Octachlorotrisilane

Ecotoxicological Summary

Additional information

Introduction and read across justification

 

No data are available for octachlorotrisilane. However, the substance reacts with water and very rapidly hydrolyses to hydrogen chloride and octahydroxytrisilane (half-life <5 seconds at 25°C and pH 7), which reacts further to give monosilicic acid (t_1/2 assumed to be <12 h). Given the speed of the hydrolysis, the exposure of aquatic species in tests will be to the hydrolysis products monosilicic acid and hydrogen chloride. The read-across is made on this basis. It should be noted however that the aquatic testing of the parent substance would be not possible due to its reactivity with water.

The registered substance and the substances used as surrogates 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 some other 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. Octahydroxytrisilane is also likely to form condensation products (polyhydroxy-polysilanes) at similar concentrations (in terms of SiO₂ equivalents).

Monosilicic 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).

Effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media.

Read-across from trimethoxysilane to octachlorotrisilane:

Trimethoxysilane, HSi(OMe)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 at 2°C of 14 seconds at pH 4, 17 seconds at pH 7, and 14 seconds at pH 9 were obtained. Methanol is produced by this reaction. The Si-H bond is also unstable and reacts with an estimated half-life of <12 h under environmentally relevant conditions to give monosilicic acid and hydrogen.

The registered substance and the substance 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 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.

The non-silanol hydrolysis products are discussed below.

Short-term toxicity data for fish, invertebrates and algae for trimethoxysilane indicate that this substance is of low toxicity to aquatic organisms (see Table 1 below). The organisms would have been exposed predominantly to the monosilicic acid and methanol hydrolysis products in the available studies.

Read-across from tetraethyl orthosilicate to octachlorotrisilane:

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. The registered substance and the substance 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 other non-Si hydrolysis products. 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.

The non-silanol hydrolysis products are discussed below.

Short-term toxicity data for fish, invertebrates and algae with tetraethyl orthosilicate indicate that this substance is of low toxicity to aquatic organisms (see Table 1 below). The organisms would have been exposed predominantly to the monosilicic acid and ethanol hydrolysis products in the available studies.

 

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 (2004a - SIDS for methanol, CAS 67-56-1, 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. 2004c, 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;

·         octahydroxytrisilane and monosilicic acid have first dissociation constants around 10 (reported in literature for monosilicic acid, estimated for octahydroxytrisilane) and so do not significantly affect the pH of an aqueous solution;

·         the silicon-containing hydrolysis products have L(E)C50s in excess of 100 mg/L in short-term aquatic toxicity studies based on appropriate read-across substances.

  

Table 1: Summary of physico-chemical and ecotoxicological properties of the registered and surrogate substances.

 

CAS Number	13596 -23 -1	2487-90-3	78-10-4
Chemical Name	Octachlorotrisilane	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) (g/mol)	367.88	122.2	208.33
Molecular weight (hydrolysis product) (g/mol)	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 solubility (parent)	n/a (reacts in contact with water)	170 000 mg/L (QSAR)	8600 mg/L (QSAR)
Water solubility (silanol hydrolysis product))	1E+06 mg/L (predicted; in reality limited to 100-150 mg/L as SiO2 by condensation reactions)	1E+06 mg/L (predicted; in reality limited to 100-150 mg/L as SiO2 by condensation reactions)	1E+06 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 at 25°C (QSAR)	<1E-06 Pa at 25°C (QSAR)	<1E-06 Pa at 25°C (QSAR)
Hydrolysis t1/2 at pH 7 and 25°C	<5 seconds (analogue group read-across)	14 seconds at 2°C (measured)	4.4 hours (measured)
Hydrolysis t1/2 at pH 4 and 25°C	<5 seconds (analogue group read-across)	17 seconds at 2°C (measured)	0.11 hours (measured)
Hydrolysis t1/2 at pH 9 and 25°C	<5 seconds (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	>245 mg/L
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>100 mg/L	>844 mg/L
Algal inhibition (ErC50and 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:

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

UNEP PUBLICATIONS 2004 - SIDS Initial Assessment Report SOLUBLE SILICATES

OECD 2004a - UNEP PUBLICATIONS - SIDS Initial Assessment Protfile METHANOL CAS N°: 67-56-1

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

 

Conclusion on classification

Reliable short-term toxicity data are read across from analogous substances (on the basis of common hydrolysis products). On this basis it is proposed that octachlorotrisilane should not be classified in the EU under EC Regulation No 1272/2008 (CLP Regulation, as adapted) for acute or chronic toxicity on the grounds that short-term EC50 values reported in 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 registered substance rapidly hydrolyses to hydrogen chloride and inorganic silicate moieties. The 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 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).