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Ecotoxicological Summary

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Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.008 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
0.027 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.001 mg/L
Assessment factor:
100
Extrapolation method:
assessment factor
PNEC marine water (intermittent releases):
0.003 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:
0.05 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.005 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:
0.005 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 soluble 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 released to the aquatic 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 or from aerial deposition of substance lost via fugitive emissions. The proportion of the substance that adsorbs to the WWTP sludge will either be burnt or spread on agricultural 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.

Short-term aquatic tests and a long-term aquatic invertebrate test have been conducted with the registration substance, 3-(triethoxysilyl)propanethiol (CAS 14814-09-6), to assess the toxicity of the parent substance, in accordance with ECHA Final Decision No. TPE-D-2114432420-65-01/F. The long-term toxicity to fish test has been waived on the basis that the Daphnia are the most sensitive species. PNECs are derived using the chronic Daphnia data and the subsequent RCRs are <1. Please see the long-term fish toxicity data waiver for further discussion.

Supporting data are also read across from 3-(trimethoxysilyl)propane-1-thiol (CAS 4420-74-0, EC No. 224-588-5), which hydrolyses rapidly in contact with water (half-life 2.6 hours at pH 7 and 20-25°C) to form 3-(trihydroxysilyl)propanethiol and methanol. Measured short-term toxicity studies with fish, Daphnia and algae are available. These supporting read-across data are included in order to help assess the sensitivity of each tested species, and to demonstrate that the substance is significantly less toxic to fish than to Daphnia. The read-across data are also used to derive an indicative PNECaquatic for use in the equilibrium partitioning method for sediment and soil PNECs for the silanol hydrolysis product 3-(trihydroxysilyl)propanethiol.

READ-ACROSS JUSTIFICATION

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 Kow 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) to support the data with the registered substance. Both the registered 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 >2.55 mg/l for fish, 2.71 mg/l for Daphnia and 32.2 mg/l for algae. It has reliable EC10 and NOEC values of 11.8 mg/l and 4.8 mg/l, respectively, for algae and a NOEC value of 0.081 mg/l for long-term toxicity to Daphnia magna (as the parent substance).

The substance hydrolyses moderately in water (32.1 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 1

An M-Factor of 1 applies.

NOTE TO REVIEWER: The new long-term Daphnia toxicity test result of 0.081 mg/l leads to classification Chronic 1. This has not yet been discussed/agreed by the members.