Transformation of ferrihydrite to goethite

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... 

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).

Influence of Tetravalent Cations The influence of the Ti ion presence during the formation of goethite from ferric solution in an alkaline medium was investigated by Mossbauer spectroscopy and other techniques [246]. The presence of Ti " " ions partially suppressed the transformation of ferrihydrite to goethite and as a result a quadrupole doublet, corresponding to the low-crystalline phase, emerged in the Mossbauer spectrum. 

The evolution of the relative spectral areas for the different iron-bearing phases as a function of depth is represented in Fig. 3.41, which thus provides an idea of the transformation of ferrihydrite to goethite and hematite in the different horizons. From this picture is clear that in the deeper layers ferrihydrite is nearly completely transformed to goethite and hematite. 

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 ... 

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. 

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). 

ABSTRACT The kinetics and mechanisms of the phase transformation of 2-line ferrihydrite to goethite and hematite are being assessed as a function of pH, temperature and Fe/As, Fe/Se, Fe/Mo molar ratios using batch experiments, BET analyses, XRD, and XANES. Initial results from XRD analyses show that ferrihydrite is stable at high pH ( 10) for up to seven days at 25°C, but considerable crystallization occurs at elevated temperatures. Specifically, XRD data show that ferrihydrite is transformed to a mixture of hematite and goethite at 50°C (-85% hematite and -15% goethite) and 75°C (-95% hematite and -5% goethite) after 24 hours and these ratios remain constant to the end of the experiments (seven days). 

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... 

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).

Fig.14.20 Effect of various clay minerals on the transformation of 2-line ferrihydrite to goethite and hematite at 25 °C and pH 5 after 16 yr as measured by the ratio of oxalate to dithionite soluble Fe (Feo/Fed) (Schwertmann et al.

Fig. 14.21 Effect of cysteine (cyst) alone and cysteine +silicate (Si/Fe = 0.1) or cysteine + Mn (Mn/(Fe+ Mn) = 0.1) on the transformation of 2-line ferrihydrite to goethite (Cornell, unpubl.)...

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. 

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. 

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 ... 

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