Formation of Eight Classes of DBPs from Chlorine, Chloramine, and Ozone: Mechanisms and Formation Pathways

Disinfection byproducts (DBPs) are formed during drinking water treatment from the reaction of chemical disinfectants with natural organic matter (NOM), anthropogenic contaminants, and inorganic bromide and iodide. DBPs are of public concern due to their carcinogenic and genotoxic effects and adverse effects observed in many epidemiologic studies. Formation mechanisms have been studied in order to identify precursors, reaction intermediates, and reaction kinetics and to predict new classes of DBPs. By understanding formation mechanisms, steps can be taken to remove DBP precursors and adjust treatment conditions to minimize DBP formation. This paper presents a critical review of formation mechanisms for eight classes of DBPs, including trihalomethanes (THMs), haloacetic acids (HAAs), haloketones (HKs), haloacetaldehydes (HALs), haloacetonitriles (HANs), haloacetamides (HAMs), halonitromethanes (HNMs), and nitrosamines (NAs) from the popular disinfectants chlorine, chloramine, and ozone. Important precursors in the formation of many of these DBPs include phenolic, β-dicarbonyl, and oxopentadioic acid groups found in humic NOM species. Likewise, amino acids are also precursors for several classes of DBPs. HANs can form by chloramination of aldehydes, and HAMs can form from the hydrolysis of HANs. HNMs can form by the oxidation/halogenation of amines as well as by nitration and halogenation of humic substances in the presence of nitrite. Preozonation followed by chlorine or chloramine can significantly increase HNM formation. Finally, dichloramine and aqueous oxygen are important reactants in the formation of nitrosamines from chloramine. Nitrosamines can also be formed by N,N-dimethylsulfamide. Detailed formation mechanisms for 8 classes of drinking water DBPs formed by chlorine, chloramine, and ozone are presented.