Ferrihydrite substituted

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

Aluminium in the ferrihydrite system not only suppresses goethite in favour of hematite (see chap. 14) but also affects the morphology of hematite, probably by entering the structure. At temperatures of between 70 and 150 °C, a shift was noticed from rhombohedra to plates whose diameter and thickness were at a maximum at an Al/(Fe-i-Al) ratio of 0.05 (Schwertmann et al., 1979 Barron et al., 1984 Barron Torrent, 1984 Wolska Szajda, 1987). At higher levels of substitution, the plates became extremely thin and structural strain increased (Stanjek Schwertmann, 1992)... 

Al-hematites formed slowly from Al-ferrihydrite at 25 °C over 20 years, varied between rhombohedra at low substitution and multidomainic ellipsoids ca. 100 nm across with a grainy interior at higher substitution (Al/(Al-rFe) = 0.15) (Fig. 4.20e f) (Schwertmann et al. 2000). Allophane as a source of A1 had the same effect (Schwert-mann et al. 2000a). Mn substituted hematites grown from ferrihydrite were ellipsoidal in the presence of oxalate and platy in the presence of NaHCOa buffer (Cornell Gio-vanoli, 1987 Cornell et al., 1990). Cu substituted (0.09 mol mol" ) hematite grows as large (0.2 pm) rhombohedral crystals the crystal faces are most probably 102 or 104 (Fig. 4.20d) (Cornell Giovanoli, 1988). 

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... 

Chromate adsorbed from solution by green rust was reduced to Cr with the green rust being simultaneously converted to ferrihydrite or possibly a Cr substituted ferrihydrite (Loyaux-Launiezak et al., 2000). 

Evanko and Dzombak (1998) report that phenol does not adsorb on goethite which contrasts with the findings of McBride and Rung (1991) who found limited adsorption on ferrihydrite as well. The latter authors reported that substituted phenols adsorb on ferrihydrite and goethite in acidic media in the order ... 

At temperatures > 600 °C, ferrihydrite and also 5-FeOOH which have been partly substituted with divalent transition metals, transform to a... 

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). 

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... 

Giovanoli, R. Briitsch, R. (1974) Dehydration of y-FeOOH Direct observation of the mechanism. Chimia 28 188-191 Giovanoli, R. Briitsch, R. (1975) Kinetics and mechanisms of the dehydration of y-FeOOH. Thermochim. Acta 13 15-36 Giovanoli, R. Cornell, R.M. (1992) Crystallization of metal substituted ferrihydrites. Z. Pflanzenemahr. Bodenk. 155 455-460 Giovanoli, R. Briitsch, R. Stadelmann, W. (1975) Thermal decomposition of y- and a-FeOOH. In Barrett, P. (ed.) Reaction kinetics in heterogeneous systems. Elsevier Amsterdam, 302-313... 

Schwertmann, U. Friedl, J. Stanjek, H. (1999) From Fe(III) ions to ferrihydrite and then to hematite. J. Coll. Interface Sci. 209 215-223 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000) The effect of A1 on Fe oxides. XIX. Formation of Al-substituted hematite from ferrihydrite at 25°C and pH 4 to 7. Clays Clay Miner. 48 159-172 Schwertmann, U. Friedl, J. Stanjek, H. Schulze, D.G. (2000a) The effect of clay minerals on the formation of goethite and hematite from ferrihydrite after 16 years ageing at 25 °C and pH 4-7. Clay Min. 35 613-623... 

When other metals (M) substitute for Fe in the structure of an Fe oxide the mole ratio of substitution is given by Mt/(Mt + Fet)(mol/mol), where Mt and Fet (t = total) are expressed in mol. Fe and other metals present at the surface of the iron oxide or in separate phases must be determined separately to correct the extent of substitution. Ferrihydrite as a separate phase can be selectively dissolved an with acid oxalate solution (see p. 50). This treatment also dissolves any separate Mn or Cr oxides. Alternatively, a short extraction (30 min, 25 °C) with 0.4 M HCl removes adsorbed surface species this method is useful if the solubility of the substituting ion in acid oxalate solution is not known or if the iron oxide under consideration (for example magnetite) is soluble in acid oxalate solution. The total Fet and Mt have then to be corrected for the oxalate soluble Fe and M. 

For Fe oxides, FTIR spectroscopy provides a rapid means of identification. It can detect traces (1-2%) of goethite in a sample of hematite. In addition, low levels of impurities arising from insufficient washing, for example nitrate in ferrihydrite (band at 1384 cm ) and carbonate in goethite (ca. 1300 and 1500 cm ) can be detected. Broadening of the absorption bands reflects a decrease in crystal perfection (Cambier, 1986). Structural substitution of Fe by A1 (Schulze Schwertmann, 1984), Mn (Stiers Schwertmann 1985), and Cr (Schwertmann et al. 1989) causes a shift in the positions of the bands. 

Fig. 5-1. Electron micrographs of Al-substituted acicular goethites illustrating the decrease in crystal size with increasing degree of Al-substitution (given as Al/(AI+Fe) moEmoI). The goethites were produced by aging 2-Iine Al-contain-ing-ferrihydrites in 0.35-0.4 M KOH for 14 days at 70 °C (Cornell and Schwert-marm 1996 with permission)..

Fig. 10-2. X-ray diffractograms of hematites. From top to bottom Hematite with 15 mol% A1 substitution grown from 2-line ferrihydrite at pH 7 and 80 °C (The peak shift due to A1. substitution is too small to be visible on this figure). Hematite from forced hydrolysis of Fe(N03)3 (Method 1). Hematite from 2-line ferrihydrite in the presence of oxalate at pH 6.5 (Method 6) Hematite from forced hydrolysis of FCCI3 (Method 3).

Al-substituted hematites were also produced from Al-containing 2-line ferrihydrite at room temperature and pH 4-7, although at a much lower rate (months to years) (Schwertmann et al. 2000). For the same initial Al/ (Al+Fe) ratios the substitution is lower than at higher temperatures. Electron micrographs show rhombic crystals at low substitution (Fig. 10-5 a), identical to unsubstituted ones, and framboidal or spindle-shaped crystals with a grainy or layered interior at higher substitution (Fig. 10-5b). The latter diffracted X-rays as single crystals. 

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