Kinetic and Mechanistic Aspects of the Reactions of Iodide and Hypoiodous Acid with Permanganate: Oxidation and Disproportionation
Oxidation kinetics of iodide and HOI/OI- by permanganate were studied in the pH range of 5.0-10.0. Iodide oxidation and iodate formation were faster at lower pH. The apparent second-order rate constants (k(obs)) for iodide oxidation by permanganate decrease with increasing pH from 29 M-1 s(-1) at pH 5.0 and 6.9 s(-1) at pH 7.0 to 2.7 M-1 s(-1) at pH 10.0. keys for HOI abatement are 56 M-1 s(-1) at pH 5.0, 2.5 M-1 s(-1) at pH 7.0, and 173 M-1 s(-1) at pH 10.0. Iodate yields over HOI abatement decrease from 98% at pH 6.0 to 33% for pH > 9.5, demonstrating that HOI disproportionation dominates HOI transformation by permanganate at pH >= 8.0. MnO2 formed as a product from permanganate reduction, oxidizes HOI to iodate for pH < 8.0, and promotes HOI disproportionation for pH > 8.0. The rate of HOI oxidation or disproportionation induced by MnO2 is much lower than for permanganate. During treatment of iodide-containing waters, the potential for iodinated disinfection byproducts (I-DBPs) formation is highest at pH 7.0-8.0 due to the long lifetime of HOI. For pH < 6.0, HOI/I-2 is quickly oxidized by permanganate to iodate, whereas for pH > 8.0, HOI/OI- undergoes a fast permanganate-mediated disproportionation.