Ferrihydrite preparation

Two-line ferrihydrite, for use as a catalyst, can also be synthesized by thermal decomposition of iron penta carbonyl, Fe(CO)5, in a stream of moist air at 500°C (Kosowski, 1993 Zhao et al.l993). It is a free flowing, reddish brown powder with a much lower bulk density than freeze-dried ferrihydrite prepared as above, probably because of much weaker aggregation. Small changes in the reaction conditions (water content of the air, duration of heating) may induce hematite formation. An analogous recipe involved slow thennal decomposition of trinuclear aceto-hy-droxy Fe "-nitrate for 20-40 hr in air in an attempt to simulate the red pigment on the Martian surface (Morris et al. 1991). 

Figure 23. Example of WAXS scattering analysis for ferrihydrite particles with sorbed arsenate. Left Raw scattering data for ferrihydrite prepared with coprecipitated arsenate in different proportions. Right S1(S) function extracted from the scattering data. From Waychunas et al. (1996).

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

In soils and other geoenvironments, water soluble humic compounds are also candidates for complexation and release of Fe from Fe oxides and thereby may provide Fe for plants. For example, compared to the control, water extractable humics from peat (1.7 mmol C/L) doubled the amount of Fe extracted from a freshly prepared 2-line ferrihydrite over 24 hr (Cesco et al. 2000). 

There is a number of synthetic substitutes for natural ferritin and the properties of these have been compared with those of ferritin. The synthetic polysaccharide iron complex (PIC), has a magnetic blocking temperature of 48K (Mohie-Eldin et al. 1994). Iron-dextran complexes are used as a substitute for ferritin in the treatment of anaemia. The iron cores of these complexes consist not of ferrihydrite, but of very poorly crystalline akaganeite with magnetic blocking temperatures of between 150 and 290 K (Muller, 1967 Knight et al. 1999) which were lowered from 55K to 35 and 25K, if prepared in the presence of 0.250 and 0.284 Al/(A1 -i- Fe), respectively (Cheng et al.2001). 

B. (1998) A study of the structural and catalytic effect of sulphation on iron oxide catalysts prepared from goefhite and ferrihydrite precursors for methane oxidation. Catalysis Letters 53 7—13... 

Cornell, R.M. (1991) Simultaneous incorporation of Mn, Ni and Co in the goethite (a-FeOOH) structure. Clay Min. 26 427-430 Cornell, R.M. (1992) Preparation and properties of Si substituted akaganeite (P-FeOOH). Z. Pflanzenemahr. Bodenk. 155 449-453 Cornell, R.M. Giovanoli, R. Schindler, P.W. (1987) Effect of silicate species on the transformation of ferrihydrite into goethite and hematite in alkaline media. Clays Clay Min. 35 12-28... 

Catalysts used in this work7,18,21 22 were prepared either by deposition-precipitation or coprecipitation, and were pretreated in air at 673 K. Depending on the way in which the a-Fe2C>3 support was made, the precursor before calcination may have been either ferrihydrite ( 2-line or 6-line 6) or 7-Fe2C>3 (magehmite), the former giving the better results.6... 

Titania, goethite and silica were used as adsorbents. Titanium dioxide (anatase) and silica powders were obtained commercially. Goethite was prepared from an FefNC solution by the precipitation of ferrihydrite.8 The suspension was held in a closed flask at 70°C for 60 hours. During this period the red brown suspension of ferrihydrite was converted to a yellow brown goethite. These oxides were washed with double distilled water to remove impurities, until the supernatant conductivity was below 2pS x cnT1. 

Virion templates of TMV were also used in combination with different synthetic routes for CdS, PbS, and Fe oxide nanoparticles. Nanoparticle-virion tubules were prepared by reacting a buffered solution of TMV in CdCl2 (pH 7) or TMV in Pb(N03)2 (pH 5) with H2S gas. The formation of metal sulfide nanoparticles occurred over 6 hours as observed by a uniform coating of CdS and PbS nanocrystals on the TMV surface from TEM analysis. Selected area electron diffraction of the mineralized products indicated a zinc blende crystal stracture for CdS particles and a rock salt structure for single domain PbS nanocrystals. The iron oxide nanoparticles were mineralized by the TMV templates by the oxidative hydrolysis of an Fe VFe acidic solution with NaOH. Consequently, a mineral coating of irregular ferrihydrite particles grew on the surface to a thickness of 2 nm. 

In vitro a crystalline iron core can be laid down in apoferritin by the addition of an oxidant, such as O2, to an aqueous solution of a ferrous salt and apoferritin (32, 132, 140). The reconstituted core of horse ferritin prepared in the absence of phosphate and with O2 as oxidant is very similar to the native core in terms of its size and Mossbauer properties (85). Electron microscopy, however, reveals that it is less well ordered. Reconstitution in the presence of phosphate leads to smaller cores. Reconstituted A. vinelandii cores in the absence of phosphate were more ordered than were the native cores, and clearly contained ferrihydrite particles and, in some cases, crystal domains (85). Thus the nature of the core is not determined solely by the protein coat the conditions of core formation are also important. This is also indicated by Mossbauer spectroscopy studies of P. aeruginosa cells grown under conditions different than those employed for the large-scale pu-... 

Goethite may be synthesized from either Fe or Fe systems. In the Fe system goethite can form over a wide pH range. Methods of synthesis in both acidic and alkaline media are provided in the following section. In alkaline media, synthesis involves holding ft-eshly prepared ferrihydrite (obtained by neutralizing a Fe salt solution with alkali) in KOH at pH > 12 for several days. This method was first described by Bohm in 1925. 

Preparation from a Cysteinc/2-Iine Ferrihydrite System... 

Hematite can also be prepared from 2-line ferrihydrite. This demonstration should be started the previous day. The suspension of ferrihydrite must be refluxed at 90-100 °C for 30 hr after which there is a color change from the dark reddish brown of the ferrihydrite to the deep blood red of the hematite. 

Modified recipe. Properties Two-line ferrihydrite [741,1633,1634], BET specific surface area 245 in7g. X-ray micrographs at different levels of preparation available [1633]. 

Ferrous sulfate (feosol, others) is the hydrated salt, FeSO 7Hff), which contains 20% iron. Ferrous fumarate (feostat, others) contains 33% iron and is moderately soluble in water, stable, and almost tasteless. Ferrous gluconate (fergon, others), which contains 12% iron, also is used in the therapy of iron-deficiency anemia. Polysaccharide—iron complex (niferex, others), a compound of ferrihydrite and carbohydrate, has comparable absorption. The effective dose of these preparations is based on iron content. 

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