Iron Sulfide Oxidation as Influenced by Calcium Carbonate Application

Two overburden materials, with different FeS 2 contents (1.9 and 4.1%) and low acid neutralization potential, were limed with CaCO 3 at rates of 0, 25, 50, 75, 100, and 125% based on the amount of CaCO 3 needed to provide an acid–base account deficit (A/B a ) of zero (A/B a = neutralization potential − potential acidity − exchangeable acidity). The limed overburden materials were inoculated with Thiobacillus ferrooxidans and leached weekly with deionized water. Residual FeS 2 and CaCO 3 were determined in samples over a 378‐d period. Oxidation followed zero‐order kinetics with respect to FeS 2 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 μmol g −1 d −1 in the overburden with 1.9% FeS 2 and from 0.01 to 0.22 μmol g −1 d −1 in the overburden with 4.1% FeS 2 Oxidation following the first‐order rate law had a first‐order rate constant of 0.03 d −1 in the 1.9% FeS 2 overburden and 0.01 d −1 in the 4.1% FeS 2 overburden. The calculated half‐life was 23 d for the 1.9% FeS 2 overburden and 69 d for the 4.1% FeS 2 overburden. Additions of CaCO 3 affected FeS 2 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 CaCO 3 was found to be faster than the oxidation of FeS 2 at pH values greater than 4. It was projected that at lime rates up to 125%, the CaCO 3 would dissolve and leach out of the system before all the FeS 2 oxidized, leaving the potential for acid minesoil formation.