Water microdroplets: A catalyst-free source of reactive oxygen species for pollutants removal

Reactive oxygen species (ROS) formation occurs frequently under ambient conditions in enzymatic or nanomaterial-based catalysis and is vitally important during advanced oxidation processes (AOPs). In this work, we demonstrated a spontaneous formation ROS of •OH, H2O2 and •O2− in water microdroplets under ambient conditions without any catalysts presenting a real green source of ROS. Control experiments revealed that at the interface of water microdroplets electrons were transferred from OH− to O2, leading to the formation of •OH (230.25 μM/min), H2O2 (152.5 μM/min) and 1O2 (•O2−) (0.11 μM/min), respectively. The generation of the ROS was applied for the oxidative removal of organic pollutants, such as rhodamine B (RhB), methylene blue, nile blue, and fluorescein. A multi-nozzle spraying of water produced large amounts of ROS capable of removing 2 mL RhB solution at 100 μg/mL in 4 h. In sum, compared to the previous works, although this strategy is limited by the production of the ROS, water microdroplets also will be a significant and real green source of ROS without any additives (catalysts, nanomaterials, irradiation UV light, especially chemical reagents) at all. And we believe the limitation will be greatly improved by the designing spray array in the future.