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Basic AMD Chemistry

There are four commonly accepted chemical reactions that represent the chemistry of pyrite weathering to form AMD. An overall summary reaction is as follows:

4 FeS2 + 15 O2 + 14 H2O => 4 Fe(OH)3 ¯+ 8 H2SO4
Pyrite + Oxygen + Water => "Yellowboy" + Sulfuric Acid

The first reaction in the weathering of pyrite includes the oxidation of pyrite by oxygen. Sulfur is oxidized to sulfate and ferrous iron is released. This reaction generates two moles of acidity for each mole of pyrite oxidized.

2 FeS2 + 7 O2 + 2 H2O => 2 Fe2+ + 4 SO42- + 4 H+
Pyrite + Oxygen + Water => Ferrous Iron + Sulfate + Acidity

The second reaction involves the conversion of ferrous iron to ferric iron. The conversion of ferrous iron to ferric iron consumes one mole of acidity. Certain bacteria increase the rate of oxidation from ferrous to ferric iron. This reaction rate is pH dependent with the reaction proceeding slowly under acidic conditions (pH 2-3) with no bacteria present and several orders of magnitude faster at pH values near 5. This reaction is refered to as the "rate determining step" in the overall acid-generating sequence.

4 Fe2+ + O2 + 4 H+ => 4 Fe3+ + 2 H2O
Ferrous Iron + Oxygen + Acidity => Ferric Iron + Water

The third reaction which may occur is the hydrolysis of iron. Hydrolysis is a reaction which splits the water molecule. Three moles of acidity are generated as a byproduct. Many metals are capable of undergoing hydrolysis. The formation of ferric hydroxide precipitate (solid) is pH dependant. Solids form if the pH is above about 3.5 but below pH 3.5 little or no solids will precipitate.

4 Fe3+ + 12 H2O => 4 Fe(OH)3 ¯ + 12 H+
Ferric Iron + Water => Ferric Hydroxide (yellowboy) + Acidity

The fourth reaction is the oxidation of additional pyrite by ferric iron. The ferric iron is generated in the reactions described above. This is the cyclic and self propagating part of the overall reaction and takes place very rapidly and continues until either ferric iron or pyrite is depleted. Note that in this reaction, iron is the oxidizing agent, not oxygen.

FeS2 + 14 Fe3+ + 8 H2O => 15 Fe2+ + 2 SO42- + 16 H+
Pyrite + Ferric Iron + Water => Ferrous Iron + Sulfate + Acidity


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