WATER CHLORINATION CHEMISTRY - NONMETAL REDOX KINETICS OF CHLORAMINE AND NITRITE ION

The first step in NH2Cl oxidation of NO2- is the general acid-assisted formation of nitryl chloride, where k(HA) is 7.6 X 10(6) M-2 s-1 for H3O+ and is 46 M-2 s-1 for H2PO4- (25.0-degrees-C, mu = 0.50 M). This is the rate-limiting step at very low [NH3] and high [NO2-] concentrations: HA + NH2Cl + NO2- -->(kHA) A- + NH3 + NO2Cl, where the subsequent reactions of NO2Cl are rapid. Under conditions of higher [NH3] and lower [NO2-], the overall rate of loss of NH2Cl is suppressed, and a complex rate expression is observed. This rate dependence is derived from an analogous mechanism found for the reaction between OCl- and NO2-: (-d[NH2Cl])/dt = {k1'(1 + (k2/k4)[NO2-])[NO2-][H+]}/ {(k-1'/k4) [NH3] + (1 + (k2/k4)[NO2-])}. In this mechanism, NO2Cl forms reversibly in the first step (k1'/k-1') and either reacts with a second NO2- (k2) to give N2O4 or loses Cl- (k4) to give NO2+; both N2O4 and NO2+ hydrolyze rapidly to give NO3-. The relative reaction rate of OCl- + H+ + NO2- is estimated to be 1.8 x 10(5) greater than for NH2Cl + H+ + NO2-. An equilibrium formation constant, K(f) = [NH2Cl]/([NH3][HOCl]) = 3.8 x 10(10) M-1, is calculated from the kinetic data.