4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane

Toxicity to aquatic algae and cyanobacteria

Description of key information

EL₅₀ (72 h): > 500 mg/l (nominal) and EL₁₀ (72 h): 280 mg/l (nominal) based on: growth rate in Scenedesmus subspicatus (new name: Desmodesmus subspicatus) in the presence of an ethanol carrier.

This result is supported by: EL₅₀ (96 h) >1000 mg/l (nominal) and EL₁₀ (96 h): >1000 mg/l (nominal) based on: biomass and yield in Scenedesmus subspicatus (new name: Desmodesmus subspicatus), in media prepared by direct addition, and an EL₅₀ of 700 mg/l (nominal) based on: yield in Scenedesmus subspicatus (new name: Desmodesmus subspicatus), in media prepared by water accommodated fraction (it is likely that the organisms would have been exposed to a mixture of the monomeric silanol hydrolysis product and siloxane oligomers; the effects seen are attributed to precipitated oligomeric or cross-linked by-products; however the results are conservatively interpreted as representative of the parent substance loading because of the need to define an indicative PNEC and assess risk).  

Additional information

A 72-h EL₅₀ value of >500 mg/l and a NOELR of 32 mg/l have been determined for the effects of the test substance on growth rate of Scenedesmus subspicatus in the presence of ethanol as a vehicle. The EL₅₀ and NOELR values significantly exceed the solubility of the substance (approximately 0.01 mg/l) and should therefore be taken to indicate that the test substance is unlikely to be toxic at its solubility limit. Effects on biomass at concentrations ≥64 mg/l may be attributable to the presence of undissolved test material or precipitated by-products.

A 96-h EL50 value of >1000 mg/l (nominal) and EL10 of >1000 mg/l (nominal) have been determined for the effects of polysulfides, bis[3-triethoxysilylpropyl] (CAS 211519-85-6) on yield of Scenedesmus subspicatus as determined by fluorescence measurement. This test was conducted in media prepared by direct addition. Based on the static exposure regime, it is likely that the organisms would have been exposed to a mixture of the dissolved and undissolved parent substance, monomeric silanol hydrolysis product and siloxane oligomers. Insoluble oligomers may have precipitated out of solution to an extent in the course of the test. The test results have been interpreted in this context. No analytical verification was performed. In view of the media preparation method and duration of study, the effects seen are attributed to a mixture of parent, silanol hydrolysis product and oligomeric or cross-linked by-products. Physical effects could have contributed.

A 96-h EL₅₀ value of 700 mg/L and a 96-h EL₁₀ of 60 mg/L have been determined for the effects of polysulfides, bis[3-triethoxysilylpropyl] (CAS 211519-85-6) on yield of Scenedesmus subspicatus as determined by fluorescence measurement. A NOELR of 1000 mg/l has been estimated from the test results by the report authors but does not seem realistic given the EL₅₀ and EL₁₀ values. The results are above the expected water solubility of the test substance (approximately 0.01 mg/L). There was evidence of undissolved test material (turbidity) in the water-accommodated fractions and this may have contributed to effects observed at the high test substance loading rates utilised in the test. Based on the test media preparation methods and exposure regime, it is likely that the organisms would have been exposed to a mixture of the monomeric silanol hydrolysis product and siloxane oligomers. Insoluble oligomers may have precipitated out of solution to an extent during the media preparation and in the course of the test. The test results have been interpreted in this context. This is reflected in the observation of turbidity in test media, although no analytical verification was performed. In view of the media preparation method and duration of study, the effects seen are attributed to precipitated oligomeric or cross-linked by-products. Physical effects are likely to have dominated any intrinsic toxic effects.

The only quantified NOECs and EL₁₀ values are considerably higher than the limit of solubility. The registration substance itself appears to be non-toxic to algae at the solubility limit. Given the half-life of the hydrolysis reaction at pH7 and 25˚C, the chemical safety assessment focusses on the parent form.