Acid Mine Drainage

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Acid Mine Drainage
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Mining the Environment

• Mine overburden waste soils (mine tailings) are
  waste products generated by the mining industry.
• When these tailings are exposed to the
  atmosphere, precipitation and ground or surface
  water, they can react with oxygen water to
  generate products which affect the pH heavy
  metal composition of soils streams

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Mine Tailings
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Acid Mine Drainage

• When mineral deposits containing sulfides are
  mined, they have the potential to produce acid
  mine drainage.
• Coal, copper, gold, silver, zinc, lead uranium
• AMD is caused by the physical chemical
  weathering of the common mineral pyrite (FeS2)

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Pyrite

• Physical weathering of pyrite is necessary to
  reduce the grain size of the mineral.
• Miners often accelerated this process by grinding
  up ores and dumping the overburden in the mine
  tailings piles
• When exposed to water oxygen, pyrite forms
  sulfuric acid.

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Oxidation of Pyrite

• 4FeS2(s) 14O2(g) 4H2O(l) 4Fe 2(aq)
  8SO42-(aq) 8H
• The ferrous hydrogen ions are released into the
  waters that runoff from mine drainage tunnels or
  tailings piles.
• The ferrous ions are oxidized to form ferric ions
• 4Fe 2(aq) O2(g) 4H(aq)
  4Fe3(aq) 2H2O(l)

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Oxidation of Pyrite

• The ferric ion hydrolyzes win water to form an
  insoluble yellow-orange precipitate called
  yellow boy.
• 4Fe3(aq) 12H2O(l) 4Fe(OH)3(s)
  12 H(aq)

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AMD in the High Andes, Peru
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AMD in Colorado
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Yellow boy precipitation smothers aquatic
plants and animals
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• 4FeS2(s) 14O2(g) 4H2O(l) 4Fe 2(aq)
  8SO42-(aq) 8H
• 4Fe 2(aq) O2(g) 4H(aq)
  4Fe3(aq) 2H2O(l)
• 4Fe3(aq) 12H2O(l) 4Fe(OH)3(s)
  12 H(aq)
• 4FeS2(s) 15O2(g) 14H2O(l) 4Fe(OH)3(s)
  8SO42-(aq) 16H
• smothers organisms living on the stream bottom

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Microbial Influences

• Abiotic oxidation of pyrite is slow.
• The bacterial microbe Thiobacillus ferrooxidans
  catalyzes the oxidation of FeS2 to ferric ions
  and hydrogen ions

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Microbial Influences

• The pH of AMD can less than 3.
• Other heavy metal ions (zinc, copper, lead,
  arsenic and manganese) are also soluble in acidic
  solution are mobilized
• Streams are often devoid of life for miles
  downstream of an AMD source

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T. ferrooxidans

• Acidophilic
• capable of surviving at low pHs
• Autotrophic
• obtains its carbon by fixing atmospheric CO2

Viewed by electron microscope magnified 30,000
times
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T. ferrooxidans

• Obtains its energy by the oxidation of either
  iron or sulfur
• Fe 2 0.25 O2 H Fe 3 0.5
  H2O
• H2S 2O2 SO4 2- 2H
• So H2O 1.5 O2 SO4 2- 2H
• S2O3 2- H2O 2O2 2SO4 2- 2H

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T. ferrooxidans

• T. ferrooxidans is generally assumed to be
  obligately aerobic, but under anaerobic
  conditions, it can be grown on elemental sulfur
  using ferric iron as an electron acceptor.
• S 6Fe3 4H2O H2SO4 6Fe 2
  6H
• ?G-314 KJ/mol

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T. ferrooxidans

• Important in bioleaching processes where
  anaerobic conditions exist
• Can also obtain energy from oxidizing Cu, Se2,
  from oxidation of Sb, U Mo compounds

Red-orange color due to production of Fe(III)
as T. ferrooxidans oxidizes Fe(II)
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T. ferrooxidans

• Experiments show that T. ferrooxidans accelerates
  extraction of copper from ores

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Coal Mining and AMD

• Upper Conemaugh River Basin, PA

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A Little History

• Nature bestowed Cambria Somerset Counties, PA a
  mixed blessing with an abundance of coal a
  topography which made it easy to extract
• Five minable seams of coal provided the energy
  needed for the Industrial Revolution which made
  Johnstown one of the largest iron steel
  production centers in the world

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A Little History

• The Cambria Iron Company (Andrew Carnegies first
  still mill) was located in Johnstown
• It later grew into the largest integrated Steel
  Mill in the world (stretched 14 mi along the
  Conemaugh Little Conemaugh Rivers
• Steel mills used large amount of coal to make
  coke (fuel for the clast furnaces)

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Types of Coal Mines

• Drift or Slope Mines
• driven into valley walls near level of coal
• drain excess water encountered by gravity flow
  out the entry
• Shaft Mines
• pumps used to remove water
• boreholes drilled to relieve water pressure

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Types of Coal Mines

• Surface Mines
• uses draglines which can remove up to a depth of
  200 ft in a single pass
• miners left the overburden rock where it acid and
  metals into streams to add to the discharges from
  the abandoned deep mines

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Water Flows

• Underground mines may produce thousand gallon per
  minute flows
• Strip mines produce less flow

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Mine Drainage Wasteland

• Iron mound precipitated from water discharging
  from a 300 deep borehole.
• Precipitate (up to 9 ft deep) has killed trees

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Open Mine Entry

• Water discharging from drift mine.
• Discharges from these types of mines
• 200-800 gpm
• pH range 2.7-3.2
• Metal concentrations
• 58mg/l Fe
• 20.9 mg/l Mn
• 55.4 mg/l Al