Adsorption of molybdates on doped γ-aluminas in alkaline solutions

Abstract

The adsorption of MoO 4 2− at pH 8.50, 25°C and at constant ionic strength, 0.1 M in KNO 3 on a number of γ-alumina based carriers containing various amounts of Na + and Li + ions has been investigated. Adsorption of these species may, in all cases, be described by an equation derived previously for describing the adsorption of polymeric Mo xO y z− ions at pH 5.00, thus suggesting the same mechanism of adsorption is followed at both pH values. Accordingly, MoO 4 2− ions are adsorbed at alkaline pH on distinct adsorption sites of the inner Helmholtz plane (I.H.P.) of the electrical double layer developed between the surface of the solid particles of the adsorbent and the solution. In this plane relatively weak lateral interactions are exerted between the adsorbed MoO 4 2− ions. Moreover, it was found that the doping of γ-alumina with various amounts of Na + and Li + ions resulted in a significant increase of the sorptive capacity of γ-alumina. This was mainly attributed to the increase in the concentration of the surface protonated hydroxyls, considered to be responsible for the creation of adsorption sites, caused by the dopants. Finally, it was observed that the intensity of the lateral interactions between the adsorbed MoO 4 2− ions increased with the extent of adsorption.