Tungstate versus Molybdate Adsorption on Oxidic Surfaces:  A Chemical Approach

The differences between the chemistry of tungstate adsorption on oxidic supports and that of molybdate (viz. γ-Al2O3, TiO2 and amorphous silica alumina (ASA)) were studied by measuring equilibrium adsorption isotherms at various conditions (pH, (co)-adsorption of tungstate and molybdate), by determining which part was reversibly adsorbed, and by structurally characterizing them using FTIR and Raman spectroscopy. It can be concluded that most of the tungstate reacts irreversibly with acidic and neutral OH groups, and the other part adsorbs reversibly by electrostatic interactions with protonated OH groups, whereas molybdate irreversibly reacts in a reaction with the basic OH groups. However, as soon as these groups are protonated, molybdate also starts to reversibly adsorb by electrostatic interactions. In addition, molybdate adsorbs on coordinatively unsaturated sites whereas tungstate does not.