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Received for review March 26,1992. Revised manuscript received September 28,1992. Accepted October 5,1992. This research was partially funded by Contract DE-ACOS-76SR00819 between the University of Georgia and the U.S. Department of Energy.
Distribution of Aqueous Chlorine with Nitrogenous Compounds: Chlorine Transfer from Organic Chloramines to Ammonia Jeyong Yoon and James N. Jensen"
Department of Civil Engineering, 212 Ketter Hail, State University of New
York
at Buffalo, Buffalo, New
York 14260
Scheme I. Chlorine-Transfer Reactions"
A kinetic model for chlorine transfer in the aqueous chlorine/ammonia/organic nitrogen system was constructed and used to explain the distribution of the chlorine species over time. Observed second-order rate constants for chlorine transfer from organic chloramines to M-' s-l for Nammonia were found to be 3.84 X chloroglycine, 1.55 X M-l s-l for N-chloroglycylglycine, M-' s-l for N-chloromethylamine (pH 6.8). and 5.85 X Chlorine transfer from organic chloramines to ammonia was found to be significant under simulated conditions of wastewater disinfection. The rates of chlorine transfer from organic chloramines to ammonia and chloramine hydrolysis both decreased with increasing basicity of the chloramine.
" k i jis the rate constant for the reaction of species i with species j . Unless otherwise stated, the rate constants are observed constants; i.e., kc,,, is the rate constant for the reaction of NH3 +
Background Chlorine usually is employed for the disinfection of wastewaters in the United States. In wastewater disinfection, aqueous chlorine combines with ammonia to form inorganic chloramines (e.g., monochloramine, NH,Cl). Chlorine also combines with nitrogenous organics (e.g., amino acids and proteinaceous material) to form organic chloramines. Since most organic chloramines have little or no bactericidal activity ( 1 , 2 ) ,it is necessary to understand the distribution of chlorine between inorganic chloramines and organic chloramines to achieve adequate
wastewater disinfection with minimal chlorine addition. The distribution of chlorine species depends not only on the relative chlorination rates and concentrations of nitrogenous compounds but also on the chlorine-transfer reactions between chloramines and nitrogenous compounds. Previous work by numerous investigators suggests that the distribution of active chlorine in a chlorine/ammonia/organic nitrogen system at chlorine/nitrogen molar ratios less than unity is controlled by the six reactions in
HOCl
HOCl
+ NHBkae. NHzCl + H 2 0 khm
+ RNHz
NHzC1 + RNHz
(1)
+ H20
(2)
k.te. NH3 + RNHCl ,k
(3)
ha:
RNHCl
NH,+ and HOCl + OCl-. The following abbreviations are used: C1, HOCl; a, ammonia; h, water (hydrolysis); m, monochloramine; 0,nitrogenous organic; oc, organic chloramine.
0013-938X/93/0927-04~3~04.0Ql00 1993 American Chemical Society
Environ. Sci. Technol., Vol. 27, No. 2, 1993
403
Scheme I. The reactions in this system can be divided into three major categories. First, chlorine reacts with ammonia and the nitrogenous organic (forward reactions in eqs 1 and 2). The reactions between aqueous chlorine and nitrogenous compounds have been studied more extensively than any of the other reactions in this system (3, 5). Chlorination rate constants generally are very large (>lo4M-' s-' at pH 7). Second, chloramines may hydrolyze back to liberate free chlorine (backward reactions in eqs 1and 2). The hydrolysis rates of monochloramine (6)and a few organic chloramines (5, 7) have been studied. Hydrolysis rate constants for organic chloramines are gens-l at pH 7). erally small Third, active chlorine may be transferred between chloramines and nitrogenous compounds (eq 3). The transfer of active chlorine between chloramines and nitrogenous compounds can occur by two routes: (1)chloramine hydrolysis to form free chlorine (kh,m and Ith,oc processes) with subsequent N-chlorination (kCl,,and Itcl,, processes) or (2) direct chlorine transfer (km,, and I t , , , processes). Hussain and colleagues (7) suggested that chlorine transfer between nitrogenous bases in water is not mediated by hydrolysis of the N-chloro compounds to hypochlorous acid, but rather involves a direct reaction between the nitrogenous chlorine donor and acceptor molecules. Several researchers reported second-order rate constants (k,,,) for chlorine transfer from monochloramine to organic amines (8,9).Isaac and Morris (9) suggested that hydrolysis is an important mechanism of chlorine transfer only at very low chloramine concentrations (
