Formation of Toxic Products During the Chlorination of Aspartic Acid

Chlorination is the most widely applied disinfection method in drinking water treatment, but it may lead to the formation of harmful disinfection by-products (DBPs). Amino acids present in natural waters and algal biomass are regarded as important precursors of DBPs. In this study, the chlorination of aspartic acid (Asp), one of the most abundant amino acids in natural waters, was investigated to elucidate the kinetics and mechanism of haloacetic acid formation. Stopped-flow spectrophotometry revealed that the reaction begins with the rapid formation of N-chloroaspartic acid, followed by chlorination to N,N-dichloroaspartic acid. Kinetic analysis shows that the second chlorination step involves dichlorine monoxide (Cl₂O), which is an effective chlorinating agent under neutral conditions. The dichloro derivative subsequently decomposes in a first-order process forming a relatively stable intermediate, 3-chloroimino-propanoic acid (CIPA). Time-resolved ¹H NMR spectroscopy and ion chromatography demonstrated that this intermediate transforms into the final products through multiple reaction paths over several hours. The dominant pathway involves the formation of cyanoacetic acid (CNAA), which is oxidized to dichloroacetonitrile (DCACN) and ultimately to dichloroacetic acid (DCAA) which is the main product of the reaction. The minor pathway leads to the formation of chloral hydrate (ChH) and subsequently trichloroacetic acid (TCAA). Nitrate ion is the major nitrogen-containing final product. The proposed kinetic model consistently explains all experimental observations and provides new insight into the formation of haloacetic acids from amino acids during water chlorination.