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 12FeS2 + 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 24Fe2+ + 7O2 + 2H2O 4Fe3+ + 2H2O• oxidation of Fe(II) to Fe(III)

• rate determining step

Chemistry of AMD (cont’d)

Reaction 32Fe3+ + 12H2O 4Fe(OH)3 + 12H+

• hydrolysis of Fe(III)

• precipitation of iron(III) hydroxide if pH > 3.5

Reaction 4FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO4

2- + 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 Reaction4FeS2 + 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.