Iodate and Iodo-Trihalomethane Formation during Chlorination of Iodide-Containing Waters: Role of Bromide

Abstract

The kinetics of iodate formation is a critical factor in mitigation of the formation of potentially toxic and off flavor causing iodoorganic compounds during chlorination. This study demonstrates that the formation of bromine through the oxidation of bromide by chlorine significantly enhances the oxidation of iodide to iodate in a bromide-catalyzed process. The pH-dependent kinetics revealed species specific rate constants of k(HOBr + IO-) = 1.9 × 106 M-1 s-1, k(BrO- + IO-) = 1.8 × 103 M-1 s-1, and k(HOBr + HOI) < 1 M-1 s-1. The kinetics and the yield of iodate formation in natural waters depend mainly on the naturally occurring bromide and the type and concentration of dissolved organic matter (DOM). The process of free chlorine exposure followed by ammonia addition revealed that the formation of iodo-trihalomethanes (I-THMs), especially iodoform, was greatly reduced by an increase of free chlorine exposure and an increase of the Br-/I- ratio. In water from the Great Southern River (with a bromide concentration of 200 µg/L), the relative I-incorporation in I-THMs decreased from 18 to 2% when the free chlorine contact time was increased from 2 to 20 min (chlorine dose of 1 mg Cl2/L).This observation is inversely correlated with the conversion of iodide to iodate, which increased from 10 to nearly 90%. Increasing bromide concentration also increased the conversion of iodide to iodate: from 45 to nearly 90% with a bromide concentration of 40 and 200 µg/L, respectively, and a prechlorination time of 20 min, while the I-incorporation in I-THMs decreased from 10 to 2%.