Bacteria Sulfide oxidation, microbial

Measured rates of microbial oxidation of sulfide in lakes range from 0 to over 100,000 mmol/m2 per year (Table IV). These rates, which are comparable to measured rates of sulfate reduction (Table I), suggest that microbial oxidation of sulfide is capable of supplying sulfate at rates needed to sustain sulfate reduction. The majority of measurements are for photosynthetic bacteria in the water column. Symbiotic sulfate reduction and sulfide oxidation are known to occur and lead to dynamic cycling of S within anaerobic water... 

The pre-1991 research involving microbial oxidation of 29 sulfide minerals of iron, copper, arsenic, antimony, gallium, zinc, lead, nickel, and mercury was compiled by Nordstrom and Southam (1997). The importance of microbially mediated sulfide oxidation has been recognized for several decades (Nordstrom and Southam, 1997). Bacteria catalyze the oxidative dissolution of sulfide minerals, increasing the production of acidity in mine wastes. In the absence of bacteria, the rate of sulfide oxidation stabilizes as the pH decreases below 3.5 (Singer and Stumm, 1970). 

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). 

A microbial contacting process for the oxidation of H2S was disclosed [22], in which a chemoautotrophic bacterium T. thiooxidants or T. ferroxidans) is used to remove sulfides from gaseous streams, at aerobic conditions and low pH. The low pH is preferred since the optimum pH for growth of the bacteria is below 4.0. and for the elimination of undesired contaminant bacterial strains. A contactor is employed, the flow of the sulfur-containing stream is contacted counter-currently with the biocatalytic aqueous solution. The sulfate is recovered from the aqueous solution, which contains the biocatalyst, as well. 

Mercury occurs in soils predominantly in the +2 oxidation state. Elemental Hg in the atmosphere is oxidized to Hg + and deposited in rainfall. It is a strong chalcophile and under anaerobic conditions forms the extremely insoluble sulfide cinnabar (HgS, pK = 52.7). Nonetheless it is not entirely immobilized under anaerobic conditions because it is reduced to volatile Hg° or methylated to volatile methyl mercury compounds by microbial action, and so returned to the atmosphere. The methylation is mediated by various bacteria, especially methanogens, through the reactions ... 

Oxidation of insoluble mineral sulfides to the usually water-soluble sulfates (PbS04 is an exception) can also be carried out in many cases by microbial leaching, that is, by the use of bacteria such as Thiobacillus fer-rooxidans which can use the sulfide-sulfate redox cycle to drive metabolic processes. The overall reaction still consumes oxygen... 

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). 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

The chemical oxidation of metal sulfides is controlled in part by the dissolution of sulfide minerals under acidic conditions and by the presence of oxidants (DO, Fe ) that lead to the disruption of sulfide chemical bonds. Bacteria can have a significant effect on the rate of oxidative dissolution of sulfide minerals by controlling mineral solubility and surface reactivity. Metal-enriched waters and solutions rich in sulfuric acid that form in association with mining can be directly linked to microbial activity. The majority of studies to date have focused on the reactivity and kinetics of sulfide minerals in the presence of A. ferrooxidans and L. ferrooxidans, and in some cases A. thiooxidans (Singer and Stumm, 1970 Tributsch and Bennett, 1981a,b Sand et al., 1992, 2001 Nordstrom and Southam, 1997 Sasaki et al., 1998 Edwards et al., 1998, 1999, 2000 Nordstrom and Alpers, 1999a Banfield and Welch, 2000 Tributsch, 2001). Additional studies have been conducted on other species of bacteria and archea (Edwards et al, 1998, 1999, 2000). 

However, the mechanism for direct oxidation is poorly understood (Silverman and Ehrlich, 1964). Iron is also made available for microbial oxidation after dissociation of the sulfide complexes by a chemical oxidation of the sulfide moiety of the mineral. A strong chemical oxidizing agent is the Fe ion itself. Singer eind Stumm (1970) showed that, under acidic conditions and in the absence of bacteria, Fe was a much more effective catalyst of pyrite oxidation than was ferrous iron. However, in the presence of bacteria, the rate of pyrite oxidation in the presence of Fe was higher the reduced iron was biologically oxidized to ferric iron which then oxidized the pyrite ... 

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