Characterization of Organic Nitrogen by Chlorination, Ozonation, and Stable Isotope Analysis of Nitrate

During oxidation, nitrogenous species in dissolved organic matter (DOM) are critical in the formation of nitrogenous, potentially toxic disinfection byproducts, but their chemical identity remains poorly understood. Here, we developed three complementary approaches to identify and quantify reactive amines in model compounds and DOM, including aliphatic primary and secondary amines, aryl-type primary amines, amino acids, and terminal peptidic amino groups. With the chloramine formation assay, the total reactive amines were quantified for the main subgroups. An assay with continuous ozonation quantified three types of reactive amines based on nitrate formation rate constants (k_NO3-): k_NO3- < 0.1 M^-1 s^-1 for secondary and aliphatic primary amines; k_NO3- = 0.9-1.9 M^-1 s^-1 for aryl-type primary amines; k_NO3- = 15-110 M^-1 s^-1 for amino acids and peptidic amino groups. The analysis of ¹⁵N/¹⁴N ratios of nitrate helped to distinguish reactive amines based on ¹⁵N enrichment factors (ε_N): aliphatic (or aryl-type) primary amines (ε_N:-9 to -3‰), and amino acids and peptidic amino groups (ε_N:-28 to -19‰). Overall, 23-27% of the organic nitrogen in DOM isolates comprises oxidant-reactive amines, with 5-6% secondary amines, 10-14% aliphatic primary amines, 4% aryl-type primary amines, 1-4% amino acids, and 0-2% peptidic amino groups. Based on the quantitative characterization of amine moieties in DOM, which are possible precursors of N-DBPs, the formation potential of N-DBPs upon oxidative water treatment was estimated.