Pollution: Acid Mine Drainage: Chemistry
The net equation for this set of reactions is:
Equation 1 depicts the oxidation of inorganic sulfur to sulfate and the subsequent production of sulfuric acid. The next step shows how ferrous iron is oxidized to form ferric iron and water. When drainage containing these pollutants reaches the stream, it becomes diluted and the pH of the solution rises. The increase in pH allows ferric hydroxide (Fe(OH)3; A.K.A. "yellow-boy") to precipitate out. This chemical reaction is illustrated in equation 3. As indicated in equation 4, a net of four moles of hydrogen ions (H+) are liberated for each mole of pyrite (FeS2) oxidized, making this one of the most acidic weathering reactions known.
Acid mine drainage depletes the buffering capacity of water by neutralizing carbonate and bicarbonate ions to form carbonic acid (H2CO3):
Once exposed to AMD, the affected carbonate buffering system is not able to control changes in pH as well. The buffering system of the stream is completely destroyed below a pH of 4.2, where all carbonate and bicarbonate ions are converted to carbonic acid. Carbonic acid (H2CO3) readily breaks down into water and carbon dioxide:
Some images © 2004 www.clipart.com
Center for Educational Technologies, Circuit Board/Apple graphic logo, and COTF Classroom of the Future logo are registered trademarks of Wheeling Jesuit University.