Direct versus indirect bioleaching.
A review with 81 refs. Bioleaching of metal sulfides is effected by bacteria like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus etc. via the (re)generation of iron(II) 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 EPS, where they are complexed to glucuronic acid residues. The mechanism of degrdn. is detd. by the mineral structure. The disulfides pyrite (FeS2), molybdenite (MoS2), and tungstenite (WS2) are degraded via the main intermediate thiosulfate. Iron(III) ions are exclusively the oxidizing agents for dissoln. Thiosulfate is consequently degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(I) 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(II) 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 Thiobacillus thiooxidans etc.). The two mechanisms, based on the mineral structure of a metal sulfide, are summarized in the following Figure 1.
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