Reaction mass of Magnesium peroxide and magnesium carbonate and magnesium hydroxide and magnesium oxide

Endpoint summary

Additional information

 

For this REACH registration dossier of the ‘reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide’, several ecotoxicity endpoints were assessed based on test data that are available for hydrogen peroxide and magnesium hydroxide.

As described in the section on environmental fate and pathways, hydrogen peroxide is a degradation product that is formed upon hydrolysis of magnesium peroxide. Furthermore, the hydrolysis of magnesium oxide as well as magnesium peroxide gives rise to the formation of magnesium hydroxide. In ECHA’s Guidance document on information requirements and chemical safety assessment it is mentioned that testing on the hydrolysis product can be justified if the DT₅₀ (50% disappearance time) is very short, i.e. less than 3 days.

 

MgO₂+ 2 H₂O -> Mg^{2 +}+ 2 (OH)^-+ H₂O₂

H₂O₂ + H₂O -> 2 H₂O + O₂

MgO + H₂O -> Mg^{2 +}+ 2 (OH)^-

Mg(OH)₂ -> Mg^{2 +}+ 2 OH^-

MgCO₃ -> Mg²⁺ + CO₃²

 

 

The DT₅₀ value of magnesium peroxide and magnesium oxide can be estimated from the water solubility study:

In the 10 mg/L experiment, the maximally attainable magnesium concentration is reached within 24 to 48 hours. As shown in the reactions depicted above, the dissolution of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide implies hydrolysis of the magnesium oxide and magnesium peroxide present in the reaction mass. As a consequence it can be concluded that, under the conditions of the test (double distilled water, 20°C), the DT₅₀ for the disappearance of magnesium oxide and magnesium peroxide in an aqueous environment will be well below 96 hours.

 

Therefore, the use of test results obtained for the hydrolysis products hydrogen peroxide and magnesium hydroxide for the assessment of ecotoxicological endpoints of the REACH registration dossier is considered acceptable and justified. It is to be noted that the hydrogen peroxide that is formed upon hydrolysis of magnesium peroxide itself undergoes further degradation.

 

According to the applicable chemical reaction (see above), the amount of hydrogen peroxide formed is equimolar to the amount of magnesium peroxide present in the reaction mass. As the concentration of magnesium peroxide in the reaction mass is ca. 37%, 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.

 

For example, a 96h-LC₅₀ of 16.4 mg/L for hydrogen peroxide can be extrapolated to result in the following 96h-LC₅₀ for the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide:

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

73.4 mg/L reaction mass        yields              16.4 mg hydrogen peroxide

The extrapolated 96h-LC₅₀ for the multi-constituent substance is: 73.4 mg/L.