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

Far UV light at 222 nm (UV 222 ) is gaining much attention for efficient water purification in UV 222 irradiation and UV 222 -based advanced oxidation processes (AOPs). The direct photolysis of pollutants is regraded to be their major removal mechanism by a sole UV 222 treatment. However, this paper reports the important roles of reactive oxidative species (ROS) generated from dioxygen and water under only UV 222 radiation. Multiple ROSs are identified, including hydroxyl radical (HO · ), singlet oxygen ( 1 O 2 ), superoxide radical anion ( · O 2 – ), and ozone (O 3 ). HO · is the major ROS for the degradation of 18 organic micropollutants under UV 222 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 O 3 (i.e., O( 1 D)) to form HO · . Water matrix components of HCO 3 – and natural organic matter can inhibit the HO · concentration, whereas NO 3 – significantly enhances it. In drinking water, UV 222 alone removes 18 micropollutants more efficiently than the typical UV 254 /H 2 O 2 AOP (150 μM), with reduced energy consumption. This study discloses a novel mechanism of ROS generation in UV 222 irradiation and underscores UV 222 as an emerging chemical-free AOP for water purification.