Iron oxides behaviour

In coastal environment, detrital and authigenic Fe and Mn oxides, which accumulate in oxic surface sediments, play a pivotal role in determining the geochemical behaviour of arsenic (Mucci et al., 2000) and selenium (Belzile et al., 2000). Arsenic and selenium differ in their affinities for metal oxide surfaces. Although both adsorb onto iron oxides, arsenate (As(V)) adsorbs more strongly than arsenite (As(lll)), and selenite (Se(IV)) adsorbs more strongly than selenate (Se(VI)) (Belzile et al., 2000). 

The effect of the partial pressure of oxygen and of CO2 is also very important for the decomposition behaviour of siderite and rhodochrosite. The formation of the iron oxides was followed by TG and by high temperature X-ray diffraction. Below 10-6 mm Hg oxygen pressure only Fe304 was formed. 

Isomorphous substitution of iron oxides is important for several reasons. In the electronics industry, trace amounts (dopants) of elements such as Nb and Ge are incorporated in hematite to improve its semiconductor properties. Dopants are also added to assist the reduction of iron ores. In nature, iron oxides can act as sinks for potentially toxic M", M and M heavy metals. Investigation of the phenomenon of isomorphous substitution has also helped to establish a better understanding of the geochemical and environmental pathways followed by Al and various trace elements. Empirical relationships (e. g. Fe and V) are often found between the Fe oxide content of a weathered soil profile and the levels of various trace elements. Such relationships may indicate similarities in the geochemical behaviour of the elements and, particularly for Al/Fe, reflect the environment in which the oxides have formed (see chap. 16). 

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. 

The most important physical property of the solid that will affect solubility is particle size. For crystals < 1 pm, the high surface area may increase solubility. This occurs because it is the surface properties, especially the surface free energy, rather than the properties of the bulk solid, that govern the dissolution behaviour. Because the surface free energies of iron oxides are relatively high, particle size will have a marked... 

Their electrochemical properties serve to regulate the coagulation rates, catalysis behaviour and electron transfer reactions of iron oxides (Mulvaney et ah, 1991). Two major methods of characterizing electrochemical behaviour are potentiometric titration and electrophoresis. 

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. 

The colloidal behaviour of iron oxides is of great importance in the environment. 

In general, the adsorption behaviour of gases on iron oxides has been investigated either because the gases are atmospheric pollutants (NO, SO2, CO2), or because they act as probe molecules and provide information about surface sites. The adsorption sites on iron oxides are usually the Lewis acid sites. 

The dissolution behaviour of iron oxides is usually approached in one of two ways. Corrosion chemists and metallurgists place the emphasis on the electrochemical as-... 

This section considers aspects and examples of the dissolution behaviour of individual iron oxides. Additional data are listed in Table 12.3 for a range of experimental conditions. As yet, characteristic dissolution rates carmot be assigned to the various iron oxides (Blesa Maroto, 1986). There are, however, some consistent differences between oxides with considerable stability differences, hence a comparison of the oxides is included here. In addition, the reactivity of any particular oxide may vary from sample to sample, depending on its source (natural or synthetic) and the conditions under which it formed. To illustrate this. Table 12.4 summarizes conditions and results from dissolution experiments in which a range of samples of the same oxide was compared. How the properties of the sample influence its dissolution behaviour is still not fully understood. A thorough characterization of the samples by solid state analysis, e. g. by EXAFS, to provide a basis for understanding the dissolution behaviour is, therefore, desirable. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Murad, E. (1988) Properties and behaviour of iron oxides as determined by Mossbauer spectroscopy. In Stucki, J.W. Goodman,... 

This book is aimed at collecting all aspects of the information about iron oxides into one compact volume. It provides a coherent text with a maximum of homogeneity and minimum overlap between chapters. It is structured according to topics, i. e. surface chemistry, dissolution behaviour, adsorption etc. For each topic a general introduction is followed by a section which reviews current knowledge concerning the different iron oxides. The latter section includes much detailed information and recent data from the authors own laboratories. As this is intended to be a handbook, an extensive list of references to help the reader expand various details is provided. We have also indicated some of the numerous opportunities for further research in this field. 

Mouhandess MT, Chassagneux F, Vittori O (1982) Electrochemical behaviour of a-iron oxide using carbon paste electrodes influence of particle size. J Electroanal Chem.131 367-371. 

Another property of the iron-defective molybdate is the presence of Mo=0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related to Mo6 contained in the Mo=0 as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting to note here that pure iron oxides accelerate combustion and that a W03—Fe2(W04)3 catalyst is practically inactive [254], Replacement of iron by chromium is possible but leads to a lower activity [253]. Baussart et al. [46] prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. 

SchrOder (1979) was able to improve this approach by using a refined expression of the Gibbs free energy of adhesion as a function of the various intermolecular attraction energies between the liquid and the solid. In this way, by using up to 10 different immersion liquids with known parameters, he has calculated the apparent dipole moment and polarizability characterizing the immersion behaviour of various pigment surfaces (e.g. rutile, iron oxide and several phtalocyanines). 

The crystallization behaviour of compositions in the systems CaO-Al2O2-SiO2r CaO-MgO-A 1202-8109 and CaO-Al202-8102"iron oxide relevant to coal ash compositions has been studied. The observations have been compared with the devitrification behaviour of both eastern and western type coal ashes determined under laboratory conditions. Results show that the crystallization of ash melts is well represented by the systen CaO-FeO-A 1202-8109. The quaternary systen has been constructed from literature data as planes of constant FeO content (5-30 wt% at 5% increments) and used to predict the behaviour of boiler deposits. 

Compared with nitrogen, the behaviour of phosphorus in intertidal sediments is less well known. Phosphorus associates with solid surfaces, particularly the surface of iron oxides in sediments (Prastka Malcolm,... 

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