Introduction
Upon exposure to oxygen (O2) and water (H2O), metal sulfides undergo oxidation to produce metal-rich acidic effluent. If the pH is low enough to overcome the natural buffering capacity of the surrounding rocks (‘calcium carbonate equivalent’ or ‘acid neutralising capacity’), the surrounding area may become acidic, as well as contaminated with high levels of heavy metals. Though acidophiles have an important place in the iron and sulfur biogeochemical cycles, strongly acidic environments are overwhelmingly anthropogenic in cause, primarily created at the cessation of mining operations where sulfide minerals, such as pyrite (iron disulfide or FeS2), are present.
Acid mine drainage may occur in the mine itself, the spoil heap (particularly colliery spoils from coal mining), or through some other activity that exposes metal sulfides at a high concentration, such as at major construction sites. Banks et al. provide a basic summary of the processes that occur:
- 2FeS2 + 2H2O + 7O2 = 2Fe2+ + 4SO42– + 4H+(aq)
- Pyrite + water + oxygen = ferrous iron + sulfate + acid
Read more about this topic: Acidophiles In Acid Mine Drainage
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