Photocatalytic degradation of 4-chlorophenol: The effects of varying TiO₂ concentration and light wavelength

The effects of TiO₂ surface loading and light conditions on the photocatalyzed degradation of 4-chlorophenol (4-CP) were examined, in order to address deficiencies in previous studies in which these issues were not examined satisfactorily. It is proposed that these factors explain differences in the reported reaction intermediate concentrations. Increasing the loading of TiO₂ in a slurry has little effect on the rate of 4-CP disappearance. However, the concentration of 4-chlorocatechol (4-CC), a primary degradation product, decreases with increasing TiO₂ loading. Zero order kinetics in terms of 4-CP were observed for 4-CP degradation. The rate of mineralization increased with increasing TiO₂ concentration. The rate of mineralization relative to 4-CP degradation was more rapid under conditions of lower light intensity. Significant adsorption of 4-CC on Degussa P25 TiO₂ was measured and these data are fitted well by a Langmuir isotherm. Adsorption of 4-CC accounts for most of the changes in 4-CC concentration at different TiO₂ loadings. An increased proportion of surface oxidation reactions at higher TiO₂ loadings and lower light intensity may explain the higher rates of mineralization in thicker slurries. The quantum yield of reaction of 4-CP increased with decreasing wavelength of light from φ = 0.01, at λ = 360 nm to φ = 0.07, at λ = 300 nm. A relationship of d[4-CP]/dt ∝ I^0.8 was measured.