Evaluation of a chloramine decomposition model incorporating general acid catalysis

Abstract

An existing model describing the slow decomposition of chloramines in the presence of excess ammonia was modified to incorporate general acid catalysis of monochloramine disproportionation, a key reaction in determining the rate of chloramine loss. Experiments were conducted at various reaction conditions in the presence of varying concentrations of phosphate which was used as an analog for other naturally occurring proton donors. Results were used to obtain an estimate of the specific rate constant characterizing the effect of H 2PO 4 − and to demonstrate the validity of the overall model formulation. Measured chloramine concentrations compared favorably with predicted values indicating that inclusion of the catalytic effect on monochloramine disproportionation alone appears justified in assessing the effect of phosphate. By analogy, it is expected that similar inorganic proton donors such as those of carbonate and silicate would also potentially affect only monochloramine disproportionation and that the modified model should be better capable of assessing chloramine decay in natural waters.