Morphology Control Factors of The Goethite Particles Produced by Gel Aging Method

Abstract

α-FeOOH particles have been produced by the aging of amorphous ferric oxyhydroxide colloidal solution. The effects of addition of ions: OH^-, Cl^-, CO₃^2-, PO₄^3- and Co²⁺, on the process of formation and morphology of α-FeOOH particles have been clarified.
(1) The mechanism of formation of α-FeOOH crystal particles is considered as the dissolution-recrystallization process. The formation reaction of a-FeOOH crystal particles from the amorphous ferric oxyhydroxide can be expressed by a rate equation of the first order reaction.
(2) Since OH^- and Cl^- ions are selectively adsorptive to the side face parallel to c axis of α-FeOOH particles, the crystal growth of the side face is depressed.
(3) Since CO₃^2- ion is not selectively adsorptive to the specific crystal plane, but is adsorbed on each plane of the a, b and c crystal planes homogeneously, the crystal growth of those planes is depressed. The recrystallization process of Fe(OH)₄^- ion is shifted from the surface reaction control to the diffusion control with increasing CO₃^2- and Cl^- ion concentrations.
(4) The asterisk type particles were produced in the limited narrow region of PO₄^3- ion concentration. An asterisk type particle is heterogeneously junctioned particle of an α-Fe₂O₃ and several α-FeOOH crystals.