Methylsilanetriyl triacetate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

6.9 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

4.8 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.48 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:

0.19 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hazard assessment for the environment is based on the properties of the silanol hydrolysis product, methylsilanetriol, because the hydrolysis half-life of the registered substance, methylsilanetriyl triacetate (CAS 4253-34-3), is <12 seconds at 20-25°C and pH 7. The substance will hydrolyse very rapidly in contact with water and atmospheric moisture to form methylsilanetriol and acetic acid. 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”. REACH guidance, (ECHA 2017, R.7b) also suggests that when the hydrolysis half-life is much less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Section 1.3, condensation reactions of the silanetriol are possible.

 

Methylsilanetriol is highly water soluble (1000000 mg/l at 20 °C although condensation reactions become important above approximately 1000 mg/l and can limit the concentration dissolved in water) and has low log K_ow (-2.4 at 20 °C) and low vapour pressure (0.053 Pa at 25 °C).

 

In order to reduce 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.

 

Methylsilanetriyl triacetate hydrolyses very rapidly to methylsilanetriol and acetic acid. Methylsilanetriol is considered to be a low-functionality silanetriol with no chemical groups attached that affect the toxicity of the substance. Therefore, methylsilanetriyl triacetate and its silanol hydrolysis product, as well as 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 organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the Si 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. The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substances is based on log K_ow of the resulting silanols and the chemical groups present in them. The silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.3 of the CSR.

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

 

The analogue approach for fulfilling the data requirement by read-across is discussed below, according to the Read-across Assessment Framework (RAAF).

Read-across from triethoxy(methyl)silane (CAS 2031-67-6), trimethoxy(methyl)silane (CAS 1185-55-3) and trichloro(ethyl)silane (CAS 115-21-9) to methylsilanetriyl triacetate (CAS 4253-34-3)

The registered substance (target substance), methylsilanetriyl triacetate (CAS 4253-34-3), and two of the substances used as surrogate for read-across (source substances), triethoxy(methyl)silane (CAS 2031-67-6) and trimethoxy(methyl)silane (CAS 1185-55-3), are part of a class of alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce the same Si hydrolysis product, methylsilanetriol, and another non-Si hydrolysis product.

 

In the context of the RAAF, the basis of the read-across hypothesis is “(Bio)transformation to common compound(s)”; Scenario 1 applies. The source substance hydrolyses to the same silicon containing hydrolysis product as the target substance, and the non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property.

The source substance triethoxy(methyl)silane (CAS 2031-67-6) is used to read across key data for short-term toxicity to fish, invertebrates and algae endpoints, and is fully discussed in terms of Scenario 1 of the RAAF. The second source substance, trimethoxy(methyl)silane (CAS 1185-55-3), is used to read-across to these endpoints as supporting data, therefore the justification for read-across is only superficially discussed.

A third source substance, trichloro(ethyl)silane (CAS 115-21-9), is used to read across key data for long-term toxicity to invertebrates. Trichloro(ethyl)silane (CAS 115-21-9) and methylsilanetriyl triacetate (CAS 4253-34-3) are part of a class of alkoxysilane compounds which hydrolyse rapidly to produce similar Si hydrolysis products, ethylsilanetriol and methylsilanetriol, respectively, and another non-Si hydrolysis product. This read-across is fully discussed in terms of Scenario 2 of the RAAF.

Table 7.0.1 below summarises the basic physicochemical and ecotoxicological data for the source and target substances.

Table 7.0.1: Summary of ecotoxicological and physico-chemical properties of the registered and surrogate substances.

CAS Number	4253-34-3	1185-55-3	2031-67-6	115-21-9
Chemical Name	methylsilanetriyl triacetate	trimethoxy(methyl)silane	Triethoxy(methyl)silane	Trichloro(ethyl)silane
Si hydrolysis product	methylsilanetriol	methylsilanetriol	methylsilanetriol	ethylsilanetriol
Molecular weight (parent)	220.26	136.22	178.31	163.51
Molecular weight (hydrolysis product)	94.14	94.14	94.14	108.17
log Kow(parent)	no data	0.7	2.2	no data
log Kow(silanol hydrolysis product)	-2.4	-2.4	-2.4	-1.9
Water sol (parent)	no data	2.9E+04 mg/l	2.9E+03 mg/l	no data
Water sol (silanol hydrolysis product))	1E+06 mg/l	1E+06 mg/l	1E+06 mg/l	1E+06 mg/L (but will be limited by condensation reactions)
Vapour pressure (parent)	26 Pa	10680 Pa	100 Pa	4780 Pa at 20°C
Vapour pressure (hydrolysis product)	0.05 Pa	0.05 Pa	0.052 Pa	0.02 Pa at 25°C
Hydrolysis t1/2 at pH 7 and 25°C	<12 seconds	2.2 h	5.5 h	<5 min
Hydrolysis t1/2 at pH 4 and 25°C	<12 seconds	<0.033 h	0.3 h	<1 min
Hydrolysis t1/2 at pH 9 and 25°C	<12 seconds	0.11 h	0.1 h	<1 min
Short-term toxicity to fish (LC50)	no data	>110 mg/l	>500 mg/l	No data
Short-term toxicity to aquatic invertebrates (EC50)	no data	>122 mg/l	>500 mg/l	no data
Algal inhibition (ErC50 and NOEC)	no data	EC50: >120 mg/l; NOEC =120 mg/l	EC50: >500 mg/l; NOEC: =500 mg/l	no data
Long-term toxicity to fish (NOEC)	no data	no data	no data	no data
Long-term toxicity to aquatic invertebrates (NOEC)	no data	no data	no data	=100 mg/l
Sediment toxicity (NOEC)	no data	no data	no data	no data
Short-term terrestrial toxicity (L(E)C50)	no data	no data	no data	no data
Long-term terrestrial toxicity (NOEC)	no data	no data	no data	no data

Read-across from triethoxy(methyl)silane (CAS 2031-67-6) (source) to methylsilanetriyl triacetate (CAS 4253-34-3) (target):

In the context of the RAAF, the basis of the read-across hypothesis is “(Bio)transformation to common compound(s)”; Scenario 1 applies. The source substance hydrolyses to the same silicon containing hydrolysis product as the target substance, and the non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property.

This scenario covers the analogue approach for which the read-across hypothesis is based on (bio) transformation to common compound(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach.

The read-across justification is presented according to RAAF Scenario 1 assessment elements (AE) as outlined in Tables 10 and 11 of the RAAF.

Table 1: RAAF Scenario 1 assessment elements (AE) as given in Tables 10 and 11 of the RAAF

AE A.1	Characterisation of source substance
AE 1.1	Formation of common (identical) and non-common compounds
AE 1.2	Degradation of non-common compounds
AE 1.3	Bioaccumulation potential of non-common compounds
AE 1.4	Impact of non-common compounds
AE A.2	Link of structural similarities and structural differences with the proposed prediction (presence of hypothesis)
AE A.3	Impact of impurities on the prediction
AE A.4	Consistency of properties in the data matrix
AE A.5	Reliability and adequacy of the source data
AE A.6	Bias that influences the prediction

     

AE A.1 Characterisation of source and target substances

The registered substance methylsilanetriyl triacetate (CAS 4253-34-3) (target) and the substance used as surrogate for read-across triethoxy(methyl)silane (CAS 2031-67-6) (source) are part of a class of compounds which hydrolyse rapidly to produce methylsilanetriol and another non-Si hydrolysis product.

Methylsilanetriyl triacetate (CAS 4253-34-3) is a silicon-carboxylate with a methyl side chain. Triethoxy(methyl)silane (CAS 2031-67-6) is an alkoxysilane with three ethoxy groups bound to a silicon atom. There is also a methyl group attached to the central Si atom. The target substance, methylsilanetriyl triacetate (CAS 4253-34-3), and the source substance, triethoxy(methyl)silane (CAS 2031-67-6), are closely related substances, each consisting of one Si atom linked to three acetoxy or three ethoxy groups and one methyl group, respectively.

The source substance triethoxy(methyl)silane (CAS 2031-67-6) and the target substance methylsilanetriyl triacetate (CAS 4253-34-3) are structurally similar substances. Both substances are transformed to the common compound methylsilanetriol and to the non-common compounds ethanol and acetic acid, respectively. The common compound methylsilanetriol is solely responsible for the effects to be predicted. The transformation of the parent substances is rapid and extensive and therefore, only negligible exposure to the parent substances occurs. During the tests with the source substance, test preparation included mixing the substance overnight with dilution water, therefore hydrolysis is expected to be extensive and exposure of test organisms would have been to the hydrolysis products. Exposure to the non-common compounds ethanol and acetic acid does not influence the prediction of the property under consideration. The effects of the target substance methylsilanetriyl triacetate (CAS 4253-34-3) are predicted to be equal to the effects of the source substance triethoxy(methyl)silane (CAS 2031-67-6) for the property under consideration.

Table 7.0.1 summarises the basic physicochemical and ecotoxicological data for the source and target substances.

 

AE 1.1 Formation of common (identical) and non-common compounds

Methylsilanetriyl triacetate (CAS 4253-34-3) hydrolyses very rapidly in contact with water to produce methylsilanetriol and acetic acid (half-life <12 seconds at pH 7 and 20-25°C).

Triethoxy(methyl)silane (CAS 2031-67-6) hydrolyses rapidly in contact with water to produce methylsilanetriol and ethanol (half-life 5.5 h at pH 7 and 20-25°C).

The toxicity of the substances is via a non-polar narcosis mechanism of toxicity.

Both target and source substances share the same silicon containing hydrolysis product methylsilanetriol, which is the relevant silicon containing substance for environmental assessment.

 

AE 1.2 Degradation of non-common compounds

The target substance methylsilanetriyl triacetate (CAS 4253-34-3) and source substance triethoxy(methyl)silane (CAS 2031-67-6) rapidly hydrolyse in water to produce methylsilanetriol and acetic acid and ethanol, respectively, as the non-common compounds.

According to the OECD SIDS Initial Assessment Report for Acetic Anhydride (CAS No. 108-24-7), which hydrolyses rapidly to acetic acid (4.4 min. at 25°C and 8.1 min. at 15°C), acetic acid is readily biodegradable. In the atmosphere, it is subject to photooxidative degradation (half-life 22 days). However, on account of its high solubility, acetic acid will be rapidly washed out of the atmosphere. In the static Zahn-Wellens test of biodegradability, acetic acid is degraded to more than 95% within 5 days. In the respirometer test (22-24 hours in modified MITI test) acetic acid is degraded to 99%.

In unbuffered media, the effects on aquatic organisms arising from exposure to acetic acid are thought to result from a reduction in the pH of the ambient environment to a level below their tolerable range. Aquatic ecosystems are characterized by among other factors, their pH; the resident biota is adapted to pH within a tolerable range. 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, hydrogen chloride). It is not considered appropriate or useful to derive a single aquatic PNEC for acetic 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 to be negligible in the environment because the substance and its hydrolysis products will enter the environment via a buffered WWTP, or will be buffered by the surrounding environment.

According to the SIDS INITIAL ASSESSMENT PROFILE for Ethanol (OECD 2004b), ethanol is degraded in the atmosphere by hydroxyl, NOx and SOx radical-mediated photooxidation. The total tropospheric half-life of ethanol is estimated to be 10-36 hours. Ethanol is stable to hydrolysis but is readily biodegradable (74% after 5 days) and is not likely to bioaccumulate (calculated log BCF = 0.5). Ethanol is not persistent in the environment.

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 are in excess of 10 000 mg/l (OECD 2004 - SIDS for ethanol, CAS 64-17-5). Therefore, it is considered unlikely that it’s presence significantly affected the results of the test performed with the source substance triethoxy(methyl)silane (CAS 2031-67-6).

 

AE 1.3 Bioaccumulation potential of non-common compounds

The target substance methylsilanetriyl triacetate (CAS 4253-34-3) and the source substance triethoxy(methyl)silane (CAS 2031-67-6) both rapidly hydrolyse on contact with water and moisture. Full and complete hydrolysis will have occurred prior to introduction of test organisms, during ecotoxicological tests with the source substance (test stock solution was prepared by stirring overnight, prior to addition of test organisms).

The non-common compounds, acetic acid and ethanol, are discussed in AE 1.2. They are not subject to further transformation to different compounds, they do not have the potential to bioaccumulate and neither are toxic to aquatic organisms.

 

AE 1.4 Impact of non-common compounds

As discussed in AE 1.2, acetic acid and ethanol are not hazardous at the concentrations relevant to the studies.

 

AE A.2 Link of structural similarities and structural differences with the proposed prediction (presence of hypothesis)

During the ecotoxicity studies conducted with the source substance triethoxy(methyl)silane (CAS 2031-67-6), the rapid hydrolysis (half-life 5.5 h at pH 7 and 25°C) and the media preparation (mixing of the test solution overnight prior to inclusion of test organisms) indicate that the organisms would have been exposed to the hydrolysis products methylsilanetriol and ethanol.

Due to the rapid hydrolysis of the target registration methylsilanetriyl triacetate (CAS 4253-34-3) (half-life <12 seconds at pH7 and 25°C), the Si hydrolysis product methylsilanetriol is the relevant silicon containing substance for environmental assessment.

Therefore, both source and target substances share identical substances upon which the chemical safety assessment is based. Exposure of the test organisms will have been to methylsilanetriol and the non-si hydrolysis product. This non-si hydrolysis product will not have an impact on the toxicity to aquatic organisms, as discussed above.

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported analytical purity of 98.4 - 98.9%. There are no impurities described in the studies that are read across. The boundary composition of the target substance reports a purity of >80 - =100%. The Substance Identification Profiles for both substances report impurities that are methanol/ethanol (hydrolysis products of the substances), siloxanes (products of hydrolysis and condensation of the substances) and structurally related alkoxysilanes. None of the impurities impact the classification and labelling of the substances.

It is therefore concluded that the source and target substances do not contain impurities at concentrations that would influence the effects observed in the available tests with the source substance.

 

AE A.4 Consistency of properties in the data matrix

Short-term toxicity to fish, invertebrates and algae data are read-across from triethoxy(methyl)silane (CAS 2031-67-6):

Short-term fish: 96-hour LC₅₀>500 mg/l, Danio rerio.

Short-term toxicity to invertebrates: 48-hour EC₅₀>500 mg/l, Daphnia magna;

Algae: 72-hour ErC₅₀>500 mg/l and NOEC of =500 mg/l, Pseudokirchneriella subcapitata.

No aquatic toxicity data are available for the target substance.

Short-term E(L)C₅₀ values and Chronic Values (ChV) have been calculated for the target and source substance silanol hydrolysis product using ECOSAR predictions for neutral organics. The relevant values are:

Methylsilanetriol:

Short-term fish: 96-hour LC₅₀ 5.36E+4 mg/l.

Short-term toxicity to invertebrates: 48-hour LC₅₀ 2.27E+4 mg/l.

Algae: 96-hour EC₅₀ 5.06E+3 mg/l.

Chronic fish: ChV 3.71E+3 mg/l.

Chronic Daphnia: ChV 983 mg/l.

Chronic algae: ChV 692 mg/l.

 

The predicted values indicate toxicity only at very high concentrations, not realistic for exposure in the environment. They support the results of the measured data with the source substance triethoxy(methyl)silane (CAS 2031-67-6), where an absence of toxicity was observed in the short-term tests.

 

AE A.5 Reliability and adequacy of the source data

All key data included in the chemical safety assessment have been reviewed and assigned Klimisch scores of 1 or 2.

 

AE A.6 Bias that influences the prediction

Data with the source substance, triethoxy(methyl)silane (CAS 2031-67-6), were selected because it rapidly hydrolyses to the same substance as the hydrolysis product of the target substance.

The selection of the source studies used as the basis for the prediction was based on the fact that they are reliable tests with a substance that hydrolyses to methylsilanetriol. Reliable data are also available for another substance that hydrolyses to methylsilanetriol, trimethoxy(methyl)silane (CAS 1185-55-3). However, the studies with fish and Daphnia were conducted in a flow-through test system and are likely to reflect exposure of the test organisms to the parent substance as well as the hydrolysis products. Data with triethoxy(methyl)silane (CAS 2031-67-6) has therefore been chosen as key.

 

Read-across from trichloro(ethyl)silane (CAS 115-21-9) (source) to methylsilanetriyl triacetate (CAS 4253-34-3) (target):

In the context of the RAAF, the basis of the read-across hypothesis for this substance is “Different compounds have the same type of effect(s)”; Scenario 2 applies. Both the source and target substance hydrolyse to compounds with qualitatively similar properties. The non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property.

This scenario covers the analogue approach for which the hypothesis is based on different compounds with the same type of effect(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach.

The read-across justification is presented according to RAAF Scenario 2 assessment elements (AE) as outlined in Tables 10 and 12 of the RAAF.

Table 2: RAAF Scenario 2 assessment elements (AE) as given in Tables 10 and 12 of the RAAF

AE A.1	Characterisation of source and target substance
AE A.2	Link of structural similarities and structural differences with the proposed prediction (presence of hypothesis)
AE 2.1	Degradation
AE 2.2	Bioaccumulation potential
AE 2.3	Common underlying mechanism, qualitative aspects
AE 2.4	Common underlying mechanism, quantitative aspects
AE A.3	Impact of impurities on the prediction
AE A.4	Consistency of properties in the data matrix
AE A.5	Reliability and adequacy of the source data
AE A.6	Bias that influences the prediction

AE A.1 Characterisation of source and target substances

The registered substance, methylsilanetriyl triacetate (CAS 4253-34-3) (target), and the substance used as surrogate for read-across, trichloro(ethyl)silane (CAS 115-21-9) (source), are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity. The substances are susceptible to hydrolysis; methylsilanetriyl triacetate (CAS 4253-34-3) undergoes a very rapid hydrolysis reaction to produce methylsilanetriol and acetic acid. Trichloro(ethyl)silane (CAS 115-21-9) reacts very rapidly in water to produce ethylsilanetriol and hydrochloric acid. For both the test undertaken with the source substance, and the chemical safety assessment of the target substance, the silanol hydrolysis product is the assessment entity of primary interest, as discussed below.

The hydrolysis half-life of methylsilanetriyl triacetate (CAS 4253-34-3) is <12 seconds at pH 7 and 25°C, producing methylsilanetriol and acetic acid; the substance will therefore undergo very rapid hydrolysis in contact with water to form methylsilanetriol and acetic acid.

The hydrolysis half-life of the source substance trichloro(ethyl)silane (CAS 115-21-9) at pH 7 and 25°C is <1 minute; the substance therefore reacts very rapidly in water to produce ethylsilanetriol and hydrochloric acid.

The environmental chemical safety assessment of the registered (target) substance (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, methylsilanetriol, as the primary assessment entity. The considerations for the acetic acid and hydrochloric acid hydrolysis product are discussed below (see AE A.2). The characterisation therefore focuses on the silicon containing hydrolysis products of the target substance, methylsilanetriol, and of the source substance, ethylsilanetriol.

Ethylsilanetriol and methylsilanetriol share similar physicochemical properties: low log K_ow (-1.9 and -2.4, respectively), high water solubility (both 1.0E+06 mg/l (potentially limited to 1000 mg/l by condensation reactions) and moderate molecular weight (MW 108.17 and 94.14 g mol^-1, respectively).

During the tests with the source substance, test organisms will have been exposed to the hydrolysis products of the test substance.

The non-Si hydrolysis product for the source substance is hydrochloric acid and for the target substance, acetic acid.

Table 7.0.1 summarises the basic physicochemical and ecotoxicological data for the source and target substances.

 

AE A.2 Link of structural similarities and structural differences with the proposed prediction (presence of hypothesis)

The characterisation focuses on the silicon containing hydrolysis products of the target substance, methylsilanetriol, and of the source substance, ethylsilanetriol.

Due to the rapid hydrolysis of the substance, during the long-term toxicity to invertebrates study conducted with the source substance trichloro(ethyl)silane (CAS 115-21-9), the organisms would have been exposed to the hydrolysis products ethylsilanetriol and hydrochloric acid.

The hypothesis is that close structural similarity and similar physicochemical properties leads to similar toxicity.

The silanol hydrolysis products are structurally very similar, both with one silanetriol group (silicon atom attached to three silanol hydroxy groups) and one alkyl group. The difference between the two hydrolysis products is that methylsilanetriol has one methyl group, while ethylsilanetriol has one ethyl group.

Both substances contain the same functional groups and neither substance contain any unique structural features. They do not contain any known structural alerts for ecotoxicity and do not have a known specific mode of toxic action. They are part of a class of compounds the toxicity of which is via a non-polar narcosis mechanism of toxicity.

The substances share similar physicochemical properties: both have moderate molecular weight (94.14 and 108.17 g/mol, respectively), high water solubility (both 1E+06 mg/l), low log K_ow(-2.4 and -1.9, respectively) and low vapour pressure (0.05 Pa and 0.02 Pa, respectively.

Thus, the substances are expected to have similar ecotoxicity profiles.

 

Consideration of the non-Si 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 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 7647-01-0, hydrogen chloride). 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.

Hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of dichlorosilane or trimethoxysilane is negligible, therefore the substance is not considered further.

 

Consideration of the non-Si hydrolysis product acetic acid:

In unbuffered media, the effects on aquatic organisms arising from exposure to acetic acid are thought to result from a reduction in the pH of the ambient environment to a level below their tolerable range. Aquatic ecosystems are characterized by among other factors, their pH; the resident biota is adapted to pH within a tolerable range. 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, hydrogen chloride). It is not considered appropriate or useful to derive a single aquatic PNEC for acetic 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 to be negligible in the environment because the substance and its hydrolysis products will enter the environment via a buffered WWTP, or will be buffered by the surrounding environment.

 

AE 2.1 Degradation

The target substance, methylsilanetriyl triacetate (CAS 4253-34-3), and the source substance, trichloro(ethyl)silane (CAS 115-21-9), are both susceptible to very rapid hydrolysis in (hydrolysis half-life <12 seconds at 25°C and pH 7 and <1 minute at pH 7 and 25°C, respectively).

Because both the target and source substances have such rapid hydrolysis half-lives, organisms are unlikely to be exposed to the parent substance. In the tests with the source substance, organisms are likely to have been exposed to the hydrolysis products ethylsilanetriol and hydrochloric acid.

Both methylsilanetriyl triacetate and trichloro(ethyl)silane are within a group of substances within which, in general, there is no evidence of any significant biodegradation once biodegradation of alkoxy/acetoxy groups has been taken into account.

No significant biodegradation is expected for either of the silanol hydrolysis products.

Hydrochloric acid is inorganic and biodegradation is not relevant.

Acetic acid is readily biodegradable (OECD 1992).

 

AE 2.2 Bioaccumulation potential

Methylsilanetriyl triacetate (CAS 4253-34-3) (target), and the substance used as surrogate for read-across, trichloro(ethyl)silane (CAS 115-21-9) (source), are both susceptible to rapid degradation by hydrolysis but their silanol hydrolysis products, methylsilanetriol and ethylsilanetriol, respectively, are not expected to degrade. However, the log K_ow values of the silanol hydrolysis products are low (-2.4 and -1.9, respectively) and thus have low potential for bioaccumulation, and the requirement for testing is waived.

 

AE 2.3 Common underlying mechanism, qualitative aspects

The target substance methylsilanetriyl triacetate (CAS 4253-34-3) and source substance triethoxy(methyl)silane (CAS 2031-67-6) are part of a class of organosilicon substances which act via a non-polar narcosis mechanism of toxicity. The substances do not contain any known structural alerts for ecotoxicity and do not have a known specific mode of toxic action.

Both substances are susceptible to rapid hydrolysis in water. The characterisation focuses on the silicon containing hydrolysis products of the substances, methylsilanetriol and ethylsilanetriol, respectively, both of which are structurally similar with similar physico-chemical properties.

Methylsilanetriol and ethylsilanetriol have high water solubility (both 1.0E+06 mg/l) and low log K_ow (-2.4 and -1.9, respectively). For substances acting via non-polar narcosis, acute toxicity effects are observed between log K_ow values of 1.9 and 5 (PFA 2016). Both silanol hydrolysis products have such low log K_ow values, toxicity is not expected.

The substances have structurally similar silanol hydrolysis products with similar physico-chemical properties. Thus, the substances are expected to have similar ecotoxicity profiles.

The different non-silanol hydrolysis products, acetic acid and hydrochloric acid, are discussed in section AE A.2 and are not expected to influence the outcome of the aquatic ecotoxicity tests.

 

AE 2.4 Common underlying mechanism, quantitative aspects

No ecotoxicity data are available with the target substance methylsilanetriyl triacetate (CAS 4253-34-3). Chronic Daphnia data are available with the source substance, trichloro(ethyl)silane (CAS 115-21-9). No effects at the highest test concentration (100 mg/l) were observed. As discussed previously, the substance is susceptible to rapid hydrolysis, therefore the Daphnia were exposed to the hydrolysis products ethylsilanetriol and hydrochloric acid.

The absence of toxicity demonstrated in the chronic Daphnia test is consistent with the expected toxicity profile for these substances.

The available key toxicity data are summarised in Table 7.0.1. 

Both target and source substance are part of a class of compounds which act via a non-polar narcosis mechanism of toxicity. The substances do not contain any known structural alerts for ecotoxicity and do not have a known specific mode of toxic action.

Both substances will rapidly hydrolyse to silicon containing hydrolysis products methylsilanetriol and ethylsilanetriol, both of which are structurally similar with similar physico-chemical properties. Neither substance is likely to biodegrade, however they both have low bioaccumulation potential.

It can therefore be concluded that the toxic potential of the target substance is adequately predicted by aquatic toxicity tests with the source substance and any quantitative differences between the substances will not lead to underestimation of potential toxicity.

 

AE A.3 Impact of impurities on the prediction

The source substance has a reported purity of 99.8%. There are no impurities described in the studies that are read across. The boundary composition of the target substance reports a purity of >80 - =100%. The Substance Identification Profiles for both substances report impurities that are methanol/ethanol (hydrolysis products of the substances), siloxanes (products of hydrolysis and condensation of the substances) and structurally related alkoxysilanes. None of the impurities impact the classification and labelling of the substances.

It is therefore concluded that the source and target substances do not contain impurities at concentrations that would influence the effects observed in the available tests with the source substance.

 

AE A.4 Consistency of properties in the data matrix

Long-term toxicity to invertebrates data are read-across from trichloro(ethyl)silane (CAS 115-21-9) (data are presented here prior to molecular weight adjustment):

Long-term toxicity to invertebrates: 21-day NOEC =100 mg/l (highest concentration tested), Daphnia magna.

There are no other long-term toxicity to aquatic invertebrates data available with other structural analogues.

Short-term E(L)C₅₀ values and Chronic Values (ChV) have been calculated for the target and source substance silanol hydrolysis products using ECOSAR predictions for neutral organics. The relevant values are:

Methylsilanetriol:

Short-term fish: 96-hour LC₅₀ 5.36E+4 mg/l.

Short-term toxicity to invertebrates: 48-hour LC₅₀ 2.27E+4 mg/l.

Algae: 96-hour EC₅₀ 5.06E+3 mg/l.

Chronic fish: ChV 3.71E+3 mg/l.

Chronic Daphnia: ChV 983 mg/l.

Chronic algae: ChV 692 mg/l.

 

Ethylsilanetriol:

Short-term fish: 96-hour LC₅₀ 2.23E+4 mg/l.

Short-term toxicity to invertebrates: 48-hour LC₅₀ 9.90E+3mg/l.

Algae: 96-hour EC₅₀ 2.66E+3 mg/l.

Chronic fish: ChV 1.63E+3 mg/l.

Chronic Daphnia: ChV 486 mg/l.

Chronic algae: ChV 402 mg/l.

 

The predicted values indicate toxicity only at very high concentrations, not realistic for exposure in the environment. The toxicity profiles of both silanol hydrolysis products are very similar and support the hypothesis that the substances have similar toxicity profiles. They also support the results of the measured data with the source substance trichloro(ethyl)silane (CAS 115-21-9) where an absence of toxicity was observed in the chronic Daphnia test.

 

AE A.5 Reliability and adequacy of the source data

All key data included in the chemical safety assessment have been reviewed and assigned Klimisch scores of 1 or 2.

 

AE A.6 Bias that influences the prediction

Data with the source substance, trichloro(ethyl)silane (CAS 115-21-9), were selected because it hydrolyses to a structurally similar substance to the hydrolysis product of the target substance.

There are no other long-term toxicity to aquatic invertebrates data available with other structural analogues.

 

Read-across from trimethoxy(methyl)silane (CAS 1185-55-3) (source) to methylsilanetriyl triacetate (CAS 4253-34-3) (target)

Supporting data. Trimethoxy(methyl)silane (CAS 1185-55-3) is used to read-across to short-term aquatic endpoints as supporting data, therefore the justification for read-across is only superficially discussed.

Methylsilanetriyl triacetate (CAS 4253-34-4) and trimethoxy(methyl)silane (CAS 1185-55-3) both hydrolyse to methylsilanetriol, with the non-silanol hydrolysis products being acetic acid and methanol, respectively. The hydrolysis rates of methylsilanetriyl triacetate (half-life <12 seconds at pH 4, 7 and 9 at 25°C) and trimethoxy(methyl)silane (half-life <0.033 h, 2.2 h and 0.11 h at pH 4, 7 and 9 and 25°C) indicate that, under conditions relevant to ecotoxicity assessment, the organisms will be exposed to the hydrolysis products, methylsilanetriol and acetic acid or methanol. The effects of acetic acid are limited to pH effects in unbuffered media, while methanol is non-hazardous to the environment; the non-silanol hydrolysis products are given due consideration below.

The studies with the structural analogue indicate that methylsilanetriol is of low short-term toxicity to aquatic organisms.

 

Considerations of the non-silanol hydrolysis products

In unbuffered media, the effects on aquatic organisms arising from exposure to the other hydrolysis product, acetic acid, are thought to result from a reduction in the pH of the ambient environment to a level below their tolerable range. Aquatic ecosystems are characterized by among other factors, their pH; the resident biota is adapted to pH within a tolerable range. 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, hydrogen chloride). It is not considered appropriate or useful to derive a single aquatic PNEC for acetic 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 to be negligible in the environment because the substance and its hydrolysis products will enter the environment through a buffered WWTP before reaching the effluents or will be buffered by the surrounding environment.

Methanol and ethanol are well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC₅₀and LC₅₀values for these substances are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1, 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 either would significantly affect the results of the tests.

References

OECD SIDS (1997) SIDS Initial Assessment Report for 6th SIAM, Paris, 9-11 June 1997, Acetic Anhydride, CAS 108-24-7.

OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1

OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Ethanol, CAS 64-17-5

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >500 mg/l in fish, >500 mg/l in invertebrates and >500 mg/l in algae based on read-across from a structural analogue with the same Si hydrolysis product. It has a reliable NOEC of =500 mg/l in algae and also a and reliable long-term NOEC of >100 mg/l in Daphnia, based on read-across from a structural analogue.

 

The substance hydrolyses very rapidly in water and is not readily biodegradable but the silanol hydrolysis product has low log K_ow.

 

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

Acute toxicity: Not classified.

Chronic toxicity: Not classified.