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

Far UV light at 222 nm (UV₂₂₂) is gaining much attention for efficient water purification in UV₂₂₂ irradiation and UV₂₂₂-based advanced oxidation processes (AOPs). The direct photolysis of pollutants is regraded to be their major removal mechanism by a sole UV₂₂₂ treatment. However, this paper reports the important roles of reactive oxidative species (ROS) generated from dioxygen and water under only UV₂₂₂ radiation. Multiple ROSs are identified, including hydroxyl radical (HO^·), singlet oxygen (¹O₂), superoxide radical anion (^·O₂^-), and ozone (O₃). HO^{^·} is the major ROS for the degradation of 18 organic micropollutants under UV₂₂₂ radiation, with an observed quantum yield of 0.447 and the concentration of 10^-13 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 O₃ (i.e., O(¹D)) to form HO^·. Water matrix components of HCO₃^- and natural organic matter can inhibit the HO^· concentration, whereas NO₃^- significantly enhances it. In drinking water, UV₂₂₂ alone removes 18 micropollutants more efficiently than the typical UV₂₅₄/H₂O₂ AOP (150 μM), with reduced energy consumption. This study discloses a novel mechanism of ROS generation in UV₂₂₂ irradiation and underscores UV₂₂₂ as an emerging chemical-free AOP for water purification.