Selenium

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.