(Bio) chemistry of bacterial leaching-direct vs. indirect bioleaching.
In: Hydrometallurgy (Hydrometallurgy), Jg. 59 (2001) ; Nr. 2-3, S. 159-175
Zeitschriftenaufsatz / Fach: Chemie
A review with 83 refs. Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus / Acidianus, etc., via the (re)generation of iron(III) ions and sulfuric acid. According to the new integral model for bioleaching presented here, metal sulfides are degraded by a chem. attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bacterial extracellular polymeric substances, where they are complexed to glucuronic acid residues. The mechanism and chem. of the degrdn. is detd. 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 dissoln. Thiosulfate is, consequently, degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(II) ion-oxidizing bacteria are able to oxidize these metal sulfides. The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), hauerite (MnS2), orpiment (As2S3), and realgar (As4S4) are degradable by iron(III) ion and proton attack. Consequently, the main intermediates are polysulfides and elemental sulfur (thiosulfate is only a byproduct of further degrdn. steps). The dissoln. proceeds via a H2S* +-radical and polysulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compds. (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concn. of the iron(III) ions and, in the latter case, on the soly. product of the metal sulfide.
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