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Toxicity to aquatic algae and cyanobacteria

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Description of key information

The ErC50 (72 h) of bis(3,5,5-trimethylhexanoyl) peroxide (75 % in isododecane) for growth inhibition in Desmodesmus subspicatus was determined to be 41 mg/L. This value was derived in a OECD 201 study with special pre-incubation of the test item (similar like WAF, to maximise actual test concentrations of this poorly water soluble, yet rapidly hydrolysing substance).

Key value for chemical safety assessment

EC50 for freshwater algae:
41 mg/L
EC10 or NOEC for freshwater algae:
10 mg/L

Additional information

The effects of di-(3,3,5-trimethylhexanoyl) peroxide (75 % in isododecane) on growth of freshwater algae were investigated in a study according to OECD 201 over 72 hours (Gancet et al., 2004). Algae were exposed to a range of test substance concentrations dissolved in dilution water. The toxic effect measured during the assay was the inhibition of cell multiplication over 72 hours.

As the test substance is poorly soluble in water, a water accommodated fraction (WAF) was prepared at a nominal loading of 100 mg/L for the definitive test. The cell density was measured daily. Analytical chemistry (LC-MS/MS) for the test item and physico-chemical measurements were carried out at the beginning and the end of the test. Initial and final concentrations were not equivalent to nominal concentrations, but below the limit of detection of the method (LOD = 0.01 mg/L). Thus, it was not possible to determine the ErC50in this study (NOELR = 100 mg/L).

The above test was done according to the WAF procedure, but no adverse effects were detected at the limit concentration. The measured concentration of the test item, however, was below the LOD of the analytical method (0.01 mg/L). This suggests that the WAF equilibration time (approx. 24 h) might have been insufficient for generating a concentration eliciting a biological effect. Therefore, a new study was conducted.

This new study (Muckle, 2013) for the assessment of the toxicity of bis(3,5,5-trimethylhexanoyl) peroxide (75 % in isododecane) to algae was conducted according to OECD 201 (resp. EU C.3) with the alga Desmodesmus subspicatus at five nominal concentrations ranging from 10000 to 460 mg/L over 72 hours.

Test solutions were prepared by mixing the test item with nutrient medium at a nominal loading of 10 g/L, followed by vigorous stirring at 37 ± 2 °C for 7 days in order 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. Since volatilisation from the aqueous phase (most likely attributable to the degradation products) was observed in pilot studies, the dissolution and equilibration procedure was performed in tightly sealed vessels. The lower treatments were prepared by diluting with nutrient medium.

Actual test substance concentrations were determined by measurement of the breakdown product 2,4,4-trimethyl-1-pentanol 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 percentage of the peak area of the detected breakdown products in reference to the percentage peak area and concentration of 2,4,4-trimethyl-1-pentanol. 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 19–381 mg/L, sum of breakdown products 10–382 mg/L).

Cell concentrations were determined by measuring light absorption. Effect values were estimated based on the geometric mean of the calculated concentration of breakdown products at the beginning and at the end of the study. The ErC50(72 h) for Desmodesmus subspicatus was determined to be 41 mg/L. This value is taken forward to the hazard assessment.