Trichloro(hexadecyl)silane

Sediment toxicity

Link to relevant study record(s)

Reference

Endpoint:

sediment toxicity: long-term

Data waiving:

other justification

Justification for data waiving:

other:

Description of key information

The substance hydrolyses rapidly and its silanol hydrolysis product has an estimated log Kow > 3 and an estimated log Koc of 5.6. Therefore, partitioning to the sediment compartment is expected. However, the substance has only a limited use pattern with intermediate uses and professional uses and therefore the exposure to sediment is expected to be minimal. Furthermore, in accordance with Annex XI, Section 3, exposure of aquatic organisms to the silanol hydrolysis product hexadecylsilanetriol was assessed as not significant (RCR < 1).

Additional information

The registered substance trichloro(hexadecyl)silane (CAS No. 5894-60-0) hydrolyses very rapidly in contact with water (DT50 << 2 min at pH 7 and room temperature, EU Method C.7) to hexadecylsilanetriol and hydrochloric acid. REACH guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). TGD (EC, 2003) and REACH guidance R.16 (ECHA, 2016) also suggest that when the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity testing. Therefore, the hazard and risk assessment is based on the silanol hydrolysis product hexadecylsilanetriol. 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 (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). Thus, hydrochloric acid is not considered to contribute to the overall aquatic toxicity of the registered substance. Therefore, hydrochloric acid can be disregarded in the environmental hazard assessment. In consequence, data for hydrochloric acid is not presented specifically for each environmental toxicity endpoint within this dossier.

The silanol hydrolysis product of the registered compound has high estimated log Kow and Koc values (5 and 5.6, respectively), therefore a partitioning into the sediment compartment is suggested. However, the parent compound has limited uses with only intermediate and professional uses. Hence, a release into the sediment compartment is expected to be minimal. Additionally, the silanol hydrolysis product is assessed as being inherently biodegradable based on read-across to structurally similar analogue substances and thus the substance does not persist in the environment. In accordance with Annex XI, Section 3, exposure of aquatic organisms to hexadecylsilanetriol is not significant. Furthermore, the hazard assessment based on acute aquatic toxicity data shows no toxicity toward aquatic organisms of all three trophic levels (all acute effect values, i.e. E(L)C50s were > 31.8 mg/L). Additionally, chronic toxicity tests on aquatic invertebrates and algae are available, which showed also no effects up to the highest concentrations tested (21-day NOEC ≥ 10 mg/L (daphnia), 72-h NOErC ≥ 31.8 mg/L (algae)). The substance is moderately soluble and thus, greater toxicity in long-term tests is unlikely. Even though no hazard was identified in acute aquatic studies, PNECs were derived for the purpose of risk characterization, using the threshold value of 10 mg/L (equivalent to 9 mg/L hexadecylsilanetriol) obtained in the chronic daphnia study in combination with standard safety factors as outlined in ECHA guidance R.10. As the substance hydrolyses rapidly, the environmental hazard assessment, PNEC derivation and risk characterization is based on the silanol hydrolysis product following a worst-case approach. The risk characterization for the silanol hydrolysis product indicated no risk to the aquatic environment including sediment (RCR < 1). For detailed information on the risk assessment, please see the attached documentation in chapter 13 (CSR).

References:

OECD SIDS, 2002. Hydrochloric acid - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.