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Toxicity to aquatic algae and cyanobacteria

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Description of key information

Short-term toxicity to algae: (72 h) ErC50: >32 mg/l and NOEC 3.2 mg/l (nominal) (OECD 201), read-across from an analogous/structurally related substance, [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1). The EC50 is equivalent to >25 mg/l and the NOEC is equivalent to 2.5 mg/l, when expressed in terms of the silanol hydrolysis product of the tested substance, [3 -(2,3 -epoxypropoxy)propyl]methylsilanediol. This value is read across to 2 -(3,4 -epoxycyclohexyl)ethylsilanetriol.

Key value for chemical safety assessment

EC50 for freshwater algae:
25 mg/L
EC10 or NOEC for freshwater algae:
2.5 mg/L

Additional information

There are no reliable algal toxicity data available for 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (CAS 3388-04-3), therefore good quality data for an appropriate structural analogue, [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1), have been read across. Both substances are susceptible to rapid hydrolysis and have similar silanol hydrolysis products in terms of the presence of the toxicologically active epoxide functional group, and similar physicochemical properties. The other hydrolysis products are methanol and ethanol, respectively. A 72 h ErC50 nominal value of 280 mg/L, equivalent to 2.7 to 6.8 mg/l parent substance (geometric mean), and a NOEC nominal value of 1.0 mg/L (geometric mean not available) have been determined for the effects of [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1) on the growth rate of the freshwater algae Scenedesmus subspicatus. In all the test concentrations bright green dispersions were observed by the end of the test period and in the highest three concentrations clumps of algal cells were noted. It cannot be excluded that the observed effects are related to undispersed test material, attributable by the reviewer to the formation of less soluble oligomers or polymers of the hydrolysis product of the registered substance. Therefore the results of the study should be treated with caution. At the nominal concentrations of 32 mg/l and below no algal clumps were recorded. Therefore a more reliable EC50should be based on these concentrations. A 72 h ErC50value of >32 mg/l nominal, equivalent to >17 mg/l measured initial (geometric mean not possible), and a NOEC of 3.2 mg/l nominal, 1.8 mg/l, measured initial, have thus been estimated by the current reviewer; where 19% and 7% inhibition were recorded at 17 and 1.8 mg/l respectively.

Epoxides are known to have the potential for enhanced toxicity due to a specific mode of action to higher organisms (e.g. by Lipnick, 1991); therefore even though the available experimental data provide an assessment limited to a relatively low concentration, the assessment is driven by the data on invertebrates and fish and the uncertainties in interpretation of this test result are not considered critical to the data set.

The results may be expressed in terms of concentration of the hydrolysis product of the tested substance, (2,3-epoxypropoxy)propyl]methylsilanediol, by applying a molecular weight correction: (MW of silanol=192.28 / MW of parent=248.40) * >32 mg/l = >25 mg/l. The NOEC value is converted using the same method.

2-(3,4-epoxycyclohexyl)ethylsilanetriol is susceptible to further hydrolysis reactions and the ultimate hydrolysis product 2-(3,4-dihydroxycyclohexyl)ethylsilanetriol is considered unlikely to exhibit significant ecotoxic effects based on QSAR estimated E(L)C50s >>100 mg/l (ECOSAR). The rate of reaction under environmental conditions is uncertain but the half-life is not expected to be more than a few days.

Reference

Lipnick RL. 1991. Outliers: their origin and use in the classification of molecular mechanisms of toxicity. Sci. Total. Environ. 109-110:131153.