Selenium falls below sulfur in the periodic table and resembles the latter element in many respects. Like S, in its inorganic environmental chemistry it exhibits four oxidation states; Se(VI) (selenates), Se(IV) (selenites), Se(0) (as the element), and Se(-II) (selenides). Metal selenides are found frequently with sulfides, and consequently selenium compounds may appear in the wastes from processing of sulfide ores. Reactions that produce SO2, including combustion of fossil fuels, also produce SeO2, but this compound is readily reduced by SO2 to elemental selenium. In the elemental form, Se is insoluble and largely unavailable to organisms. Selenate resembles sulfate and is the most stable oxidized form of the element, although selenite is also comparatively stable and is believed to be the most active form biologically. In any event, interconversion of the two forms is slow, and selenite is a common environmental form of this element. Salts of both tend to be soluble. Selenides, like sulfides, are often very insoluble. Selenium and its compounds are employed in electronic and photoelectric devices, in glass for color, and in photocopying equipment.

The toxic nature of selenium is well known; effects vary with species, but in humans loss of hair and nails, skin lesions, and respiratory failure have been reported. Because of the acute toxicity, there has been a considerable emphasis on reducing selenium emissions. Selenium in the soil can be taken up by plants, which may be toxic to animals that consume them, as has occurred in some areas of the western United States. Ordinary grasses and cereals can absorb toxic levels, but some plant species can achieve very high degrees of concentration. Concentration of selenium compounds leached from soils by irrigation water and entering wetlands also has been problematic for wildlife (Section 11.7). Availability of soil selenium to plants varies with the total selenium content and soil acidity. The selenium in alkaline soils is chiefly present as soluble selenates that are readily taken up by plants (hence the early name "alkali disease" given to the symptoms of cattle that have eaten vegetation high in selenium), while under more acidic conditions the selenium seems to be present as selenites or elemental selenium. The latter is the predominant state under moderate and low pE conditions, and as already mentioned is insoluble and comparatively inert. Soil organisms have been shown to be able to convert soluble selenates to the insoluble elemental form.

Bacteria, perhaps other organisms in water sediments and sewage, and some plants can convert inorganic forms of selenium to volatile organic products such as dimethyl selenide, (CH3)2Se, and dimethyl diselenide, (CH3)2Se2, much as inorganic mercury is converted to volatile methylmercury compounds. This may be a means by which these organisms protect themselves from toxic levels of selenium. Such conversion may, as well, set up a process of transport and cycling for this element. Other organoselenium compounds such as amino acids in which Se has substituted for S (e.g., selenocysteine) also are produced. Organoselenium compounds typically make up a significant fraction of the selenium in aqueous systems and sediments.

In spite of the toxicity of selenium, it is an essential trace element for a number of organisms, including humans. It is used in the glutathione peroxid-ase enzyme system to prevent the formation of peroxides and free radicals from the oxidation of unsaturated fats. Recently, evidence has been presented suggesting that selenium is effective in preventing the onset of cancer; that is, the incidence of cancer can be correlated inversely with selenium levels in the blood. Interestingly, this effect is counteracted by high levels of zinc. If the foregoing observation is valid, it illustrates the difficulty in dealing with trace elements. Eliminating selenium too thoroughly from food and water supplies may do harm, just as too high an intake certainly will.

Tellurium, the next element in the oxygen-sulfur-selenium family, is much less toxic than selenium because it is much more easily reduced and excreted as organotellurium compounds from organisms.

0 0

Post a comment