Iron Oxides in the Environment

P-Fe20s has been obtained by dehydration of P-FeOOH in high vacumn at 170 °C (Braun and Gallagher, 1972). s-FeaOs ean be produced by the reae-tion of alkaline potassium ferrieyanide solution with sodium hypochlorite. It is also obtained (together with a mixture of other iron oxides) in an electric arc under an oxidizing atmosphere (Buttner, 1961). Its magnetic and thermal properties have been investigated by Dezsi and Coey (1973). 

Wustite, FeO, is obtained by heating a pelletized mixture of hematite and iron at 837 °C in a sealed siliea tube for 24 lirs and then quenching in liquid N2 (Battle and Cheetham, 1979). It is usually non-stoichiometric and contains defect clusters approaehing the Fe304 structure. Wustite is stable only at temperatures greater than 570 °C. At lower temperatures it decomposes to Fe304 and Fe. 

Iron oxides are widespread in nature. They are ubiquitous in soils and roeks, lakes and rivers, on the sea floor, in air (e.g. admixed in aeolian Sahara dust) and in organisms, and they may be responsible for the red-dish-eolored surface of the Mars. Iron oxides are of great significance for many of the properties and processes taking place in ecosystems. 

Iron oxides may be either beneficial or undesirable. Everyone is familiar with rust, a mixture of various Fe oxides, as the end product of corrosion of iron. The modem steel industry, on the other hand, relies on the huge deposits of hematite and magnetite (iron ores) found in many parts of the world. 

For example, the formation of goethite (ot-FeOOH) from an olivine (faya-lite) (eq. 2) or from pyrite (eq. 3) ean be written as 

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Siderite is a common mineral in mires, where it is formed through microbiological reduction of iron oxides in the environment. This mechanism may explain its occurrence on artefacts that have lain exposed on the mire surface for a period of time. In these conditions they will be quickly covered by a layer of iron oxides, which will subsequently be reduced to siderite after being overgrown and embedded in an anoxic environment. However, for other artefacts (and modem samples) that have been placed directly under anoxic conditions the siderite must have formed directly from the metallic iron, and here it is still unclear exactly what cathodic reaction is responsible for the oxidation of iron. A Pourbaix diagram based on the actual soil conditions at Nydam is shown in Figure 8. The hatched area in the Pourbaix diagram demonstrates that the pH values found at Nydam are close to the lower limit for siderite stability, so the soil pH is monitored intensively to be sure that no acidification takes place. 

Ferrihydrite is generally the initial precipitate that results from rapid hydrolysis of Fe solutions. Its crystallinity, i.e. crystal size and order, is usually lower than that of any of the other Fe oxides described except feroxyhyte and schwertmannite. It is usually named according to the number of its XRD peaks, with 6-8 broad peaks for well crystalline (6-line-) ferrihydrite and only two very broad ones for the most poorly crystalline form (2-line-ferrihydrite). The 2-line ferrihydrite is commonly but incorrectly called hydrous ferric oxide (HFO) or, amorphous iron oxide . In natural environments all forms of ferrihydrite are widespread usually as yoimg Fe oxides and they play an important role as an active sorbent due to their very high surface area. 

Cowan, C. E., Zachara, J. M., and Resch, C. T. (1991). Cadmium adsorption on iron oxides in the presence of alkaline-earth elements. Environ. Sci. Technol. 25, 437-446. 

Inskeep, W.P., 1989. Adsorption of sulfate by kaolinite and amorphous iron oxide in the presence of organic ligands. J. Environ. Qual. 18, 379-385. 

Fig. 6 Plot of Fe 2p3/2 BE vs. oxidation state for iron-containing compounds. The BE also varies between compounds having the same Fe oxidation state because of changes in the environment and screening of the nuclear charge provided by the ligands...

Lafferty BJ, Loeppert RH (2005) Methyl arsenic adsorption and desorption be-hatior on iron oxides. Environ Sci Technol 39 2120—2127 Le XC (2002) Arsenic speciation in the environment and humans. In Frankenberger WT Jr (ed) Environmental chemistry of arsenic. Marcel Dekker, Inc. New York, pp 95-116... 

Iron and its various oxidation state species are common components of the environment. In addition to the oxides FeO and Fe203, it is found in minerals such as hematite, goethite, and ferrihydrite, and in a number of hydroxy and oxy compounds. Because of its common occurrence in the environment in general, and in soil in particular, the total iron content of soil is usually not a useful piece of information. 

The importance of bacteria in mediating Mn(II) oxidation in certain environments is evident. But, the mechanisms whereby bacteria oxidize Mn(II) are poorly understood. Some bacteria synthesize proteins or other materials that enhance the rate of Mn(II) oxidation (.52). Other strains of bacteria require oxidized manganese to oxidize Mn(II) (53), suggesting that they may catalyse the oxidation of Mn(II) on the manganese oxide surface. Other bacteria may catalyse the oxidation of Mn(II) on iron oxide surfaces, as iron is associated with manganese deposits on bacteria collected in the eastern subtropical North Pacific (54). 

Oxidants are present in the environment and in foods. Nitrogen oxides are oxidants present in cigarette smoke and urban smog. Other oxidants include the copper and iron salts in meat and some plants. Inhaling and ingesting oxidants such as these can increase the level of oxidants in our bodies. 

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