Effect of UV and UV/H₂O₂ in the Presence of Chloramines on NDMA Formation Potential of Tramadol

This study evaluates the effect of UV–C and UV–C/H₂O₂ in the presence of chloramines on the N-nitrosodimethylamine formation potential (NDMA FP) of tramadol as a model precursor. The experiments were performed at high initial concentrations of TMDL (i.e., 20 mg/L) in order to elucidate the structures of TMDL byproducts. Twenty-four byproducts were identified in UV–C, UV–C/monochloramine, and UV/H₂O₂/monochloramine oxidation of tramadol using MS³ capabilities of a hybrid quadrupole-linear ion trap mass spectrometer, combined with online hydrogen/deuterium (H/D) exchange experiments. Oxidative cleavage of methoxy and methoxybenzene moiety, O-demethylation, hydroxylation, and cyclohexane ring-opening were identified as major reaction mechanisms of tramadol in UV oxidation. Addition of monochloramine decreased the degradation rates of tramadol and its byproducts and yielded several monochlorinated derivatives. The oxidation rates were significantly enhanced in the presence of H₂O₂, and byproducts of oxidative benzene ring-opening were detected. The majority of the identified byproducts are likely to have a higher NDMA FP than the parent compound due to a reduced steric hindrance and/or insertion of electron-donating hydroxyl groups in the N,N-dimethylamine side chain. This was confirmed by the results of NDMA FP tests, which showed that the formation of NDMA was enhanced up to four times depending on the process conditions in UV alone and in UV and UV/H₂O₂ in the presence of monochloramine. Prolonged oxidation by hydroxyl radicals in UV/H₂O₂/monochloramine process mineralized some of the byproducts and slightly reduced the NDMA FP at the end of the treatment. The obtained degradation pathway of tramadol allowed the correlation of changes in NDMA FP during oxidation with its major oxidative transformation reactions. This manuscript demonstrates the significance of oxidation byproducts as NDMA precursors and emphasizes the need for their consideration when evaluating the evolution of NDMA FP during oxidative treatment.