Ferrihydrite particle size

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... 

Equation (9.46) has a slope of 2.35 instead of 3.0 and implies that there are 0.65 moles of nitrate in the structure of ferrihydrite. The problem with this formulation is, however, that it refers to a salt, not an oxide hydroxide and thus, cannot apply to ferrihydrite. On the other hand, Byrne and Yu (2000) suggest that their Kso = [Fe] [OH] for freshly precipitated ferrihydrite has a non integral value owing to variations in the activity of the solid phase, which as mentioned earlier, is due to the influence of pH (3-7.5) on the particle size distribution of the precipitate. Two fairly similar values of ca. -39 have been reported for soil ferrihydrite (Table 9.4). It is probable that in these soils, ferrihydrite determined the activity of dissolved Fe. 

That the reduction takes place at the surface, receives support from an experiment in which the extent of reduction of various Fe oxides by Shewanella alga after 30 days was linearly correlated with the SAbet the exception was 2-line ferrihydrite for which a surface area of 600 m /g had to be assumed in order to fit the relationship (Roden Zachara, 1996). Although experimental (BET) surface areas of ferrihydrite are substantially lower than 600 m /g, calculated values based on particle size as well as those determined from ligand adsorption experiments (see Table 5.1) are in this range. Dissolved Fe was found to create a lag phase in the reduction process (in contrast to the behaviour in inorganic systems) because Fe is adsorbed at the cell surface (Liu et al. 2001). This effect can be overcome by complexing the Fe (e. g. 

Morris et al. (1991) obtained hematite of very small particle size ( 10 nm), termed nanophase by slow thermal decomposition in air of tri-Ee -acetato-hy-droxy-nitrate. XRD shows only two broad lines as in a 2-line ferrihydrite, but the magnetic hyperfine field at 4.2 K of 50.4 T appears to be more in agreement with poorly crystalline hematite. Well-crystalline hematite and Al-hematite were produced by decomposing Ee-Al-oxinates at 700 °C (da Costa et al. 2001). 

The poorly crystalline Fe(III) mineral ferrihydrite (Fe10Ol5 9H20) forms with very small particle size, whereas geothite (a-FeOOH) and hematite (a-Fe203) form with particle sizes varying from about 5 nm to 1 /am in diameter. The effect of small particle size on these latter two Fe(III) minerals can be modeled mathematically with the empirical relationships ... 

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-... 

The better crystalline iron oxides do not have an internal surface area, hence the BET method measures the total surface area of the solid. Ferrihydrite, however, frequently contains aggregates the internal surfaces of which may not be entirely accessible to N2, hence the BET surface area is much lower than that calculated from the average particle size obtained by electron microscope observation. For example, for spherieal particles with a diameter of 3 ran and a density of 3 g/em a 2-line ferrihydrite would have a surface area of ea. 600 m /g whereas its N2-BET surface is hardly ever higher than ea 300 m /g. Indeed, ferrihydrites are usually strongly aggregated irrespeetive of whether dried in air or freeze-dried. 

Bottom right. Subrounded crystals produced from 2-line ferrihydrite in the presence of cysteine (Method 5.2.3 for particle size distribution see Fig. 5-6). 

Discuss the conditions of formation, particle size, crystallinity, moisture content, [Fe(III)/OHJ solid ratio, and thermodynamic stability of Fe(lll) oxyhydroxide solids such as ferrihydrite and goethite. What happens to the solution pH, stoichiometry, and solubility of such solids when they are precipitated and aged under acid versus under alkaline conditions ... 

This analysis has utilized roughly equivalent amounts of sample species in bulk and GI mode, but bulk experiments can also be increased in surface sensitivity by using progressively smaller particle sizes. In this way for a given surface loading the amount of sample species can be increased subject to overall sample absorption. The sensitivity obtained in this way overlooks variations in particle surfaces as size decreases, and is highly dependent on X-ray energy. For example, arsenate sorbed on 3 nm ferrihydrite particles can be studied at the bulk composition of As/Fe=0.005 or so (Waychunas et al. 

Fig. 6.9. Variation in zeta potential (y and particle size with the percentage of myo-inositol hexakisphosphate (IHP) adsorbed on goethite, ferrihydrite, calcite, illite and kaolinite at pH 4.5 in 0.01 M KCI, except for the calcite system, for which the pH was 8.0 (adapted from Celi eta/., 1999, 2000, 2003).

All Fe oxides and oxohydroxides are scarcely soluble. The solubility usually follows the order ferrihydrite > maghemite > lepidocrocite > hematite > goethite (Schwertmann and Taylor 1989), which obviously is the order of the relative thermodynamic stability of these minerals. The reported solubility products of actual samples, however, show important variations due to differences in particle size and... 

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