Hexachlorodisilane

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

The hydrolysis half-life of hexachlorodisilane is approximately 5 seconds at 25˚C and pH 7; the substance will therefore undergo rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-Cl bonds to give hexahydroxydisilane and hydrochloric acid. Further hydrolysis of the Si-Si bond is also expected to happen rapidly, giving monosilicic acid (Si(OH)₄) which condenses at concentrations above approximately 100-150 mg/l as SiO₂to give insoluble amorphous polysilicic acid. The precise rate the hydrolysis of the Si-Si bond is uncertain but the current best estimate is that it is less than 12 hours at environmentally relevant pH.

The current uncertainty does not significantly affect the outcome of the environmental hazard assessment. Read-across data are available from the substances trimethoxysilane and tetraethyl orthosilicate. Trimethoxysilane, like hexachlorodisilane, hydrolyses very rapidly (t_1/2≤0.3 min at pH 4, 7 and 9 and 2°C) to give silanetriol and methanol; silanetriol then reacts further to monosilicic acid. Under neutral conditions of the environment and buffered test media, neither chloride nor methanol will significantly influence the hydrolysis and condensation reactions of the silanetriol species formed by initial hydrolysis of hexachlorodisilane and trimethoxysilane. Tetraethyl orthosilicate hydrolyses rapidly (t_1/2≤0.11 h at pH 4, 4.4 h at pH 7 and 0.22 h at pH 9 and 25°C) to give monosilicic acid and ethanol. Therefore, regardless of the rates of these further reactions, the final silicon-containing products of trimethoxysilane, tetraethyl orthosilicate and hexachlorodisilane hydrolysis are equivalent, and produced on an equivalent timescale. Short-term toxicity studies with fish, invertebrates and algae indicate that this substance is not toxic to aquatic organisms at a loading rate of 100 mg/l. The low acute aquatic toxicity, physicochemical properties and limited exposure indicate that silanetriol is unlikely to be of concern for the environment.

REACH guidance (ECHA 2012, 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”. TGD guidance, (EC 2003a) and ECHA (2016) 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. Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silicic acid hydrolysis product in accordance with REACH guidance. As described below, condensation reactions of the silicic acid are possible.

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex VIII requirements for the registration 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.

In moist medium, hexachlorodisilane hydrolyses very rapidly (half-life 5 seconds at 25°C and pH 7), with the final hydrolysis products being polysilicic acid and hydrochloric acid. The non-silanol hydrolysis product hydrochloric acid is not expected to contribute to any adverse effects at the relevant dose levels. This is discussed further below.

The registration substance and the substances used as surrogate for read-across 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 polysilicic acid at concentrations above approximately 100-150 mg/l. 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. Further details are given in PFA 2015ao.

In the following paragraphs the read-across approach for hexachlorodisilane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physicochemical properties, presented in Table 7.0.1.

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

CAS Number	13465-77-5	2487-90-3	78-10-4
Chemical Name	Hexachlorodisilane	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)	268.89	122.2	208.33
Molecular weight (hydrolysis product)	96.1	96.1	96.1
log Kow (parent)	n/a (reacts in 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 in 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)	310 Pa at 25°C (QSAR)	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	<5 seconds(analogue group read-across)	≤0.3 min at 2°C (measured)	4.4 hours (measured)
Hydrolysis t1/2 at pH 4 and 25°C	<5 seconds (analogue group read-across)	≤0.3 min at 2°C (measured)	0.11 hours (measured)
Hydrolysis t1/2 at pH 9 and 25°C	<5 seconds (analogue group read-across)	≤0.3 min at 2°C (measured)	0.22 hours (measured)
Short-term toxicity to fish (LC50)	n/a	>100 mg/l	>245 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>100 mg/l	>844 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	EC50 >100 mg/l and NOEC <6.3 mg/l	72-hour EC50: >100 mg/l; NOEC: ≥100 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

Analogue approach justification

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). Most living organisms contain at least trace quantities of silicon. For some species Si is an essential element that is actively taken up. For example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures (OECD SIDS 2004, Soluble silicates). Silicic acid has been shown to be beneficial in protection against mildew formation in wheat and to be non-phytotoxic in non-standard studies (Côte-Beaulieu et al. 2009).

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). Some of the other non-Si hydrolysis products do have the potential to cause harm at high treatment levels and therefore the hazard assessment and Predicted No Effect Concentrations (PNECs) will be based on the properties of these where appropriate.

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 at concentrations above approximately 100-150 mg/l. 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.

Further details are given in PFA 2015ao.

Given that all substances produce silicic acid and no toxicity is observed, it is possible to read-across freely within the analogue group. (Reference PFA 2013x).

 

Read-across from trimethoxysilane to hexachlorodisilane:

Trimethoxysilane, HSi(OMe)₃ (CAS 2487-90-3), is very unstable in the presence of water. The substance contains two reactive groups: Si-OMe and Si-H. The rate of Si-OMe hydrolysis has been measured in a reliable study; half-lives of ≤0.3 min at pH 4, 7 and 9 and 2°C were obtained. Methanol is produced by this reaction. If Si-OMe were hydrolysed, but Si-H were not, silanetriol (HSi(OH)₃) would be formed. However, the Si-H bond is also expected to react, forming monosilicic acid, Si(OH)₄. The rate of this reaction has not been measured but is expected to be fast to moderately fast. 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. This is the same as the product of Si-Cl hydrolysis for hexachlorodisilane. The co-products of hydrolysis (hydrogen chloride and methanol) are not expected to significantly influence the further hydrolysis and condensation reactions of silanetriol. Therefore, regardless of the rates of these further reactions, the final silicon-containing products of trimethoxysilane and hexachlorodisilane hydrolysis are equivalent, and produced on an equivalent timescale. Further details are given in Section 4.1 of the CSR and Section 5.1 of the IUCLID 5 dataset.

Trimethoxysilane is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C₅₀ values of >100 mg/l for each endpoint have been determined.

 

 

Read-across from tetraethyl orthosilicate to hexachlorodisilane:

Tetraethyl orthosilicate (CAS 78-10-4) is an alkoxysilane that hydrolyses rapidly (t_1/2 of 4.4 h at 25°C and pH 7) to produce monosilicic acid and ethanol. Hexachlorodisilane undergoes very rapid hydrolysis with a half-life of approximately 5 seconds at 25˚C and pH 7, to form hydrochloric acid and hexahydroxydisilane, which reacts further to give monosilicic acid. The effects of hydrochloric acid are limited to effects in an unbuffered media and are assessed below. Tetraethyl orthosilicate and hexachlorodisilane are considered part of the same analogue group as they both react in water to produce 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 at concentrations above approximately 100-150 mg/l. 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. The non-silicon hydrolysis products, ethanol and hydrogen chloride respectively, do not cause effects in aquatic organisms at relevant concentrations and under appropriately buffered conditions as discussed below.

Tetraethyl orthosilicate is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C₅₀ values of >245, >844 and >100 mg/l, respectively, have been determined.

Short-term toxicity data for fish and invertebrates with tetraethyl orthosilicate indicate that this substance is of low toxicity to aquatic organisms.

Considerations on the non-silanol hydrolysis products:

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 EC₅₀ and LC₅₀ values 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).

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;
• silanetriol 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.

References:

Côté-Beaulieu C, Chain F, Menzies JG, Kinrade SD, Bélanger RR (2009) Absorption of aqueous inorganic and organic silicon compounds by wheat and their effect on growth and powdery mildew control. Environ Exp. Bot 65: 155–161.

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

OECD SIDS (2004) SIDS Initial Assessment Report for SIAM 18, Paris, France, 20-23 April, 2004, Soluble Silicates, CAS 1344-09-8 Silicic acid, sodium salt; CAS 6834-92-0 Silicic acid (H2SiO3), disodium salt; CAS 10213-79-3 Silicic acid (H2SiO3), disodium salt, pentahydrate; CAS 13517-24-3 Silicic acid (H2SiO3), disodium salt, nonahydrate; CAS 1312-76-1 Silicic acid, potassium salt.

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, 19-22 October 2004, Ethanol, CAS 64-17-5.

PFA, 2013x, Peter Fisk Associates, Analogue report - Ecotoxicity of (poly)silicic acid generating compounds , PFA.300.003.001.

PFA, 2015ao, Peter Fisk Associates, The aquatic chemistry of inorganic silicic acid generators, PFA.404.001.001.

Conclusion on classification

Reliable short-term toxicity data are read across from analogous substances (on the basis of common hydrolysis products). Short-term EC₅₀ values reported in the studies used as read-across for the silanol hydrolysis product indicate that it would not be toxic at a loading rate of 100 mg/l. A NOEC value of <6.3 mg/l is reported in the algal study with the surrogate substance trimethoxysilane.

 

The registration substance rapidly hydrolyses to hydrogen chloride and inorganic silicate moieties. The non-silanol hydrolysis product, hydrogen chloride, has a harmonised classification in Annex VI of Regulation No 1272/2008 and does not require classification for the environment.

 

The silanol 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).

 

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.