Reduction of Iron and Manganese

Microorganisms involved in oxidation-rednction reactions of iron and manganese can be grouped into the following  

Bacteria that are able to use Fe(IH) and Mn(IV) as electron acceptors and derive energy for their growth from organic and inorganic snbstrates. These organisms must have unique characteristics, which might include the ability to (1) solubilize substrates, (2) attach to substrate and directly transfer electrons to it, or (3) transport the substrate into the cell as a solid (Nealson and Myers, 1992). 

Oxidation CgHi20g Reduction 12Mn02 + 48H+ -i Oxidation-reduction CgHi20g 

FIGURE 10.6 Relative energy yield during reduction of select electron acceptors with glucose as the electron donor. Values expressed as percent energy yield under aerobic (oxygen as electron acceptor) conditions. 

Bacillus, Clostridium, Desulfovibrio, Escherichia, Ferribacterium, Geobacter, Geothrix, Lactobacillus, Pelobacter, Pseudomonas Sulfolobus, Thiobacillus Pseudomonas, Shewanella Bacillus, Clostridium, Desulfovibrio, Escherichia, Ferribacterium, Geobacter, Lactobacillus, Pseudomonas, Shewanella Geobacter 

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Lovley, D., Phillips, E.J.P., and Lonergan, D.J. (1989) Hydrogen and formate oxidation coupled to dissimilatory reduction of iron and manganese by Alteromonas putrefaciens. Appl. Environ. Microbiol. 55, 700-706. 

FIGURE 3.16 Oxidation-reduction of iron and manganese under flooded and drained soil conditions. 

As shown in these reactions, reduction of iron and manganese results in the consumption of H ions, thus resulting in a decrease in H+ ion activity or an increase in pH. For example, in paddy soils of Philippines (Table 4.4) iron comprises 0.63-4.7% of the total soil mass, and manganese content can be as high as 0.1%. The proton to electron ratio is 3 for Fe(OH)3 reduction, as compared to a ratio of 2 for Mn02 reduction. Thus, iron reduction will have a greater effect on the pH increase in acid soils. Reduction of other electron acceptors such as nitrate and sulfate can also have a similar effect on the soil pH. In acid soils, an initial increase in pH can also occur due to rapid decomposition of soil organic matter and accumulation of carbon dioxide. However, acid soils eventually increase in pH due to reduction of oxides of Fe and Mn. 

It is well established that iron reduction is coupled to soluble phosphorous release in soils dominated by iron redox couples (Figure 10.32) (see Chapter 9 for details). Although phosphorous itself is not normally involved in redox reactions, it does undergo reactions that have a pronounced effect on its reactivity. Most of this change in the reactivity of phosphorous in wetland soils and aquatic sediments is associated with the oxidation-reduction of iron and manganese. The reduction of ferric phosphate compounds results in the release of phosphorous, a major solubility mechanism in wetlands and aquatic systems. 

Unique characteristics of ferromanganese nodules and associated oxidation-reduction reactions have been used by soil scientists as morphological indicators to help identify hydric soils (see Chapter 3). These characteristics are termed by soil scientists as redoximorphic features however, various terms such as redox concentrations, redox depletions, and reduced matrix are synonymously used for the oxidation-reduction of iron and manganese and their respective concentrations. We prefer not to define these characteristics as redoximorphic features because oxidation-reduction reactions not only involve iron and manganese but also a range of elements that support biotic communities in the biosphere. 

List the oxidation states of iron and manganese. Which oxidation states are present under reducing conditions Describe the sequential reduction of iron and manganese as related to the presence of other electron acceptors. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Lovley, D. R and E. J. P. Phillips (1988), "Novel Mode of Microbial Energy Metabolism Organic Carbon Oxidation Coupled to Dissimilatory Reduction of Iron or Manganese", Applied and Environ. Microbiology 54/6, 1472-1480. 

Manganese. The reduction and oxidation processes of iron and manganese, which may be important under the conditions of the anoxic hypo-... 

Reductive dissolution of iron and manganese (oxy)(hydr)oxides... 

The oxidation-reduction potentials and reaction constants of oxidation of iron and manganese differ and these reactions can occur in different amounts of oxygen. That is why the level of appearance of particulate manganese is situated higher than that of particulate iron [63]. Bacteria have been shown to oxidize manganese [64], whereas iron oxidation is possible without bacteria but can be carried out with bacteria [50]. Reduced iron can be oxidized by particulate manganese, forming complex compounds [65]. 

Some microorganisms can catalyze certain oxido-reduction reactions like the oxidation of iron and manganese in water, the oxidation of sulfur compounds, and oxidation-reduction of nitrogen compounds. Aerobic autotrophic bacteria of the type Thiobacillus can release soluble iron, copper, and sulfuric acid as sulphates into water. These organisms can be found everywhere in nature wherever an acidic environment is maintained in the presence of sulfide-containing minerals. 

Thamdrup B., Finster K., Hansen J. W., and Bak F. (1993) Bacterial disproportionation of elemental sulfur coupled to chemical reduction of iron or manganese. Appl. Environ. Microbiol. 59, 101-108. 

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