The Element Selenium

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 (Se⁰), selenide (Se^2-), selenite (SeO₃^2-), and selenate (SeO₄^2-). 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, SeO₂, H2SeO₃, SeF₄, SeCl₂, selenic acid (H2SeO₄), Na₂SeO₃, Na₂SeO₄, 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 3¹⁺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 Se⁰ is heated above 400⁺C in air, it becomes the very pungent and highly toxic gas H2Se. This gas decomposes in air back to Se⁰ and water (10).

Reduction or oxidation of elemental selenium can be to the -2-oxidation state (Se^2-), the +4-oxidation state (SeO₃^2-), or the +6-oxidation state (SeO₄^2-). The Se^2- ion is water-soluble (270 ml per 100 ml H₂O 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 (SeO₂), SeO₃^2-, or selenious acid (H2SeO₃). Selenium dioxide is water-soluble (38.4 g per 100 ml H₂O at 14°C) and is produced when Se⁰ is burned or reacts with nitric acid. Reduction back to Se⁰ can be carried out in the presence of ammonium, hydroxylamine, or sulfur dioxide. In hot water, SeO₂ will dissolve to H2SeO₃, 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, SeO₃^2- is reduced to Se⁰ by ascorbic acid or sulfur dioxide. In the soil, SeO₃^2- 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 (H2SeO₄) or SeO₄^2- salts. Selenic acid is formed by the oxidation of H2SeO₃ and is a strong, highly soluble acid. Selenate salts are soluble, whereas SeO₃^2- salts and metal Se^2- salts are sparingly soluble. Their solubilities and stabilities are the greatest in alkaline environments. Conversion of SeO₄^2- to the less-stable SeO₃^2- and to Se⁰ occurs very slowly (10).