Acidithiobacillus ferrooxidans pyrite oxidation

Bacteria may catalyze and considerably enhance the oxidation of pyrite and Fe(II) in water, especially under acidic conditions (Welch et al., 2000, 597). Many microbial species actually oxidize only specific elements in sulfides. With pyrite, Acidithiobacillus thiooxidans is important in the oxidation of sulfur, whereas Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans (formerly Thiobacillus fer-rooxidans) oxidize Fe(II) (Gleisner and Herbert, 2002, 140). Acidithiobacillus ferrooxidans obtain energy through Reaction 3.45 (Gleisner and Herbert, 2002, 140). The bacteria are most active at about 30 °C and pH 2-3 (Savage, Bird and Ashley, 2000, 407). Acidithiobacillus sp. and Leptospirillum ferrooxidans have the ability to increase the oxidation of sulfide minerals by about five orders of magnitude (Welch et al., 2000, 597). 

Fe(III) can oxidize arsenopyrite about 10 times faster than pyrite and the rates are even more rapid if Acidithiobacillus ferrooxidans is present (Welch et al., 2000, 597 Gleisner and Herbert, 2002, 140 Evangelou, Seta and Holt, 1998, 2084). Scorodite, a product of Reaction 3.52, may form colloids in water and natural organic matter (NOM) could assist in stabilizing the colloids (Buschmann et al., 2006, 6019). 

Figure 3. Diagram of a section through the cell wall of Acidithiobacillus ferrooxidans modified from Blake et al. (1992) showing the relationship between iron oxidation and pyrite dissolution. OM =outer membrane, P = periplasm, IM = inner or (cytoplasmic) membrane, cty = cytochrome, pmf = proton motive force. Passage of a proton (driven by proton motive force) into the cell catalyzes the conversion of ADP to ATP. Ferrous iron binds to a component of the electron transport chain, probably a cytochrome c, and is oxidized. The electrons are passed to a terminal reductase where they are combined with O2 and to form water, preventing acidification of the cytoplasm. Ferric iron can either oxidize pyrite (e.g. within the ore body) or form nanocrystalline iron oxyhydroxide minerals (often in surrounding groundwater or streams).

From the shape (rods) of the bacteria, they appear to be Acidithiobacillus ferro-oxidans but not Leptospirillum ferrooxidans. Finally, when the mixture of the mudstone and the medium for the sulfur-oxidizing bacteria (pH 6.5) supplemented with powdered pyrite is shaken in air, the pH of the culture medium is lowered over time. When the pH is lowered to below 4, the amount of ferrous ion plus ferric ion in the medium increases rapidly together with a parallel increase of sulfate ion (Yamanaka et al., 2002b). These phenomena are not observed with the mudstone heated at 121°C for 20 min. The results show that the acidophilic iron-oxidizing bacteria (growing at pH lower than about 4) oxidize pyrite but the usual sulfur-... 

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