Role of dissolved oxygen in promoting indirect photolysis of organic micropollutants in far-UVC 222 nm treatment

Far-UVC technologies using KrCl* excimer lamps have emerged as a promising technology for removing organic micropollutants (OMP) in water by advanced oxidation process (AOP) and direct photolysis. Recently, preliminary calculation suggested that dissolved oxygen (DO) absorbed UV light at 222 nm similar to or stronger than OMP at environmentally relevant scenarios. However, it is unclear whether DO plays an important role on OMP photolysis at 222 nm. This study investigated the effect of DO on the photodegradation of five selected OMP compounds including 4-nitrophenol, phenol, diethyltoluamide, sulfadiazine, and nitrobenzene under UV 222 nm irradiation, at three DO concentrations including low (< 1.5 mg/L), medium (~ 4 mg/L), and high DO (> 15 mg/L). High DO concentrations increased the photolysis rate by 3- to 14-fold for all five OMPs, compared with low levels. DO photolysis can potentially form ozone (O3) and OH radical , leading to indirect photolysis of the OMPs. The relative contribution of OH radical and O3 to OMP photodegradation was elucidated using quenching experiments and a probe experiment with cinnamic acid, respectively. Overall, DO affected OMP photolysis at 222 nm, which should not be ignored in future research.