Line Ferrihydrite

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... 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

Ferrihydrite (2-line) Dissolve 40 g Fe(N03)3 9 H2O (or the appropriate amount of any other Fe salt) in 500 mL distilled water, then add, with stirring, ca. 330 mL M KOH to bring the pH to 7-8. Ferrihydrite immediately forms as a voluminous, dark, reddish brown precipitate which settles rapidly. 

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

ABSTRACT The kinetics and mechanisms of the phase transformation of 2-line ferrihydrite to goethite and hematite are being assessed as a function of pH, temperature and Fe/As, Fe/Se, Fe/Mo molar ratios using batch experiments, BET analyses, XRD, and XANES. Initial results from XRD analyses show that ferrihydrite is stable at high pH ( 10) for up to seven days at 25°C, but considerable crystallization occurs at elevated temperatures. Specifically, XRD data show that ferrihydrite is transformed to a mixture of hematite and goethite at 50°C (-85% hematite and -15% goethite) and 75°C (-95% hematite and -5% goethite) after 24 hours and these ratios remain constant to the end of the experiments (seven days). 

SEM-EDS. All the samples of fresh ochre-precipitates exhibit XRD patterns typical of poorly crystalline material, such as ferrihydrite (Schwertmann Cornell 2000). They present the strongest band centred at 2.56A, corresponding to the (110) reflection of 6-line ferrihydrite, as well as the characteristic poorly resolved reflections at d spacings 1.47 - 1.73 - 1.98 and 2.24k. 

Why mammalian ferritin cores contain ferrihydrite-like structures rather than some other mineral phase is less easy to understand, and presumably reflects the way in which the biomineral is built up within the interior of the protein shell together with the geometry of the presumed nucleation sites. The phosphate content in the intracellular milieu can readily be invoked to explain the amorphous nature of the iron core of bacterioferritins and plants. Indeed, when the iron cores of bacterioferritins are reconstituted in the absence of phosphate, they are found to be more highly ordered than their native counterparts, and give electron diffraction lines typical of the ferrihydrite structure. Recently it has been reported that the 12 subunit ferritin-like Dps protein (Figure 19.6), discussed in Chapter 8, forms a ferrihydrite-like mineral core, which would seem to imply that deposition of ferric oxyhydroxides within a hollow protein cavity (albeit smaller) leads to the production of this particular mineral form (Su et al., 2005 Kauko et al., 2006). 

Figure 10. Fe isotope compositions for total aqueous Fe (Fe,(,T) and ferrihydrite (FH) precipitate and aqueous Fe-ferrihydrite fractionations from the batch oxidation and precipitation experiment of Bullen et al. (2001). (A) Measured S Fe values from Bullen et al. (2001), compared to simple Rayleigh fractionation (short-dashed lines, noted with R ) using 10 1naFe.,-FH = 0.9%o, as well as the two-step re-equilibration model discussed in the text (i.e., Eqn. 12), shown in solid gray lines for the aqueous Fe and ferrihydrite components the predicted 5 Fe value for Fe(III), is shown in the heavy dashed line, which reflects continual isotopic equilibrium between Fe(II), and Fe(III),(. Note that in the experiment of Bullen et al. (2001), aqueous Fe existed almost entirely as Fe(II),(,. (B) Measured fractionation between total aqueous Fe and ferrihydrite precipitate, as measured, and as predicted from simple Rayleigh fractionation (black dashed line) and the two-step model where isotopic equilibrium is maintained between aqueous Fe(II),q and Fe(III),q (solid gray line).

The unit cell parameters given by Towe and Bradley (1967) for 6-line ferrihydrite are a = 0.508 nm and c = 0.94 nm. There are four formula units per unit cell. A con-... 

Further refinements of the structure are based on the agreement between experimental and simulated XRD data (Drits et al., 1993 1995) in combination with structural data from EXAFS spectra (Manceau Drits, 1993). According to these results, 6-line ferrihydrite contains three intergrown structural components (Fig. 2.9, upper) ... 

The main difference between 6- and 2-line ferrihydrite is in the size of their crystal... 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

Lower Comparison of a neutron with an X-ray diffractogram of 6-line ferrihydrite (Jansen et al. 2002, with permission)... 

Galvez et al. (1999) demonstrated that phosphorus up to a P/Fe mol ratio of 0.03 mol mol , can be incorporated into the hematite structure by heating P-con-taining 2-line ferrihydrite. Support for structural incorporation comes from a higher unit cell c (1.3776 => 1.3824 nm), IR-stretching bands of P-OH, a lowered intensity ratio of the XRD 104/113 lines and congruent release of Fe and P upon dissolution. 

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... 

Highly-broadened XRD peaks and electron diffraction patterns indicate that ferrihy-drites are characterized by small crystal size and/or low structural order. TEM shows single spherical particles, ca. 4-6 nm in size (Fig. 4.17). At higher magnification (HRTEM), 6-line ferrihydrite appeared as single crystals with a hexagonal outline and... 

Fig. 4.17 TEM of (a) 6-line ferrihy-drite produced by a 12 min acid hydrolysis of Fe(N03)3 at 75 °C and (b) a 2-line ferrihydrite formed by fast hydrolysis of Fe " solution at RTunder neutral conditions (Schwertmann, Cornell, 2000 with permission).

Fig. 4.18 H RTEM of 6-line ferrihydrite with lattice images showing its crystalline nature (Janney et al. 2000a, with permission courtesy D. E. Janney).

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). 

The IR bands are at 3615 cm (free surface OH groups), 3430 cm (bulk OH stretch) and at 650 cm and 450 cm (bulk OH deformations). Russell (1979) found that most OH groups were readily accessible to deuterium exchange which suggested that there is little distinction between the surface and the bulk OH groups. The large half width of the band at 3615 cm (60 cm ) is evidence for a disordered crystal structure this band is broader in 2-line than in 6-line ferrihydrite. Ferrihydrite containing a few percent Si shows a broad, intense band at 940 cm (Schwertmann Thalmann, 1976 Carlson Schwertmann, 1981 Parfitt et al., 1992 Hansen et al.. 

Fig. 7.6 Mossbauer spectra at 4.2 K and hyperfine field distributions of natural 6- (LC31) and 2-line (LC2) ferrihydrite from ferri-ferrous spring waters (Murad, 1988, with permission).

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. 

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). 

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