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Kinetic and mechanistic aspects of the reactions of iodide and hypoiodous acid with permanganate: oxidation and disproportionation Xiaodan Zhao, Elisabeth Salhi, Huiling Liu, Jun Ma, and Urs von Gunten Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00320 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on April 6, 2016

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Environmental Science & Technology

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Kinetic and mechanistic aspects of the reactions of iodide and

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hypoiodous acid with permanganate: oxidation and

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disproportionation

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Xiaodan Zhao † §, Elisabeth Salhi §, Huiling Liu †, Jun Ma

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*†

, Urs von Gunten

*§ ⊥

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8

†

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Technology, Harbin 150090, China

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

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§

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CH-8600 Dübendorf, Switzerland

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Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133,

⊥

School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique

Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

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*Corresponding authors:

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Jun Ma; Tel: +86-451-86282292; Fax: +86-451-86283010; Email: [email protected]

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Urs von Gunten; Tel. +41-58-765-5270; Fax: +41-58-765-5210; Email: [email protected]

1

ACS Paragon Plus Environment


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Abstract

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Oxidation kinetics of iodide and HOI/OI- by permanganate were studied in the pH

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range 5.0-10.0. Iodide oxidation and iodate formation were faster at lower pH. The

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apparent second order rate constants (kobs) for iodide oxidation by permanganate

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decrease with increasing pH from 29 M-1s-1 at pH 5.0, 6.9 M-1s-1 at pH 7.0, to 2.7 M-

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1 -1

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and 173 M-1s-1 at pH 10.0. Iodate yields over HOI abatement decrease from 98% at

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pH 6.0 to 33% for pH ≥ 9.5, demonstrating that HOI disproportionation dominates

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HOI transformation by permanganate at pH ≥ 8.0. MnO2 forms as a product from

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permanganate reduction, oxidizes HOI to iodate for pH < 8.0 and promotes HOI

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disproportionation for pH ≥ 8.0. The HOI oxidation or disproportionation induced by

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MnO2 is not comparable to permanganate. During treatment of iodide-containing

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waters, the potential for the iodinated disinfection by-products (I-DBPs) formation is

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highest at pH 7.0-8.0 due to the long lifetime of HOI. For pH < 6.0, HOI/I2 is quickly

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oxidized by permanganate to iodate whereas for pH ≥ 8.0, HOI/OI- undergoes a fast

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permanganate-mediated disproportionation.

s at pH 10.0. kobs for HOI abatement are 56 M-1s-1 at pH 5.0, 2.5 M-1s-1 at pH 7.0

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TOC ART

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Introduction

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Occurrence of iodide and its fate during oxidative water treatment

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According to a survey in the U.S.A. and in Canada, iodide is present in drinking

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waters in the range of 0.4-104 µg/L. 1 Iodide concentrations can be much higher under

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extreme conditions, such as ground-waters adjacent to halide rocks or oil field brines.

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2

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(HOI/I2), which can further be transformed to iodate or react with natural organic

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matter (NOM) to form iodinated disinfection by-products (I-DBPs).

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product of iodide oxidation by ozone, chlorine and chloramine.

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oxidation with birnessite, I2 was identified as the intermediate (pH 6.3-7.5).

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PbO2/I-/NOM system, triiodide was found to be the reactive iodine species due to

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iodide oxidation by PbO2 and high iodide concentrations (~1 mM). 10 At the air-water

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interface of microdroplets, the oxidation of iodide by ozone resulted in the formation

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of various iodine species including HOI, iodite, iodate, triiodide and molecular

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iodine.11

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HOI oxidation, disproportionation, and/or its reaction with organic compounds all

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contribute to the consumption of HOI.

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oxidants have been systematically studied under relevant water treatment conditions. 7,

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During oxidative water treatment, iodide can be oxidized to reactive iodine species

12-14

7

7

3-6

HOI is the

During iodide 8, 9

In the

Oxidation kinetics of HOI with various

The oxidation of HOI to iodate by ozone is fast with a high second order rate

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constant kO3+HOI=3.6×104 M-1s-1.

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this process is much slower due to the relatively low second order rate constants

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(kHOCl+HOI=8.2 M-1s-1; kOCl+HOI=52 M-1s-1).

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accelerated due to the formation of HOBr (kHOBr+OI=1.9×106 M-1s-1; kOBr+OI=1.8×103

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M-1s-1). 5 In chloramination, a low second order rate constant (kNH2Cl+HOI10 µM, a complete transformation of iodide to iodate is observed with a

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Mn(VII):iodate ratio of 2 (slope of straight line in Figure 1a). The corresponding

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reaction can be formulated as eq. (1).

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2MnO4-+ I- + 2H+ = IO3- + 2MnO2 + H2O

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For higher pH values (pH 7.0 and 10.0), more Mn(VII) is needed to achieve a

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complete oxidation of iodide due to other processes competing with the oxidation of

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iodide to iodate (see below).

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Stoichiometry of the permanganate-HOI reaction to iodate

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The stoichiometry for transformation of HOI by Mn(VII) to iodate was studied by

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adding 0-20 µM Mn(VII) to solutions containing HOI (5 µM) (Figure 1b). Iodate

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formation at pH 5.0 increases linearly with increasing Mn(VII) doses. 4 moles of

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Mn(VII) are consumed for forming 3 moles of iodate (slope of straight line in Figure

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1b). Based on this observation, the reaction can be formulated as eq. (2).

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4MnO4- + 3HOI + H+ = 3IO3- + 4MnO2 + 2H2O

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Similar to iodide, iodate yields decrease with increasing pH for the same

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permanganate dose (see below for explanation).

(1)

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(2)

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Evolution of reactive iodine and iodate during oxidation of iodide with

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permanganate

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Kinetic experiments were performed to investigate the evolution of reactive iodine

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from the iodide oxidation by permanganate in the pH range 5.0-10.0 (Figure 2). The

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main iodine species under our experimental conditions were I-, I2, HOI and iodate.

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Triiodide concentrations are very low under the experimental conditions with iodide

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in the µM range and they were neglected. 33

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Iodide loss is determined by a mass balance including initial I-, reactive iodine and

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IO3-. kobs for the iodide oxidation by permanganate are determined by first order plots

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(Figure S2). They decrease significantly with increasing pH (Table 1). kobs for the

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iodide oxidation for pH 5.0 and 6.0 are 29 and 10 M-1s-1, respectively. These values

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are significantly lower than previously reported (177 and 59 M-1s-1 for pH 5.16 and

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6.13, respectively).

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phosphate buffer concentrations in the previous study. The influence of phosphate on

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reactive iodine oxidation kinetics will be elucidated below.

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As illustrated in Figure 2, the maximum concentration of reactive iodine (1.7 µM) at

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pH 5.0 is reached at 10 minutes. As pH increases from 5.0 to 7.0, reactive iodine

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reaches a higher level (2.2 µM for pH 7.0) because the iodate formation rate decreases.

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For pH > 7.0, HOI dominates the reactive iodine and its maximum HOI concentration

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is much lower with 0.28 µM at pH 10.0. Iodate formation is favored at lower pH

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values and decreases with increasing pH. In the pH range 8.0-10.0, no significant

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differences for iodate formation are observed and the reactive iodine concentrations

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remain constant, however, at lower levels.

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Kinetics of the transformation of HOI by permanganate to iodate

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This significant discrepancy can be attributed to the high

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Since the oxidation of iodide to iodate by permanganate occurs via HOI, the

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transformation of HOI by permanganate was investigated as a function of pH. In

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absence of oxidants (Figure S3), a slow disproportionation is the main pathway for its

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abatement (eq. (3)).

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3HOI

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Figure 3 shows the abatement of HOI and iodate formation in presence of

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permanganate for differing pH values. Apparent second order rate constants (kobs) for

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HOI/OI- abatement by permanganate depend strongly on pH (Table 1, Figures S4a-c).

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HOI disproportionation in absence of permanganate can be neglected for the

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investigated time scale (Figure S3). At pH < 8.0, kobs for HOI/OI- abatement decreases

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with increasing pH while at pH > 8.0, a significant increase is observed.

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A plot of △IO3- vs △HOI for various pH values exhibits good linear correlations

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(Figure S4e). The slopes of these straight lines represent the iodate yields (Table 1).

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For pH ≤ 6.0, iodate yields from reactive iodine are almost 100%, indicating a

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complete oxidation of HOI by permanganate to iodate. For increasing pH values up to

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9.5, the iodate yields decrease gradually to approximately 33%, corresponding to the

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stoichiometry of iodate formation from HOI disproportionation (eq. (3)). The iodate

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yields remain fairly constant for pH > 9.5 (Table 1). Under these conditions the

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transformation of HOI/OI- to iodate in presence of permanganate is entirely controlled

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by disproportionation. In summary, oxidation of HOI to iodate dominates in the pH

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range 5.0-7.0, while disproportionation dominates at pH > 9.5. In the pH range 7.0-9.0,

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both oxidation and disproportionation contribute to HOI consumption and iodate

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formation in presence of permanganate.

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Role of the permanganate concentration on iodate formation from iodide and

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HOI

IO3- + 2I- + 3H+

(3)

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The effect of the permanganate concentration on iodate formation from iodide at pH

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7.0 is shown in Figure 4a. The rate of iodate formation increases with increasing

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permanganate concentration. Figure S5 shows the corresponding pseudo-first order

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kinetics. lnkobs from Figure S5 is linearly correlated to ln([Mn(VII)]) with a slope of 1

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(Figure S7a), which suggests a first order reaction with respect to permanganate.

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Figure 4b shows the kinetics of iodate formation in the permanganate-HOI system as

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a function of the permanganate concentration. Figure S6 shows the corresponding

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pseudo-first order plots. lnkobs from Figure S6 is linearly correlated to ln([Mn(VII)])

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with a slope of 0.59 (Figure S7b), indicating that the reaction kinetics with respect to

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the permanganate concentration is not first order. The iodate yields increase from 48%

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to 85% for increasing permanganate concentration (10-100 µM) (Table S2). This

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trend points out that at higher permanganate levels, HOI oxidation to iodate by

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permanganate becomes favorable compared to the disproportionation. For pH ≤ 7.0,

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the HOI abatement rate constant increases with decreasing pH and increasing

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permanganate concentrations with the apparent reaction order of 0.59 (Figure S7b)

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and 0.68 (Figure S4d) with respect to [Mn(VII)] and [H+], respectively. For pH ≤ 7.0,

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k for HOI abatement can be described as follows (eq. (4)). The coefficient (2192) in

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this equation is calculated from the intercept (0.79) of the straight line in Figure S4d

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taking the permanganate concentration (50 µM) into consideration.

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k = 2192×[Mn(VII)]0.59×[H+]0.68

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Eq. (4) allows calculating the apparent third order rate constants for the abatement of

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HOI as function of the permanganate concentration and the pH.

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Effect of phosphate on HOI oxidation by permanganate

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Figure 5 illustrates the effect of varying phosphate concentrations on HOI abatement

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at pH 7.0. The concentrations of HOI and iodate were measured simultaneously. An

(4)

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enhancement of HOI abatement is observed in presence of phosphate and in absence

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of permanganate (Figure 5a). Oxidation of HOI is significantly accelerated by

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permanganate with iodate yields of ~100% (Figures 5b and c). This indicates that the

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disproportionation is negligible under these conditions. Phosphate promotes iodate

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formation via a base-catalyzed HOI oxidation by permanganate.

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Role of MnO2 in the iodate formation from HOI

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It has been reported previously that metal oxides, such as cupric oxide, can enhance

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the disproportionation of HOCl, HOBr and ClO2, leading to formation of ClO3- and

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BrO3-.

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10.0 in analogy to permanganate (Figure 6a). However, the HOI abatement promoted

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by permanganate (Figure 6b) is more effective than for MnO2 (Figure 6a). The half-

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lives of HOI (5 µM) at pH 10.0 are 20 min and 4 min for MnO2 (10 µM) and

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permanganate (10 µM), respectively. Therefore, the presence of MnO2 plays a minor

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role for the Mn(VII)-induced HOI abatement (Figure S13).

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Figure S8 shows that △IO3- and △HOI are linearly correlated for varying pH values

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with two different slopes (numerical data in Table S3). Two main mechanisms can be

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postulated. For pH 5.0-7.0, HOI oxidation is the major pathway with an iodate yield

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of ca. 90%. For the pH range 8.0-11.0, the MnO2-promoted HOI disproportionation is

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dominant with an iodate yield of ~33%. These findings are in contrast to CuO, for

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which no HOI disproportionation was found. 35 The maximum rate for HOI abatement

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is observed close to the pKa of HOI (10.4) (Figure S12).

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HOI and OI- are involved in the MnO2-catalyzed reaction.

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Mechanistic aspects for the MnO2-HOI reactions

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To better understand the underlying processes, a kinetic model is proposed for the

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MnO2-HOI system (model 1, SI). It is hypothesized that MnO2, which can be

34-36

The HOI abatement is significantly promoted in presence of MnO2 at pH

19

12


Thus, it seems that both

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considered as a Lewis acid, leads to a complexation of HOI, thereby enhancing its

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reactivity towards another HOI/OI-. The MnO2-promoted HOI transformation is

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initiated by a fast pre-equilibrium step with the formation of a MnO2-HOI complex

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(reactions A7-A8, Table S4). For pH ≤ 7.0, the formation of IO2- is rate-limiting

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(reaction A9, Table S4). IO2- is subsequently transformed to iodate (reaction A10,

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Table S4). For pH ≥ 8.0, the reaction between MnO2-HOI and OI- is the dominant

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reaction pathway (reactions A11-A13, Table S4). According to this model, HOI

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transformation and iodate formation in presence of MnO2 as a function of pH and

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MnO2 concentration can be well simulated (Figures 6a (pH 10.0) and S12, lines).

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Mechanistic aspects of the permanganate-iodide system

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Scheme 1 shows the potential iodate formation mechanisms for iodide-containing

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waters in presence of permanganate based on the above experimental data. To

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simplify the discussions of this two-step process, the HOI transformation in presence

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of permanganate will be explained first (Scheme 1, reactions 4-5). Thereafter, the

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oxidation of iodide to reactive iodine by permanganate will be discussed (Scheme 1,

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reactions 1-3).

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Oxidation and disproportionation of HOI in presence of permanganate (Scheme 1,

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reactions 4-5)

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In various oxidation processes of iodide, bromide, nitrous acid and sulfite with

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permanganate, a rapid complex formation between permanganate and the substrate

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has been proposed.

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model is proposed for HOI transformation by permanganate (model 2, SI). HOI/OI-

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forms Mn(VII)-HOI/OI- complexes with permanganate in a fast pre-equilibrium step

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(reactions B7-B10, Table S5). For pH ≤ 7.0, the rate-determining step is a pH-

24, 37-39

In analogy to these previous studies, a conceptual kinetic

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dependent (reaction B11, Table S5). Thereby, HOI is directly oxidized by

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permanganate to iodate in a two-step process (Scheme 1, reaction 4).

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During disproportionation (pH ≥ 8.0), the electrophilic character of HOI/OI- is

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enhanced due to a Mn(VII)-HOI/OI- complex formation (Scheme 1, reaction 5).

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Conceptually, Mn(VII)-HOI/OI- reacts predominantly with OI- (reactions B13 and

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B14, Table S5). The second order rate constant for the reaction of Mn(VII)-HOI and

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OI- (reaction B13) was determined to be three orders of magnitude higher than for the

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reaction between Mn(VII)-HOI and HOI (reaction B15, Table S5). Reaction B15

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plays an important role below pH 8.0 and reaction B13 is dominant in the pH range

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8.0-10.5. For pH > 10.5, the rate for HOI abatement increases (Figure 3), which can

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be explained by the reaction of Mn(VII)-OI- with OI- (reaction B14, Table S5).

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Reaction B14 is the dominant reaction for pH > 10.5 due to the high fraction of OI-.

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With the proposed model (Table S5), the experimental data for HOI abatement and

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iodate formation can be reproduced quite well (Figure 3, lines). Iodate formation from

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HOI as function of permanganate concentration at pH 7.0 and 10.0 can also be

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modelled (Figures 4b and 6b). In Figure 4b, the simulations are within a factor of 2,

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which is still reasonable considering the complexity of the model.

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Oxidation of iodide to reactive iodine by permanganate (Scheme 1, reactions 1-3)

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It has been reported that a rapid pre-equilibrium occurs between Mn(VII) and I- to

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form a Mn(VII)-I- complex, which decomposes to HOI and Mn(V).

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information, a conceptual kinetic model is proposed (Table S6, model 3). The two

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reaction pathways for HOI formation from the Mn(VII)-I- complex are pH-dependent

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(pH ≤ 7.0) and pH-independent (pH ≥ 9.0), respectively. In the pH range 5.0-7.0, I2 is

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an important species (Scheme 1, reaction 2) while at pH > 7.0, HOI is the major

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product (Scheme 1, reaction 1). It is hypothesized that I2 exhibits a similar mechanism 14


24

Based on this

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as HOI toward permanganate. Because I2 dominates the reactive iodine at lower pH, it

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is postulated that I2 is oxidized by permanganate to iodate (Scheme 1, reaction 3).

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Based on models 2 and 3, iodide loss, reactive iodine evolution and iodate formation

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in presence of permanganate as a function of pH can be well simulated (Figure 2,

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lines). The simulations for the effect of the permanganate concentration on iodate

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formation from iodide are acceptable (Figure 4a, lines). The model represents the

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experimental trends quite well. The modeled concentrations are within a factor of two

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compared to the results. Even though this is a reasonable agreement, there might still

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be some additional reactions that are not covered in the model.

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Implications for water treatment. The experimental results from this study allow

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estimating the evolution of reactive iodine during permanganate treatment of iodide-

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containing water as function of the pH and the permanganate concentration. This

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includes reactive iodine formation from iodide and its further oxidation or

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disproportionation to iodate. Our findings suggest that reactive iodine species are

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more stable at circumneutral pH than under more acidic and basic conditions. To

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assess permanganate treatment relative to other common oxidants, half-lives of iodide

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and HOI for various oxidants are listed in Table 2. The lifetime of HOI is crucial for

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the potential formation of I-DBPs (see introduction). For a typical drinking water

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treatment with a concentration of 10 µM (~1.6 mg/L) KMnO4 26, the half-life of HOI

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is 2.8 or 5.2 h for pH 7.0 or 8.0, respectively, calculated from the kinetic parameters

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determined in this study. For permanganate, the half-life time of HOI is lower than for

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chloramine, however, significantly higher than for ozone or chlorine. HOI is quickly

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further oxidized by ozone to iodate, with no formation of I-DBPs.

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chlorination, this reaction also occurs, however, significant concentration of mixed I-

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DBPs can be formed depending on the water quality parameters. 15


40

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During

During


334

chloramination, HOI has the highest half-life, enabling the formation of I-DBPs. In

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this case only I-DBPs and no mixed Cl-Br-I-DBPs are formed, which leads to a lower

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overall DBP formation in certain cases than for chlorine. 41 This case is probably quite

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similar to permanganate for which also only I-DBPs are expected. Therefore, the

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formation of I-DBPs should be considered if waters containing higher iodide levels

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are treated with permanganate.

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(a)

5 ∆ Mn(VII) =2 ∆ Iodate

4

pH5.0 pH7.0 pH10.0

3 2 1

Iodate concentration/(µM)

Iodate concentration/(µM)

5

0

4 3 2

pH5.0 pH7.0 pH10.0

∆Mn(VII) 4 = 3 ∆Iodate

1 0

0

341 342

(b)

5

10 [Mn(VII)]/(µM)

15

20

0

5

10 [Mn(VII)]/(µM)

15

20

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Figure 1. Iodate formation: stoichiometry of (a) the permanganate–iodide and (b)

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permanganate–HOI reactions at differing pH values. Experimental conditions: [I-]0=5

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µM or [HOI]0=5 µM; reaction time (iodide): 48 h, reaction time (HOI): 72 h,

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T=23±1 ℃. Error bars represent the standard deviation of duplicate experiments

347

(invisible bars are within the symbols).

17



pH5.0

Iodine/(µM)

4 3 Iodide Reactive iodine Iodate

2 1

0

20

40

60 80 Time/(min)

100

3

2

2

1

1 0 0

120

20

40

60 80 Time/(min)

100

120

pH9.0

(e)

pH8.0 5

(d)

5

0

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3

3

3

2

2

2

1

1

1

Iodine/(µM)

0

0 0

20

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60 80 Time/(min)

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120

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60 80 Time/(min)

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120

pH10.0

(f)

5

4

4

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4

3

0

0

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4

pH7.0

(c)

pH6.0 5

(b)

5

Iodine/(µM)

(a)

5

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0

0

20

40

60 80 Time/(min)

100

120

0

20

40

60 80 Time/(min)

100

120

350 351

Figure 2. Oxidation of iodide by permanganate as a function of pH (5.0-10.0):

352

experimental data (symbols) and model calculations (lines) for the evolution of iodine

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species. Experimental conditions: [I-]0=5 µM, [Mn(VII)]=50 µM, T=23±1 ℃. Error

354

bars represent the standard deviations of duplicate experiments (invisible bars are

355

within the symbols).

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4

4

4

3

3

3

2

2

2

1

1

1

0

0

HOI Iodate

0

2

4

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6 8 10 Time/(min)

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(d)

5

14

16

pH8.0

40

60 80 Time/(min)

100

120

2

2

1

1

0

0 0

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40

60 80 Time/(min)

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(g)

120

Iodine/(µM)

3

2

2

1

1

0

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0

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6 8 10 Time/(min)

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60 80 Time/(min)

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(f)

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pH9.5

3 2

0 20

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60 80 Time/(min)

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0

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pH10.5

(h)

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pH11.0

(i)

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pH7.0

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pH9.0 Iodine/(µM)

Iodine/(µM)

3

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358 359

20

(e)

5

3

(c)

0 0

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4

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pH6.0 5

(b)

4 Iodine/(µM)

Iodine/(µM)

pH5.0 5

(a)

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3 2 1 0

0

2

4

6 8 10 Time/(min)

12

14

16

0

2

4

6 8 10 Time/(min)

12

14

16

360

Figure 3. Evolution of reactive iodine (HOI) and iodate from HOI transformation in

361

presence of permanganate as a function of pH (5.0-11.0): experimental data (symbols)

362

and model simulations (lines). Experimental conditions: [HOI]0=5 µM, [Mn(VII)]=50

363

µM, T=23±1 ℃. Error bars represent the standard deviations of duplicate experiments

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19



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365

Table 1. Apparent second order rate constants (kobs) for iodide/HOI abatement in

366

presence of permanganate and iodate yields (△IO3-/△HOI) during HOI abatement in

367

presence of permanganate for various pH values. Experimental conditions:

368

[Mn(VII)]=50 µM, [I-]0 or [HOI]0=5 µM, T=23±1 ℃. pH

kobs

for

iodide

oxidation (M-1s-1)

kobs

for

HOI

abatement (M-1s-1)

△IO3-/△HOI

R2 of △IO3-/△HOI

(%)

(Coeffecient of the linear correlation)

5.0

29±1.5

56±2.3

87

0.994

5.5

13±1.1

24±1.6

92

0.992

6.0

10±1.1

9.3±1.5

98

0.999

7.0

6.9±1.3

2.5±0.5

80

0.998

8.0

5.3±0.5

3.8±0.5

51

0.992

9.0

3.3±0.5

12±1.4

41

0.996

9.5

2.3±0.2

55±1.2

34

0.997

10.0

2.7±0.3

173±4.5

31

0.997

10.5

32

0.999

11.0

32

1.000

20


Page 21 of 30


2.5

(a) 10µM 25µM 50µM 75µM 100µM

Iodate formation/(µM)

2.0 1.5 1.0 0.5 0.0

10µM 25µM 50µM 75µM 100µM

1.5 1.0 0.5 0.0

0

369

(b)

2.0 Iodate formation/(µM)

2.5

25

50

75 100 125 Time/(min)

150

175

200

0

25

50

75 100 125 Time/(min)

150

175

200

370 371

Figure 4. Role of permanganate concentrations (10 – 100 µM) for the kinetics of

372

iodate formation from (a) iodide and (b) HOI at pH 7.0 (symbols: experimental data;

373

lines: model simulations). Experimental conditions: [I-]0=2.5 µM, [HOI]0=2.5 µM,

374

T=23±1 ℃. Error bars represent the standard deviations of duplicate experiments

375


21



HOI concentration/(µM)

HOI concentration/(µM)

4 3 2 1

0mM 10mM 20mM 50mM

4 3 2 1 0

0

376

(b)

5

0

20

40

60 80 Time(min)

100

120

(c)

5 Iodate concentration(uM)

(a)

5

Page 22 of 30

4 3 2 1 0

0

20

40

60 80 Time(min)

100

120

0

20

40

60 80 Time(min)

100

120

377 378

Figure 5. Effect of phosphate concentrations (0 – 50 mM) on HOI transformation in

379

absence or presence of permanganate at pH 7.0. (a) HOI loss in absence of

380

permanganate; (b) HOI abatement in presence of permanganate; (c) iodate formation

381

in presence of permanganate. Experimental conditions: [HOI]0=5 µM, [Mn(VII)]=50

382

µM, T=23±1 ℃. Error bars represent the standard deviations of duplicate experiments

383


22


Page 23 of 30


(a)

5

0µM 5µM 10 µM

3 2 1

3 2 1

0

0 0

384

0 µM 5 µM 10 µM

4 Iodine/(µM)

Iodine/(µM)

4

(b)

5

20

40

60 80 Time/(min)

100

120

0

2

4

6 8 10 Time/(min)

12

14

16

385 386

Figure 6. HOI (closed symbols) oxidation and iodate (open symbols) formation at pH

387

10.0. (a) Effect of MnO2 dosage (0 – 10 µM); (b) effect of permanganate dosage (0 –

388

10 µM) (symbols: experimental data; lines: model simulations); Experimental

389

conditions: [HOI]0=5 µM, T=23±1 ℃. Error bars represent the standard deviations of

390

duplicate experiments (invisible bars are within the symbols).

23



391 392

Scheme 1. Iodate formation mechanism for the permanganate-iodide system: I- is

393

oxidized to reactive iodine (HOI (1) or I2 (2)), which reacts with Mn(VII) as follows:

394

(3) direct oxidation of I2 to iodate, (4) direct oxidation of HOI to iodate and (5)

395

Mn(VII)-promoted HOI disproportionation to iodate.

24


Page 24 of 30

Page 25 of 30


396

Table 2. Comparison of the apparent second order rate constants and half-lives for the

397

oxidation of iodide or HOI by permanganate and various other oxidants (ozone,

398

chlorine, monochloramine) at pH 7.0 and 8.0 with oxidant concentrations of 10 µM. kapp (M-1s-1) Oxidants

O3

HOCl

I-

of I-

9

2.0×10

4.3×108 2.4×103

NH2Cl

2

kapp (M-1s-1)

Half-life

HOI

of HOI

7.0

3.7×104

1.8 s

8.0

4.2×10

4

1.6 s

7.0

21

56 min

8.0

42

28 min

< 2×10-3

> 1 year

Half-life

References

pH

< 1 ms

< 1 ms

< 15

7.0

2.4×10

min

8.0

7.0

2.7 h

7.0

6.9

2.8 h

5.3

3.6 h

8.0

3.7

5.2 h

7, 42

7, 43

7, 44

This study

KMnO4

399

25



400

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401

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