Principles of Sulfide Oxidation and Acid Rock Drainage

Oxidation of sulfide minerals releases sulfuric acid and dissolved metals, with iron sulfides pyrite (FeS₂) and pyrrhotite (Fe_(1−x)S) recognized as the most common acid-forming minerals. Several factors control the oxidation rate including: the oxidant type, sulfide morphology, microbial action, and trace element contents. Whilst metal sulfides such as galena and sphalerite are less acid-forming, they are typically sources of environmentally significant elements such as Cd, Pb and Zn. Common sulfide oxidation reaction products are metal-sulfate efflorescent salts. Dissolution of these minerals is critical to the storage and transport of acids and metals released upon weathering of mineralized rock or mine wastes. Acid formed by sulfide oxidation can be consumed through reaction with gangue minerals. Neutralization is primarily offered by dissolution of carbonate minerals with calcite and dolomite the most effective. Factors affecting carbonate reactivity include: grain size, texture and the presence of trace elements which can influence a mineral’s resistance to weathering. Silicate minerals such as olivine, wollastonite and serpentinite are recognized as effective longer term neutralizers. Lesser neutralizing potential contributions from phyllosilicates, pyroxenes, amphiboles and feldspars have been reported. Micas, clays and organic matter can temporarily adsorb H⁺ ions through cation exchange reactions, with gibbsite and ferric hydroxide recognized as offering neutralizing capacity under acidic conditions. Ultimately, the balance of acid producing and acid consuming chemical reactions will determine the production of acid rock drainage (ARD).