Overlooked Generation of Reactive Oxidative Species from Water and Dioxygen by Far UV Light.

Far UV light at 222 nm (UV) is gaining much attention for efficient water purification in UV irradiation and UV-based advanced oxidation processes (AOPs). The direct photolysis of pollutants is regraded to be their major removal mechanism by a sole UV treatment. However, this paper reports the important roles of reactive oxidative species (ROS) generated from dioxygen and water under only UV radiation. Multiple ROSs are identified, including hydroxyl radical (HO), singlet oxygen (O), superoxide radical anion (O), and ozone (O). HO is the major ROS for the degradation of 18 organic micropollutants under UV radiation, with an observed quantum yield of 0.447 and the concentration of 10 M at pH 7. Dioxygen is the initial source of ROS, while water mainly serves as a medium to react with the photolytic intermediate of O (i.e., O(D)) to form HO. Water matrix components of HCO and natural organic matter can inhibit the HO concentration, whereas NO significantly enhances it. In drinking water, UV alone removes 18 micropollutants more efficiently than the typical UV/HO AOP (150 μM), with reduced energy consumption. This study discloses a novel mechanism of ROS generation in UV irradiation and underscores UV as an emerging chemical-free AOP for water purification.