KINETICS OF OXIDATIVE PRECIPITATION OF IRON-OXIDE PARTICLES

Conditions for the formation of Fe3O4, alpha- and gamma-FeOOH by the oxidation of Fe(OH)2 have been studied. In a system containing Fe(OH)2(s), Fe2O3.xH2O and excess Fe(II)(aq), the evolution to delta-FeOOH, Fe3O4, green rust, alpha-FeOOH or gamma-FeOOH depends on the kinetics regime, controlled principally by the oxidation rate, initial ratio R0 = [Fe2+]0/[OH-]0, temperature and the anions present. Measurements of the rate of proton release at constant pH during the course of the auto-oxidative precipitation of Fe(II) demonstrate a decoupling of the precipitation and oxidation processes. The precipitation rates are initially auto-acceleratory, owing to sensitivity to the solid mass. A large fraction of the oxidation reaction takes place later, by a redissolution-oxidation reprecipitation process. The results are explained in terms of simple kinetic mechanisms of heterogeneous reactions.