Goethite surface area

Lengweiler et al. (1961) found that the solubility of goethite, like that of ferrihydrite, increased as the pH rose above 12. For ferrihydrite, equilibrium between the solid and Fe(OH)4 was reached quite rapidly, whereas for goethite, equilibrium was not reached even after 40 days (25 °C). A value of 1.40 + 0.1 for of goethite (surface area ca. 100 m g" ) was only reached after 3 years (Fig. 9.5) (Bigham et al., 1996). As expected on thermodynamic grounds, the solubility of goethite was 10 to 10 times less than that of ferrihydrite. 

Fig. 10.12 The effect of increasing amounts of goethite (surface area 51 m g ) on the electrophoretic mobility of kaolinite at various pH. The figures on the curves indicate the amount of goethite added (mg g ) (Venema and Glasauer, unpubl.).

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

Measured surface areas (11-point BET analyses) for pure phases such as ferrihydrite, goethite and hematite are in the range as proposed by Cornell Schwertmann (2003) (Table 1). Preliminary XRD analyses showed that temperature impacts the kinetics of phase transformation of ferrihydrite. Data indicated that after seven days, the rate of transformation from ferrihydrite to more crystalline forms, if it was occurring, was too slow to be measured at 25°C (Fig. 1). In contrast to the 25°C experiment, significant, transformations were observed at 50 (Fig. 2) and 75°C (Fig. 3) after 24... 

The surface areas of both natural and synthetic goethites range from ca. 8 to 200 m g . XRD line broadening techniques indicate that soil goethites have surface areas of from 20 to 200 m g (Schwertmann, 1988). On the other hand, massive (museum) specimens may consist of mm sized crystals with surface areas well below 1 m g- ... 

High surface areas (80-150 m g ) are also reported for goethites formed by oxidation of Fe systems at pH 6-7 and room temperature, whereas those grown at pH 12 have lower areas of around 30 m g (Torrent et al., 1990). 

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... 

It is obvious that the standard deviation for hematites is greater than that for goethites, mainly due to the greater variety of crystal faces. Al-for-Fe substitution did not directly influence the P adsorption for either synthetic goethites or hematites the surface area tends to increase with rising A1 incorporation and this, in turn, increases adsorption/unit weight (Ainsworth et al., 1985). 

Fig. 12.18 Dissolved Fe and change of the sample s surface area with time during the dissolution of a synthetic goethite at 24 °C in 6 M HCI (Weidler, 1995, with permission).

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). 

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. 12.22 Relationship between the dissolution rate per unit surface area in Na-dithionite/citrate/bicarbonate at 25 °C and the Al substitution of 28 synthetic goethites (upper) and 24 synthetic hematites (lower) (Torrent et al., 1987, with permission).

Dos Santos Alfonso and Stumm (1992) suggested that the rate of reductive dissolution by H2S of the common oxides is a function of the formation rate of the two surface complexes =FeS and =FeSH. The rate (10 mol m min ) followed the order lepidocrocite (20) > magnetite (14) > goethite (5.2) > hematite (1.1), and except for magnetite, it was linearly related to free energy, AG, of the reduction reactions of these oxides (see eq. 9.24). A factor of 75 was found for the reductive dissolution by H2S and Fe sulphide formation between ferrihydrite and goethite which could only be explained to a small extent by the difference in specific surface area (Pyzik Sommer, 1981). 

Body, J.E. Persson, P. Sjdberg, S. (2000) Benzene carboxylate surface complexation at the goethite (a-EeOOH)/water interface. III. The influence of particle surface area and the significance of modelling parameters. J. Cod. 

Weidler, P.G. (1997) BET sample pretreatment of synthetic ferrihydrite and its influence on the determination of surface area and porosity. J. Porous Materials 4 165-169 Weidler, P.G. Degovics, G. Laggner, P. (1998) Surface roughness created by acidic dissolution of synthetic goethite monitored with SAXS and N2 adsorption isotherms. J. Colloid Interface Sd. 197 1-8... 

Adsorption of anions at mineral surfaces is important in soils because of the limit this process imposes on the availability of plant nutrients such as P, S, and Mo which occur naturally as anions and are added in anionic form in fertilizers. Anion adsorption is also relevant in geochemistry, ore processing, and other fields where minerals with high surface areas are brought into contact with aqueous solutions of anions. Selenite and goethite were chosen for this study because in Western Australia a selenium deficiency in pastures has been shown to be related to the incidence of white muscle disease in sheep (3), and according to workers quoted by Rosenfeld and Beath (9) selenium in soils of higher... 

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