Ferrihydrite

Ferrihydrite is naturally present in various water systems, sediments and soils. It is also the product coming from mine wastes and acid mine drainage. Ferrihydrite plays an important role in the geochemical cycling of iron. Specifically, it is an 

23 MOSSBAUERSPECTROSCOPY IN THE INVESTIGATION OFTHE PRECIPITATION OF IRON OXIDES 

XRD pattern of two-XRD-line ferrihydrite. (Reproduced from Ref. 106 with permission of Elsevier.) 

Both synthetic and natural ferrihydrite (named by F.V. Chukhrov see Chukhrov et al., 1973) are poorly ordered. The degree of ordering is variable and a range of XRD 

The structure of ferrihydrite has been the object of numerous studies in the past and several different structures have been proposed. The main difficulty affecting elucidation of the structure is the low degree of order. The original models of Towe and Bradley (1967) and Chukhrov et al. (1976) are based on XRD data and involve a defective hematite structure based on an hep array of anions with vacant Fe sites and a considerable amount of water. The Fe ions are distributed randomly over the interstices and there is more OH and H2O and less Fe in ferrihydrite than in hematite, i. e. there is a lower Fe/O ratio ( 2/3). 

The unit cell parameters given by Towe and Bradley (1967) for 6-line ferrihydrite are a = 0.508 nm and c = 0.94 nm. There are four formula units per unit cell. A con- 

Further refinements of the structure are based on the agreement between experimental and simulated XRD data (Drits et al., 1993 1995) in combination with structural data from EXAFS spectra (Manceau Drits, 1993). According to these results, 6-line ferrihydrite contains three intergrown structural components (Fig. 2.9, upper)  

The main difference between 6- and 2-line ferrihydrite is in the size of their crystal 

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Some work has also been achieved with heterogeneous catalysis. These catalysts include Amberlyst-15, Nafion-H, montmorillonite KSF clay, ferrihydrite silica gel aerogels containing 11-13% iron, silica sulfuric acid, and zeolites. ... 

Figure 6.13 shows the Mossbauer spectra of ferritin [51], which is an iron-storage protein consisting of an iron-rich core with a diameter around 8 nm with a structure similar to that of ferrihydrite and which is surrounded by a shell of organic material. At 4.2 K essentially all particles contribute to a magnetically split component, but at higher temperatures the spectra show the typical superposition of a doublet and a sextet with a temperature dependent area ratio. At 70 K the sextet has disappeared since all particles have fast superparamagnetic relaxation at this temperature. 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

Ageing of Amorphous Ferrihydrite to More-crystalline Products... 

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). 

The biological mineralizing systems for iron that have been studied the most extensively are the ferrihydrite (and, in prokaryotic ferritins, the amorphous... 

When pea seed apoferritin is reconstituted in vitro in the absence of phosphate, the reconstituted mineral core consists of crystalline ferrihydrite (Rohrer et ah,1990 Wade et ah, 1993 Waldo et ah, 1995). Conversely, horse spleen ferritin reconstituted in the presence of phosphate produces an amorphous core (Rohrer et ah,1990 St. Pierre et ah, 1996)... 

The initial stages of iron incorporation requires the ferroxidase sites of the protein. Thereafter the inner surface of the protein shell provides a surface which supplies ligands that can partially coordinate iron but which leave some coordination spheres available for mineral phase anions, thereby enabling the biomineralization process to proceed, with formation of one or more polynuclear ferrihydrite crystallites. Iron is transferred from the ferroxidase sites to the core nucleation sites by the net reaction (Yang et ah, 1998) ... 

On the basis of a number of physico-chemical methods (Mossbauer spectroscopy, electron diffraction, EXAFS) the iron cores of naturally occurring haemosiderins isolated from various iron-loaded animals and man (horse, reindeer, birds and human old age) were consistently shown to have ferrihydrite-like iron cores similar to those of ferritin (Ward et ah, 1992, 2000). In marked contrast, in the tissues of patients with two pathogenic iron-loading syndromes, genetic haemochromatosis and thalassaemia, the haemosiderins isolated had predominantly amorphous ferric oxide and goethite cores, respectively (Dickson etah, 1988 Mann etah, 1988 ... 

Sorption of methyl arsenic onto metal oxides has received scant attention. Lafferty and Loeppert (2005) found that MMAs(III) and DMAs(III) were not appreciably sorbed onto goethite or ferrihydrite within the pH range of 3 to 11, while arsenite was strongly sorbed to both the oxides. In contrast, MMAs(V) and arsenate were sorbed from pH 3 to 10 in great... 

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