N ‐Nitrosodimethylamine ( NDMA ) Formation Mechanisms in Drinking Water Systems

Abstract N ‐nitrosodimethylamine (NDMA) occurrence and formation pathways in drinking water systems are reviewed and NDMA yields are compared on the basis of disinfectant type, water chemistry, and precursor category. While NDMA formation is primarily associated with chloramination, it can form by ozonation, in which it is enhanced by bromide, and during chlorine dioxide disinfection. The reaction pathway for NDMA formation during ozonation involves reactive intermediates such as hydroxylamine, which may also be relevant to NDMA formation with other disinfectant types. In chloramination, despite monochloramine being the predominant species between pH 7 and 9, evidence suggests that dichloramine is the primary species involved in NDMA formation. This is somewhat confounding as NDMA yields are maximal at pH 9, yet at pH 9 dichloramine decays faster than it forms and hence is present at trace levels; additionally, the proposed mechanism involves a spin‐forbidden incorporation of dissolved oxygen as a triplet, which is presumably kinetically slow. This article reveals that kinetic data for NDMA formation is lacking, and its influence on chloramine chemistry has not been carefully considered.