Selenium in the environment

Abstract

Selenium (Se) is one of the dispersed trace elements occurring in all rocks, soils, plants, and animals but rarely in concentrations above 100 ppm. Estimates of Se levels in the earth's crust range from 0.03 to 0.08 ppm. The distribution of Se parallels that of sulfur. The average S Se ratio is calculated at 6000:1. Commercial production of Se is entirely as a by product of electrolytic copper refining from sulfide ores. Industrial uses of the element include fabrication of components of rectifiers, photocells, and other electronic devices. Small amounts of Se are used in pigment production. Se has not been used in pesticide formulations for many years because of potential toxic hazards. Toxicity of the element and its compounds for human beings has only rarely been a problem in industry because of rigid enforcement of hygienic measures. Animals forced to graze exclusively in certain high Se areas may develop symptoms of Se toxicity and death may result. Fortunately toxic areas are not widespread in the world. Natural waters are usually low in the element with average concentration of 0.25 ppb commonly reported. Maximum allowable Se concentration in potable waters is 10 ppb (0.01 ppm). Irrigation waters may contain up to 0.05 ppm. Low concentrations of the element in water sources are attributed to formation of the very insoluble basic ferric selenite. The basic ferric selenite contributes to the high Se concentrations found in sediments and sedimentary rocks, especially shales. Uplift of the sediments and subsequent weathering to soils results in seleniferous soils. Se concentrations in plants growing on these soils depend on soil pH, soil leaching, and plant species. There is no evidence that plants require Se for their growth; however all plants contain some Se. Concentrations of Se in plants range from as little as 0.01 ppm to as high as 10,000 ppm, depending on the amount of plant available Se in the soil and the plant species. Plant materials containing more than 5 ppm of Se are regarded as potentially toxic to animals. The majority of plants contain from 0.05 to 5.0 ppm of Se. These concentrations provide adequate and safe amounts of the element for animal and human nutrition. Animal diets containing less than 0.05 ppm of Se do not provide amounts amoujts of the element for animal health. It is necessary to supplement sufficient amounts of animals in many areas of the world to prevent large economic losses from Se deficiency. Because of interregional movement of human foods it appears that the human diet is adequate in Se.