| Abstract
| - The photocatalytic degradation of polychlorinateddibenzo-p-dioxins (PCDDs), which include mono-, tetra-,hepta-, and octachlorinated congeners (MCDD, TCDD, HpCDDand OCDD), was carried out on TiO2 films under UV (λ >300 nm) or solar light irradiation in the air. All the dioxincongeners tested were successfully degraded on TiO2while the direct photolysis of them in the absence of TiO2was negligible. The photocatalytic degradation rates ofPCDDs decreased with the number of chlorines and weredescribed 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 increasedwith light intensity and TiO2 coating weights up to 2 mW/cm2 (300 < λ < 400 nm) and 200 μg of TiO2/cm2, respectively.Further increases in either the light intensity or theTiO2 coating mass did not much affect the degradationrate. The natural solar light under clear sky conditions wasas effective as a 200-W mercury lamp irradiation (filteredthrough a Pyrex filter) in degrading OCDD on TiO2. Thephotocatalytic degradation of PCDDs seems to be initiatedby OH radical attack and not by conduction band electrontransfer. From a diffuse reflectance FTIR study, thedegradation of OCDD was found to involve the cleavageof the aromatic ring. Neither stable intermediates nordechlorinated PCDDs were detected during the photocatalyticdegradation.
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