How do MgO and H2O react with each other

Magnesium is contained in seawater in an amount of around 1300 ppm. In addition to sodium, it is also the most abundant cation in the oceans. The magnesium content in river water is 4 ppm, in marine algae at 6000-20,000 ppm and in marine fish and oysters at 1200 ppm.
Together with other alkaline earth ions, magnesium is responsible for the hardness of the water (total hardness). Water with a high content of alkaline earth ions is called hard, and water with a few alkaline earth ions is called soft.

How and in which compounds does magnesium react with water?

Magnesium metal is not attacked by water at room temperature. Magnesium is a relatively inert element anyway, but it tarnishes when exposed to air.
It also reacts with water vapor to form magnesium oxide or magnesium hydroxide and hydrogen gas:

Mg (s) + 2H2O (g) → Mg (OH)2(aq) + H2(G)

Burning magnesium must not be extinguished with water. Magnesium even burns when there is no more oxygen. It then combines with nitrogen from the air to form magnesium nitride (Mg3N2). When attempting to extinguish the fire with water, it reacts aggressively with the released hydrogen gas. Therefore magnesium fires have to be covered with sand.
An example of a magnesium compound is magnesium phosphide (Mg3P.2), a foul-smelling gray solid. When it comes into contact with water or humidity, it is decomposed and poisonous phosphine is formed. This is self-igniting in the air due to the diphosphine that is also formed.

Water solubility of magnesium and / or its compounds

Magnesium is mainly present as Mg in aqueous solution2+(aq), but also in the form of MgOH+(aq) or Mg (OH)2(aq) present. In sea water it also occurs as MgSO4 in front.
The water solubility of magnesium hydroxide is 12 mg / L. Other magnesium compounds are much more soluble, such as magnesium carbonate with 600 mg / L. Magnesium sulfate, which causes a bitter taste in water, has a water solubility of even 309 g / L at 10OC.

Solubility and how it can be influenced

How can magnesium get into the water?

Many minerals contain magnesium, such as dolomite (calcium magnesium carbonate; CaMg (CO3)2) and magnesite (magnesium carbonate; MgCO3). The magnesium can be washed out of the rock and washed into water.
Magnesium can find many different uses and thus get into the water in e.g. production processes. In the chemical industry it is added to plastics and other materials as a fire retardant or as a filler. As a component of fertilizers and an additive in animal feed, it is particularly easy to get into the environment. Magnesium sulfate is used in breweries and magnesium hydroxide is used as a flocculant in wastewater treatment. Magnesium also acts as a mild laxative. As an alloy with other metals, it can be used for the bodies of cars and airplanes.
During the Second World War, magnesium was used for incendiary bombs to cause large-scale fires in cities. The development of these bombs eventually led to the development of a method to extract magnesium from seawater.

What environmental problems can water contamination with magnesium cause?

Magnesium is essential for almost all living things except insects. It plays an important role as a central atom in the chlorophyll molecule and is therefore necessary for the photosynthesis of plants.
Since magnesium is not only found in seawater, but also in rivers and rainwater, it naturally finds its way into the environment almost everywhere.
There are three naturally occurring isotopes of magnesium, all of which are stable and therefore non-radioactive. There are also eight unstable isotopes. Guidelines for the magnesium content of drinking water are not known, since no negative effects for humans are to be feared. There is therefore no danger to animals either.
Environmental problems that can arise indirectly through magnesium are consequences of the use of water softeners. As described above, magnesium is partly responsible for the hardness of the water. Calcium and magnesium ions (but above all calcium ions -> calcium and water) have a negative effect on the washing effect of detergents, as they form almost water-insoluble salts with soaps. That is why up to 40% water softener is added to detergents. In the past, these were often phosphates, which were so difficult to break down and which led to the eutrophication of water. In the meantime, substitutes for this have been found, which are other complexing agents (e.g. sodium citrate, EDTA and NTA) or ion exchangers (e.g. zeolite A). These do not cause eutrophication and are considered non-toxic. Nitrilotriacetate (NTA), however, may be mutagenic and, like other substitutes, is difficult to remove in the sewage treatment plant. Zeolite A increases the amount of sewage sludge produced. In addition, some of the complexing agents, such as ethylenediaminetetraacetate (EDTA), have the side effect that they can dissolve heavy metals from their normally poorly soluble compounds. Since EDTA cannot (well) be removed in the sewage treatment plant, mobile heavy metal ions can eventually end up in water. In contrast to calcium, magnesium is hardly bound by zeolites under washing conditions.
Since the hardness of the water can be very different in different regions, the amount of water softeners present in detergents is actually superfluous for regions with relatively soft water. In regions with hard water, on the other hand, detergent may be dosed in high amounts in order to add a lot of water softener to the washing process. In this case, other washing substances are superfluously dosed and pollute the wastewater. A possible solution to this problem are modular systems in which the various ingredients can be dosed depending on the water quality.

What health effects can magnesium cause in water?

Magnesium is contained in the human body in an amount of around 25 g, 60% of it in the skeleton and 40% in muscles and other tissues.
It is an essential substance for humans, which is partly responsible for membrane functions, stimulus transmission, muscle contractions, the structure of proteins and the replication of DNA. Magnesium, for example, is a component of numerous enzymes. Magnesium and calcium often have the same functions in the human body, but they often also act as antagonists.
There are no known cases of magnesium poisoning. In the case of very high levels of oral intake, vomiting and diarrhea occur. However, overdose of magnesium in medication and supplements can cause muscle weakness, sluggishness, and confusion. Humans need around 200-300 mg of magnesium daily, which can be easily achieved with a normal diet, as the element is contained in many foods. However, vegetables can lose half of their magnesium in cooking water. Too little magnesium can cause sluggishness, muscle weakness, increased neuromuscular irritability, but also depression, personality changes or a chronic fatigue syndrome.
As already mentioned, it is no longer customary to specify limit values ​​for magnesium in drinking water, as there is no scientific evidence of a health hazard.
In other compounds, e.g. as a component of asbestos, magnesium can in turn be dangerous.

Which water purification technologies can be used to remove magnesium?

Magnesium compounds are often removed from the water because they are partly responsible for the water's degree of hardness being too high. This is mainly done with the help of water softeners.
Magnesium itself can be used as a flocculant in wastewater treatment in the form of magnesium hydroxide.

References


To the periodic table of the elements

To the overview of the elements and water