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Trimethoxy(2-methylpropyl)silaneis part of a group of organosilicon substances which hydrolyse in contact with moisture and the toxicity of which is determined by a non-polar 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.

Data are available on the short-term toxicity of the registered substance to fish, aquatic invertebrates and algae. A 96-hour LC50value of >100 mg/l has been determined for effects of the test substance on the mortality of the freshwater fish Danio rerio(tested as Brachydanio rerio). A 48-hour EC50value of >865 mg/l has been determined for effects of the test substance on mobility of the freshwater invertebrate Daphnia magna. A 72-hour EC50 value of >1170 mg/l and NOEC value of 220 mg/l have been determined for effects of the test substance on the growth rate of freshwater algae Desmodesmus subspicatus (tested as Scenedesmus subspicatus). The results of the tests are expressed in terms of the nominal concentration of the test substance. However, it is likely that the test organisms were primarily exposed to the hydrolysis products of the substance.

These short-term aquatic toxicity studies used aqueous stock solutions at concentration of 1000 to 1300 mg/l. At this concentration the silanol hydrolysis product (which is formed in significant quantities over the time-scale of the media preparation) can undergo condensation reactions to give siloxane and siloxanol dimers, oligomers and polymers.

The stock solution used for the test medium was filtered and no condensation products were observed and after filtration, the measured DOC (dissolved oxygen concentrations) were within 20% of the nominal concentration. Although polymerisation products such as oligomers could still be present in the stock solution as filtration is limited to solid, high viscous particles. It is considered reasonable to assume that the concentrations at theLC50 or EC50 predominantly represent the concentration of the test substance’s hydrolysis product (and not the polymer).

At concentrations around 100 mg/l and below the polymerisation of this substance is understood to be a reversible process. Therefore, while the stock solution preparation was not carried out under ideal conditions for this substance, the studies do indicate that the registered substance is of low short-term toxicity to the tested organisms. In addition, predicted values using a validated QSARs are in agreement with the measured data.

ECOSAR predictions for the hydrolysis product (2-methylpropyl)silanetriol confirm the low short-term toxicity to aquatic organisms: EC50/LC50values 8500, 3100 and 410 mg/l for fish, invertebrates and algae respectively.

  • Considerations on the non-silanol hydrolysis product:

Methanol and is well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC50and LC50values for methanol are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol). Therefore, at the loading rates experienced in these tests it is unlikely that the presence of either would significantly affect the results of the tests.