To clarify the formation mechanism of acid mine drainage and to propose a method to alleviate this environmental problem, the microbially-mediated dissolution of pyrite and its suppression was investigated. First, the dissolution of pyrite with the iron-oxidizing bacteria, Thiobacillus ferrooxidans, was investigated by both a solution analysis (ICP-AES) and a mineral-surface analysis (XRD, XPS, FTIR, and XMA). A microbial attack on pyrite by T. ferrooxidans proceeds mainly by the "indirect contact mechanism". An abiotic dissolution of pyrite with Fe(III) ions around pH 2 was found to be suppressed by the complexation of Fe(III) ions with anionic ligands, such as oxalate, and also by a preferential adsorption of Cu(II) and Fe(II) ions. Tannic and fulvic acids were found to be more effective inhibitors of T. ferrooxidans and T. thiooxidans than oxalic acid, reported by others. Finally, originally isolated fulvic acids were used to elucidate the suppression of the microbially mediated dissolution of pyrite by T. ferrooxidans and T. thiooxidans. The suppression was due to the inhibition of microorganisms by organic acids and their adsorption to active sites of the pyrite surface, as well as the reduction and complexation of Fe(III) ions by them. The results indicate that humic substances would be useful for the remediation and prevention of damage caused by acid mine drainage.
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