Promoting multiple reactive oxygen species generation for deep oxidation of VOCs by UV/persulfate/permanganate

Advanced oxidation processes (AOPs) could effectively remove volatile organic compounds (VOCs) when coupled with wet scrubbing process. However, it still faces challenges in achieving deep oxidation of VOCs due to the limited production of reactive oxygen species (ROS). Herein, we present the utilization of permanganate (KMnO4) as a co-oxidant to enhance VOCs oxidation in UV/persulfate (PDS)/KMnO4 system. The removal efficiency of a targeted VOC, toluene, kept high at above 90% throughout the entire long-term reaction of 4 h, which was much higher than that of UV/PDS and UV/KMnO4. Notably, the in-situ formed manganese oxide (MnOx) acts as a catalyst in UV/MnOx and UV/MnOx/PDS, facilitating the generation of various ROS and thereby enhancing VOCs degradation. Electron paramagnetic resonance (EPR) and quenching experiments indicated that sulfate radical (SO4−), hydroxyl radical (HO), superoxide radical (O2−) and singlet oxygen (1O2) were the main ROS generated in the UV/PDS/KMnO4 system. The overall oxidation process can be divided into three stages: the UV/KMnO4-dominated stage, the UV/MnOx/PDS-dominated stage, and the UV/MnOx-dominated stage. Correspondingly, HO played a major role in toluene oxidation in the UV/KMnO4-dominated and UV/MnOx-dominated stages, with the contributions of 48% and 52%, respectively. Meanwhile, SO4− played a more significant role in the UV/MnOx/PDS-dominated stage, with a contribution of 59%. Three main reaction pathways involving electron abstraction, HO addition and benzene ring opening were proposed based on the intermediates identified by proton-transfer-reaction mass spectrometry (PTR-MS). This study provided a deep understanding of KMnO4 in improving the oxidation capacity of UV-based AOPs for VOCs degradation, emphasizing the importance of in-situ formed MnOx for promoting multiple ROS generation.