Ferrihydrite structure

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

Why mammalian ferritin cores contain ferrihydrite-like structures rather than some other mineral phase is less easy to understand, and presumably reflects the way in which the biomineral is built up within the interior of the protein shell together with the geometry of the presumed nucleation sites. The phosphate content in the intracellular milieu can readily be invoked to explain the amorphous nature of the iron core of bacterioferritins and plants. Indeed, when the iron cores of bacterioferritins are reconstituted in the absence of phosphate, they are found to be more highly ordered than their native counterparts, and give electron diffraction lines typical of the ferrihydrite structure. Recently it has been reported that the 12 subunit ferritin-like Dps protein (Figure 19.6), discussed in Chapter 8, forms a ferrihydrite-like mineral core, which would seem to imply that deposition of ferric oxyhydroxides within a hollow protein cavity (albeit smaller) leads to the production of this particular mineral form (Su et al., 2005 Kauko et al., 2006). 

Waychunas, G.A., Fuller, C.C., Rea, B.A. and Davis, J.A. (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochimica et Cosmochimica Acta, 60(10), 1765-81. 

The generally accepted structure for the crystalline iron core of ferritin is the ferrihydrite structure proposed by Towe Bradley.This consists of oxygen layers with iron in octahedral sites between the layers. Ford et al and PowelF have sununarized the evidence in support of this model for ferritin, which includes electronic spectroscopic data and EXAFS measurements confirming the presence of six-coordinate Fe(III) and indicating that four-coordinate Fe(III) is present, if at aU, at low levels only. This latter point is important because the X-ray powder diffraction patterns of... 

Polyhedral representation of the ideal ferrihydrite structure viewed along the c axis. The central FeOg octahedra. (From Michel et al, 2007. Copyright 2007 with permission from AAAS.)... 

Rapid Hydrolysis of FeCI, at pH 7-8 by KOH Properties One-point BET specific surface area 269 mVg, two-line ferrihydrite structure confirmed by XRD [173],... 

Although Liu and Millero (1999) presented stabUity constant data for Fe(OH)4 in NaCl media, no analysis of these data has been undertaken in the present review. The experimental procedure adopted to obtain these stability constants was from solubUity measurements of ferrihydrite (Fe(OH)3(s)). It is possible that chloride could have been incorporated into the ferrihydrite structure, as identified by Byrne and Luo (2000), and consequently, the derived hydrolysis and solubility constants may have some inherent error (Stefansson, 2007). 

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. 

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). 

XANES to ensure the quality of the synthates. Three batches of ferrihydrite were synthesized and precipitates were washed 5-6 times to ensure a chloride-free synthate. Ferrihydrite precipitates were redispersed in 200 mL of double deionized (DDI) water at (1) room temperature (25°C), as well as preheated in water baths to temperatures of (2) 50°C and (3) 75°C. For all of these slurries, pH was kept constant at 10 using 1M KOH. 40 mL samples were pipetted from each reaction vessel after 0, 1,2, 3, and 7 days. Slurries were centrifuged, washed three times with DDI water and air dried for analyses (BET, XRD, and XANES). BET analyses were used to evaluate the decrease in surface areas with increasing crystallinity, and XRD and XANES were used to detail the structural and speciation changes in iron. 

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. 

Almost all the iron oxides, hydroxides and oxide hydroxides are crystalline. The degree of structural order and the crystal size are, however, variable and depend on the conditions under which the crystals were formed. All Fe oxides display a range of crystallinities except for ferrihydrite and schwertmannite which are poorly crystalline. 

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). 

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 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). 

An exact formula for ferrihydrite has yet to be established because a precise separation of structural OH and H2O from adsorbed water has not been successful to date. Towe Bradley (1967) originally suggested the bulk formula Fes. H0g 4H20 which is certainly to be preferred over Fe(OH)3. Infrared measurements using D2O exchange have suggested that ferrihydrite contains OH and... 

Galvez et al. (1999) demonstrated that phosphorus up to a P/Fe mol ratio of 0.03 mol mol , can be incorporated into the hematite structure by heating P-con-taining 2-line ferrihydrite. Support for structural incorporation comes from a higher unit cell c (1.3776 => 1.3824 nm), IR-stretching bands of P-OH, a lowered intensity ratio of the XRD 104/113 lines and congruent release of Fe and P upon dissolution. 

Numerous coprecipitates of ferrikydrite with different cations (and anions) have been synthesized and exist in nature, but so far, no definite proof of structural incorporation has been produced, probably because of the very low particle size and crystallinity of the (2-line) ferrihydrite which makes the distinction between a position at... 

Highly-broadened XRD peaks and electron diffraction patterns indicate that ferrihy-drites are characterized by small crystal size and/or low structural order. TEM shows single spherical particles, ca. 4-6 nm in size (Fig. 4.17). At higher magnification (HRTEM), 6-line ferrihydrite appeared as single crystals with a hexagonal outline and... 

Aluminium in the ferrihydrite system not only suppresses goethite in favour of hematite (see chap. 14) but also affects the morphology of hematite, probably by entering the structure. At temperatures of between 70 and 150 °C, a shift was noticed from rhombohedra to plates whose diameter and thickness were at a maximum at an Al/(Fe-i-Al) ratio of 0.05 (Schwertmann et al., 1979 Barron et al., 1984 Barron Torrent, 1984 Wolska Szajda, 1987). At higher levels of substitution, the plates became extremely thin and structural strain increased (Stanjek Schwertmann, 1992)... 

The small, spherical particles of ferrihydrite often pack together to form aggregates >0.1 pm across. The aggregated structure and interparticle porosity create difficulties in measurement of surface area because the internal area is not fully accessible to all measurement techniques. It is rare for more than two methods of area measurement to have been applied to the same sample. Pyman and Posner (1978) obtained an area of 250 m g" using both N2 and water BET measurements. With the EG ME method, however, the same sample had an area of 600 m g". These... 

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