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

Abstract

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 TiO2 films under UV (λ > 300 nm) or solar light irradiation in the air. All the dioxin congeners tested were successfully degraded on TiO2 while the direct photolysis of them in the absence of TiO2 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 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 the TiO2 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 TiO2. 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.

Mathematical Model for Photocatalytic Destruction of Organic Contaminants in Air

“…However, a further increase in the light intensity had only slight impact on the toluene degradation rate, which tended to become constant at light intensities above 2.2 mW/cm 2 . A similar effect was reported by Chang et al 21 and Choi et al 22 An increase in light intensity yields a higher generation of valence band holes of catalysts, thus a higher formation of surface reactive species and hydroxyl radicals leading to a high toluene elimination rate. However, at the high light intensity, self-recombination of hydroxyl radical may become significant.…”

Pure and modified TiO2 photocatalysts and their environmental applications

“…PCDDs can be also degraded by TiO 2 . Figure 3 compares the direct photolytic and photocatalytic degradation of four PCDDs (loaded on a glass plate or a TiO 2 -coated glass plate) under UV irradiation and ambient air [14]. The direct photolysis with k > 300 nm did not induce any noticeable degradation of PCDDs but their photocatalytic degradation on TiO 2 was greatly enhanced, and resulted in 85% conversion within 15-h irradiation for OCDD (octachlorodibenzo-p-dioxin).…”

“…Photocatalytic reactions occurring on semiconductor surface have many applications including chemical fuel synthesis (e.g., H 2 production through water splitting) [2], selective oxidation [3,4], degradation of organic compounds [5,6], disinfection [7,8], metal corrosion prevention [9][10][11], lithography [12,13], etc. In particular, their application to the remediation of polluted water and air has been demonstrated to be a technically viable process and TiO 2 has been the most popular and successful photocatalyst for this purpose [5,6,[14][15][16]. This technology has a unique versatility in that it can be applied to various environmental media of water, air, and even solid phases.…”

Photocatalytic degradation of aqueous methyl-tert-butyl-ether (MTBE) in a supported-catalyst reactor

“…TiO 2 has to be separated, usually by filtration, from the decontaminated water after the photocatalytic treatment. In supported-catalyst reactors, TiO 2 is fixed onto a structural surface, such as glass mesh (Choi et al 2000) and optical fiber (Miller et al 1999), which eliminates the need for recovery of catalyst from treated water. Moreover, it is difficult to achieve a uniform light intensity on all catalyst surfaces in a slurry-catalyst reactor and the white TiO 2 itself can be an effective shield to the light.…”