Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe2O3 and the corresponding ferrites in 0.1 mol dm−3 oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (kMe; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k−w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me–O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe2O4>CoFe2O4⪢NiFe2O4. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe2O3 and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni2+ labilizes less the corresponding ferrite. The correlation between log kM and log k−w is somewhat blurred and displaced to lower kM values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.