Formation mechanisms of chlorophenylacetonitriles from aromatic amino acids in chloramine disinfection

Abstract

Chlorophenylacetonitriles (CPANs), a class of nitrogenous disinfection byproducts (DBPs) with high cytotoxicity, are increasingly detected in chloraminated water systems. However, their formation mechanisms from aromatic amino acids (ArAAs), critical nitrogen-rich precursors in proteinaceous waters, remain poorly understood. This study systematically investigated the intermediates, reactive sites, nitrogen sources, and pathways involved in CPANs formation from phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) under chloramine disinfection. A total of 26 DBPs were identified, including 7 CPANs, with Trp exhibiting the highest diversity (18 species). ¹⁵N isotope experiments revealed dual nitrogen sources for CPANs: over 50% of nitrogen was derived from chloramine via aldehyde intermediates (phenylacetaldehyde/indole-3-acetaldehyde), while decarboxylation pathways contributed the remaining residual nitrogen from amino acid backbones. Notably, chloramine dosage governed pathway competition: Increasing chloramine concentrations (C[ArAAs]:C[NH₂Cl] ≤ 1:10) progressively shift pathway dominance toward the aldehyde route during CPAN formation. Molecular electrostatic potential calculations pinpointed α-amino groups (ALIE: 0.30∼0.46 Hartree; ESP: −0.03∼0.01 Hartree) and aromatic rings as primary reactive sites, driving sequential chlorination and cyclization. These findings challenge the perception of chloramination as a "safer" alternative, emphasizing the need for precursor-specific control strategies to mitigate CPAN risks in nitrogen-rich waters.