Ferrihydrite transformation

Arthur, S.E., Brady, P.V., Cygan, R.T., Anderson, H.L., Westrich, H.R. 1999. Irreversible sorption of contaminants during ferrihydrite transformation. WM 99 Conference, February 28-March 4. 

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). 

Figure 8.3. Ferrihydrite transformations upon reaction with Fefll) produced biologically.

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). 

KEYWORDS ferrihydrite, goethite, hematite, phase transformation. 

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... 

Although additional analyses of the existing data and additional experiments are required to reach definitive conclusions on the phase changes of ferrihydrite in uranium mine tailings, preliminary XRD data suggest that in deionized water at elevated pH (pH=10) phase transformation of ferrihydrite can occur at elevated temperatures. In both elevated temperature experiments, hematite appeared to be the dominant transformation product. At room temperature, however, ferrihydrite remains stable after the duration of the experiment (seven days). 

Johnston, J. Lewis, D.G. 1983. A detailed study of the transformation of ferrihydrite to hematite in an aqueous medium at 92°C. Geochimica et Cosmochimica Acta, 47, 1823-1831. 

Mineralogical characterization of microbial ferrihydrite and schwertmannite and non-biogenic Al-sulfate precipitates from acid mine drainage in the Donghae mine area, Korea. Environmental Geology, 42, 19-31. Knorr, K. Blodau, C. 2007. Controls on schwertmannite transformation rates and products. Applied Geochemistry, 22, 2006-2015. 

Appelo CAJ, VanDerWeiden MJJ, Toumassat C, Charlet L (2002) Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environ Sci Techno 36 3096-3103 Ardizzone S, Formaro L (1983) Temperature induced phase transformation of metastable Fe(OH), in the presence of ferrous ions. Mat Chem Phys 8 125-133... 

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

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).

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

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... 

Ferrihydrite precipitates directly from rapidly hydrolysed Fe " salt solutions. At pH >3, 2-line ferrihydrite precipitates, whereas at lower pH and temperatures close to 100 °C, the 6-line variety forms. Ferrihydrite also forms as a result of oxidation of a Fe" salt solution. A full range of intermediate ferrihydrites may be produced in the Fe "system by varying the rate of hydrolysis, or in the Fe" system, in the presence of low levels of Si (Schwertmann Cornell, 2000). Two-line ferrihydrite does not transform to 6-line ferrihydrite. 

Hematite forms by holding Fe " salt solutions at temperatures close to 100 °C ( forced hydrolysis ) (Matijevic Scheiner, 1978), from 2-line ferrihydrite in aqueous media at around pH 7, by high temperature solid-state transformation of var-... 

Although this type of transformation can take place in solution, usually under hydrothermal conditions, it has been most intensively investigated in the dry state. A precise separation of a transformation in the dry state from that in the presence of vater is, ho vever, often difficult because the minimum amount of water with which a via-solution transformation is still possible may be very small (see 14.3.5). This applies especially to poorly ordered and nano-sized oxides, such as ferrihydrite, with high surface areas and, therefore, high amounts of adsorbed water. 

At temperatures > 600 °C, ferrihydrite and also 5-FeOOH which have been partly substituted with divalent transition metals, transform to a... 

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