| A novel thin-film rotating-disk TiO2 photocatalytic reactor (RDPR) was developed and evaluated for the complete elimination of traces of recalcitrant halogenated aromatic and other organic contaminants in drinking water and industrial wastewater. The RDPR incorporates important features including immobilization of the catalyst on the rotating disk and the degradation and mineralization of organic impurities in a thin film of contaminated water using only TiO2 catalyst, UV radiation, and oxygen from the atmosphere. The study included detailed investigation of the hydrodynamics of flow in the RDPR, determination of the liquid carrying capacity of the disk, immobilization and characterization of the catalyst, and quantification of the light intensity distribution. Detailed experimental work accompanied by mathematical modeling investigated the erects of disk angular velocity, incident light irradiance, type and concentration of electron acceptors including hydrogen peroxide and oxygen, initial contaminant concentration, and solution matrix characteristics. The effects of these parameters were evaluated considering initial rates of degradation and extent of mineralization of the parent contaminant, reaction rate constants in the thin liquid film exposed to TiO2 and UV, and efficiencies of light utilization. Determination of the dimensionless Damköhler number (Da), which accounts for the influence of mass transfer on the photocatalytic process, revealed that degradation reactions were not mass transfer limited when the disk angular velocity surpassed a critical value. The latter was mainly due to the increase of mass transfer coefficient for the transport of the contaminant from the bulk of the thin liquid film carried by the disk to the surface of the catalyst. On the other hand, photocatalytic reactions were limited by the intensity of UV radiation. The influence of initial contaminant concentration on the degradation reactions follow Langmuir-Hinshelwood kinetics. Similarly, the effect of oxygen concentration on the degradation reactions was found to obeyed the Langmuir-Hinshelwood model. The effect of hydrogen peroxide in the feed solution during the continuous-mode operation of the RDPR revealed the existence of an optimum concentration of hydrogen peroxide and that high concentrations of this electron acceptor inhibited the photocatalytic reactions mainly due to the predominance of other reactions involving scavenging of hydroxyl radicals, the major oxidizing species in the photocatalytic process. |
