Iron oxides ferrihydrite

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

Almost all the iron oxides, hydroxides and oxide hydroxides are crystalline. The degree of structural order and the crystal size are, however, variable and depend on the conditions under which the crystals were formed. All Fe oxides display a range of crystallinities except for ferrihydrite and schwertmannite which are poorly crystalline. 

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. 

Ghromate adsorption by iron oxides is suppressed by a large excess of carbonate or silicate species. Gompetition between silicate and borate adsorption on ferrihydrite was reported by McPhail et ak (1972). Low levels of sulphate suppress uptake of phthalic and chelidimic acids by goethite (Ali Dzombak, 1996 a). 

To date, only the second form of ternary adsorption has been observed for iron oxides. Davis and Leckie (1978 a) found that thiosulphate adsorbed on ferrihydrite in acid media with adsorption decreasing to zero as the pH rose to ca. 7, whereas the adsorption edge of silver lay between pH 7 and 8. In the presence of thiosulphate, adsorption of silver was enhanced in the pH range 4-6.5 (Fig. 10.10), i. e. 

In addition to oxalate, malonate and citrate accelerate the dissolution of iron oxides in the presence of Fe (Sulzberger et al., 1989). Fe " also promotes the dissolution of magnetite in sulphuric acid (Bruyere Blesa, 1985). Small amounts of Fe in solution speed up the transformation of ferrihydrite to goethite at 50 °C (see Fig. 14.24) by promoting the dissolution of ferrihydrite (Fischer, 1972). Adsorption... 

Cations differ from ligands in that they influence the crystallization of ferrihydrite over a wider pH range than do ligands. They usually require mol ratios (M/(M + Fe)) of 0.05-0.1 to influence the kinetics and products of the reaction, whereas ligands are often effective at hundredfold lower concentrations. In addition, cations are often incorporated in the iron oxide structure (see Chap. 3). The effects of Ti, VO , Pb ", Cr , and the first row divalent transition elements have been investigated. These effects vary widely, although retardation predominates. 

Fig. 16.20 Electron micrographs of synthetic associations between iron oxides and Si-miner-als. Normal (a, b) and shadowed (c, d) kaolinite - 6-line ferrihydrite associations at pH 3 (a, d) and 9 (b, c) (Saleh Jones, 1984 with permis-...

Ferrihydrite is the iron oxide with the most widespread distribution in living organisms. In the form of ferritin, an iron storage protein, it is found in all organisms from bacteria through to man (in heart, spleen and liver). It occurs in plants as phytoferritin (review by Seckback, 1982). Ferritin plays a key role in iron metabolism it maintains... 

The magnetite is considered to form from a ferrihydrite precursor by interaction of this phase with dissolved Fe" ions (Kirschvink Lowenstam, 1979 Lowenstam, 1981 Nesson Lowenstam, 1985). The same mechanism operates for inorganic synthesis at around pH 7 (see chap. 13). Most probably the other iron oxides in the teeth form by a similar mechanism, but under conditions of slightly lower pH and/ or higher redox potential. The separation of these minerals in time and space suggests local variations in growth conditions. 

Laboratory studies have indicated an increasing number of further processes for which iron oxides may be used as catalysts. A sodium promoted iron oxide on a support of Si02 catalyses the gas phase oxidation (377-427 °C) by nitrous oxide, of pro-pene to propene oxide (Duma and Honicke, 2000). Ferrihydrite or akaganeite can be used to catalyse the reduction (at 55-75 °C) by hydrazine, of aromatic nitro compounds to aromatic amines (which are the starting materials for a huge range of chemicals) these Fe oxides have the potential to provide a safe and economical pathway to the production of these important organics (Lauwiner et al., 1998). 

Hydrothermal processes, i. e. the heating of suspensions of ferrihydrite in alkaline media under pressure, have been used to produce large platy crystals of hematite. This process gives vell formed crystals, but is expensive. The crystals can be reduced to produce isomorphous magnetite plates. Flame hydrolysis involves burning Fe " chloride at 400-800 °C to iron oxide. Owing to the many technical difficulties associated with this process, it is not commercially important. 

Production of iron oxides on substrates or in confined spaces Goethite, hematite and ferrihydrite... 

B. (1998) A study of the structural and catalytic effect of sulphation on iron oxide catalysts prepared from goefhite and ferrihydrite precursors for methane oxidation. Catalysis Letters 53 7—13... 

Chiarizia, R. Horwitz, E.P. (1991) New formulations for iron oxides dissolution. Hydrometallurgy 27 339-360 Childs, C.W Wilson, A.D. (1983) Iron oxide minerals in soils of the Ha apai Group, Kingdom of Tonga. Aust. J. Soil Res. 21 489-503 Childs, C.W. (1992) Ferrihydrite A review of structure, properties and occurrence in relation to soils. Z. Pflanzenemahr. Bodenk. 155 441-448... 

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