Photocatalytic oxidation of a model halogenated aromatic compound: A mechanistic study

The TiO{dollar}sb2{dollar} photocatalyzed oxidation of 4-chlorophenol (4-CP), a model halogenated aromatic water pollutant, was examined using a variety of methods, to gain mechanistic understanding of TiO{dollar}sb2{dollar} photocatalysis, and ultimately to improve reactor design for this potential water treatment method.; An in-situ FTIR technique was employed to probe the adsorption and photocatalytic degradation of 4-chlorophenol on TiO{dollar}sb2{dollar} particles in a gas/solid system. Adsorbed 4-CP was degraded to hydroquinone (HQ) when illuminated, and HQ was subsequently mineralized when irradiated with UV in an oxygen atmosphere.; 4-CP was photocatalytically degraded in aqueous TiO{dollar}sb2{dollar} slurries, following approximately zero order kinetics. Increased TiO{dollar}sb2{dollar} loading did not increase the rate of 4-CP degradation, but did increase the rate of mineralization. The quantum yield of reaction depended on the wavelength of the light, and approached 0.08 for 300 nm light. 4-Chlorocatechol (4-CC) was adsorbed to TiO{dollar}sb2{dollar} according to Langmuir isotherm kinetics.; 4-CP was oxidized by {dollar}gamma{dollar}-radiolysis in conditions favoring hydroxyl radical oxidation ({dollar}cdot{dollar}OH) at pH 3-6, forming intermediates 4-CC and HQ. Aromatic intermediates were not detected when 4-CP was oxidized at pH 9 or with azide radicals. Pulse radiolysis was used to characterize the radical transient species formed during the oxidation of 4-CP, and measure the second order rates of reaction of 4-CP and 4-CC with {dollar}cdot{dollar}OH.; Photocatalytic degradation produces intermediates consistent with hydroxyl radical oxidation, but the concentration of aromatic intermediates indicates that the course of photocatalytic transformation of 4-CP does not exclusively involve hydroxyl radical oxidation.; A model of the TiO{dollar}sb2{dollar} photocatalyzed degradation of 4-CP was developed using photocatalytic and radiolytic data recorded in this study and from the literature. The model combined expressions for the rates of hydroxyl radical oxidation, substrate and intermediate sorption, oxidation on the TiO{dollar}sb2{dollar} surface, electron/hole formation, electron scavenging, and recombination. The resulting system of equations was solved using computer numerical methods. The model predicted the rate of 4-CP oxidation, rate of TOC reduction, and intermediate concentrations for a variety of different light intensities, TiO{dollar}sb2{dollar} loadings in the slurry, and reactor geometries, indicating how the selection of these parameters influences the course of degradation.