Tungstate versus molybdate adsorption on oxidic surfaces: A chemical approach

The differences between the chemistry of tungstate adsorption on oxidic sup ports and that of molybdate (viz. gamma-Al2O3 TiO2 and amorphous silica alu mina (ASA)) were studied by measuring equilibrium adsorption isotherms at v arious conditions (pH, (co)-adsorption of tungstate and molybdate), by dete rmining which part was reversibly adsorbed, and by structurally characteriz ing them using FTIR and Raman spectroscopy. It can be concluded that most o f the tungstate reacts irreversibly with acidic and neutral OH groups, and the other part adsorbs reversibly by electrostatic interactions with proton ated OH groups, whereas molybdate irreversibly reacts in a reaction with th e basic OH groups. However, as soon as these groups are protonated, molybda te also starts to reversibly adsorb by electrostatic interactions. In addit ion, molybdate adsorbs on coordinatively unsaturated sites whereas tungstat e does not.