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Bulletin de la Societe Geologique de France; May 2002; v. 173; no. 3; p. 265-270; DOI: 10.2113/173.3.265
© 2002 Societe Geologique de France
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Effect of carbonate ions on pyrite (FeS2) dissolution

Michaël Descostes1,2, Catherine Beaucaire3, Florence Mercier4, Sébastien Savoye3, Joachim Sow4 and Pierpaolo Zuddas2

1 CEA-DEN, Service de Chimie et Physique Analytique, CE Saclay, 91191 Gif-sur-Yvette, France. Tel. : +33-1-69-08-73-60; fax : +33-1-69-08-32-42 ; e-mail : michael.descostes{at}cea.fr.
2 Université Paris 7, Laboratoire de Géochimie des Eaux, 75251 Paris cedex 05, France.
3 IPSN, Service d’Etudes et de Recherches sur la Géosphère et l’élimination des Déchets, 92265 Fontenay-aux-Roses, France.
4 CNRS-UMR 8587, CE Saclay, 91191 Gif-sur-Yvette, France.

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 {Delta}pH=5.06 and {Delta}pH=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 goethite, 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.

Key Words: Pyrite • Oxidation • Carbonate pH buffer • FTIR • Redox






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