Effect of Natural Organic Matter on Monochloramine Decomposition:  Pathway Elucidation through the Use of Mass and Redox Balances

Monochloramine is often employed as a drinking water disinfectant for systems where free chlorine residuals are difficult to maintain or where disinfection byproduct formation is significant. Monochloramine is, however, unstable and decomposes, leading to nitrogen oxidation and chlorine reduction (auto-decomposition). The role of natural organic matter (NOM) in monochloramine loss is unclear. NOM could catalyze monochloramine auto-decomposition, or it could act as an external reductant. This study elucidates the decay pathways of monochloramine in the presence and absence of NOM. When monochloramine decomposes in the absence of NOM, ammonia and nitrogen gas are the primary nitrogen decay products. When NOM is present, the product speciation changes such that little nitrogen gas production occurs, yet production of ammonia and nitrate increases. This product speciation shift indicates that under these conditions, NOM acts primarily as a reductant and not as a catalyst. This conclusion was verified using a redox balance which compares oxidized product, N2 and NO3 -, production to monochloramine loss. The number of electrons accounted for by oxidized product production correlates well with monochloramine loss in the absence of NOM (60−100% recovery). However, there is a deficit in the presence of NOM (25−60% recovery). Clearly, much of the oxidizing capacity of monochloramine goes toward NOM oxidation.