(Bio) chemistry of bacterial leaching - direct vs. indirect bioleaching

Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus fe rrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus, etc., via th e (re)generation of iron(III) ions and sulfuric acid. According to the new integral model for bioleaching presented here, metal s ulfides are degraded by a chemical attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bact erial extracellular polymeric substances, where they are complexed to glucu ronic acid residues. The mechanism and chemistry of the degradation is dete rmined by the mineral structure. The disulfides pyrite (FeS2), molybdenite (MoS2), and tungstenite (WS2) are degraded via the main intermediate thiosulfate. Exclusively iron(III) ions are the oxidizing agents for the dissolution. Thiosulfate is, consequently , degraded in a cyclic process to sulfate, with elemental sulfur being a si de product. This explains, why only iron(II) ion-oxidizing bacteria are abl e to oxidize these metal sulfides. The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), h auerite (MnS2), orpiment (As2S3), and realgar (As4S4) are degradable by iro n(III) ion and proton attack. Consequently, the main intermediates are poly sulfides and elemental sulfur (thiosulfate is only a by-product of further degradation steps). The dissolution proceeds via a H2S*(+)-radical and poly sulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compounds (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concentration of the i ron(III) ions and, in the latter case, on the solubility product of the met al sulfide. (C) 2001 Elsevier Science B.V. All rights reserved.