Acid mine drainage or AMD is a major environmental concern at coal mines, where sulfur-containing minerals, such as pyrites are found. Pyrite (`FeS_2`) oxidizes to generate ferrous ions, as per the following equation:
`2FeS_2 + 2H_2O + 7O_2 -> 2Fe^(2+) + 4H^+ + 4SO_4^(2-)`
The ferrous ions are biologically oxidized to ferric ions.
`4Fe^(2+) + O_2 + 4H^+ -> 4Fe^(3+) + 2H_2O`
The ferric ions catalyze the oxidation of pyrite, generating more ferrous ions and thus autocatalyzing the process. The sulfate ions convert to sulfuric acid, giving rise to AMD.
A number of bacteria have been found to catalyze the dissolution of pyrite and oxidation of ferrous ions. Some of these bacteria include, Thiobacillus ferrooxidans, Acidithiobacillus caldus and Leptospirillum ferrophilum. Without the bacterial catalysis, these reaction would be very slow (by a factor of 5-6) to be of much concern.
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