Octachlorotrisilane

Toxicity to aquatic algae and cyanobacteria

Description of key information

There are no test data for the substance. However, read across data for related substances gives the following results: A 72-hour EC50 of >100 mg/L and a NOEC of <6.3 mg/L have been determined for the effects of related substance, trimethoxysilane (CAS 2487-90-3), on growth rate and biomass of Pseudokirchneriella subcapitata. It is likely that the test organisms were exposed to the hydrolysis products of the substance. A 72-hour EC50 value of >22 mg/L and NOEC of ≥22 mg/L have been determined for the effects of tetraethyl orthosilicate on growth rate and biomass of Pseudokirchneriella subcapitata based on geometric mean measured concentrations (>100 mg/L and ≥100 mg/L respectively based on nominal concentration and >5.8 mg/L and ≥5.8 mg/L respectively based on measured final concentrations). It is likely that the test organisms were exposed to the hydrolysis products of the substance. The results of this test were selected in preference to those of a second test (72-hour EC50 value >1039.3 mg/L for Scenedesmus subspicatus (new name: Desmodesmus subspicatus)) because they were considered to be more reliable. The appropriate interpretation of these data is to use nominal concentrations, because rapid hydrolysis during test media preparation means the test organisms would have been exposed predominantly to hydrolysis products. The low recoveries based on GC analysis are due to removal of test substance by hydrolysis. The nominal results in terms of hydrolysis product are calculated by MW conversion to be an EC50 value of >63 mg/L and NOEC of ≥63 mg/L.

Additional information

No test data for toxicity to aquatic algae and cyanobacteria are available for octachlorotrisilane. 

However, algal growth inhibition studies are available for the related substances trimethoxysilane and tetraethyl orthosilicate.

Octachlorotrisilane (CAS 13596-23-1) and trimethoxysilane (CAS 2487-90-3) will hydrolyse to octahydroxytrisilane and silanetriol respectively, then both to monosilicic acid. The hydrolysis half-lives of octachlorotrisilane and trimethoxysilane indicate that, under conditions relevant to ecotoxicity assessment, both will hydrolyse to the same silicon hydrolysis products over the course of the test.

The other hydrolysis products are hydrogen chloride and methanol, respectively; the properties of these substances are well characterised. Methanol is non-toxic to the environment and it is considered unlikely that its presence significantly affected the results of the test performed with trimethoxysilane. Effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media. 

Tetraethyl orthosilicate (78-10-4) hydrolyses very rapidly to monosilicic acid and ethanol. Monosilicic acid is a naturally occurring substance which is not harmful to aquatic organisms at relevant concentrations (Reference PFA 2013x).

Ethanol is non-toxic to the environment and it is considered unlikely that its presence significantly affected the results of the test performed with trimethoxysilane. Effects of hydrogen chloride on aquatic organisms are limited to those that result from changes to pH in unbuffered media. 

References:

PFA, 2013x, Peter Fisk Associates, Analogue report - Ecotoxicity of (poly)silicic acid generating compounds, PFA.300.003.001