Chlorination and bromination of nucleobases: the role of cytosine and adenine in the formation of disinfection byproducts

Abstract

Freshwater sustainability is increasingly challenged by population growth, climate change, and rising economic demands—prompting greater reliance on alternative water sources such as recycled wastewater. Water disinfection is used to inactivate pathogens but can also unintentionally generate disinfection byproducts (DBPs), which pose significant health risks with long-term exposure. Wastewater contains elevated levels of ammonia and organic nitrogen sources—such as nucleobases—promoting the formation of toxic unregulated nitrogen-containing DBPs (N-DBPs). Chlorine (HOCl) and in situ disinfectants like bromine (HOBr) can react with nitrogenous compounds to produce N-halamines which may serve as precursors to N-DBPs. The present work investigated the formation and stability of N-halamines from reactions between chlorine or bromine and two nitrogen-rich model compounds (cytosine and adenine) under varying pH and stoichiometric ratios. Using ultraviolet-visible (UV-vis) spectrophotometry and tandem mass spectrometry, it was elucidated that the reaction between chlorine with both adenine or cytosine produced mono- and possibly di-substituted N-chloramines, whereas reactions with bromine produced carbon-substituted brominated final products. DBP formation potential experiments showed that chlorinated and brominated nucleobases continued to react with organic matter (humic acid) to produce both C- and N-DBPs at moderate concentrations similar to monochloramine. This work provides insight into the role of N-halamines and brominated nitrogenous compounds in the production of N-DBPs and should inform future policy working towards further regulating N-DBPs in our potable water.