Environ. Sci. Technol. 2004, 38, 1753-1758
Production of Macromolecular Chloramines by Chlorine-Transfer Reactions M A R Y B E D N E R , †,‡ WILLIAM A. MACCREHAN,‡ AND G E O R G E R . H E L Z * ,† Department of Chemistry and Biochemistry, University of Maryland, College Park Maryland 20742, and Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Chlorination of treated wastewaters is undertaken to prevent dispersal of human pathogens into the environment. Except in well-nitrified effluents, the primary agents in chlorination, Cl2(g) or NaOCl(aq), are short-lived and quickly transfer oxidative chlorine to secondary agents (Nchloramines), which then participate in the disinfection process. Maturation of residual chlorine resulting from chlorine-transfer reactions is still poorly characterized. Using gel permeation and reversed-phase liquid chromatography combined with a novel, oxidant-specific detector, unanticipated trends during the maturation of residual chlorine in wastewater are identified. Within 2 min after addition of NaOCl, and continuing for several hours at least, significant amounts of oxidative chlorine are transferred to secondary agents that are moderately to strongly hydrophobic and to agents that have high relative molecular masses (Mr 1300-25000). It is hypothesized that hydrophobic stabilization of organic chloramines (RNHCl°) thermodynamically drives these transfers, making macromolecular chloramines the ultimate oxidative chlorine carriers. Macromolecular chloramines are expected to be sluggish oxidants, as observed in their reduction by sulfite, and are expected to be poor disinfectants. If transfer of oxidative chlorine to high Mr components occurs widely at treatment plants, then this phenomenon offers a new, physicochemical explanation for the well-known impotency of organic chloramines in wastewater disinfection.
Introduction Reactive chlorine species play pivotal roles in disinfection of wastewater and drinking water as well as in inflammatory responses in mammalian tissue. Chlorination unleashes a cascade of chemical reactions that transfer oxidative chlorine (Cl+) rapidly from Cl2 and HOCl/OCl- to amines and then from one amine to another. The more labile (i.e., more basic) amines tend to acquire Cl+ quickly and may pass it on to less labile amines as the system drifts toward metastable equilibrium (1). Consequently, the assemblage of Cl+-bearing molecules that constitute total residual chlorine matures with time. With few exceptions (2, 3), our current understanding of how transfer reactions influence maturation of total * Corresponding author phone: (301) 405-1797; fax: (301) 3149121; e-mail:
[email protected]. † University of Maryland. ‡ National Institute of Standards and Technology. 10.1021/es035064z CCC: $27.50 Published on Web 01/31/2004
2004 American Chemical Society
residual chlorine relies on laboratory studies of low relative molecular mass (Mr) amino compounds (4-11). In many cases, chloramines (e.g., NH2Cl, RNHCl) become the principal molecular carriers of oxidative chlorine in chlorinated wastewaters (2, 3). Concentrations of soluble reduced nitrogen in treated municipal wastewaters fall in the 10-3 to 10-5 mol/L range (5, 12-15), whereas chlorine doses are usually
