Hematite surface hydroxyls

Where represents phthalate ion, which are considered here as potential determining ions. Khl and Kl, are the equilibrium constants, obtained by fit to adsorption data. The adsorption density as a function of phthalate aqueous concentration is illustrated in Figure 2, where the solid line shows SCF/DLM model calculation with log A // , = 16.45, log/i , = 11.28, in addition to the constants for hematite surface hydroxyl species. The model yields a good estimation at higher adsorption densities, but it overestimates the adsorbed organics at lower solution concentration. 

Small organic molecules, for example phthalate, show specific interaction with hematite surface hydroxyl groups. Phthalate forms strong complexes on the hematite surface, thereby reducing the initially positive surface potential. Wlien sufficient phthalate is present in the suspension, hematite surface potential can be reversed as a result of phthalate adsorption. 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

Fig. 10.3 Surface hydroxyl configuration ofthe hematite [100], [110], [012], [104], [018], [113], and [001] faces. Distances of O and Fe ions to the projection plane are indicated next to the corresponding row of ions. Rows of singly, doubly and triply...

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Barron,V. Torrent, J. (1996) Surface hydroxyl configuration of different crystal faces of hematite and goethite. J. Colloid Interface Sci. 177 407-411... 

Robl, R. (1958) t)ber ferromagnetische Eisenox-yde Angew. Chem. 12 367-371 Rochester, C.H. Topham, S.A. (1979) Infrared study of surface hydroxyl groups on goethite. J. Chem. Soc. Faraday Trans. I. 75 591-602 Rochester, C.H. Topham, S.A. (1979a) Infrared study of surface hydroxyl groups on hematite. J. Chem. Soc. Faraday Trans. I. 75 1073-1088... 

Watanabe. 11. and Seto, J., The intrinsic equihbrium constants of the surface hydroxyl groups of maghemite and hematite. Bull. Chem. Soc. Jpn, 63, 2916, 1990. 

Certain acids with hydroxylic and carboxylic groups have been shown (Schwert-mann and Cornell, 1991) to induce in Fe(HI) solutions the formation of hematite because these acids may act as templates for the nucleation of hematite. These examples illustrate that a complete understanding and quantitative description of the rate of heterogeneous nucleation will have to include surface complexation and other adsorption processes. 

Figure 9. Simplified model of the (111) surface of the corundum-type structure, (a) A view of the surface from a direction slightly shifted from <111>. Only metal ions of the zeroth, first, and second layers are shown, (b) A section of the surface along the arrows depicted in part a. Hexagonally close-packed oxide ion layers are shown with lines. Surface protons are not shown, (c) A divalent Co-57 or pentavalent Sb-119 ion on the zeroth metal ion layer, (d) Aquo or hydroxyl complex of divalent Co-57 or pentavalent Sb-119 hydrogen-bonded to the surface oxide ion layers of hematite.

Yapp. C.J. (2000) Climatic implications of surface domains in arrays of 5D and 5180 from hydroxyl minerals Goethite as an example. Geochim. Cosmochim. Acta 64 2009-2025 Yariv, S. Mendelovid, E. Villalba, R. (1980) Thermal transformation of goethite into hematite in alkali halide discs. J. Chem. Soc. Faraday Trans. I. 76 1442-1454 Yariv, S. Mendelovid, E. Villalba, R. Cohen, M. (1979) Transformation of goethite to maghemite in Csl discs. Nature 279 519-520... 

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