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EC number: 238-883-1
CAS number: 14814-09-6
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
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
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
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.
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.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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