Trichlorosilane

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 trichlorosilane (CAS 10025-78-2) is approximately 5 seconds at 25°C and pH 4, 7 and 9 (based on read-across data); 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 silanetriol and hydrochloric acid. The Si-H bond is also expected to react rapidly, forming monosilicic acid (Si(OH)₄) and hydrogen as the ultimate hydrolysis products. The precise rate of this reaction is uncertain but the half-life for Si-H reactivity of trichlorosilane is estimated as <12 hours (possibly much less than 12 hours) at pH 7 and 25°C.

Monosilicic acid (Si(OH)₄) exists only in dilute aqueous solutions and readily condenses at concentrations above approximately 100-150 mg/l as SiO₂ to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

 

The water solubility is approximately 100-150 mg/l (limited by condensation reactions) (see Section 4.8 of IUCLID for further discussion).

 

The consideration of the non-silanol hydrolysis product, hydrochloric acid, is discussed below.

 

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”. 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 longer) with an average temperature of 15°C (assumed to be at neutral pH). Significant degradation by hydrolysis would be expected before the substance is released to the receiving waters.

 

The environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is therefore based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

 

As described below and in Section 4.8 of IUCLID, condensation reactions of the monosilicic acid are possible.

 

READ-ACROSS JUSTIFICATION

No measured data are available for the registration substance. Data have therefore been read across from relevant substances to assess the toxicity of the silanol hydrolysis product to aquatic organisms.

 

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex X 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, trichlorosilane (CAS 10025-78-2) hydrolyses very rapidly (half-life approximately 5 seconds at 20-25°C and pH 7), with the final hydrolysis products being monosilicic 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. 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 supporting reports (PFA 2015ao and PFA 2013x) attached in Section 13 of the IUCLID dataset.

 

Reliable data have been read across from trimethoxysilane (CAS 2487-90-3). This substance, like trichlorosilane (CAS 10025-78-2), hydrolyses very rapidly (t_1/2 <0.3 min at pH 4, 7 and 9 and 2°C) to form silanetriol. Methanol is also produced. Silanetriol then reacts further to monosilicic acid. Under neutral conditions of the environment and buffered test media, neither hydrogen chloride nor methanol will significantly influence the hydrolysis and condensation reactions of the silanetriol species formed by initial hydrolysis of both trichlorosilane (CAS 10025-78-2) and trimethoxysilane (CAS 2487-90-3). Although there is some uncertainty around the rate of reaction from silanetriol to monosilicic acid, the reaction is expected to be fairly rapid (<12 hours) and the final silicon-containing products of trimethoxysilane and trichlorosilane hydrolysis are equivalent and produced on an equivalent timescale.

Short-term toxicity studies with fish, invertebrates and algae have been read across from trimethoxysilane (CAS 2487-90-3).

LC₅₀ or EC₅₀ values for the three organisms are all >100 mg/l indicating that the substance is not acutely toxic to aquatic organisms. These data are used as key data.

 

Short-term toxicity data are also available with tetraethyl orthosilicate (CAS 78-10-4) which rapidly hydrolyses (t_1/24.4 h at 25°C and pH 7) to produce monosilicic acid and ethanol.

As discussed above, trichlorosilane (CAS 10025-78-2) also rapidly hydrolyses (half-life approximately 5 seconds at 20-25°C and pH 7) to produce monosilicic acid as the silanol hydrolysis product. Tetraethyl orthosilicate (CAS 78-10-4) and trichlorosilane (CAS 10025-78-2) are considered part of the same analogue group as they both react in water to produce (poly)silicic acid. The non-silicon hydrolysis products, ethanol and hydrochloric acid, respectively, do not cause effects in aquatic organisms at relevant concentrations as discussed below.

Short-term toxicity data for fish and algae with tetraethyl orthosilicate (CAS 78-10-4) indicate that this substance is of low toxicity to aquatic organisms (E(L)C₅₀values >100 mg/l). These data are used as supporting data.

 

Silicic acid producers analogue group

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 2004c, 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).

 

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).

 

Please see the attached report in IUCLID Section 13 for the analogue approach to address ecotoxicity of trichlorosilane (CAS 10025-78-2).

 

Considerations on the non-silanol hydrolysis product:

Hydrogen chloride

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 2002 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;

•            monosilicic acid has a predicted first dissociation constant around 10 and so does 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.

 

Methanol and Ethanol

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 and OECD 2004b, respectively).

 

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.

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

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.

OECD SIDS (2004c). 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.

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 data are read across from analogous substances (on the basis of common hydrolysis product). On this basis it

is proposed that trichlorosilane 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 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 very rapidly hydrolyses to hydrogen chloride and inorganic silicate moieties. Hydrolysis product hydrogen chloride has a harmonised classification in Annex VI of Regulation No 1272/2008 and does not require classification for the environment. 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).