Acid Mine Drainage: From Formation to Remediation - PowerPoint PPT Presentation

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Acid Mine Drainage: From Formation to Remediation

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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. – PowerPoint PPT presentation

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Title: Acid Mine Drainage: From Formation to Remediation


1
Acid Mine Drainage From Formation to Remediation
  • CE 367 - Aquatic Chemistry
  • Julie Giardina
  • Dominike Merle

2
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.

3
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 mines
  • dewatering system
  • tailings disposal facilities
  • waste heaps
  • Water table rebound after pumping equipment is
    removed.

4
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

5
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

6
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

7
Effects of AMD (contd)
  • 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

8
Effects of AMD (contd)
  • 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

9
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

10
Chemistry of AMD (contd)
  • Reaction 3
  • 2Fe3 12H2O ? 4Fe(OH)3 12H
  • hydrolysis of Fe(III)
  • precipitation of iron(III) hydroxide if pH gt 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

11
Chemistry of AMD (contd)
  • Overall Reaction
  • 4FeS2 15O2 14H2O ? 4Fe(OH)3 8H2SO4

12
Typical Case Manila Creek, VA
  • Iron content567 mg/L, pH3.5, flow from mine of
    42GPM.
  • Wetlands were used to increase pH.
  • pH increased to 5.1, iron contents reduced to 67
    mg/L.

13
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.

14
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 Ca2 HCO3
  • Soil, clay, synthetic covers.
  • Chemical additives

15
Remediation Procedures
16
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.

17
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.
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