Acid Sulfate Soil

Acid sulfate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that are formed under waterlogged conditions.

These soils contain iron sulfide minerals (predominantly as the mineral pyrite) or their oxidation products. In an undisturbed state below the water table, acid sulfate soils are benign.

However, if the soils are drained, excavated or exposed to air by a lowering of the water table, the sulfides react with oxygen to form sulfuric acid.

Release of this sulfuric acid from the soil can in turn release iron, aluminium, and other heavy metals (particularly arsenic) within the soil.

Once mobilized in this way, the acid and metals can create a variety of adverse impacts : killing vegetation, seeping into and acidifying groundwater and surface water bodies, killing fish and other aquatic organisms, and degrading concrete and steel structures to the point of failure.

Chemical reaction

When drained, pyrite- (FeS2) containing soils (also called cat-clays) may become extremely acidic (pH < 4) due to the oxidation of pyrite into sulfuric acid (H₂SO₄)..

2FeS₂ + 9O₂ + 4H₂O 8H⁺ + 4SO₄^2- + 2Fe(OH)₃

Fe(OH)₃, iron(III) hydroxide (orange), precipitates as a solid, insoluble mineral by which the alkalinity component is immobilized, while the acidity remains active in the sulfuric acid.

The process of acidification is accompanied by the formation of high amounts of aluminium (Al³⁺, released from clay minerals under influence of the acidity), which are harmful to vegetation.

Other products of the chemical reaction are:

1. Hydrogen sulfide (H₂S), a smelly gas

2. Sulfur (S), a yellow solid

3. Iron(II) sulfide (FeS), a black/gray/blue solid

4. Hematite (Fe₂O₃), a red solid

5. Goethite (FeO.OH), a brown mineral

6. Schwertmannite, a brown mineral

7. Iron sulfate compounds (e.g. jarosite)

8. H-Clay (hydrogen clay, with a large fraction of adsorbed H⁺ ions, a stable mineral, but poor in nutrients)