Selenium speciation analyses in water and sediment matrices
Résumé
From the time selenium was discovered in 1817 by Berzelius and Gahn [1] until the recognition in the mid 1950s of its toxicity and, subsequently, its essentiality for animal well-being, a large number of its properties have been elucidated. This element ranks seventieth in abundance among the elements and constitutes approximately 105% of the earth's crust. Selenium is located in the VIa Group of the Periodic Table between sulphur and tellurium, and its chemical and physical properties are between these elements, i.e. intermediate between nonmetals and the metals, although similarity in behaviour of selenium and sulphur is more pronounced than that of selenium and tellurium. Its atomic number is 34 and its mass of 78.96 results from the six natural isotopes which exist under normal conditions 74Se (0.87%), 76Se (9.02%), 77Se (7.58%), 78Se (23.52%), 80Se (49.82%) and 82Se (9.19%). Other artificial short-lived man-made isotopes such as 75Se, 77Se are utilized in neutron activation and radiology [2]. Selenium has a melting point of 217 °C and a boiling point of 685 °C. Like sulphur it can exist in three allotropic forms: a gray or “metallic”,thermodynamically stable, hexagonal form; a red monoclinic form; and a vitreous amorphous form. Selenium is mainly found in metal sulphide deposits, mostly of the metals Cu, Zn, Ag, Hg, and Pb and is obtained industrially as a by-product of the processing of these elements. It is mainly used in the electrical and electronic (23%), rubber, plastics and lubricants (13%), ceramics and glass 27%) and other chemical industries [3], Table 1. Selenium pollution is caused by these industries and by fossil fuel combustion which releases the element into the atmosphere. Analytical methodologies and applications for the determination of total selenium at trace level have been developed and used extensively in the last thirty years. The reason for determining selenium are multiple: a) the long established selenium poisoning of domestic animals foraging on seleniferous plants, b) disorders in humans and animals resulting from selenium deficiencies and excesses, c) the nutritional essentiality of the element, d) the protective effect of selenium against the toxicity of the metals such as Hg and As, e) the metabolic interaction between selenium and vitamin E and other antioxidants and f) the reported carcinogenicity, anticarcinogenicity, antimutagenicity and anticlastogenicity and its effect on dietary intake of selenium [4] (Fig.l). Table 1: Industrial uses of different Se compounds (adapted from ref. [3]) It is now well known that it is the chemical form of elements rather than their concentration which determines their role in the living organism. This is particularly so for elements such as, Se, Hg, etc., which exist in many different forms and oxidation states. Natural waters, sediments and air are the most commonly studied environmental media. Robberecht and Van Grieken [5] have extensively reviewed the literature on total selenium in natural waters of river (content 0.016-20 ng.l1), lake (0.1-1.85ng.11), open ocean (0.025-0.2ng.11)and coastal or harbour environments, but an exhaustive review of selenium species in environmental samples has not yet been published. © 1995, Elsevier B.V.