Abstract
The relationship between rate constant, activation energy for formation of acicular α-FeOOH particles and addition quantity of CO32- to colloidal mother liquid was discussed under the conditions: the pH and FeCl3 concentration of mother liquid are constant 12.0-12.1 and 0.040-0.044 kmol/m3 respectively. The rate constant decreased with increasing addition quantity of CO32-upto 0.25kmol/m3, and was constant over 0.25 kmol/m3 for the aging at 80°C. At the temperature lower than 55°C, it did not show the dependence on CO32- concentration. Similarly, the activation energy decreased with increasing addition quantity of Co32-upto 0.25 kmol/m3, and further addition did not affect the activation energy. The observed dependence of the rate constant on the addition quantity of CO32-can be attributed to the following reason: the crystal growth of α-FeOOH is restrained by the adsorption of CO32- on the specified crystal plane. The fact that activation energy decreases with increasing addition quantity of CO32- indicates that recrystallization process of Fe(OH)4- changes from the surface reaction controlled process to the diffusion controlled process. Both crystallite size and median diameter of acicular, α-FeOOH particles decreased with increasing addition quantity of CO32-. Both lengths of major axis and minor axis of acicular particles similarly decreased with increasing addition quantity of CO32-. These dependencies indicate that the CO32- ions adsorb on the crystal planes homogeneously without any preference of the crystal planes among a, b, and c planes. This resulted in the decrease of dimension of the acicular α-FeOOH formed with similar aspect ratio.
