Dichloroacetonitrile and Dichloroacetamide Can Form Independently during Chlorination and Chloramination of Drinking Waters, Model Organic Matters, and Wastewater Effluents

The increasing usage of organic nitrogen-rich wastewater- or algal-impacted waters, and chloramines for secondary disinfection, raises concerns regarding the formation of haloacetonitriles, haloacetamides and other nitrogenous disinfection byproducts (N-DBPs). Previous research obtained contradictory results regarding the relative importance of chlorination or chloramination for promoting these byproducts, but applied chlorine and chloramines at different doses and exposure periods. Additionally, mechanistic work, mostly using model precursors, suggested that haloacetonitrile and haloacetamide formation should be correlated because hydrolysis of haloacetonitriles forms haloacetamides. In this work, the formation of dichloroacetonitrile (DCAN) and dichloroacetamide (DCAcAm) were compared across a range of chlorine and chloramine exposures for drinking waters, wastewater effluents, algal extracellular polymeric substances (EPS), NOM isolates and model precursors. While chlorination favored formation of DCAN over DCAcAm, chloramination nearly always formed more DCAcAm than DCAN, suggesting the existence of haloacetamide formation pathways that are independent of the hydrolysis of haloacetonitriles. Experiments with asparagine as a model precursor also suggested DCAcAm formation without a DCAN intermediate. Application of N-15-labeled monochloramine indicated initial rapid formation of both DCAN and DCAcAm by pathways where the nitrogen originated from organic nitrogen precursors. However, slower formation occurred by pathways involving chloramine incorporation into organic precursors. While wastewater effluents and algal EPS tended to be more potent precursors for DCAN during chlorination, humic materials were more potent precursors for DCAcAm during chlorination and for both DCAN and DCAcAm during chloramination. These results suggest that, rather than considering haloacetamides as haloacetonitrile hydrolysis products, they should be treated as a separate N-DBP class associated with chlorarnination. While use of impaired waters may promote DCAN formation during chlorination, use of chloramines may promote haloacetamide formation for a wider array of waters.