Reactive Nitrogen Species Are Also Involved in the Transformation of Micropollutants by the UV/Monochloramine Process

The UV/monochloramine (NH₂Cl) process is an emerging advanced oxidation process (AOP) in water treatment via radicals produced from the UV photolysis of NH₂Cl. This study investigated the degradation of micropollutants by the UV/NH₂Cl AOP, with ibuprofen (IBP) and naproxen (NPX) selected as representative micropollutants. Hydroxyl radical (HO^•) and chlorine atom (Cl^•) were identified in the process, and unexpectedly, we found that reactive nitrogen species (RNS) also played important roles in the transformation of micropollutants. The electron paramagnetic resonance (EPR) analysis proved the production of ^•NO as well as HO^•. The concentrations of HO^•, Cl^•, and ^•NO in UV/NH₂Cl remained constant at pH 6.0-8.6, resulting in the slightly changed UV fluence-based pseudo-first-order rate constants (k') of IBP and NPX, which were about 1.65 × 10^-3 and 2.54 × 10^-3 cm²/mJ, respectively. For IBP, the relative contribution of RNS to k' was 27.8% at pH 7 and 50 μM NH₂Cl, which was higher than that of Cl^• (6.5%) but lower than that of HO^• (58.7%). For NPX, the relative contribution of RNS to k' was 13.6%, which was lower than both Cl^• (23.2%) and HO^• (46.9%). The concentrations of HO^•, Cl^•, and ^•NO increased with the increasing NH₂Cl dosage. Water matrix components of natural organic matter (NOM) and bicarbonate can scavenge HO^•, Cl^•, and RNS. The presence of 5 mg/L NOM decreased the k' of IBP and NPX by 66.9 and 57.6%, respectively, while 2 mM bicarbonate decreased the k' of IBP by 57.4% but increased the k' of NPX by 10.5% due to the contribution of CO₃^•- to NPX degradation. Products containing nitroso-, hydroxyl-, and chlorine-groups were detected during the degradation of IBP and NPX by UV/NH₂Cl, indicating the role of nitrogen oxide radical (^•NO) as well as HO^• and Cl^•. Trichloronitromethane formation was strongly enhanced in the UV/NH₂Cl-treated samples, further indicating the important roles of RNS in this process. This study first demonstrates the involvement of RNS in the transformation of micropollutants in UV/NH₂Cl.