Water As Solvent

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					Acid Mine Drainage
    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 &
Mine Tailings
         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

• 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.
             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
   4Fe 2+(aq) + O2(g) + 4H+(aq)        4Fe3+(aq) + 2H2O(l)
           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)
AMD in the High Andes, Peru
AMD in Colorado
“Yellow boy” precipitation smothers aquatic
           plants and animals
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
Microbial Influences
          • Abiotic oxidation of
            pyrite is slow.
          • The bacterial microbe
            Thiobacillus ferrooxidans
            catalyzes the oxidation of
            FeS2 to ferric ions and
            hydrogen ions
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
                  T. ferrooxidans
                                • Acidophilic
                                  – capable of surviving at
                                    low pH’s
                                • Autotrophic
                                  – obtains its carbon by
                                    fixing atmospheric

Viewed by electron microscope
magnified 30,000 times
             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+
             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

    S + 6Fe3+ + 4H2O   H2SO4 + 6Fe 2+ + 6H+
                              G=-314 KJ/mol
                  T. ferrooxidans
                                  • Important in
                                    bioleaching processes
                                    where anaerobic
                                    conditions exist
                                  • Can also obtain energy
                                    from oxidizing Cu+,
                                    Se2+, & from oxidation
Red-orange color due to             of Sb, U & Mo
production of Fe(III) as            compounds
T. ferrooxidans oxidizes Fe(II)
            T. ferrooxidans

• Experiments show that T. ferrooxidans
  accelerates extraction of copper from ores
 Coal Mining and AMD

Upper Conemaugh River Basin, PA
            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
             A Little History
• The Cambria Iron Company (Andrew
  Carnegie’s first still mill) was located in
• 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)
         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
        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
            Water Flows
• Underground mines may produce thousand
  gallon per minute flows
• Strip mines produce less flow
Mine Drainage Wasteland
                • Iron mound
                  precipitated from
                  water discharging
                  from a 300’ deep
                • Precipitate (up to
                  9 ft deep) has
                  killed trees
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

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