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Magnesium (Mg) and water

Magnesium and water: reaction mechanisms, environmental impact and health effects

Magnesium is present in seawater in amounts of about 1300 ppm. After sodium, it is the most commonly found cation in oceans. Rivers contains approximately 4 ppm of magnesium, marine algae 6000-20,000 ppm, and oysters 1200 ppm.
Dutch drinking water contains between 1 and 5 mg of magnesium per liter.
Magnesium and other alkali earth metals are responsible for water hardness. Water containing large amounts of alkali earth ions is called hard water, and water containing low amounts of these ions is called soft water.

In what way and in what form does magnesium react with water?

Magnesium metals are not affected by water at room temperature. Magnesium generally is a slow-reacting element, but reactivity increases with oxygen levels. Furthermore, magnesium reacts with water vapor to magnesium hydroxide and hydrogen gas:

Mg (s) + 2H2O(g) -> Mg(OH)2(aq) + H2(g)

Magnesium fires cannot be extinguished by water. Magnesium continues to burn after oxygen is depleted. It than reacts with nitrogen from air to form magnesium nitride (Mg3N2). When attempts are made to extinguish magnesium fires with water, magnesium aggressively reacts with hydrogen gas. To prevent any damage, a magnesium fire must be covered in sand.
An example of a magnesium compound is magnesium phosphide (Mg3P2), an odorous, grey solid. When this compound comes in contact with water or moist air, it is decomposed and phosphine (PH3) is formed. This is a toxic compound, and it is also very flammable in air.

Solubility of magnesium and magnesium compounds

Magnesium is mainly present as Mg2+ (aq) in watery solutions, but also as MgOH+ (aq) and Mg(OH)2 (aq). In seawater it can also be found as MgSO4.
Water solubility of magnesium hydroxide is 12 mg/L. Other magnesium compounds are clearly more water soluble, for example magnesium carbonate (600 mg/L). Magnesium sulphate adds a bitter flavour to water, and has a water solubility of 309 g/L at 10oC.

Why is magnesium present in water?

A large number of minerals contains magnesium, for example dolomite (calcium magnesium carbonate; CaMg(CO3)2) and magnesite (magnesium carbonate; MgCO3). Magnesium is washed from rocks and subsequently ends up in water.
Magnesium has many different purposes and consequently may end up in water in many different ways. Chemical industries add magnesium to plastics and other materials as a fire protection measure or as a filler. It also ends up in the environment from fertilizer application and from cattle feed. Magnesium sulphate is applied in beer breweries, and magnesium hydroxide is applied as a flocculant in wastewater treatment plants. Magnesium is also a mild laxative. Magnesium alloys are applied in car and plane bodies.
During World War II magnesium was applied in fire bombs, to cause major fires in cities. The development of these bombs introduced a method to extract magnesium from seawater.

What are the environmental effects of magnesium in water?

Magnesium is a dietary mineral for any organism but insects. It is a central atom of the chlorophyll molecule, and is therefore a requirement for plant photosynthesis. Magnesium cannot only be found in seawater, but also in rivers and rain water, causing it to naturally spread throughout the environment.
Three magnesium isotopes occur naturally, which are all stable and consequently not radioactive. There are also eight instable isotopes.
Guidelines for magnesium content in drinking water are unlikely, because negative human and animal health effects are not expected.
Environmental problems indirectly caused by magnesium in water are caused by applying softeners. As was described earlier, hardness is partially caused by magnesium. Calcium and magnesium ions (particularly calcium) negatively influence cleansing power of detergents, because these form nearly insoluble salts with soap. Consequently, about 40% softener is added to soap. This used to be phosphates, but it was discovered that these where hardly biodegradable, and caused eutrophication. Today, alternative chemicals are applied, mainly complexing agents such as sodium citrate, EDTA and NTA, or ion exchangers such as zeolite A. These substances do not cause eutrophication and are not toxic. Nitriloacetic acid (NTA) may be mutagenic, and is difficult to remove during water purification. Zeolite A increases the amount of sludge. Additionally, other complexing agents such as ethylenediaminetetraacetic acid (EDTA) have the audacity to remove metals from compounds that are otherwise difficult to decompose. Mobile heavy metals may end up in water ways, because EDTA is difficult to remove in wastewater purification plants. Contrary to calcium, magnesium is not bound to zeolites under washing conditions.
Water hardness may differ per region, therefore adding softeners to detergents is unnecessary for regions that only contain soft water. In regions containing hard water higher doses of detergent may be applied, in order to add more softener. This causes other substances in detergents to be dosed to high, thereby complicating the wastewater treatment process. A possible solution to this problem is dosing different compounds in detergents yourself.

What are the health effects of magnesium in water?

he human body contains about 25 g of magnesium, of which 60% is present in the bones and 40% is present in muscles and other tissue. It is a dietary mineral for humans, one of the micro elements that are responsible for membrane function, nerve stimulant transmission, muscle contraction, protein construction and DNA replication. Magnesium is an ingredient of many enzymes. Magnesium and calcium often perform the same functions within the human body and are generally antagonistic.
There are no known cases of magnesium poisoning. At large oral doses magnesium may cause vomiting and diarrhoea. High doses of magnesium in medicine and food supplements may cause muscle slackening, nerve problems, depressions and personality changes.
As was mentioned before, it is unusual to introduce legal limits for magnesium in drinking water, because there is no scientific evidence of magnesium toxicity. In other compounds, for example asbestos, magnesium may be harmful.

Which water purification technologies can be applied to remove magnesium from water?

Magnesium compounds are usually removed from water, because of the role magnesium plays in water hardness. This is achieved by means of water softening.
Magnesium hydroxide is applied as a flocculant in water purification.

Literature and the other elements and their interaction with water

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