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Ecotoxicological Summary

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Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.64 mg/L
Assessment factor:
50
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
6.4 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.064 mg/L
Assessment factor:
500
Extrapolation method:
assessment factor

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
1 mg/L
Assessment factor:
100
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
4.3 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.43 mg/kg sediment dw
Assessment factor:
10
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:
0.48 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
PNEC oral
PNEC value:
10 mg/kg food
Assessment factor:
300

Additional information

The hydrolysis half-life of triethoxy(2,4,4-trimethylpentyl)silane is 43 hours at pH 7 and 20-25°C. The registered substance will therefore hydrolyse at a moderately fast rate in contact with water and atmospheric moisture to form (2,4,4-trimethylpentyl)silanetriol and ethanol.

The parent substance, triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3), has a low water solubility of <0.1 mg/L at 20°C and high log Kow of >6.5 (estimated).

The silanol hydrolysis product, (2,4,4-trimethylpentyl)silanetriol, has a high calculated water solubility of 2.4E+05 mg/L, although this is limited to approximately 200 mg/L by condensation reactions. It has a low log Kow of 0.9 (QSAR).

REACH guidance R.16 (ECHA, 2016) 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”. Technical Guidance Document on Risk Assessment (EC 2003) also suggests that when the hydrolysis half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.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.

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 or days longer) with an average temperature of 15°C (assumed to be at neutral pH). Due to the moderate hydrolysis half-life of 43 hours, it is expected that the receiving waters in the environment will be exposed to both the parent substance and its hydrolysis products. The substance is expected to act via a non-polar narcosis mechanism and as such, toxicity is related to log Kow. Due to the high log Kow of the parent substance, it is expected to be more toxic than the silanol hydrolysis product, thus assessment of the parent substance should be protective of the environment and further aquatic assessment of the hydrolysis product is not required. The environmental hazard assessment for the aquatic compartment should therefore be based on the properties of the parent substance, in accordance with REACH guidance.

Significant hydrolysis is expected before the sediment is exposed to the substance. The sediment can act as a sink for chemicals, and the silanol hydrolysis product, (2,4,4-trimethylpentyl)silanetriol, is not readily biodegradable, therefore it may persist in the sediment compartment for longer than the parent substance. Accordingly, exposure and chemical safety of the sediment should be based on the 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, significant hydrolysis is expected before the soil is exposed to the substance, therefore the hydrolysis products are relevant for chemical safety assessment.The terrestrial chemical safety assessment should therefore be based on the properties of the hydrolysis products.

Short-term ecotoxicity data are available for the registered substance with supporting data read across from a structural analogue of the registered substance. Long-term invertebrate toxicity data are available for the registered substance as well as an analogous substance that shares the same hydrolysis product as the registered substance. 

The tests with the registered substance included at least 24 hours test solution preparation time and were conducted above the water solubility of the test substance. Despite filtering of the test solution, the presence of undissolved material was reported in the Daphnia study. The tests are likely to reflect exposure of the test organisms to a mixture of parent substance and its hydrolysis products, as well as potential undissolved material. The data are therefore supported by data read across from a structural analogue, triethoxy(octyl)silane (CAS 2943-75-1), which was not tested above its limit of water solubility. The tests with this substance reflect exposure to the parent substance for the fish and invertebrate tests and a mixture of the parent substance and its hydrolysis products for the algae test.

Currently, the available chronic data with the registration substance reflect exposure of the test organisms to the hydrolysis products of the substance. Consequently, in order to assess the toxicity of the parent substance, long-term aquatic toxicity tests are being conducted in accordance with ECHA Final Decision TPE-D-2114455990-41-01/F. The OECD TG 210 Fish, Early Life-Stage Toxicity test and the OECD TG 211 Daphnia Reproduction test are ongoing. The substance dataset and risk assessment will be updated once results of the chronic studies are available and the studies are finalised. An update is planned to be completed within three months from when the last final report is received. The draft reports are expected in January/February 2021, with the final reports following in March/April 2021.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil REACH Annex VII-X 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

The registered substance and the substances used as surrogate for read-across are part of a class of low-functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the silicon atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. Additional information is given in a supporting report (PFA 2016y) attached in Section 13.

In moist medium, triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3) hydrolyses at a moderate rate (half-life 43-hours at pH 7 and 20-25°C) to (2,4,4-trimethylpentyl)silanetriol and ethanol. (2,4,4-Trimethylpentyl)silanetriol is a silanetriol with a branched alkyl side chain and a log Kow of 0.9.

The effects of ethanol are not thought to affect the outcome of the studies and are discussed below.

As described in Section 4.8 of IUCLID and Section 1.4 of the CSR, silanols may undergo condensation reactions to give siloxane dimers, oligomers and polymers at high enough concentrations. Condensation reactions become significant at approximately 200 mg/l in the case of (2,4,4-trimethylpentyl)silanetriol.

Short-term data are available for the registered substance and a structural analogue of the registered substance. Long-term invertebrate data are available for the registered substance and an analogous substance that shares the same hydrolysis product as the registered substance.  

In the following paragraphs the read-across approach for triethoxy(2,4,4‑trimethylpentyl)silane (CAS 35435-21-3) is assessed for the surrogate substance taking into account structure, hydrolysis rate and physicochemical properties. Table 7.0.1 presents relevant physicochemical properties and the available ecotoxicological data.

Table 7.0.1: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number

35435-21-3

2943-75-1

18379-25-4

Chemical Name

Triethoxy(2,4,4-trimethylpentyl)silane

Triethoxyoctylsilane

Trichloro(2,4,4-trimethylpentyl)silane

Si hydrolysis product

(2,4,4-trimethylpentyl)silanetriol

Octylsilanetriol

(2,4,4-trimethylpentyl) silanetriol

Molecular weight (g/mol) (parent)

276.5

276.5

247.7

Molecular weight (g/mol) (hydrolysis product)

192.3

192.3

192.3

log Kow (parent)

>6.5

6.4

n/a

log Kow (silanol hydrolysis product)

0.9 (QSAR prediction)

1.1 (QSAR prediction)

0.9 (QSAR prediction)

Water solubility (parent)

<0.1 mg/L

<0.13 – 0.79 mg/L (measured and QSAR prediction, respectively)

n/a (hydrolysis too rapid)

Water solubility (silanol hydrolysis product))

2.4E+05 mg/L (QSAR prediction) (limited to approximately 200 mg/L by condensation reactions)

59000 mg/L (QSAR prediction) (limited to approximately 100 mg/L by condensation reactions)

2.4E+05 mg/L (QSAR prediction) (limited to approximately 200 mg/L by condensation reactions)

Vapour pressure (parent)

0.14 Pa at 20°C and 0.22 Pa at 25°C

0.11 Pa at 25°C

(QSAR prediction)

45 Pa at 25°C

(QSAR prediction)

Vapour pressure (hydrolysis product)

1.2E-04 Pa at 25°C (QSAR prediction)

2.7E-05 at 25°C

(QSAR prediction)

1.2E-04 Pa at 25°C (QSAR prediction)

Hydrolysis t1/2 at pH 7 and 20-25°C

43 hours (QSAR prediction)

30 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Hydrolysis t1/2 at pH 4 and 20-25°C

0.9 hours (QSAR prediction)

0.7 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Hydrolysis t1/2 at pH 9 and 20-25°C

0.5 hours (QSAR prediction)

0.4 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Short-term toxicity to fish (LC50)

>100 mg/L (Greater than the limit of solubility)

>0.055 mg/L

>100 mg/L

Short-term toxicity to aquatic invertebrates (EC50)

12 mg/L (unfiltered test solution), <100 mg/L (filtered test solution)(Greater than the limit of solubility)

>0.049 mg/L

n/a

Algal inhibition (ErC50 and NOEC)

>1.2 mg/L and 0.28 mg/L (Greater than the limit of solubility)

>0.13 and ≥0.13 mg/L

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

32 mg/L (nominal)

0.058 mg/L

(measured)*(reflecting exposure to hydrolysis products), and testing currently ongoing with parent substance 

Testing currently ongoing

32 mg/L (measured)

 

 Long-term toxicity to fish (NOEC)  Testing currently ongoing  Testing currently ongoing  No data

* The test in which these results were obtained is subject to some uncertainty. 

Read-across from triethoxy(octyl)silane (CAS 2943-75-1) to triethoxy(2,4,4 trimethylpentyl)silane (CAS 35435-21-3) (supporting data)

Triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3) and triethoxy(octyl)silane (CAS 2943-75-1) both hydrolyse moderately rapidly in contact with water (43 hours at pH 7, 20-25°C, and 30 hours at pH 7, 20-25°C, respectively). The organosilicon hydrolysis products are (2,4,4-trimethylpentyl)silanetriol and octylsilanetriol respectively. Triethoxy(2,4,4-trimethylpentyl)silane and triethoxy(octyl)silane are structural analogues; both are triethoxysilanes with an octyl side chain, which is branched for the registration substance and linear for the read-across substance. Both substances have very low water solubility (<0.1 mg/l and <0.13 - 0.79 mg/l at 20°C respectively), high log Kow (>6.5 and 6.4 respectively), low vapour pressure (0.22 Pa and 0.11 Pa at 25°C respectively) and the same molecular weight (276.5). Their hydrolysis products, (2,4,4-trimethylpentyl)silanetriol and octylsilanetriol, are silanetriols with branched or linear octyl side chains and have very similar physicochemical properties: high water solubility (24400 and 59000 mg/l, respectively, predicted), low log Kow (0.9 and 1.1, respectively), low vapour pressure (2.7E-05 Pa and 1.2E-04 Pa at 25°C, respectively) and the same molecular weight (192.33 g/mol). Therefore it is considered appropriate to read-across between the two substances.

Triethoxy(octyl)silane is used as supporting data to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C50 values of >0.005, >0.049 and >0.13 mg/L respectively have been determined.

Read-across from trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) to triethoxy(2,4,4 trimethylpentyl)silane (CAS 35435-21-3)

Triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3) and trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) both hydrolyse in contact with water (43 hours at pH 7, 20-25°C, and <1 minute at pH 7, 25°C respectively) to form the same organolsilicon hydrolysis product (2,4,4-trimethylpentyl)silanetriol. As the hydrolysis rate for trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) is so rapid, the test organisms will have been exposed to its hydrolysis products,(2,4,4-trimethylpentyl)silanetriol and hydrogen chloride (which forms hydrochloric acid on contact with water). It is therefore considered appropriate to read-acrossfrom trichloro(2,4,4-trimethylpentyl)silane to evaluate the toxicity of the silanol hydrolysis product.

Trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) is used to read-across to long-term toxicity to invertebrates endpoint. A 21-day NOEC value of 32 mg/l has been determined. These data are used for chemical safety and risk assessment of the silanol hydrolysis product.

 

Considerations on the non-silanol hydrolysis product ethanol

Ethanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC50 and LC50 values for this substance is in excess of 1000 mg/l (OECD 2004b - SIDS for ethanol, CAS 64-17-5). Therefore, at the loading rates experienced in these tests it is unlikely that the presence of ethanol would significantly affect the results of the tests.

Considerations on the non-silanol hydrolysis product hydrochloric acid

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e. g. OECD SIDS for CAS No. 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are considered in the risk management measures (e. g. neutralisation) for effluents/aqueous waste.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity.

References

EC 2003. European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, European Chemicals Bureau, 2003.

ECHA 2016. REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016.

ECHA 2017. European Chemicals Agency. Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.7b: Endpoint specific guidance. Version 4.0 June 2017.

OECD 2002. SIDS Initial Assessment Report for SIAM 15, Boston, USA, 22-25th October 2002, Hydrochloric acid, CAS 7647-01-0.

OECD 2004b. SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.

PFA 2016y: Peter Fisk Associates, Analogue report - ecotoxicity of low functionality groups. PFA.404.003.004.

Conclusion on classification

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