ABSTRACT
Selenium (Se) is an essential nutritional element, but excessive Se can be toxic to animals and humans. Selenium has an atomic number of 34, an atomic weight of 78.94 and occupies a position in Group VIA of the periodic table between the metal tellurium and the non-metal sulfur. Selenium’s chemical and physical properties are intermediate between those of metals and non-metals (Table 1). Selenium has a valence of 2 in combination with hydrogen or metals, and in oxygenated compounds it can exist as the þ4 or the þ6 oxidation states giving rise to an array of Se compounds.[1] Six stable Se isotopes occur with varying degrees of abundance: 74Se (0.87%), 76Se (9.02%), 77Se (7.58%), 78Se (23.52%), 80Se (49.82%), and 82Se (9.19%) and a short-lived isotope (75Se) used in neutron activation, radiology, and tracer applications.[2]
The average Se concentration in the earth’s crust is about 0.05-0.09 mg kg1.[3] Selenium concentrations range from 0.004-1.5 mg g1 in igneous rocks to 0.6103 mg g1 in shales of the cretaceous period.
