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 ([NO3–]), which increased as [NO3–] increased to 2–5 mg-N/L but decreased by 7–52% when [NO3–] 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 [NO3–], the apparent quantum yield in forming RNS changed, causing a low steady-state concentration of •NO2 ([•NO2]ss) and high formation of other RNS. Fulvic acid inhibited photolysis due to the scavenging of •OH, while bicarbonate enhanced photolysis by preferentially generating CO3•– over •NO2. For toxic byproducts, the formation of nitrite and 4-chloro-2-nitrophenol (4-CNP) from 4-chlorophenol varied upon [NO3–]. 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.