Acid Mine Drainage

Acid mine drainage (AMD) or acid rock drainage (ARD) is acidic water that comes from either coal mines or metal mines (AMD) or from areas of disturbed earth, coal storage areas, coal processing areas, etc. (ARD). The acidity of the water in acid mine drainage is a result of a complex chemical reaction between a metal sulfide, water and oxygen; typically, the metal sulfide is iron pyrite (FeS2) although other metal sulfides can be involved.

Acid mine drainage chemical reaction

In the case of iron pyrite, the chemical reactions are as follows:

Persulfide ion (S₂^2-) is first oxidized to sulfate ion, iron (II) and hydrogen ion (acidity).

2FeS₂(s) + 7O₂(g) + 2H₂O(l) = 2Fe²⁺(aq) + 4SO₄^2-(aq) + 4H⁺(aq)

The iron (II) can be further oxidized to iron (III).

4Fe²⁺(aq) + O₂(g) + 4H⁺(aq) = 4Fe³⁺(aq) + 2H₂O(l)

Very often, the above oxidation reactions are catalyzed by microorganisms (e.g. Thiobacillus ferrooxidans).

The iron (III) or ferric irons produced can also react with more pyrite and produce ferrous ions, sulfate ion and more acidity (H+):

FeS₂(s) + 14Fe³⁺(aq) + 8H₂O(l) = 15Fe²⁺(aq) + 2SO₄^2-(aq) + 16H⁺(aq)

The high acidity (low pH) in acid mine drainage keeps the iron (II) or ferrous ion in solution but ferric ion is less soluble and tends to precipitate as a yellow/orange solid (iron oxide/hydroxide).

Acid mine drainage treatment

Treatment usually involves acid neutralization with limestone (CaCO₃), lime (CaO) or sodium hydroxide (NaOH) and oxidation either by aeration or addition of an oxidizing agent (sodium permanganate or hydrogen peroxide). Interestingly, it usually requires more base to neutralize the stream than would be predicted by the pH (H+ alone) since hydroxide ion is also needed to precipitate the metal hydroxides.

In the case or acid mine drainage from coal mines, the metals involved are usually, iron, aluminum and manganese. Iron and aluminum can precipitate below pH 7 but precipitation of manganese requires higher pH, significant oxidation and residence time. When iron and manganese are found together, the iron must be removed before the manganese can be removed since iron (II) tends to reduce and redissolve insoluble manganese dioxide (MnO₂).