ClO2 pre-oxidation impacts the formation and nitrogen origins of dichloroacetonitrile and dichloroacetamide during subsequent chloramination

Abstract

Chlorine dioxide (ClO2) can be used as a pre-oxidant when chloramination is performed in water treatment plants. However, the effects of ClO2 pre-oxidation on the formation of nitrogenous disinfection by-products, such as dichloroacetonitrile (DCAN) and dichloroacetamide (DCAcAm), during chloramination are not well understood. In this study, the effects of ClO2 pre-oxidation on the formation of DCAN and DCAcAm during chloramination of 28 model compounds and seven real water samples were investigated. The sources of nitrogen for DCAN and DCAcAm formation were investigated using 15N-labeled monochloramine. ClO2 pre-oxidation affected DCAN and DCAcAm formation during chloramination of model compounds in different ways. ClO2 pre-oxidation increased unlabeled and 15N-labeled DCAN and DCAcAm formation during chloramination of six amino acids and peptides and five indoles and tertiary amines. ClO2 pre-oxidation decreased DCAN formation but increased DCAcAm formation during chloramination of three hydroxybenzamide compounds, but had the opposite effects for four tetracyclines. ClO2 pre-oxidation generally decreased DCAN and DCAcAm formation during chloramination of the phenolic compounds that are precursors not containing nitrogen. 2-Aminoacetophenone, formamid-trans-muconic acid, and unsaturated ketones were found to be transformation products of ClO2 oxidation of 3-methylindole, salicylamide, and resorcinol, respectively. Possible DCAN and DCAcAm formation pathways during chloramination after ClO2 oxidation were identified. For most of the water samples, ClO2 pre-oxidation decreased the amounts of DCAN and DCAcAm formed during chloramination by 36%–70% and 11%–59%, respectively. This may have been caused by ClO2 oxidation destroying phenolic precursors and macromolecular proteins rather than amino acids in the water samples.