Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

Trimethoxyphenylsilane hydrolyses rapidly in contact with water (half-lifeapproximately 0.4 h at 25°C and pH 7), to produce phenylsilanetrioland methanol. Methanol is considered non-hazardous at the concentrations at which it would be present in the studies assessed. 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.

The registered substance is part of a group of organosilicones, the toxicity of which is determined by a non-polar narcosis mechanism of toxicity, and as such log Kow drives toxicity. Substances in this group include alkoxy- and chloro- silanes having secondary features that do not affect the toxicity of the substances. The side chain of the registered substance is an aryl. There is evidence that the phenyl (aryl) group’s toxicity is related to log Kow alone as for other neutral organics and thus does not enhance toxicity. Additional information is given in a supporting report Application of Category - Analogue - QSAR for Reconsile (PFA, 2016b) attached in Section 13 of the IUCLID dossier.

 

Measured short-term data are available for fish, invertebrates and algae for the toxicity of trimethoxyphenylsilane (CAS 2996-92-1).

 

A limit test was conducted for each of the three trophic levels. No effects were observed at the nominal test concentration of 0.20 mg/l for Oncorhynchus mykiss (OECD Guideline 203 Fish, Acute Toxicity Test, Springborn Smithers 2008a), Daphnia magna (OECD Guideline 202 Daphnia sp. Acute Immobilisation Test, Springborn Smithers 2008b) and Pseudokirchnerella subcapitata (OECD Guideline 201 Alga, Growth Inhibition Test, Springborn Smithers 2008c).

 

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 used (0.2 mg/l) was well below the predicted solubility of the substance (1700 mg/l) and so does not provide an accurate representation of the likely toxicity of the substance.Therefore good quality data for the analogue substance, trichloro(phenyl)silane, have been read across. Both substances share the same silanol hydrolysis product, phenylsilanetriol; the other hydrolysis products being methanol respectively, which is readily biodegradable andare well characterised and methanol is expected to be not hazardous at the concentrations relevant to the study.It is therefore likely that the test organisms were primarily exposed to hydrolysis product retained in the test media.

 

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physico-chemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physico-chemical properties and the structure is altered.

 

In the following paragraphs the read-across approach fortrimethoxyphenylsilaneis assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

 

·        Read-across from trichloro(phenyl)silane to trimethoxyphenylsilane:

Trimethoxyphenylsilane and Trichloro(phenyl)silane share the same silanol hydrolysis product, phenylsilanetriol; the other hydrolysis products being methanol and hydrochloric acid.

Trichloro(phenyl)silane (CAS 98-13-5) is a trichlorosilane with an aryl side chain that hydrolyses very rapidly (<1 min at 25°C and pH 4, 7 and 9) to produce phenylsilanetriol and hydrochloric acid. The effects of hydrochloric acid are related to effects in an unbuffered media and are assessed below.

 

A limit test was conducted with trichlorophenylsilane (98-13-5) at a test concentration of 100 mg/l. No effects were observed at this concentration to fish, invertebrates and algae.

 

·        Considerations on the non-silanol hydrolysis products:

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 No. 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.

Methanol 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 these substance is in excess of 1000 mg/l (OECD2004a - SIDS for methanol).

 

·        QSAR:

QSAR predictions for the hydrolysis product phenylsilanetriol confirm the low short-term toxicity of the substance to aquatic organisms: EC50/LC50values of 1400, 600 and 120 mg/l for fish, invertebrates and algae were calculated, respectively. These QSAR data were used to derive the PNECs, as exposure to the hydrolysis product is more environmentally relevant.

Table7.1.1Key physicochemical parameters and ectoxicological data for the registered and surrogate substances.

CAS Number

2996-92-1

98-13-5

Chemical Name

Trimethoxyphenylsilane

Trichloro(phenyl)silane

Si hydrolysis product

Phenylsilanetriol

Phenylsilanetriol

Molecular weight (parent)

198.3

211.6

Molecular weight (silanol hydrolysis product)

156.2

156.2

log Kow(parent)

2.0

n/a

log Kow(silanol hydrolysis product)

-0.02

-0.02

Water sol (parent)

1700 mg/l

n/a

Water sol (silanol hydrolysis product))

1 000 000 mg/l

1 000 000 mg/l

Vapour pressure (parent)

18.2 Pa

44.3 Pa

Vapour pressure (hydrolysis product)

0.000053 Pa

0.000053 Pa

Hydrolysis t1/2at pH 7 and 25°C

0.4 hours

<1 min

Hydrolysis t1/2at pH 4 and 25°C

n/a

<1 min

Hydrolysis t1/2at pH 9 and 25°C

n/a

<1 min

Short-term toxicity to fish (LC50)

>0.2 mg/l (The maximum concentration used (0.2 mg/l) was well below the predicted solubility of the substance (1700 mg/l)).

>100 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

>0.2mg/l (The maximum concentration used (0.2 mg/l) was well below the predicted solubility of the substance (1700 mg/l)).

>100 mg/l

Algal inhibition (ErC50and NOEC)

>0.2mg/l (The maximum concentration used (0.2 mg/l) was well below the predicted solubility of the substance (1700 mg/l)).

>100 mg/l

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

n/a