An Electron-Optical Study of Iron Oxide Phases Produced in Alkaline and Acidic Ferric Solution in the Presence of Selected Additive Species

The presence of foreign ions (both anionic and cationic species) has been found to exert a profound influence on the composition and morphology of crystalline products formed on ageing the unstable ferric oxide-hydrate ferrihydrite. In the present work, electron-optical techniques have been used to study the effect of Zn{II) and Ni(II) species on the above transformat ion. The results obtained have been compared with previous studies on the influence of Mn(II),Co(II) and Cu{II) ions. Depending on the growth conditions, the uncontaminated oxide-hydrate transforms to hematite (alpha-Fe2O3),by a largely solid-state rearrangement, and/or goethite (alpha-FeOOH) through a dissolution/reprecipitation mechanism. Zinc species adsorb strongly on ferrihydrite and were found to strongly inhibit the rate of dissolution,thereby promoting hematite format ion. Although data indicated that as much as 9-10 mol% zinc had been incorporated in the hematite structure, the additive species were not found to be associated with goethite.This was in accord with a situation in which readsorption of zinc on undissolved ferrihydrite was favoured over release into solution. By contrast,the less strongly-adsorbing nickel species did not promote formation of hematite at high pH and,since readsorption on ferrihydrite was less favoured, as much as 8-9 mol% of the additive was found to be incorporated in goethite. These transition metal cations appeared to direct the composition and morphology of goethite/hematite-containing products through two extreme types of behaviour: ions such as Cu(II) and Zn(II) which interact strongly with ferrihydrite will block the release of soluble ferric species, thereby influencing hematite development directly and goethite formation indirectly. By contrast, species such as Mn(II) are more easily released into solution and interact directly with the oxyhydroxide. Co{II) and Ni{II) may show intermediate behaviour. A number of other factors,including the amphoteric behaviour of the additive hydroxides, should also be considered when discussing the action of these additiives. Spinel phases formed in the presence of divalent cationic additives were thought to nucleate either in solution or on the surface of ferrihydrite. In the present work,results pointed to a mechanism by which spinel crystals nucleated on the precursor and were fed by soluble species during growth. The proportion of additive required to induce spinel formation at high pH was found to be lowest for zinc and copper.Growth of goethite crystals in precipitates which had been "seeded" with species capable of acting as goethite nuclei was strongly inhibited,while spinel formation was not retarded to a great extent. Data indicated that many of the spinel crystals contained insufficient additive ions to give a fully-substituted stoichiometry.Some evidence for extended ordering was obtained for defect zinc-magnetites