Vitamin B2-Initiated Hydroxyl Radical Generation under Visible Light in the Presence of Dissolved Iron

The generation of hydroxyl radical (•OH) under solar irradiation from natural photosensitizers is widely recognized to play a key role in the photochemical degradation of biorefractory organic pollutants. Solar-light-mediated •OH production has been supposed as a green and sustainable strategy in water and wastewater treatment. In this study, an efficient visible-light-mediated •OH generation method using the photosensitization of vitamin B2 (VB2) catalyzed by dissolved iron was proposed. •OH generation was validated by electron paramagnetic resonance spectroscopy combined with spin trapping, and was further confirmed using the hydroxylation of nitrobenzene. VB2 plays a dual role of in the process of •OH generation. In the initial step, VB2 works as an excellent photosensitizer. Upon absorption of a photon, VB2 gives rise to its excited singlet state (1VB*) and, through intersystem crossing, to its excited triplet state (3VB*). In the subsequent step, VB2 works as an effective electron mediator to mediate O2•– generation. However, O2•– is sensitive to proton and is converted to H2O2 quickly in water. Ultimately, the •OH generation occurs through dissolved-iron catalyzed decomposition of H2O2. All ingredients involved in this process are nontoxic, environmentally benign, and easily available. Thus, this process might have potential environmental implications.