Trichloro(hexadecyl)silane

Long-term toxicity to fish

Link to relevant study record(s)

Reference

Endpoint:

fish early-life stage toxicity

Data waiving:

exposure considerations

Justification for data waiving:

other:

Description of key information

A long-term toxicity study on fish is not necessary as

- exposure considerations in accordance with Annex XI, Section 3, indicate that exposure of aquatic organisms to trichloro(hexadecyl)silane is absent or not significant

and

- the substance has a low hazard potential based on the available toxicity data.

Additional information

In accordance with Annex XI, Section 3, exposure of aquatic organisms to trichloro(hexadecyl)silane (CAS No. 5894-60-0) is absent or not significant. 

 

The environmental hazard assessment, PNEC derivation and risk characterization is based on the silanol hydrolysis product, hexadecylsilanetriol, as the substance hydrolyses very rapidly under environmental conditions (DT50 << 2 min at pH 7 and room temperature). The other 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 (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized 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). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. 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, hydrochloric acid is not relevant for the assessment of aquatic toxicity.

 

No acute and chronic aquatic toxicity data is available for the registered compound trichloro(hexadecyl)silane (CAS No. 5894-60-0) or its silanol hydrolysis product. Therefore, good quality data was read-across from the structurally analogue substances trimethoxy(hexadecyl)silane (CAS No. 16415-12-6). The source substance hydrolyses slowly under environmental conditions (DT50 > 14 d at pH 7), but it forms the identical silanol hydrolysis product as the target compound and methanol as the second hydrolysis product instead of hydrochloric acid (target). The second hydrolysis product formed by the source substance, methanol, is also well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies (OECD SIDS, 2004). Thus, methanol is not considered contributory to the overall aquatic toxicity of the source substance and can be disregarded in the assessment. Additional information on the source substance CAS No. 16415-12-6 is given in a supporting read-across justification attached in IUCLID Section 13.

 

Short-term aquatic toxicity data is available for all three trophic levels (fish, algae, aquatic invertebrates) on the source compound. The hazard assessment showed no short-term aquatic hazard to all three trophic levels (all acute effect values > 31.8 mg/L). Additionally, no chronic effects were observed in aquatic invertebrates with a 21-d NOEC of ≥10 mg/L and algae with a 72-h NOEC of ≥ 31.8 mg/L. Consequently, no PNECs aqua (freshwater/marine water) were derived as no effects were observed on the read-across source substance. However, for the purpose of exposure-based adaptation as in the present endpoint, PNECs were derived in the Chemical Safety Report (CSR, Chapters 9 and 10) for the required risk characterization even though no hazard was identified in the available aquatic toxicity studies. The PNECs aqua (freshwater/marine water) were derived using a worst-case threshold value of 10 mg/L obtained in the chronic study on daphnia (equivalent to 9 mg/L when based on the silanol hydrolysis product hexadecylsilanetriol) in combination with the standard assessment factors of 50 and 500, respectively, as outlined in ECHA guidance R.10 (2008). The result was obtained under semi-static test conditions. In view of slow hydrolysis rate and exposure regime it is likely that the test organisms were exposed predominantly to the parent of the tested substance.

The silanol hydrolysis product hexadecylsilanetriol is moderately water-soluble (4.7 mg/L at 20 °C) and is expected to be inherently biodegradable based on a read-across to the source substance chloro(dimethyl)octadecylsilane, which shares a structurally similar silanol hydrolysis product compared to the silanol hydrolysis product of the registered substance. Significant deviations from this overall ecotoxicity profile are not expected and greater long-term toxicity in an additional test with fish is considered unlikely.

The risk characterization for the silanol hydrolysis product of the registered compound indicated no risk to the aquatic environment (RCR < 1).

For detailed information on the risk assessment please refer to the attached documentation in Section 13.

 

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.