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Ecotoxicological information

Ecotoxicological Summary

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Administrative data

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 tetraethyl orthosilicate (TEOS, CAS 78-10-4; EC No. 201-083-8) is approximately 4.4 hours at 25°C and pH 7 (based on measured data (OECD TG 111)); the substance will therefore undergo rapid hydrolysis in contact with water to form monosilicic acid (CAS 10193-36-9; EC No. 233-477-0) and ethanol (CAS 64-17-5; EC No. 200-578-6). 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.

Log Kow is not relevant for inorganic compounds such as silicic acid. However, on the basis of structure, monosilicic acid has a high affinity with water and low affinity for lipids and organic carbon. The water solubility of monosilicic acid is approximately 100-150 mg/L (limited by condensation reactions) (see Section 4.8 of the IUCLID dataset for further discussion).

The non-silanol hydrolysis product, ethanol, 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. 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.

Direct releases of the registration substance to air are expected to be low. TEOS will hydrolyse on contact with atmospheric moisture to form monosilicic acid and ethanol,

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.

 

Silicic acid is a naturally-occurring substance which is not harmful to aquatic organisms at relevant concentrations. Monosilicic 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, soluble 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, the available aquatic toxicity studies with organosilicon substances that hydrolyse to monosilicic acid report no effects at 100 mg/l nominal loading in short-term toxicity studies (PFA 2103x).

The non-Si hydrolysis product, ethanol, does not have the potential to cause harm at high treatment levels and therefore the hazard assessment and Predicted No Effect Concentrations (PNECs) are concluded as ‘no hazard identified’.

 

Considerations on the non-silanol hydrolysis product, ethanol:

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 substance are in excess of 1000 mg/L (OECD 2004b).

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.

PFA 2013x: Peter Fisk Associates, Analogue grouping report: Ecotoxicity of (poly)silicic acid generating compounds. PFA.300.003.001.

Conclusion on classification