Kinetics of thermal dehydroxylation of aluminous goethite

The kinetics of dehydroxylation of synthetic aluminous goethite was studied using isothermal and non-isothermal thermogravimetry. The complete isothermal dehydroxylation can be described by the Johnson-Mehl equation with up to three linear regions in plots of lnln [1/(1-y)] vs. lnt. Kinetics for the initial stage of dehydroxylation changed from diffusion to first-order through the temperature range 190 to 260 degrees C. The rate of dehydroxylation was reduced by Al-substitution and increased with temperature. Activation energy for dehydroxylation, calculated from the time to achieve a given dehydroxylation extent, varied depending on the extent of dehydroxylation and Al-substitution, Non-stoichiometric OH existed in goethite and some remained in hematite after the complete crystallographic transition.