42
Selenium [Se, 34]
42.1 INTRODUCTION
Selenium (Se) is a nonmetal element of group 16 in the periodic table of elements, and
its properties are similar to Te, an element of the same group. Its mean abundance in
the Earth’s crust is estimated to be 0.05 mg/kg, but sometimes higher values, up to
0.5 mg/kg, are cited. It may be a bit concentrated in mafic rocks. In sedimentary rocks
Se is associated with clay fraction, and thus its highest contents are in argillaceous
sediments. Its enriched concentrations in Cretaceous rocks (above 100  mg/kg) are
derived from volcanic gases and dust, deposited with rain into the Cretaceous sea.
Coal contains Se within the range of 0.2–10.7 mg/kg, and in fly ash it is accumulated up to about 1.5 mg/kg. Some coal of China contain Se at about 6500 mg/kg
(Plant et al. vide Kabata-Pendias 2011). Increased Se concentrations, up to 0.1%, were
observed in some U deposits, and in some metal sulfide ores. Its elevated amounts are
also reported for phosphate rocks (Bech et al. 2010).
Selenium has variable oxidation state, from −2 to +6. The most common is +4,
whereas in organic compounds it is mainly −2. It rarely occurs in its elemental state
in nature, or as pure ore compounds. It exhibits variable geochemical properties,
chalcophilic and siderophilic, that affects its easy change of oxidation states, and
complex behavior in geochemical processes. Common Se species, selenites (Se 3+ )
and selenates (Se 6+ ), do not form stable compounds, and are preferably absorbed by
minerals, mainly clay minerals, and Fe and Mn oxyhydroxides, which are involved
in several exchange reactions. Selenium is associated also with P and S compounds.
Approximately 50 Se minerals are known, of which the relatively common are
klockmanite (CuSe), berzelianite (Cu 2–x Se), clausthalite (PbSe), tiemannite (HgSe),
ferroselenite (FeSe 2 ), crookesite [(Cu,Tl,Ag) 2 Se], and cobaltomenite [Co(SeO 3 )
2H 2 O]. Commonly Se occurs in association with some host minerals, such as pyrite,
chalcopyrite, and sphalerite.
Global Se production (rounded) in 2010 was 2260 t, of which 780 t was produced in
Japan, 680 t in Germany, 200 t in Belgium, 170 t in Canada, and 140 t in Russia (USGS
2011). Impure selenium is found in metal sulfide ores, and, commercially, it is produced as a by-product in the refining of these ores, most often during Cu production.
Various compounds of Se are used in photoelectric cells, some batteries, and in
solar cell technologies. Selenides (e.g., HgSe, PbSe, ZnSe) are good semiconductors.
It is used as a pigment (maroon and orange colors), mainly in the production of glass
and plastic, is also added to some steel alloys, and is used in lubricants for metals.
Some Se organic compounds are applied in organic synthesis.
A high proportion of Se (around 20% of the total production) is used as dietary supplement for humans and livestock. It is added, mainly as sodium selenite (Na 2 SeO 3 ),
to fertilizers, insecticides, and as supplement in the plant–animal–human food
chain. Although small amounts of Se are considered beneficial, it can be hazardous
281