Analysis of Chlorinated Microbially Derived Dissolved Organic Nitrogen: Linking Molecular Structure to Toxicological Risk

Dissolved organic nitrogen (DON) is an important precursor of nitrogenous disinfection byproducts (N-DBPs) during chlorination. As soluble microbial products (SMP) and extracellular polymeric substances (EPS) constitute important sources of DON in the water environment, understanding how their distinct compositional profiles influence DBP formation is critical for water safety assessment. This study systematically explored the chlorination reaction network of SMP and EPS, elucidating the structure and toxicological profiles of generated N-DBPs. The results showed that SMP, characterized by elevated aromaticity and humification, preferentially formed aromatic chlorinated DBPs (Cl-DBPs) through electrophilic aromatic substitution pathways. In contrast, EPS contained more labile protein with less ordered secondary structure, promoting N-DBP generation. Molecular network analysis showed that SMP primarily formed aromatic Cl-DBPs via lignin/tannin chlorination, whereas EPS generated more N-DBPs from proteins and lipids. Structural characterization via ultrahigh performance liquid chromatography-high resolution mass spectrometry proposed two predominant N-DBP classes: heterocyclic aromatic amines (high mammalian oral toxicity and developmental effects) and long-chain aliphatic amines (pronounced aquatic toxicity but minimal mammalian effects). This study provides critical insights into differential DBP formation pathways in chlorinated DON-rich water and highlights the necessity of multiend point toxicity assessment for water quality evaluation.