Dichlorosilane

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:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Introduction

Dichlorosilane (CAS 4109-96-0) is a gas at normal temperature and pressure and is expected to react violently and rapidly with water. This violent and rapid reaction of the gas with water would render aquatic ecotoxicology testing impossible and thus testing is waived on the basis of the technical difficulty to conduct such tests. The read-across information is added here as supporting evidence and adding to the weight of evidence for not doing any further testing.

The hydrolysis half-life of dichlorosilane is estimated to be ≤17 seconds 1.5˚C and pH 7; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-Cl bonds to give silanediol and hydrochloric acid. The Si-H bonds of silanediol are also unstable and react with an estimated half-life of <12 h under environmentally relevant conditions to give monosilicic acid and hydrogen. Both silanediol and monosilicic acid readily and rapidly (within minutes) condense at concentrations above approximately 100-150 mg/l SiO₂ to give amorphous polysilicic acid. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure. Given the relevant timescales and concentrations, the relevant species for environmental hazard assessment is therefore monosilicic acid.

Read-across Justification

The registered substance (target substance) and the substances used as surrogates for read-across (source substances) are part of a class of chlorosilane and alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce the same Si hydrolysis product, monosilicic acid (Si(OH)₄), and another non-Si hydrolysis product. Si(OH)₄ has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses at concentrations above approximately 100-150 mg/l to give amorphous polysilicic acid. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure.

Silicic acid is a naturally-occurring substance which is not harmful to aquatic organisms at relevant concentrations. Silicic acid is the major bioavailable form of silicon for aquatic organisms and plays an important role in the biogeochemical cycle of silicon (Si). Silicic acid is therefore not expected to be harmful to organisms present in the environment. To support this view, all the available studies with aquatic organisms report no effects at 100 mg/l nominal loading in short-term toxicity studies (see Table 2 in PFA 2013x for key studies, attached in Section 13 of the IUCLID dossier).

In the context of the RAAF, the basis of the read-across hypothesis is (Bio)transformation to common compound(s); Scenario 1 is expected to apply. Both source substances rapidly hydrolyse 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.

 

Read-across from trimethoxysilane (CAS 2487-90-3) to dichlorosilane (CAS 4109-96-0):

AE 1.1 Formation of common (identical) compound(s)

The hydrolysis half-life of dichlorosilane (CAS 4109-96-0) at pH 7 and 25°C is estimated to be approximately 5 seconds; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-Cl bonds to give silanediol and hydrochloric acid. The Si-H bonds of silanediol are also unstable and react with an estimated half-life of <12 h under environmentally relevant conditions to give monosilicic acid and hydrogen.

The hydrolysis half-life of trimethoxysilane (CAS 2487-90-3) is ≤0.3 min at pH 4, 7 and 9 and 2°C; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-OCH3 bonds to give silanetriol and methanol. The Si-H bond is also expected to react fairly rapidly, giving monosilicic acid (Si(OH)₄) and hydrogen. The precise rate of this reaction is uncertain but the current best estimate is that it is less than 12 hours at environmentally relevant temperature and pH.

Monosilicic acid (Si(OH)₄) has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous polysilicic acid. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure.

AE 1.2 The biological targets for the common compound(s)

Short-term toxicity data for fish, invertebrates and algae with trimethoxysilane (CAS 2487-90-3) indicate that this substance is of low toxicity to aquatic organisms (see the Table below). The organisms would have been exposed predominantly to the monosilicic acid hydrolysis product in the available studies.

 

AE 1.3 Exposure of the biological target(s) to the common compound(s)

The registered substance (target) and the substance used as surrogate for read-across (source) are part of a class of chlorosilane and alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce monosilicic acid (Si(OH)₄) and another non-Si hydrolysis product. Si(OH)₄ has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous silica at concentrations above approximately 100-150 mg/l. Depending on the pH and concentration, solutions will contain varying proportions of monomeric silicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure.

 

AE 1.4 The impact of parent compounds

The hydrolysis half-life of dichlorosilane is approximately 5 seconds at 25°C and pH 4, 7 and 9. The hydrolysis half-life of trimethoxysilane is also only a matter of seconds.

Due to the rapid hydrolysis reactions, exposure to the parent substances is expected to be negligible.

 

AE 1.5 Formation and impact of non-common compounds

The hydrolysis half-life of dichlorosilane is approximately 5 seconds at 25°C and pH 4, 7 and 9. The initial products of hydrolysis are hydrogen chloride and silanediol. The silanediol is expected to react rapidly to produce hydrogen and monosilicic acid. The hydrolysis half-life of trimethoxysilane is also only a matter of seconds, producing the initial hydrolysis products of methanol and silanetriol. Silanetriol is expected to react rapidly to produce monosilicic acid.

The intermediate hydrolysis products, silanediol and silanetriol, are structural analogues with similar physico-chemical properties. The low acute aquatic toxicity, physicochemical properties and limited exposure indicate that silanetriol is unlikely to be of concern for the environment; ecotoxicological effects of silanetriol and silandiol are not expected to differ. Exposure to these intermediates is expected to be limited based on hydrolysis rates.

 

Methanol 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 2004a - SIDS for methanol, CAS 67-56-1).

 

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.

 

 

Read-across from tetraethyl orthosilicate (CAS 78-10-4) to dichlorosilane (CAS 4109-96-0): 

AE 1.1 Formation of common (identical) compound(s)

The hydrolysis half-life of dichlorosilane (CAS 4109-96-0) at pH 7 and 25°C is estimated to be approximately 5 seconds; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-Cl bonds to give silanediol and hydrochloric acid. The Si-H bonds of silanediol are also unstable and react with an estimated half-life of <12 h under environmentally relevant conditions to give monosilicic acid and hydrogen.

The hydrolysis half-life of tetraethyl orthosilicate (CAS 78-10-4) is 4.4 h at 25°C and pH 7; the substance will therefore undergo rapid hydrolysis in contact with water to produce monosilicic acid and ethanol.

Monosilicic acid (Si(OH)₄) has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous polysilicic acid. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure.

 

AE 1.2 The biological targets for the common compound(s)

Short-term toxicity data for fish, invertebrates and algae with tetraethyl orthosilicate (CAS 78-10-4) indicate that this substance is of low toxicity to aquatic organisms (see Table below). The organisms would have been exposed predominantly to the monosilicic acid hydrolysis product in the available studies.

 

AE 1.3 Exposure of the biological target(s) to the common compound(s)

Tetraethyl orthosilicate and dichlorosilane are considered part of the same analogue group as they both react in water to produce the same Si hydrolysis product, monosilicic acid. Si(OH)₄ has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous silica at concentrations above approximately 100-150 mg/l. Depending on the pH and concentration, solutions will contain varying proportions of monomeric silicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure.

 

AE 1.4 The impact of parent compounds

The hydrolysis half-life of dichlorosilane is approximately 5 seconds at 25°C and pH 4, 7 and 9. The hydrolysis half-life of tetraethyl orthosilicate is 4.4 hours at 25°C and pH 7.

Due to the rapid hydrolysis reactions, exposure to the parent substances is expected to be negligible.

 

AE 1.5 Formation and impact of non-common compounds

The hydrolysis half-life of dichlorosilane is approximately 5 seconds at 25°C and pH 4, 7 and 9. The initial products of hydrolysis are hydrogen chloride and silanediol. The silanediol is expected to react rapidly to produce hydrogen and monosilicic acid.

Due to the rapid hydrolysis reactions, exposure to the intermediate hydrolysis product, silanediol, is expected to be limited, and is addressed by read-across from trimethoxysilane (CAS 2487-90-3).

 

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).

 

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.

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;

- silanediol and monosilicic acid have predicted first dissociation constants around 10 and so do not significantly affect the pH of an aqueous solution;

- the silicon-containing hydrolysis products are not toxic to aquatic organisms at 100 mg/l in short-term studies.

 

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 is negligible, therefore the substance is not considered further.

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

CAS Number	4109 -96 -0	2487-90-3	78-10-4
Chemical Name	Dichlorosilane	Trimethoxysilane	Tetraethyl orthosilicate
Si hydrolysis product	(Poly)silicic acid (note, properties below refer to monomeric monosilicic acid under dilute conditions)	(Poly)silicic acid   (note, properties below refer to monomeric monosilicic acid under dilute conditions)	(Poly)silicic acid   (note, properties below refer to monomeric monosilicic acid under dilute conditions)
Molecular weight (parent)	101.01	122.2	208.33
Molecular weight (hydrolysis product)	96.1	96.1	96.1
log Kow (parent)	n/a (reacts on contact with water)	0.2 (QSAR)	1.4 (QSAR)
log Kow (silanol hydrolysis product)	-4 (QSAR)	-4 (QSAR)	-4 (QSAR)
Water sol (parent)	n/a (reacts on contact with water)	170 000 mg/l (QSAR)	8600 mg/l (QSAR)
Water sol (silanol hydrolysis product))	1000000 mg/l (predicted; in reality limited to 100-150 mg/l as SiO2 by condensation reactions)	1000000 mg/l (predicted; in reality limited to 100-150 mg/l as SiO2 by condensation reactions)	1000000 mg/l (predicted; in reality limited to 100-150 mg/l as SiO2 by condensation reactions)
Vapour pressure (parent)	n/a	11370 Pa at 20°C (measured)	110 Pa at 25°C (QSAR)
Vapour pressure (hydrolysis product)	<1E-06 Pa (QSAR)	<1E-06 Pa (QSAR)	<1E-06 Pa (QSAR)
Hydrolysis t1/2 at pH 7 and 25°C	≤17 seconds at 1.5°C (analogue group read-across)	14 seconds at 2°C (measured)	4.4 hours (measured)
Hydrolysis t1/2 at pH 4 and 25°C	≤10 seconds at 1.5°C (analogue group read-across)	17 seconds at 2°C (measured)	0.11 hours (measured)
Hydrolysis t1/2 at pH 9 and 25°C	≤9 seconds at 1.5°C (analogue group read-across)	14 seconds at 2°C (measured)	0.22 hours (measured)
Short-term toxicity to fish (LC50)	n/a	>100 mg/l	>245mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>100 mg/l	>844 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	>100 and <6.3 mg/l	72-hour EC50: >63 mg/l; NOEC: ≥63 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a	n/a
Long-term sediment toxicity (NOEC)	n/a	n/a	n/a
Short-term terrestrial toxicity (L(E) C50)	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a	n/a

 

References:

OECD 2004 - UNEP PUBLICATIONS - SIDS Initial Assessment Report SOLUBLE SILICATES

OECD 2004b - UNEP PUBLICATIONS - SIDS Initial Assessment Report ETHANOL CAS N°: 64-17-5

OECD 2004c - UNEP PUBLICATIONS - SIDS Initial Assessment Report HYDROGEN CHLORIDE CAS N°: 7647-01-0

OECD 2004a - SIAM 19, 19-22 October 2004 US/ICCA SIDS INITIAL ASSESSMENT PROFILE METHANOL

PFA, 2013x, Peter Fisk Associates, Analogue grouping report - Ecotoxicity of (poly)silicic acid generating compounds, PFA.300.003.003

 

Conclusion on classification

Reliable data are read across from analogous substances (on the basis of common hydrolysis products).

On this basis it is proposed that dichlorosilane should not be classified in the EU under Regulation (EC) No 1272/2008 (CLP Regulation, as adapted) for acute or chronic toxicity on the grounds that reliable studies read-across for the silanol hydrolysis product indicate that it would not be toxic at a loading rate of 100 mg/l.

The substance rapidly hydrolyses to hydrogen chloride and inorganic silicate moieties.

The hydrolysis product hydrogen chloride has a harmonised classification in Annex VI of Regulation (EC) No 1272/2008 and does not require classification for the environment.

The hydrolysis product monosilicic acid is a naturally-occurring substance which is not harmful to aquatic organisms at relevant concentrations.

All available studies with aquatic organisms report no effects at 100 mg/L in short-term toxicity studies (reference PFA 2013x).