Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid and 4-chloro-2-methylphenoxyacetic acid in water by using TiO2.

The photocatalytic degradation of many chlorinated organic compounds by semiconductor particles, has been widely recognised as a promising method of water and wastewater treatment process. In the present work the photocatalytic transformation of 2,4-D and MCPA in aqueous solution with 2 g l(-1) suspended TiO2 is demonstrated. The formation of several intermediates has been observed. The photodegradation rate increases with increasing pH. The photocatalytic transformation of 2,4-D and MCPA over TiO2 in solution containing hydrogen peroxide has been also studied. H2O, accelerated significantly the reaction rate of 2, 4-D and MCPA. The partial inhibition by ethanol is attributed to the scavenging of OH radicals involved in the first step of the reaction. Finally, from these results, a mechanism is proposed. This photocatalytic method has good potential for application on a large scale.

Phototransformation of diuron in aqueous solution by UV irradiation in the absence and in the presence of H2O2.

The phototransformation of diuron has been studied by photolysis at 253.7 nm at 20 degrees C, in the absence and in the presence of H2O2. Experiments were conducted in batch and in continuous-flow reactors. In the absence of H2O2, the value of the quantum yield of photolysis of diuron at 253.7 nm was found to be equal to be 0.0125 +/- 0.0005 (using a molar absorption coefficient of 16500 +/- 500 M(-1) cm (-1) at 253.7 min) and insensitive to pH in the range 2-8.5. Oxidation rates of diuron by H2O2/UV could be predicted successfully by a kinetic model including photochemical and OH*-oxidation reactions using a value of 4.6 x 10(9) M(-1) s(-1) for the rate constant of the reaction of OH* with diuron. The model was verified for the various reactors used and under a wide range of conditions in pure water (pH: 2-8, [H2O2] : 0-0.1 M) and in the presence of hydrogenocarbonate ions (0-35 mM, pH = 8.3-8.4). The contribution of the carbonate radicals to the degradation rates of diuron was found to be insignificant under our experimental conditions.