Ferrihydrite surface area

Natural Fe oxides have also been used for water purification. In a laboratory study, a natural ferrihydrite (surface area of 243 m g ) originating from a ferriferrous acid spring turned out to be capable of removing > 95 % of the inorganic phosphate from water with 0.1 mg P L (WeiE et al., 1992). The so-called Red Mud, a waste product of the alumina industry, containing 330 g Fe/kg was also effective, whereas a tropical soil with 80 g kg Fe was comparatively less so (WeiE et al., 1992a). Nine... 

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). 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Since laboratories follow different aging procedures, results of their studies can be difficult to compare. Values reported for surface area and site densities vary over a relatively broad range (Dzombak and Morel, 1990). It is not clear, furthermore, how closely the synthetic material resembles sorbing ferric oxides (e.g., ferrihydrite) encountered in nature. 

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. 

Measured surface areas (11-point BET analyses) for pure phases such as ferrihydrite, goethite and hematite are in the range as proposed by Cornell Schwertmann (2003) (Table 1). Preliminary XRD analyses showed that temperature impacts the kinetics of phase transformation of ferrihydrite. Data indicated that after seven days, the rate of transformation from ferrihydrite to more crystalline forms, if it was occurring, was too slow to be measured at 25°C (Fig. 1). In contrast to the 25°C experiment, significant, transformations were observed at 50 (Fig. 2) and 75°C (Fig. 3) after 24... 

The BET method requires that the sample be dried and outgassed to remove adsorbed water. This procedure may, if the outgassing temperature is too high, lead to a phase change at the surface of the oxide hydroxide and hence, an alteration in the specific surface area of the sample. Clausen and Eabricius (2000) recommend that ferrihydrite be outgassed at room temperature, at which temperature, a stable BET surface area is obtained after 19 hr of outgassing. 

The effect of aluminium on the surface area of goethite depends on the level of Al in the system and on the source of iron. Other conditions being equal, Al reduces both the rate of growth and the crystal size its effect on surface area depends on which of these two effects predominates. The surface area (EGME) of goethite grown from ferrihydrite in 0.3 M KOH at 25 °C dropped from 52 to 26 m g as the extent of Al substitution rose from 0 to 0.16 mol mol (Schulze and Schwertmann, 1987). This effect was attributed to an increase in crystal thickness along the [001] direction... 

Surface areas of ferrihydrite have been determined by a wide range of methods and are reported as varying between 100 and 700 m g (Table 5.1 see also Jambor Du-trizac, 1998). The BET method is widely used for natural samples this gives areas of between 200-400 m g . ... 

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... 

The surface area of synthetic hematite depends upon whether the oxide was produced by calcination or grown in solution. The temperature of (dry) heating influences the surface area. Hematites produced at 800-900 °C have areas < 5 m g due to sintering of the particles. Hematites obtained by dehydroxylation of the various polymorphs of FeOOH or ferrihydrite at temperatures lower than 500-600 °C are mesoporous and have much higher surface areas - up to 200 m g". Commercial hematites are usually produced by calcination and hence have a low surface area. 

Lengweiler et al. (1961) found that the solubility of goethite, like that of ferrihydrite, increased as the pH rose above 12. For ferrihydrite, equilibrium between the solid and Fe(OH)4 was reached quite rapidly, whereas for goethite, equilibrium was not reached even after 40 days (25 °C). A value of 1.40 + 0.1 for of goethite (surface area ca. 100 m g" ) was only reached after 3 years (Fig. 9.5) (Bigham et al., 1996). As expected on thermodynamic grounds, the solubility of goethite was 10 to 10 times less than that of ferrihydrite. 

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. 

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

Although this type of transformation can take place in solution, usually under hydrothermal conditions, it has been most intensively investigated in the dry state. A precise separation of a transformation in the dry state from that in the presence of vater is, ho vever, often difficult because the minimum amount of water with which a via-solution transformation is still possible may be very small (see 14.3.5). This applies especially to poorly ordered and nano-sized oxides, such as ferrihydrite, with high surface areas and, therefore, high amounts of adsorbed water. 

No interaction between ferrihydrite and kaolinite was found at pH 9 because both compounds are negatively charged at this pH (Fig. 16.20b, c). Boiling kaolinite and montmorillonite in a Fe(N03)3 solution for 8 min resulted in clays containing up to ca. 55 mg oxalate soluble Fe/g clay. The BET surface area of kaolinite increased from 18 to 34 m /g and that of montmorillonite from 11 to 62 m g . Whereas kaolinite shows only a small decrease in > 10 pm pores, montmorillonite lost about half of its >10 pm pores even with the lowest Fe oxide content (6.6 mg ECq g clay). It has been speculated that in contrast to kaolinite, the Fe oxide, in the presence of montmorillonite, remained highly disorderd and active due to A1 and Si dissolved from... 

GrefFie, C. Amouric, M. Parron, C. (2001) HRTEM study of freeze-dried and untreated synthetic ferrihydrites consequences of sample processing. Clay Min. 36 381-387 Gregg, S.J. Sing, K.S.W. (1991) Adsorption, surface area and porosity. 2"" ed.. Academic Press, London, 371 pp. 

Stanjek, H. Weidler, P.G. (1992) The effect of dry heating on the chemistry, surface area, and oxalate solubility of synthetic 2-line and 6-line ferrihydrites. Clay Min. 27 397-412... 

Weidler, P.G. (1997) BET sample pretreatment of synthetic ferrihydrite and its influence on the determination of surface area and porosity. J. Porous Materials 4 165-169 Weidler, P.G. Degovics, G. Laggner, P. (1998) Surface roughness created by acidic dissolution of synthetic goethite monitored with SAXS and N2 adsorption isotherms. J. Colloid Interface Sd. 197 1-8... 

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