Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment

Oxidative treatment of iodide-containing waters can form toxic iodinated disinfection byproducts (I-DBPs). To better understand the fate of iodine, kinetics, products, and stoichiometries for the reactions of ferrate­(VI) with iodide (I–) and hypoiodous acid (HOI) were determined. Ferrate­(VI) showed considerable reactivities to both I– and HOI with higher reactivities at lower pH. Interestingly, the reaction of ferrate­(VI) with HOI (k = 6.0 × 103 M–1 s–1 at pH 9) was much faster than with I– (k = 5.6 × 102 M–1 s–1 at pH 9). The main reaction pathway during treatment of I–-containing waters was the oxidation of I– to HOI and its further oxidation to IO3 – by ferrate­(VI). However, for pH > 9, the HOI disproportionation catalyzed by ferrate­(VI) became an additional transformation pathway forming I– and IO3 –. The reduction of HOI by hydrogen peroxide, the latter being produced from ferrate­(VI) decomposition, also contributes to the I– regeneration in the pH range 9–11. A kinetic model was developed that could well simulate the fate of iodine in the ferrate­(VI)-I– system. Overall, due to a rapid oxidation of I– to IO3 – with short-lifetimes of HOI, ferrate­(VI) oxidation appears to be a promising option for I-DBP mitigation during treatment of I–-containing waters.