Transcript Selenium Nutrient or Toxicant

Selenium
Nutrient or Toxicant
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
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
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
Nutrient
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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:
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can inhibit growth,
limit reproductive capability,
reduce appetite,
possibly lead to death.
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.
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.
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• 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
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.
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.