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Selenium

Selenium. Nutrient or Toxicant. Periodic Table of the Elements. Chemistry of selenium. Group VIB metalloid Commonly occurs with and replaces sulfur Four oxidation states: -2, H 2 Se: toxic and reactive gas

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Selenium

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  1. Selenium Nutrient or Toxicant

  2. Periodic Table of the Elements

  3. Chemistry of selenium • Group VIB metalloid • Commonly occurs with and replaces sulfur • Four oxidation states: -2, H2Se: toxic and reactive gas 0 elemental selenium; red amorphous or black crystalline solid: insoluble and unreactive +4 selenite (SeO32-): Soluble and toxic but easily reduced to elemental Se +6 selenate (SeO42-): Most soluble and toxic, and easily taken up by plants

  4. Occurrence of Se • Crustal abundance 0.05 ppm • Se substitutes for S in pyrite, chalcopyrite, and bornite in copper and copper-lead-zinc sulfide deposits • In rare Se minerals crooksite (Cu7(Tl,Ag)Se4) and clausthalite (PbSe). • In sedimentary uranium and phosphatic deposits • In siltstones and shales. • In volcanic gases • Concentration ranges from 0.1 to 1200ppm. • Few deposits contain high enough concentrations of selenium for economic mining. • Se obtained from the refining of anode slimes, from electro-winning of copper. • Or from the leaching of flue dusts from sulfide ore smelters • Annual production of selenium over 2100 metric tons. Black botryoidal clausthalite

  5. Uses of Se Glass manufacturing, electronics, agriculture, metal alloy production, and in chemical and pigment production. • Agricultural/biological applications (e.g., as an additive to animal feeds and fertilizers). • Demand for selenium may increase in the future due to the possibility of using selenium to replace lead in plumbing brass and other lead alloys. • Canada is among the world's largest producers

  6. Nutrient • O.04 ppm Se essential • 0.04 - 0.1 ppm Se beneficial • 3 ppm toxic • Se deficiency causes muscular degeneration, impeded growth, fertility problems, anaemia, liver disease • Selenium is a micronutrient required by fish, birds, and mammals (including humans) to maintain good health • Plants will uptake selenium and make it available to foraging animals, but no nutritional requirement has been found for selenium in plants. • Animals require approximately 0.1 mg/kg Se to maintain levels of the enzyme glutathione peroxidase (SeGSHpx), which assists in the conversion of free radicals into other harmless products. • Deficiencies in selenium: • can inhibit growth, • limit reproductive capability, • reduce appetite, • possibly lead to death.

  7. EC Guidlines • The Canadian Soil Quality Guideline (CSoQG) for Se • in agricultural and residential/park lands is 1.0 mg Se /kg of soil. • on commercial and industrial lands is 3.9 mg Se/kg of soil. • US EPA • Maximum contaminant level (MCL) for drinking water is 10 μg/L and 5 μg/L for chronic exposure for aquatic life. • Waters containing more than 1000 μg/L are considered toxic waste by the EPA.

  8. Toxicant • Small margin of safety between levels of Se compounds that will cause dietary deficiency and those that result in toxicity. • 3-15 mg Se/kg food can cause chronic or acute selenosis and death. • Many symptoms of Se toxicity are similar to those observed in cases of Se deficiency. • In general, very high levels of dietary Se produces dizziness, fatigue, irritation, collection of fluid in the lungs, and severe bronchitis, stillbirths, and malformation of offspring. • Se compounds on the skin cause rashes, swelling, and pain. • High blood levels of Se can also result in selenosis, • symptoms are garlic odour breath, thickened and brittle nails, hair and nail loss, gastro-intestinal problems reduced hemoglobin, mottled teeth, skin lesions, and pain or numbness in the limbs. • Selenite is 100% bioavailable. • In 1987, the USA EPA designated selenium as a priority pollutant based on the narrow range between beneficial and toxic concentrations, • Toxic effects of Se found in bacteria, fungi, algae, plants, and invertebrates.

  9. Locoweed • Cattle eating plants rich in selenium (loco-weeds) get the "blind staggers" muscular dystrophy appear drugged. e.g. in San Joaquin Valley in California • Locoweed is a species of poisonous plants of the genera Astragalus and Oxytropis, in the pea family (Fabaceae), native to the prairies of north central and western North America. • Locoweed is eaten during the early spring and late fall, when it is often the only green plant available to grazing animals. • Ingestion causes symptoms similar to BSE, including erratic behavior, aggression, lethargy, depression, loss of balance, nervousness, and abortion, among others. • Although symptoms reduce with time after removing the animal from exposure to locoweed, some nerve damage is permanent. In horses, Horses are especially susceptible to selenosis and have necrotic hoof and brain damage can make them dangerous to ride.

  10. Mining Issues • Selenium behaves in a similar way to S oxidation, producing selenite (SeO32-) and selenate (SeO42-) anions. • Kinetically favored selenate is the most common in oxidized waters, but neutral to acidic environments will favor the selenite and biselenite forms. • Selenate is much more mobile in an aqueous environment and is much harder to treat than selenite.

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