Trichloro(hexadecyl)silane

Ecotoxicological Summary

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:

PNEC soil

PNEC value:

2.78 mg/kg soil dw

Assessment factor:

10

Extrapolation method:

assessment factor

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Conclusion on classification

Classification related key information

The registered compound trichloro(hexadecyl)silane (CAS No. 5894-60-0) hydrolyses rapidly in contact with water under environmentally relevant conditions (half-life << 2 min at room temperature and pH 7). Therefore, the environmental hazard and risk assessment is based on the silanol hydrolysis product, i.e. hexadecylsilanetriol in accordance to REACH guidance R.16 (ECHA, 2016). The second hydrolysis product, hydrochloric acid, is well characterised in the public domain literature and effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (caused by an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterised 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 7647-01-0, hydrochloric acid). Generally the changes in pH of the receiving water should stay within the natural range of the pH, and for this reason, adverse effects on the aquatic environment are not expected due to anthropogenic or naturally occurring hydrochloric acid (OECD SIDS, 2002). Therefore, it is not expected to affect the environmental hazard profile of the parent substance (OECD SIDS, 2002) and thus can be disregarded in the environmental hazard assessment.

Because no ecotoxicity data is available on the registered compound or its silanol hydrolysis product, good quality data was read-across from the structurally analogue substance trimethoxy(hexadecyl)silane (CAS No. 16415-12-6). The source substance trimethoxy(hexadecyl)silane (CAS No. 16415-12-6) shares the same silanol hydrolysis product as the target compound, hexadecylsilanetriol. The second hydrolysis product is methanol instead of hydrochloric acid (target). Methanol is known for the absence of environmental effects up to the highest concentrations that were tested in the present studies (OECD SIDS, 2004). Therefore, methanol can be disregarded in the environmental hazard and risk assessment. However, the hydrolysis rate is much slower for the source substance (DT50 > 14 d at pH 7) compared to the target substance. In consideration of the study design and the duration of the acute and chronic studies (48 h – 96 h acute and 72 h – 21 d chronic studies) it can be assumed that the organisms were predominantly exposed to the parent form of the source substance.

Additional information on the source substance trimethoxy(hexadecyl)silane (CAS No. 16415-12-6) is given in a supporting read-across justification attached in IUCLID Section 13.

Abiotic degradation - Hydrolysis

DT50 << 2 min at pH 7 and room temperature (EU Method C.7)

Biodegradation in water

Inherently biodegradable (65% and 81% after 28 d and 60 d, OECD 301F, RA CAS No. 18643-08-8 and CAS No. 16415-12-6, respectively)

Bioaccumulation

Trichloro(hexadecyl)silane (parent): Log Kow not applicable due to fast hydrolysis

hexadecylsilanetriol (hydrolysis product): Log Kow = 5 (QSAR)

Hydrochloric acid (hydrolysis product): Log Kow not applicable to inorganic substances

Aquatic acute toxicity:

Fish

LC50 (96 h) > 1000 mg/L (nominal, OECD 203; RA CAS No. 16415-12-6)

Aquatic invertebrates

EC50 (48 h) > 100 mg/L (nominal, OECD 202, RA CAS No. 16415-12-6)

 Algae

ErC50 (72 h) > 31.8 mg/L (nominal, methanol as solvent) (OECD 201; RA CAS No. 16415-12-6)

Chronic aquatic toxicity

Algae

NOErC (72 h) ≥ 31.8 mg/L (nominal, methanol as solvent) (OECD 201; RA CAS No. 16415-12-6)

 Aquatic invertebrates

NOEC (21-d) ≥ 10 mg/L (nominal, OECD 211; RA CAS No. 16415-12-6)

Classification according to CLP

Trichloro(hexadecyl)silane (CAS No. 5894-60-0) hydrolyses rapidly in water (DT50 << 2 min at pH 7 and room temperature), to the hydrolysis products hexadecylsilanetriol and hydrochloric acid. The silanol hydrolysis product is considered to be inherently biodegradable, while the second hydrolysis product hydrochloric acid is an inorganic substance and therefore not susceptible to biodegradation as such. Acute aquatic toxicity data are available for all three trophic levels (fish, aquatic invertebrates, algae) and chronic data is available for algae and aquatic invertebrates based on read-across to the structurally analogue substance trimethoxy(hexadecyl)silane (CAS No. 16415-12-6). No acute effects were observed towards the species of three trophic levels up to the highest tested concentrations. Also no adverse chronic effects were observed in aquatic invertebrates and algae. On this basis, trichloro(hexadecyl)silane and its silanol hydrolysis product are not classified as hazardous to the environment according to the consolidated version of Regulation (EC) No 1272/2008 (CLP) and all further amendments (ATPs).

 

References:

OECD, 2002. Hydrogen Chloride - SIDS Initial Assessment Report for SIAM 15, Boston, USA: UNEP Publications.

OECD SIDS, 2004. Methanol - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.