Photocatalytic oxidation of carbamazepine in triclinic-WO₃suspension: Role of alcohol and sulfate radicals in the degradation pathway

The photocatalytic oxidation of carbamazepine (CBZ, an antiepileptic drug frequently detected in surface water and WWTP effluent) in WO3suspension under the irradiation of 420 nm lamps has been investigated. The addition of S2O82-to this process can significantly accelerate the CBZ degradation. It was interesting to observe that conventional radical quenchers like methanol and ethanol promoted the photocatalytic oxidation of CBZ at low concentration (<100 mM), while the tert-butanol inhibits or accelerates the CBZ degradation depending on its concentration. To explore this, the decomposition of CBZ with and without the presence of methanol was investigated in details through the comparison of evolution of intermediates/products, and the comparison of generated radicals by ESR spectroscopy in the process. Sulfate radicals are believed to play a major role during CBZ decay with the presence of methanol in this process. It was found the rate enhancement of methanol is likely due to the competition between quenching effect and cleaning effect on the catalyst surface. In addition, the reaction mechanism reveals that CBZ degradation is mainly dominant by the electrophilic attack of radicals at olefinic double bond in the central heterocyclic ring, leading to ring-opening and formation of various intermediates.