FT-infrared spectroscopic study of the effect of aluminium substitution on dehydroxylation of synthetic goethite

Full text: Goethite (α-FeOOH) and Hematite (α-Fe2O3) are the most common forms of pedogenic crystalline iron oxides; they often contain appreciable amount of aluminium as an isomorphous substitution in the natural environment. Dehydroxylation of goethite to form hematite occurs due to natural and managed fires, and is a component of some manufacturing processes. Aluminium substitution in goethite is an important factor in controlling the stability of the lattice, affecting the thermal decomposition of the crystal structure, the rate of cation migration and the release of hydroxyl groups (Ruan and Gilkes, 1995,1996, Grygar et al., 1999). X-ray diffraction lines indicated that the temperature range of goethite to hematite transformation was 200-230 deg C for the 0 mole%, 210-250 deg C for the 10 and 20 mole% and 220-260 deg C for the 30 mole% aluminium substituted goethite, respectively heated for one hour. The intensity of goethite infrared absorption bands of the hydroxyl stretching region at 3208-3222 cm-1, the hydroxyl deformation and water bending region at 1682- 1684, 1645-1647 and 1413-1418 cm-1, and the low frequency region at 888-910, 795-809, 562-616 and 454-480 cm-1 decreased as dehydroxylation temperature increased. - decreased as dehydroxylation temperature increased. Some of these bands completely disappeared at a heating temperature of 270 deg C. The decrease of intensity in the low frequency region indicated that different stages of the transformation of goethite to hematite responded to various heating temperatures. The 454-489, 536-554, 1327-1348 and 1502-1520 cm-1 bands appeared after heating at 230 deg C have confirmed the formation of hematite. This type of hematite has been reported to contain about 3% excess hydroxyls (Ruan and Gilkes, 1995). The variation of bands in the hydroxyl deformation region (1200-1800 cm-1) indicated a sustained release of excess hydroxyl groups from the structure of goethite and the resultant hematite as affected by dehydroxylation temperature. The excess water band of 3450 cm1 in the hydroxyl stretching region remained after heating at 270 deg C, indicating that goethite altered to hematite without intermediate phases. This is why hematite formed by heating of goethite inherited the crystallographic characteristics from the precursor goethite (Ruan and Gilkes, 1995). The effect of aluminium substitution on dehydroxylation of goethite is to increase the dehydroxylation temperature (Ruan and Gilkes, 1996). This trend was indicated by the infrared absorption spectra. A decrease in intensity of the infrared absorption bands at 220 deg C for the 0 mole%, 230 deg C for the 10 and 20 mole% and 240 deg C for the 30 mole% aluminium substituted goethite were observed. These assignments corresponded to the maxima of H2O/OH release and lattice vibrations of the FeO6 octahedra. The band central of 454, 795 and 888 cm-1 for goethite and 449, 536 and 1327 cm-1 for hematite shifts to a higher frequency number systematically as aluminium substitution increased. An exception of the 616 cm-1 band for goethite was shown by a decrease in frequency number as aluminium substitution increased. Other band components remained unchangeable with the variation of aluminium substitution