Photodegradation kinetics, products and mechanism of timolol under simulated sunlight

Abstract

The photodegradation of β-blocker timolol in fulvic acid (FA) solution was investigated under simulated sunlight. The triplet excited state of FA (3FA*) and singlet oxygen (1O2) were the main reactive species responsible for the degradation of timolol in the aerated FA solutions. Both dissolved oxygen and iodide ions (I−) are the efficient quenchers of 3FA*. The photodegradation was drastically accelerated after removing the dissolved oxygen. The presence of I− inhibited the photosensitized degradation of timolol in the deoxygenated FA solutions, whereas the role of I− in the reaction was concentration-dependent in the aerated solutions. The other halide ions such as chloride (Cl−) and bromide (Br−) exhibited less effect on the photodegradation of timolol in both aerated and deoxygenated solutions. By LC-DAD/ESI-MS/MS analysis, the photoproducts of timolol in both aerated and deoxygenated FA solutions were identified. Electron transfer interaction occurred between 3FA* and amine moiety of timolol, leading to the cleavage of C–O bond in the side chain and oxidation of the hexatomic ring. These findings suggest the photosensitized degradation was a significant pathway for the elimination of timolol in natural waters.