Oxidant-assisted photocatalytic reaction of o-ClA in TiO2

Abstract

The removal of organic matter through the illumination of UV light has proven to be effective in the fields of air and wastewater treatment. However, direct photolysis only causes little or slow decay with insignificant number of radicals while a higher reaction rate is always observed with photon-assisted catalysts. The application of heterogeneous photocatalysts has received great consent because of its effectiveness and the wide adaptability in organic destruction. Titanium dioxide, TiO2 is considered one of the best catalysts for the photo-oxidation process. It could assist the degradation of organic without altering itself under the UV light illumination. Excited conduction-band electrons (e-CB) and valance-band holes (h+VB) induce large amount of hydroxyl radicals (*OH), or other radicals (*O2). However, fast recombination of conduction-band electrons and valence-band holes would limit the degradation rates. To tackle with the problem, the use of inorganic oxidants has been proposed to prevent the occurrence of recombination. The photocatalytic degradation of an anthropogenic organic, o-chloroaniline (o-ClA), has been extensively studied. Comparison indicated that oxhalogens (especially IO3-) were found to the most effective oxidant among various selections (i.e., H2O2, rose bengal and oxhalogens) in the UV/TiO2 system. With the addition of oxygen sources provided by the oxidants, TiO2 particles acted as an electron donor to form the superoxide radical ion (O2*-) which could yield higher degradation rate. The use of oxidants in UV/TiO2 has proven as a pH dependent process. The change of surface charges on the TiO2 surface is one of the explanations. In addition, the identification and quantification of photo-products (aromatic intermediates and ionic end-products) were conducted in UV/TiO2 and UV/TiO2/IO3- system. The degradation of o-ClA was mainly attributed to the *OH attack. The existence of aromatic intermediates would not accumulate in the solution and degradation was observed. The generation of NH4+, Cl- and NO2- has also been detected. With the inclusion of the photo-products, the degradation pathway of o-ClA has been proposed accordingly. The photocatalytic process was brought into practice by immobilizing TiO2 particles onto supported substrate via sol-gel process. Degradation of o-ClA by the TiO2 film can be achieved but affected by the calcination temperature, layers of coating and the use of light intensities. Also, recycling performance of film was reported to have high percentage of repeatability after nine consecutive usages.