Tetramethyl orthosilicate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

5 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

50 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.5 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

20 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

2 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

1.12 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The registered substance will hydrolyse very rapidly (half-life <3 min at pH 4, 7 and 9 at approximately 20°C) in contact with water and atmospheric moisture tosilicic acid and methanol. REACH guidance (ECHA 2010A, 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 ECHA guidance, (EC 2003, ECHA 2010A) 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 hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

As described in Section 1.3, the silicic acid may precipitate as insoluble silica (SiO₂) at high enough concentrations. Relevant low molecular weight tetra-alylsilicates are described in section 1.4.

 

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX 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.

Tetramethyl orthosilicate and tetraethyl orthosilicate are members of a group of substances that hydrolyse rapidly (<12 hours) to form silicic acid, a bioavailable form of silicate.

Silicic acid is a naturally occurring substance which is not harmful to aquatic organisms. It is the major bioavailable form of silica for aquatic organisms and plays an important role in the biogeochemical cycle of Si. Most living organisms contain at least trace quantities of silicon. For some species Si is essential element taken up actively, for example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures.In addition, silicic acid has been shown to be beneficial in protection against mildew formation in wheat and to not be phytotoxic in non-standard studies (Côte-Beaulieu et al. 2009).Therefore, it is not expected to be harmful to the environment nor the organisms it will be exposed to. However, methanol has the potential to cause some harmful effects at high dose levels and PNECs will be based on the properties of methanol.

In the following paragraphs the read-across approach for tetramethyl orthosilicate is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

Additional information is given in a supporting report (PFA, 2013x) attached in Section 13 of the REACH IUCLID 5 dossier.

·        Read-across from tetraethyl orthosilicate to tetramethyl orthosilicate

The registered substance, tetramethyl orthosilicate (CAS 681-84-5), hydrolyses rapidly (half-life <3 min at pH 4, 7 and 9 at approximately 20°C) to produce silicic acid and methanol (CAS 67-56-1). Tetraethyl orthosilicate (CAS 78-10-4) also hydrolyses rapidly (half-life 4.4 h at pH 7 and at 25°C) to silicic acid and ethanol (CAS 64-17-5).Therefore the surrogate and registered substance share the same silanol hydrolysis product and read-across is considered to be directly relevant. The non-silanol hydrolysis products are given due consideration below.

Short-term toxicity data available for the structurally-analogous substance do not report any effects below 100 mg/l. 

·        Considerations on the non-silanol hydrolysis products:

Ethanol is well characterised in the public domain literature and is of low toxicity at the concentrations relevant to the studies; the short-term EC₅₀and LC₅₀values for this substances is in excess of 1000 mg/l (OECD 2004 - SIDS for ethanol, CAS 64-17-5).

Aquatic toxicity data for propanol have been reviewed as part of a European Union Risk assessment Report (EC 2008) and show the low toxicity of the substance to aquatic organisms. The lowest available effect concentration identified was a 48-hour EC₅₀value of 1000 mg/l for effects on the invertebrate species Gammarus pulex.

CAS Number	681-84-5	78-10-4
Chemical Name	Tetramethyl orthosilicate	tetraethyl orthosilicate
Si hydrolysis product	Silicic acid	Tetraethyl orthosilicate
Non-Si hydrolysis product	Methanol	Ethanol
Molecular weight (parent)	152.22	208.33
Molecular weight (silanol hydrolysis product)	96.1	96.1
Molecular weight (non-silanol hydrolysis product)	32.04	46.07
log Kow(parent)	-0.5	3.18
log Kow(silanol hydrolysis product)	n/a	n/a
log Kow (non-silanol hydrolysis product)	-0.82	-0.3
Water sol (parent)	n/a	approx. 1500 mg/l
Water sol (silanol hydrolysis product))	1E+06 mg/l	1E+06 mg/l
Water sol (non-silanol hydrolysis product)	Soluble	Soluble
Vapour pressure (parent)	1800 Pa	1.1 hPa
Vapour pressure (silanol hydrolysis product)	<1E-10 Pa	<1E-10 Pa
Vapour pressure (non-silanol hydrolysis product)	12790 Pa	57.3 hPa
Hydrolysis t1/2at pH 7 and 25°C	<3 min	4.4 hrs
Hydrolysis t1/2at pH 4 and 25°C	<3 min	0.11 hours
Hydrolysis t1/2at pH 9 and 25°C	<3 min	0.22 hours
Short-term toxicity to fish (LC50)	>245mg/l	>245mg/l
Short-term toxicity to aquatic invertebrates (EC50)	>75 mg/l	>75 mg/l
Algal inhibition (ErC50and NOEC)	(72-hour) EC50: >22 mg/l; NOEC: ≥22 mg/l	72-hour EC50: >22 mg/l; NOEC: ≥22 mg/
Long-term toxicity to fish (NOEC)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a
Sediment toxicity (NOEC)	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a

PNEC derivation No data are available for the registered substance. However, the rapid hydrolysis of tetramethyl orthosilicate means that it is appropriate to base PNEC derivation on the data for its hydrolysis products.

The read-across data for tetraethyl orthosilicate indicate that the common hydrolysis product, tetrahydroxysilane (silicic acid), is not toxic at the loading rates used.

For the other hydrolysis product, methanol, short-term aquatic toxicity data reviewed under the OECD SIDS programme indicate that L(E)C₅₀values for fish, invertebrates and algae exposed are all ≥10 000 mg/l. The low toxicity of methanol to aquatic organisms is confirmed by QSAR predictions for short-term L(E) C50 values of >1000 mg/l for the same three taxonomic groups with a lowest 96-hour EC₅₀value of 5000 mg/l reported for algae. There are no long-term data for methanol.

The QSAR-predicted algal 96-hour EC50 value for methanol of 5000 mg/l is used to derive the PNECs as follows because it is the lowest available non-limit value:

·        A PNEC_{aquatic(freshwater)} of 5 mg/l methanol (equivalent to 6 mg/l TMOS) is derived using an assessment factor (AF) of 1000 because short-term data are only available.

·        A PNEC_{aquatic(marine water)} of 0.5 mg/l methanol (equivalent to 0.6 mg/l TMOS) is derived by applying an assessment factor 10000.

·        A PNEC_{aquatic(intermittent releases)} of 50 mg/l methanol (equivalent to 60 mg/l TMOS) is derived by applying an assessment factor of 100.

PNECs for sediment and soil compartments are determined from the PNECs for the aquatic compartment using the equilibrium partitioning method.

Conclusion on classification

Tetramethyl orthosilicate hydrolyses very rapidly in contact with moisture. The hydrolysis product is inorganic silicic acid.

 

Tetramethyl orthosilicateis notclassifiedfor the environment in the EU according to Regulation (EC) No. 1272/2008 because short-term data available for an appropriate analogue indicate the EC and LC₅₀ to be >100 mg/l.