EFFECTS OF COMMON INORGANIC ANIONS ON RATES OF PHOTOCATALYTIC OXIDATION OF ORGANIC-CARBON OVER ILLUMINATED TITANIUM-DIOXIDE

Effects of pH and common inorganic anions on the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO2 over glass-supported near-UV illuminated TiO2 have been investigated by using conductivity detection of evolved CO2. In the presence of chlorides and the absence of organics, lowering the pH of the solution below 3.6 caused an increase in conductivity with increasing circulation time. At pH 4.0 ± 0.2, the rate of oxidation decreased with increasing concentration of chlorides. Perchlorates and nitrates have very little effect. Sulfates or phosphates even at millimolar concentrations are rapidly adsorbed by the catalyst and reduce the rates of oxidation by 20-70%. The catalyst can be readily regenerated by washing with 0.1 M NaHCO3 solution. The data are considered in terms of kinetic salt effects, surface active sites having discrete adsorption properties, and Langmuir-Hinshelwood kinetic models. The model providing the best fit to the data assumes that inorganic anions may compete with organic solutes for oxidizing sites on the surface to form oxidizing inorganic radical anions. Some of these radical anions also react with surface-adsorbed organics to give CO2 but at decreased rates. The implications for water purification processes based on photocatalytic oxidation using titanium dioxide are discussed. © 1990 American Chemical Society.