Effect of carbonate ions on pyrite (FeS2) dissolution

Abstract

Abstract Neutralization by carbonate of acidification generated by pyrite (FeS2) oxidation was investigated by both solution (iron and sulfur speciation, pH and Eh) and solid (FT-IR) characterizations. Batch dissolution experiments were carried out in contact with atmospheric oxygen (20 %) in four different bicarbonated solutions ([NaHCO3]=10–3, 1,12.10−2, 10−1 and 1 mol/L). Five different contact duration were selected : 6 hours, 1, 3, 8 and 30 days. Ferrous carbonate complexes (FeOHCO3− and Fe(CO3 )22− ) tend to maintain iron in solution (up to 152.2 μmol/L in [NaHCO3]=1 mol/L solution) and to increase pyrite oxidation rate by preventing surface coating. Acidification is thus more intense in diluted and concentrated carbonate medium ([NaHCO3− ] =10–3 and 1 mol/L) with respectively ΔpH=5.06 and ΔpH=1.99 at 30 days whereas pH remains buffered in [NaHCO3− ] =1.12.10−2 and 0.1 mol/L solutions. Siderite appears to be the first solid precipitating, transforming into gœthite, oxyhydroxy ferric sulfate incorporating sulfite and thiosulfate, and then lepidocrocite. Sulfur chemistry controls the acidification observed. Thiosulfate is the first sulfoxyanion released in solution and its oxidation into sulfite then sulfate seems to be the key of acidification production. Thus, carbonate pH buffer properties seem to be limited and effective for moderated carbonate concentrations.