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

Dissolution rates of NiO, CoO, ZnO, alpha-Fe(2)O(3) and the corresponding ferrites in 0.1 mol dm(-3) oxalic acid at pH 3.5 were measured at 70 degrees 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 (k(Me); 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 (384nm) 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 ZnFe(2)O(4)>CoFe(2)O(4)>>NiFe(2)O(4). Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe(2)O(3) 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 Ni(2+) labilizes less the Corresponding ferrite. The correlation between logk(M) and logk(-w) is somewhat blurred and displaced to lower k(M) Values. Fe(II), either photogenerated or added as salt, enhances the Fate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data. (C) 2008 Elsevier Inc. All rights reserved.