The photocatalytic degradation of dicamba in TiO 2 suspensions with the help of hydrogen peroxide by different near UV irradiations

Abstract

The direct photolysis and the photocatalytic degradations of dicamba in TiO 2 suspensions with and without the use of hydrogen peroxide were studied using two different monochromatic UV irradiations (300 and 350 nm). Both the direct photolysis and photocatalytic degradations of dicamba follow pseudo-first-order decay kinetics. Photolysis reactions were slow but the corresponding photocatalysis rates were increased by about 3 and 5 times in the presence of TiO 2 at 300 and 350 nm of UV, respectively. Photocatalytic rates were increased with the pH at acidic to neutral ranges because of the increase of hydroxide ions, but the reaction was gradually retarded at the alkaline medium due to the effect of charges repulsion. The different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. The results of H 2O 2-assisted photocatalysis experiments showed that a low H 2O 2 dosage in photocatalysis using UV 300 nm would enhance the decay rate of dicamba by 2.4 times, but an overdose of H 2O 2 will retard the rate because of the expenditure of hydroxyl radicals. However, this process was found impracticable at UV 350 nm due to the absorption characteristic of H 2O 2. A neutral initial pH level was found to favour the H 2O 2-assisted photocatalysis at UV 300 nm. The reactions were highly retarded at the alkaline medium due to the unstable properties of H 2O 2.