Odorous disinfection byproducts from sweet peptides during chlorination

Odorous disinfection byproducts (DBPs) are important contributors to taste and odor (T&O) problems in drinking water and directly affect consumer perception of water quality and safety. Synthetic peptides are frequently detected in aquatic environments, yet their role in the formation of odorous DBPs is largely overlooked. In this study, aspartame (APM) and neotame (NEO), two widely used sweet peptides, were selected as representative synthetic peptides. They were identified for the first time as previously overlooked precursors of odorous DBPs during chlorination. Sensory analysis defined the odor features of the chlorinated products, while comprehensive two-dimensional gas chromatography-time-of-flight high-resolution mass spectrometry (GC × GC-TOF-MS) characterized the temporal evolution of total volatile DBP composition. On this basis, gas chromatography-olfactometry-mass spectrometry (GCO-MS) confirmed phenylacetaldehyde, phenylacetonitrile, and N-chlorophenylacetaldimine as the main odorous DBPs, with maximum combined molar yields of 29.38% from APM and 23.66% from NEO. Ultra-performance liquid chromatography-quadrupole TOF-MS (UPLC-QTOF-MS) was used to further identify key intermediates and clarify potential formation pathways, which were further validated by density functional theory (DFT) calculations of reaction thermodynamics and electronic properties. Results show N-chlorination as the initial step, followed by substitution, dehydrohalogenation, β-elimination, hydrolysis, and decarboxylation, ultimately leading to the formation of odorous DBPs. In addition, the effects of different chlorination conditions on odorous DBP formation were examined, showing that increasing Cl:N ratios promoted odorous DBP formation during chlorination, whereas chloramination consistently suppressed their formation. In treated water from two drinking water treatment plants, the actual concentrations of phenylacetaldehyde, phenylacetonitrile, and N-chlorophenylacetaldimine were 2.67, 8.43, and 6.52 μg·L⁻¹ for river water, and 3.31, 9.82, and 8.22 μg·L⁻¹ for lake water, respectively. Using real raw waters, ammonia addition before NaClO reduced the total concentration of the three odorous DBPs by at least 85.4% compared with free chlorine alone. These findings advance the mechanistic understanding of odorous DBP formation and introduce a new analytical strategy for identifying chlorination-derived odorous compounds in drinking water.