Metal contamination:
A variety of metal contaminents exist in groundwater, surface water, and soils resulting from industrial and agricultural activity. Toxic metals such as lead, mercury, cadmium, arsenic, chromium, and uranium can cause damage to human health and the environment.
How does metal contamination occur?
Perhaps the most prevalent and problematic form of metal pollution is acid mine drainage. This occurs when the mining of coal and metal ores exposes metals and radionuclides to the atmosphere allowing them to be oxidized by certain bacteria (Thiobacillus ferrooxidans). For example pyrite (often exposed in coal mining) can be oxidized to iron hydroxide and sulfuric acid:
FeS2 + 15/4 O2 + 7/2 H2O = Fe(OH)3 + 2H+ + 2HSO4-
The effluent produced, known as acid mine drainage, is highly toxic since it is not only acidic but also contains toxic metals. Such effluents have been known to contaminate nearby watersheds, killing much of the aquatic life. Streams containing acid mine effluent (pictured below) are often discolored due to Fe(OH)3 or “yellow-boy” contamination.
Why use bioremediation?
While metals cannot be broken down into non-toxic components like organic compounds, bioremediation can be used to stabalize, extract, or reduce the toxicity of soil and groundwater contaminated by acid mine drainage. Bioremediation strategies are often more beneficial than traditional "pump and treat" strategies because it can be implemented in situ, providing a simpler, less intrusive, and cheaper method of remediation.
Sources:
Frank, Paul Website; Spiro (2003)