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On the radiation chemistry of methyl iodide in aqueous solution
Radiation Physics and Chemistry 67 (2003) 623–637
On the radiation chemistry of methyl iodide in aqueous solution G.V. Buxtona,*, H.E. Simsb a
School of Chemistry, University of Leeds, Leeds LS2 9JT, UK b AEA Technology, Harwell, Didcot, OX11 0RA, UK Received 18 June 2002; accepted 15 November 2002
Abstract d New results on the reactions of COd 2 and O2 with methyl iodide in aqueous solution are presented. These results, together with the published experimental data under a wide range of conditions (pH, concentration, saturating gas), d d show that CH3I is readily decomposed to dCH3 and I by the strong reductants e aq, H and CO2 , but is unreactive d d d with the weaker reductant O2 . This means that oxygen, which reacts rapidly with eaq, H and COd 2 to form O2 , can protect methyl iodide from radiation-induced reductive decomposition in aerated neutral and alkaline aqueous solution. The available evidence indicates that the strong oxidant dOH reacts with CH3I by addition to form the more selective oxidant (CH3IOH)d without decomposing the molecule. A set of reactions has been devised to simulate the experimental data. In most cases the trends shown by the data under a wide variety of conditions are reproduced quite well. r 2002 Published by Elsevier Science Ltd.
Keywords: Methyl iodide; Radiolysis; Modelling methyl iodide chemistry; Nuclear safety
1. Introduction The radiation chemistry of methyl iodide in aqueous solution is potentially important for nuclear safety, for example in a loss of coolant accident (LOCA) in a PWR. Whilst there have been several investigations of various aspects of the radiation chemistry of methyl iodide in aqueous solution (Thomas, 1967; Shankar et al., 1969; Bruhlmann . et al., 1973; Buchler . et al., 1977; Habersbergerova and Sistek, 1982; Mohan and Asmus, 1987, 1988; Paquette and Ford, 1989; Mezyk and Madden, 1996; Maity and Mohan, 2001), none has provided complete information on the products and their yields. As a result, our knowledge of the reaction mechanism is still lacking; in particular, the products of the reaction of the hydroxyl radical have still to be established. Ideally, one would like to achieve conditions in a LOCA where *Corresponding author.
volatile methyl iodide is decomposed reductively to nonvolatile iodide ion. The radiolysis of water is described by H2 O
d d 2:8e aq ; 0:62lH ; 2:8 OH; 0:47H2 ;
0:78H2 O2 ; 2:7Hþ
ð1Þ
The numbers are typical G-values in units of 107 mol J1 applicable to dilute aqueous solution. However, they are dependent on the scavenging power of the solutes and so must be adjusted for each system of interest. Here information about the reactions of the d d primary radicals e aq, H and OH with methyl iodide is required. In addition, reactions of secondary radicals that are likely to be formed under ambient conditions must also be considered. For example, in the presence of d d d air, e aq and H are rapidly converted to O2 /HO2 : d e aq þ O2 -O2
k2 ¼ 1:9 1010 dm3 mol1 s1
0969-806X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. doi:10.1016/S0969-806X(02)00498-X
ð2Þ ðBuxton et al:; 1988Þ:
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G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Hd þ O2 -HOd2
ð3Þ
k3 ¼ 2:1 1010 dm3 mol1 s1 þ HOd2 "Od 2 þH
ðBuxton et al:; 1988Þ:
pKa ¼ 4:8 ðBielski et al:; 1985Þ: ð4Þ
d Thus, the reactions of Od 2 /HO2 come into play in aerated solutions (Bielski et al., 1985). In this paper we present new experimental data, including the reactions of COd and Od with methyl 2 2 iodide; we also summarise, evaluate and attempt to model these and the published data that have been obtained under a range of different conditions.
2. Experimental Experiments were carried out at (a) the University of Leeds and (b) AEA Technology, Harwell. (a) Methyl iodide (BDH laboratory reagent grade) was washed with aqueous 0.1 mol dm3 potassium iodide solution and then with triply distilled water. Solutions for irradiation were prepared by injecting ca. 0.1 cm3 of the washed CH3I into 100 cm3 of triply distilled water and diluting as necessary. The final concentration of CH3I was determined from absorbance measurements at 250 nm taking e250 ¼ 34 m2 mol1 (Habersbergerova and Sistek, 1982). These solutions were prepared immediately before use. When they were required to be saturated with a gas, this was achieved by saturating the water or aqueous solution before injecting the methyl iodide. All other reagents were of analytical grade; the gases were of the highest purity available and were used straight from the cylinder. Phosphate buffer (2 103 mol dm3) was used to maintain pH 7. The solutions (20 cm3) were irradiated with 60Co grays at a dose rate of 1.03 Gy s1 in a pyrex tube (i.d. 2 cm) which had attached to it a sidearm carrying a suprasil spectrophotometer cell (optical path length 1 cm). Thus, any spectral changes could be measured without opening the irradiation vessel. The CH3I was equilibrated between the gas (ca. 20 cm3) and liquid phases by filling and emptying the sidearm until the absorbance at 250 nm became constant. Irradiation was carried out at ambient temperature (ca. 221C). After irradiation, H2O2 and CH3O2H were measured by the Ghormley method (Allen et al., 1952). This method also measures any other species capable of oxidising iodide ion, and also free iodine in the form of I 3. (b) Experiments to measure the radiolytic destruction of methyl iodide were carried out under atmospheres of air, argon and nitrous oxide, respectively. The solutions were buffered at pH 4.7 with boric acid solution (0.1 mol dm3) or pH 7 with phosphate solution
(0.1 mol dm3). These high concentrations of buffer were chosen to represent the likely conditions in a LOCA. For experiments under Ar or N2O, the solution was saturated with the required gas and then CH3I was added and equilibrated. Samples for irradiation were drawn into a 10 cm3 glass syringe so that there was no gas space, sealed with a glass cap and irradiated with 60 Co g-rays at a dose rate of 0.026 Gy s1. Before irradiation, a small aliquot was taken from the syringe to determine the initial [CH3I]. Samples were also taken periodically during the irradiation to measure changes in [CH3I]. A control experiment was also carried out with an unirradiated aerated solution that was kept in the dark between samplings. The concentration of CH3I was determined by solvent extraction into toluene, followed by analysis of the toluene solution by gas chromatography, using a Pye Unicam PU 4550 gas chromatograph equipped with SGE BP5-1.0 column and electron capture detector. Quantification was by comparison of the sample peak heights with those of freshly prepared standards analysed under the same conditions. Correction was made for the efficiency of solvent extraction which was measured as 92%. In both sets of experiments, (a) and (b), the dose rate was determined by Fricke dosimetry (Spinks and Woods, 1964).
3. Results of experiments (a) 3.1. Solutions of CH3I+I+O2 The results of these experiments, which are qualitatively similar to those reported by Shankar et al. (1969), are presented in Table 1. When the irradiated solutions were analysed by the Ghormley method (Allen et al., 1952), an immediate yield of I 3 was observed and a further amount was produced over a period of ca. 20 min. The immediate yields, which are assigned to H2O2+I2, have been corrected for the slowly formed I 3 , assumed to be due to CH3O2H, during the time (2–3 min) taken to measure them. Also shown in Table 1 are the data calculated using the reaction set for the radiolysis of aqueous solutions of CH3I, presented in the appendix. The two sets of data agree quite closely. 3.2. Reaction of CH3I with COd 2 Under the conditions used (0.01–0.05 mol dm3 HCO 2 , pH 7) essentially all the primary radicals of water radiolysis are converted to the strong reductant via: COd 2 d e aq þ N2 O-N2 þ OH þ OH
ð5Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
625
Table 1 Comparison of experiment and simulation for irradiated solutions containing CH3I and O2 at pH 7 (2 103 mol dm3 phosphate). Dose rate 1.03 Gy s1 Time (s)
[I2]+[H2O2]a (105 mol dm3)
[CH3O2H]b (105 mol dm3)
Totalc (105 mol dm3)
Expt
Expt
Expt
Calc
Calc
Calc
[CH3I]=4.8 104, [I]=3.6 103, [O2]=2.4 104 mol dm3 120 2.41 2.96 1.65 240 4.42 5.63 3.03 360 6.80 8.00 4.10 480 8.60 10.07 5.80
1.70 3.32 4.83 6.25
4.06 7.45 10.90 14.40
4.66 8.95 12.83 16.32
[CH3I]=4.8 104, [I]=3.7 103, 120 1.79 240 3.77 360 4.22 480 6.03
[O2]=1.3 103 mol dm3 1.51 0.80 2.93 1.48 4.26 2.06 5.50 3.02
0.88 1.74 2.57 3.38
2.59 5.25 6.28 9.05
2.39 4.67 6.83 8.88
[CH3I]=2.4 102, [I]=3.0 103, [O2]=2.4 104 mol dm3 120 4.47 4.75 2.82 240 7.58 8.63 6.42 360 11.87 11.65 7.98 480 12.45 13.91 10.31 600 14.20 15.55 11.87
2.46 4.91 7.32 9.63 11.78
7.29 14.00 19.85 22.76 26.07
7.21 13.54 18.97 23.54 26.33
[CH3I]=2.4 102, [I]=3.2 103, [O2]=1.3 103 mol dm3 120 5.25 4.64 2.72 240 9.53 8.77 6.42 360 10.89 11.96 10.12 480 13.23 14.72 12.06 600 18.09 17.26 13.81
2.46 4.91 7.35 9.79 12.25
7.97 15.95 21.01 25.29 31.90
7.10 13.68 19.31 24.51 29.51
[CH3I]=4.8 103, [I]=3.6 103, [O2]=1.3 103 mol dm3 120 3.27 3.75 2.33 240 6.03 7.17 5.06 480 12.45 12.93 10.12
2.16 4.32 8.57
6.80 11.09 22.57
5.91 11.49 21.40
[CH3I]=4.8 103, [I]=3.6 103, [O2]=2.4 104 mol dm3 120 4.67 4.40 2.92 240 6.42 8.21 6.03 600 14.20 16.00 10.89
2.23 4.44 10.72
7.39 12.45 25.09
6.63 12.65 26.72
Measured as the immediate yield of I 3 (see text). Measured as the slowly formed yield of I 3. c Sum of the immediate and slowly formed yields of I 3. a
b
k5 ¼ 9:1 109 dm3 mol1 s1 d
ðBuxton et al:; 1988Þ:
d OH þ HCO 2 -CO2 þ H2 O 9
3
1 1
k6 ¼ 3:2 10 dm mol
s
ð6Þ
Hd þ CH3 I-d CH3 þ I þ Hþ
ð8Þ
k8 ¼ 1:2 1010 dm3 mol1 s1 ðBuxton et al:; 1988Þ:
d Hd þ HCO 2 -CO2 þ H2
k7 ¼ 2:1 108 dm3 mol1 s1
unchanged:
ðMezyk and Bartels; 1994Þ: ð7Þ
ðBuxton et al:; 1988Þ:
Under conditions where reactions (8) and/or (9) would compete with reaction (7), the mechanistic outcome is
Hd þ N2 O-N2 þd OH k9 ¼ 2:1 106 dm3 mol1 s1
ð9Þ ðBuxton et al:; 1988Þ:
The data are presented in Table 2 and show that CH3I is decomposed by a chain reaction, consistent with a
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
626
Table 2 Comparison of G(I)expt with G(I)calc in N2O-saturated CH3I solutions containing HCO 2 3 [HCO 2 ] (mol dm )
Dose rate (Gy s1)
[CH3I]
G(I) (107 mol J1) Expt
Calc
0.012 0.012 0.012 0.012
55 93 130 220
49 112 110 250
2.5 104
0.053
40
23 (24.3)
1 102 1 102 5 102
2.5 104 1.3 103 2.5 104
1.03 1.03 1.03
11 15 20
6 (6.8) 12 (12.1) 12 (11.6)
1 103
1 105
0.026
—
2
1 10 1 102 5 102 5 102
4
2.5 10 1.3 103 2.5 104 1.3 103
1 102
(53.6) (105) (110) (249)
2.1
The values in bold type are predicted by Eq. (I) for low [CH3I]. The values in brackets are calculated using the full reaction set (see Appendix).
3.3. Reaction of CH3I with Od 2
mechanism comprising reactions (5)–(12): d COd 2 þ CH3 I- CH3 þ I þ CO2
d
d
ð10Þ
d CH3 þ HCO þ CH4 2 -CO
ð11Þ
CH3 þ COd 2 -CH3 CO2
ð12Þ
Application of the steady-state approximation to d [COd 2 ] and [ CH3], and assuming that G(I )b d G(CO2 ), leads to Eq. (I) ( 1=2 k10 k11 GðCOd 2 Þ GðI Þ ¼ ½CH I ; ðIÞ ½HCO 3 2 2k12 D where D is the dose rate. A reasonable fit to the experimental data is obtained with k10 k11 =2k12 ¼ 0:19 dm3 mol1 s1 as shown in Table 2. Taking 2k12 to be 1 1010 dm3 mol1 s1 gives k10 k11 ¼ 1:9 109 dm6 mol2 s2. Chain termination by reaction (12) indicates that the rate of this reaction is much greater than the self-reactions of dCH3 and COd 2 , which have rate constants of 1.2 109 dm3 mol1 s1 and 5 108 dm3 mol1 s1, respectively (Ross et al., 1998). For short chains there is an extra term –G(COd 2 )/2 in the expression for [COd 2 ]. If one neglects this, then Eq. (I) can be used to estimate G(I) under any conditions. The last row in bold type in Table 2 shows what can be expected at low concentrations. Also shown in Table 2 are the values of G(I) calculated using the full reaction set listed in the appendix. Again there is good accord between the three sets of data.
A convenient way of converting all the primary radicals to Od 2 is to use O2-saturated formate solutions where reactions (2)–(4), (6), (7) and (13) are rapid: d COd 2 þ O2 -O2 þ CO2
k13 ¼ 2 109 dm3 mol1 s1
ð13Þ ðBuxton et al:; 1976Þ:
This system was employed to test whether Od reacts 2 with CH3I. When a dose of 185 Gy was given to an O2solution at pH 7 containing 102 mol dm3 HCO 2 and 2.9 104 mol dm3 CH3I, the yields of the observed products were G(H2O2)=2.9 107 mol J1, G(I)=5.2 108 mol J1 and another product which slowly oxidised I to I2 in the Ghormley analysis (Allen et al., 1952) for H2O2 with G(I2)=4.1 108 mol J1. This product is assumed to be CH3O2H, which is expected to be formed through reactions (14) and (15) (Shankar et al., 1969): d
CH3 þ O2 -CH3 Od2
k14 ¼ 4:1 109 dm3 mol1 s1
ð14Þ ðMarchaj et al:; 1991Þ:
CH3 Od2 þ Od 2 -O2 þ CH3 O 2 ðþH2 O-CH3 O2 H þ OH Þ
ð15Þ
The identity of H2O2 as the major product was confirmed by the fact that it was destroyed by catalase, whereas the product assumed to be CH3O2H was not. If Od 2 does not react with CH3I, then the expected yield of d d H2O2 from reaction (16) is {G(e aq)+G( OH)+G(H )}/ 7 1 2, i.e. 2.8 10 mol J . d Od 2 þ O2 þ2H þ -H2 O2 þ O2
ð16Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
To this must be added G(H2O2)=8 108 mol J1 formed directly in reaction (1), making a total expected yield of G(H2O2)=3.6 107 mol J1. The smaller observed yield can be explained by the competition between reactions (3) and (8), and (2) and (17), respectively: d e aq þ CH3 I- CH3 þ I
k17 ¼ 1:6 1010 dm3 mol1 s1
ð17Þ ðBuxton et al:; 1988Þ:
From the known rate constants under the conditions used, reaction (8) is expected to account for G(Hd)=9 109 mol J1 and reaction (17) for 8 mol J1 and result in the formation G(e aq)=4.3 10 of I with G(I )=5.2 108 mol J1, which equals the measured yield. Thus the shortfall in G(H2O2) should be equal to 1/2(G(I)+G(CH3O2H)), i.e. 4.7 108 mol J1. This is of a similar magnitude to the observed deficit of 7 108 mol J1. Computer simulation shows 2 3 1 1 that k(Od s 2 +CH3I) must be less than 10 dm mol to accommodate G(I)=5.2 108 mol J1, and so it is clear that Od does not react with CH3I to any 2 measurable extent under these conditions.
4. Results of experiments (b) Relatively low concentrations of CH3I were chosen in these experiments to represent the likely conditions in a loss-of-coolant accident.
627
These experiments were carried out to test if Od 2 reacts with CH3I. It can be seen that the decomposition yields are very low; G(CH3I) is typically 4 109 mol J1 at an initial [CH3I] of 4 106 mol dm3, i.e. equating to 8 ca. 1% of G(e mol J1 at an aq)+G(H), and 2.7 10 5 3 initial [CH3I] of 2.3 10 mol dm , i.e. ca. 8% of G(e aq)+G(H). These results are consistent with CH3I reacting only with e aq and H, since the expected yields in this case, calculated from the rate constants in the appendix, are 1.6% and 8.4% of G(e aq)+G(H), respectively. Thus, they support the conclusion drawn in Section 4.3 that Od does not react at a measurable 2 rate with methyl iodide. They also imply that CH3I is not decomposed by reaction with dOH. The rate constant of this reaction is estimated to be 1.5 109 dm3 mol1 s1 (Shankar et al., 1969). In 0.1 mol dm3 phosphate buffer at pH 7, k(dOH+phosphate)[phosphate]B8 103 s1 (Buxton et al., 1988), so that phosphate will only begin to compete significantly (10%) with CH3I for dOH when [CH3I] o4 105 mol dm3. On the other hand, the presence in the phosphate buffer of an impurity that scavenges dOH very efficiently would prevent the reaction of this radical with CH3I under these conditions, but this was not tested experimentally. 4.2. Dilute solutions of CH3I saturated with N2O
The results from experiments in air are shown in Fig. 1, which also shows data for O2-saturated solutions.
The data in Fig. 2 confirm that CH3I is not decomposed to any significant extent by reaction with d OH when its concentration is very low. The values of G(CH3I) estimated from these data are 5.5 109 and 1.8 108 mol J1 for initial concentrations of CH3I of 2.3 106 and 1.6 105 mol dm3, respectively.
Fig. 1. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated O2-saturated (K, ’) and airsaturated (m) solution buffered at pH 7 with 0.1 mol dm3 phosphate.
Fig. 2. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated N2O-saturated solution buffered at pH 7 with 0.1 mol dm3 phosphate for initial [CH3I]=2.3 106 (’) and 1.3 105 (K) mol dm3.
4.1. Dilute solutions of CH3I saturated with air or O2
628
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
4.3. Dilute solutions of CH3I saturated with argon At pH 7 the solutions contained 0.1 mol dm3 2 phosphate buffer ([H2PO 4 ]:[HPO4 ]=1:1) so that eaq is converted to Hd in 2 d e aq þH2 PO4 -HPO4 þ H
k18 ¼ 1:7 107 dm3 mol1 s1
ð18Þ ðBuxton et al:; 1988Þ:
2 3 d e aq þ HPO4 -PO4 þ H
k19 ¼ 1:5 105 dm3 mol1 s1
ð19Þ ðBuxton et al:; 1988Þ:
The decomposition yield of CH3I should therefore reflect the competition between reactions (8) and (20), but Fig. 3 shows that the literature values of k8 and k20 give calculated values of G(CH3I) that are much larger than the experimental ones. d Hd þ H2 PO 4 -HPO4 þ H2
k20 ¼ 5 105 dm3 mol1 s1
Fig. 3. A similarly good fit is obtained if, instead of increasing k20 ; k8 is decreased to 1.2 109 dm3 mol1 s1. Whilst there is no reason, a priori, to doubt the values of k8 and/or k20 ; they have no independent confirmation and there seems to be no other obvious explanation for the low values of G(CH3I) in Fig. 3.
ð20Þ ðBuxton et al:; 1988Þ:
These results suggest that reaction (20) competes more effectively with reaction (8) than is expected from the literature values of k8 and k20 . The presence of an impurity in the phosphate buffer that competes with CH3I for Hd to form an unreactive product could account for this observation. However, calculations show that the level of impurity would have to be equivalent to 2 105 mol dm3 O2, which seems unlikely. In fact, a good fit between simulation and experiment is obtained when k20 is increased to 7 106 dm3 mol1 s1, as shown by the full lines in
5. Discussion 5.1. Reactions of the primary radicals The hydrated electron reacts with methyl iodide in reaction (17) by dissociative electron capture (Symons and Smith, 1981) so that the reactions of dCH3 must be taken into account when this reaction occurs. Reaction (8) takes place at the diffusion-controlled rate (Mezyk and Bartels, 1994), indicating that it does not involve abstraction of H from CH3I, leaving electron transfer as the likely alternative, which probably proceeds via addition of Hd to the iodine moiety followed by dissociation of HI: Hd þ CH3 I-ðCH3 IHÞd -d CH3 þ I þ Hþ
ð80 Þ
From the observations made in pulse radiolysis studies (Bruhlmann . et al., 1973; Buchler . et al., 1977; Mohan and Asmus, 1987) it has been proposed that the reaction of dOH with methyl iodide proceeds via a sequence of equilibria involving the formation of an adduct as d
d OH þ CH3 I"ðCH3 I:OHÞd "ðCHþ 3 :IOH Þ
ð21Þ
Mohan and Asmus (1987) concluded that the adduct (CH3I.OH)d is formed at pH>5 at the diffusion controlled rate, in accord with the report (Shankar et al., 1969) that k(dOH+CH3OH)/k(dOH+CH3I)= 0.65; they found that it decayed exponentially with a half-life of ca. 10 ms in an N2O-saturated solution of 103 mol dm3 CH3I at pH 5.7. A different species was observed at pH 3.5 and was attributed by Mohan and Asmus (1987) to ðCH3 I:OHÞd þ Hþ -CH3 Idþ þH2 O
ð22Þ
followed by formation of a dimer radical cation in CH3 Idþ þ CH3 I"ðCH3 I‘ICH3 Þþ Fig. 3. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated Ar-saturated solution buffered at pH 7 with 0.1 mol dm3 phosphate for initial [CH3I]=4 106 (K) and 5.7 106 (&) mol dm3. For the broken lines k20 ¼ 5 105 dm3 mol1 s1; for the full lines for the dotted lines k20 ¼ 7 106 dm3 mol1 s1; k20 ¼ 5 105 dm3 mol1 s1 and an impurity equivalent to 2 105 mol dm3 O2 is assumed to be present (see text).
k23 ¼ 161 dm3 mol1
ð23Þ
ðMaity and Mohan; 2001Þ:
There is no evidence to show that the products of reactions (21)–(23) react with CH3I. Mohan and Asmus (1987) showed that (CH3I‘ICH3)+ is a strong oxidant which reacts rapidly with I (k ¼ 7:7 109 dm3 mol1 s1) and with (CH3)2S (k ¼ 3:6 109 dm3 mol1 s1). However, in the context of LOCA,
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
(CH3I‘ICH3)+ and CH3Id+ are unlikely to be important species. Information about the fate of the more relevant species (CH3I.OH)d is rather sparse. Maity and Mohan (2001) obtained a value of 41.4 kJ mol1 for the stability of (CH3I.OH)d from semi-empirical quantum chemical calculations. Bruhlmann . et al. (1973) measured products formed by a single pulse of radiation (300 Gy) in N2Osaturated 103 mol dm3 CH3I solution and obtained G(CH3OH)=0.6 107 mol J1 and G(I2)=0.27 107 mol J1. The mechanism proposed by Buchler . et al. (1977) to account for these results comprises following reactions: ðCH3 I:OHÞd þ ðCH3 I:OHÞd -H2 O2 þ 2CH3 I 9
3
1 1
k24 ¼ 6 10 dm mol
s
ðBuchler . et al:; 1977Þ:
d d þ þ ðCHþ 3 :IOH Þ þ H -ð I:H2 OÞ þ CH3
k25 ¼ 2 1010 dm3 mol1 s1 2ðd I:H2 OÞ-I2 þ CHþ 3 þ H2 O-CH3 OH þ H
ð24Þ
ð25Þ
ðBuchler . et al:; 1977Þ: ð26Þ ð27Þ
Since, under these conditions, G(dOH)=6 107 mol J1, G(H2O2) is expected to be ca. 3 107 mol J1. However, this product does not appear to have been measured. It is pertinent to emphasise that CH3OH and I2 were determined under conditions where radical–radical reactions dominated over possible radical–solute reactions since Buchler . et al. (1977) observed the kinetics of the decay of the transient species to be second-order for a dose per pulse of 300 Gy. This contrasts with the firstorder kinetics reported by Mohan and Asmus (1987) for a dose per pulse of only 1–2 Gy. Subsequently, Mohan and Asmus (1988) reported that (CH3I.OH)d is a good one-electron oxidant that reacts with compounds such as (CH3)2S and I with k ¼ 2 109 dm3 mol1 s1 under conditions where CH3I was in at least 30-fold excess. Radical–radical reactions will generally be of much less significance, compared with radical–solute reactions, in g-radiolysis studies where the dose rates, and hence steady-state concentrations of the radicals, are orders of magnitude lower. Results of g-radiolysis studies are considered in the next sections. 5.2. g-Radiolysis studies of aqueous solutions of methyl iodide 5.2.1. Aerated solutions containing iodide ion In solutions at pHB6, Shankar et al. (1969) showed that the major products are H2O2, I2 and CH3O2H (see Table 3) and proposed that the mechanism comprises
629
reactions (1)–(4), (17), (8) together with (14), (15) and (28)–(31): d
CH3 þ O2 -CH3 Od2
CH3 Od2 þ Od 2 -O2 þ CH3 O 2 ðþH2 O-CH3 O2 H þ OH Þ d
CH3 þ I2 -CH3 I þ Id
ð28Þ
d
OH þ I -OH þ Id
ð29Þ
Id þ I -Id 2
ð30Þ
CH3 Od2 þ Id 2 -CH3 O2 H þ I2
ð31Þ
G(CH3O2H) was found to decrease with decreasing [CH3I], which is attributable to reactions (2) and (3) competing with reactions (17) and (8), respectively. The conclusion that reaction (31) occurs rather than (32) was based on the lack of a dose rate effect and the observation (see Table 3) that G(CH3O2H) is independent of [I] in 0.1 mol dm3 CH3I solution (Shankar et al., 1969). CH3 Od2 þ I -CH3 O2 H þ Id 0
ð32Þ 0
ðI2 =I 2Þ
¼ 0:21 V Since E ðI=I Þ ¼ 1:33 V and E (Stanbury, 1989) it is not unreasonable to invoke reaction (31) rather than (32) as a major route to CH3O2H in neutral and alkaline solution. 5.3. Aerated solutions in the absence of added iodide ion Habersbergerova and Sistek (1982) studied the radiation-induced decomposition of methyl iodide in aerated solutions buffered at pH 9 with borate. There was no gas space in the irradiation vessel to provide a reservoir of O2. They observed that the only iodine-containing product was I with G(I)=G(CH3I) and that these G-values increased practically linearly with the initial concentration of CH3I from 7.5 109 mol J1 at 1.5 105 mol dm3 to 2 107 mol J1 at 3.5 104 mol dm3, and then more gradually to 3 107 mol J1 at 1.8 103 mol dm3 (see Fig. 4). With increasing dose, typically 2–10 kGy depending on the initial concentration of CH3I (0.3–1.5 mmol dm3), [I] and [CH3I] attained constant values after ca. 70% decomposition; this was attributed by Habersbergerova and Sistek (1982) to reaction (29) competing with reaction (21). The assumption was made that reaction (21) produces CH3OH and I2 cf. Bruhlmann . et al. (1973), with I2 being reduced to I by reaction with H2O2 and Od (Janovsk!y, 1980). It is remarkable that 2 both [CH3I] and [I] reached a stationary level during irradiation for it implies that these substances are then being destroyed and regenerated at the same rate. CH3O2H was produced under conditions where reaction (17) competes with reaction (2) but G(CH3O2H)
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
630
Table 3 Comparison of experimental (Shankar et al., 1969) and simulated G-valuesa for air-saturated solutions of CH3I at natural pH [CH3I] (mol dm3)
4
10 5 104 5 104 103 1.5 103 4 103 102 102 101 101 101 101 101 b 101 c
[I] (mol dm3)
3
10 0 103 103 103 103 104 103 0 104 103 102 103 0
G(H2O2)
G(I2) (107 mol J1)
G(CH3O2H) (107 mol J1)
Expt
Calc
Expt
Calc
Expt
Calc
0.73 0.73 0.62 0.67 0.41 0.41 0.36 0.36 0.78 0.52 0.52 0.52 — 1.14
0.73 0.46 0.64 0.61 0.45 0.43 0.35 0.39 0.66 0.41 0.41 0.33 0.18 0.88
0.05 0.29 0.62 1.14 1.40 1.76 1.81 1.81 2.12 3.00 3.00 3.00
0.38 0.36 0.98 1.21 1.31 1.47 1.64 1.71 1.65 2.04 2.13 2.26
1.50
0.57
0.1 0.83 0.83 1.19 1.76 2.12 2.18 2.18 2.08 2.18 2.18 2.18 0.24 1.86
0.64 0.87 1.28 1.42 1.49 1.55 1.74 1.76 1.54 2.12 2.15 2.19 0.64 1.81
a
Calculated at 40 min after irradiation to match the experimental conditions. Saturated with I2. c Contains 0.1 mol dm3 CH3OH. b
Fig. 4. Comparison of measured (open points) (Habersbergerova and Sistek, 1982) and calculated (solid points and lines) G-values for I (circles), CH3O2H (squares) and H2O2+ HOI+3IO 3 (triangles) as a function of [CH3I] in g-irradiated air-saturated solution at pH 9.
Fig. 5. Dependence of G(–CH3I) on [CH3I] in air-saturated solution: (’) pH 7 (this work); (K) pH 9 (Habersbergerova and Sistek, 1982); (m) pH 3 (Paquette and Ford, 1989). The solid line shows the calculated yield for decomposition by d reaction with e aq and H only.
accounts for only a third of the carbon-containing products. Small amounts of methane were found and were thought to be due to a small fraction of Hd reacting with CH3I by displacement:
about 30% decomposition. No explanation was given for this result; however, it is clear that the radiationinduced rate of decomposition of methyl iodide is very low under these conditions with G(CH3I) of the order of 109 mol J1. This low value is in agreement with those obtained in the present work (see Section 5.2). In fact, Fig. 5 shows that the dependence of G(CH3I) on log[CH3I] in aerated solutions at pH 7 and 9 follows a smooth curve. The solid line has been calculated by assuming that CH3I is decomposed only by reaction with e aq and H, with the dependence of G(CH3I) on [CH3I] accounted for by the competition
Hd þ CH3 I-CH4 þ Id
ð33Þ
No attempt was made to determine the remaining carbon products. On prolonged irradiation (doses of the order of 1 MGy), complete destruction of ca. 103 mol dm3 CH3I was achieved in a time when hydrolysis in the corresponding unirradiated solutions accounted for only
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
between reactions (17) and (2), and (8) and (3), respectively. The good agreement in Fig. 4 between calculation and experiment validates this assumption and reinforces the conclusion that Od is not reactive 2 towards methyl iodide. It also indicates that dOH does not contribute to G(CH3I) under these conditions. This is consistent with the occurrence of reaction (34), which is known to be efficient: ðCH3 IOHÞd þ I -CH3 I þ Id þ OH k34 ¼ 2 109 dm3 mol1 s1
ð34Þ
ðMohan and Asmus; 1988Þ:
On the other hand, Paquette and Ford (1989) reported G(CH3I)=1.8 107 mol J1 in air-saturated 3 103 mol dm3 CH3I solution at pH 3 (see Table 7). They also found I2 to be the major product at doses up to 2 kGy, with I also a product at higher doses, but no values of G(I2) or G(I) were given. The relatively high value of G(CH3I) was attributed to HOd2 þ CH3 I-CH3 O2 H þ Id
ð35Þ
where HOd2 is produced via reactions (36) and (2)–(4): þ d e aq þ H -H
ð36Þ
k36 ¼ 2:3 1010 dm3 mol1 s1
ðBuxton et al:; 1988Þ:
However, under these conditions G(HOd2 ) is only ca. 3 108 mol J1 so it does not play a significant role in the radiation chemistry. In fact, >90% of the sum of e in reactions (17) and (8) so the aq+H generate I absence of I as a product at low dose is consistent with its oxidation by (CH3I‘ICH3)d+. Fig. 5 shows that the measured value of G(CH3I)=1.8 107 mol J1 at pH 3 for [CH3I]=3 103 mol dm3 is much lower than the value of 2.9 107 mol J1 that can be derived from the data at pH 7 and 9. 5.4. Solutions saturated with argon In this case, Paquette and Ford (1989) found that both I2 and I are produced in 3 103 mol dm3 CH3I solution at pH 3, and G(CH3I)=1.0 107 mol J1.
631
Similar results were obtained at pH 6 where G(–CH3I)= 1.36 107 mol J1 (see Table 4). Addition of 2-methyl-2-propanol at pH 3 to remove d OH resulted in G(CH3I)=0.5 107 mol J1 and I was the only iodine-containing product. Thus it would appear from their experiments that I2 arises from the reaction of dOH with CH3I and that this reaction contributes to the overall decomposition. Indeed, Paquete and Ford concluded that CH3I is decomposed by all the radicals, HOd2 , dOH, Hd and e aq. The results given in Table 5 indicate that G(CH3I)=1.9 108 and 3.6 108 mol J1 in solutions at pH 7 with the initial [CH3I]=4.0 106 and 5.7 106 mol dm3, respectively. Thus, taking these results together with those of Paquette and Ford (1989), there is an indication that G(CH3I) increases with [CH3I] as in air-saturated solution (Fig. 5). 5.5. Solutions saturated with N2O Paquette and Ford (1989) reported that I2 is a major product in 3 103 mol dm3 CH3I at pH 3 with G(CH3I)=5 108 mol J1 (see Table 4). Under these conditions, however, the fractions of e aq taking part in reactions (5), (17) and (36) are 0.76, 0.16 and 0.08, respectively. Thus, G(dOH) is ca. 5 107 mol J1 so that reaction with dOH is not the cause of the decomposition of CH3I. In fact, G(CH3I)= d 5 108 mol J1 is smaller than the sum of e aq+H that react with CH3I, which is estimated to be 1.2 107 mol J1. Simulations show that regeneration of CH3I in reaction (28) accounts for these low yields.
6. Conclusions From the data presented above it is evident that CH3I d d is reduced rapidly by e aq, H and CO2 , but it does not d react at a measurable rate with O2 . This mirrors the reduction potentials of these radicals which are 2.9, 2.3, 1.8 and 0.33 V, respectively (Stanbury, 1998).
Table 4 Comparison of experimental (Paquette and Ford, 1989) and simulated G-valuesa in 103 mol dm3 CH3I solution Gas
N2O Ar Ar Air a b
pH
3 3 6 3
G(I2) (107 mol J1)
G(I) (107 mol J1)
G(CH3I) (107 mol J1)
Calca
Calca
Expt
1 kGy
5 kGy
1 kGy
5 kGy
0.24 0 0 0.64
0.30 0 0 0b
0 1.23 1.25 0.97
0 1.17 1.12 1.08
Integral G-values. All O2 is consumed at doses > 4 kGy.
0.50 1.00 1.36 1.74
Calca 1 kGy
5 kGy
0.75 1.25 1.28 2.37
0.70 1.23 1.18 1.10
632
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Table 5 Comparison of experimental (Shankar et al., 1969) and simulated G-valuesa for air-saturated solutions of CH3I in acidic solution System
[CH3I]=0.1 mol dm3 [I]=0.01 mol dm3
[CH3I]=5 104 mol dm3 [I]=0.001 mol dm3 a
pH
5 3.1 2.7 1.4 1 6 2.3
G(H2O2)+G(I2) (107 mol J1)
G(H2O2) (107 mol J1)
G(I2) (107 mol J1)
G(CH3O2H) (107 mol J1)
Expt
Calc
Expt
Calc
Expt
Calc
Expt
Calc
3.6 3.5 3.7 3.9 3.9 — —
3.13 4.46 4.46 4.46 4.46 — —
— — — — — 0.62 0.88
0.60 1.26 1.26 1.26 1.26 0.64 0.93
— — — — — 0.62 1.45
2.53 3.20 3.20 3.20 3.20 0.98 1.15
2.12 2.18 2.18 2.12 2.18 0.83 0.88
2.19 2.19 2.19 2.19 2.19 1.28 1.24
Calculated at 40 min after irradiation to match the experimental conditions.
There is no good evidence that CH3I is decomposed by reaction with dOH. On the other hand, the work of Mohan and Asmus (1988) shows that (CH3IOH)d, CH3Id+ and (CH3I‘ICH3)+ produced in reactions (21)–(23) are all good one-electron oxidants with reduction potentials X2 V.
k37 ¼ 4 108 dm3 mol1 s1
ðNikolaev et al:; 1992Þ:
Schuchmann and von Sonntag (1984) have shown that the major products of this reaction are CH2O, CH3OH and H2O2, which can be accounted for by 2CH3 Od2 -2CH2 O þ H2 O2
ð38Þ
7. Computer modelling
2CH3 Od2 -CH3 OH þ CH2 O þ O2
ð39Þ
Because there is not yet established a complete mechanism for the radiation chemistry of aqueous solutions of methyl iodide, attempts have been made here to devise one that accounts for the experimental observations. The reaction set adopted here is listed in the appendix. It has been applied to all the experimental conditions described above and the results are compared with the available data.
There are three main groups of reactions in the set: (a) Reactions pertaining to the radiolysis of water and d d reactions of the primary products e aq, H and OH with added solutes and reaction products. The rate constants for these reactions are well established and are taken from the compilation by Ross et al. (1998) when they are listed therein. (b) Reactions of secondary organic radicals derived from the decomposition of CH3I. The primary organic radical is dCH3, formed in reactions (8) and (17). In aerated solution reaction (14) is rapid so that the principal secondary organic radical is CH3Od2 . Following Shankar et al. (1969), we assume that reactions (15) and (31) account for the formation of CH3O2H. Since these are reactions between two radicals in steady-state concentrations, it follows that the competing reaction is likely to be (Nikolaev et al., 1992):
Other reactions that were considered by Schuchmann and von Sonntag (1984) to be responsible for the minor products CH3O2H, HCO2H and CH3O2CH3, which they observed, are not included in our reaction set. By trial and error, we found that k38 ¼ 3 108 dm3 mol1 s1 and k39 ¼ 1 108 dm3 mol1 s1 gave the most satisfactory agreement between calculation and experiment. The data that are most sensitive to the values of k38 and k39 are those of Shankar et al. (1969) in Table 3 (see Section 7.2). The simulations of their data also show that reaction (15) occurs to a negligible extent, accounting for 4 o0.1% of Od mol dm3, even 2 , when [CH3I]=10 7 when k15 is increased from 7 10 dm3 mol1 s1 to 2 109 dm3 mol1 s1. At higher [CH3I] the extent of reaction (15) is even smaller. (c) Reactions of inorganic iodine species. Rate constants for most of these reactions are known or have been estimated (Sellers, 1985 and references therein). Of particular importance in the present context is the reduction of I2 by H2O2 because these species are both products of the radiolysis of methyl iodide solutions and their post-irradiation reaction must be included in the simulations. The kinetics and mechanism of this reaction has been discussed in detail by Ball et al. (1996). A simple mechanism proposed by them, which accounts for their experimental measurements, is the following:
2CH3 Od2 -products
HO 2 þ I2 " HO2 I þ I
7.1. The reaction set
ð37Þ
ð40Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
633
HO2 I þ OH -I þO2 þ H2 O
ð41Þ
7.3. Aerated solution in the presence of iodide ion
HO 2 þ HOI-HO2 I þ OH
ð42Þ
The data shown in Table 1 at pH 7 in 2 103 mol dm3 phosphate buffer can be modelled quite well by making k40 ¼ 6 106 dm3 mol1 at this buffer concentration (see Section 7.1(c)). This implies that k40 is even smaller in the absence of a buffer. Attempts to simulate the data obtained by Shankar et al. (1969) in aerated solutions at natural pH were only partially successful; the results are compared in Table 3 and Fig. 6. Shankar et al. (1969) do not indicate in their paper how the pH was controlled so we have used k39 ¼ 6 106 dm3 mol1 in the simulations. Also, Shankar et al. (1969) did not measure the products in solution
From simulations of measurements made in solutions buffered with phosphate, Ball et al. (1996) found k40 ¼ ð1:870:2Þ 108 and 6.1 107 dm3 mol1 s1 at [phosphate]=0.05 and 0.02 mol dm3, respectively, i.e. approximately proportional to [phosphate], whereas k40 ¼ ð3:970:9Þ 105 ; k41 ¼ 2 109 and k42 ¼ ð1:070:5Þ 109 dm3 mol1 s1 were independent of [phosphate]. Ball et al. (1996) concluded that reaction (40) is catalysed by phosphate. Based on these data, we have assigned values to these rate constants in our simulations as follows: k40=3 109 [phosphate] dm3 mol1 s1, k40=4 105 dm3 mol1 s1, 9 3 1 1 k41=2 10 dm mol s and k42=1 109 dm3 mol1 s1. 7.2. Aerated solution in the absence of iodide ion The agreement between calculation and measurement in Fig. 1 for very dilute solutions at pH 7, saturated with O2 or air, is quite satisfactory. For the more concentrated solutions studied at pH 9 by Habersbergerova and Sistek (1982), the calculated yields in Fig. 4 were obtained at 5 min after the end of the irradiation to allow for thermal reactions between I2 and H2O2. Habersbergerova and Sistek (1982) used a range of doses up to 400 Gy, depending on [CH3I], to obtain the G-values in Fig. 4. The simulated yield–dose plots are not quite linear so the calculated points in Fig. 4 are the means and spread of the integral G-values for doses of 100 and 400 Gy. The calculations reproduce the experimental data quite well. However, they show that H2O2 is replaced by HOI and IO when [CH3I]> 3 103 mol dm3; this change in the products probably would not have been detected in the analysis used by Habersbergerova and Sistek (1982). Whilst the reaction set does conform to the experimental results that G(–CH3I)=G(I) over the concentration range 1 105p[CH3I]p2 103 mol dm3, it does not show the plateau values in these yields at ca. 75% decomposition of CH3I that was observed by Habersbergerova and Sistek (1982). Instead, the calculations show [CH3I] going smoothly to zero and [I] increasing to equal the initial [CH3I]. This discrepancy between experiment and simulation remains unresolved. At the higher [CH3I], the computer simulation predicts that the oxygen is completely depleted before all the CH3I is decomposed. The simulation G-values obtained at pH 3 are shown in Table 4, and all are dependent on dose. Paquette and Ford (1989) reported that I2 is a major product at doses below 2 kGy and that I is also a significant product at higher doses, but this is not reflected in the simulations.
Fig. 6. Comparison of measured (open points and broken lines, Shankar et al., 1969) with calculated (solid points and lines) G-values for I2 (squares), CH3O2H (triangles) and H2O2+HOI+3IO 3 (circles) as a function of [CH3I] in girradiated air-saturated solution at pH B6 containing 103 mol dm3 I.
Fig. 7. Comparison of experimental (solid points, Shankar et al., 1969) with calculated (open points) yields for I2 (circles), CH3O2H (triangles) and H2O2 (squares) from air-saturated solutions at natural pH (B6) containing 0.1 mol dm3 CH3I and 0.01 mol dm3 I. The calculation shows [O2]o2.5 106 mol dm3 for doses>1.2 kGy.
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G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Table 6 Reaction
Rate constant (dm3 mol1 s1)
Reference
Reactions in the radiolysis of water Hd+Hd-H2 Hd+e aq(+H2O)-H2 (+OH ) e aq+eaq(+2H2O)-H2 (+2OH ) d + OH-OH e aq d e aq+H2O2- OH+OH + d e +H -H aq d OH+dOH-H2O2 d OH+Hd-H2O d OH+H2-Hd+H2O d OH+H2O2-HOd2 +H2O d d OH+HO 2 -O2 +H2O d OH+OH"Od+H2O + HOd2 "Od 2 +H d OH+HOd2 -O2+H2O d OH+Od 2 -O2+OH HOd2 +Od -O +HO 2 2 2 d d HO2 +HO2 -O2+H2O2 + H +OH "H2O
7.8e9 2.5e10 5.5e9 3e10 1.1e10 2.3e10 5.5e9 1.5e10 4.2e7 2.7e7 7.5e9 1.3e10; 1.8e6 8e5; 5e10 1e10 1e10 9.7e7 8e5 1.3e11; 2.3e-5 s1
Caldin (1964)
Reactions of water radicals with solutes and products d e aq+O2-O2 Hd+O2-HOd2 d e aq+N2O-N2+O Hd+N2O-N2+dOH d e aq+CH3I- CH3+I d e +CH O H-CH OH+ OH aq 3 2 3 d e aq+I2-I2 d e aq+HOI-HOI d2 e aq+IO3 -IO3 Hd+CH3I-dCH3+H++I d OH+CH3I-(CH3IOH)d d OH+I-HOId d OH+HOI-IOd+H2O d OH+CH2O-dCHO+H2O d Od 2 +I2-I2 +O2 d Od +HOI-HOI +O2 2 d H +HCO2 -H2+COd 2 d d OH+HCO 2 -H2O+CO2
1.9e10 2.1e10 9.1e9 2.1e6 1.6e10 1e10 5.3e10 2e10 8e9 1.2e10 1.5e9 1.1e10 7e9 1e9 6e9 1e6 2.1e8 3.2e9
Reactions of methyl radicals d CH3+O2-CH3Od2 d CH3+I2-CH3I+Id d CH3+dCH3-C2H6 d CH3+Id-CH3I
3e9 2.8e9 1e9 1e10
Assumed value
2.5e9
Best fit value
7e7 3e8
Assumed value Split path giving the best fit
1e8
Split path giving the best fit
Reactions of peroxymethyl radicals (+H2O) CH3Od2 +Id 2 -CH3O2H+I2 (+OH) CH3Od2 +Od 2 (+H2O) -CH3O2H+O2 (+OH) CH3Od2 +CH3Od2 -2CH2O+H2O2 CH3Od2 +CH3Od2 -CH3OH+CH2O+O2
Assumed value
Shankar et al. (1969)
Assumed value (o1e7; Schwarz and Bielski, 1986)
Mezyk and Madden (1996)
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
635
Table 6 (continued) Reaction
Rate constant (dm3 mol1 s1)
Reference
Reactions of (CH3IOH)d (CH3IOH)d+Id 2 -CH3I+I2+OH (CH3IOH)d+I-CH3I+HOI (CH3IOH)d+HOI-CH3I+IOd+H2O (CH3IOH)d+(CH3IOH)d-2CH3I+H2O2 (CH3IOH)d+H+"CH3Id++H2O CH3Id++CH3I"(CH3I‘ICH3)d+ CH3Id++I-CH3I+Id (CH3I‘ICH3)d++I-2CH3I+Id d (CH3IOH)d"(CH+ 3 .IOH ) d (CH+ 3 .IOH ) +I -CH3I+HOI
2e9 2e9 2e9 5e9 1.6e8; 1e3 4e8; 2.5e6 7.7e9 7.7e9 1e8; 4e8 2e9
Assumed value Mohan and Asmus (1987) Assumed value Buchler . et al. (1977) Mohan and Asmus (1987)
Reactions of iodine species I2+OH"HOI 2 HOI+I"HOI 2 + I2+H2O"HOI 2 +H HOId-dOH+I HOId-Id+OH HOId+I-Id 2 +OH HOId+H+-Id+H2O Id+I"Id 2 Id+H2O2-HO2+I+H+ Id+Id-I2 d Id 2 +I2 -I3 +I d d I2 +O2 -O2+2I IOd+IOd-I2O2 IOd2 +H2O"HIOd 3 3 +OH d HIOd +HIO -IO +IO 3 3 3 2 +H2O I2+I"I 3 + I2O2+H2O-HOI+IO 2 +H + HOI+HOI-IO2 +I +2H + HOI+IO 2 -IO3 +I +H I2+HO2 "HO2I+I HOI+H2O2-HO2I+H2O HOI+HO 2 -HO2I+OH HO2I+OH-O2+I+H2O HO2I-O2+I+H+
1e10; 1.9e7 6e8; 4.6e7 2.1; 1e10 20 s1 1.2e8 s1 2.5e4 5e10 1e10; 1e5 5e6 1e10 4e9 5e8 2e9 4.7e7; 1.3e10 2e9 6e9; 8.4e6 1e4 6 1e5 6e6; 5e5 37 2.1e9 2e9 0.2 s1
c
Reactions in the presence of formate ion d COd 2 +CH3I- CH3+CO2+I d d CH3+HCO2 -CH4+CO2 d CH3+COd 2 - CH3CO2
4e4 5e4 5e9
f
a
Mohan and Asmus (1988) Mohan and Asmus (1988) b
Assumed value
c
Sellers (1985) Shiraishi et al. (1992) Sellers (1985) Sellers (1985) Sellers (1985) c
Buxton and Sellers (1985) Sellers (1985) Assumed value d
e
Sellers (1985) Dickinson and Sims (1996) Dickinson and Sims (1996) Dickinson and Sims (1996)
f f
Based on 161 dm3 mol1 (Maity and Mohan, 2001). . Based on K ¼ 0:25 (Buchler et al., 1977). c Best fit values derived from modelling the radiolysis of iodide solutions (e.g. Fig. 11, Buxton and Sellers, 1985). d This reaction is catalysed by OH and borate (Buxton and Sellers, 1985). e The forward reaction is catalysed by phosphate (Ball et al., 1996). f Assumed values derived from fitting Eq. (I) to the data in Table 2. a
b
for at least 40 min after irradiation, so our simulation values allow for this time lapse. In this case, the simulation values of G(H2O2) and G(CH3O2H) agree reasonably well with the experimental data but the model fails to predict the large value of G(I2) for [CH3I]=0.1 mol dm3. Shankar et al. (1969) themselves
were unable to account for such high yields of I2. A similar discrepancy between experiment and simulation is shown by the yield–dose plots in Fig. 7. Shankar et al. (1969) also reported data for aerated solutions at lower pH and the simulations are compared with these in Table 5. At all pH and for [CH3I]=
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G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
0.1 mol dm3, it was found that G(H2O2) is very sensitive to the values of k38 and k39 : Thus when k38 ¼ 1 108 dm3 mol1 s1 and k39 ¼ 3 108 dm3 mol1 s1, G(H2O2) fell to zero. For that reason, we have chosen to make k38 ¼ 3 108 dm3 mol1 s1 and k39 ¼ 1 108 dm3 mol1 s1 in the reaction set used to model all the data reported in this paper. 7.4. N2O-saturated solution Fig. 2 shows the results obtained in dilute solution. Here the agreement between calculation and experiment is fairly good. However, the reaction set predicts that O2 is the major product with G(O2) ca. 107 mol J1 and [H2O2] approaches a steady-state value of ca. 2.5 106 mol dm3 with increasing dose. This is the expected result for a system containing insufficient scavenger to protect H2O2 from reaction with dOH, which is consistent with CH3I being rather stable against attack by dOH. Paquette and Ford (1989) reported that I2 is a major product at pH 3 but did not quantify it. The simulation predicts that I2 is a major product (see Table 4). 7.5. Argon-saturated solution Although there is no agreement between the calculated and measured data for dilute solutions of CH3I at pH 7 in Fig. 3, Table 4 shows that the measured and calculated values of G(CH3I) do agree quite well at pH 3 and 6 when [CH3I] is 1000-fold larger. In this case, the simulations predict that G(I2) is close to zero at these pH whereas Paquette and Ford (1989) reported that both I2 and I were major products, although they did not quantify their yields. The simulations show that H2O2 is produced in these solutions with G(H2O2)E 4 108 mol J1 up to a dose of 4 kGy. Thus some thermal oxidation of I by H2O2 might have occurred at pH 3 to account for the I2 observed by Paquette and Ford (1989). Possible reasons for the discordant results in Fig. 3 have been discussed in Section 3.3.
8. Conclusions The radiation chemistry of methyl iodide in aqueous solution is complex, involving as it does the chemistry of iodine, which itself is complicated enough, as well as the chemistry ensuing from the organic moiety. Nevertheless, a number of elements of the mechanism appear to be well grounded, although further experiments are required to resolve the remaining discrepancies. In the context of a LOCA, the most relevant scenario is likely to comprise aerated, neutral to alkaline solutions containing various levels of organic and
inorganic impurities which may be sufficient to protect CH3I from radiation-induced decomposition. Firstly, there is protection from the potentially efficient scavend ging of e aq and H by O2 to form the unreactive radical Od . Secondly, there is protection by organic impurities 2 from attack by dOH because of the generally high rates of reaction of this radical with such compounds (Buxton et al., 1988), especially as CH3I itself seems to be, to a significant extent, just a carrier of dOH in the form of the adduct (CH3IOH)d. Escape of methyl iodide from solution in an ionising radiation field will be minimised at low [O2] and high pH.
Acknowledgements We are grateful to Mr. I. Colling (University of Leeds) and Mr. G. Baston (AEA Technology, Harwell) for carrying out the experimental work, and to the Health and Safety Executive for financial support.
Appendix Reaction set and rate constants at ambient temperature used to simulate the radiolysis of aqueous solutions of methyl iodide. Except where stated, the rate constants are from Ross et al. (1998) (see Table 6).
References Allen, A.O., Hochanadel, C.J., Ghormley, J.A., Davis, T.W., 1952. Decomposition of water and aqueous solutions under mixed fast neutron and gamma radiation. J. Phys. Chem. 56, 575. Ball, J.M., Hnatiw, J.B., Sims, H.E., 1996. The reduction of I2 by H2O2 in aqueous solution. In: Guntay, . S. (Ed.), Proceedings of the Fourth CNSI Workshop on the Chemistry of Iodine in Reactor Safety. Paul Scherrer Institut, Wurenlingen, . Switzerland, 1996, p. 169. Bielski, B.H.J., Cabelli, D.E., Ross, A.B., 1985. Reactivity of HO2/O 2 radicals in aqueous solution. J. Phys. Chem. Ref. Data 14, 1041. Bruhlmann, . U., Buchler, . H., Marchetti, F., Buhler, . R.E., 1973. Transient ion pairs from OH-radical reactions: pulse radiolysis of aqueous alkyl iodide solutions. Chem. Phys. Lett. 21, 412. Buxton, G.V., Sellers, R.M., 1985. Radiation-induced redox reactions of iodine species in aqueous solution. J. Chem. Soc., Faraday Trans. I 81, 449. Buchler, . H., Bruhlmann, . U., Buhler, . R.E., 1977. A transient equilibrium of outer–inner charge transfer complexes from OH-radical reactions with aqueous alkyl iodide. Chem. Phys. 19, 43. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637 (dOH/Od) in aqueous solution. J. Phys. Chem. Ref. Data 18, 513. Caldin, E.F., 1964. Fast reactions in solution. Blackwell Scientific Publications, Oxford. Dickinson, S., Sims, H.E., 1996. INSPECT-96: a data set for the calculation of iodine speciation and volatility on irradiated solution, AEAT-0176, AEA technology. Winfrith Technology Centre, Dorchester. Habersbergerova, A., Sistek, I., 1982. Radiolysis of methyl iodide in aerated borate spray solution. Radiat. Phys. Chem. 19, 409. Janovsky, I., 1980. Radiolysis of aqueous borate solutions containing iodide. Radiochem. Radioanal. Lett. 44, 115. Maity, D.K., Mohan, H., 2001. Kinetic and thermodynamic parameters of three-electron bonded species formed in the reaction of hydroxyl radicals with alkyl iodides and 1,ndiiodoalkanes. Phys. Chem. Chem. Phys. 3, 5505. Mezyk, S.P., Bartels, D.M., 1994. Rate constant and activation energy measurement for the reaction of atomic hydrogen with methanol, iodomethane, iodoethane, and 1-iodopropane in aqueous solution. J. Phys. Chem. 98, 10578. Mezyk, S.P., Madden, K.P., 1996. Arrhenius parameter determination for the reaction of methyl radicals with iodine species in aqueous solution. J. Phys. Chem. 100, 9360. Mohan, H., Asmus, K.-D., 1987. Radical cations from alkyl iodides in aqueous solution. J. Chem. Soc. Perkin Trans. II, 1795. Mohan, H., Asmus, K.-D., 1988. Rate constants for oxidation reactions by radical cations from methyl iodide. J. Chem. Soc. Perkin Trans. II, 118. Nikolaev, A.I., Safiullin, R.L., Enikeeva, L.R., Komissarov, V.D., 1992. Kinetics of the loss of methylperoxyl radicals in a liquid phase. Khim. Fiz. 11, 69. Paquette, J., Ford, B.L., 1989. The radiation-induced formation of iodoalkanes and the radiolysis of iodomethane. In: Vikis,
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A.C. (Ed.), Proceedings of the Second CNSI Workshop on Iodine Chemistry in Reactor Safety. AECL-9923/CSNI-149, p. 48. Ross, A.B., Mallard, W.G., Helman, W.P., Buxton, G.V., Huie, R.E., Neta, P., 1998. NDRL-NIST Solution Kinetics Database—Ver. 3, NIST Standard Reference Data. Gaithersburg, MD. Schuchmann, H.-P., von Sonntag, C., 1984. Methylperoxyl radicals: a study of the g-radiolysis of methane in oxygenated aqueous solutions. Sellers, R.M., 1985. Reaction kinetics of iodine compounds in aqueous solution. CEGB Report no. TPRD/B/0688/N85. Schwarz, H.A., Bielski, B.H.J., 1986. Reactions of HO2 and O 2 with iodine and bromine and the I 2 and I reduction potentials. J. Phys. Chem. 90, 1445. Shankar, J.K., Rama Rao, V.S., Shastri, L.V., 1969. Peroxide formation in the g radiolysis of aerated aqueous solutions of methyl iodide. J. Phys. Chem. 73, 52. Shiraishi, H., Okuda, H., Morinaga, Y., Ishigure, K., 1992. Measurement on the rate of some reactions relevant to iodine chemistry in the aqueous phase. In: Ishigure, K., Saeki, M., Soda, K., Sigimoto, J. (Eds.), Proceedings of the Third CSNI Workshop on Iodine Chemistry in Reactor Safety. Japan Atomic Energy Research Institute, Tokaimura, Japan, 1991, p. 152. Stanbury, D.M., 1998. Reduction potentials involving inorganic free radicals in aqueous solution. Prog. Inorg. Chem. 47, 511. Symons, M.C.R., Smith, I.G., 1981. Unstable intermediates. Part 194. Alkyl radical adducts: an electron spin resonance study of the radiolysis of a range alkyl bromides and iodides. J. Chem. Soc. Perkin Trans. II, 1180. Thomas, J.K., 1967. Pulse radiolysis of aqueous solutions of methyl iodide and methyl bromide. The reactions of iodine atoms and methyl radicals in water. J. Phys. Chem. 71, 1919.
On the radiation chemistry of methyl iodide in aqueous solution G.V. Buxtona,*, H.E. Simsb a
School of Chemistry, University of Leeds, Leeds LS2 9JT, UK b AEA Technology, Harwell, Didcot, OX11 0RA, UK Received 18 June 2002; accepted 15 November 2002
Abstract d New results on the reactions of COd 2 and O2 with methyl iodide in aqueous solution are presented. These results, together with the published experimental data under a wide range of conditions (pH, concentration, saturating gas), d d show that CH3I is readily decomposed to dCH3 and I by the strong reductants e aq, H and CO2 , but is unreactive d d d with the weaker reductant O2 . This means that oxygen, which reacts rapidly with eaq, H and COd 2 to form O2 , can protect methyl iodide from radiation-induced reductive decomposition in aerated neutral and alkaline aqueous solution. The available evidence indicates that the strong oxidant dOH reacts with CH3I by addition to form the more selective oxidant (CH3IOH)d without decomposing the molecule. A set of reactions has been devised to simulate the experimental data. In most cases the trends shown by the data under a wide variety of conditions are reproduced quite well. r 2002 Published by Elsevier Science Ltd.
Keywords: Methyl iodide; Radiolysis; Modelling methyl iodide chemistry; Nuclear safety
1. Introduction The radiation chemistry of methyl iodide in aqueous solution is potentially important for nuclear safety, for example in a loss of coolant accident (LOCA) in a PWR. Whilst there have been several investigations of various aspects of the radiation chemistry of methyl iodide in aqueous solution (Thomas, 1967; Shankar et al., 1969; Bruhlmann . et al., 1973; Buchler . et al., 1977; Habersbergerova and Sistek, 1982; Mohan and Asmus, 1987, 1988; Paquette and Ford, 1989; Mezyk and Madden, 1996; Maity and Mohan, 2001), none has provided complete information on the products and their yields. As a result, our knowledge of the reaction mechanism is still lacking; in particular, the products of the reaction of the hydroxyl radical have still to be established. Ideally, one would like to achieve conditions in a LOCA where *Corresponding author.
volatile methyl iodide is decomposed reductively to nonvolatile iodide ion. The radiolysis of water is described by H2 O
d d 2:8e aq ; 0:62lH ; 2:8 OH; 0:47H2 ;
0:78H2 O2 ; 2:7Hþ
ð1Þ
The numbers are typical G-values in units of 107 mol J1 applicable to dilute aqueous solution. However, they are dependent on the scavenging power of the solutes and so must be adjusted for each system of interest. Here information about the reactions of the d d primary radicals e aq, H and OH with methyl iodide is required. In addition, reactions of secondary radicals that are likely to be formed under ambient conditions must also be considered. For example, in the presence of d d d air, e aq and H are rapidly converted to O2 /HO2 : d e aq þ O2 -O2
k2 ¼ 1:9 1010 dm3 mol1 s1
0969-806X/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. doi:10.1016/S0969-806X(02)00498-X
ð2Þ ðBuxton et al:; 1988Þ:
624
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Hd þ O2 -HOd2
ð3Þ
k3 ¼ 2:1 1010 dm3 mol1 s1 þ HOd2 "Od 2 þH
ðBuxton et al:; 1988Þ:
pKa ¼ 4:8 ðBielski et al:; 1985Þ: ð4Þ
d Thus, the reactions of Od 2 /HO2 come into play in aerated solutions (Bielski et al., 1985). In this paper we present new experimental data, including the reactions of COd and Od with methyl 2 2 iodide; we also summarise, evaluate and attempt to model these and the published data that have been obtained under a range of different conditions.
2. Experimental Experiments were carried out at (a) the University of Leeds and (b) AEA Technology, Harwell. (a) Methyl iodide (BDH laboratory reagent grade) was washed with aqueous 0.1 mol dm3 potassium iodide solution and then with triply distilled water. Solutions for irradiation were prepared by injecting ca. 0.1 cm3 of the washed CH3I into 100 cm3 of triply distilled water and diluting as necessary. The final concentration of CH3I was determined from absorbance measurements at 250 nm taking e250 ¼ 34 m2 mol1 (Habersbergerova and Sistek, 1982). These solutions were prepared immediately before use. When they were required to be saturated with a gas, this was achieved by saturating the water or aqueous solution before injecting the methyl iodide. All other reagents were of analytical grade; the gases were of the highest purity available and were used straight from the cylinder. Phosphate buffer (2 103 mol dm3) was used to maintain pH 7. The solutions (20 cm3) were irradiated with 60Co grays at a dose rate of 1.03 Gy s1 in a pyrex tube (i.d. 2 cm) which had attached to it a sidearm carrying a suprasil spectrophotometer cell (optical path length 1 cm). Thus, any spectral changes could be measured without opening the irradiation vessel. The CH3I was equilibrated between the gas (ca. 20 cm3) and liquid phases by filling and emptying the sidearm until the absorbance at 250 nm became constant. Irradiation was carried out at ambient temperature (ca. 221C). After irradiation, H2O2 and CH3O2H were measured by the Ghormley method (Allen et al., 1952). This method also measures any other species capable of oxidising iodide ion, and also free iodine in the form of I 3. (b) Experiments to measure the radiolytic destruction of methyl iodide were carried out under atmospheres of air, argon and nitrous oxide, respectively. The solutions were buffered at pH 4.7 with boric acid solution (0.1 mol dm3) or pH 7 with phosphate solution
(0.1 mol dm3). These high concentrations of buffer were chosen to represent the likely conditions in a LOCA. For experiments under Ar or N2O, the solution was saturated with the required gas and then CH3I was added and equilibrated. Samples for irradiation were drawn into a 10 cm3 glass syringe so that there was no gas space, sealed with a glass cap and irradiated with 60 Co g-rays at a dose rate of 0.026 Gy s1. Before irradiation, a small aliquot was taken from the syringe to determine the initial [CH3I]. Samples were also taken periodically during the irradiation to measure changes in [CH3I]. A control experiment was also carried out with an unirradiated aerated solution that was kept in the dark between samplings. The concentration of CH3I was determined by solvent extraction into toluene, followed by analysis of the toluene solution by gas chromatography, using a Pye Unicam PU 4550 gas chromatograph equipped with SGE BP5-1.0 column and electron capture detector. Quantification was by comparison of the sample peak heights with those of freshly prepared standards analysed under the same conditions. Correction was made for the efficiency of solvent extraction which was measured as 92%. In both sets of experiments, (a) and (b), the dose rate was determined by Fricke dosimetry (Spinks and Woods, 1964).
3. Results of experiments (a) 3.1. Solutions of CH3I+I+O2 The results of these experiments, which are qualitatively similar to those reported by Shankar et al. (1969), are presented in Table 1. When the irradiated solutions were analysed by the Ghormley method (Allen et al., 1952), an immediate yield of I 3 was observed and a further amount was produced over a period of ca. 20 min. The immediate yields, which are assigned to H2O2+I2, have been corrected for the slowly formed I 3 , assumed to be due to CH3O2H, during the time (2–3 min) taken to measure them. Also shown in Table 1 are the data calculated using the reaction set for the radiolysis of aqueous solutions of CH3I, presented in the appendix. The two sets of data agree quite closely. 3.2. Reaction of CH3I with COd 2 Under the conditions used (0.01–0.05 mol dm3 HCO 2 , pH 7) essentially all the primary radicals of water radiolysis are converted to the strong reductant via: COd 2 d e aq þ N2 O-N2 þ OH þ OH
ð5Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
625
Table 1 Comparison of experiment and simulation for irradiated solutions containing CH3I and O2 at pH 7 (2 103 mol dm3 phosphate). Dose rate 1.03 Gy s1 Time (s)
[I2]+[H2O2]a (105 mol dm3)
[CH3O2H]b (105 mol dm3)
Totalc (105 mol dm3)
Expt
Expt
Expt
Calc
Calc
Calc
[CH3I]=4.8 104, [I]=3.6 103, [O2]=2.4 104 mol dm3 120 2.41 2.96 1.65 240 4.42 5.63 3.03 360 6.80 8.00 4.10 480 8.60 10.07 5.80
1.70 3.32 4.83 6.25
4.06 7.45 10.90 14.40
4.66 8.95 12.83 16.32
[CH3I]=4.8 104, [I]=3.7 103, 120 1.79 240 3.77 360 4.22 480 6.03
[O2]=1.3 103 mol dm3 1.51 0.80 2.93 1.48 4.26 2.06 5.50 3.02
0.88 1.74 2.57 3.38
2.59 5.25 6.28 9.05
2.39 4.67 6.83 8.88
[CH3I]=2.4 102, [I]=3.0 103, [O2]=2.4 104 mol dm3 120 4.47 4.75 2.82 240 7.58 8.63 6.42 360 11.87 11.65 7.98 480 12.45 13.91 10.31 600 14.20 15.55 11.87
2.46 4.91 7.32 9.63 11.78
7.29 14.00 19.85 22.76 26.07
7.21 13.54 18.97 23.54 26.33
[CH3I]=2.4 102, [I]=3.2 103, [O2]=1.3 103 mol dm3 120 5.25 4.64 2.72 240 9.53 8.77 6.42 360 10.89 11.96 10.12 480 13.23 14.72 12.06 600 18.09 17.26 13.81
2.46 4.91 7.35 9.79 12.25
7.97 15.95 21.01 25.29 31.90
7.10 13.68 19.31 24.51 29.51
[CH3I]=4.8 103, [I]=3.6 103, [O2]=1.3 103 mol dm3 120 3.27 3.75 2.33 240 6.03 7.17 5.06 480 12.45 12.93 10.12
2.16 4.32 8.57
6.80 11.09 22.57
5.91 11.49 21.40
[CH3I]=4.8 103, [I]=3.6 103, [O2]=2.4 104 mol dm3 120 4.67 4.40 2.92 240 6.42 8.21 6.03 600 14.20 16.00 10.89
2.23 4.44 10.72
7.39 12.45 25.09
6.63 12.65 26.72
Measured as the immediate yield of I 3 (see text). Measured as the slowly formed yield of I 3. c Sum of the immediate and slowly formed yields of I 3. a
b
k5 ¼ 9:1 109 dm3 mol1 s1 d
ðBuxton et al:; 1988Þ:
d OH þ HCO 2 -CO2 þ H2 O 9
3
1 1
k6 ¼ 3:2 10 dm mol
s
ð6Þ
Hd þ CH3 I-d CH3 þ I þ Hþ
ð8Þ
k8 ¼ 1:2 1010 dm3 mol1 s1 ðBuxton et al:; 1988Þ:
d Hd þ HCO 2 -CO2 þ H2
k7 ¼ 2:1 108 dm3 mol1 s1
unchanged:
ðMezyk and Bartels; 1994Þ: ð7Þ
ðBuxton et al:; 1988Þ:
Under conditions where reactions (8) and/or (9) would compete with reaction (7), the mechanistic outcome is
Hd þ N2 O-N2 þd OH k9 ¼ 2:1 106 dm3 mol1 s1
ð9Þ ðBuxton et al:; 1988Þ:
The data are presented in Table 2 and show that CH3I is decomposed by a chain reaction, consistent with a
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
626
Table 2 Comparison of G(I)expt with G(I)calc in N2O-saturated CH3I solutions containing HCO 2 3 [HCO 2 ] (mol dm )
Dose rate (Gy s1)
[CH3I]
G(I) (107 mol J1) Expt
Calc
0.012 0.012 0.012 0.012
55 93 130 220
49 112 110 250
2.5 104
0.053
40
23 (24.3)
1 102 1 102 5 102
2.5 104 1.3 103 2.5 104
1.03 1.03 1.03
11 15 20
6 (6.8) 12 (12.1) 12 (11.6)
1 103
1 105
0.026
—
2
1 10 1 102 5 102 5 102
4
2.5 10 1.3 103 2.5 104 1.3 103
1 102
(53.6) (105) (110) (249)
2.1
The values in bold type are predicted by Eq. (I) for low [CH3I]. The values in brackets are calculated using the full reaction set (see Appendix).
3.3. Reaction of CH3I with Od 2
mechanism comprising reactions (5)–(12): d COd 2 þ CH3 I- CH3 þ I þ CO2
d
d
ð10Þ
d CH3 þ HCO þ CH4 2 -CO
ð11Þ
CH3 þ COd 2 -CH3 CO2
ð12Þ
Application of the steady-state approximation to d [COd 2 ] and [ CH3], and assuming that G(I )b d G(CO2 ), leads to Eq. (I) ( 1=2 k10 k11 GðCOd 2 Þ GðI Þ ¼ ½CH I ; ðIÞ ½HCO 3 2 2k12 D where D is the dose rate. A reasonable fit to the experimental data is obtained with k10 k11 =2k12 ¼ 0:19 dm3 mol1 s1 as shown in Table 2. Taking 2k12 to be 1 1010 dm3 mol1 s1 gives k10 k11 ¼ 1:9 109 dm6 mol2 s2. Chain termination by reaction (12) indicates that the rate of this reaction is much greater than the self-reactions of dCH3 and COd 2 , which have rate constants of 1.2 109 dm3 mol1 s1 and 5 108 dm3 mol1 s1, respectively (Ross et al., 1998). For short chains there is an extra term –G(COd 2 )/2 in the expression for [COd 2 ]. If one neglects this, then Eq. (I) can be used to estimate G(I) under any conditions. The last row in bold type in Table 2 shows what can be expected at low concentrations. Also shown in Table 2 are the values of G(I) calculated using the full reaction set listed in the appendix. Again there is good accord between the three sets of data.
A convenient way of converting all the primary radicals to Od 2 is to use O2-saturated formate solutions where reactions (2)–(4), (6), (7) and (13) are rapid: d COd 2 þ O2 -O2 þ CO2
k13 ¼ 2 109 dm3 mol1 s1
ð13Þ ðBuxton et al:; 1976Þ:
This system was employed to test whether Od reacts 2 with CH3I. When a dose of 185 Gy was given to an O2solution at pH 7 containing 102 mol dm3 HCO 2 and 2.9 104 mol dm3 CH3I, the yields of the observed products were G(H2O2)=2.9 107 mol J1, G(I)=5.2 108 mol J1 and another product which slowly oxidised I to I2 in the Ghormley analysis (Allen et al., 1952) for H2O2 with G(I2)=4.1 108 mol J1. This product is assumed to be CH3O2H, which is expected to be formed through reactions (14) and (15) (Shankar et al., 1969): d
CH3 þ O2 -CH3 Od2
k14 ¼ 4:1 109 dm3 mol1 s1
ð14Þ ðMarchaj et al:; 1991Þ:
CH3 Od2 þ Od 2 -O2 þ CH3 O 2 ðþH2 O-CH3 O2 H þ OH Þ
ð15Þ
The identity of H2O2 as the major product was confirmed by the fact that it was destroyed by catalase, whereas the product assumed to be CH3O2H was not. If Od 2 does not react with CH3I, then the expected yield of d d H2O2 from reaction (16) is {G(e aq)+G( OH)+G(H )}/ 7 1 2, i.e. 2.8 10 mol J . d Od 2 þ O2 þ2H þ -H2 O2 þ O2
ð16Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
To this must be added G(H2O2)=8 108 mol J1 formed directly in reaction (1), making a total expected yield of G(H2O2)=3.6 107 mol J1. The smaller observed yield can be explained by the competition between reactions (3) and (8), and (2) and (17), respectively: d e aq þ CH3 I- CH3 þ I
k17 ¼ 1:6 1010 dm3 mol1 s1
ð17Þ ðBuxton et al:; 1988Þ:
From the known rate constants under the conditions used, reaction (8) is expected to account for G(Hd)=9 109 mol J1 and reaction (17) for 8 mol J1 and result in the formation G(e aq)=4.3 10 of I with G(I )=5.2 108 mol J1, which equals the measured yield. Thus the shortfall in G(H2O2) should be equal to 1/2(G(I)+G(CH3O2H)), i.e. 4.7 108 mol J1. This is of a similar magnitude to the observed deficit of 7 108 mol J1. Computer simulation shows 2 3 1 1 that k(Od s 2 +CH3I) must be less than 10 dm mol to accommodate G(I)=5.2 108 mol J1, and so it is clear that Od does not react with CH3I to any 2 measurable extent under these conditions.
4. Results of experiments (b) Relatively low concentrations of CH3I were chosen in these experiments to represent the likely conditions in a loss-of-coolant accident.
627
These experiments were carried out to test if Od 2 reacts with CH3I. It can be seen that the decomposition yields are very low; G(CH3I) is typically 4 109 mol J1 at an initial [CH3I] of 4 106 mol dm3, i.e. equating to 8 ca. 1% of G(e mol J1 at an aq)+G(H), and 2.7 10 5 3 initial [CH3I] of 2.3 10 mol dm , i.e. ca. 8% of G(e aq)+G(H). These results are consistent with CH3I reacting only with e aq and H, since the expected yields in this case, calculated from the rate constants in the appendix, are 1.6% and 8.4% of G(e aq)+G(H), respectively. Thus, they support the conclusion drawn in Section 4.3 that Od does not react at a measurable 2 rate with methyl iodide. They also imply that CH3I is not decomposed by reaction with dOH. The rate constant of this reaction is estimated to be 1.5 109 dm3 mol1 s1 (Shankar et al., 1969). In 0.1 mol dm3 phosphate buffer at pH 7, k(dOH+phosphate)[phosphate]B8 103 s1 (Buxton et al., 1988), so that phosphate will only begin to compete significantly (10%) with CH3I for dOH when [CH3I] o4 105 mol dm3. On the other hand, the presence in the phosphate buffer of an impurity that scavenges dOH very efficiently would prevent the reaction of this radical with CH3I under these conditions, but this was not tested experimentally. 4.2. Dilute solutions of CH3I saturated with N2O
The results from experiments in air are shown in Fig. 1, which also shows data for O2-saturated solutions.
The data in Fig. 2 confirm that CH3I is not decomposed to any significant extent by reaction with d OH when its concentration is very low. The values of G(CH3I) estimated from these data are 5.5 109 and 1.8 108 mol J1 for initial concentrations of CH3I of 2.3 106 and 1.6 105 mol dm3, respectively.
Fig. 1. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated O2-saturated (K, ’) and airsaturated (m) solution buffered at pH 7 with 0.1 mol dm3 phosphate.
Fig. 2. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated N2O-saturated solution buffered at pH 7 with 0.1 mol dm3 phosphate for initial [CH3I]=2.3 106 (’) and 1.3 105 (K) mol dm3.
4.1. Dilute solutions of CH3I saturated with air or O2
628
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
4.3. Dilute solutions of CH3I saturated with argon At pH 7 the solutions contained 0.1 mol dm3 2 phosphate buffer ([H2PO 4 ]:[HPO4 ]=1:1) so that eaq is converted to Hd in 2 d e aq þH2 PO4 -HPO4 þ H
k18 ¼ 1:7 107 dm3 mol1 s1
ð18Þ ðBuxton et al:; 1988Þ:
2 3 d e aq þ HPO4 -PO4 þ H
k19 ¼ 1:5 105 dm3 mol1 s1
ð19Þ ðBuxton et al:; 1988Þ:
The decomposition yield of CH3I should therefore reflect the competition between reactions (8) and (20), but Fig. 3 shows that the literature values of k8 and k20 give calculated values of G(CH3I) that are much larger than the experimental ones. d Hd þ H2 PO 4 -HPO4 þ H2
k20 ¼ 5 105 dm3 mol1 s1
Fig. 3. A similarly good fit is obtained if, instead of increasing k20 ; k8 is decreased to 1.2 109 dm3 mol1 s1. Whilst there is no reason, a priori, to doubt the values of k8 and/or k20 ; they have no independent confirmation and there seems to be no other obvious explanation for the low values of G(CH3I) in Fig. 3.
ð20Þ ðBuxton et al:; 1988Þ:
These results suggest that reaction (20) competes more effectively with reaction (8) than is expected from the literature values of k8 and k20 . The presence of an impurity in the phosphate buffer that competes with CH3I for Hd to form an unreactive product could account for this observation. However, calculations show that the level of impurity would have to be equivalent to 2 105 mol dm3 O2, which seems unlikely. In fact, a good fit between simulation and experiment is obtained when k20 is increased to 7 106 dm3 mol1 s1, as shown by the full lines in
5. Discussion 5.1. Reactions of the primary radicals The hydrated electron reacts with methyl iodide in reaction (17) by dissociative electron capture (Symons and Smith, 1981) so that the reactions of dCH3 must be taken into account when this reaction occurs. Reaction (8) takes place at the diffusion-controlled rate (Mezyk and Bartels, 1994), indicating that it does not involve abstraction of H from CH3I, leaving electron transfer as the likely alternative, which probably proceeds via addition of Hd to the iodine moiety followed by dissociation of HI: Hd þ CH3 I-ðCH3 IHÞd -d CH3 þ I þ Hþ
ð80 Þ
From the observations made in pulse radiolysis studies (Bruhlmann . et al., 1973; Buchler . et al., 1977; Mohan and Asmus, 1987) it has been proposed that the reaction of dOH with methyl iodide proceeds via a sequence of equilibria involving the formation of an adduct as d
d OH þ CH3 I"ðCH3 I:OHÞd "ðCHþ 3 :IOH Þ
ð21Þ
Mohan and Asmus (1987) concluded that the adduct (CH3I.OH)d is formed at pH>5 at the diffusion controlled rate, in accord with the report (Shankar et al., 1969) that k(dOH+CH3OH)/k(dOH+CH3I)= 0.65; they found that it decayed exponentially with a half-life of ca. 10 ms in an N2O-saturated solution of 103 mol dm3 CH3I at pH 5.7. A different species was observed at pH 3.5 and was attributed by Mohan and Asmus (1987) to ðCH3 I:OHÞd þ Hþ -CH3 Idþ þH2 O
ð22Þ
followed by formation of a dimer radical cation in CH3 Idþ þ CH3 I"ðCH3 I‘ICH3 Þþ Fig. 3. Comparison of measured (points) and calculated (lines) values of [CH3I] in g-irradiated Ar-saturated solution buffered at pH 7 with 0.1 mol dm3 phosphate for initial [CH3I]=4 106 (K) and 5.7 106 (&) mol dm3. For the broken lines k20 ¼ 5 105 dm3 mol1 s1; for the full lines for the dotted lines k20 ¼ 7 106 dm3 mol1 s1; k20 ¼ 5 105 dm3 mol1 s1 and an impurity equivalent to 2 105 mol dm3 O2 is assumed to be present (see text).
k23 ¼ 161 dm3 mol1
ð23Þ
ðMaity and Mohan; 2001Þ:
There is no evidence to show that the products of reactions (21)–(23) react with CH3I. Mohan and Asmus (1987) showed that (CH3I‘ICH3)+ is a strong oxidant which reacts rapidly with I (k ¼ 7:7 109 dm3 mol1 s1) and with (CH3)2S (k ¼ 3:6 109 dm3 mol1 s1). However, in the context of LOCA,
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
(CH3I‘ICH3)+ and CH3Id+ are unlikely to be important species. Information about the fate of the more relevant species (CH3I.OH)d is rather sparse. Maity and Mohan (2001) obtained a value of 41.4 kJ mol1 for the stability of (CH3I.OH)d from semi-empirical quantum chemical calculations. Bruhlmann . et al. (1973) measured products formed by a single pulse of radiation (300 Gy) in N2Osaturated 103 mol dm3 CH3I solution and obtained G(CH3OH)=0.6 107 mol J1 and G(I2)=0.27 107 mol J1. The mechanism proposed by Buchler . et al. (1977) to account for these results comprises following reactions: ðCH3 I:OHÞd þ ðCH3 I:OHÞd -H2 O2 þ 2CH3 I 9
3
1 1
k24 ¼ 6 10 dm mol
s
ðBuchler . et al:; 1977Þ:
d d þ þ ðCHþ 3 :IOH Þ þ H -ð I:H2 OÞ þ CH3
k25 ¼ 2 1010 dm3 mol1 s1 2ðd I:H2 OÞ-I2 þ CHþ 3 þ H2 O-CH3 OH þ H
ð24Þ
ð25Þ
ðBuchler . et al:; 1977Þ: ð26Þ ð27Þ
Since, under these conditions, G(dOH)=6 107 mol J1, G(H2O2) is expected to be ca. 3 107 mol J1. However, this product does not appear to have been measured. It is pertinent to emphasise that CH3OH and I2 were determined under conditions where radical–radical reactions dominated over possible radical–solute reactions since Buchler . et al. (1977) observed the kinetics of the decay of the transient species to be second-order for a dose per pulse of 300 Gy. This contrasts with the firstorder kinetics reported by Mohan and Asmus (1987) for a dose per pulse of only 1–2 Gy. Subsequently, Mohan and Asmus (1988) reported that (CH3I.OH)d is a good one-electron oxidant that reacts with compounds such as (CH3)2S and I with k ¼ 2 109 dm3 mol1 s1 under conditions where CH3I was in at least 30-fold excess. Radical–radical reactions will generally be of much less significance, compared with radical–solute reactions, in g-radiolysis studies where the dose rates, and hence steady-state concentrations of the radicals, are orders of magnitude lower. Results of g-radiolysis studies are considered in the next sections. 5.2. g-Radiolysis studies of aqueous solutions of methyl iodide 5.2.1. Aerated solutions containing iodide ion In solutions at pHB6, Shankar et al. (1969) showed that the major products are H2O2, I2 and CH3O2H (see Table 3) and proposed that the mechanism comprises
629
reactions (1)–(4), (17), (8) together with (14), (15) and (28)–(31): d
CH3 þ O2 -CH3 Od2
CH3 Od2 þ Od 2 -O2 þ CH3 O 2 ðþH2 O-CH3 O2 H þ OH Þ d
CH3 þ I2 -CH3 I þ Id
ð28Þ
d
OH þ I -OH þ Id
ð29Þ
Id þ I -Id 2
ð30Þ
CH3 Od2 þ Id 2 -CH3 O2 H þ I2
ð31Þ
G(CH3O2H) was found to decrease with decreasing [CH3I], which is attributable to reactions (2) and (3) competing with reactions (17) and (8), respectively. The conclusion that reaction (31) occurs rather than (32) was based on the lack of a dose rate effect and the observation (see Table 3) that G(CH3O2H) is independent of [I] in 0.1 mol dm3 CH3I solution (Shankar et al., 1969). CH3 Od2 þ I -CH3 O2 H þ Id 0
ð32Þ 0
ðI2 =I 2Þ
¼ 0:21 V Since E ðI=I Þ ¼ 1:33 V and E (Stanbury, 1989) it is not unreasonable to invoke reaction (31) rather than (32) as a major route to CH3O2H in neutral and alkaline solution. 5.3. Aerated solutions in the absence of added iodide ion Habersbergerova and Sistek (1982) studied the radiation-induced decomposition of methyl iodide in aerated solutions buffered at pH 9 with borate. There was no gas space in the irradiation vessel to provide a reservoir of O2. They observed that the only iodine-containing product was I with G(I)=G(CH3I) and that these G-values increased practically linearly with the initial concentration of CH3I from 7.5 109 mol J1 at 1.5 105 mol dm3 to 2 107 mol J1 at 3.5 104 mol dm3, and then more gradually to 3 107 mol J1 at 1.8 103 mol dm3 (see Fig. 4). With increasing dose, typically 2–10 kGy depending on the initial concentration of CH3I (0.3–1.5 mmol dm3), [I] and [CH3I] attained constant values after ca. 70% decomposition; this was attributed by Habersbergerova and Sistek (1982) to reaction (29) competing with reaction (21). The assumption was made that reaction (21) produces CH3OH and I2 cf. Bruhlmann . et al. (1973), with I2 being reduced to I by reaction with H2O2 and Od (Janovsk!y, 1980). It is remarkable that 2 both [CH3I] and [I] reached a stationary level during irradiation for it implies that these substances are then being destroyed and regenerated at the same rate. CH3O2H was produced under conditions where reaction (17) competes with reaction (2) but G(CH3O2H)
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
630
Table 3 Comparison of experimental (Shankar et al., 1969) and simulated G-valuesa for air-saturated solutions of CH3I at natural pH [CH3I] (mol dm3)
4
10 5 104 5 104 103 1.5 103 4 103 102 102 101 101 101 101 101 b 101 c
[I] (mol dm3)
3
10 0 103 103 103 103 104 103 0 104 103 102 103 0
G(H2O2)
G(I2) (107 mol J1)
G(CH3O2H) (107 mol J1)
Expt
Calc
Expt
Calc
Expt
Calc
0.73 0.73 0.62 0.67 0.41 0.41 0.36 0.36 0.78 0.52 0.52 0.52 — 1.14
0.73 0.46 0.64 0.61 0.45 0.43 0.35 0.39 0.66 0.41 0.41 0.33 0.18 0.88
0.05 0.29 0.62 1.14 1.40 1.76 1.81 1.81 2.12 3.00 3.00 3.00
0.38 0.36 0.98 1.21 1.31 1.47 1.64 1.71 1.65 2.04 2.13 2.26
1.50
0.57
0.1 0.83 0.83 1.19 1.76 2.12 2.18 2.18 2.08 2.18 2.18 2.18 0.24 1.86
0.64 0.87 1.28 1.42 1.49 1.55 1.74 1.76 1.54 2.12 2.15 2.19 0.64 1.81
a
Calculated at 40 min after irradiation to match the experimental conditions. Saturated with I2. c Contains 0.1 mol dm3 CH3OH. b
Fig. 4. Comparison of measured (open points) (Habersbergerova and Sistek, 1982) and calculated (solid points and lines) G-values for I (circles), CH3O2H (squares) and H2O2+ HOI+3IO 3 (triangles) as a function of [CH3I] in g-irradiated air-saturated solution at pH 9.
Fig. 5. Dependence of G(–CH3I) on [CH3I] in air-saturated solution: (’) pH 7 (this work); (K) pH 9 (Habersbergerova and Sistek, 1982); (m) pH 3 (Paquette and Ford, 1989). The solid line shows the calculated yield for decomposition by d reaction with e aq and H only.
accounts for only a third of the carbon-containing products. Small amounts of methane were found and were thought to be due to a small fraction of Hd reacting with CH3I by displacement:
about 30% decomposition. No explanation was given for this result; however, it is clear that the radiationinduced rate of decomposition of methyl iodide is very low under these conditions with G(CH3I) of the order of 109 mol J1. This low value is in agreement with those obtained in the present work (see Section 5.2). In fact, Fig. 5 shows that the dependence of G(CH3I) on log[CH3I] in aerated solutions at pH 7 and 9 follows a smooth curve. The solid line has been calculated by assuming that CH3I is decomposed only by reaction with e aq and H, with the dependence of G(CH3I) on [CH3I] accounted for by the competition
Hd þ CH3 I-CH4 þ Id
ð33Þ
No attempt was made to determine the remaining carbon products. On prolonged irradiation (doses of the order of 1 MGy), complete destruction of ca. 103 mol dm3 CH3I was achieved in a time when hydrolysis in the corresponding unirradiated solutions accounted for only
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
between reactions (17) and (2), and (8) and (3), respectively. The good agreement in Fig. 4 between calculation and experiment validates this assumption and reinforces the conclusion that Od is not reactive 2 towards methyl iodide. It also indicates that dOH does not contribute to G(CH3I) under these conditions. This is consistent with the occurrence of reaction (34), which is known to be efficient: ðCH3 IOHÞd þ I -CH3 I þ Id þ OH k34 ¼ 2 109 dm3 mol1 s1
ð34Þ
ðMohan and Asmus; 1988Þ:
On the other hand, Paquette and Ford (1989) reported G(CH3I)=1.8 107 mol J1 in air-saturated 3 103 mol dm3 CH3I solution at pH 3 (see Table 7). They also found I2 to be the major product at doses up to 2 kGy, with I also a product at higher doses, but no values of G(I2) or G(I) were given. The relatively high value of G(CH3I) was attributed to HOd2 þ CH3 I-CH3 O2 H þ Id
ð35Þ
where HOd2 is produced via reactions (36) and (2)–(4): þ d e aq þ H -H
ð36Þ
k36 ¼ 2:3 1010 dm3 mol1 s1
ðBuxton et al:; 1988Þ:
However, under these conditions G(HOd2 ) is only ca. 3 108 mol J1 so it does not play a significant role in the radiation chemistry. In fact, >90% of the sum of e in reactions (17) and (8) so the aq+H generate I absence of I as a product at low dose is consistent with its oxidation by (CH3I‘ICH3)d+. Fig. 5 shows that the measured value of G(CH3I)=1.8 107 mol J1 at pH 3 for [CH3I]=3 103 mol dm3 is much lower than the value of 2.9 107 mol J1 that can be derived from the data at pH 7 and 9. 5.4. Solutions saturated with argon In this case, Paquette and Ford (1989) found that both I2 and I are produced in 3 103 mol dm3 CH3I solution at pH 3, and G(CH3I)=1.0 107 mol J1.
631
Similar results were obtained at pH 6 where G(–CH3I)= 1.36 107 mol J1 (see Table 4). Addition of 2-methyl-2-propanol at pH 3 to remove d OH resulted in G(CH3I)=0.5 107 mol J1 and I was the only iodine-containing product. Thus it would appear from their experiments that I2 arises from the reaction of dOH with CH3I and that this reaction contributes to the overall decomposition. Indeed, Paquete and Ford concluded that CH3I is decomposed by all the radicals, HOd2 , dOH, Hd and e aq. The results given in Table 5 indicate that G(CH3I)=1.9 108 and 3.6 108 mol J1 in solutions at pH 7 with the initial [CH3I]=4.0 106 and 5.7 106 mol dm3, respectively. Thus, taking these results together with those of Paquette and Ford (1989), there is an indication that G(CH3I) increases with [CH3I] as in air-saturated solution (Fig. 5). 5.5. Solutions saturated with N2O Paquette and Ford (1989) reported that I2 is a major product in 3 103 mol dm3 CH3I at pH 3 with G(CH3I)=5 108 mol J1 (see Table 4). Under these conditions, however, the fractions of e aq taking part in reactions (5), (17) and (36) are 0.76, 0.16 and 0.08, respectively. Thus, G(dOH) is ca. 5 107 mol J1 so that reaction with dOH is not the cause of the decomposition of CH3I. In fact, G(CH3I)= d 5 108 mol J1 is smaller than the sum of e aq+H that react with CH3I, which is estimated to be 1.2 107 mol J1. Simulations show that regeneration of CH3I in reaction (28) accounts for these low yields.
6. Conclusions From the data presented above it is evident that CH3I d d is reduced rapidly by e aq, H and CO2 , but it does not d react at a measurable rate with O2 . This mirrors the reduction potentials of these radicals which are 2.9, 2.3, 1.8 and 0.33 V, respectively (Stanbury, 1998).
Table 4 Comparison of experimental (Paquette and Ford, 1989) and simulated G-valuesa in 103 mol dm3 CH3I solution Gas
N2O Ar Ar Air a b
pH
3 3 6 3
G(I2) (107 mol J1)
G(I) (107 mol J1)
G(CH3I) (107 mol J1)
Calca
Calca
Expt
1 kGy
5 kGy
1 kGy
5 kGy
0.24 0 0 0.64
0.30 0 0 0b
0 1.23 1.25 0.97
0 1.17 1.12 1.08
Integral G-values. All O2 is consumed at doses > 4 kGy.
0.50 1.00 1.36 1.74
Calca 1 kGy
5 kGy
0.75 1.25 1.28 2.37
0.70 1.23 1.18 1.10
632
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Table 5 Comparison of experimental (Shankar et al., 1969) and simulated G-valuesa for air-saturated solutions of CH3I in acidic solution System
[CH3I]=0.1 mol dm3 [I]=0.01 mol dm3
[CH3I]=5 104 mol dm3 [I]=0.001 mol dm3 a
pH
5 3.1 2.7 1.4 1 6 2.3
G(H2O2)+G(I2) (107 mol J1)
G(H2O2) (107 mol J1)
G(I2) (107 mol J1)
G(CH3O2H) (107 mol J1)
Expt
Calc
Expt
Calc
Expt
Calc
Expt
Calc
3.6 3.5 3.7 3.9 3.9 — —
3.13 4.46 4.46 4.46 4.46 — —
— — — — — 0.62 0.88
0.60 1.26 1.26 1.26 1.26 0.64 0.93
— — — — — 0.62 1.45
2.53 3.20 3.20 3.20 3.20 0.98 1.15
2.12 2.18 2.18 2.12 2.18 0.83 0.88
2.19 2.19 2.19 2.19 2.19 1.28 1.24
Calculated at 40 min after irradiation to match the experimental conditions.
There is no good evidence that CH3I is decomposed by reaction with dOH. On the other hand, the work of Mohan and Asmus (1988) shows that (CH3IOH)d, CH3Id+ and (CH3I‘ICH3)+ produced in reactions (21)–(23) are all good one-electron oxidants with reduction potentials X2 V.
k37 ¼ 4 108 dm3 mol1 s1
ðNikolaev et al:; 1992Þ:
Schuchmann and von Sonntag (1984) have shown that the major products of this reaction are CH2O, CH3OH and H2O2, which can be accounted for by 2CH3 Od2 -2CH2 O þ H2 O2
ð38Þ
7. Computer modelling
2CH3 Od2 -CH3 OH þ CH2 O þ O2
ð39Þ
Because there is not yet established a complete mechanism for the radiation chemistry of aqueous solutions of methyl iodide, attempts have been made here to devise one that accounts for the experimental observations. The reaction set adopted here is listed in the appendix. It has been applied to all the experimental conditions described above and the results are compared with the available data.
There are three main groups of reactions in the set: (a) Reactions pertaining to the radiolysis of water and d d reactions of the primary products e aq, H and OH with added solutes and reaction products. The rate constants for these reactions are well established and are taken from the compilation by Ross et al. (1998) when they are listed therein. (b) Reactions of secondary organic radicals derived from the decomposition of CH3I. The primary organic radical is dCH3, formed in reactions (8) and (17). In aerated solution reaction (14) is rapid so that the principal secondary organic radical is CH3Od2 . Following Shankar et al. (1969), we assume that reactions (15) and (31) account for the formation of CH3O2H. Since these are reactions between two radicals in steady-state concentrations, it follows that the competing reaction is likely to be (Nikolaev et al., 1992):
Other reactions that were considered by Schuchmann and von Sonntag (1984) to be responsible for the minor products CH3O2H, HCO2H and CH3O2CH3, which they observed, are not included in our reaction set. By trial and error, we found that k38 ¼ 3 108 dm3 mol1 s1 and k39 ¼ 1 108 dm3 mol1 s1 gave the most satisfactory agreement between calculation and experiment. The data that are most sensitive to the values of k38 and k39 are those of Shankar et al. (1969) in Table 3 (see Section 7.2). The simulations of their data also show that reaction (15) occurs to a negligible extent, accounting for 4 o0.1% of Od mol dm3, even 2 , when [CH3I]=10 7 when k15 is increased from 7 10 dm3 mol1 s1 to 2 109 dm3 mol1 s1. At higher [CH3I] the extent of reaction (15) is even smaller. (c) Reactions of inorganic iodine species. Rate constants for most of these reactions are known or have been estimated (Sellers, 1985 and references therein). Of particular importance in the present context is the reduction of I2 by H2O2 because these species are both products of the radiolysis of methyl iodide solutions and their post-irradiation reaction must be included in the simulations. The kinetics and mechanism of this reaction has been discussed in detail by Ball et al. (1996). A simple mechanism proposed by them, which accounts for their experimental measurements, is the following:
2CH3 Od2 -products
HO 2 þ I2 " HO2 I þ I
7.1. The reaction set
ð37Þ
ð40Þ
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
633
HO2 I þ OH -I þO2 þ H2 O
ð41Þ
7.3. Aerated solution in the presence of iodide ion
HO 2 þ HOI-HO2 I þ OH
ð42Þ
The data shown in Table 1 at pH 7 in 2 103 mol dm3 phosphate buffer can be modelled quite well by making k40 ¼ 6 106 dm3 mol1 at this buffer concentration (see Section 7.1(c)). This implies that k40 is even smaller in the absence of a buffer. Attempts to simulate the data obtained by Shankar et al. (1969) in aerated solutions at natural pH were only partially successful; the results are compared in Table 3 and Fig. 6. Shankar et al. (1969) do not indicate in their paper how the pH was controlled so we have used k39 ¼ 6 106 dm3 mol1 in the simulations. Also, Shankar et al. (1969) did not measure the products in solution
From simulations of measurements made in solutions buffered with phosphate, Ball et al. (1996) found k40 ¼ ð1:870:2Þ 108 and 6.1 107 dm3 mol1 s1 at [phosphate]=0.05 and 0.02 mol dm3, respectively, i.e. approximately proportional to [phosphate], whereas k40 ¼ ð3:970:9Þ 105 ; k41 ¼ 2 109 and k42 ¼ ð1:070:5Þ 109 dm3 mol1 s1 were independent of [phosphate]. Ball et al. (1996) concluded that reaction (40) is catalysed by phosphate. Based on these data, we have assigned values to these rate constants in our simulations as follows: k40=3 109 [phosphate] dm3 mol1 s1, k40=4 105 dm3 mol1 s1, 9 3 1 1 k41=2 10 dm mol s and k42=1 109 dm3 mol1 s1. 7.2. Aerated solution in the absence of iodide ion The agreement between calculation and measurement in Fig. 1 for very dilute solutions at pH 7, saturated with O2 or air, is quite satisfactory. For the more concentrated solutions studied at pH 9 by Habersbergerova and Sistek (1982), the calculated yields in Fig. 4 were obtained at 5 min after the end of the irradiation to allow for thermal reactions between I2 and H2O2. Habersbergerova and Sistek (1982) used a range of doses up to 400 Gy, depending on [CH3I], to obtain the G-values in Fig. 4. The simulated yield–dose plots are not quite linear so the calculated points in Fig. 4 are the means and spread of the integral G-values for doses of 100 and 400 Gy. The calculations reproduce the experimental data quite well. However, they show that H2O2 is replaced by HOI and IO when [CH3I]> 3 103 mol dm3; this change in the products probably would not have been detected in the analysis used by Habersbergerova and Sistek (1982). Whilst the reaction set does conform to the experimental results that G(–CH3I)=G(I) over the concentration range 1 105p[CH3I]p2 103 mol dm3, it does not show the plateau values in these yields at ca. 75% decomposition of CH3I that was observed by Habersbergerova and Sistek (1982). Instead, the calculations show [CH3I] going smoothly to zero and [I] increasing to equal the initial [CH3I]. This discrepancy between experiment and simulation remains unresolved. At the higher [CH3I], the computer simulation predicts that the oxygen is completely depleted before all the CH3I is decomposed. The simulation G-values obtained at pH 3 are shown in Table 4, and all are dependent on dose. Paquette and Ford (1989) reported that I2 is a major product at doses below 2 kGy and that I is also a significant product at higher doses, but this is not reflected in the simulations.
Fig. 6. Comparison of measured (open points and broken lines, Shankar et al., 1969) with calculated (solid points and lines) G-values for I2 (squares), CH3O2H (triangles) and H2O2+HOI+3IO 3 (circles) as a function of [CH3I] in girradiated air-saturated solution at pH B6 containing 103 mol dm3 I.
Fig. 7. Comparison of experimental (solid points, Shankar et al., 1969) with calculated (open points) yields for I2 (circles), CH3O2H (triangles) and H2O2 (squares) from air-saturated solutions at natural pH (B6) containing 0.1 mol dm3 CH3I and 0.01 mol dm3 I. The calculation shows [O2]o2.5 106 mol dm3 for doses>1.2 kGy.
634
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
Table 6 Reaction
Rate constant (dm3 mol1 s1)
Reference
Reactions in the radiolysis of water Hd+Hd-H2 Hd+e aq(+H2O)-H2 (+OH ) e aq+eaq(+2H2O)-H2 (+2OH ) d + OH-OH e aq d e aq+H2O2- OH+OH + d e +H -H aq d OH+dOH-H2O2 d OH+Hd-H2O d OH+H2-Hd+H2O d OH+H2O2-HOd2 +H2O d d OH+HO 2 -O2 +H2O d OH+OH"Od+H2O + HOd2 "Od 2 +H d OH+HOd2 -O2+H2O d OH+Od 2 -O2+OH HOd2 +Od -O +HO 2 2 2 d d HO2 +HO2 -O2+H2O2 + H +OH "H2O
7.8e9 2.5e10 5.5e9 3e10 1.1e10 2.3e10 5.5e9 1.5e10 4.2e7 2.7e7 7.5e9 1.3e10; 1.8e6 8e5; 5e10 1e10 1e10 9.7e7 8e5 1.3e11; 2.3e-5 s1
Caldin (1964)
Reactions of water radicals with solutes and products d e aq+O2-O2 Hd+O2-HOd2 d e aq+N2O-N2+O Hd+N2O-N2+dOH d e aq+CH3I- CH3+I d e +CH O H-CH OH+ OH aq 3 2 3 d e aq+I2-I2 d e aq+HOI-HOI d2 e aq+IO3 -IO3 Hd+CH3I-dCH3+H++I d OH+CH3I-(CH3IOH)d d OH+I-HOId d OH+HOI-IOd+H2O d OH+CH2O-dCHO+H2O d Od 2 +I2-I2 +O2 d Od +HOI-HOI +O2 2 d H +HCO2 -H2+COd 2 d d OH+HCO 2 -H2O+CO2
1.9e10 2.1e10 9.1e9 2.1e6 1.6e10 1e10 5.3e10 2e10 8e9 1.2e10 1.5e9 1.1e10 7e9 1e9 6e9 1e6 2.1e8 3.2e9
Reactions of methyl radicals d CH3+O2-CH3Od2 d CH3+I2-CH3I+Id d CH3+dCH3-C2H6 d CH3+Id-CH3I
3e9 2.8e9 1e9 1e10
Assumed value
2.5e9
Best fit value
7e7 3e8
Assumed value Split path giving the best fit
1e8
Split path giving the best fit
Reactions of peroxymethyl radicals (+H2O) CH3Od2 +Id 2 -CH3O2H+I2 (+OH) CH3Od2 +Od 2 (+H2O) -CH3O2H+O2 (+OH) CH3Od2 +CH3Od2 -2CH2O+H2O2 CH3Od2 +CH3Od2 -CH3OH+CH2O+O2
Assumed value
Shankar et al. (1969)
Assumed value (o1e7; Schwarz and Bielski, 1986)
Mezyk and Madden (1996)
G.V. Buxton, H.E. Sims / Radiation Physics and Chemistry 67 (2003) 623–637
635
Table 6 (continued) Reaction
Rate constant (dm3 mol1 s1)
Reference
Reactions of (CH3IOH)d (CH3IOH)d+Id 2 -CH3I+I2+OH (CH3IOH)d+I-CH3I+HOI (CH3IOH)d+HOI-CH3I+IOd+H2O (CH3IOH)d+(CH3IOH)d-2CH3I+H2O2 (CH3IOH)d+H+"CH3Id++H2O CH3Id++CH3I"(CH3I‘ICH3)d+ CH3Id++I-CH3I+Id (CH3I‘ICH3)d++I-2CH3I+Id d (CH3IOH)d"(CH+ 3 .IOH ) d (CH+ 3 .IOH ) +I -CH3I+HOI
2e9 2e9 2e9 5e9 1.6e8; 1e3 4e8; 2.5e6 7.7e9 7.7e9 1e8; 4e8 2e9
Assumed value Mohan and Asmus (1987) Assumed value Buchler . et al. (1977) Mohan and Asmus (1987)
Reactions of iodine species I2+OH"HOI 2 HOI+I"HOI 2 + I2+H2O"HOI 2 +H HOId-dOH+I HOId-Id+OH HOId+I-Id 2 +OH HOId+H+-Id+H2O Id+I"Id 2 Id+H2O2-HO2+I+H+ Id+Id-I2 d Id 2 +I2 -I3 +I d d I2 +O2 -O2+2I IOd+IOd-I2O2 IOd2 +H2O"HIOd 3 3 +OH d HIOd +HIO -IO +IO 3 3 3 2 +H2O I2+I"I 3 + I2O2+H2O-HOI+IO 2 +H + HOI+HOI-IO2 +I +2H + HOI+IO 2 -IO3 +I +H I2+HO2 "HO2I+I HOI+H2O2-HO2I+H2O HOI+HO 2 -HO2I+OH HO2I+OH-O2+I+H2O HO2I-O2+I+H+
1e10; 1.9e7 6e8; 4.6e7 2.1; 1e10 20 s1 1.2e8 s1 2.5e4 5e10 1e10; 1e5 5e6 1e10 4e9 5e8 2e9 4.7e7; 1.3e10 2e9 6e9; 8.4e6 1e4 6 1e5 6e6; 5e5 37 2.1e9 2e9 0.2 s1
c
Reactions in the presence of formate ion d COd 2 +CH3I- CH3+CO2+I d d CH3+HCO2 -CH4+CO2 d CH3+COd 2 - CH3CO2
4e4 5e4 5e9
f
a
Mohan and Asmus (1988) Mohan and Asmus (1988) b
Assumed value
c
Sellers (1985) Shiraishi et al. (1992) Sellers (1985) Sellers (1985) Sellers (1985) c
Buxton and Sellers (1985) Sellers (1985) Assumed value d
e
Sellers (1985) Dickinson and Sims (1996) Dickinson and Sims (1996) Dickinson and Sims (1996)
f f
Based on 161 dm3 mol1 (Maity and Mohan, 2001). . Based on K ¼ 0:25 (Buchler et al., 1977). c Best fit values derived from modelling the radiolysis of iodide solutions (e.g. Fig. 11, Buxton and Sellers, 1985). d This reaction is catalysed by OH and borate (Buxton and Sellers, 1985). e The forward reaction is catalysed by phosphate (Ball et al., 1996). f Assumed values derived from fitting Eq. (I) to the data in Table 2. a
b
for at least 40 min after irradiation, so our simulation values allow for this time lapse. In this case, the simulation values of G(H2O2) and G(CH3O2H) agree reasonably well with the experimental data but the model fails to predict the large value of G(I2) for [CH3I]=0.1 mol dm3. Shankar et al. (1969) themselves
were unable to account for such high yields of I2. A similar discrepancy between experiment and simulation is shown by the yield–dose plots in Fig. 7. Shankar et al. (1969) also reported data for aerated solutions at lower pH and the simulations are compared with these in Table 5. At all pH and for [CH3I]=
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0.1 mol dm3, it was found that G(H2O2) is very sensitive to the values of k38 and k39 : Thus when k38 ¼ 1 108 dm3 mol1 s1 and k39 ¼ 3 108 dm3 mol1 s1, G(H2O2) fell to zero. For that reason, we have chosen to make k38 ¼ 3 108 dm3 mol1 s1 and k39 ¼ 1 108 dm3 mol1 s1 in the reaction set used to model all the data reported in this paper. 7.4. N2O-saturated solution Fig. 2 shows the results obtained in dilute solution. Here the agreement between calculation and experiment is fairly good. However, the reaction set predicts that O2 is the major product with G(O2) ca. 107 mol J1 and [H2O2] approaches a steady-state value of ca. 2.5 106 mol dm3 with increasing dose. This is the expected result for a system containing insufficient scavenger to protect H2O2 from reaction with dOH, which is consistent with CH3I being rather stable against attack by dOH. Paquette and Ford (1989) reported that I2 is a major product at pH 3 but did not quantify it. The simulation predicts that I2 is a major product (see Table 4). 7.5. Argon-saturated solution Although there is no agreement between the calculated and measured data for dilute solutions of CH3I at pH 7 in Fig. 3, Table 4 shows that the measured and calculated values of G(CH3I) do agree quite well at pH 3 and 6 when [CH3I] is 1000-fold larger. In this case, the simulations predict that G(I2) is close to zero at these pH whereas Paquette and Ford (1989) reported that both I2 and I were major products, although they did not quantify their yields. The simulations show that H2O2 is produced in these solutions with G(H2O2)E 4 108 mol J1 up to a dose of 4 kGy. Thus some thermal oxidation of I by H2O2 might have occurred at pH 3 to account for the I2 observed by Paquette and Ford (1989). Possible reasons for the discordant results in Fig. 3 have been discussed in Section 3.3.
8. Conclusions The radiation chemistry of methyl iodide in aqueous solution is complex, involving as it does the chemistry of iodine, which itself is complicated enough, as well as the chemistry ensuing from the organic moiety. Nevertheless, a number of elements of the mechanism appear to be well grounded, although further experiments are required to resolve the remaining discrepancies. In the context of a LOCA, the most relevant scenario is likely to comprise aerated, neutral to alkaline solutions containing various levels of organic and
inorganic impurities which may be sufficient to protect CH3I from radiation-induced decomposition. Firstly, there is protection from the potentially efficient scavend ging of e aq and H by O2 to form the unreactive radical Od . Secondly, there is protection by organic impurities 2 from attack by dOH because of the generally high rates of reaction of this radical with such compounds (Buxton et al., 1988), especially as CH3I itself seems to be, to a significant extent, just a carrier of dOH in the form of the adduct (CH3IOH)d. Escape of methyl iodide from solution in an ionising radiation field will be minimised at low [O2] and high pH.
Acknowledgements We are grateful to Mr. I. Colling (University of Leeds) and Mr. G. Baston (AEA Technology, Harwell) for carrying out the experimental work, and to the Health and Safety Executive for financial support.
Appendix Reaction set and rate constants at ambient temperature used to simulate the radiolysis of aqueous solutions of methyl iodide. Except where stated, the rate constants are from Ross et al. (1998) (see Table 6).
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