The influence of mineralization products on the coagulation of TiO2 photocatalyst

We have investigated the influence of common inorganic and organic anions formed during TiO2 photocatalysis of organic compounds on the coagulation of the photocatalyst. Chloride, sulfate, phosphate, formate, and oxalate were added to aqueous suspensions of TiO2 over a range of pH values and the rates of coagulation determined using photon correlation spectroscopy. The rates of coagulation generally increase with the concentration of ions and are fastest at pH values near the zero point charge (zpc) of the photocatalyst, where electrostatic repulsion among particles is relatively small except for that for the phosphate-containing suspensions. At high concentrations of phosphate the coagulation is relatively slow near the zpc of the TiO2 in clean water. This suggest the characteristics of the TiO2 surface properties are modified through ligand exchange between the surface hydroxyl groups and phosphate or strong surface adsorption of phosphate, such that the zero point charge shifts and the resulting modified particles possess a negative charge when the pH is near the pH(zpc) of unmodified TiO2. Stirring of the solution dramatically accelerates the coagulation of TiO2 particles, decreasing the available surface area of the catalyst, which is expected to reduce the activity of the catalyst. Sonication was effective for inhibiting coagulation of the photocatalyst, hence maintaining a high surface area which should improve the performance of the photocatalyst relative to mechanical stirring.