Codeposition of Mo (VI) Species and Ni 2+ Ions on the γ-Alumina Surface: Mechanistic Model

Abstract

The codeposition of Mo (VI) species and Ni 2+ ions on the γ-alumina surface, using the "equilibrium deposition followed by filtration" (EDF), has been studied in the pH range 4.1-6.2. An increase in the adsorptivity of the γ-alumina for both the Mo (VI) species and Ni 2+ ions was observed at all pH values studied. This was attributed to the stronger lateral interactions exerted between the codeposited Mo (VI) and Ni 2+ ions as they compared with those exerted between the deposited Mo (VI) species in the absence of the Ni 2+ ions and between the deposited Ni 2+ ions in the absence of the Mo (VI) species. A codeposition model was developed and tested on the basis of the results obtained from deposition experiments, potentiometric titrations, and microelectrophoresis. According to this model the deposition of the Mo (VI) species on the γ-alumina surface in the presence of the Ni 2+ ions takes place mainly via adsorption of the MoO 2- 4 and Mo 7O 6- 24 ions on sites created by the protonated surface hydroxyls. Each adsorption site is created by one protonated surface hydroxyl. These sites are located in the inner Helmholtz plane (IHP) of the double layer developed between the surface of the support and the impregnating solution. The extent of deposition through reaction of one MoO 2- 4 ion with two neutral surface hydroxyls was proved to be negligible. Moreover, it was found that the extent of adsorption of MoOn 2- 4 (Mo 7O 6- 24) decreased (increased) as pH decreased. Finally, it was shown that the total deposition of Mo (VI) increased as pH decreased. Concerning the Ni deposition, according to the established model, it takes place through adsorption of one Ni 2+ ion on a site created by one deprotonated surface hydroxyl in the IHP. As pH increased, the concentration of the deprotonated surface hydroxyls and thus the amount of the deposited Ni increased.