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Introduction

In this section a summary is provided with regard to the chemical reactivity and fate in the environment of the substance “Reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide”.

The composition of the reaction mass can be characterized as follows:

·      ca. 37% magnesium peroxide (MgO2)

·     ca. 33% magnesium hydroxide (Mg(OH)2)

·     ca. 15% magnesium oxide (MgO)

·     ca. 10% magnesium carbonate (MgCO3)

 

  

Chemical reactivity of the reaction mass of magnesium carbonate, magnesium hydroxide, magnesium oxide and magnesium peroxide

In aqueous solution, the magnesium peroxide in the reaction mass decomposes into hydrogen peroxide and magnesium hydroxide. The hydrogen peroxide will subsequently further decompose into water and oxygen. Furthermore, the magnesium oxide present in the reaction mass transforms into magnesium hydroxide upon reaction with water, and the magnesium hydroxide and magnesium carbonate dissociate when brought into aqueous medium. The different chemical reactions are:

 

MgO2+ 2 H2O -> Mg2++ 2 (OH)-+ H2O2

H2O2+ H2O -> 2 H2O + O2

MgO + H2O -> Mg2++ 2 (OH)-

Mg(OH)2-> Mg2++ 2 OH-

MgCO3-> Mg2++ CO32 -

 

Water solubility of the relevant molecular species

 

According to the CRC Handbook of chemistry and physics (73rd edition, 1992-1993) the water solubility of magnesium hydroxide and magnesium oxide is 9 and 6.2 mg/l, respectively. However the water solubility of magnesium carbonate is 106 mg/l according to this handbook.

Due to the fact that magnesium carbonate is not hazardous (no physicochemical, toxicological orecotoxicological classification following the criteria of the CLP regulation), this substance has been omitted from the risk assessment. Furthermore this constituent is only present in low concentrations in the reaction mass. Magnesium hydroxide is present in much higher concentrations in the reaction mass and it can also be formed due to the hydrolysis of magnesium peroxide.

 

Water solubility testing of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide

A water solubility study was performed that used the reaction mass as the test substance (2012, GLP, OECD 105). In this test, known amounts of the reaction mass (10, 100 and 1000 mg/L) were stirred with double distilled water for 24, 48 or 96 hours. Subsequently the water phase was centrifuged to remove all undissolved test substance and the concentration of magnesium and hydrogen peroxide in the supernatant was determined.

The results from the 10 mg/L test run show that the theoretical maximum concentration of magnesium is obtained within 24-48 hours. This means that 10 mg of the test substance was dissolved. The water solubility of the reaction mass is thus > 0.01 g/L.

Moreover, the magnesium concentrations in the 100 and 1000 mg/L experiments suggest that an equilibrium is reached during these experiments. The overall water solubility for the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide, when calculated from the experimentally determined equilibrium concentration of magnesium, is 0.031 g/L.

 

 

Hydrolysis of magnesium peroxide and subsequent decomposition of hydrogen peroxide

The water solubility test on the reaction mass of magnesium carbonate, magnesium hydroxide, magnesium oxide and magnesium peroxide also demonstrates the formation of hydrogen peroxide upon reaction of the reaction mass of magnesium carbonate, magnesium hydroxide, magnesium oxide and magnesium peroxide with water.

 

 

Presence of the hydroxyl anions

From the chemical reactions depicted in the figure above it is clear that hydroxyl anions will be formed upon addition of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide to water. Those hydroxyl anions originate from the hydrolysis of the magnesium peroxide and magnesium oxide, and from the magnesium hydroxide present in the reaction mass. This results in a solution with a high pH value, as can also be seen in the water solubility experiment described above.

 

From a toxicological point of view, the high pH value implies that the substance could show the typical local effects associated with basic solutions. The systemic effects of the hydroxyl anions are mitigated by the buffer capacity of the blood. Likewise, the buffer capacity of the environment will also attenuate the ecotoxicological effects of the hydroxyl anions.

Biodegradation and bioaccumulation

As the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide is a multi-constituent substance containing only inorganic constituents, biodegradation is not a relevant parameter. The removal of the different constituents from the environment is driven by the abiotic processes described above.

 

Bioaccumulation of the species formed upon dissolution of the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide in water can be excluded based on the following argumentation:

·     Hydrogen peroxide has not only a short half life and a low log Kow, it is also destroyed by the enzyme “catalase” which is ubiquitously present in biotic systems.

·     Both the pH effect cause by the presence of the hydroxyl anions and the intake of the essential element magnesium are actively regulated in biotic systems.

 

Transport and distribution

As the reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide rapidly hydrolyses and dissociates in aqueous environment, the potential of the thus formed chemical species to adsorp to soil and sediment is examined:

·     For hydrogen peroxide no remarkable adsorption to soil and sediment is expected based on its high polarity and water solubility. An estimated Koc value of 1.58 L/kg is available in the REACH registration dossier for hydrogen peroxide. Furthermore, Mackay Level 1 modelling indicates that the substance will partition into the aquatic compartment for > 99% at 20°C, and that negligible fractions will be found in the air, soil or sediment compartment.

·     For the hydroxyl anions a Koc or Kd value is not relevant as the behaviour of this anion depends on the buffer capacity of the receiving environment.

 

Conclusion

The reaction mass of magnesium carbonate and magnesium hydroxide and magnesium oxide and magnesium peroxide is an inorganic multi-constituent substance that will not persist in the environment nor bioaccumulate in living organisms. Abiotic processes such as hydrolysis and dissociation are responsible for the fate of this multi-constituent substance in the environment. Magnesium peroxide is hydrolysed to magnesium hydroxide and hydrogen peroxide. Magnesium oxide is hydrolysed to magnesium hydroxide. The alkaline constituents magnesium hydroxide and magnesium carbonate will be neutralized in the environment, while hydrogen peroxide will be degraded by abiotic and biotic processes to oxygen and water.

 

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