PHOTODISSOLUTION OF IRON-OXIDES .4. A COMPARATIVE-STUDY ON THE PHOTODISSOLUTION OF HEMATITE, MAGNETITE, AND MAGHEMITE IN EDTA MEDIA

The thermal and 254-nm photochemical dissolution reactions of magnetite (Fe3O4), maghemite (gamma-Fe2O3), and hematite (alpha-Fe2O3) suspended in EDTA aqueous solutions were compared. Gamma-Fe2O3 and Fe3O4 are thermally and photochemically more reactive than alpha-Fe2O3. Both thermal and photochemical dissolution reactions are governed by an initiation step, which involves the production of Fe(II)aq, and a subsequent thermal reaction of these ions with the solid, to produce Fe(III)aq. The initiation step under UV irradiation involves the photoreduction of surface >Fe(III) - EDTA complexes to yield Fe(II)aq and the photooxidation of adsorbed EDTA to yield CH2O and other oxidation products. After Fe(III) - EDTA complexes build up in solution through the following step, homogeneous photolysis is the main source of Fe(II) and CH2O. Oxides with spinel type structure are characterized by faster rates in the two processes, and 02 MaY inhibit the dissolution processes by changing the stoichiometry of the initiation step to that of the autooxidation of EDTA. The relative importance of autooxidation and photodissolution depends on the nature of the oxide and the experimental conditions. Photooxidation reaction rates parallel those of the photodissolution initiation steps, and long-term stability towards photocorrosion (dissolution) implies low photocatalytic activity for the oxidation of EDTA. The set of differential equations describing all the reaction rates is discussed and applied to the different cases.