3-(triethoxysilyl)propanethiol

Endpoint summary

Additional information

Approach to the Terrestrial Chemical Safety Assessment

The registered substance will hydrolyse (half-life 0.0533, 32.1 and 4.69 hours at pH 4, 7 and 9 respectively, at approximately 20°C) in contact with water and atmospheric moisture to 3-(trihydroxysilyl)propanethiol and ethanol. 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 aquatic hazard assessment is based on the parent substance, and the sediment and soil compartments are based on the properties of both the parent substance and the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol.

The non-silanol hydrolysis product, ethanol, is not expected to contribute to toxicity to terrestrial organisms.

Parent: 3-(triethoxysilyl)propanethiol

No terrestrial toxicity data are available with the registered substance.

The substance is water soluble and is not readily biodegradable, but has low bioavailability and low potential for adsorption (based on log K_ow <3 (2.7) and log K_oc 1.4).

3-(Triethoxysilyl)propanethiol is classed as hazard category 2 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 2017) based on low potential for adsorption (log K_ow < 5), no indication of high persistence (DT₅₀ <180 days), but evidence that the substance is very toxic to aquatic organisms (21-day Daphnia NOEC 0.081 mg/l). In this situation, the approach for screening assessment for the parent substance is to conduct a PEC/PNECscreen based on the Equilibrium Partitioning Method and conduct a confirmatory short-term soil toxicity test with the most sensitive organism group as indicated from aquatic toxicity data.

The PNECscreen based on the equilibrium partitioning method (EQPM) for 3-(triethoxysilyl)propanethiol is 0.0052 mg/kg dry weight and is calculated from the PNECaquatic freshwater of 0.0081 mg/l, derived from the long-term test result with aquatic invertebrates. Based on the exposure assessment, the highest agricultural soil RCR available for the parent substance is 0.265, with a corresponding Predicted Environmental Concentration (PEC) of 0.00138 mg/kg dwt. Therefore, the PNECsoil calculated by the Equilibrium Partitioning Method for the parent substance generates a risk characterisation ratio that is below 1.

For this substance, invertebrates were the most sensitive organisms in the aquatic toxicity tests, indicating a preference to conduct a confirmatory terrestrial test with soil invertebrates. A short-term terrestrial toxicity test in accordance with EU Method C.8 (Toxicity for Earthworms: Artificial Soil Test) has been read across from the structural analogue, 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0, EC No. 224-588-5). A 14-day LC₅₀ value of >1000 mg/kg soil dry weight has been reported for the effects of 3-(trimethoxysilyl)propane-1-thiol on mortality of Eisenia fetida (Scholz, 1996)).

In accordance with ECHA Guidance Chapter R.7c (ECHA 2017), because the PEC/PNECscreen is < 1 and there is no indication of risk from a confirmatory short-term soil toxicity test, no further toxicity testing for soil organisms is required.

Additionally, the aquatic PNEC derived for 3-(triethoxysilyl)propanethiol is already conservative in terms of its use in the Equilibrium Partitioning Calculation for PNECsoil, because the substance contains a structural alert for Daphnia and therefore exhibits excess toxicity to Daphnia that would be unlikely to occur in terrestrial organisms.  

Overall, it is concluded that the risk characterisation conclusion is sufficiently conservative and therefore further testing is not considered necessary.

Hydrolysis product: 3-(trihydroxysilyl)propanethiol

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 bioavailability and low potential for adsorption (based on log K_ow <3 (-1.4) and log K_oc 0.29).

Some toxicity was observed in short-term aquatic tests. The substance contains a thiol group which is associated with excess toxicity in Daphnia. The occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would therefore be considered unlikely.

3-(Trihydroxysilyl)propanethiol is classed as hazard category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 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.

In the event that terrestrial invertebrate and plant studies need to be conducted, the definitive terrestrial risk characterisation would use a PNEC_soil based on the lower of the two test results with an assessment factor of 50 (unless soil microorganism data are available as well, in which case, the assessment factor would be 10).

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 PNEC_screen(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/PNEC_{screen (EQPM)}).Based on the exposure assessment, the highest agricultural soil RCR available for the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol, is 0.961, with a corresponding Predicted Environmental Concentration (PEC) of 0.000865 mg/kg dwt. The (PECx10)/PNECscreen(EQPM) for 3-(trihydroxysilyl)propanethiol is (0.000865*10)/ 0.0009 = screening RCR 9.61.  

A confirmatory long-term terrestrial toxicity test is therefore required in accordance with the recommendations for hazard category 3 substances. If a confirmatory long-term terrestrial test were to be conducted, an assessment factor of 100 would be applied to derive PNECsoil from one long-term test. A confirmatory test with either terrestrial plants or invertebrates would result in a new value for PNECsoil. This value could only be more conservative than the value of PNECscreen(EQPM) in the situation that standard testing in terrestrial plants or invertebrates exhibited a dose response with a NOEC/EC₁₀ < 0.09 mg/kg dw (and applying an assessment factor of 100). There is no basis to expect such toxicity for 3-(trihydroxysilyl)propanethiol because the PNECaquatic is already conservative in terms of its use in the Equilibrium Partitioning Calculation for PNECsoil, because the substance contains a structural alert for Daphnia and therefore exhibits excess toxicity to Daphnia that would be unlikely to occur in terrestrial organisms.  

 

Additionally, in a short-term toxicity test conducted with the structural analogue, 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0), which hydrolyses to the same silanol hydrolysis product, no toxicity was observed to Eisenia fetida (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 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 CSR Section 7, 9 and 10.