Sulfuric Acid - Occurrence

Occurrence

Pure sulfuric acid is not encountered naturally on Earth in anhydrous form, due to its great affinity for water. Dilute sulfuric acid is a constituent of acid rain, which is formed by atmospheric oxidation of sulfur dioxide in the presence of water – i.e., oxidation of sulfurous acid. Sulfur dioxide is the main byproduct produced when sulfur-containing fuels such as coal or oil are burned.

Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as iron sulfide. The resulting water can be highly acidic and is called acid mine drainage (AMD) or acid rock drainage (ARD). This acidic water is capable of dissolving metals present in sulfide ores, which results in brightly colored, toxic streams. The oxidation of pyrite (iron sulfide) by molecular oxygen produces iron(II), or Fe2+:

2 FeS₂ (s) + 7 O₂ + 2 H₂O → 2 Fe2+ (aq) + 4 SO2−
4 (aq) + 4 H+

The Fe2+ can be further oxidized to Fe3+:

4 Fe2+ + O₂ + 4 H+ → 4 Fe3+ + 2 H₂O

The Fe3+ produced can be precipitated as the hydroxide or hydrous oxide:

Fe3+ (aq) + 3 H₂O → Fe(OH)₃ (s) + 3 H+

The iron(III) ion ("ferric iron") can also oxidize pyrite:

FeS₂ (s) + 14 Fe3+ + 8 H₂O → 15 Fe2+ (aq) + 2 SO2−
4 (aq) + 16 H+

When iron(III) oxidation of pyrite occurs, the process can become rapid. pH values below zero have been measured in ARD produced by this process.

ARD can also produce sulfuric acid at a slower rate, so that the acid neutralizing capacity (ANC) of the aquifer can neutralize the produced acid. In such cases, the total dissolved solids (TDS) concentration of the water can be increased from the dissolution of minerals from the acid-neutralization reaction with the minerals.

Sulfuric acid is used as a defence by certain marine species, for example, the phaeophyte alga Desmarestia munda (order Desmarestiales) concentrates sulfuric acid in cell vacuoles.