Selenium
Selenium is a non metallic chemical element, member of the group XVI
of the periodic table. In chemical activity and physical properties it
resembles sulfur and
tellurium. Selenium appearsin a number of
allotropic forms: the most popular are a red amorphous powder, a red crystalline material, and
a gray crystalline metallike form called metallic selenium. This last
form conducts electricity better in the light than in the dark and is
used in photocells. Selenium burns in air and is uneffected by water,
but dissolves in concentrated nitric acid and alkalis.
Applications
Selenium has good photovoltaic and photoconductive properties, and it is used extensively in electronics, such as photocells, light meters and
solar cells. The second largest use of selenium is in the glass
industry: selenium is used to remove colour from glass, to give a red
colour to glasses and enamels. The third min use, taking about 15% is
sodium selenite for animal feeds and food supplements. Selenium can also
find applications in photocopying, in the toning of photographs. Its
artistic use is to intensify and extend the tonal range of black and
white photographic images. Other uses of selenium are in metal alloys
such as the lead plates used in storage batteries and in rectifiers to
convert AC current in DC current. Selenium is used to improve the
abrasion resistance in vulcanized rubbers. Some selenium compounds are
added to anti-dandruff shampoos.
Selenium in the environment
Selenium is among the rarer elements on the surface of this planet,
and is rarer than silver. Selenium is present in the atmosphere as metyl
derivatives. Uncombined selenium is occasionally found and there are
around 40 known selenium-conaining minerals, some of which can have as
much as 30% selenium - but all are rare and generally they occur
together with sulfides of metals such as copper, zinc and lead. The main
producing countries are Canada, USA, Bolivia and Russia. Global
industrial production of selenium is around 1500 tonnes a year and about
150 tonnes of selenium are recycled from industrial waste an reclaimed
from old photocopiers.
Selenium occurs naturally in the environment.
It is released through both natural processes and human
activities. Well fertilized agricultural soil generally has about
400 mg/ton since the element is naturally present in phosphate
fertilizers ans is often added as a trace nutrient. In its natural form as an element selenium cannot be
created or destroyed, but selenium does have the ability to change
form.
Selenium levels in soils and waters increase, because selenium
settles from air and selenium from waste also tends to end up in
the soils of disposal sites. When selenium in soils does not react
with oxygen it remains fairly immobile. Selenium that is immobile
and will not dissolve in water is less of a risk for organisms.
The oxygen levels in the soil and the acidity of the soil will
increase mobile forms of selenium. Higher oxygen levels and
increased acidity of soils is usually cause by human activities,
such as industrial and agricultural processes.
When selenium is more mobile, the chances of exposure to its
compounds will be greatly enhanced. Soil temperatures, moisture,
concentrations of water-soluble selenium, the season of the year,
organic matter content and microbial activity determine how fast
selenium will move through soil. In other words, these factors
determine its mobility.
Agriculture cannot only increase the selenium content in soil; it
can also increase selenium concentrations in surface water, as
selenium is brought along in irrigation drainage water.
|
Humans may be exposed to selenium in several
different ways. Selenium exposure takes place either through food
or water, or when we come in contact with soil or air that
contains high concentrations of selenium. This is not very
surprising, because selenium occurs naturally in the environment
extensively and it is very widespread.
The exposure to selenium mainly takes place through food, because
selenium is naturally present in grains, cereals and meat. Humans
need to absorb certain amounts of selenium daily, in order to
maintain good health. Food usually contains enough selenium to
prevent disease caused by shortages.
Selenium uptake through food may be higher than usual in many
cases, because in the past many selenium-rich fertilizers have
been applied on farmland.
People that live near hazardous waste-sites will experience a
higher exposure through soil and air. Selenium from hazardous
waste-sites and from farmland will end up in groundwater or
surface water through irrigation. This phenomenon causes selenium
to end up in local drinking
water, so that exposure to selenium
through water will be temporarily increased.
People that work in metal industries, selenium-recovery industries
and paint industries also tend to experience a higher selenium
exposure, mainly through breathing. Selenium is released to air
through coal and oil combustion.
People that eat a lot of grains that grow near industrial sites
may experience a higher exposure to selenium through food.
Exposure to selenium through drinking water may be increased when
selenium from hazardous waste disposals ends up in water wells.
Exposure to selenium through air only comes about in the workplace
usually. It can cause dizziness, fatigue and irritations of the
mucous membranes. When the exposure is extremely high, collection
of fluid in the lungs and bronchitis may occur.
Selenium uptake through food is usually high enough to meet human
needs; shortages rarely occur. When shortages occur people may
experience heart and muscle problems.
When selenium uptake is too high health effects will be likely to
come about. The seriousness of these effects depends upon the
concentrations of selenium in the food and how often this food is
eaten.
The health effects of various forms of selenium can vary from
brittle hair and deformed nails, to rashes, heat, swelling of the
skin and severe pains. When selenium ends up in the eyes people
experience burning, irritation and tearing.
Selenium poisoning may become so severe in some cases that it can
even cause death.
Overexposure of selenium fumes may
produce accumulation of fluid in the lungs,
garlic breath, bronchitis, pneumonitis, bronchial asthma, nausea,
chills, fever, headache, sore throat, shortness
of breath, conjunctivitis, vomiting, abdominal
pain, diarrhea and enlarged liver. Selenium is an eye and upper respiratory irritant and a sensitizer.
Overexposure may result in red
staining of the nails, teeth and hair. Selenium dioxide reacts
with moisture to form selenious acid, which is
corrosive to the skin and eyes. Carcinogenicity-
The International Agency for Research on Cancer (IARC) has listed selenium within Group 3 (The agent is
not classifiable as to its carcinogenicity
to humans.)
|
|
Low levels of selenium can end up in soils or water through
weathering of rocks. It will than be taken up by plants or end up
in air when it is adsorbed on fine dust particles. Selenium is
most likely to enter the air through coal and oil combustion, as
selenium dioxide. This substance will be converted into selenium
acid in water or sweat.
Selenium substances in air are usually broken down to selenium and
water fairly quickly, so that they are not dangerous to the health
of organisms.
The behaviour of selenium in the environment strongly depends upon
its interactions with other compounds and the environmental
conditions at a certain location at a certain time.
There is evidence selenium can accumulate in the body tissues of
organisms and can than be passed up through the food chain.
Usually this bio magnification of selenium starts when animals eat
a lot of plants that have been absorbing large amounts of
selenium, prior to digestion. Due to irrigation run-off
concentrations of selenium tend to be very high in aquatic
organisms in many areas.
When animals absorb or accumulate extremely high concentrations of
selenium it can cause reproductive failure and birth defects.
|
Sources
of periodic table.
Back to the
periodic table of elements.
Recommended daily intake of
selenium
|
|
