9360
J. Phys. Chem. 1996, 100, 9360-9364
Arrhenius Parameter Determination for the Reaction of Methyl Radicals with Iodine Species in Aqueous Solution† Stephen P. Mezyk* AECL Whiteshell Laboratories, Pinawa, Manitoba, R0E 1L0 Canada
Keith P. Madden* Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: October 24, 1995; In Final Form: February 13, 1996X
The techniques of electron pulse radiolysis and direct ESR detection have been used to determine Arrhenius parameters for the recombination reaction of methyl radicals and methyl radical reaction with iodine in aqueous solution. At 22.8 °C, rate constants of 2k7 ) (1.77 ( 0.16) × 109 dm3 mol-1 s-1 and k1 ) (2.75 ( 0.43) × 109 dm3 mol-1 s-1, with corresponding activation energies of 14.89 ( 0.87 and 13.10 ( 0.71 kJ mol-1 (5.739.6 °C), were obtained respectively for these two reactions. The analogous reaction of methyl radicals with iodide or iodate was found to be much slower, with the room temperature rate constant for both reactions estimated as k < 106 dm3 mol-1 s-1.
•
Introduction In the event of a severe nuclear reactor accident in which fuel failure occurs, radioactive material could escape to the environment by leakage through the containment building and from filtered-air discharge systems. Based on a combination of inventory, half-life, volatility, and biological activity, radioactive iodine is generally recognized as the most hazardous fission product that could be released in this manner.1 Thermodynamic calculations and various experiments have established that the initial release of iodine from fuel would be mostly as CsI,2,3 which would be readily dissolved in the large quantities of water also present in most accidents at water-cooled reactors. Consequently, a good knowledge of iodide chemistry under these containment conditions is essential. These considerations have been responsible for a recent resurgence of interest in the radiation-induced chemistry of iodine-containing compounds.4-6 The high radiation fields expected following a reactor accident have been shown to significantly affect the iodine chemistry that occurs.6 Of particular concern are the reactions that lead to oxidation of the nonvolatile iodide to volatile products, such as low molecular weight iodoalkanes.7 Although qualitative demonstration of their formation in irradiated iodide or iodine solutions containing organic impurities has been reported,7-9 little quantitative information on rate constants or mechanisms is presently available. Previously,7 the steady state γ-radiolysis of iodide, iodine, and iodate solutions containing methane was investigated, with yields of volatile species determined. From this work two main reaction mechanisms were postulated for the formation of iodomethane; the reaction of methyl radicals with molecular •
CH3 + I2 f CH3I + I
(1)
CH3 + •I f CH3I
(2)
iodine. For the irradiation of iodate solutions, it was postulated that these two reactions also occurred but were preceded by methyl radical reaction with iodate itself,
CH3 + IO3- f CH2O + HIO2•-
•
(3)
where the HIO2•- species further reacted to give HOI and eventually I2. However, no kinetic information was given for any of these reaction pathways. To understand the radiolytic processes occurring in these systems, the confirmation of proposed mechanistic schemes by computer modeling is required. Such modeling is dependent on accurate rate constant data being available; however, for the above three reactions, only one approximate measurement10 for k1 (ca. 6 × 109 dm3 mol-1 s-1) has been previously reported in aqueous solution. This paper reports on our experimental study of the direct measurement of the temperature-dependent rate constants for methyl radical reaction with iodine (k1) and iodate (k3) in aqueous solution. For comparative purposes, the reaction with iodide was also investigated. Elucidation of these values required the rate constants for the self-reaction of methyl radicals to be accurately known over the temperature range of study; consequently, these values have also been determined in this work. Direct ESR detection of the decay of the methyl radicals, formed by the fast reaction of electron pulse radiolysis generated hydroxyl radicals with dimethyl sulfoxide (DMSO) •
OH + (CH3)2SO f •CH3 + (CH3)SO(OH)
(4)
(k4 ) 7.0 × 109 dm3 mol-1 s-1 11), was the monitoring method of choice.
and atomic Experimental Section * To whom correspondence should be addressed. † The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL 3884 from the Notre Dame Radiation Laboratory. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03134-0 CCC: $12.00
Stock acid solutions at room temperature were prepared by addition of HClO4 (Mallinkrodt, A.R. Grade, 69.05%) to N2O saturated Millipore Milli-Q reagent grade water containing 0.10 mol dm-3 DMSO (Ficher Certified) to give a pH of 3.60 ( 0.05 © 1996 American Chemical Society
Reaction of Methyl Radicals with Iodine Species
J. Phys. Chem., Vol. 100, No. 22, 1996 9361
as measured by a pH meter. This pH value was chosen in order to minimize molecular iodine hydrolysis that occurs via the equilibrium
I2 + H2O h HOI + I- + H+
K5 ) 4.3 × 10-13 dm6 mol-2 12 (5)
Scavenging experiments were performed by successive addition of known amounts of sodium iodide (Aldrich, 99.999%), potassium iodate (Aldrich, 99.5%), or molecular iodine (Aldrich, 99.999%) to the stock solutions. To facilitate dissolution of the molecular iodine, volumes of concentrated stock iodine solutions in pure DMSO were used. The accuracy of all these concentrations is estimated at better than 3%. Experiments at nonambient temperatures were performed by flowing the room temperature solutions through a short, temperature-controlled condenser tube immediately before irradiation. The exact solution temperature was measured by a thermocouple placed directly above the irradiation cell. The temperature stability of this method was found to be better than 0.3 °C. In situ radiolysis ESR data were recorded by irradiating flowing aqueous solutions with a 2.8 MeV electron beam from a Van de Graaff accelerator.13,14 A pulsed 150 mA beam of 0.5 µs duration and 25 Hz repetition rate was used for these kinetic experiments. The initial radical concentration for the kinetic experiments was approximately 2.0 × 10-5 mol dm-3, as determined from the decay of the sulfite anion under basic conditions (pH 11),
(6) mol-1 s-1,15
has been whose rate constant, 2k6 ) 1.4 × determined by both conductivity and absorption spectroscopy methods. Corroboration of this initial radical concentration was also obtained by absolute measurement of nitroxide depletion under the same experimental conditions.16 The solution flow rate was sufficiently fast to ensure the irradiation volume of the ESR flat cell was replenished completely between electron beam pulses. ESR kinetic curves were recorded at X-band (9.2 GHz) using nonsaturating levels of microwave power. Magnetic field measurements were performed via NMR methods,13 using the ESR spectrometer and procedures of Madden et al.17 Timeresolved ESR kinetic traces were recorded at the line position corresponding to the low-field three line of the 1:3:3:1 quartet of the methyl radical.18 The line width of the methyl radical corresponds to submicrosecond relaxation times, and as no CIDEP was observed for this radical, the growth and decay of the ESR kinetic traces accurately reflects the chemical kinetics of the radical. Traces were fitted using the LevenbergMarquardt nonlinear least-squares fitting module within the Origin computer program (MicroCal Software, Inc., Northampton, MA) for simple second-order decays or by the numerical differential equation solving code FACSIMILE19 to allow elucidation of individual rate constants in the mixed order decays. dm3
TABLE 1: Summary of Measured Rate Constants, for Self-Reaction of Methyl Radicals and Reaction of Methyl Radical with Molecular Iodine, at pH 3.6 in Aqueous Solution temp (°C) 5.7
2SO3•- f products 108
Figure 1. Experimental data and fitted decays (solid lines) of the methyl radical in aqueous solution at pH 3.60 and 5.7 °C: (0) no added iodine, fit of second-order decay with 2k7 ) 1.19 × 109 dm3 mol-1 s-1; (O) [I2] ) 3.09 × 10-5 mol dm-3, numerically calculated best fit values, obtained as described in the text, using measured 2k7 rate constant and k1 ) 2.18 × 109 dm3 mol-1 s-1; (4) [I2] ) 6.89 × 10-5 mol dm-3, calculated values using 2k7 value and k1 ) 1.87 × 109 dm3 mol-1 s-1.
Results and Discussion Under the conditions of these experiments, the decay of the methyl radicals in the presence of molecular iodine gave mixed order kinetics (see Figure 1). In order to deconvolute the rate constant for reaction 1 from these decay curves, accurate temperature-dependent rate constants for the recombination reaction for methyl radicals
16.5 22.8 32.2 39.6
2•CH3 f C2H6 10-9 2k7/dm3 mol-1 s-1
• CH3 + I2 f products 10-9 k1/dm3 mol-1 s-1
1.19 ( 0.22 1.22 ( 0.11 1.47 ( 0.15 1.65 ( 0.26 1.77 ( 0.16 2.19 ( 0.20 2.35 ( 0.21
1.93 ( 0.47
2•CH3 f C2H6
2.53 ( 0.35 2.75 ( 0.43 3.18 ( 0.54 3.64 ( 0.74
(7)
had to be known. Consequently, these rate constants were first determined in this work. Methyl Radical Recombination. A typical decay curve for irradiated 0.10 mol dm-3 DMSO aqueous solution at 5.7 °C is shown in Figure 1, along with the fit obtained for pure secondorder decay. The measured initial radical yield under these conditions was used to convert the observed signal intensity to absolute concentrations. The rate constant obtained at this temperature was 2k7 ) (1.19 ( 0.22) × 109 dm3 mol-1 s-1. These measurements were repeated over the temperature range 5.7-39.6 °C, with the individual rate constants determined listed in Table 1. The corresponding Arrhenius plot is shown in Figure 2 and is well described by the equation
log 2k7 ) (11.86 ( 0.16) - [(14890 ( 870)/(2.303RT)] (8) corresponding to an activation energy of 14.89 ( 0.87 kJ mol-1. The measured rate constant of this study at 22.8 °C, 2k7 ) (1.77 ( 0.16) × 109 dm3 mol-1 s-1, is lower than the two previous determinations in aqueous solution; at room temperature, 2k ) (2.48 ( 0.40) × 109,20 and at 25 °C, 2k ) (3.2 ( 0.4) × 109 dm3 mol-1 s-1.21 These studies both used absorption spectroscopy at very short wavelengths (2.0 V),30 and therefore electron abstraction reactions to give iodide and the DMSO cation cannot occur. Therefore, the only other pathway for this radical is recombination11
2I• f I2
2k ) 2.0 × 1010 dm3 mol-1 s-1
(12)
To determine the importance of this reaction on the monitored methyl radical decay, modeling studies were performed. Using reactions 1, 2, 7, and 12 with the rate constants listed above (k2 was assumed to be 1.0 × 1010 dm3 mol-1 s-1) and taking the room temperature (22.8 °C) “worst case” experiment of [•OH] ) 2.0 × 10-5 mol dm-3 and [I2] ) 3.0 × 10-5 mol dm-3, it was found that the inclusion of reaction 12 caused a 15% increase in the final (50 µs) concentration of I2. From the combined order fitting of this calculated methyl radical concentration time profile, again keeping the 2k7 value constant to be consistent with the experimental analysis, this concentration increase translated to a fitted k1 value that was about 35% higher. However, when the initial iodine concentration was increased, the calculated difference quickly became much smaller. By modeling all of the iodine concentrations measured at this temperature, an overall error of ∼15% in the second-order rate constant for reaction 1 was obtained. This value is comparable to the experimental scatter. These calculations imply that our determined k1 ) 2.75 × 109 dm3 mol-1 s-1 value is a lower bound rate constant; however, given the uncertainty in the literature reaction rate constants and the relatively large scatter in the data, we believe that a correction to the experimental values is not warranted. When the modeling was performed for the other temperatures of this study, with diffusion-controlled activation energies being assumed for the reactions without experimental values, similar results were obtained. To make allowance for this potential error in the k1 values summarized in Table 1, the errors stated are simply the larger of the experimental scatter or the calculated fitting differences. There is another potential pathway that could be important, where the formed •CH3I2 complex dissociates to produce CH3I•+ and iodide, •
CH3I2 f CH3I•+ + I-
(13)
The methyl iodide radical cation has been well characterized in aqueous solution31,32 and is known to react with DMSO by electron abstraction to give methyl iodide.33 This mechanism would therefore give the anticipated products and also be consistent with the experimental observations. Unfortunately, the elucidation of the dominant mechanisms was beyond the scope of this study. The addition of iodine to elucidate radiation-induced reaction mechanisms in hydrocarbons has been used for many years.34 In contrast, there has been only one previous, indirect, determination of the rate constant for hydrocarbon radical reaction with iodine in aqueous solution.10 This study, which determined the value for k1 by competition with peroxy radical formation in aerated aqueous solutions, calculated an approximate rate constant of 6 × 109 dm3 mol-1 s-1 at room temperature, significantly faster than our measured value at 22.8 °C of k1 ) (2.75 ( 0.23) × 109 dm3 mol-1 s-1. The reaction of hydrocarbon radicals with iodine in cycloalkanes (C5 to C10) and alkanes (C6 to C17) was again found to be accurately described by eq 9,22 with the parameters ka ) 2.44 × 1010 dm3 mol-1 s-1 and Φkd ) 1.56 × 1010/η dm3 mol-1 mPa. This gives the predicted value in water as 1.02 × 1010
dm3 mol-1 s-1, much faster than our observed value of (2.75 ( 0.23) × 109 dm3 mol-1 s-1. This discrepancy is possibly due to the value of Φkd found for iodine reaction in hydrocarbons, which has been noted to be very high,22 greater even than the diffusion-controlled rate constant. This was attributed to the Φ value being almost unity in hydrocarbons, which may not be correct for this reaction in water. Other Reactions. Following the methodology established for molecular iodine reaction with methyl radicals, similar experiments were conducted to determine the extent of methyl radical reaction with iodate, reaction 3, and iodide in aqueous solution. In contrast to the iodine results, the addition of 10-2 mol dm-3 of either of these substances produced no significant change in the second-order decay of the methyl radical. From the fitting of a combined first- and second-order decay to these curves, upper rate constants for both iodate and iodide reaction with the methyl radical at 23 °C were estimated as k < 106 dm3 mol-1 s-1. Conclusion Direct ESR measurement of methyl radicals in aqueous solution has been used to determine Arrhenius parameters for the recombination reaction
2•CH3 f C2H6
(7)
and the reaction of the methyl radical with molecular iodine •
CH3 + I2 f CH3I + I•
(1)
in aqueous solution. Over the temperature range 5.7-39.2 °C, the experimental data were found to be well fitted by the Arrhenius equations
log 2k7 ) (11.86 ( 0.16) - [(14890 ( 870)/(2.303RT)] (8) and
log k1 ) (11.75 ( 0.13) - [(13100 ( 710)/(2.303/RT)] (11) respectively. The corresponding rate constant for methyl radical reaction with iodate or iodide at room temperature was found to be
