Kinetics and formation of disinfection byproducts during iohexol chlor(am)ination

Abstract

In this study, the degradation kinetics, and the formation of disinfection byproducts (DBPs) during iohexol chlor(am)ination were studied. The chlorination kinetics can be described using a second-order model. As the pH of the solution increased, the apparent second-order rate constant of iohexol chlorination increased considerably. The reaction rate constants of iohexol with HOCl and OCl− were calculated as (3.75±0.70)×10-4 and (1.39±1.19)×10-4 M−1 s−1, respectively. In addition, the chloramination of iohexol followed tertiary kinetics, and the maximum apparent rate constant of 3.78×10-6 M−1 s−1 occurred at pH 7. NH4+ can promote the degradation of iohexol during chlorination, whereas Br− inhibits iohexol degradation during chloramination. The formation of conventional carbonated DBPs and emerging nitrogenated DBPs during chlor(am)ination of iohexol was measured. However, in the presence of bromide, the conversion of iohexol to toxic iodinated DBPs increased significantly during chlorination, and chlorination resulted in higher formation of toxic I-DBP compared to chloramination. Therefore, chloramine disinfection is preferred for treating raw water containing high bromide concentrations in terms of DBP formation control.