PHOTOCATALYTIC DEGRADATION OF PHENOL IN THE PRESENCE OF NEAR-UV ILLUMINATED TITANIUM-DIOXIDE

The photocatalytic oxidation of aqueous phenol solutions in the presence of near-UV illuminated titanium dioxide has been studied at concentrations of 1-100-mu-M. The titanium dioxide was used as a free suspension and attached to sand. Under identical illumination conditions the degradation rate with the free suspension was approximately 2.6 times faster than with the immobilized catalyst. The quantum yield for the disappearance of phenol from a 100-mu-M solution in 0.1% TiO2 suspension illuminated with 'Blacklight Blue' 360-370 nm maximum emission radiation was approximately 0.6%. Direct solar illumination gave degradation rates comparable with that achieved with a 100 W medium pressure mercury lamp. In less than 1 h the midday summer sun degraded 90% of a 100-mu-M phenol solution, of volume 0.5 l in a 145 mm diameter open dish. The disappearance of phenol and the formation of carbon dioxide with illumination time was quantitatively accounted for by numerical integration of simultaneous differential equations based on Langmuir-Hinshelwood kinetics. An unexpected observation emerging from this analysis was a dependence of the adsorption equilibrium constant on the reciprocal of the initial phenol concentration. This dependence occurred at each solution pH studied. A possible explanation for the observation is given in terms of phenol reformation reactions competing with irreversible phenol degrading reactions and free diffusion of OH radicals from the catalyst surface at low solute concentrations.