Ferrihydrite aggregation

Hematite, Aggregation, short-range crystallization within ferrihydrite aggregate Aqueous solution at pH 6-8... 

In summary, there is considerable evidence to support the concept that in the presence of water, hematite forms from aggregated ferrihydrite by a short-range crystallization process within the ferrihydrite aggregate, with even adsorbed water being sufficient for the transformation to occur. The evidence is ... 

Hematite production by transformation offerrihydrite (methods 4 and 5) starts with the precipitation of 2-line ferrihydrite which is then converted into hematite in aqueous suspension by a short-range crystallization process within the ferrihydrite aggregates. 

This transformation takes place under weakly acid to weakly alkaline conditions and requires the presence of some water within the ferrihydrite aggregates (see chapter 4) (Feitknecht and Michaelis, 1962 Schwertmann.and Fischer, 1966, Schwertmann et ah, 1999). 

Arsenate adsorption on ferrihydrite consisted of a period of rapid uptake followed by slow adsorption for at least 8 days 43). The rate of the slow adsorption reaction is considered to be limited by diffusion into the ferrihydrite aggregates. Slow adsorption kinetics similar to those for phosphate are expected for arsenate because of the similar chemistry of these two anions. Arsenate adsorption data adhere to the Elovich kinetic model indicating a diffusion limited reaction. Arsenate desorption rates were much slower than arsenate adsorption rates, also consistent with a diffusion limited process. A model was developed that assumes that 63% of adsorbing sites are located at the exteriors of aggregates and reach arsenate equilibrium rapidly, while 37% of adsorbing sites are located in the interiors of aggregates with access being diffusion limited. 

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... 

Due to aggregation of particles, ferrihydrite is microporous, i. e. the porosity is interparticular. Ferrihydrite precipitated at pH 8 from Fe " solution displayed a type IV isotherm with type E hysteresis (Crosby et al., 1983). The freshly precipitated material contained ink bottle pores 2-5 nm in diameter. Larger pores (ca. 20 nm) developed over an 11 day period. Between 83 and 95% of the total pore volume of a 2-line ferrihydrite was found to be due to micropores (Weidler, 1995). 

Ferrihydrites from ferritin also show a range of TbS, depending on the source organism (human > limpet > bacterial) (Webb St.Pierre, 1989) (see Fig. 17.3). The Tb of a 2-line ferrihydrite dropped from 50 to 25K when citrate was added at a level of ci-trate/Fe -0.08 (Zhao et al. 1994). Particle interactions in aggregates also affect the Mossbauer parameters. 

Migration of phosphate into aggregated particles of ferrihydrite. J. Soil Sci. 39 275-282... 

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. 

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 2. Cartoon illustrating features of the structure of ferrihydrite proposed by Drits et al. (1993). Note the severd octahedra-wide spaces. Although not discussed by Drits et al., these might arise if the solid forms by aggregation of smaller iron oxyhydroxide subunits.

Fuller et al. (1993) measured desorption rates for As(V) from ferrihydrite, aged for 24 h, as a result of increasing pH. Arsenate was first equilibrated with ferrihydrite for 144 h at pH 8.0 The molar ratio of As(V) adsorbed to Fe in ferrihydrite was 0.10 (Fig. 7). Desorption was initiated by rapidly increasing the pH to 9.0. Within a few hours, the molar ratio of As(V)/Fe had decreased to about 0.08. The rate of desorption then slowed as the rate became limited by diffusion of As(V) from pores within mineral aggregates. Within 96 h, the concentration of As(V) still adsorbed was only about 5% greater than the adsorbed concentration of As(V) determined in a separate adsorption experiment at pH 9.0. Similar desorption behavior for As(V) from ferrihydrite were measured by Fuller and Davis (1989). 

Properties Two-line ferrihydrite [539,1510], BET specific surface area 327 m7g [1510], aggregates 235 jiun in diameter [539]. 

Iron Core Only a small fraction of the iron atoms in ferritin bind directly to the protein. The core contains the bulk of the iron in a polynuclear aggregate with properties similar to ferrihydrite, a mineral found in nature and formed experimentally by heating neutral aqueous solutions of Fe(III)(N03)3. X-ray diffraction data from ferritin cores are best fit by a model with hexagonal close-packed layers of oxygen that are interrupted by irregularly incomplete layers of octahedrally coordinated Fe(III) atoms. The octahedral coordination is confirmed by Mossbauer spectroscopy and by EXAFS, which also shows that the average Fe(III) atom is surrounded by six oxygen atoms at a distance of 1.95 A and six iron atoms at distances of 3.0 to 3.3 A. 

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