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The registered substance, 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0, EC No. 224-588-5), will hydrolyse rapidly (2.6 hours at pH 7 and 20-25°C (QSAR)) in contact with water and atmospheric moisture to form 3-(trihydroxysilyl)propanethiol and methanol.

 

3-(Trihydroxysilyl)propanethiol is soluble in water (1.0E+06 mg/l at 20°C (QSAR)) and has low log Kow (-1.4 (QSAR)).

 

REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT50 is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. ECHA Guidance Chapter R.7b (ECHA, 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis. Therefore, in accordance with REACH guidance, the terrestrial chemical safety assessment for 3-(trimethoxysilyl)propane-1-thiol is based on its silanol product 3-(trihydroxysilyl)propanethiol. The non-silanol hydrolysis product, methanol, is not expected to contribute to toxicity to terrestrial organisms. 

 

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long or short-term terrestrial toxicity to invertebrates/higher plants 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 substance is highly water soluble and is not readily biodegradable but has low potential for bioaccumulation and low potential for adsorption (based on log Kow <3 (-1.4) and low log Koc 0.29). The environmental compartment of concern is therefore expected to be the aquatic compartment.

 

Toxicity was observed in short-term aquatic toxicity tests, with Daphnia being the most sensitive. The occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.

 

3-(Trihydroxysilyl)propanethiol is classed as hazard category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R.7c, 2017) based on potential for high persistence (DT50 >180 days), lack of ready biodegradability and low toxicity to aquatic organisms (EC/LC50 not <1 mg/l).

 

In this situation, a screening approach is applied: a confirmatory long-term terrestrial test is usually appropriate, in addition to the equilibrium partitioning approach with an extra factor of ten in order to determine whether further full tests are necessary.

 

A confirmatory test would be conducted with the most sensitive organism group based on short-term aquatic testing. For this substance, invertebrates were the most sensitive organisms, indicating a preference to conduct a confirmatory test with terrestrial invertebrates.

 

The current PNECscreen(EQPM) for 3-(trihydroxysilyl)propanethiol is derived from the short-term test results with invertebrates and has a value of 0.0009 mg/kg dwt. For the purpose of the screening assessment comparison only, an extra factor of ten is applied (PECx10/PNECscreen (EQPM)).Based on the exposure assessment, the highest PECx10/PNECscreen(EQPM) for 3-(trihydroxysilyl)propanethiol is 5.53.

 

This high RCR would usually trigger the requirement for long-term terrestrial toxicity tests in accordance with Annex X testing requirements. However, there is an ongoing long-term toxicity to aquatic invertebrates study being conducted with the analogue substance 3-(triethoxysilyl)propanethiol (CAS 14814-09-6, EC No. 238-883-1), in accordance with an appropriate test guideline and GLP. The results from this study will be read across to the 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0). The PNECs, including the PNECscreen(EQPM) will be updated when the new data become available.

 

In the event that terrestrial invertebrate and plant studies were to be conducted, the definitive terrestrial risk characterisation would use a PNECsoil based on the lower of the two test results with an assessment factor (AF) of 50 (unless soil microorganism data are available as well, in which case, the assessment factor would be 10). This PNEC would only be more conservative than the current value of PNECscreen(EQPM) if standard testing in terrestrial plants or invertebrates exhibited a dose response with a NOEC/EC10 ≤ 0.045 mg/kg dw (current PNEC of 0.0009 mg/kg * AF 50 for two long-term tests). There is no basis to expect such toxicity in the terrestrial compartment for 3-(trihydroxysilyl)propanethiol based on the absence of significant toxicity observed in aquatic tests (EC/LC50 not <1 mg/l). Additionally, no toxicity was observed in a short-term toxicity to Eisenia fetida test with the registered substance (14-day LC50 >1000 mg/kg soil dry weight (Scholz, 1996)).

 

In the case of 3-(trihydroxysilyl)propanethiol, the registrants consider that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such, the registrants propose that further testing (including the confirmatory study) is not necessary.

 

Overall, it is concluded that the risk characterisation conclusion is sufficiently conservative and therefore further in vivo 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 Section 7, 9 and 10 of the Chemical Safety Report.