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Strontium is a soft, Silver-yellow, alkaline-earth metal. It has three allotropic
crystalline forms and in its physical and
chemical properties it is similar to Calcium and
Barium. Strontium reacts
vigorously with water and quickly tarnishes in air, so it must be stored
out of contact with air and water. Due to its extreme reactivity to air,
this element always naturally occurs combined with other elements and
compounds. Finely powdered Strontium metal will ignite spontaneously in
air to produce both Strontium oxide and Strontium nitride.
Applications
Strontium has uses similar to those of Calcium and Barium, but
it is rarely employed because of its higher cost. Principal uses of
Strontium compounds are in pyrotechnics, for the brilliant reds in
fireworks and warning flares and in greases. A little is used as a
getter in vacuum tubes to remove the last traces of air. Most Strontium
is used as the Carbonate in special glass for television screens and
visual display units. Although Strontium-90 is a dangerously radioactive
isotope, it is a useful by-product of nuclear reactors from whose spent
fuel is extracted. Its high-energy radiation can be used to generate an
electric current, and for this reason it can be used in space vehicles,
remote weather stations and navigation buoys.
Strontium in the environment
Strontium is commonly occurs in nature, formung about 0.034% of all igneous
rock and in the form of the sulfate mineral celestite
(SrSO4) and the Carbonate strontianite (SrCO3). Celestite occurs frequently in sedimentary deposits
of sufficient size, thus the development of mining facilities attractive.
The main mining areas are UK, Mexico, Turkey and Spain. World production
of Strontium ores is about 140.000 tonnes per year from an unassessed
total of reserves.
Foods containing Strontium range from very low e.g. in corn (0.4 ppm
and oranged (0.5 ppm) to high, e.g. in cabbage (45 ppm), onions (50 ppm)
and lattuce (74 ppm).
Health effects of Strontium
Strontium compounds that are water-insoluble
can become water-soluble, as a result of chemical reactions. The
water-soluble compounds are a greater threat to human health than
the water-insoluble ones. Therefore, water-soluble forms of
Strontium have the opportunity to pollute drinking water.
Fortunately the concentrations in drinking water are usually quite
low.
People can be exposed to small levels of (radioactive) Strontium
by breathing air or dust, eating food, drinking water, or by
contact with soil that contains Strontium. We are most likely to
come in contact with Strontium by eating or drinking.
Strontium concentrations in food contribute to the Strontium
concentrations in the human body. Foodstuffs that contain
significantly high concentrations of Strontium are grains, leafy
vegetables and dairy products.
For most people, Strontium uptake will be moderate. The only
Strontium compound that is considered a danger to human health,
even in small quantities, is Strontium chromate. The toxic
Chromium that it contains mainly causes this. Strontium chromate
is known to cause lung cancer, but the risks of exposure have been
greatly reduced by safety procedures in companies, so that it is
no longer an important health risk.
The uptake of high Strontium concentrations is generally not known
to be a great danger to human health. In one case someone
experienced an allergic reaction to Strontium, but there have been
no similar cases since. For children exceeded Strontium uptake may
be a health risk, because it can cause problems with bone growth.
Strontium salts are not known to cause skin rashes or other skin
problems of any kind.
When Strontium uptake is extremely high, it can cause disruption
of bone development. But this effect can only occur when Strontium
uptake is in the thousands of ppm range. Strontium levels in food
and drinking water are not high enough to be able to cause these
effects.
Radioactive Strontium is much more of a health risk than stable
Strontium. When the uptake is very high, it may cause anaemia and
Oxygen shortages, and at extremely high concentrations it is even
known to cause cancer as a result of damage to the genetic
materials in cells.
Strontium in its elemental form occurs
naturally in many compartments of the environment, including
rocks, soil, water, and air. Strontium compounds can move through
the environment fairly easily, because many of the compounds are
water-soluble.
Strontium is always present in air as dust, up to a certain level.
Strontium concentrations in air are increased by human activities,
such as coal and oil combustion. Dust particles that contain
Strontium will settle to surface water, soils or plant surfaces at
some point. When the particles do not settle they will fall back
onto earth when rain or snow falls. All Strontium will eventually
end up in soils or bottoms of surface waters, where they mix with
Strontium that is already present.
Strontium can end up in water through soils and through weathering
of rocks. Only a small part of the Strontium in water comes from
dust particles from the air. Most of the Strontium in water is
dissolved, but some of it is suspended, causing muddy water at
some locations. Not much Strontium ends up in drinking water.
When Strontium concentrations in water exceed regular
concentrations, this is usually caused by human activities, mainly
by dumping waste directly in the water. Exceeded Strontium
concentrations can also be caused by settling of dust particles
from air that have reacted with Strontium particles from
industrial processes.
Strontium concentrations in soil may also be increased by human
activities, such as the disposal of coal ash and incinerator ash,
and industrial wastes. Strontium in soil dissolves in water, so
that it is likely to move deeper into the ground and enter the
groundwater. A part of the Strontium that is introduced by humans
will not move into groundwater and can stay within the soil for
decades.
Because of the nature of Strontium, some of it can end up in fish,
vegetables, livestock and other animals.
One of the isotopes of Strontium is radioactive. This isotope is
not likely to occur naturally in the environment. It ends up in
the environment, though, as a result of human activities, such as
nuclear bomb testing and radioactive storage leaking. The only way
to decrease concentrations of this isotope is through radioactive
decay to stable Zirconium.
The concentrations of radioactive Strontium in the environment are
relatively low and the particles will always end up in soils or
water-bottoms eventually, where they mix with other Strontium
particles. It is not likely to end up in drinking water.
Certain deep-sea creatures incorporate
Strontium into their shells as Strontium sulphate, and stony
corals require it, which is why it needs to be added in the
water in aquaria.
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