Stabilities of iron oxides

The stability of iron oxide suspensions is relevant to fields as varied as the paint industry, extraction of iron from its ores, the structure of soils, hydrometallurgy and waste water treatment. The ease of homogensisation of paint, for example, is controlled by proper adjustment of the stability of the pigment suspensions. In ground waters, the settling behaviour of small iron oxide particles influences transportation of trace elements and radio-nuclides. The stability of a dispersion of magnetic particles can determine the quality of ferrofluids and magnetic tapes. 

Bigham, J.M. Heckendorn, S.E. Jaynes,W.E. Smeck, N.E. (1991) Stability of iron oxides in two soils with contrasting colors. Soil Sd. 

Apart from microenvironments, an explanation for the concurrent dissimilatory sulfate and iron reduction was provided by Postma and Jakobsen (1996). They demonstrated that the stabilities of iron oxides are decisive with respect to iron and/or sulfate reduction assuming that the fermentative step and not the overall energy yield is overall rate limiting. Additionally, it shall be noted that the typical sulfate reducing bacteria Desulfovibrio desulfuricans was found to reduce iron oxide enzymatically contemporarily or optionally (Coleman et al. 1993). When only very small concentrations of as sole electron donor were available iron oxide instead of sulfate was used as electron acceptor by D. desulfuricans. 

Tipping E., Ohnstad M. (1984b), Colloid stability of iron oxide particles from a freshwater lake. Nature, 308, 266-268. 

The percolation argument is based on the idea that with an increasing Cr content an insoluble interlinked cliromium oxide network can fonn which is also protective by embedding the otherwise soluble iron oxide species. As the tlireshold composition for a high stability of the oxide film is strongly influenced by solution chemistry and is different for different dissolution reactions [73], a comprehensive model, however, cannot be based solely on geometrical considerations but has in addition to consider the dissolution chemistry in a concrete way. 

Dead-burned dolomite is a specially sintered or double-burned form of dolomitic quicklime which is further stabilized by the addition of iron oxides. Historically, it was used as a refractory for lining steel furnaces, particularly open hearths, but as of this writing is used primarily in making dolomite refractory brick (see Refractories). 

PefractoTy lime is synonymous with dead-burned dolomite, an unreactive dolomitic quicklime, stabilized with iron oxides, that is used primarily for lining refractories of steel furnaces, particularly open hearths. 

The stringent requirements covering oxidation stability are defined by the test method DIN 51352, Part 2, known as the Pneurop Oxidation Test (POT). This test simulates the oxidizing effects of high temperature, intimate exposure to air, and the presence of iron oxide, which acts as catalyst - all factors highly conducive to the chemical breakdown of oil, and the consequent formation of deposits that can lead to fire and explosion. 

C) for cast iron and up to 140 °F for marstenitic SS (60 °C). It is widely used where silicates are present with the iron oxides. Typically, 5 to 7.5% HC1 is employed. The ammonium bifluoride normally is present at 0.5%, but it may be increased to a maximum of 1.5% for a boiler that has not been cleaned for many years. The presence of hydrofluoric acid (HF), which is formed by the reaction of ammonium bifluoride with HC1 (see equation), tends to increase the rate of iron oxide dissolution and reduce the corrosion rate of exposed steel, when compared to using HC1 alone. This is due to the stability of the hexafluoroferric ion (FeFg3 ), which prevents the ferric ion from corroding exposed steel. 

Addition of 10% of iron oxide reduces the thermal stability of the salt. 

Except for phthalic acid, all other carboxylic acids studied induce considerable increases in the light compared to the dark values (the relatively high rate of iron oxide dissolution induced by oxalic acid has been extensively studied (5,8). Phthalic acid actually appears to stabilize the iron oxide against photodissolution despite the solution phase complex exhibiting some photoactivity. 

The H64Y variant of Mb is an example of the former situation in that the tyrosyl side chain coordinates to the heme iron of the oxidized variant. As expected for a variant with an anionic phenolate ligand, the reduction potential of this variant is 40 mV lower than that of the wild-type protein (Table I). Although this change is consistent with stabilization of the oxidized form of the protein, the fact that the tyrosyl ligand is not coordinated in the reduced protein complicates quantitative interpretation of this shift in potential. 

Fig.1. Eh-pH diagram for the system Fe-U-S-C-H2O at 25 °C showing the mobility of uranium under oxidizing conditions, the relative stability of iron minerals, and the distribution of aqueous sulfur species. Heavy line represents the boundary between soluble uranium (above), and insoluble conditions (below), assuming 1 ppm uranium in solution.

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

Solubility diagrams have nearly always been calculated using solubility and stability constants. Experimental determination of the solubility of iron oxides as a function of pH has been concerned predominately with ferrihydrite. Lengweiler et al. 

A similar oxidation-reduction mechanism in the carbon monoxide oxidation reaction on oxide catalysts has been proposed by Benton (71), Bray (72), Frazer (73), and Schwab (74). In this reaction also, Mooi and Selwood (57) found that a decrease in the percentage of iron oxide, manganese oxide or copper oxide on the alumina support first increased the rate, and then at lower percentages decreased the rate, of carbon monoxide oxidation, indicating that valence stabilization is again operative in these cases. 

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