Reaction of phenazone-type drugs and metabolites with chlorine and monochloramine, Science of the Total Environment, 757 (2021), 143770 

This work studies the chlorination and monochloramination reaction kinetics of two phenazone-type drugs (phenazone – Phe and propyphenazone – PrPhe) and three metabolites of phenazone-type drugs (4-formylaminoantipyrine – FAA, 4-aminoantipyrine - AA and 4-acetoamidoantipyrine - AAA). Kinetics were faster with chlorine (apparent second-order constants between 100 and 66,500 times higher) than with monochloramine. For FAA and AAA, no significant reaction was observed during monochloramination. Further, apparent rate constants decreased as the pH increased from pH 5.7 to 8.3, except during chlorination of AA. The transformation products (TPs) formed were also elucidated by liquid chromatography-high resolution mass spectrometry. The main transformation pathway for Phe and PrPhe consisted of halogenations, hydroxylations and dealkylations, while AAA and FAA were firstly transformed to AA, then followed by pyrazole ring opening and hydroxylations. The extend of the reaction was also tested in real water samples, where, in general, slower reaction kinetics were obtained during monochloramination, while the chlorination reaction showed similar half-lives to ultrapure water. Finally, acute and chronic toxicity of the TPs were estimated using two quantitative structure-activity relationship (QSAR) software (ECOSAR and TEST), showing that some TPs could be more toxic than their precursor compounds.