| Microbes in Mines |
Pyrite (FeS2), otherwise known as "fool's gold," may not look like lunch to you but it does to the chemolithotrophic bacteria Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans). These bacteria extract energy from the oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+). Pyrite is one of the most common forms of iron in nature, and is very common in bituminous coals and in many ore bodies. When pyrite is exposed, as in a mining operation, it reacts with oxygen to generate ferrous ions, sulfate, and hydrogen ions.
2FeS2 + 7O2 + 2H2O ---> 2Fe2+ + 4SO42- + 4H+
Ferrous ions - lunch! And, the hydrogen ions generated in this reaction do not faze A. ferrooxidans. This acidophile prefers a pH below 3.5. It is able maintain a relatively neutral internal pH by actively pumping protons between the cytoplasm and external environment against a steep pH gradient.
Acid mine drainage, which causes serious ecological damage to rivers and lakes is in part a result of the presence of A. ferrooxidans. The ferric ions generated by the bacteria are soluble in the acid environment and easily react with additional pyrite.
FeS2 +14Fe3+ + 8H2O ---> 15Fe2+ + 2SO42- + 16H+
The additional acid formed from this reaction is just one of the resultant pollutants. The ferric ions (Fe2+) that are generated precipitate in a complex mineral called jarosite [HFe3(SO)4,(OH)6]. The unsightly stains in mine drainages, called "yellow boy" by U.S. miners, are jarosite.
Acid mine drainage and its associated pollution do not form unless pyrite is exposed to oxygen. Only upon mining does the initial reaction generating ferrous ions provide an environment in which A. ferrooxidans will thrive.
To reduce toxic metal content in acid mine drainage, scientists are turning to sulfate-reducing bacteria, which occur naturally in anoxic soils. These bacteria use sulfate as an electron acceptor instead of oxygen, in a form of metabolism known as anaerobic respiration. Hydrogen sulfide is generated in the process. At a bioremediation site in southeast Idaho Dan Kortansky and his colleagues set up a series of ponds separated by berms (embankments) of crushed limestone, straw, and manure. The goal was to convert the sulfate in the drainage to sulfide. This reacts with the dissolved metals to form metal sulfides, such as ferrous sulfide. The limestone in Kortansky's bioremediation system lowers the pH as metal-laden water passes through the berms and sulfate-reducing bacteria thrive. Results have been encouraging. Iron concentrations in the drainage at this site were reduced 65% and copper residues were reduced by nearly 100 percent.