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Toxicity to microorganisms

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Description of key information

No studies on magnesium carbonate are available.
The toxicity of magnesium chloride hexahydrate to aquatic microorganisms has been investigated in a GLP study performed in accordance with OECD Test Guideline 209. The effect of magnesium chloride hexahydrate on the respiration of activated sewage sludge micro-organisms gave a 3 h EC50 of >900 mg/L, the highest concentration tested, and is considered to be non-toxic under the conditions of the study.
The result from this study can be read across to magnesium carbonate and hence the equivalent EC50 for magnesium carbonate can be estimated to be >372 mg/L.

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Additional information

Rationale for read across:

Magnesium chloride hexahydrate is significantly more soluble in water than magnesium carbonate and therefore represents the worst case in terms of bioavailability in natural waters. Although magnesium carbonate is less soluble, both magnesium chloride hexahydrate and magnesium carbonate are expected to dissociate to their ionic forms in water and therefore the toxicity results can be read across.

A GLP guideline study was performed which assessed the toxicity of magnesium chloride hexahydrate (Eurofins-GAB GmbH, 2010) to aquatic microorganisms. The 3 h EC50 for magnesium chloride hexahydrate was found to be >900 mg/L, the highest concentration tested, which is equivalent to >372 mg/L for magnesium carbonate. Although slight inhibitory effects were noted in the main limit test, the test material can be regarded as non toxic to activated sludge organisms.

The water solubility of magnesium chloride hexahydrate is significantly greater than magnesium carbonate and hence the toxicity study with the more soluble chloride salt can be considered the worst case. The lack of toxicity seen in this study therefore suggests that magnesium carbonate would not be toxic up to the limit of its solubility in water (110 mg/L at 20 °C).

Magnesium is naturally abundant in the environment meaning that aquatic microorganisms are constantly exposed to magnesium without suffering from any adverse or detrimental effects. Indeed, each of the Mg2+, Cl- and CO3^2- ions are ubiquitous and are not considered to pose a risk of toxicity.Magnesium is present in all natural waters and is a major contributor to water hardness. Water from areas rich in magnesium-containing rocks may contain magnesium in the concentration range 10 to 50 mg/L. The sulfates and chlorides of magnesium are very soluble, and water in contact with such deposits may contain several hundred milligrams of magnesium per litre.

The abundance of magnesium in the environment and the lack of toxicity in the presented study demonstrate that any further testing is scientifically unjustified.