Iron oxides and hydroxides

Cluster molecular-orbital methods were first applied to iron oxides by Tossell et al. (1973a, 1974) and by Vaughan et al. (1974) using the MS- 

In this cluster, which has C2, symmetry, the calculated molecular-orbital energy levels should be compcu-ed with those of the FeO cluster in Fig. 4.26(a). As for the latter cluster, the molecular orbitals can be grouped into sets in relation to their atomic-orbital character, with the dominantly Fe 3d (crystal-field-type) orbitals being at the top of the valence band. Now, however, as shown in Fig. 4.29, the orbitals below these in energy fall into four groups O 2p nonbonding orbitals, Fe-(9 

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Sulfides are intermixed with iron oxides and hydroxides on carbon steels and cast irons. The oxides are also produced in the corrosion process (Reaction 6.6). Although theoretical stoichiometry of 1 to 3 is often suggested between sulfide and ferrous hydroxide, empirically the ratio of iron sulfide to ferrous hydroxide is highly variable. Sulfide decomposes spontaneously upon exposure to moist air. Additionally, corrosion-product stratification is marked, with sulfide concentration being highest near metal surfaces. 

Normal mill coolant pH was near 5. The upset caused large amounts of iron corrosion products to be swept into the coolant. Settling of iron oxides and hydroxides fouled many mill components. 

Manganese and iron oxidation are coupled to cell growth and metabolism of organic carbon. Microbially deposited manganese oxide on stainless and mild steel alters electrochemical properties related to the potential for corrosion. Iron-oxidizing bacteria produce tubercles of iron oxides and hydroxides, creating oxygen-concentration cells that initiate a series of events that individually or collectively are very corrosive. 

Iron oxides and hydroxides are used as the starting material in the production of ferrites. Hard ferrites (high coercivity) e. g. BaFei20i9 and SrFei20i9 are made from hematite by a sintering process, i. e. 

Diakonov, I. Khodakovsky I. Schott, J. Sergeeva, E. (1994) Thermodynamic properties of iron oxides and hydroxides. I. Surface and bulk thermodynamic properties of goethite (a-FeOOH) up to 500 K. Fur. J. Min. 6 967-983... 

Mackay, A.L. (1961) Some aspects of the topo-chemistry of iron oxides and hydroxides. In de Boer, J.H. (ed.) Reactivity of solids. Proc. 4 Int. Symp. Reactivity of Solids, Amsterdam, 571—583... 

Peak, D. Sparks, D.L. (2002) Mechanisms of selenate adsorption on iron oxides and hydroxides. Envir. Sci. Techn. 36 1460-1466 Peak, D., Ford, R.G. Sparks, D.L. (1999)... 

Bargar et al. 991a,b,c) have explored the affinity of the surface oxygen atoms in aluminum and iron oxides and hydroxides for Pb(II) and Co(II) using... 

Reactions (2) and (3) indicate processes that regenerate Fe2+ in the catalytic cycle. As long as peroxide is available in the system, the iron species continually cycle between Fe2+ and Fe3+, unless additional reactions result in formation of insoluble iron oxides and hydroxides. The rate of formation of hydroxyl radical can be expressed as ... 

Adamo, P., Colombo, C. Violante, P. (1997). Iron oxides and hydroxides in the weathering interface between Stereocaulon vesuvianum and volcanic rock. Clay Minerals,... 

Carbonate iron sediments are formed when iron is precipitated in the presence of dissolved carbonic acid or as a result of interaction of the primary sediments with organic matter in the course of diagenesis. The determination of the stability of primary iron carbonates with respect to iron oxides and hydroxides was made on the basis of using the functional dependence of and on pH, calculated by the method of Garrels and Christ (1968) for the system carbonate-water. According to this dependence, in conditions of FeCOj in equilibrium with water the value of partial pressure of carbon dioxide decreases in proportion to increasing pH. 

Iron oxides and hydroxides E172 77491 Iron oxide red... 

All iron oxides and hydroxides consist of Fe, O and/or OFl. They differ in composition, in the valence of Fe and, above all, in crystal structure. 

Fe(OH)2 is prepared from Fe° solutions by precipitation with alkali. When freshly precipitated under an inert atmosphere (in a Schlenck apparatus for example) Fe(OH)2 is white (Bernal et al., 1959). It is, however, readily oxidized by air or even water upon which it darkens. Fe(OH)2 has the CdL type structure with hep anions and half of the octahedral interstices being filled with Fe ions. The crystals form hexagonal platelets. In solution Fe(OH)2 transforms by a combination of oxida-tion/de-hydration/hydrolysis reactions to other iron oxides and hydroxides. The end product depends both upon the order in which these processes occur and upon their rates. 

Almost all the iron oxides and hydroxides ean be prepared by a number of synthesis pathways. The route followed frequently influences the properties of the product, particularly crystal morphology, degree of crystallinity, sample surface area and water eontent. The main synthesis methods are described in the following seetions. 

Rustad, J.R., A.R. Felmy, and B.P. Hay. 1996a. Molecular statics calculations for iron oxide and hydroxide minerals Toward a flexible model of the reactive mineral-water interface. Geochim. Cosmochim. Acta 60 1553-1562. 

Tabk 10.1 Mossbauer parameters for binary iron oxides and hydroxides... 

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