Oxidation of Iodide and Hypoiodous Acid in the Disinfection of Natural Waters

In aqueous oxidative processes with ozone (O3), chlorine, or chloramine, naturally occurring iodide (I-) can easily be oxidized to hypoiodous acid (HOI) which can react with natural organic matter (NOM) or be further oxidized to iodate (IO3-). Such processes can be of importance for the geochemistry of iodine and for the fate of iodine in industrial processes (drinking water treatment, aquacultures). Whereas IO3- is the desired sink for iodine in drinking waters, iodoorganic compounds (especially iodoform, CHI3) are problematic due to their taste and odor. To assess the sink for iodine during oxidation of natural waters, we determined the kinetics of several oxidation reactions of HOI. Ozone, chlorine, and chloramine have been tested as potential oxidants. Ozone oxidized both HOI and hypoiodite (OI-) (kO3+HOI = 3.6 × 104 M-1 s-1; kO3+OI− = 1.6 × 106 M-1 s-1) in a fast reaction. Chlorine species oxidized HOI by a combination of second- and third-order reactions (k' 'HOCl+HOI = 8.2 M-1 s-1; k' ''HOCl+HOI = 8.3 × 104 M-2 s-1; kOCl-+HOI = 52 M-1 s-1). Monochloramine did not further oxidize HOI. The probability of the formation of iodoorganic compounds during drinking water disinfection therefore increases in the order O3 < Cl2 < NH2Cl. In aquacultures, I- is transformed to IO3- within seconds to minutes in the presence of chlorine or ozone. In the surface boundary layer of seawater, O3 oxidizes I- to HOI but not to IO3-.