Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units.

Dehydroxylation of goethite as affected by aluminium substitution was investigated using Fourier transform infrared spectroscopy (FT-IR) in conjunction with X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA). The band intensities of hydroxyl vibrations were indicative of the degree of dehydroxylation and the changes in band parameters due to aluminium substitution were observed. The effect of aluminium substitution on band parameters of FT-IR spectra of goethite and its partially and fully dehydroxylated products, the mixture of goethite/hematite and hematite, were interpreted. The results of this study have confirmed that aluminium substituted goethite is thermally more stable than non-substituted goethite and is in harmony with the results of XRD and DTGA. A larger amount of non-stoichiometric hydroxyl units is associated with a higher aluminium substitution. A shift to a higher wavenumber of bending and hydroxyl stretching vibrations is attributed to the effects of aluminium substitution associated with non-stoichiometric hydroxyl units on the a-b plane relative to the b-c plane of goethite. The results provide information for the characterisation of activated bauxite containing hematite and goethite.