Pyrite Oxidation

Pyrite (FeS2) is an iron sulfide mineral that is formed through the process of sulfate reduction. Pyrite is stable under anoxic conditions. However, on exposure to air, a mole of pyrite oxidises to form 2 moles of sulfuric acid in a 3 step reaction [1]:

(Eq. 1) FeS2 + 7/2O2 + H2O = Fe2+ + 2SO42- + 2H+
(Eq. 2) Fe2+ + 1/4O2 + 3/2H2O = FeOOHppt + 2H+
(Eq. 3) FeS2 + 15/4O2 + 7/2H2O = Fe(OH)3ppt + 2SO42- + 4H+

Aluminium and Iron Mobilisation

The sulfuric acid formed in sulfate reduction may then react with clay minerals to release silica and metals of which aluminum (Al3+), iron (Fe2+), potassium (K+), sodium (Na+) and magnesium (Mg2+) are most abundant [1]:

(Eq. 4) (K0.5N0.36Ca0.05)(Al1.5Fe0.25Mg0.3)(Al0.45Si3.46)O10(OH)2 + 7.41H+ + 2.59H2O = 0.5K+ + 0.36Na+ + 0.05Ca2+ + 0.3Mg2+ + 0.25Fe(OH)3 + 1.95Al3+ + 3.46H4SiO4

Manganese and metal contaminants may also be released in the process [1].

  1. Sammut, J., White, I., Melvilles, M.D. 1996. Acidification of an estuarine tributary in eastern Australia due to drainage of acid sulphate soils, Marine and Freshwater Research 47, 669-684.
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