AMD (Acid Mine Drainage) is caused by a chemical reaction between water and sulfide minerals, such as pyrite (FeS2), that are exposed to air and water during mining activities. The reaction is as follows:
FeS2 + 7O2 + H2O → Fe2+ + 2SO4^2- + 2H+
This reaction is an oxidation reaction, where the sulfide mineral is oxidized to sulfate ions and ferrous ions are released. The ferrous ions can then react with water and oxygen to form ferric hydroxide (Fe(OH)3), which is a yellow-orange solid that contributes to the characteristic color of AMD.
The overall reaction can be written as:
4FeS2 + 15O2 + 14H2O → 4Fe(OH)3 + 8SO4^2- + 16H+
This reaction shows that four molecules of pyrite react with 15 molecules of oxygen and 14 molecules of water to produce four molecules of ferric hydroxide, eight molecules of sulfate ions, and 16 molecules of hydrogen ions. The reaction is balanced to ensure that the number of atoms of each element is the same on both sides of the equation.