Selenium (Se), a beneficial element, is one of the most widely distributed elements on Earth, having
an average soil abundance of 0.09 mg kg -1 (1). It is classified as a Group VI A metalloid, having
metallic and nonmetallic properties. Selenium was identified in 1818 by the Swedish chemist Jöns
Jacob Berzelius as an elemental residue during the oxidation of sulfur dioxide from copper pyrites
in the production of sulfuric acid (2). The name selenium originates through its chemical similarities
to tellurium (Te), discovered 35 years earlier. Tellurium had been named after the Earth (tellus in
Latin), so selenium was named for the moon (selene in Greek) (3). Although selenium is not considered
as an essential plant micronutrient (4), it is essential for maintaining mammalian health (5).
Selenium deficiency or toxicity in humans and livestock is rare, but can occur in localized areas (5,6) owing to low selenium contents in soils and locally produced crops (7). Recently, much attention has
been given to the role of selenium in reducing certain types of cancers and diseases. Efforts in plant
improvement have begun to enhance the selenium content of dietary food sources. |
Selenium Chemistry
Selenium has an atomic number of 34 and an atomic mass of 78.96. The atomic radius of Se is 1.40 �,
the covalent radius is 1.16 �, and the ionic radius is 1.98 �. The ionization potential is 9.74 eV, the
electron affinity is - 4.21 eV, and the electronegativity is 2.55 on the Pauling Scale
(8). The chemical
and physical properties of selenium are very similar to those of sulfur (S). Both have similar
atomic size, outer valence-shell electronic configurations, bond energies, ionization potentials, electron
affinities, electronegativities, and polarizabilities
(8). Selenium can exist as elemental selenium
(Se
0), selenide (Se
2-), selenite (SeO
32-), and selenate (SeO
42-). There are six stable isotopes of selenium
in nature:
74Se (0.87%),
76Se (9.02%),
77Se (7.58%),
78Se (23.52%),
80Se (49.82%), and
82Se
(9.19%)
(8). Some of the commercially available forms of selenium are H2Se, metallic selenides,
SeO
2, H2SeO
3, SeF
4, SeCl
2, selenic acid (H2SeO
4), Na
2SeO
3, Na
2SeO
4, and various organic Se
compounds
(9).
In the elemental form, selenium exists in either an amorphous state or in one of three crystalline
states. The amorphous form of selenium is a hard, brittle glass at 3
1+C, vitreous at 31 to 230
+C, and
liquid at temperatures above 230°C
(10). The first of three crystalline states takes the form of flat
hexagonal and polygonal crystals called a-monoclinic or red selenium. The second form is the prismatic
or needle-like crystal called �-monoclinic or dark-red selenium. The third crystalline state is
made up of spiral polyatomic chains of Sen, often referred to as hexagonal or black selenium.
The black forms of crystalline Se are the most stable. At temperatures above 110
+C, the monoclinic
amorphous forms convert into this stable black form. Conversion of the amorphous form into the
black form occurs readily at temperatures of 70 to 210
+C. When Se
0 is heated above 400
+C in air,
it becomes the very pungent and highly toxic gas H2Se. This gas decomposes in air back to Se
0 and
water
(10).
Reduction or oxidation of elemental selenium can be to the -2-oxidation state (Se
2-), the +4-oxidation state (SeO
32-), or the +6-oxidation state (SeO
42-). The Se
2- ion is water-soluble
(270 ml per 100 ml H
2O at 22.
5+C) and will react with most metals to form sparingly soluble metal
selenides. Selenium in the +4-oxidation state can occur as selenium dioxide (SeO
2), SeO
32-, or
selenious acid (H2SeO
3). Selenium dioxide is water-soluble (38.4 g per 100 ml H
2O at 14°C) and is
produced when Se
0 is burned or reacts with nitric acid. Reduction back to Se
0 can be carried out in
the presence of ammonium, hydroxylamine, or sulfur dioxide. In hot water, SeO
2 will dissolve to
H2SeO
3, which is weakly dibasic. Organic selenides, which are electron donors, will oxidize readily
to the higher oxidation states of selenium. Selenites are electron acceptors. At low pH, SeO
32-
is reduced to Se
0 by ascorbic acid or sulfur dioxide. In the soil, SeO
32- is bound strongly by hydrous
oxides of iron and is sparingly soluble at pH 4 to 8.5
(10).
In the +6-oxidation state, selenium is in the form of selenic acid (H2SeO
4) or SeO
42- salts.
Selenic acid is formed by the oxidation of H2SeO
3 and is a strong, highly soluble acid. Selenate salts
are soluble, whereas SeO
32- salts and metal Se
2- salts are sparingly soluble. Their solubilities and
stabilities are the greatest in alkaline environments. Conversion of SeO
42- to the less-stable SeO
32-
and to Se
0 occurs very slowly
(10).
