Environ. Sci. Technol. IOQ4, 28, 331-337
Water Chlorination Chemistry: Nonmetal Redox Kinetics of Chloramine and Nitrite Ion Dale W. Margerum,. Lynn M. Schurter, JoLynn Hobson,? and Elizabeth E. Moore?
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 The first step in NHzCl oxidation of NOz- is the general acid-assisted formation of nitryl chloride, where km is 7.6 X loe M-2 s-l for H30+and is 46 M-2 s-l for HzP04- (25.0 "C, p = 0.50 M). This is the rate-limiting step at very low [NHJ and high [NOz-I concentrations: HA + NHzCl+
-
klu
+
NOzANH3 + NOZC1, where the subsequent reactions of NOzCl are rapid. Under conditions of higher [NH,] and lower [NOz-], the overall rate of loss of NHzCl is suppressed, and a complex rate expression is observed. This rate dependence is derived from an analogous mechanism found for the reaction between OC1- and NOz-: (-d[NHzClI)/dt = {ki'(l + (kz/k4)[NOz-l)[NOz-l [H+lj/ ((k-i'lk4) (1 + (kz/kd)[NOz-])]. In this mechanism, NOzCl forms reversibly in the first step (k~'/k-l')and either reacts with a second NOz- (kz)to give Nz04 or loses C1(k4) to give NOz+;both Nz04 and N02+ hydrolyze rapidly to give NO,-. The relative reaction rate of OC1- + H+ + NO2- is estimated to be 1.8 X 105 greater than for NH2Cl + H+ + NOz-. An equilibrium formation constant, Kf= [NHZC~]/([NHJ[HOC~I) = 3.8 X 1O1O M-l, is calculated from the kinetic data.
+
Introduction
-d[NHzC1] =
dt ["I, To'9o [NH4+]). Fractional reaction orders of this type are unusual except in cases of free radical chain mechanisms (3, 4). This suggests the reaction follows a complex mechanism. Valentine tested a mechanism (eqs 3 and 4), but found that it did not fit his data. It is an impossible NHzC1+ HzO s HOCl + NH,
+ HzO
-
NH4++ NO;
+ C1-
+ Undergraduate research students. 0013-938X194/0928-0331$04.50/0
0 1994 American Chemical Society
C1- + NO;
(3)
+ H+
(4)
mechanism because some of his observed rate constants are 105 times larger than the known rate constant ( 5 ) for the hydrolysis of chloramine (eq 3). We concur with this conclusion and propose a mechanism that explains the apparent fractional reaction orders. Recent studies by Johnson and Margerum (6) have shown that hypochlorite oxidation of nitrite proceeds by C1+ transfer from HOCl to NOz- to give NOzCl (nitryl chloride) as an intermediate. Nitryl chloride then reacts with a second NOz- to form NzO4 or dissociates into the very reactive nitronium ion (NOz+). The complete mechanism given by eqs 5-10 is well documented (6-8). Kh
+ HzO F? HOCl + OHki HOCl + NO; 8 NOzC1+ OHOC1-
(1)
very favorable. This reaction is of environmental importance in wastewater chlorination processes, where it is possible to have mixtures that contain both nitrite and chloramine (1,2).The principle nitrogenous pollutants in municipal wastewaters fall into three main categories: ammonia, organic nitrogen compounds, and oxoanions (nitrite and nitrate). Organic nitrogen is predominantly in the form of amino acids, polypeptides, and proteins. Nitrite and nitrate are present in wastewaters that have undergone biological nitrification; nitrite is formed as an intermediate in this process. Industrial or agricultural discharges may also contain nitrite and nitrate (2). Chlorination of a biologically treated industrial or agricultural effluent could potentially give solutions that contain monochloramine and nitrite. Direct and indirect reactions of nitrite with chloramine could be a source of chlorine demand ( I , 2). Although the reaction between nitrite and chloramine is relatively slow, it is important to understand the reaction kinetics. Knowledge of the kinetics and mechanism of the chloramine-nitrite reaction will aid in predicting which reactions can and cannot be discounted under environmental conditions. Valentine studied the reaction kinetics of the reaction in eq 1 and reported (1) the empirical rate = [NH31 + expression given in eq 2 (where ["BIT
-
HOCl + NO;
Chloramine can oxidize nitrite ion to nitrate ion, where the overall reaction given by eq 1is thermodynamically NHzC1+ NO;
[ [H~11~73[N0;11~65~NHzC110~95
(5)
(6)
k-i
ka
NOzC1+ NO;
8
-
k-a
N204+ OH-
ks
NO;
Nz04+ C1-
+ NO; + H+
k4
(7) (fast)
(8)
+ C1-
NOzC1 e NO: k-4
+
k6
NOz+ OH- NO; + H+ (fast) (10) We propose a similar mechanism for the chloramine reaction with nitrite. The first step is given by eq 11,and H+ + NHzC1+ NO;
ki'
NH,
+ N0,Cl
(11)
k-1'
the subsequent steps are shown in eqs 7-10. Under all conditions studied, the reactions are first-order in "$21. The rate expression in eq 12 is derived from the proposed -d[NHzC1l dt
- kl'[H+l [NHZCll[NO;I(k, + k,[NO;l) k-1'[",1 + (k4 + kz[NO;l) (12)
mechanism on the basis of a steady-state approximation Envlron. Scl. Technol., Vol. 28, No. 2, 1994 331
1.4
Table 1. pK. Values at 25 O C , p = 0.50 M species
PKa
1
ref
NH4+ H2P04HOCl
9.32 U 6.46 b 7.50 C H20 pK, 13.61 d "02 2.97 e Nozaki, T.;Mse, T.;Torii,K. Nippon Kagaku Kaishi 1973,2030. Mesner,R.E.;Baes, C. F. J.Solution Chem. 1974,3,307. Fogelman, K. D.; Walker, D. W.; Margerum, D. W. Znorg. Chem. 1989,28,986. Molina,M.; Melios, C.;Tognolli, J. 0.;Luchiari, L. C.;Jafelicci, M., Jr. J. Electroanal. Chem. Interfacial Electrochem. 1979,105, 237. e Tummavuori, J.; Lumme, P. Acta Chem. Scand. 1968, 22, 2003.
*
0 0
r.
U
f!
0.7
0 (I)
n Q:
for the NOzCl concentration. All data are shown to fit this rate expression rather than the fractional reaction orders in eq 2. The rate dependence in [NOz-I is complex because of the dual pathway for nitryl chloride decomposition (eqs 7 and 9). The rate of the reaction between NHzCl and NOz- is much slower than the rate between HOCl and NOz-. As a result, at low [NH31 and high [NOz-I, where k-x'[NH~l
