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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