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

Abstract

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.