A review of synergistic effect of photocatalysis and ozonation on wastewater treatment

For the treatment of wastewater that contain recalcitrant organic compounds, such as organo-halogens, organic pesticides, surfactants, and colouring matters, wastewater engineers are now required to develop advanced treatment processes. A promising way to perform the mineralization of this type of substance is the application of an advanced oxidation process (AOP). Photocatalytic oxidation and ozonation appear to be the most popular treatment technologies compared with other advanced oxidation processes (AOPs) as shown by the large amount of information available in the literature. The principal mechanism of AOPs function is the generation of highly reactive free radicals. Consequently, combination of two or more AOPs expectedly enhances free radical generation, which eventually leads to higher oxidation rates. The use of combine photocatalysis and ozonation is an attractive route because of the enhancement of the performance for both agents by means of the hydroxyl radical generation, a powerful oxidant agent that can oxidize completely the organic matter present in the aqueous system. The scope of this paper is to review recently published work in the field of integrated photocatalysis and ozonation on wastewater treatment. In this review the chemical effects of various variables on the rate of degradation of different pollutants are discussed. The mechanism and kinetics has also been reported. It can be concluded that photocatalytic oxidation in the presence of ozone is a process that is qualitatively and quantitatively different from the well-known photocatalytic oxidation with oxygen and the ozonation without photocatalyst. The reason for the higher oxidation rate is probably a photocatalytic induced decay of ozone, initiated by the combination of titanium dioxide and UV-A radiation. (c) 2006 Elsevier B.V. All rights reserved.