Ferrihydrite transformation into goethite

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). 

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. 

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).

Precipitate ferrihydrite by adding 180 mL 5 M KOH to 100 mL M Fe(N03)3 solution. Dilute the suspension to 2 L vith bidistilled vater and hold in a closed polypropylene flask in a 70 °C oven for 60 h. During this time the voluminous redbrown ferrihydrite suspension transforms into a compact yellow precipitate of goethite. Wash well and dry at 50 °C. Around 9 g goethite should be obtained. 

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... 

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-... 

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. 

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. 

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). 

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. 

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