Iron Sulfide Oxidation as Influenced by Calcium Carbonate Application

Two overburden materials, with different FeS2 contents (1.9 and 4.1%) and low acid neutralization potential, were limed with CaCO3 at rates of 0, 25, 50, 75, 100, and 125% based on the amount of CaCO3 needed to provide an acid–base account deficit (A/Ba) of zero (A/Ba = neutralization potential - potential acidity exchangeable acidity). The limed overburden materials were inoculated with Thiobacillus ferrooxidans and leached weekly with deionized water. Residual FeS2 and CaCO3 were determined in samples over a 378-d period. Oxidation followed zeroorder kinetics with respect to FeS2 concentration at pH values greater than 4 and first-order kinetics at pH values less than 4. Zero-order oxidation rates ranged from 0.0l to 0.46 (mu)mol g-1 d-1 in the overburden with 1.9% FeS2 and from 0.01 to 0.22 (mu)mol g-1 d-1 in the overburden with 4.1% FeS2. Oxidation following the first-order rate law had a first-order rate constant of 0.03 d-1 in the 1.9% FeS2 overburden and 0.01 d-1 in the 4.1% FeS2 overburden. The calculated half-life was 23 d for the 1.9% FeS2 overburden and 69 d for the 4.1% FeS2 overburden. Additions of CaCO3 affected FeS2 oxidation by controlling the pH of the system. Liming to greater than 50% of the acid–base account deficit did not significantly affect the zero-order oxidation rate. Dissolution of the applied CaCO3 was found to be faster than the oxidation of FeS2 at pH values greater than 4. It was projected that at lime rates up to 125%, the CaCO3 would dissolve and leach out of the system before all the FeS2 oxidized, leaving the potential for acid minesoil formation.