Tetrakis(2-butoxyethyl) orthosilicate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

10 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

1 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

463 mg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

63.6 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

6.4 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:

0.57 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hydrolysis half-life of tetrakis(2-butoxyethyl) orthosilicate (CAS 18765-38-3) is approximately 4 h at 25˚C and pH 7; the substance will therefore undergo rapid hydrolysis in contact with water to form monosilicic acid (Si(OH)₄ and 2-butoxyethanol (CAS 111-76-2). Monosilicic acid 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 polysilicic acid. The water solubility of the parent substance is 6.6 mg/l and the log K_ow is 4.3.

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”. REACH Technical Guidance 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 hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the hydrolysis products in accordance with REACH guidance.

(Poly)silicic acid is a naturally occurring substance which is not harmful to aquatic organisms at relevant concentrations. (Poly)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 2004, silicates). (Poly)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).

Monosilicic acid, oligomeric and polysilicic acids are therefore not expected to be harmful to organisms present in the environment, as confirmed by the available data. 2-Butoxyethanol does have the potential to cause harm at high dose levels and therefore the hazard assessment and Predicted No Effect Concentrations (PNECs) will be based on the properties of 2-butoxyethanol.

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 polysilicic acid at concentrations above approximately 100-150 mg/l. Depending on the pH and concentration, solutions will contain varying proportions of monomeric monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure. Further details are given in a Report attached to Section 13 of the IUCLID data set (PFA 2013x).

Consideration of the non-Si hydrolysis product

The non-silicon-containing hydrolysis product of tetrakis(2-butoxyethyl) orthosilicate is 2-butoxyethanol (CAS 111-76-2). 2-butoxyethanol is well-characterised in the public domain literature. The lowest result reported for 2-butoxyethanol is a NOEC value of 100 mg/l for long-term toxicity to aquatic invertebrates. Therefore, PNECs in the chemical safety assessment of tetrakis(2-butoxyethyl)orthosilicate will be based on the properties of 2-butoxyethanol. 2-Butoxyethanol has a log K_ow of 0.81 and is highly soluble in water. It has a molecular weight of 118.176 g/mol.

Refer to the endpoint summaries for discussion of individual results.

Conclusion on classification

The substance has reliable short-term E(L)C50 values of >201 mg/l in fish, >90 mg/l in invertebrates and >161 mg/l in algae. It has a NOEC value ca. 110 mg/l in algae. There are no long-term fish or invertebrate data available.

The available short-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

The substance hydrolyses rapidly in water and is readily biodegradable.

 

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.