Substituted goethite

Al substitution (0.09-0.16 mol mol ) had no definite effect on the photochemical dissolution of substituted goethite in oxalate at pH 2.6 (Cornell Schindler, 1987). On the other hand, Al substitution depressed the initial (linear) stage of dissolution of synthetic goethites and hematites in mixed dithionite/citrate/bicarbonate solutions (Fig. 12.22) (Torrent et al., 1987). As the variation in initial surface area has already been accounted for, the scatter of data in this figure is presumably due to variations in other crystal properties such as disorder and micropores. Norrish and Taylor (1961) noted that as Al substitution in soil goethites increased, the rate of reductive dissolution dropped (see also Jeanroy et al., 1991). 

Other substituents also influence the dissolution rate. Of these, and, in particular, Cr substantially stabilized goethite against proton attack, whereas the opposite effect was found for Mn (Fig. 12.23) (Schwertmann, 1991). Similar results were obtained by Lim-Nunez and Gilkes (1987). Unsubstituted and Cr-goethites had 

AICI3 solutions (Ruan, Gilkes, 1995, with permission). 

The effect of these substituents has not been fully explained. It is not, in most cases, due to formation of a protective layer of the substituent at the surface because (M = trivalent substituent) and Fe usually dissolve congruently. Crystal defects (e. g. vacancies) created by substitution should accelerate dissolution, but this is not 

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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).

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. 

Fig. 12.20 Dissolution-time curves of synthetic Al substituted goethites in 6 M HCI at 25 °C. The figures at the curves indicate Al substitution expressed as AI/(Fe-i-AI) mol mol and the figures in () are surface areas in m g (EGME) (Schwertmann, 1991, with permission).

Fig. 12.23 Dissolution-time curves for metal (M)-substituted goethites (M/(Fe-i-M)) as compared to pure goethite without substitution Mn (0.057) (0.060) (0.079) Cr -" (0.078)...

Jonas, K Solymar, K. (1970) Preparation, X-ray derivatographic and infrared study of aluminium-substituted goethites. Acta Chim. 

Scheinost, A.C. Schulze, D.G. Schwertmann, U. (1999) Diffuse reflectance spectra of A1 substituted goethite a ligand field approach. Clays Clay Miner. 47 156-164 Scheinost, A.C. Schwertmann, U. (1999) Color identification of iron oxides and hydroxysul-fates Use and limitations. Soil Sci. Soc. Am. 

Schuler D. (2000 a) Die Biomineralistion von magnetischen Nanokristallen in magnetotak-tischen Bakterien. Biospektrum 6 445-449 Schulze, D.G. Schwertmann, U. (1984) The influence of aluminum on iron oxides. X. Properties of Al-substituted goethites. Clay Min. 19 521-539... 

E. Stiers,W. (1986) Fe Mossbauer effect study of Mn-substituted goethite and hematite. Hyperfme Interactions 29 1157-1160... 

Clays Clay Min. 37 273-279 Vempati, R.K. Loeppert, R.H. Dufner, D.C. Cocke, D.L. (1990) X-ray photoelectron spectroscopy as a tool to differentiate siliconbonding state in amorphous iron oxides. Soil Sci. Soc. Am. J. 54 695-698 Vempati, R.K. Morris, R.V. Lauer, H.V Helmke, P.A. (1995) Reflectivity and other physiochemical properties of Mn-substituted goethites and hematites. J. Geophys. Res. 

Grygar T (1997) Dissolution of pure and substituted goethites controlled by the surface reaction under conditions of abrasive stripping voltammetry. J Sohd State Electrochem 1 77-82. 

Gonzalez, J. M., and Laird, D. A. (2004). Role of smectites and Al-substituted goethites in the catalytic condensation of arginine and glucose. Clays Clay Miner. 52,443-450. 

FIGURE 11.11 (a) TEM image, (b) selective-area electron diffraction pattern, and (c) EDS spectra for synthetic Al-substituted goethite. 

Fe and Fe solutions should be used immediately after preparation. If Fe solutions are allowed to stand, some nucleation of goethite can take place, even at room temperature. If Al-substituted goethite is being prepared, for example, the presence of such seed crystals of goethite in an aged Fe solution will prevent a imiformly substituted product from being obtained. Aged Fe° solutions will, of course, be partly oxidized. 

Fig. 5-2. Electron micrographs of pure and Al-substituted goethites grown in strongly alkaline conditions at 70 °C (a, b) or 25 C (c, d). a no substitution b 7.9 mol% substitution c no substitution d 11.6 mol% substitution. Bar = 100 nm (see also Schulze and Schwertmann, 1984).

Fig. 5-6. X -ray diffractograms of pure and Al-substituted goethites prepared in 0.3 M KOH at 70 or 25 °C. The vertical line at the 212 peak illustrates the peak shift to higher angles as Al-for-Fe substitution increases.

Fig. 5-7. Infra red spectra of pure and Al-substituted goethites prepared at 25 and 70 "C The vertical line illustrates the shift in the position of the OH bending mode due to Al-for Fc substitution.

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