Ferrihydrite transformation into hematite

Over time, two-line ferrihydrite normally transforms into goethite or hematite in laboratory or natural environments (Rancourt et al., 2001, 839). However, extensive sorption of As(V) could delay the transformation (Ford, 2002). The crystallization of arsenic-bearing amorphous iron compounds often releases arsenic from the compounds (Welch et al., 2000, 599). In particular, while aging in seawater from Ambitle Island near Papua New Guinea, two-line ferrihydrites transformed into less arsenic-rich six-fine varieties. The arsenic released by the transformation of the ferrihydrites produced distinct crystals of claudetite (As203) (Rancourt et al., 2001, 838-839). 

Chukhrov et al. assign a special role to ferrihydrite (2.5 Fe203-4.5 H2O), which is believed to be the typical product of rapid oxidation of Fe + in slightly acid, neutral, and slightly alkaline solutions with the participation of iron bacteria. The oxidation process also is accelerated by the catalytic action of silica. In the course of time ferrihydrite spontaneously converts to hematite, but in solutions with Fe " ions ferrihydrite is transformed into stable goethite in the absence of significant amounts of oxygen. 

Iron oxides are most conveniently stored as dry powders. However, after prolonged storage in an air-dry state some metastable forms may transform into more stable ones. For example, ferrihydrite will gradually turn into hematite and goethite when kept in contact with the atmosphere, presumably owing to the presence of adsorbed non-stoichiometric water Fig. 2-1 shows an X-... 

Hematite production by transformation offerrihydrite (methods 4 and 5) starts with the precipitation of 2-line ferrihydrite which is then converted into hematite in aqueous suspension by a short-range crystallization process within the ferrihydrite aggregates. 

Ferrihydrite obtained by hydrolysis of Fe2(S04)3 is very slowly transformed into goethite at pH 7 [20], After one year only 7% of the initially formed ferrihydrite was transformed, while for ferrihydrite obtained by hydrolysis of FeCls or Fe(N03)3 the degree of transformation into mixture of goethite and hematite was 30% at the same pH. On the other hand at pH 11 the transformation into goethite was almost complete after one year and the difference in the transformation kinetics between ferrihydrite obtained by hydrolysis of Fe2(S04>3, FeCU and Fe(N03)3 was less significant. At pH 8 10 substantial amount of hematite is present after a one year aging. These results show that in adsorption experiments with fresh precipitates we can deal with two different adsorbents in the beginning and in the end the experiment. 

Noncrystalline oxides, particularly ferrihydrite, are common in soils because the presence of soluble silica and organic matter tends to inhibit crystallization into more stable, better ordered oxides of Fe. Even so, ferrihydrite is considered to be unstable, gradually transforming to hematite in tropical or subtropical climates or to goethite in humid temperate climates. 

Samples from the site contained considerable amounts of freshly precipitated iron hydroxides. Their transformation into thermodynamically more stable minerals such as goethite or hematite has a very slow kinetics, thus ferrihydrite was chosen as the major adsorbing surface. The Diffuse Double Layer model (Dzombak and Morel, 1990) was selected to describe surface complexation. The respective intrinsic surface parameters and the reaction constants for the ions competing with uranium(VI) for sorption sites were taken from a database mainly based on Dzombak and Morel, 1990, with the urani-um(Vl) sorption parameters as determined by Dicke and Smith, 1996. The results, based on runs with 1000 varied parameter sets, are summarized in Table 5.2. 

Fig. 14.19 The effect of silicate (Si/Fe = 0.005) on the transformation of 2-line ferrihydrite into goethite and hematite at 70 °C (Cornell et al., 1987 with permission).

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. (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. Giovanoli, R. and Schindler, P. W. (1987) Effect of silicate species on the transformation of ferrihydrite into goethite and hematite in alkaline media. Clays Clay Min. 35 21-28. 

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