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Ecotoxicological information

Ecotoxicological Summary

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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 trimethoxy(vinyl)silane (CAS 2768-02-7) is 0.1 h at pH 7 and 20-25°C. The registered substance will hydrolyse in contact with water and atmospheric moisture to form vinylsilanetriol and methanol. Vinylsilanetriol and methanol are both miscible in water and have low log Kow (-2.0 and -0.82 respectively).

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”. TGD and REACH guidance (EC 2003) and R.16 (2016) also suggest 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 chemical safety assessment (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product,vinylsilanetriol, in accordance with REACH guidance. As described in Section 4.8 of IUCLID and Section 1.5 of the CSR, condensation reactions of the silanetriol are possible.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex X 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.

The registered substance, its silanol hydrolysis productand the substance used as a surrogate for read-across (see detailed discussion below) are part of a class of non-functional compounds which act via a non-polar narcosis mechanism of toxicity.The group of 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 silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.5 of the CSR. The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on log Kow of the resulting silanols and the chemical groups present in them.

Additional information is given in a supporting report (PFA 2016y), attached in Section 13.

In the following paragraphs the read-across approach for trimethoxy(vinyl)silane is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

There are no reliable microorganism toxicity data available for trimethoxy(vinyl)silane (CAS 2768-02-7), therefore good quality data for the structurally-related substance, trichloro(vinyl)silane (CAS 75-94-5), have been read across. Both substances share the same silanol hydrolysis product, vinylsilanetriol. The other hydrolysis products are methanol and hydrogen chloride / chloride salts respectively.

Trimethoxy(vinyl)silane (CAS 2768-02-7) and trichloro(vinyl)silane (CAS 75-94-5) are withinan analogue group of substances within which, in general, there is no evidence of significant toxicity to microorganisms.

This analogue group for the toxicity to microorganisms endpoint consists of a number of sub-classes of substances with different main codes. However, read-across is carried out between substances with the same main code in most cases. In this case read-across from a different sub-class is used, fromthe sub-class I-2-T-C (Trichlorosilanes)to the sub-class I-2-T-Me-H (Trimethoxysilanes hydrolysing rapidly at pH 7). This is because the I-2 -T-Me-H sub-class does not contain any substances with a vinyl side-chain, and it was thought important to cover this structural feature. The two substances have a very rapidrate of hydrolysis in common.

It is therefore considered valid to read-across the results for trichloro(vinyl)silane (CAS 75-94-5) to fill the data gap for the registered substance.

Additional information is given in a supporting report (PFA, 2013j) attached in Section 13. Please also see Section 6.1.7 for further details on read-across of microorganism toxicity.

·        Considerations on the non-silanol hydrolysis products:

Methanol is well-characterised in the public domain literature and is not hazardous to microorganisms at the concentrations relevant to the studies (OECD 2004 - SIDS for methanol).

Hydrogen chloride is not expected to have adverse effects on microorganisms in a sewage treatment plant, where the pH is maintained within a favourable range.

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;

- silanetriols have a 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.

Conclusion on classification

The substance has reliable short-term E(L)C50 values of 191 mg/L in fish, 169 mg/L in invertebrates and >89 mg/L in algae. It has a reliable NOEC of =89 mg/L in algae and reliable long-term NOEC of 28.1 mg/L in Daphnia magna

The substance hydrolyses rapidly in water. The silanol hydrolysis product is not considered to be readily biodegradable but has low potential for bioaccumulation (i.e. low log Kow). 

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.