Transformation of ferrihydrite

This is a very versatile method eapable of producing goethite, lepidocro-cite, magnetite, ferrihydrite and feroxyhyte. Which product forms depends on the reaction conditions. Careful control of pH, rate of oxidation, suspension concentration, temperature, and concentration of foreign species is needed to ensure that a pure product is obtained (Feitknecht, 1959 Sehwertmann, 1959 b). In some cases an inert atmosphere of N2 must be provided. Industrial pigments are usually produeed by this method (Buxbaum, 1993). 

The reaction can be carried out over the pH range 6-14. Between pH 6-7 goethite and lepidocrocite result a pure product of either ean be obtained by adjusting the rate of oxidation and the concentration of carbonate in the system (Sehwertmann, 1959 b Carlson and Sehwertmann, 1990). At pH 8 magnetite is obtained and at pH 14, pure goethite is produced. With very rapid oxidation (e.g. by H2O2) feroxyhyte is obtained. 

The oxidation/hydrolysis reaction releases protons, so that as the transformation proceeds and if no extra base is added, the pH drops to about 3 and the transformation rate falls to practically zero, thus leading to incomplete oxidation  

In order, therefore, to obtain a higher yield of reasonably well crystalline product, the pH must be held constant by continual addition of alkali to 

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Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

Measured surface areas (11-point BET analyses) for pure phases such as ferrihydrite, goethite and hematite are in the range as proposed by Cornell Schwertmann (2003) (Table 1). Preliminary XRD analyses showed that temperature impacts the kinetics of phase transformation of ferrihydrite. Data indicated that after seven days, the rate of transformation from ferrihydrite to more crystalline forms, if it was occurring, was too slow to be measured at 25°C (Fig. 1). In contrast to the 25°C experiment, significant, transformations were observed at 50 (Fig. 2) and 75°C (Fig. 3) after 24... 

Although additional analyses of the existing data and additional experiments are required to reach definitive conclusions on the phase changes of ferrihydrite in uranium mine tailings, preliminary XRD data suggest that in deionized water at elevated pH (pH=10) phase transformation of ferrihydrite can occur at elevated temperatures. In both elevated temperature experiments, hematite appeared to be the dominant transformation product. At room temperature, however, ferrihydrite remains stable after the duration of the experiment (seven days). 

Johnston, J. Lewis, D.G. 1983. A detailed study of the transformation of ferrihydrite to hematite in an aqueous medium at 92°C. Geochimica et Cosmochimica Acta, 47, 1823-1831. 

Fig. 6.10 Left Placement of various synthetic goethites (G), lepidocrocites (L) and hematites (H) in CIE L a b colour space. Right Development of a and b in the CIE L a b colour space during the transformation of ferrihydrite (common starting point) to goethite or hematite, respectively (Nagano et al., 1994, with permission).

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... 

The transformation of ferrihydrite to hematite by dry heating involves a combination of dehydration/dehydroxylation and rearrangement processes leading to a gradual structural ordering within the ferrihydrite particles in the direction of the hematite structure. This transformation may or may not be facilitated by the postulated structural relationship between the two phases. EXAFS studies have shown, for example, that some face sharing between FeOg octahedra, characteristic of hematite, also exists in 6-line ferrihydrite (see chap. 2). 

Fig. 1417 Transmission electron micrographs documenting the transformation of ferrihydrite to hematite (Fischer. Schwert-mann, 1975 with permission).

In general, foreign species in the system can have two different effects on the transformation of ferrihydrite to other Fe oxides they can either modify the rate of the transformation, usually by slowing the process, or change the composition (mainly the hematite/goethite ratio) and properties of the end product. Two principal mechanisms of interaction operate ... 

Although titanium retards the transformation of ferrihydrite (pH 6-11), it enhances the formation of goethite over hematite (Fitzpatrick Le Roux, 1976 Fitzpatrick et al., 1978). The opposite was found for trivalent chromium (Schwertmann et al., 1989) and vanadium (Schwertmann Pfab, 1994) besides retarding the transformation, higher concentrations of both ions led to enhanced hematite formation. 

Fig. 14.24 Transformation of ferrihydrite to goethite with time at 50 °C in the presence of 5 10 M Fe " at various pHs (pH values given on the curves). Insert Fe " concentration in solution after 30 min vs. pH (Fischer, 1972 with permission).

The laboratory derived model of hematite formation in soils via ferrihydrite has received general acceptance. So far, it is the only way to produce hematite at ambient temperatures and in the pH range of soils. Support from soil analysis, however, is meagre. Hematite is usually associated with other Fe oxides, mainly with goethite but not with ferrihydrite. There seems to be only one report of a ferrihydrite-hema-tite association (based on XRD and Mossbauer spectra) viz. in several andisols formed from basalt in the warm and moist climate of Hawaii (Parfitt et al., 1988). In this case, in addition to the low age of the soils, high release of Si may retard the transformation of ferrihydrite to hematite, whereas normally, the rate of transformation of ferrihydrite seems to be higher than that of ferrihydrite formation, so that this mineral does not persist. 

The rapid oxidation of Fe " close to the surface and in the presence of a fair supply of organic matter and dissolved Si, conditions which hinder crystallization, leads to ferrihydrite instead of goethite. The ferrihydrite is, however, often associated with goethite and it is still unknown whether the two minerals have formed simultaneously or in sequence. Simultaneous formation seems more likely for two reasons in the first place, low-temperature hydrolysis of Fe " or oxidation of Fe ", both, led to mixtures of the two oxides in different proportions if the rate of hydrolysis/oxidation was varied (Schwertmann et al. 1999 Schwertmann Cornell, 2000). Secondly, the transformation of ferrihydrite, especially in the presence of Si, appears to be extremely sluggish. 

Hydrothermal transformation of ferrihydrite in a teflon bomb at 180 °C for several days yields platy crystals up to several pm in size (Schwertmann and Cornell, 2000). 

Cornell, R.M. Giovanoli, R. (1988) The influence of copper on the transformation of ferrihydrite (5 Fe203 9 H2O) into crystalline products in alkaline media. Polyhedron 7 385-391... 

Cornell, R.M. (1985) Effect of simple sugars on the alkaline transformation of ferrihydrite into goethite and hematite. Clays Clay Min. 33 219-227... 

Cornell, R.M. (1987) Comparison and dassifica-tion of the effects of simple ions and molecules upon the transformation of ferrihydrite into more crystalline products. Z. Pflanzener-nahr. Bodenk. 150 304-307... 

Cornell, R.M. (1991) Simultaneous incorporation of Mn, Ni and Co in the goethite (a-FeOOH) structure. Clay Min. 26 427-430 Cornell, R.M. (1992) Preparation and properties of Si substituted akaganeite (P-FeOOH). Z. Pflanzenemahr. Bodenk. 155 449-453 Cornell, R.M. Giovanoli, R. Schindler, P.W. (1987) Effect of silicate species on the transformation of ferrihydrite into goethite and hematite in alkaline media. Clays Clay Min. 35 12-28... 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

Cornell, R. M. and Giovanoli, R. (1987) Effect of manganese on the transformation of ferrihydrite into goethite and jacobsite in alkaline media. Clays Clay Min. 35 11-20. 

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