Mineral oxidation, iron oxidizing bacterial

Sulfide-mineral oxidation by microbial populations has been postulated to proceed via direct or indirect mechanisms (Tributsch and Bennett, 1981a,b Boon and Heijnen, 2001 Fowler, 2001 Sand et al., 2001 Tributsch, 2001). In the direct mechanism, it is assumed that the action taken by the attached cell or bacterium on a metal sulfide will solubilize the mineral surface through direct enzymatic oxidation reactions. The sulfur moiety on the mineral surface is oxidized to sulfate without the production of any detectable intermediates. The indirect mechanism assumes that the cell or bacteria do not act directly on the sulfide-mineral surface, but catalyze reactions proximal to the mineral surface. The products of these bacterially catalyzed reactions act on the mineral surfaces to promote oxidation of the dissolved Fe(II) and S° that are generated via chemical oxidative processes. Ferrous iron and S°, present at the mineral surface, are biologically oxidized to Fe(III) and sulfate. Physical attachment is not required for the bacterial catalysis to occur. The resulting catalysis promotes chemical oxidation of the sulfide-mineral surface, perpetuating the sulfide oxidation process (Figure 1). 

Several mechanisms have been proposed to explain the antimicrobial properties of tannins. The antimicrobial effects of phenolic compounds are probably related to the inhibition of bacterial enzymes, alterations in cell wall permeability, an increase in the hydrogen ion activity of the microbial environment, a reduction in the surface and/or interfacial tension and perhaps chelation of essential minerals, particularly iron with a concomitant impairment of the microbial oxidative metabolic system (Chung et al. 1998). The antimicrobial activities of tannins are ascribed to the interactions of goats tannins with the extracellular enzymes secreted and... 

It can be seen, therefore, that ferrous iron and chalcopyrite oxidation are acid-consuming reactions, while pyrite oxidation and iron hydrolysis are acid-producing reactions. Thus, whether the overall reaction in a dump is acid producing or acid-consuming depends on the relative proportions of chalcopyrite and pyrite and on the pH conditions. In practice, sulfuric acid additions to the leach solution applied to the dump are usually required to overcome the acid consuming reactions of the gangue minerals and to keep the pH in a suitable range, typically 2 to 2.4, to optimize bacterial activity and minimize iron hydrolysis. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

This bacterial production occurs in the pore fluids of sediments and in stagnant basins (seas, lakes, rivers and fiords). At the interface between anoxic and oxic waters the H2S can be oxidized. This oxidation is frequently coupled to changes in the redox state of metals (1.2) and non-metals (2). Another major interest in the H-jS system comes from an attempt to understand the authigenic production of sulfide minerals as a result of biological or submarine hydrothermal activity and the transformation and disappearance of these minerals due to oxidation (4). For example, hydrothermally produced H2S can react with iron to form pyrite, the overall reaction given by... 

A major contribution of this paper was pointing out the importance of bioturbation and bioirrigation on chemical processes associated with carbonate dissolution. In the movement of sulfidic sediment from depth to near the interface by biological processes, oxidation of the sediment produces sulfuric acid which ends up titrating alkalinity, lowering pH, and thus lowers saturation state (e.g., Berner and Westrich, 1985). Actually this process is very complex, involving many reactive intermediate compounds such as sulfite, thiosulfate, polythionates, etc. Aller and Rude (1988) demonstrated an additional complication to this process. Mn oxides may oxidize iron sulfides by a bacterial pathway that causes the saturation state of the solution to rise with respect to carbonate minerals, rather than decrease as is the case when oxidation takes place with oxygen. 

SABA [Spherical Agglomeration-Bacterial Adsorption] A microbiological process for leaching iron pyrite from coal. The bacterium Thiobacillus ferrooxidans adsorbs on the surface of the pyrite crystals, oxidizing them with the formation of soluble ferrous sulfate. Developed by the Canadian Center for Mineral and Energy Technology, Ottawa. In 1990, the process had been developed only on the laboratory scale, using coal from eastern Canada. 

Mineral saturation indices for melanterite and amorphous iron hydroxide agree quite well with field occurrences of the same minerals. Jarosite, however, appears to be supersaturated for nearly all of the samples regardless of the presence or absence of the mineral in these streams. Field observations indicate that jarosite precipitation occurs in the microenvironment of bacterial colonies where the chemical conditions may be quite different from the bulk solution. These considerations lead us to suggest that there is a kinetic barrier which hinders jarosite precipitation but does not hinder ferric hydroxide precipitation and that this barrier is overcome by the surfaces of bacterial colonies (both iron-oxidizers and unidentified nonoxidizers ). ... 

Habicht and Canheld (2001) provide evidence that the pyrite forming in marine sediments is inhuenced by such a pathway by simultaneously determining the isotopic composition of H2S produced by SRB and that of iron-sulhde minerals coexisting in the same sediments. Their measured bacterial fractionations could only explain 41-85% of the observed depletion in iron sulhdes from a range of marine environments. They calculated a closed-system model showing that microbial-disproportionation fractionations involving partially oxidized sulfur species, as documented in the laboratory, could explain their data. Their calculations assumed that all H2S initially produced by SRB was... 

The existence of these chemical pathways for the oxidation of minerals during bacterial leaching does not exclude a direct role for bacteria. Both pathways may occur simultaneously, the relative importance of each depending upon the rate constants for the reactions and the concentration of Fe(III) present in solution. Experiments carried out on synthetic iron-free cobalt and nickel sulfides, using carefully washed cells of T. ferrooxidans to ensure the absence of Fe(III), showed consumption of oxygen and the solubilization of the metal as sulfate. Solubilization rates were appreciably increased on the addition of Fe(III). These results present support for the presence of both direct and indirect pathways (116). Recently, attempts have been made to compare the rates of these pathways for the oxidation of pyrite by T. ferrooxidans (74). 

The investigations of Landesman et al. (1966a, b) clarify the effects of the various conditions controlling the optimum oxidation rates of ferrous iron, sulfur and reduced sulfur compounds by T. ferrooxidans. Experiments on soluble iron, sulfur and iron-containing sulfide minerals (chalcopyrite, CuFeS2, bornite, CUsFeS4, and pyrite) established that iron and sulfur can be oxidized simultaneously. With a mixed iron-sulfur substrate a rate of oxidation, equal to that of the sum of the maximum rates of oxidation of the two substrates individually was observed with both S-adapted and Fe-adapted cells. Subsequently, Duncan et al. (1967) established the differential susceptibility of the bacterial oxidation of ferrous iron and sulfur to N-ethyl maleimide and sodium azide, and determined the effect of these inhibitors on pyrite and chalcopyrite oxidation. Decreased rates... 

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