Tetrapropyl orthosilicate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

10 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

100 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

1 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

9 600 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

52 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

5.2 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:

4.5 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 rapidly (6.7 h at pH 7 and 25⁰C) in contact with water and atmospheric moisture to silicic acid and propanol. REACH guidance (ECHA 2010A, R.16) states that “for substances where hydrolytic DT₅₀is 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 silicate hydrolysis product, silicic acid, and propanol, in accordance with REACH guidance. However, for completeness it is important to look also at any data from tests of this and related substances.

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.

Tetrapropyl orthosilicate, tetrakis(2-butoxyethyl) orthosilicate and tetraethyl orthosilicate are members of a group of substances that hydrolyse rapidly to form silicic acid, a bioavailable form of silicate.

Silicic acid is a naturally occurring substance which is not harmful to aquatic organisms at the concentrations resulting from use of the registration substance. 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 that will be exposed to it.

In the following paragraphs the read-across approach fortetrapropyl orthosilicateis 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 fromtetraethyl orthosilicate to tetrapropyl orthosilicate

The registered substance has a hydrolysis half life of6.7 h at pH 7 and 25⁰C and will formsilicic acid and propanol upon hydrolysis. Tetraethyl orthosilicate (CAS 78-10-4) hydrolyses to produce silicic acid and ethanol with a half-life of 4.4 h at pH 7 and at 25°C. 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. 

 

·        Read-across from tetrakis(2-butoxyethyl) orthosilicate to tetrapropyl orthosilicate

The hydrolysis of tetrakis(2-butoxyethyl) orthosilicate (CAS 18765-38-3) results in the formation of silicic acid and 2-butoxyethanol. The half-life of this reaction at pH 7 and at 25°C is not known, however it is expected to be moderately fast. 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:

Aquatic toxicity data for propanol have been reviewed as part of an European Union Risk Assessment Report (EU RAR, EC 2008) and show the low toxicity of the substance to aquatic organisms. The lowest available effect concentration identified in reviewed studies covering fish, invertebrates and algae was a 48-hour EC₅₀ value of 1,000 mg/l for effects on mortality of the invertebrate species Gammarus pulex.

Ethanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for this substances is in excess of 1000 mg/l (OECD 2004 - SIDS for ethanol, CAS 64-17-5).

Aquatic toxicity data for 2-butoxyethanol have been reviewed as part of an EU RAR (EC, 2006) and show the low toxicity of the substance to aquatic organisms. The lowest reliable available effect concentration identified in reviewed short-term studies covering fish, invertebrates and algae was a 48-hour EC₅₀ value of 540 mg/l for effects on mortality of the invertebrate species Hydra attenuata or H. vulgaris. The lowest reliable effect concentration derived in reliable reviewed long-term toxicity studies was a 21-day NOEC value of 100 mg/l with Daphnia magna.

It is therefore unlikely that any of the non-silanol hydrolysis products would affect the outcome of the studies with the registered and surrogate substances.

 

Table 7.1Key physicochemical parameters for the registered and surrogate substances.

CAS Number	682-01-9	78-10-4	18765-38-3
Chemical Name	Tetrapropyl orthosilicate	Tetraethyl orthosilicate	Tetrakis(2-butoxyethyl) orthosilicate
Si hydrolysis product	Silicic acid	Silicic acid	Silicic acid
Non-Si hydrolysis product	Propanol	Ethanol	2-Butoxyethanol
Molecular weight (parent)	264.44	208.33	496.76
Molecular weight (silanol hydrolysis product)	96.1	96.1	96.1
Molecular weight (non-Si hydrolysis product)	60.1	46.07	118.17
log Kow(parent)	3.4	3	4.3
log Kow(silanol hydrolysis product)	n/a (inorganic)	n/a (inorganic)	n/a (inorganic)
log Kow(non-Si hydrolysis product)	0.34	-0.3	0.8
Water sol (parent)	78 mg/l	Ca. 1500 mg/l	1490 mg/l
Water sol (silanol hydrolysis product)	1E+06 mg/l	1E+06 mg/l	1E+06 mg/l
Water sol (non-Si hydrolysis product)	soluble	soluble	soluble
Vapour pressure (parent)	0.26 Pa	1.1 hPa	1.7E-06 Pa
Vapour pressure (silanol hydrolysis product)	<1E-06 Pa	<1E	06 Pa
Vapour pressure (non-Si hydrolysis product)	19 400 Pa	57.3 hPa	141 Pa
Hydrolysis t1/2at pH 7 and 25°C	6.7 h	4.4 h	n/a
Hydrolysis t1/2at pH 4 and 25°C	0.22 h	0.11 h	n/a
Hydrolysis t1/2at pH 9 and 25°C	0.44 h	0.22 h	n/a
Short-term toxicity to fish (LC50)	n/a	>245 mg/l	>201 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>75 mg/l	>90 mg/l
Algal inhibition (ErC50and NOEC)	n/a	ErC50: >22 mg/l and NOEC and NOEC ≥22 mg/l	ErC50: >161 mg/l and NOEC: approx. 110 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 toxicity to sediment organisms (NOEC)	n/a	n/a	n/a
Short-term terrestrial toxicity (L/EC50)	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a	n/a

 

Conclusion on classification

The substance is not classified for the environment in the EU according to Regulation (EC) No. 1272/2008 on the basis that data on structurally analogous substances to the hydrolysis products, silicic acid and propanol, indicate no effects at 100 mg/l.