Photocatalytic Oxidation as a Secondary Treatment Method following Wet Air Oxidation

Photocatalytic oxidation is investigated as a potential secondary treatment method to follow wet air oxidation (WAO). The rates of photocatalytic oxidation of acetic acid and 2-chlorobiphenyl in aqueous solution were studied under continuous flow conditions in a small diameter cylindrical borosilicate reactor packed with highly porous TiO2 particles at temperatures up to 160 °C and pressures up to 20 atm. These compounds are of particular interest because they are recalcitrant to conventional WAO. Specific attention is paid to the effects of oxygen concentration, pH, and temperature on reaction rates. Results show that solution pH may play an important role in the selective oxidation of a compound in a mixed waste stream. 2-Chlorobiphenyl was selectively oxidized in the presence of much higher concentrations of acetic acid by maintaining a neutral to basic pH. Moreover, the dependence of the rate of mineralization of acetic acid on oxygen is strongly influenced by pH. At low pH, high dissolved oxygen concentrations were required to achieve the limiting oxidation rate. Temperature affects the intrinsic rate of reaction, the extent of substrate adsorption, and the light intensity. Each of these effects can be accurately modeled using a modified form of a Langmuir−Hinshelwood−Hougen−Watson kinetic rate expression that pertains to a mechanism involving noncompetitive adsorption on two different types of sites.