Mn-goethite

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).

XANES spectra provide information about contaminants present at too low a level to produce EXAFS spectra. It has been used to investigate the oxidation of As on Mn-goethite (Sun et al. 1999). This technique has also provided information about the valence of Fe in Fe oxide films during cathodic reduction in a borate buffer (Schmicki et al. 1996), about the dissolution of Fe oxide films in acidic solutions (Vir-tanen et al. 1997) and about the orientation of styrene molecules adsorbed on FeO (111) and Fe304 (111) (Wuehn et al. 2000). 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

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. 3.6 Relationship between the unit cell edge length b of synthetic goethites and the level of structurally incorporated Al ", Mn ", Cr ", Co " and V " (data from Schulze Schwertmann, 1984 Stiers Schwertmann, 1985 Schwertmann et al., 1989 Gerth, 1990 and Schwertmann Pfab, 1994, respectively).

Although Fe "and Mn " have similar ionic radii, Mn " does not fit as readily into the goethite structure as does Fe " because owing to its four d electrons, Mn " has a tetragonally distorted coordination sphere on an octahedral site Jahn-Tdler effect). 

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. 

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. 

The carboxylate free radical rapidly decarboxylates to -hydroxy or -amine radicals which are oxidized by oxygen to formaldehyde or acetaldehyde. Substitution of Mn in the goethite structure reduces the photoredox reactivity of goethite. 

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... 

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)...

Fig. 14.13 Extent of transformation at 70 °C of akaganeite to goethite and hematite versus time, a) rod-shaped akaganeite in M KOH b-d) spindle-shaped akaganeite b) M KOH c) 0.1 M KOH d) 0.1 M KOH -h Mn -" (Mn/(Fe-hMn) = 0.1) (Cornell Giovanoli, 1990,1991 with permission).

Fig. 14.21 Effect of cysteine (cyst) alone and cysteine +silicate (Si/Fe = 0.1) or cysteine + Mn (Mn/(Fe+ Mn) = 0.1) on the transformation of 2-line ferrihydrite to goethite (Cornell, unpubl.)...

Fig. 14.23 The effect of Mn, Co, Ni and Cu on the amounts of hematite and goethite (Hm/(Hm + Gt) formed from 2-line ferrihy-drite at various pH and 70°C. M/Fe = 0.1 (Giovanoli Cornell, 1992 with permission).

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