Role of Nitrate Concentration on Photolysis of Organic Micropollutants by Far-UVC Light (222 nm)

KrCl* excimer lamps emitting at 222 nm are promising for removing organic micropollutants (OMPs) in water treatment. Previous studies mainly studied the nitrate effect at limited concentrations (e.g., 5–10 mg-N/L), which did not represent the diverse contamination scenarios. Hence, this study evaluated six OMPs at 0–20 mg-N/L nitrate. OMP photolysis rate varied with nitrate concentrations (NO 3 – ), which increased as NO 3 – increased to 2–5 mg-N/L but decreased by 7–52% when NO 3 – further increased to 20 mg-N/L. Mechanism investigation revealed that light screening exerted a minor impact on overall photolysis rate, and the formation of hydroxyl radical ( • OH) and reactive nitrogen species (RNS) contributed to the enhancement. However, at high NO 3 – , the apparent quantum yield in forming RNS changed, causing a low steady-state concentration of • NO 2 ( • NO 2 ss ) and high formation of other RNS. Fulvic acid inhibited photolysis due to the scavenging of • OH, while bicarbonate enhanced photolysis by preferentially generating CO 3 •– over • NO 2 . For toxic byproducts, the formation of nitrite and 4-chloro-2-nitrophenol (4-CNP) from 4-chlorophenol varied upon NO 3 – . Lastly, a kinetic model was developed to predict the 4-CNP concentration. These findings suggested the critical role of the nitrate concentration in enhanced OMP photolysis at 222 nm.