Abstract
The relationship between rate constant, activation energy and growth rate of crystallite for formation of acicular α-FeOOH particles, and pH of colloidal mother liquid containing carbonate ion was discussed for the case, in which addition quantity of CO32- and FeCl3 concentration were constant. Both rate constant and growth rate of crystallite increased with increasing pH, and increased steeply above pH11.6 and pH11.0, respectively. The activation energy decreased and reached minimum values at pH of about 11.6, and increased at higher pH region exceeding 11.6. The supersaturation of solute, Fe(OH)4- increases with increasing pH. At the same time, the singular adsorption of OH- on the side face of acicular particles is restrained by the coating of coexisting CO32- on their surfaces. These may be the reasons for the pH dependencies of rate constant and growth rate of crystallite observed. Both crystallite size and median diameter of the formed α-FeOOH particles increased with increasing pH, then α-FeOOH particles grew larger with keeping needle shape similar according to TEM observation. Moreover, raft-shaped particles which single crystal coalesced with each other, were formed at higher pH region exceeding 11.6. The increase of activation energy may be caused by the formation of raft-shaped particles. The granular hematite particles were also formed with acicular α-FeOOH particles at lower pH region below 11.0.
