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)

• 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)

Oxidation of Pyrite

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 source

T. ferrooxidans

• Acidophilic– capable of surviving at

low pH’s

• Autotrophic– obtains its carbon by

fixing atmospheric CO2

Viewed by electron microscopemagnified 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 acceptor.

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 of Sb, U & Mo compounds

Red-orange color due to production of Fe(III) asT. 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 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