Ferrihydrite properties

Like X-ray diffraction patterns, neutron and electron diffraction patterns provide averaged information about the structure of a compound. Details of these techniques are given in works by Hirsch et al. (1965) and West (1988). Neutron diffraction involves interaction of neutrons with the nuclei of the atoms. As the neutrons are scattered relatively evenly by all the atoms in the compound, they serve to indicate the positions of the protons in an oxide hydroxide. This technique has been applied to elucidation of the structure and/or magnetic properties of goethite (Szytula et al., 1968 Forsyth et al., 1968), akaganeite (Szytula et al., 1970), lepidocrocite (Oles et al., 1970 Christensen Norlund-Christensen, 1978), hematite (Samuelson Shirane, 1970 Fernet et al., 1984) and wiistite (Roth, 1960 Cheetham et al., 1971 Battle Cheetham, 1979). A neutron diffractogram of a 6-line ferrihydrite was recently produced by Jansen et al. (2002) and has helped to refine its structure (see chap. 2). 

In general, foreign species in the system can have two different effects on the transformation of ferrihydrite to other Fe oxides they can either modify the rate of the transformation, usually by slowing the process, or change the composition (mainly the hematite/goethite ratio) and properties of the end product. Two principal mechanisms of interaction operate ... 

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). 

Chiarizia, R. Horwitz, E.P. (1991) New formulations for iron oxides dissolution. Hydrometallurgy 27 339-360 Childs, C.W Wilson, A.D. (1983) Iron oxide minerals in soils of the Ha apai Group, Kingdom of Tonga. Aust. J. Soil Res. 21 489-503 Childs, C.W. (1992) Ferrihydrite A review of structure, properties and occurrence in relation to soils. Z. Pflanzenemahr. Bodenk. 155 441-448... 

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... 

Pankhurst, Q.A. Pollard, R.J. (1992) Structural and magnetic properties of ferrihydrite. Clays Clay Min. 40 268-272... 

Schwertmann, U. Stanjek, H (1998) Stirring effects on properties of A1 goethite formed from ferrihydrite. Clays Clay Miner. 46 317-321... 

Although arsenic-bearing ferrihydrites are mostly amorphous, Rancourt et al. (2001, 849) concluded that As(V) is generally tetrahedrally coordinated with Fe(III) in the compounds. The chemical properties of the marine ferrihydrites described in Rancourt et al. (2001, 848) more closely resemble synthetic ferrihydrites formed by the coprecipitation of arsenic and Fe(III) rather than compounds that had sorbed arsenic sometime after their precipitation. 

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-... 

Other physical properties also show that the iron cores of native ferritins and bacterioferritins are different. Mossbauer spectra of ferritins measured as a function of temperature (Fig. 1) show quadrupole split doublets, with an isomer shift typical of Fe +, gradually being replaced as the temperature is lowered (between about 50 and 15 K) by a magnetic hyperfine spectrum (30, 31). The transition temperature, Tb, is lower than the ordering temperature, Tord (240 K) observed for bulk ferrihydrite (32), because of fluctuations in the direction of mag-... 

Madsen MB, Morup S, Koch CJW (1986) Magnetic properties of ferrihydrite. Hyperfine Interactions 27 329-332... 

Accurate predictions of the transport of As in groundwater requires site specific data to model adsorption/desorption reactions. In complex mixtures of minerals, it may not be possible to quantify the adsorption properties of individual minerals. Therefore, it has been suggested that adsorption properties of composite materials should be characterized as a whole (Davis and Kent, 1990). Previously published data for adsorption by pure mineral phases such as the surface complexation database for adsorption by ferrihydrite (Dzombak and Morel, 1990) can be a useful starting point for modeling adsorption of solutes in groundwater however, these equilibrium constants may not reflect the adsorption properties of composite oxide coatings on aquifer solids. For example, incorporation of Si, and to a lesser extent, A1 into Fe oxyhydroxides has been shown to decrease adsorption reactivity towards anions (Ainsworth et al., 1989 Anderson and Benjamin, 1990 Anderson et al, 1985). Therefore, equilibrium constants will likely need to be modified for site-specific studies. 

Properties 60% lepidocrocite, 40% ferrihydrite, XRD results available, BET specific surface area 222 m /g [1611]. 

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

Properties Two-line ferrihydrite [157,1629], structure confirmed by XRD, three-point BET specific surface area 202 m-/g [1630]. 

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],... 

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]. 

Properties Two-line ferrihydrite with traces of goethite [1644], amorphous, BET specihc surface area 200 rn /g [1488]. 

Properties 36% ferrihydrite (FejOj 2.2H2O), XRD pattern available, BET specific surface area 213 mVg, TEM image available [1611]. 

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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