Acid dissolution of synthetic aluminous goethite before and after transformation to hematite by heating

Measurements of the kinetics of acid dissolution of synthetic aluminous goethites and corresponding hematites produced by heating of parent AI-goethites at various temperatures were carried out in 1 M HCl at 30, 40 and 50 degrees C. Dissolution-time curves show sigmoidal shapes for the goethites (110 degrees C), whereas deceleratory shapes were obtained for most of the partly and fully dehydroxylated samples. The dissolution rate for all materials decreased with increasing Al substitution and increased with increasing dissolution temperature, specific surface area and heating temperature. On a unit surface area basis, hematite dissolved similar to 2-8 times faster than goethite. Dissolution kinetics of most heated goethite samples (200-260 degrees C) were quite well described (R(2) > 0.96) by the modified first-order Kabai equation. The activation energy and frequency factor for dissolution increased with increasing Al substitution.