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 &
streams
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
(FeS2)
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
ions
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) + O2(g) + 4H+(aq)
4Fe3+(aq) + 12H2O(l)
4Fe 2+(aq) + 8SO42-(aq) + 8H+
4Fe3+(aq) + 2H2O(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
source
T. ferrooxidans
• Acidophilic
– capable of surviving at
low pH’s
• Autotrophic
– obtains its carbon by
fixing atmospheric
CO2
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+
H2S + 2O2
So + H2O + 1.5 O2
S2O3 2- + H2O + 2O2
Fe 3+ + 0.5 H2O
SO4 2- + 2H+
SO4 2- + 2H+
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
acceptor.
S + 6Fe3+ + 4H2O
H2SO4 + 6Fe 2+ + 6H+
G=-314 KJ/mol
T. ferrooxidans
Red-orange color due to
production of Fe(III) as
T. ferrooxidans oxidizes Fe(II)
• Important in
bioleaching processes
where anaerobic
conditions exist
• Can also obtain energy
from oxidizing Cu+,
Se2+, & from oxidation
of Sb, U & Mo
compounds
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
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)
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
borehole.
• 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