Goethite substitution

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

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... 

Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

Fig. 3.3 Relationship between the edge length b of the unit cell and the AI-for-Fe substitution of synthetic goethites.

Fig. 3.4 Electron micrographs of goethites with Al substitution increasing from 0 to 0.167 mol mol" (produced from 2-line ferri-hydrite in alkaline solution at 70°C.

Al-for-Fe substitution in natural goethites was originally discovered, with the aid of X-ray diffraction, in marine, oolithic, iron ores from the Jurassic era by Correns von Engelhardt as early as in 1941 and twenty years later found in soils by Norrish and Taylor (1961). Since then, a large number of studies has revealed that Al substitution in goethites from the weathered zone, e.g. in soils (see chap. 16), appears to be the rule rather than the exception. It should be noted that Al located in the struc-... 

Table 3.3 Cations substituting for Fe " in goethite. Maximum substitution and corres ponding unit...

Fig. 3.7 Top Relationship between the unit cell edge length a of synthetic goethites and various structurally incorporated metals. Bottom Rate of change of a per mol of substituted metal (= slope of the upper curves) vs. ionic radius of the respective metal cations (Gerth, 1990, with permission).

Fig. 3.8 Fraction of metals dissolved vs. fraction of Fe dissolved from synthetic metal substituted goethites. Left Ni-, Co- and Mn-goethites in 0.5 M HCI at 75 °C (Giovanoli Cornell, 1992, with permission). Right Four synthetic V-goethites in 6 M HCI at 25 °C (Schwertmann, Pfab, 1994, with permission).

Conditions which promote multi-domainic goethites are high ionic strength (either [KOH] or salt) and also low synthesis temperature (<40°C). In alkaline solutions, multi-domainic character decreases and domain width increases as Al substitution increases to Al/(Fe-i-Al) of 0.15, whereas at Al/( Al-nFe) >0.15 single domain crystals result (Schulze Schwertmaim, 1984 Mann et al., 1985). Multidomainic goethites can recrystallize to single domain crystals as a result of hydrothermal treatment at 125-180 °C (Fig. 4.9) (Schwertmann et al., 1985). 

Additives usually alter only the length-to-width or width-to-thickness ratio of the aci-cular crystals. Growth of long, thin crystals (aspect ratio >12) is induced by high levels (>0.1) of Mn or Co and is attributed to adsorption rather than substitution. These ions have the same influence on aspect ratio whether goethite is grown from Fe" or Fe " systems and over the pH range 7-13. 

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)... 

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... 

Fig. 6.13 Munsell hue of goethite as a function of substitution with transition metals ( Courtesy A.C.Scheinost Schwertmann Cornell, 2000, with permission)...

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai " Ti transition from 943 to 985 nm and that of the Ai " T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn "-substituted goethites showed bands arising from Mn " near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V "-substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn "-substituted hematite shifted to 545 nm and that of the Ai " T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

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