TRANSFORMATION OF AKAGANEITE INTO MAGNETITE IN AQUEOUS HYDRAZINE SUSPENSIONS

Akaganeite suspended in hydrazine-containing aqueous solutions at pH 9. 5-11. 5 and 373 K, is transformed into magnetite through a dissolution-recrystallization mechanism. The rate of conversion is constant with time, suggesting a fast solubility pre-equilibrium followed by homogeneous iron(III) reduction by hydrazine as the rate-determining step. Nucleation of magnetite apparently takes place on the akaganeite surface. The dependence of rate on pH is explained in terms of changes in solubility and the different reactivities of Fe(OH)** plus //2, Fe(OH)//3 and Fe(OH)** minus //4 towards hydrazine. If nuclei of hematite or goethite are introduced, these phases form through a competitive removal of iron(III) from solution.