Acid Mine Drainage: From
Formation to Remediation
CE 367 - Aquatic Chemistry
Julie Giardina
Dominike Merle
Introduction: What is
Acid Mine Drainage (AMD)?
• Highly acidic water with elevated levels of
dissolved metals.
• Drainage from surface or deep coal or metal
mines and coal refuse piles.
• An important environmental issue in many
areas where mining has taken place.
Sources of Acid Mine Drainage
• Mining of gold, silver, copper, iron, zinc,
lead (or combined metals), and coal
– Past and present
– During exploration, operation, and closure of
mine, from the mine’s:
• dewatering system
• tailings disposal facilities
• waste heaps
– Water table rebound after pumping equipment
is removed.
Process of Acid Mine Drainage
• Geochemical and microbial reactions during
weathering of sulfide minerals (pyrite) in
coal, refuse, or mine overburden
– Oxidation of sulfide minerals in the presence of
air, water, and bacteria
– Formation of sulfuric acid and increase in
acidity
– Solubilization of metals due to low pH
A Side Note: Acid Rock Drainage
• Formation of acidic waters
– Occurs naturally due to weathering of sulfide
minerals in rocks
– Occurs at a much slower rate
Effects of Acid Mine Drainage
• Water resources
– Increased acidity
– Depleted oxygen
– Increased weathering of minerals  release of
heavy metals/toxic elements into stream
– Precipitation of Fe(OH)3  bright orange color
of water and rocks
Effects of AMD (cont’d)
• Biological resources
– Low pH and oxygen content  water
unsuitable for aquatic life
– Precipitation of Fe(OH)3
• Increased turbidity and decreased photosynthesis
• Gill-clogging, smothering of bottom dwellers and
food supply, and direct toxicity (benthic algae,
invertebrates, and fish)
• Clogging of interstitial pore space in coars aquatic
substrate habitat
Effects of AMD (cont’d)
• Biological resources
– Elimination of aquatic plants  change in
channel hydraulics
– Stress on other biota associated with aquatic
habitats
• Human resources
– Corrosion of pipes, pumps, bridges, etc.
– Degradation of drinking water supplies
– Harm to fisheries
Chemistry of Acid Mine Drainage
Reaction 1
2FeS2 + 7O2 + 2H2O  4Fe 2+ + 4SO4 + 4H+
• weathering of pyrite in the presence of oxygen and water to
produce iron(II), sulfate, and hydrogen ions
Reaction 2
4Fe2+ + 7O2 + 2H2O  4Fe3+ + 2H2O
• oxidation of Fe(II) to Fe(III)
• rate determining step
Chemistry of AMD (cont’d)
Reaction 3
2Fe3+ + 12H2O  4Fe(OH)3 + 12H+
• hydrolysis of Fe(III)
• precipitation of iron(III) hydroxide if pH > 3.5
Reaction 4
FeS2 + 14Fe3+ + 8H2O  15Fe2+ + 2SO42- + 16H+
• oxidation of additional pyrite (from steps 1 and 2) by Fe(III) -here iron is the oxidizing agent, not oxygen
• cyclic and self-propagating step
Chemistry of AMD (cont’d)
Overall Reaction
4FeS2 + 15O2 + 14H2O  4Fe(OH)3 + 8H2SO4
Typical Case: Manila Creek, VA
• Iron content:567 mg/L,
pH:3.5, flow from mine of
42GPM.
• Wetlands were used to
increase pH.
• pH increased to 5.1, iron
contents reduced to 67
mg/L.
Extreme Case: Iron Mountain, Ca
• Extreme pH
measurements from
1.51 to –3.6 over a
temperature range of
29-47oC.
• Total iron from 2.67
to 141 g/L.
• SO4: 14-50 g/L
• Zn: 0.058-23 g/L.
• Regulatory actions
initiate to increase pH
and reduce metal
concentrations.
Remediation
• Use of acid generating rocks to segregate/blend
waste.
• Bacteria Desulfovibrio and Desulfotomaculum
– SO4-2 + 2 CH2O = H2S + 2 HCO3-
• Alkaline Materials (CaCO3, NaOH, NaHCO3,
anhydrous ammonia).
– CaCO3 + H+ = Ca+2 + HCO3
• Soil, clay, synthetic covers.
• Chemical additives
Remediation Procedures
Future/Ongoing Research
• Prediction of acid generation
– Acid\base accounting
– Weathering tests
– Computer models
• Prevention/Mitigation
–
–
–
–
Rock phosphate to inhibit pyrite oxidation.
Coatings and sealant to inhibit acid production.
Improve time for bactericide leaching.
Encapsulation of pyrite material.
Conclusions
• AMD is an environmental problem results from the
oxidation of pyrite by bacteria air, and water.
• Oxidation of pyrite decrease pH and increase
concentrations of dissolve metals in water.
• The latter results in the pollution of water, which can
be harmful for the environment and living species.
• Several methods such as wetlands have been done to
increase pH and decrease metal concentrations in
water.
• AMD research continues in order to find better ways to
mitigate pollution and reduce the overall effects in the
environment such as global warming.