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 whe re disinfection byproduct formation is significant. Monochloramine is, however, unstable and decomposes, leading to nitrogen oxidation and c hlorine reduction (auto-decomposition). The role of natural organic ma tter (NOM) in monochloramine loss is unclear. NOM could catalyze monoc hloramine auto-decomposition, or it could act as an external reductant . This study elucidates the decay pathways of monochloramine in the pr esence and absence of NOM. When monochloramine decomposes in the absen ce of NOM, ammonia and nitrogen gas are the primary nitrogen decay pro ducts. When NOM is present, the product speciation changes such that l ittle nitrogen gas production occurs, yet production of ammonia and ni trate increases. This product speciation shift indicates that under th ese conditions, NOM acts primarily as a reductant and not as a catalys t. This conclusion was verified using a redox balance which compares o xidized product, N-2 and NO3-, production to monochloramine loss. The number of electrons accounted for by oxidized product production corre lates well with monochloramine loss in the absence of NOM (60-100% rec overy). However, there is a deficit in the presence of NOM (25-60% rec overy). Clearly, much of the oxidizing capacity of monochloramine goes toward NOM oxidation.