Role of reactive nitrogen species in ranitidine degradation in UV/chloramine process: Transformation pathways and NDMA formation

Chloramine under UV photolysis could not only produce the widely known hydroxyl radical (HO center dot) and reactive chlorine species (RCS; e.g., ClO center dot, Cl-2(center dot-) and Cl-center dot), but also produce reactive nitrogen species (RNS; e.g., (NH2)-N-center dot, (NO)-N-center dot and (NO2)-N-center dot). In this study, the kinetic mechanisms, degradation products, N-Nitrosodimethylamine (NDMA) formation of RAN in the UV/chloramine process were investigated. The RAN degradation by UV/chloramine process well fitted the pseudo-first order kinetic model and exhibited a synergistic improvement compared with UV photolysis and chloramination alone. HO center dot is the predominant radical that contributes to RAN degradation in the range of solution pH from 6.0 to 8.0 (from 43.4% to 56.3%), and RNS was confirmed to contribute to RAN degradation through experiments. As the concentrations of HCO3-, Cl- and NO3- (0 similar to 4 mM), chloramine dosage (200 similar to 300 mu M), solution pH (6.0 similar to 8.0) and natural organic matter (0 similar to 4 mg-C L-1) increased, the RAN degradation in UV/chloramine process was inhibited. Besides, the second-order rate constant between CO3 center dot- and RAN was determined to be 8.05 x 10(6) M-1 s(-1) in this study. A possible pathway and reaction schemes of RAN degradation by UV/chloramine process were proposed, which could also be used to explain the role of RNS in the NDMA formation. During the treatment of RAN by chloramination alone and UV/chloramine, NDMA has a better formation potential at both pH = 7.0 and pH = 8.0. Although RNS were responsible for NDMA formation during the RAN degradation by UV/chloramine process, UV photolysis and extending the photolysis time from 5 to 10 min could degrade NDMA and its precursors. Overall, the high yield and toxicity of NDMA should be concerned when choosing the UV/chloramine process.