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

The adsorption of phosphate ions from aqueous solution onto thin films of colloidal TiO2 has been studied for the first time by in situ internal reflection infrared spectroscopy. Phosphate binds strongly to TiO2, 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 TiO2 at pH = 2.3 is (3.8 ± 0.8) × 104 dm3 mol-1, which is similar to the binding constants onto TiO2 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 TiO2 surface.