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Water Pollution: Acid Mine Drainage: Chemistry
Groundwater often flows through the cracks in the crushed sulfides, or sulfur-rich minerals, that remain after an area has been mined. One of the most important sulfides found in the waste rock of mines is iron sulfide or pyrite (FeS2). This sulfide is more commonly known as "fool's gold." When pyrite is exposed to water and oxygen, it undergoes the following oxidation and reduction reactions to form acid mine drainage (AMD):

2FeS2 + 7O2 + 2H2O 2FeSO4 + 2H2SO4 (1);

2Fe2+ + 1/2 O2 + 2H+ 2Fe3+ + H2O (2);

Fe3+ + 3H2O Fe(OH)3 + 3H+ (3).

The net equation for this set of reactions is:

FeS2 (s) + 15/4 O2+ 7/2 H2O 4H+ + 2SO4- + Fe(OH)3 (s) (4).

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):

H+ + CO32- HCO3-, then

H+ + HCO3- 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:

H2CO3 H2O + CO2.

 

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