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Trimethoxyphenylsilane (CAS No. 2996 -92 -1) hydrolyses rapidly in contact with water (DT50 = 0.4 h at 25°C and pH 7) to produce phenylsilanetriol (CAS No. 3047 -74 -3) and methanol (CAS No. 67 -56 -1). Methanol is considered non-hazardous at the concentrations at which it would be present in the studies assessed. The ECHA 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). The ECHA guidance R.16 also suggests that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Therefore, the hazard assessment for the environment, including sediment and soil compartments, is based on the properties of the hydrolysis product phenylsilanetriol, in accordance with REACH guidance.

Experimental (supporting) studies for the aquatic toxicity of the target substance trimethoxyphenylsilane (CAS No. 2996 -92 -1) are available for all three trophic levels, which were conducted according to standard OECD guidelines and GLP. The studies for the short-term toxicity to fish and aquatic invertebrates were performed as limit tests with a maximum nominal test item concentration of 0.2 mg/L. The study for the toxicity to aquatic algae tested 5 concentrations up to a nominal test item concentration of 0.2 mg/L.

No effects were observed at the nominal test concentration of 0.20 mg/l for Oncorhynchus mykiss (OECD 203), Daphnia magna (OECD 202) and Pseudokirchnerella subcapitata (OECD 201). The results are expressed relative to nominal test substance concentration. The corresponding mean measured concentration of the substance in the treated test medium over the course of the test was 0.074 mg/L for fish study and 0.0029 mg/L in the Daphnia study. In the algae study, analysis of the test media showed that measured concentrations of the parent substance declined to below the Limit of Quantification (LOQ) of the analytical method in all treatments within 24 hours of the start of the test.

The maximum concentration tested in these studies (0.2 mg/L) were slightly above the functional solubilities of the substance in the respective test media, which had been determined in preliminary solubility trials. However, the highest tested concentration is also well below the predicted solubility of the substance (1700 mg/L).

Therefore, it was considered appropriate to read-across from reliable and good quality data for the structurally analogous source substance trichloro(phenyl)silane (CAS No. 98 -13 -5) as both the target and source substance hydrolyse rapidly in contact with water (half-lives < 1 h) and form a common silanol hydrolysis product, namely phenylsilanetriol (CAS No 3047 -74 -3). The other hydrolysis products being methanol (target) and hydrochloric acid (source), respectively, are not expected to be hazardous at the concentrations relevant to the studies.

The effect concentrations derived for methanol in standard guideline studies are all well above 100 mg/L (OECD SIDS, 2004). Thus, methanol is not considered to contribute to the overall aquatic toxicity of the target substance. In consequence, data for methanol are not presented specifically for each environmental toxicity endpoint within this dossier.

By means of the read-across approach, experimental data are available for the short-term toxicity of trimethoxyphenylsilane to all three trophic levels, i.e. fish, invertebrates and algae. All studies were performed according to accepted standard OECD guidelines and GLP and the experimental study design favored exposure to the hydrolysis products.

No short-term aquatic toxicity was observed towards aquatic organisms of all three trophic levels up to the highest concentration tested, i.e. 100 mg/L (nominal). Furthermore, an activated sludge respiration inhibition test according to OECD guidelines and GLP indicates no effects towards microorganisms of activated sludge.