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Short-term toxicity to aquatic invertebrates

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Description of key information

The acute toxicity of the test item bis (3,5,5-trimethylhexanoyl) peroxide in isododecane was evaluated in two separate tests in accordance with OECD guideline 202 and EU method C.2. In the first test the water-accommodated fraction (WAF) procedure was applied to prepare the test solutions due to the poor water solubility of the test item. The 24 and 48-hour EL50s were not calculated since immobilization at the highest loading rate (1000 mg/L) was lower than 50% at T24 and T48 hours (0% and 20%, respectively). 
As test results from WAF tests cannot be used for PNEC derivation, a second study was conducted. Due to the poor water solubility but rapid hydrolysis of the test item, the test item was mixed with dilution water at a nominal loading of 10 g/L, followed by vigorous stirring at 37 ± 2 °C for 7 days. This was done to maximise concentrations of the test material hence generating a solution of several, largely non-identifiable, breakdown products. Determination of the biological effect values was based on the geometric mean of the calculated concentrations of breakdown products at the beginning and at the end of the test period. The 48-h EC50 for Daphnia magna was determined to be 32 mg/L. This value is carried forward to the risk assessment.

Key value for chemical safety assessment

EC50/LC50 for freshwater invertebrates:
32 mg/L

Additional information

The acute toxicity of the test item bis(3,5,5-trimethylhexanoyl) peroxide in isododecane to aquatic invertebrates was evaluated in two different tests, both in accordance with OECD guideline 202 and EU method C.2.

As the test item is a mixture of poorly water-soluble components, the water-accommodated fraction (WAF) procedure was applied to prepare the test solutions in the first test. Nominal loading rates were 0, 100, 178, 316, 562 and 1000 mg/L. The highest loading rate resulting in no immobilisation at 48 hours was 562 mg/L. The lowest loading rate resulting in 100 % immobilisation at 48 hours was > 1000 mg/L. The 24 and 48-hour EL50values could not be calculated since immobilisation at the highest loading rate (1000 mg/L) was lower than 50 % at 24 and 48 hours (0 and 20 %, respectively).The above test was done according to the WAF procedure. However, analytical determinations of the peroxide resulted in test concentrations below the LOD (0.1 mg/L).This suggests that the WAF equilibration time (approx. 23 h) might have been insufficient for generating a concentration eliciting a biological effect. Therefore, a new study was conducted.Therefore, a new study was conducted.

In this second study, test item solutions were prepared by mixing the test item with dilution water at a nominal loading of 10 g/L, followed by vigorous stirring at 37 ± 2 °C for 7 days. This procedure was chosen to maximise concentrations of the test material, which in itself is known to be poorly soluble, but hydrolyses rapidly, hence generating a solution of several, largely non-identifiable, breakdown products.

The concentration of the test item in the test solutions was monitored by measurement of the breakdown product 2,4,4-trimethyl-1-pentanol, and of dissolved organic carbon (DOC) at the beginning and at the end of the test.

Altogether seven breakdown products could be detected, but only two products (2,4,4-trimethyl-1-pentanol and 3,5,5-trimethylhexanoic acid) could be identified, and only the decomposition product 2,4,4- trimethyl-1-pentanol was quantified. Because toxicity was caused not only by 2,4,4-trimethyl-1-pentanol, the concentration of total dissolved breakdown products was estimated based on the percentages of the chromatographic peak areas of the detected breakdown products in reference to the per cent peak area and concentration of 2,4,4- trimethyl-1-pentanol.

Determination of the biological effect values was based on the geometric means of the calculated concentrations of breakdown products at the beginning and at the end of the test. Additionally, the content of dissolved organic carbon in the test solutions was determined by DOC measurement at the beginning and at the end of the test. The estimated content of breakdown products and DOC were in a similar range (DOC: 7.5–220 mg/L, sum of breakdown products: 8.0–229 mg/L).

The 48-h EC50 for Daphnia magna was determined to be 32 mg/L. This value is carried forward to the risk assessment.