Photodegradation of polychlorinated dibenzo-p-dioxins:: comparison of photocatalysts

1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (1,2,3,6,7,8-HxCDD) and octachlorodibenzo-p-dioxin (OCDD) were photocatalytically degraded using immobilized TiO2, ZnO and SnO2 films under ultraviolet (UV) with the wavelength between 300 and 450 nm. The specific surface areas of TiO2, ZnO and SnO2 powders were calculated as 54.2, 4.6 and 4.8 m(2) /g, and the band gap energies were determined to be 3.17, 2.92 and 4.13 eV, respectively. The light source used had wavelengths between 300 and 450 run, and the main wavelength was approximately 365 nm; hence, the smaller quantum efficiency of SnO2 with wider band gap due to poor utilization of the UV light was expected. X-ray diffraction (XRD) revealed that sintered photocatalysts at 400degreesC did not alter their characteristics. The first-order rate constants of OCDD in UV/TiO2, UV/ZnO and UV/SnO2 were 5.30, 0.74 and 0.28 h(-1), respectively; those of 1,2,3,6,7,8-HxCDD in UV/ZnO and UV/SnO2 were 3.28 and 3.19 h(-1), respectively. As expected, photodegradation rates declined as the number of chlorine atoms increased. Due to the low dosage (50 ng) of target compounds used, the amounts of Cl- and total organic carbon were too small for quantification. No 2,3,7,8-substituted congeners was identified during the photodegradation, and the UV/photocatalyst treatment might offer an effective treatment for PCDDs. (C) 2004 Elsevier B.V. All rights reserved.