Acid mine drainage
Introduction
Acid mine drainage (AMD) occurs when metal sulfides, most commonly pyrite, are exposed to and react with air and water. When water flows over or through sulfur-bearing mine tailings, a chemical reaction occurs between the water and rocks resulting in metal-rich water. AMD typically has a bright orange, yellow, or bownish-red color to it. The mine drainage is acidic, as its name states, and found around ore and coal mines. Abandoned and currently operating mining activities contribute to AMD. Metal rich drainage can also occur in areas that have not been mined. Areas like construction sites and other places that have been highly disturbed can exacerbate natural rock weathering processes leading to acidic drainage.
An area heavily mined for coal, therefore greatly impacted by AMD, is the mid-Atlantic region in the middle of the Appalachian Mountains. Here, and everywhere AMD occurs, has a highly altered ecosystem. The greatest consequence of AMD is water pollution. This in turn results in contaminated drinking water, damage to aquatic flora and fauna, and corrosion of man-made infrastructure.
Microorganisms like bacteria and archaea significantly affect AMD. When metal sulfides, usually pyrite, that are contained in rock are exposed to water and air, an oxidation reaction takes place. Microbes speed up the decomposition of these metal ions. Microorganisms also play a huge part in the bioremediation of AMD. Techniques that are being researched include using metal-immobilizing bacteria, biocontrol with bacteria and archaea, and bioleaching.
Physical environment
Physical & Chemical Characteristics
AMD is effected by characteristics such as pore size, particle size, and mineral composition of the materials being oxidized. Water and oxygen availability are the most important factors though.
Oxidation of Pyrite
A lot of mines use the technique of sub-surface mining, where extraction occurs beneath the earth’s surface. With this technique mines are commonly below the water table, requiring water to be pumped out of the mine to stop flooding. When a mine is deserted, water once again fills in its natural position beneath the surface. This water is one of the necessary ingredients for the oxidation of pyrite leading to AMD formation.
Exposing pyrite to oxygen and water leads to an oxidation reaction, where hydrogen and sulfate ions and soluble metal cations are created:
- 2FeS2(s) + 7O2(g) + 2H2O(l) → 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)
Pyrite oxidation occurs naturally at a slow rate in undisturbed rock. However, the acidity created is buffered by water. Because mining exposes more surface area of these sulfur-bearing rocks, additional acid is produced that is beyond the water’s usual buffering capabilities.
When enough oxygen is available either from dissolved oxygen in the water or the atmosphere, further oxidation of ferrous iron (Fe+2) to ferric iron (Fe+3) occurs:
- 4Fe2+(aq) + O2(g) + 4H+(aq) → 4Fe3+(aq) + 2H2O(l)
Ferric iron (Fe+3) can either precipitate as ochre (Fe(OH)3 ), the reddish-orange precipitate observed in acid mine drainage waters:
- 2Fe3+(aq) + 6H2O(l) <→ 2Fe(OH)3¬(s) + 6H+(aq)
or it can react directly with pyrite to make additional ferrous iron and hydrogen ions:
- FeS2(s) + 14Fe3+(aq) + 8H2O(l) → 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq)
Overall, these reactions release hydrogen ions, which decreases pH leading to an acidic environment. [2]
Metal Contamination
AMD is also known to be contaminated by toxic metals such as copper and nickel and lesser trace metals like lead, aluminum, arsenic, and manganese.
Biological interactions
Are there important biological interactions that are important in this environment? Do these interactions influence microbial populations and their activities? How do these interactions influence other organisms? Describe biological interactions that might take place in this environment, using as many sections/subsections as you require. Look at other topics available in MicrobeWiki. Create links where relevant.
Subsection 1
Subsection 1a
Subsection 1b
Subsection 2
Role of Microbes
Catalyze Oxidation
The general agreement among scientists of microbial-induced pyrite oxidation is that these microorganisms raising the amount of available ferric iron which increases the rate of pyrite oxidation. Acidithiobacillus ferrooxidans, for example, use reduced ferrous iron in AMD areas as an electron donor for energy creation at low pH.
Treatment Methods
Subsection 1b
Subsection 2
Key Microorganisms
Named extremophiles because of their ability to live in severe conditions, microbes living in AMD usually have to deal with low pH levels (extreme acidophiles) and high temperatures.
Bacteria
- Acicithiobacillus ferrooxidans
- Acidithiobacillus thiooxidans
- Leptospirillum ferrooxidans
- Sulfobacillus thermosulfidooxidans
Archaea
Ferroplasma acidiphilum
Current Research
Enter summaries of recent research here--at least three required
References
Edited by student of Angela Kent at the University of Illinois at Urbana-Champaign.
