Simultaneous Deposition of W(VI) Species and Ni(II) Ions on the γ-Alumina Surface: Mechanistic Model

The simultaneous deposition (co-deposition) of W(VI) species and Ni(II) ions on the γ-alumina surface, from aqueous solutions using “equilibrium deposition followed by filtration” (EDF), has been studied in the pH range 4.2−6.2. It was found that the deposition of W(VI) species and Ni(II) ions on the γ-alumina surface in the presence of Ni(II) ions and W(VI) species, respectively, takes place mainly through adsorption of W x O y z - or Ni 2+ ions on energetically equivalent sites at the inner Helmhotz plane (IHP), with strong attractive lateral interactions between the adsorbed ions. A mutual promotion in the deposition of W(VI) and Ni(II) species was observed at all pH values investigated. The protonated and deprotonated surface hydroxyls were found to be mainly responsible for the adsorption of the W x O y z - and Ni 2+ ions, respectively, thus resulting in a decrease [increase] of the adsorbed amount of W(VI) species (Ni(II) ions) with increasing pH. Moreover, it was found that the presence of Ni(II) ions minimized the contribution of the neutral surface hydroxyls to the total W(VI) deposition. Finally, a model developed to describe the simultaneous deposition of W(VI) species and Ni(II) ions on the γ-alumina surface showed that as pH increased from 4.2 to 6.2 the deposition of W(VI) through adsorption of WO 4 2- ions increased at the expense of deposition of W(VI) through adsorption of HW 6 O 20 (OH) 2 5- ions, which decreased.