Iron sulfides reoxidation

The strong coupling between sulfur and iron chemistry becomes obvious in this example. Conservation of alkalinity within the system is achieved only if the sulfide formed is prevented from reoxidation, a process that would restore the acidity. Prevention of reoxidation occurs through the ultimate storage of sulfide in sediments, either as organic sulfur or as iron sulfides (12, 13). The overall reaction of pyrite formation proceeds via formation of FeS ... 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

Sulfate reduction occurs mainly in sediments, with rates controlled by the quality and quantity of organic matter, S042- availability, bioturbation, abundance of dissolved O2 in overlying waters, rates of sulfide reoxidation, and availability of iron-sulfide minerals. 

The sulfite (SOf ) anion can undergo reoxidation by oxygen closer to the sediment surface to produce polysulfur anions or even sulfate. Elemental sulfur may be found in marine sediments as a product of the oxidation of iron sulfides (Schippers and Jorgensen, 2001). The incorporation of pyrite and organosulfides, formed as a by-product of sulfate reduction in marine sediments, is a major sink for seawater S04. Over geological time we have seen that this is an important process in controlling the concentrations of seawater sulfate. 

A similar study to the one just described was undertaken by Jorgensen (1977c) on the coastal marine sediments of Limfjorden, Denmark. Sulfate reduction (on average 9.5 mmol m d ) was largely confined to the upper 20 cm of sediment and over 90% of the HjS produced appeared to be reoxidized at the sediment-water interface only about 7% of HjS was converted to iron sulfides. The development of the sulfur cycle in sediments has been described by Jorgensen and Fenchel (1974) using the experimental system described earlier (see p. 303). The results, illustrated in Fig. 6.1.3, showed that the rate of sulfate reduction remained almost constant over the period studied and was accompanied by increasing pools of HjS, FeS and S ... 

Measured rates of sulfate reduction can be sustained only if rapid reoxidation of reduced S to sulfate occurs. A variety of mechanisms for oxidation of reduced S under aerobic and anaerobic conditions are known. Existing measurements of sulfide oxidation under aerobic conditions suggest that each known pathway is rapid enough to resupply the sulfate required for sulfate reduction if sulfate is the major end product of the oxidation (Table IV). Clearly, different pathways will be important in different lakes, depending on the depth of the anoxic zone and the availability of light. All measurements of sulfate reduction in intact cores point to the importance of anaerobic reoxidation of sulfide. Little is known about anaerobic oxidation of sulfide in fresh waters. There are no measurements of rates of different pathways, and it is not yet clear whether iron or manganese oxides are the primary electron acceptors. 

Recently we presented (23) the results of an experimental study on the kinetics and mechanisms of the reaction of lepidocrocite (y-FeOOH) with H2S. With respect to the interaction between iron and sulfur, lepidocrocite merits special attention. It forms by reoxidation of ferrous iron under cir-cumneutral pH conditions (24), and it can therefore be classified as a reactive iron oxide (19). The concept of reactive iron was established by Canfield (19), who differentiated between a residual iron fraction and a reactive iron fraction (operationally defined as soluble in ammonium oxalate). The reactive iron fraction is rapidly reduced by sulfide or by microorganisms. 

FeS can still be found in such environments because of an insufficient supply of an oxidant to react with sulfide. This insufficiency may happen when either no sulfide-reactive iron exists or the rate of reoxidation of (mi-... 

The CRS AVS ratio reflects the trophic state of a lake (cf. Urban (28), Table V). This observation may be explained by the preceding kinetic considerations. The high organic matter supply in eutrophic lakes leads to an intensive mineralization rate by both iron- and sulfate-reducing bacteria. However, reoxidation of Fe2+ to ferric oxide or of sulfide to sulfate does not take place because an anoxic hypolimnion prevents penetration of oxygen. Therefore FeS can build up, but the sediment becomes depleted with respect to reactive iron. 

Substances in the bulk solution diffuse into the biofilm, where they are consumed (such as oxygen, point 1 in Figure 6) or recycled (such as sulfate through stepwise reoxidation of H2S from sulfate reduction, point 5). Within the biofilm, very steep gradients exist for oxygen or hydrogen sulfide and also for ferrous iron from reductive dissolution of ferric oxides. These gradients result from the coexistence of anaerobic and aerobic metabolisms such as aerobic respiration (point 1), reduction of ferric oxides (point 3), and sulfate... 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

Iron Sponge Also called Dry box. An obsolete process for removing hydrogen sulfide from gas streams by reaction with iron oxide monohydrate. The ferric sulfide formed is periodically reoxidized to regenerate ferric oxide and elemental sulfur. When this process becomes inefficient because of pore-blockage, the sulfur is either oxidized to sulfur dioxide, for conversion to sulfuric acid, or is extracted with carbon disulfide. 

These microbially mediated redox processes utilize electron acceptors and produce reduced species. This will generate more reduced environments as long as there are electron donors available. The microbial population thus strongly affects their environment in the core of the plume. At the boundaries of the plume, complex microbial communities may exist, and steep redox gradients are created when dissolved electron acceptors are consumed. In addition, reoxidation of sulfides or ferrospecies by oxygen diffusing into the plume may increase the concentration of sulfate and ferric iron, which can stimulate sulfate and iron reduction in these zones as observed at Norman Landhll (Cozzarelli et al., 2000). 

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