Article pubs.acs.org/JPCA
Investigation of the Halogenate−Hydrogen Peroxide Reactions Using the Electron Paramagnetic Resonance Spin Trapping Technique Maja C. Pagnacco,*,† Miloš D. Mojović,† Ana D. Popović-Bijelić,† and Attila K. Horváth‡ †
Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12-16, P.O. Box 47, 11158 Belgrade, Serbia Department of Inorganic Chemistry, University of Pécs, Ifjúság u. 6, H-7624 Pécs, Hungary
‡
ABSTRACT: The differences in the mechanism of the halogenate reactions with the same oxidizing/reducing agent, such as H2O2 contribute to the better understanding of versatile halogen chemistry. The reaction between iodate, bromate, and chlorate with hydrogen peroxide in acidic medium at 60 °C is investigated by using the electron paramagnetic resonance (EPR) spin trapping technique. Essential differences in the chemistry of iodate, bromate, and chlorate in their reactions with hydrogen peroxide have been evidenced by finding different radicals as governing intermediates. The reaction between KIO3 and H2O2 is supposed to be the source of IO2• radicals. The KBrO3 and H2O2 reaction did not produce any EPR signal, whereas the KClO3−H2O2 system was found to be a source of HO• radical. Moreover, KClO3 dissolved in sulfuric acid without hydrogen peroxide produced HO• radical as well. The minimal-core models explaining the origin of obtained EPR signals are proposed. Current findings suggested the inclusion of IO2• and HOO• radicals, and ClO2• and HO• radicals in the particular kinetic models of iodate−hydrogen peroxide and chlorate−hydrogen peroxide systems, as well as possible exclusion of BrO2• radical from the kinetic scheme of the bromate−hydrogen peroxide system. Obtained results may pave the way for understanding more complex, nonlinear reactions of these halogen-containing species.
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X•, X−, etc.2,3 Because of the lack of thermodynamic data, as well as sensitive, selective, and fast experimental technique for monitoring very reactive, unstable, and short-lived halogen intermediates, reactions of these species are still a subject of intensive research.18,26−30 Nevertheless, understanding the oxyhalogen oscillatory systems is greatly hampered by the absence of detailed information on a potential difference in the chemistry of iodate, bromate, and chlorate analogues. Furthermore, the existing rate laws correctly describe the broad features of halogen reactions, but information on particular reactions involved in one- and two electron-transfer processes, which cause or eliminate the oscillatory behavior, is often omitted.2 According to all the facts mentioned above, the electron paramagnetic resonance (EPR) spin trapping technique for detection and identification of short-lived species with unpaired electrons seems to be a promising tool to investigate the intimate details of those systems where reactive radical species are involved. Understanding the chemistry of halogenate ions is also important to prevent, or at least to reduce, their formation in wastewater treatments. It is well-known that iodide, bromide, and chloride ions naturally present in water are easily oxidized by several disinfectants (usually chlorine or ozone) used in wastewater treatments, resulting in the formation of iodate, bromate, and chlorate ions.14,31 These compounds may be
INTRODUCTION One point of mechanistic interest in halogen reactions is the relative activity of oxy-halogens when they react with common oxidizing and/or reducing agents. Due to the stability of these reactants, reactions of XO3− (X = I, Br or Cl) with hydrogen peroxide (H2O2) in acidic solution are suitable model systems for such investigations. The chemistry of iodate, bromate, and chlorate species should receive considerable attention because they easily undergo electron-transfer reaction.1,2 This characteristic related to the significant role of oxy-halogens in complex oscillatory reactions,3−5 atmospheric,6−8 and marine9,10 chemistry, as well as important role in living cells11,12 and during water purification treatment.13,14 Numerous homogeneous oscillators have been discovered, involving halogen-containing oxyanions as oxidizing agents, such as iodate,5,15−18 bromate,19−22 and chlorite23−25 driven oscillators. The oxy-halogen species (iodate, bromate, and chlorate ions) are inevitable constituents of these oscillatory solutions. Potential differences in the chemistry of the iodate, bromate, and chlorate analogues lead to significantly different dynamical features of these corresponding oscillatory systems. Although these oscillators have been intensively investigated during a long period of time, their exact oscillatory mechanism still remains a challenge for nonlinear kineticists. Because oxyhalogen species easily undergo electron-transfer reaction, the corresponding one-electron versus two-electron-transfer processes play an important role in a detailed oscillatory mechanism.2 Highly complicated oscillatory redox systems involve species such as HXO3, HXO2, XO2•, XO•, HXO, X2, © XXXX American Chemical Society
Received: March 2, 2017 Revised: April 8, 2017 Published: April 12, 2017 A
DOI: 10.1021/acs.jpca.7b02035 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 1. Reaction Mixture Content and the Obtained EPR Signal after 25 min of the Reaction EPR signal after 25 min KClO3
KBrO3
+ + + +
KIO3
H2SO4
DEPMPO
H2O2
yes
+ +
+ + + + + + +
+ + + + + + +
+
+
+
carcinogenic and highly toxic, especially bromate.13,32,33 To avoid hazardous bromate concentration in drinking water, hydrogen peroxide has been used as an effective reducing agent.34 Understanding the corresponding oxy-halogen processes with hydrogen peroxide may contribute to the improvement of water purification treatment as well. The aim of this work was to examine the presence of free radicals in the reactions between iodate, bromate, and chlorate separately with hydrogen peroxide in acidic solutions, using the EPR spin trap technique, to reveal potential differences in the chemistry of these analogues.
comment DEPMPO/BL radical adduct
+ +
+ +
no
+ +
DEPMPO/OH radical adduct DEPMPO/OH radical adduct + +
peroxide and chlorate−hydrogen peroxide and for chlorate in sulfuric acid solution as well (Table 1). The measured EPR spectra for the reactions between iodate and chlorate with hydrogen peroxide in acidic solution after 25 min are shown in Figures 1 and 2, respectively. For the reaction
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Figure 1. EPR spectrum of the DEPMPO/iodine-centered adduct obtained in reaction between iodate and hydrogen peroxide in acidic solution at 60 °C. Hyperfine splitting constants: a(P) = 32.5 G, a(N) = 9.8 G, and a(H) = 12.5 G.35
EXPERIMENTAL SECTION The nitrone spin-trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) was used for the EPR spin-trapping measurements. Purity of DEPMPO was checked with 0.5 mM K3Fe(CN)6. The iodate−hydrogen peroxide reaction mixture was carried out in a volume of 120 μL at 60 °C that initially contained 7.00 × 10−2 M KIO3, 4.80 × 10−2 M H2SO4, 9.90 × 10−2 M H2O2, and 5 × 10−2 M DEPMPO. The appropriate bromate (KBrO3)− and chlorate (KClO3)−hydrogen peroxide reaction mixtures were of the same volume and initial concentrations. All the chemicals were obtained from Merck, except DEPMPO, which was obtained from Enzo Life Sciences. All solutions were prepared in Milli-Q deionized water (18.2 MΩ cm at 21 °C). The reaction mixture that contains the DEPMPO spin trap, the specific XO3−, and sulfuric acid was thermostated in a small cuvette of 1 mL at 60 °C for 5 min, prior to the addition of hydrogen peroxide. Subsequently, the reaction solution was thermostated at 60 °C for an additional 20 min. A 30 μL aliquot of the reaction mixture was drawn into gas-permeable capillary Teflon tubes (Zeus Industries, Raritan) and EPR spectra were immediately recorded. The same conditions (the mixture was 25 min at 60 °C and volume was 120 μL) and initial concentrations of halogen XO3− (7.00 × 10−2 M), sulfuric acid (4.80 × 10−2 M), and hydrogen peroxide (9.90 × 10−2 M) are used for particular measurement (Table 1). The EPR spectra were recorded on the Bruker Elexsys II E540 EPR spectrometer at room temperature under the following conditions: field center 3410 G, scan range 200 G, microwave frequency 100 kHz, modulation amplitude 2 G, microwave power 10 mW, and time constant 0.032 s.
Figure 2. EPR spectrum of the DEPMPO/OH radical adduct obtained in the reaction between potassium chlorate and hydrogen peroxide in acidic solution at 60 °C. The same signal was obtained in sulfuric acid solution of potassium chlorate. Hyperfine splitting constants: a(P) = 46.7 G, a(N) = 13.64 G, and a(H) = 12.78 G.36
between iodate and hydrogen peroxide, the obtained EPR signal (Figure 1) was assigned to the previously reported DEPMPO/iodine-centered radical adduct observed in Bray− Liebhafsky (BL) oscillatory reaction.35 EPR signal termed DEPMPO/BL adduct and simulated with the following hyperfine splitting constants: a(P) = 32.5 G, a(N) = 9.8 G, and a(H) = 12.5 G.35 When KIO3 was replaced with KBrO3, no EPR signal could be registered (Table 1). In the case of the reaction between KClO3 and hydrogen peroxide in acidic solution the DEPMPO/OH adduct is detected (Figure 2).36,37 It should be pointed out that the source of HO• radicals is certainly not the hydrogen peroxide because exactly the same signal is obtained when a sulfuric acid solution of KClO3 was investigated (Table 1). Regarding the temperature at which the reactions were carried out and studied, an elevated temperature of 60 °C was chosen for two reasons: (i) An increase in temperature accelerates the rate of the elementary steps of a complex chemical reaction, resulting generally in the increase of the overall rate of reaction. If reaction takes place via a slow step (or steps) including production of radicals, formation of these species will be accelerated and trapping the radicals
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RESULTS AND DISCUSSION To investigate the presence of free radicals formed during the iodate, bromate, and chlorate reaction, separately, with hydrogen peroxide in an acidic medium, a set of experiments at 60 °C was performed using the EPR spin-trap DEPMPO (Table 1). The EPR signal is obtained for iodate−hydrogen B
DOI: 10.1021/acs.jpca.7b02035 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
zero. Therefore, the reaction (eq 3) could also be considered as an additional source of HOO• radicals. The possible reactions of termination are shown in eqs 4 and 5:
produced increases the amount of DEPMPO-radical adducts (ii) The Bray−Liebhafsky (BL) oscillatory reaction,16 which involves only iodate and hydrogen peroxide in acidic medium, is usually studied at 60 °C; the identical temperature is taken for the purpose of direct comparison of the other halogen analogues, especially due to the fact that oscillatory behavior in bromate or chlorate−hydrogen peroxide system has not been found. Essential differences in the dynamical behavior of these systems may be the consequence of the presence or absence of certain short-lived radical species. Our experimental setup, where the DEPMPO spin trap was present in reaction mixture at the beginning of the reaction, provides a better interception of radical potentially originated from the initial stage of the reaction. The DEPMPO residence time, 25 min in the reaction mixture, is also tentatively responsible for good spin−adduct accumulation. Furthermore, the small reaction volume (120 μL) providing high radical concentration is also confirmed to be suitable for successful spin−adduct accumulation. Iodate−Hydrogen Peroxide Reaction. In accordance with the obtained EPR spectrum of the DEPMPO/radical adduct (Figure 1), the minimal iodate−hydrogen peroxide model is proposed. Here we focus on the initial stage of the reaction investigated, especially on the processes where iodate is involved, because iodate is present in high excess relative to other iodine-containing intermediates (HIO2, HIO, I2, I−). It is assumed that the obtained EPR signal from DEPMPO/BL radical adduct, most probably originates from one electron reduction of iodate and slow IO2• formation (eq 1):35 IO3− + H+ + H 2O2 → IO2• + HOO• + H 2O
(1)
IO2• + IO2• → I 2O4
(5)
BrO3− + H+ + H 2O2 → HBrO2 + O2 + H 2O
(6)
HBrO2 + H 2O2 → HBrO + O2 + H 2O
(7)
2HBrO2 → HBrO + BrO3− + H+
(8)
2HBrO ↔ Br2O + H 2O
(9)
HBrO + H 2O2 → O2 + H 2O + Br − + H+
(10)
Br − + BrO3− + H+ → HBrO + HBrO2
(11)
+
−
HBrO + H + Br ↔ Br2 + H 2O
(12)
Reactions 6−8 and 10 and 11 are potential subsequent oxygentransfer processes leading finally to the formation of elementary bromine in the fast reaction between hypobromous acid and bromide (eq 12).43 The disproportionation of hypobromous acid begins probably with reaction 9 and the intermediate Br2O is formed.42 The intermediate compound Br2O was also proposed in the kinetic model of the bromate driven, oscillating Belousov−Zhabotinsky (BZ) reaction.43,44 The absence of the EPR signal in the bromate−hydrogen peroxide reaction may suggest that it does not proceed via the intermediate BrO2• radical; thus, eqs 13 and 14 are not likely to occur under our experimental condition.
(2)
−1 −1
Due to the very slow rate constant (k = 0.5 M s ) in acidic media (pH = 0.5−3.5),40 the possible reaction between hydroperoxyl radicals and hydrogen peroxide is neglected. The obtained IO2• (eqs 1 and 2) predominantly reacted with the components present in high concentration such as H2O2 (eq 3): IO2• + H 2O2 → HIO2 + HOO•
(4)
These termination reactions (eqs 4 and 5) are second-order reactions, and a small concentration of radical species makes these reactions the slow ones. Consequently, not only IO2• but also HOO• radical may be accumulated in detectable amounts. A straightforward question can be immediately raised why the HOO• radical is not trapped by spin-trap DEPMPO and subsequently detected by EPR. Reaction 2 is crucial for this discussion. Due to the assumption, supported by the standard reduction potential for couple E0r(O2/HOO•) = −0.186 V, the HOO• radical is a reducing agent, oxidized easily by high excess of iodate.38,39 Thus, reaction 2 provides a fast and efficient removal of HOO• and at the same time it serves as an additional source of IO2• radical production. Bromate−Hydrogen Peroxide Reaction. The reaction between bromate and hydrogen peroxide in acidic medium at 60 °C gave no measurable EPR signal, from which we concluded that this reaction may not proceed via one-electrontransfer mechanism. One of the possible sets of reactions is shown by eqs 6−12:1,4,20,22,42,43
The low rate of this reaction (eq 1) should be the consequence of unfavorable change of free energy ΔG0 > 0, i.e., the negative standard reduction potential. The long accumulation period (>10 min), after which a detectable EPR signal DEPMPO/BL radical adduct was formed,35 is a possible experimental evidence for the very slow formation of IO2• (eq 1). The initial step (eq 1) in the radical reaction is followed by the propagation steps (eqs 2 and 3). During the propagation step (eq 2), the hydroperoxyl radical HOO•, formed in eq 1, known as a good reducing agent (E0r(O2/HOO•) = −0.186 V),38,39 can effectively reduce iodate, to produce oxygen and additional • IO2 : HOO• + IO3− + H+ → IO2• + O2 + H 2O
HOO• + HOO• → H 2O2 + O2
(3)
E0r(HOO•/
BrO3− + H+ + H 2O2 → BrO2• + HOO• + H 2O
(13)
BrO3− + H+ + HBrO2 ↔ 2BrO2• + H 2O
(14)
Furthermore, this idea is supported by the small rate coefficient of BrO2• formation (44.70 mol−2 dm−6 s−1), as well as the high rate coefficient (6.70 × 107 mol−2 dm−6 s−1) of the reverse reaction of (14).44 The reaction (eq 14) is also part of the previously mentioned BZ reaction models.22,43,44 In addition, the comparison of experimental results with computer simulations for BZ reaction showed that free radicals play no
The standard reduction potential of the couple H2O2) is 1.436 V38 and for the IO2•/IO2H couple could be only estimated to be E0r(IO2•/IO2H) < 1.8 V on the basis of the relatively good positioning of IO2H in the Latimer diagram.41 Thus, thermodynamical consideration on the reaction (eq 3) suggests a favorable process; i.e., the standard reduction potential for the reaction (eq 3) ΔE0 is larger than C
DOI: 10.1021/acs.jpca.7b02035 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A role in the mechanism of the oxygen-atom-transfer reactions.43 These findings are in favor of the EPR signal absence and our assumption that the bromate−hydrogen peroxide reaction should undergo via the sequence of eqs 6−12. The absence of the BrO2• radical may also explain that in contrast to the oscillatory iodate−hydrogen peroxide reaction, its counterpart, the bromate−hydrogen peroxide system does not display periodic behavior. Chlorate−Hydrogen Peroxide Reaction. Surprisingly, in the case of the chlorate−hydrogen peroxide reaction in acidic medium at 60 °C hydroxyl radical (HO•) was detected (Figure 2), despite the fact that this radical was not found in the previous systems. Due to the fact that the same signal of the DEPMPO/OH adduct is also obtained in acidic chlorate solution in the absence of hydrogen peroxide (Table 1), the direct chlorate−hydrogen peroxide can be ruled out as the major source of HO• radical. A convenient explanation of the HO• radical source is the homolytic dissociation of chloric acid, generated in the reaction between potassium chlorate and sulfuric acid (eqs 15 and 16): 2KClO3 + H 2SO4 → 2HClO3 + K 2SO4
(15)
HClO3 ↔ HO• + ClO2•
(16)
we have performed experiments in the presence and absence of daylight at 60 °C placing a filter paper immersed in potassium iodide solution right above the reaction vessel containing chlorate and sulfluric acid. As expected, the filter paper became blue after 30 min only when the reaction was carried out in the presence of daylight. In addition, no DEPMPO/radical adduct EPR signals were detected, for this reaction, in the absence of daylight. Thus, we concluded that homolytic dissociation of chloric acid is indeed enhanced by light exposure to produce HO• and ClO2• radicals and escape of ClO2• plays a crucial role under our experimental condition. To the best of our knowledge this is the first experimental evidence of the HO• radical existence in the oxy-chlorine system and it can be of importance in verification of the kinetic models including the HO• radical. Additionally, the investigated reaction (eq 16) is a novel, very approachable and inexpensive source of hydroxyl radicals and it will definitely be the subject of further investigation.
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CONCLUSION This study investigates the presence of free radicals in reactions between iodate, bromate, and chlorate analogues, particularly with hydrogen peroxide in acidic solution at 60 °C, using the EPR spin trapping technique with the DEPMPO spin trap. These challenging reactions presented a permanent task, because of their complex kinetics and stoichiometry. The results obtained reveal a number of differences in the chemistry of the iodate, bromate, and chlorine analogues in terms of the sequence of elementary reactions. Namely, the reaction between iodate and hydrogen peroxide in acidic solution is a potential source of IO2• radical, whereas the reaction of bromate with hydrogen peroxide did not produce any EPR signal, suggesting the lack of decisive role of the radical pathway. Furthermore, the production of the hydroxyl radical in the chlorate−peroxide system can most probably be attributed to the homolytic cleavage of chloric acid because the DEPMPO/OH radical signal was also detected in the absence of hydrogen peroxide. The minimal-core models explaining the origin of the acquired EPR signals are proposed. On the basis of these results, the involvement of IO2• and HOO• radicals (in iodate− hydrogen peroxide reaction), as well as ClO2• and HO• radicals (for sulfuric acid solution of potassium chlorate), and the possible exclusion of BrO2• radical (in bromate− hydrogen peroxide reaction) from the modeling of particular and related processes were suggested. Moreover, the results from this study indicate a significant difference in the chemical behavior of the iodate, bromate, and chlorate analogues under the same experimental conditions. Additionally, the oxy-halogen species have the ability to participate in different mechanistic pathways, and to generate radicals and other highly reactive intermediate species, but results obtained here appear to suggest that generalization of oxy-halogen reactions mechanism should be avoided.
In favor of the presence of the homolytic dissociation of chloric acid the following arguments should be emphasized. First, ab initio studies of HClO3 revealed that its dissociation may take place via four different pathways including eq 16 as a possible route.45 Second, the elevated temperature along with the photochemically induced dissociation of chloric acid may also account this behavior. As an example to support this idea, reaction of chlorate with iodine was experimentally proven to be a photoinitiated process having no direct reaction in the absence of light.46,47 Last, but not least, indirect evidence of the analogous homolytic dissociation of chlorous acid can also be mentioned.48 As Horváth et al. demonstrated, it was necessary to include the homolytic bond cleavage of chlorous acid in the kinetic model to describe quantitatively the most important characteristics of the kinetic curves even though this reaction is thermodynamically unfavorable (+128 kJ/mol).48 It was concluded that this process has to be driven photochemically by the light source of the spectrophotometer. The accumulation of the HO• radical may be rationalized via the following arguments. First, the recombination of HO• and chlorine dioxide radical is partially prevented by the escape of chlorine dioxide from the solution at elevated temperature. Second, the reaction between HO• radical and the excess of chlorate to produce perchlorate was ruled out by density functional calculation.49 ClO3− + HO• → ClO4 − + H+ + 1e−
(17)
ClO3−
Because oxidation of does not take place (eq 17), the system may be well described with one- or two-electron reduction of ClO3− with HO• radicals, producing H2O2 or water and some oxy-chlorine products. These reactions, even though they include radicals, are relatively “slow” processes (rate constant k < 106)50 and thus HO• radicals are not so rapidly consumed, paving the way for the reaction with the DEPMO spin-trap. A word is also in an order here to clarify why the ClO2• radical has not been detected. Because elevated temperatures significantly decrease the solubility of the ClO2• gas, this results in its easy escape from the solution. To check this possibility,
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AUTHOR INFORMATION
Corresponding Author
*M. C. Pagnacco. E-mail: maja.milenkovic@ffh.bg.ac. ORCID
Maja C. Pagnacco: 0000-0002-1299-7974 Attila K. Horváth: 0000-0002-1916-2451 D
DOI: 10.1021/acs.jpca.7b02035 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Serbian Ministry of Education, Science and Technological Development (project no. 172015) and the Hungarian National Research Fund NKFIH-OTKA grant no.: K116591. A.K.H. is also grateful for the financial support of the GINOP-2.3.2-15-2016-0030 grant. The present scientific contribution is dedicated to the 650th anniversary of the foundation of University of Pécs, Hungary. The EPR spectra were acquired in the EPR laboratory of the Center for physical chemistry of biological systems, University of Belgrade, Faculty of physical chemistry. The authors are thankful to Prof. Dragomir R. Stanisavljev (University of Belgrade, Faculty of physical chemistry) for his useful suggestions and comments.
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