Hematite formation from ferrihydrite

A number of observations help to understand the mechanism of hematite formation from ferrihydrite in aqueous systems i. e. under conditions essentially different from those for solid-state transformation by dry heating (see 14.2.6). Air-dry storage of ferrihydrite containing 100-150g H20/kg of water (found by weight loss) at room temperature for 20.4 years in closed vessels led to partial transformation to fairly well crystalline hematite with a little goethite (Schwertmann et al., 1999). In contrast, no hematite was formed from ferrihydrite if the content of adsorbed water was substantially reduced (Stanjek and Weidler, 1992 Weidler, 1997) as seen from the following examples ... 

Iron extraction values show that iron speciation varies significantly between layers in the cave (Fig. 6A). Values for amorphous, total, and ferrous iron range from 2.4 to 84 pmol/g. Extraction results indicate a significant amount of goethite in the lower layers of the sequence as determined by total minus ammonium-oxalate extractable iron (52 pmol/g in the bottom yellow layer) (Schwertmann and Taylor, 1977). The upper layers have total iron values represented almost entirely by ammonium-oxalate extractable iron (83 pmol/g in the red layer and 59 pmol/g in the top orange layer) suggestive of ferrihydrite (a necessary precursor to hematite formation). The ferrihydrite in the upper layers is indicative of formation by rapid oxidation of ferrous iron (Schwertmann, 1993). The black layer contains the only cave sediment with a significant amount (42 pmol/g) of extracted ferrous iron. 

Schwertmann, U. Murad, E. 1983. Effect of pH on the formation of goethite and hematite from ferrihydrite. Clays and Clay Minerals, 31(4), 277-284. 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

The mechanism by which hematite is formed from ferrihydrite in an aqueous system, appears more complicated than that by which goethite forms. If hematite crystals are added to the system they do not function as seeds for hematite formation but induce epitaxial growth of goethite instead (Atkinson et al. 1968 Cornell Giovanoli, 1985). 

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. 

Schwertmann, U. Friedl, J. Stanjek, H. (1999) From Fe(III) ions to ferrihydrite and then to hematite. J. Coll. Interface Sci. 209 215-223 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000) The effect of A1 on Fe oxides. XIX. Formation of Al-substituted hematite from ferrihydrite at 25°C and pH 4 to 7. Clays Clay Miner. 48 159-172 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000a) The effect of clay minerals on the formation of goethite and hematite from ferrihydrite after 16 years ageing at 25 °C and pH 4-7. Clay Min. 35 613-623... 

Ferrihydrite catalysis of hydroxyl radical formation from peroxide has also shown experimental results consistent with a surface reaction [57]. The yield of hydroxyl radical formation was lower for ferrihydrite than for dissolved iron, resulting in a higher peroxide demand to degrade a given amount of pollutant. As mentioned above, although ferrihydrite exhibited a faster rate of peroxide decomposition than goethite or hematite, the rate of 2-chlorophenol degradation with these catalysts was fastest for hematite [55], In other studies, quinoline oxidation by peroxide was not observed when ferrihydrite was used as catalyst [53]. 

Both goethite and hematite can form from ferrihydrite. Under alkaline conditions, hematite formation is less likely the higher the pH and the lower the temperature (Schwertmann and Murad, 1983 Cornell and Gio-vanoli, 1985). At < 70 C, a pH >12 is usually sufficient to avoid hematite (Lewis and Schwertmann, 1980). 

The influence of the V " " ion presence during the formation of goethite from ferrihydrite in an alkaline medium was investigated using Mossbauer spectroscopy and other techniques by Kaur et al. [249]. The presence of ions reduced HMF values in the Mossbauer spectra due to a substitution of Fe by V ions (ionic radius of 0.64 A almost equal to the radius of Fe " ") in the goethite structure. Small quantities of hematite and superparamagnetic goethite were also formed. 

How factors such as the degree of ordering of ferrihydrite, and solution conditions, particularly pH and temperature affect the goethite/hematite ratio can provide information about the details of the process and also the conditions under which these two oxides might have formed in nature. The proportion of hematite formed after 15 hr at 100 °C increased from 43% to 95 % as the temperature at which the ferrihydrite was precipitated rose from 0 to 100 °C (Schwertmann Fischer, 1966). This suggests that with increasing temperature of precipitation, ferrihydrite dissolves less readily and the rate of dissolution (and hence goethite formation) falls. 

Fig. 14.22 Fields of formation of goethite and hematite from Al-ferrihydrite at 70 °C as a function of [OH] and [Al] (Lewis Schwertmann, 1979a with permission).

Fig. 7.10 Recrystallization products of a ferrihydrite suspension after 441 days. Aging causes a formation of hematite (Ht) and goethite (Gt) depending on the pH of ambient water (adopted from Schwertmann and Murad 1983).

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