Iron cycle

Even though iron is the fourth most abundant element in the Earth s crust, only a small portion is available for biogeochemical cycling which consists largely of oxidation—reduction reactions of ferric to ferrous ions and vice versa. These reactions are important in organic and inorganic iron-containing compounds. 

As discussed in Chapter 5, ferric and ferrous ions have very different solubility properties. Ferric ions precipitate in neutral to alkaline environments as ferric hydroxide, and under anaerobic conditions they may be reduced to the more soluble ferrous form. However, under certain anaerobic conditions enough H2S may be present so as to precipitate iron as ferrous sulfide. 

In organic compounds, iron is often attached to organic ligands by chelation and, thereby, it can undergo oxidation-reduction transformations that are utilized in electron-transport processes. Cytochromes in electron-transport chains contain chelated iron that undergoes such redox transformations. 

Virtually all microorganisms—with the exception of certain lactobacilli— require iron as cofactor of many metabolic enzymes and regulatory proteins because of its ability to exist in two stable oxidation states. Although iron is one of the most abundant elements in the environment, it is often a limiting factor for bacterial growth. This is so because of the formation of insoluble ferric hydroxide complexes under aerobic conditions at neutral pH, which impose severe restrictions on the availability of the element. Consequently, bacteria have evolved specialized high-affinity transport systems in order to acquire sufficient amounts of this essential element. 

Most bacteria have the ability to produce and secrete molecules—called siderophores—to fulfill their iron requirements. Siderophores are special iron-chelating agents that facilitate iron solubilization and uptake. They are water-soluble, low-molecular weight molecules that bind ferric ions strongly. The ability of bacteria to utilize siderophores is associated with the presence of transport systems that can recognize and mediate uptake of the ferric-siderophore complexes into the cell. These iron-acquisition systems are regulated in response to iron availability, and their action thus increases under iron limitation conditions. 

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Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

The large specific surface areas of the Fe solid phases (Fe(II,III)(hydr)oxides, FeS2, FeS, Fe-silicates) and their surface chemical reactivities facilitate specific adsorption of various solutes. This is one of the causes for the interdependence of the iron cycle with that of many other elements, above all with heavy metals, some metalloids, and oxyanions such as phosphate. 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

The cycle of iron solubilization will continue as long as bacteria and/or plants produce organic ligands.The cycle will stop when sulfate reduction rates are high and organic ligand production is low. At this point soluble hydrogen sulfide reacts with Fe(II) to form sulfide minerals. The iron cycle shown in Fig. 10.15 for salt marsh sediments may also occur in other marine sedimentary systems. 

The Iron Cycle in the Photic Zone of Surface Waters In the photic zone the formation of iron(II) occurs as a photochemical process. The photochemical iron II) formation proceeds through different pathways 1) through the photochemical reductive dissolution of iron(III)(hydr)oxides, and 2) through photolysis of dissolved iron(lll) coordination compounds, Fig. 10.16. 

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Giblin, A.E., and Howarth, R.W. (1984) Porewater evidence for dynamics sedimentary iron cycle in salt marshes. Limnol. Oceanogr. 29, 47-63. 

Swarzenski, P.W., McKee, B.A., Sorenson, K., and Todd, J.F. (1999) 210Pb and 210Po, manganese and iron cycling across the O2/H2S interface of a permanently anoxic fjord Framvaren, Norway. Mar. Chem. 67, 199-217. 

Calcium-Bromine-Iron Cycle The calcium-bromine-iron (Ca-Br, or UT-3) cycle involves solid-gas interactions that may facilitate the reagent-product separations, as opposed to the all-fluid interactions in the SI cycle, but it will introduce the problems of solids handling, support, and attrition. This process is formed of the following reactions (Doctor et al., 2002) ... 

Luther III, G. W., J. E. Kostka, T. M. Church, B. Sulzberger, and W. Stumm. 1992. Seasonal iron cycling in the salt-marsh sedimentary environment the importance of ligand complexes with Fell and Felll in the dissolution of Felll minerals and pyrite, respectively. Marine Chem. 40 81-103. 

Similarly, by adding metal oxide catalysts to the Ispra Mark 13 sulfnr-bromine cycle. General Atomics snlfnr-iodine cycle and sulfur-iron cycle (Reactions (56) to (64)), a number of new, modified metal sulfate based... 

Moore, J. K., Doney, S. C., and Lindsay, K. (2004). Upper ocean ecosystem dynamics and iron cycling in a global three-dimensional model. Global Biogeochemical Cycles. 18, 10.1029/2004GB002220. 

Parekh, P., Follows, M. J., and Boyle, E. (2004). Modeling the global ocean iron cycle. Global Biogeochem. Cycles 18(1), doi 10.1029/2003GB002061. 

Moore JK, Doney SC, Glover DM, Fung lY. Iron cycling and 37. nutrient-limitation patterns in surface waters of the World Ocean. Deep-Sea Res. II 2001 49 463-507. 

Archer D. E. and Johnson K. (2000) A model of the iron cycle in the ocean. Global Biogeochem. Cycles 14, 269—279. 

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