Chloramination has gained increasing popularity in drinking water disinfection but may lead to increasing formation of nitrogenous disinfection byproducts (N-DBPs). Transformation pathways of chloramination of nitrogenous organic compounds are largely unclear. This thesis work investigated transformation pathways of chloramination of three model nitrogenous organic compounds including 4-amino-2-chlorobenzoic acid (ACBA), phenylalanine (PHE) and cytosine (CYT) with variations in reaction time, monochloramine dosages, and pH. Structures of the intermediates and products were elucidated by a numbers of mass spectrometry techniques. The number and origins of nitrogen atoms in the products are determined by a labeled ¹⁵N-monochloramine method.

Chloramination of the model compounds...[ Read more ]

Chloramination has gained increasing popularity in drinking water disinfection but may lead to increasing formation of nitrogenous disinfection byproducts (N-DBPs). Transformation pathways of chloramination of nitrogenous organic compounds are largely unclear. This thesis work investigated transformation pathways of chloramination of three model nitrogenous organic compounds including 4-amino-2-chlorobenzoic acid (ACBA), phenylalanine (PHE) and cytosine (CYT) with variations in reaction time, monochloramine dosages, and pH. Structures of the intermediates and products were elucidated by a numbers of mass spectrometry techniques. The number and origins of nitrogen atoms in the products are determined by a labeled ¹⁵N-monochloramine method.

Chloramination of the model compounds underwent several cycles of chlorine substitution and hydrolysis, which were affected by reaction time, monochloramine dosages, and pH. Chlorine atoms firstly substituted hydrogen atoms either on the aromatic rings containing electron donating functional groups (e.g., ACBA and CYT) or on the amine groups on the aliphatic side chain (e.g., PHE). Most of these chlorine-substituted intermediates formed within 4 hours to 1 day and decayed with further increasing reaction time. During the continuous chloramination, large molecular weight compounds containing aromatic rings underwent ring cleavage to form low molecular weight aliphatic compounds. At high Cl/P molar ratios, more chlorine atoms are added to the products. At lower pH, a larger number of products were formed and identified. Chloroimino intermediates were identified in chloramination of PHE and CYT, and their nitrogen atoms mainly originated from the model nitrogenous compounds.

This thesis work reveals that chloramination of nitrogenous organic compounds can initiate with chlorine substitution of the ring structures, in addition to commonly known chlorine substitution of the amine groups. The weak monochloramine attack on the chlorine substituted compounds can also lead to ring opening and finally formation of more nitrogenous DBPs, with increasing contact time. Therefore, special attention shall be paid to nitrogenous DBP formation in large water distribution systems (with long holding time) using monochloramine for residual protection. The N-DBP formation can be reduced by controlling the source of nitrogenous organic compounds. The formation can also be reduced by maintaining lower monochloramine concentrations and/or higher pH.