Reaction Pathways Involved in the Reduction of Monochloramine by Ferrous Iron

The maintenance of disinfectants in distribution systems is critical to ensure the safety of drinking water. Reduced iron is one type of reactive constituent believed to play a central role in disinfectant loss as the water travels through the pipe. Specifically, reactions involving Fe(II), both in solution and at the pipe−water interface, are felt to be important processes that account for this loss. This work shows that the rate-limiting reactions responsible for the disappearance of monochloramine involve a direct reaction between molecular monochloramine and Fe(II), leading to the formation of the amidogen radical ( • NH 2 ) as a reactive intermediate. In addition, the mechanism was found to be autocatalytic with the ferric oxide precipitate acting to accelerate the overall reduction of monochloramine. A variable stoichiometry was observed for this system, and this was rationalized by accounting for radical scavenging in the reaction scheme.