Acid mine drainage

Acid mine drainage

The Rio Tinto river, affected by acid mine drainage

Acid mine drainage (AMD) refers to the outflow of acidic water from metal or coal mines. This phenomenon occurs when sulfide minerals in exposed rock surfaces react with oxygen and water to produce sulfuric acid. The acid can leach additional metals from the surrounding rock, leading to environmental and ecological damage.

Formation

Acid mine drainage is primarily formed through the oxidation of pyrite (FeS_), a common sulfide mineral found in many mining areas. The chemical reaction can be summarized as follows:

2 FeS_ + 7 O_ + 2 H_O _ 2 Fe__ + 4 SO___ + 4 H_

This reaction produces ferrous iron, sulfate, and hydrogen ions, which contribute to the acidity of the water.

Environmental impact

Rocks stained by acid mine drainage in Shamokin Creek

The environmental impact of acid mine drainage is significant. The acidic water can lead to the mobilization of heavy metals such as iron, aluminum, and manganese, which can be toxic to aquatic life. The low pH and high metal concentrations can devastate ecosystems, leading to the loss of biodiversity and the degradation of water quality.

Prevention and treatment

Several methods are employed to prevent and treat acid mine drainage. These include:

• Source control: Limiting the exposure of sulfide minerals to air and water by covering waste rock and tailings with impermeable materials.
• Neutralization: Adding alkaline substances such as lime to neutralize the acidity.
• Constructed wetlands: Using natural processes to treat AMD by promoting the growth of plants and microorganisms that can absorb and precipitate metals.

Case studies

Acid mine drainage in a pyrite-rich area

One of the most well-known examples of acid mine drainage is the Rio Tinto in Spain, where mining activities have been ongoing for thousands of years. The river is characterized by its reddish color due to high concentrations of iron oxides.

Another example is the Shamokin Creek in Pennsylvania, USA, where historical coal mining has led to significant acid mine drainage issues.

Related pages

• Mining
• Environmental impact of mining
• Water pollution
• Sulfide mineral

Gallery

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  Iron hydroxide precipitate in a stream affected by acid mine drainage

Acid mine drainage

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  Rio Tinto river affected by acid mine drainage
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  Rocks on Shamokin Creek showing effects of acid mine drainage
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  Acid mine drainage in a pyrite-rich area
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  Iron hydroxide precipitate in stream due to acid mine drainage