Selenium (Se) is both beneficial and toxic to animals, plants, and humans. Selenium poisoning due to an excessive exposure has been well documented in the literature. Selenium poisoning was reported in livestock in western China as early as 1295, and in livestock, chickens, and children in Columbia, South America, in 1560 (Eisler 1985).
2.1 Physical and Chemical Properties
Selenium has an atomic number of 34, melting point (gray) of 217 _C, boiling point (gray) of 684.9 _C, and specific gravity ranging from 4.28 (vitreous) to 4.79 (gray) (Chemical Rubber Co., CRC, 1968). As a member of Group VIa of the periodic table, it is located between non-metallic sulphur and metallic tellurium. Naturally occurring stable isotopes include 74Se (0.87%), 76Se (9.02%), 77Se (7.58%), 78Se (23.52%), 80Se (49.82%), and 82Se (9.19%), resulting in a composite molecular weight of 78.96 (Adriano 1986).
Selenium can be prepared either with an amorphous or crystalline structure. The colour of amorphous Se is either red (in powder form) or black (in vitreous form). The crystalline monoclinic form of Se is deep red in colour while the crystalline hexagonal variety, the most stable form of Se, is metallic gray. Selenium is a member of the sulphur family and resembles sulphur both in its various forms and its compounds (CRC 1968).
Selenium, like sulphur, exists in the 2- (Se-2 or selenide), 0 (Se0 or elemental Se), 4+ (SeO3-2 or selenite), and 6+ (SeO4-2 or selenate) oxidation states. Each oxidation state exhibits different chemical behaviour. The concentration, speciation, and association of Se in a given environment depend upon pH and redox conditions, the solubility of its salts (selenates are more soluble than selenites), the complexing ability of soluble and solid ligands, biological interactions, and reaction kinetics. Selenide and elemental Se occur in acidic, reducing, and organic-rich environments. Metallic selenides, Se-sulphides, and elemental Se are insoluble, and therefore, biological unavailable. For the pH and redox conditions of most soil and aquatic environments, SeO3-2 and SeO4-2 should be the dominant forms of Se (McNeal and Balistrieri 1989).
2.2 Production and Uses
Selenium is considered a rare element because there are no large deposits anywhere in the world. It is mainly obtained as a by-product of the electrolytic refining of copper. Canada, Japan, United States of America, Mexico, Sweden, and Belgium are among the leading producers of selenium (Adriano 1986). According to Statistics Canada, the quantities of Se produced in Canada were 321 000 kg in 1988, 369 000 kg in 1990, 227 000 kg in 1991, and 286 000 kg in 1992 (Canada Year Book 1994).
Compounds of selenium are used in many ways:
· in photoelectric cells (e.g., photographic exposure meters, photometers, counting devices, and light-controlled switches);
· as maroon and orange pigments, in combination with cadmium sulphide, for plastics and ceramics;
· in increasing the resistance of rubber to heat, oxidation, and abrasion (as an accelerator and vulcanizing agent in rubber production);
· as lubricants to increase the machinability of stainless steel.
· Selenium sulphide is also used as a catalyst in the preparation of pharmaceuticals including niacin and cortisone, as an ingredient in anti-dandruff shampoos (selenium sulphide) for certain scalp conditions, and as a constituent of fungicides (selenium sulphide).
· Radioactive selenium is used in diagnostic medicine and aids in the visualization of difficult-to-study malignant tumours.
· Selenium is also used in glass manufacturing and as an alloy of steel and copper (Adriano 1986, USDHHS 1994).
In the early 1900s selenium was used as a pesticide to control cotton pests, mites and spiders that attack citrus, and mites that damage apples. The use of selenium pesticides has been discontinued, due to their stability in soils and the resultant contamination of food crops, their high price, and their proven toxicity to mammals and birds (Eisler 1985).