Goethite 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 ... 

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. 

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. 

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). 

Cornell, R.M. SchneidepW. (1989) Formation of goethite from ferrihydrite at physiological pH under the influence of cysteine. Polyhedron 8 149-155... 

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]. 

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. 

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. 

Dissolution of goethite and ferrihydrite at pH 6 by M-EDTA (M = Pb, Zn, Cu, Co, Ni) is slower than that by EDTA alone (Nowack Sigg, 1997). Dissolution was considered to involve the formation of a ternary surface complex which then dissociated releasing M into solution after which Fe was detached from the oxide as Fe-EDTA. For ferrihydrite, the rate of dissolution depended on the nature of M, because the rate determining step was dissociation of M-EDTA. For goethite, on the other hand, this step was fast, hence the rate of dissolution was independent of M. 

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).

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