3-(triethoxysilyl)propanethiol

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.007 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.067 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.001 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

PNEC marine water (intermittent releases):

0.007 mg/L

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

3.3 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.2 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.02 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.04 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hydrolysis half-life of 3-(triethoxysilyl)propanethiol (CAS 14814-09-6) is approximately 0.0533 hours at pH 4, 32.1 hours at pH 7, and 4.69 hours at pH 9 and 20°C (OECD 111). The registered substance will hydrolyse in contact with water and atmospheric moisture to 3-(trihydroxysilyl)propanethiol and ethanol. 3-(Trihydroxysilyl)propanethiol and ethanol are both miscible in water and have low log Kow (-1.4 and -0.3, respectively).

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 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.

The substance will be exposed to the environment through wastewater treatment plant (WWTP) effluent only. The minimum residency time in the wastewater treatment plant is approximately 7 hours (although this is a conservative figure and wastewater treatment time may be hours to days longer) with an average temperature of 15°C (assumed to be at neutral pH). Although some degradation by hydrolysis would be expected before the substance is released to the receiving waters, the hydrolysis half-life indicates that the environment is likely to be exposed to significant quantities of the parent substance. The environmental hazard assessment for the aquatic compartments is therefore based on the parent substance.

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 i.e. parent substance. As the substance resides and settles in the sediment compartment, it will be susceptible to further hydrolysis. Accordingly, exposure and chemical safety of the sediment is based on both the parent and hydrolysis products.

 

Exposure of soil is via the partitioning of the substance to the wastewater treatment plant (WWTP) sludge. The proportion of the substance that adsorbs to the WWTP sludge will either be burnt or spread on soil, depending on the practices of the WWTP. For sludge that is spread onto soil, the substance will have undergone further hydrolysis by the time it reaches the soil. However, given the hydrolysis half-life, it is possible that parent substance will be present. The terrestrial chemical safety assessment is therefore based on the properties of both the parent substance and hydrolysis products.

Currently, the available data, read across from a structural analogue, reflect exposure of the test organisms to the hydrolysis products of the registration substance. Consequently, aquatic toxicity tests with the registration substance are being conducted, in accordance with ECHA Final Decision No. TPE-D-2114432420-65-01/F, to assess the toxicity of the parent substance. A stepwise testing strategy is proposed: acute toxicity studies with fish, invertebrates and algae, and a chronic invertebrate study are being conducted with 3-(triethoxysilyl)propanethiol (CAS 14814-09-6). The  results of these studies will determine which trophic level is acutely the most sensitive to the test substance and whether a long-term fish study will be commissioned.

 

The study results will not be available in time for the final decision deadline of 26th January 2021. These study results will therefore be submitted at a later date. The acute toxicity study reports are expected to be available in March 2021. Further information on the stepwise testing strategy is available in IUCLID Section 13.

 

The chemical safety assessment and PNECs will be revised once further data are available.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

The registered substance, its silanol hydrolysis product and the substance used as a surrogate for read-across are part of a class of thiol compounds which contain a terminal thiol (SH) group present on a Silicon side chain. A methoxy, ethoxy or hydroxyl group will be attached directly to the silicone. Additional information is given in a supporting report (PFA 2013w), attached in IUCLID Section 13.

The registered substance hydrolyses in water and therefore the selection of surrogate substance is based on log K_ow of the resulting silanols and the chemical groups present in them.

Short-term toxicity to aquatic organisms data and terrestrial toxicity data have been read-across from the structural analogue 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0). Both the registration substance and the read-across substance hydrolyse to 3-(trihydroxysilyl)propanethiol, and ethanol and methanol, respectively.

Considerations on the non-silanol hydrolysis products: ethanol and methanol are well characterised in the public domain literature and are not hazardous to aquatic organisms at the concentrations relevant to the studies (OECD 2004a - SIDS for methanol; OECD 2004b - SIDS for ethanol).

Please see the attached report in Section 13 of IUCLID and Annex 5 of the CSR for the analogue approach to address ecotoxicity of 3-(triethoxysilyl)propanethiol (CAS 14814-09-6).

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >100 mg/L in fish and algae. It has a reliable EC₅₀ of 6.7 mg/L in invertebrates (as the parent substance).

The substance hydrolyses moderately in water (32 hours at pH 7 and 20°C). Both the parent substance and the silanol hydrolysis product, 3-(trihydroxysilyl)propanethiol, are not readily biodegradable but have a log Kow <3.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified

Chronic toxicity: Category Chronic 2

A long-term Daphnia toxicity test has been commissioned and a long-term fish study is planned depending on the outcome of the acute studies. The classification and labelling will be updated once the results of this test are available.