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Short-term toxicity to aquatic invertebrates

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Description of key information

A reliable key study is available that assesses the short-term toxicity of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide to aquatic invertebrates. The resulting 48h-EC50 is 56 mg/L.
From a supporting study on hydrogen peroxide it can be concluded that the ecotoxicological effects observed for the reaction mass are less pronounced than would be expected based on the amount of hydrogen peroxide that can theoretically be generated from the magnesium peroxide present in the reaction mass. From these results it can furthermore be concluded that, for the ecotoxicological assessment of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide, the read-across to data available for hydrogen peroxide is justified.

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates
Effect concentration:
56 mg/L

Additional information

The short-term toxicity of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide to aquatic invertebrates has been assessed in a recent study according to OECD guideline 202 and other internationally accepted guidelines (Tobor-Kaplon, 2012).

In this key study, the daphnia were exposed to nominal concentrations of 1.0, 3.2, 10, 32 and 100 mg/L. As the test material was not completely soluble in the test medium at these initial loading rates, the test solutions were filtered through a 0.45 µm membrane filter to remove the undissolved particles. The concentration of the test substance in the test media was verified based on measurements of the magnesium and hydrogen peroxide concentration. The average exposure concentrations were calculated to be 0.50, 1.2, 2.9, 5.8 and 12 mg/L, respectively. The 48-h EC50 derived in this study is 56 mg/L.

 

Furthermore, some supporting information is available on hydrogen peroxide (H2O2) and magnesium hydroxide (MgOH)2), the primary hydrolytic degradation products of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide.

 

For magnesium hydroxide, the short-term toxicity test with Daphnia magna (Heinen, 2005) was carried out according to the OECD 202 guideline taking into account GLP. A control plus seven test substance concentrations were tested, ranging from 96.88 mg/L to 6200 mg/L. The test substance contained 61% of magnesium hydroxide in an aqueous suspension. Survival of the daphnia after 48 hours ranged from 0% in the highest dose group to 96.88% in the lowest dose group. The LC50 was calculated to be 284.76 mg/L based on the test material containing 61% magnesium hydroxide. This value was recalculated towards an LC50for pure magnesium hydroxide: 170.86 mg/L.

The acute toxicity of hydrogen peroxide to invertebrates was tested in a 48h experiment with Daphnia pulex (Shurtleff, 1989). The test was performed in 1989 according to US EPA TSCA guidelines. The 48h-LC50 value determined from this study was 2.4 mg/L. As the 48h-LC50value for hydrogen peroxide is lower than the value found for magnesium hydroxide, the former was selected for the assessment of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide. From this 48h-LC50 value a corresponding 48h-LC50 value can be calculated for the magnesium peroxide reaction mass, by taking into account the applicable chemical reaction (i.e. the amount of hydrogen peroxide formed is equimolar to the amount of magnesium peroxide present in the reaction mass) and the composition of the magnesium peroxide reaction mass (i.e. the reaction mass contains ca. 37% MgO2).

 

Thus: 100 mg of the reaction mass contains 37 mg of magnesium peroxide,which corresponds to 0.66 mmol of magnesium peroxide. Therefore, 0.66 mmol (= 22.35 mg) of hydrogen peroxide is formed upon dissolution of 100 mg of the reaction mass.

 

As a consequence, the 48h-EC50 for the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide can be calculated from the 48h-EC50 for hydrogen peroxide(2.4 mg/L):

100 mg/L reaction mass         yields              22.35 mg/L hydrogen peroxide

10.7 mg/L reaction mass        yields              2.4 mg hydrogen peroxide

==> the 48h-EC50 (reaction mass) = 10.7 mg/L

 

Comparing the EC50 value derived from the 48h study on the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide with the EC50 value recalculated from the study using hydrogen peroxide, it is clear that the latter is a factor of 5 lower. The toxic effects exerted by the magnesium peroxide reaction mass are thus less pronounced than would be expected based on the amount of hydrogen peroxide that can theoretically be generated from the magnesium peroxide present in the reaction mass. This effect can possibly be explained by the fact that that hydrogen peroxide is released slowly from the magnesium peroxide present in the test medium, while the hydrogen peroxide itself is rapidly degraded in the system. As a consequence, the maximum concentration of hydrogen peroxide that is reached in the test medium is on every point in time below the theoretical maximum concentration.

From these results it can furthermore be concluded that, for the ecotoxicological assessment of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide, the use of data available for hydrogen peroxide is acceptable. Therefore, the read-across approach used in this dossier is considered justified.

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