Phosphate Adsorption onto TiO₂ from Aqueous Solutions:  An in Situ Internal Reflection Infrared Spectroscopic Study

Abstract

The adsorption of phosphate ions from aqueous solution onto thin films of colloidal TiO 2 has been studied for the first time by in situ internal reflection infrared spectroscopy. Phosphate binds strongly to TiO 2, as evidenced by the large changes in the PO stretching band structure in the infrared spectrum of the adsorbed species compared with the solution species. The Langmuir binding constant for phosphate onto TiO 2 at pH = 2.3 is (3.8 ± 0.8) × 10 4 dm 3 mol -1, which is similar to the binding constants onto TiO 2 for bidentate ligand species such as oxalate and catechol. The strength of the binding is also apparent in the kinetics of adsorption, showing fast adsorption and much slower desorption, as expected for a strongly bound species. The kinetics data at pH = 8.3 have indicated coverage-dependent phosphate adsorption and desorption. Experiments with substituted phosphate species have confirmed the bidentate binding of phosphate to Ti(IV) ions at the TiO 2 surface.