Mechanism of deposition of Co2+ and Ni2+ ions on the interface between pure and F–-doped γ-alumina and the impregnating solution

Abstract

The recently studied mechanism of adsorption of Co²⁺ and Ni²⁺ ions on the interface between pure or F^–-doped γ-alumina and electrolyte solution has been investigated further. A quite general model has been developed and tested by comparing the experimental deposition isotherms obtained over various pH values and dopant concentrations with the corresponding ones calculated by applying to these models an interaction computer program for calculating chemical equilibria in aqueous systems (SURFEQL). The selected model showed that the deposition of the Co²⁺ or Ni²⁺ ions takes place by adsorption, following the extremely simple mechanism: AlO^–+ Co²⁺(Ni²⁺)⇄ AlO^–⋯Co²⁺(Ni²⁺), where AlO^– represents deprotonated surface hydroxy groups of the support and AlO^–⋯Co²⁺(Ni²⁺) a sorptive complex with the right-hand side being in the inner Helmholtz plane of the double layer developed between the support surface and the electrolyte solution. Moreover, the simple model selected has been confirmed using two additional independent methods. First, by comparing calculated and experimental values of the ζ potential in the presence of Co²⁺ and Ni²⁺ ions, and secondly, by comparing calculated and experimental values of the difference in γ-alumina adsorptivity for hydrogen cations in the absence and presence of Co²⁺ or Ni²⁺ ions.