Practice exam #3 - QUESTION 2

a) Low pH indicates a high concentration of hydrogen ions. The reaction of pyrite with water and oxygen releases just such a preponderance of hydrogen: 2FeS2 + 9O2 + 4H2O -> 8H^+ + 4SO4^2- + 2Fe(OH)3

b) The acidity is a product of the dissolution of pyrite in water. Therefore, programs to limit the amount of water in the soil may reduce the acidity. This could be done by improving drainage, potentially by adding sand or larger size-fraction particles to the soil. Alternately, alkaline compounds such as lime could be added to the soil to raise the pH.

Spencer, please see Maja's explanation about S oxidation above, and refer to lecture #24.So when answering part b) limiting the amount of water would not reduce the oxidation of the sulfides (and thus would not reduce the acidity). In fact, drainage would make the problem worse (not better) - see comments above e.g. from Blaise.