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Ecotoxicological information

Short-term toxicity to aquatic invertebrates

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Description of key information

The short-term toxicity of the substance towards Daphnia magna is characterized by  an EC50(48 h) of 58.2 mg/L (Currenta, 2010f).

Key value for chemical safety assessment

EC50/LC50 for freshwater invertebrates:
58.2 mg/L

Additional information

The substance is characterized by a poor (not measurable) water solubility and, based on the presence of isocyanate structures, only a transient existence in water. Isocyanates are known to rapidly hydrolyse under the formation of the corresponding amines and inert, insoluble polyurea compounds. The amount of amine and polyurea formed strongly depends on the method used to dissolve the substance in the test media. A low stirring velocity promote the formation of polyurea compounds, whereas at high dispersion methods the formation of the environmental relevant amine is preferred. But to prepare the test solutions for the present study, solutions were stirred for 24 h on a magnetic stirrer as high dispersion solution techniques do not reflect situations which might occur in the environment.

Biological assays are designed to assess effects of the dissolved (bioavailable) fraction rather than physical hazards caused by particles present in the test medium. Thus, test solutions were filtrated prior to test begin to remove undissolved particles preferably consisting of polyurea compounds. However, the insoluble inert polyurea are not assumed to cause environmental hazards due to a reduced uptake of high molecular mass compounds.

As the test item is hydrolytically not stable, an accompanying analysis by a feasible method is commonly required to prove the test item concentration throughout the test. But no selective and sensitive chromatographical method for the determination of the test item in aqueous solutions could be established as the substance is insoluble or only poorly soluble in water and hydrolyses in a very short time (within hours), which could be shown in a separate hydrolysis test (Currenta, 2009). Nevertheless, the test item concentrations in the growth medium were quantified by the means of nitrogen analysis to get a rough impression of exposure conditions over time. It can be seen that the actual concentrations in the aqueous phase are below the nominal ones, but the nitrogen values remain stable throughout the test duration.

The test was conducted testing concentrations up to 100 mg/L. An achievement of such high concentration is not feasible due to the following facts: (i) the test item is characterized by a poor water solubility and (ii) removal of undissolved particles prior to the test begin, consisting of inert, insoluble polyurea compounds, through filtration prior to test begin. Concluding, the discrepancy between the nominal and measured concentrations can be explained by the very low test item water solubility and the removal of insoluble inert polyurea.

All results are expressed in terms of nominal concentrations as the nitrogen analysis is not substance specific detecting hydrolysis products besides the parent substance, nitrogen containing medium components as well as excretion products of the test organisms causing erroneous results. An assignment of the measured concentrations to the effective parent substance concentration is not possible in the absence of a fixed correlation between the test item and hydrolysis product concentrations. Furthermore, evoked effects are caused by the formed mixture rather than byividual components. In the absence of a substance specific analysis, effects cannot be linked to the presence of certain substances.

Nevertheless, the chosen test design well addresses situations which might occur in the environment (no high dispersion stirring, formation of transformation products and the assessment of ecotoxicological effects thereof) enabling a proper description of the environmental behaviour of the substance.