Photocatalytic Degradation of Polychlorinated Dibenzo-p-dioxins on TiO2 Film under UV or Solar Light Irradiation

The photocatalytic degradation of polychlorinated dibenzo- p -dioxins (PCDDs), which include mono-, tetra-, hepta-, and octachlorinated congeners (MCDD, TCDD, HpCDD and OCDD), was carried out on TiO 2 films under UV (λ > 300 nm) or solar light irradiation in the air. All the dioxin congeners tested were successfully degraded on TiO 2 while the direct photolysis of them in the absence of TiO 2 was negligible. The photocatalytic degradation rates of PCDDs decreased with the number of chlorines and were described by the first-order reaction kinetics with half-lives of 5.8, 3.9, 0.71, and 0.38 h for OCDD, HpCDD, TCDD, and MCDD, respectively. The degradation rates increased with light intensity and TiO 2 coating weights up to 2 mW/cm 2 (300 < λ < 400 nm) and 200 μg of TiO 2 /cm 2, respectively. Further increases in either the light intensity or the TiO 2 coating mass did not much affect the degradation rate. The natural solar light under clear sky conditions was as effective as a 200-W mercury lamp irradiation (filtered through a Pyrex filter) in degrading OCDD on TiO 2 . The photocatalytic degradation of PCDDs seems to be initiated by OH radical attack and not by conduction band electron transfer. From a diffuse reflectance FTIR study, the degradation of OCDD was found to involve the cleavage of the aromatic ring. Neither stable intermediates nor dechlorinated PCDDs were detected during the photocatalytic degradation.