Homogeneous photodegradation study of 2-mercaptobenzothiazole photocatalysed by sodium decatungstate salts: Kinetics and mechanistic pathways

The photochemical degradation of a benzothiazole derivative, 2-mercaptobenzothiazole (MBT) has been studied in aqueous solution in the presence of a polyoxometalate (POM): sodium decatungstate salts Na4W10O32 (DTA). In aerated conditions, the photodegradation rate of MBT clearly increased in the presence of DTA by a factor six when compared with the direct photolysis with k(MBT) = 0.25 h(-1) and t(1/2)(MBT)= 2.8 h. For the total comprehension of the degradation mechanism, the oxygen influence has been investigated. Oxygen appeared essential for DTA regeneration, its absence induced a three times inhibition of MBT disappearance and completely stopped the photocatalytic cycle. The main photoproducts were identified with LC-ESI-MS and LC-DAD techniques and using some calculations obtained by B3LYP/6-21G method in Gaussian 4.1 software. All the results allowed to propose a mechanistic pathway. Electron transfer and H atom abstraction processes involving W10O324-center dot excited state species were involved in the degradation. In the primary step of the degradation, the hydroxylation of the aromatic ring leading to four OH-MBT isomers and the formation of disulfide form of MBT were observed. For longer irradiation time, a secondary electron transfer permitted the oxidation of OH-MBT isomers and the formation of sulfoxide derivatives. For prolonged exposure (around 100 h), the complete mineralization was noticed in the presence of sodium decatungstate salts. (C) 2010 Elsevier B.V. All rights reserved.