed in the RAAF report for ecotoxicity, attached in Section 13 of IUCLID and Annex 4 of the CSR.

 

To support the proposed long-term read-across data, short-term aquatic toxicity data with 3-(triethoxysilyl)propanethiol (CAS 14814-09-6) are included, for comparison of aquatic effects between the registered and read-across substances.

The relevant short-term ecotoxicity values for 3-(triethoxysilyl)propanethiol (CAS 14814-09-6) are:

Fish LC50 (96 h): >2.55 mg/l, equivalent to >1.65 mg/l when expressed in terms of the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol (threshold toxicity test).

Invertebrates EC50 (48 h): 2.71 mg/l, equivalent to 1.75 mg/l when expressed in terms of the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol.

Algae ErC50 (72 h): 32.2 mg/l, ErC10: 11.8 mg/l and NOErC: 4.8 mg/l, equivalent to 20.8, 7.6 and 3.1 mg/l, respectively, when expressed in terms of the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol.

 

During the tests with 3-(triethoxysilyl)propanethiol (CAS 14814-09-6), it is likely that the fish and Daphnia were exposed predominantly to the parent substance, especially at the beginning of the tests, with increasing amounts of hydrolysis products towards the end of the test media renewal period. Due to the static nature of the algal test, it is likely that the algae were exposed to the parent at the beginning of the test, with the hydrolysis products dominating towards the end.

 

The short-term aquatic toxicity data with both the registered substance (3-(trimethoxysilyl)propane-1-thiol) and the read-across substance (3-(triethoxysilyl)propanethiol) show that invertebrates are the most sensitive trophic level for both substances, regardless of whether the organisms were exposed to parent substance or hydrolysis products. Although the data with the read-across substance indicates the parent substance may exert slightly higher toxicity than the hydrolysis product, the short-term Daphnia effect concentration values for both read-across and registered substance are in the same order of magnitude.

 

The chemical safety assessment and PNECs will be revised once the long-term data, to be read across from 3-(triethoxysilyl)propanethiol (CAS 14814-09-6, EC No. 238-883-1), are available.