Sunlight-mediated degradation of methyl orange sensitized by riboflavin: Roles of reactive oxygen species

Abstract Natural photosensitizer has been reported to play an important role in the sunlight-mediated degradation of organic pollutants and the self-purification of natural waters. Riboflavin (RF), a model natural photosensitizer, is widely present in-vivo and natural environments. RF has been proposed to generate reactive oxygen species (ROS) for degradation of pollutants, but it is susceptible to photodegradation because of instability of its ribose chain. In this work, the sunlight-mediated degradation of methyl orange (MO), as a model pollutant, was assessed at different pHs and initial RF concentrations. The MO photodegradation was found to be sensitized mainly by the photoproducts of RF, rather than RF itself. The highest MO photodegradation efficiency was achieved at pH 11.0. In order to further reveal its functional mechanisms, experiments were carried out in the presence of singlet oxygen ( 1 O 2 ) quencher (1,4-diazabicyclo[2.2.2]octane) and superoxide anion radical (O 2 − ) quencher (1,4-benzoquinone). Only 1,4-benzoquinone was able to inhibit the photodegradation, suggesting that the photooxidation of MO was mainly initiated by O 2 − . The generation of O 2 − was further validated by electron paramagnetic resonance (EPR) spectroscopy combined with spin trapping. Possible pathways were proposed for the RF-sensitized photodegradation of MO under solar irradiation.