Iodate and iodo-trihalomethane formation during chlorination of iodide-containing waters: Role of bromide

Justine Criquet (1) , Sebastien Allard (2) , Elisabeth Salhi (3) , Cynthia Joll (2) , Anna Heitz (2) , Urs von Gunten (3)

1 LASIRE - Laboratoire Avancé de Spectroscopie pour les Intéractions la Réactivité et l'Environnement - UMR 8516
2 Curtin University
3 EAWAG - Swiss Federal Insitute of Aquatic Science and Technology [Dübendorf]

The kinetics of iodate formation is a critical factor in mitigation of the formation of potentially toxic and off flavour 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-catalysed 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 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 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 minutes (chlorine dose 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%.

Domaines: Autre; Ingénierie de l'environnement

Justine Criquet, Sebastien Allard, Elisabeth Salhi, Cynthia Joll, Anna Heitz, et al.. Iodate and iodo-trihalomethane formation during chlorination of iodide-containing waters: Role of bromide. Environmental Science and Technology, 2012, 46 (13), pp.7350-7357. ⟨10.1021/es301301g⟩. ⟨hal-02592865⟩