Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 211,No. 2 (1996) 461-471
Jointly published by Elsevier Science S. A., Lausanne and Akad~miai Kiadr, Budapest
VALENCY STABILIZATION OF POLYVALENT IONS DURING GAMMA-IRRADIATION OF THEIR AQUEOUS SOLUTIONS BY SACRIFICIAL PROTECTION II. VALENCY STABILIZATION OF Ce(IV) BY BROMATE IONS M. F. BARAKAT*, M. M. ABDEL-HAMID
Nuclear Chemistry Department, Hot Lab. Centre, Atomic Energy Authority, Cairo (Egypt) (Received September 5, 1996) The various aspects of valency stabilization of polyvalent ions during ~/-radiolysis have been further investigated. Ce(IV) ions, which are normally reduced in their aqueous solution, were found to be stabilized for increasing periods of time when they were irradiated in the presence of increasing amounts of bromate ions. It was found that the addition of about fifteen times excess of bromate ions tO a 10-3 N Ce(IV) solution stabilized the cerium ions in the tetravalent state for about 120 hours during irradiation at a dose rate of 336 Gy/h. Increasing the amount of bromate used resulted in a subsequent increase in the protection time. It has been also noted that while bromate ions protected Ce(IV) in solution, the latter ions showed a clear protective effect on the bromate used, i.e., there is a mutual protective effect. The probable mechanisms, conditions and limitations of the protection process have been discussed. Based on the data obtained in the present work, it has been suggested that the protection of Ce(IV) ions by bromate ions in aqueous solutions during ~/-radiolysis is very probably due to the preferential interaction of bromate with the reducing radiolysis products of water which are capable of reducing Ce(IV) to Ce(IIl).
Irradiation of polyvalent ions in their aqueous systems, usually leads to changes in their valency states. It has been suggested that when subjected to ionizing radiations, all polyvalent ions with standard electrode potentials, E ~ higher than 0.8 V are reduced and those with E ~ values lower than 0.8 V are oxidised while in the range 1.15-0.7 V different investigators obtained conflicting results, i.e., both oxidation and reduction3 In the course of our studies aiming at investigating the nature of various protection processes of polyvalent ions during "t-irradiation, the protection of Fe(II), has been treated in a previous publication. 2 In that work, sodium sulphite, being a powerful reducing agent, was used as a protective agent. It has been demonstrated that the processes leading to valency stabilization of Fe(II) ions, in aqueous acidic solutions, involved the preferential interaction of the protecting agent, i.e., sulphite ions, with the *Present address: Arab Atomic Energy Agency, P.O.Box 402, E1-Manzah-1004 Tunis, Tunisia.
0236-5731/96/US $15.0 Copyright 9 1996 Akad~miai Kiadr, Budapest All rights reserved
M. F. BARAKAT, M. M. ABDEL-HAMID: VALENCY STABILIZATION, II.
oxidising radicals, formed as a result of water radiolysis and which are capable of oxidising iron ions to the trivalent state. In order to get a more comprehensive idea about the nature of the valency stabilization processes of polyvalent ions during "/-radiolysis, it is worthwhile to investigate the case of a polyvalent ion that is normally reduced during T-irradiation. Thus, in the present work, the valency stabilization of Ce(IV) ions, which are normally reduced when irradiated by T-rays, has been studied. Sodium bromate, being a strong oxidising agent (with E ~ = 1.45 V) was chosen as a protective agent for Ce(IV) ions against radiolytic reduction to Ce(III). It is expected that bromate ions can actively participate in the competitive reactions with the reducing species formed in the irradiated medium due to water radiolysis. This can finally lead to the valency stabilization of the cerium ions in the tetravalent state. The conditions, limitations and the nature of the protection process have been studied in detail.
Experimental Chemicals: Extra pure Ce(W) sulphate (Ce(SO4)2 94H20 ) and Ce(III) sulphate (Ce2(SO4)3 98I-I20) were obtained from Prolabo Co., France. Analytical grade sodium bromate (NaBrO3) was obtained from the British Drug Houses, U.K. Preparation and analysis of solutions: Exactly about 4.04 g/Cerium(IV) sulphate were quantitatively transferred into a glass beaker followed by 45 ml of 1 : 1 H2SO4 and the solution was heated with frequent addition of double distilled water until the solid completely dissolved. The resultant solution was cooled and quantitatively transferred and completed to the mark in a 1 litre volumetric flask. The concentration of Ce(W) was determined titrimelrically with a standard 0.01N FeSO4 solution using ferroin as indicator. 3 Accurately, about 7.12 g of pure cerium(HI) sulphate were quantitatively transferred into a glass Maker followed by 55 ml of 1 : 1 H2SO4 and the solution was heated with frequent addition of double distilled water until the solid completely dissolved. After cooling, the solution was quantitatively transferred and completed to the mark in a volumetric flask. The exact concentration of Ce(III) was determined titrimetrically by the persulphate method.4 A stock standard sodium bromate solution was prepared by dissolving exactly about 15.091 g of dry NaBrO 3 in double distilled water and the resultant solution was quantitatively transferred and completed to the mark in a 1 lille volumetric flask. The exact bromate concentration was then calculated.5 Preparation and irradiation of samples: The irradiated aqueous samples were prepared by introducing appropriate amounts of Ce(IV) solution together with varying 462
M. F. BARAKAT,M. M. ABDEL-HAMID:VALENCYSTABILIZATION,II. amounts of the bromate solution into 50 ml glass irradiation tubes of 15 cm long and 2.5 cm in diameter having a short neck and provided with a ground glass stopper. Samples were irradiated in a Canadian 6~ y-cell which was frequently calibrated by the Fricke dosimeter. 6 Analysis of irradiated samples: Analysis of cerium ions in solutions was carded out spectrophotometrically using a Shimadzu UV-VIS double beam spectrophotometer type UV-210 A. For Ce(IV) determination in the irradiated solutions a calibration curve was used, within the concentration range 2- 10-5-3 9 10-4 N Ce(IV) in 0.8 N I-I2SO4. Measurements were carded out at 320 nm whereby the molar absorptivity was found to be 5830 which compares well with the value (5800) reported in the literature.7 For Ce(III) determination a calibration curve was used within the concentration range 1.6.10-4-2.4 9 10-3 N in 1.0 N H2SO4. The optical density of Ce(III) solutions was measured at 253 nm whereby the molar absorptivity was found to be 708 which is in fair agreement with the value reported in the literature.8 Determination of the bromate ions in the irradiated solutions was carded out by adding a slight excess of solid KI to an aliquote of the irradiated solution (10 ml) followed by 5 ml conc. H2SO4 in a glass stoppered conical flask. After keeping the solution for 15 minutes, the liberated iodine was determined titrimelrically against a standard sodium thiosulphate solution using starch as indicator near the end point.9 Ce(IV), if present, did not interfere with the bromate determination.
Results and discussions Ce(IV) aqueous solutions are normally reduced when ilTadiated by y-radiations. Figure 1 shows that 10-3 N Ce(IV) solutions, containing sulphuric acid within the concentration range 2-5 N could be almost completely reduced in about 12 hours on using a y-dose rate of 336 Gy/h. At rather lower acidities, i.e., at 1 N H2SO4 the reduction occurred more rapidly. The mechanism of radiolytic reduction of Ce(IV) has been amply documented in the literature.7,1~ Thus, it has been reported that Ce(IV) can be reduced by the reaction Ce(IV) + H-->Ce(III) + H + The yield of Ce(III) did not vary whether 02 was intially present or not,6 since in presence of oxygen the following reactions can also occur: 02 + H--->HO2 Ce(IV) + HO2--->Ce(III) + H + + 02 463
M. F. BARAKAT, M. M. ABDEL-HAMID: V A L E N C Y STABILIZATION, II.
A )
1.0 G 0 43 0 43
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0.5
m
%
% %
%
%
9
% %
0
[ 2
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% 9
I 9 9 I F 8 10 12 Irrodiation time ,h
Fig. 1. Reduction kinetics of 10-3N Ce(IV) solutions during y-irradiation in presence of different H2SO 4 acid concentrations: A - IN, • - 2N, O - 3.5N, 9 1 - 5N; dose ,rate: 336 Gy/h
which implies that the overall Ce(III) yield will not differ. At lower Ce(IV) concentrations HO 2 can react with H radicals to give rise to H202 which, in turn, can reduce another Ce(IV) as follows: Ce(IV) + HzO2~Ce(III) + HO 2 + H + HO 2 formed can in principle further reduce another Ce(IV) ion. The reaction of Ce(IV) with hydrogen atoms leaves an excess of OH radicals. Since no traces of Ce(IV) ions were observed on irradiating Ce(III), OH radicals were assumed to combine, OH + OH--)HzO z or disproportionate, OH + OH~I-I20 + 1/2Oz, with the disproportionation reaction more feasible. 6 This shows that Ce(IV) is much more reduced basically by the action of hydrogen atoms. Intermediates such as HO 2 or 1-I202 can also participate but the overall yield is not enhanced. The above mentioned mechanism can probably explain the observed decrease in the reduction of Ce(1V) ions upon increasing the acidity of the medium. Valency stabilization of Ce(IV) in irradiated aqueous solutions, in principle, couldbe achieved by using an oxidizing agent that is capable of either effectively interacting with the reducing species formed in the irradiated solution as a result of water radiolysis or 464
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50
[
1
2
t
I
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Fig, 2. Effect of acidity on the oxidation of a 10-3N Ce(IIl) solution using different bromate concentrations. Curves: 1 - 0.96N, 2 - 0.48N, 3 - 0.12N, Reaction time: 45 minutes
effectively oxidising Ce(III) ions formed in the irradiated solution as a result of the radiolytic reduction of Ce(IV), i.e., it is possible either to have a true protection mechanism or a simple oxidation mechanism. Judging by the values of the standard electrode potentials of Ce(IV)/Ce(III) and BrOffBr- systems, amounting to 1.44 V and 1.45 V, respectively, 12 the capacity of bromate ions in oxidising Ce(III) is expected to be very limited. Nevertheless, it has been found that on adding a 940 fold excess of bromate ions to 10-3 N Ce(llI) solution, in 4N I-I2SO4, almost complete oxidation of Ce(III) took place in about 45 minutes. On using only 230 fold excess of bromate, almost complete oxidation occurred in about 24 hours. These results show clearly that Ce(III) oxidation into the tetravalent state requires the existence of excess bromate. The effect of acidity on the Ce(III) oxidation process by bromate is graphically shown in Fig. 2. From these results it could be concluded that at relatively high acidities, 5-6N H 2 S O 4 , m o r e effective oxidation of Ce(III) takes place on using 960-120 fold excess of bromate relative to Ce(III) concentration. Even under these conditions the reaction requires about 45 minutes. A preliminary experiment was carried out to investigate the possibility of protecting cerium(IV) by bromate. Upon irradiating 10-3N Ce(IV) in presence of a 960 fold excess of bromate, in 4N H2SO4 acid solution, using a dose rate 336 Gy/h and following the Ce(IV) concentration during irradiation, it was found that the tetravalent state of cerium was retained for a very long time, up to about 3200 hours. This clearly demonstrated that bromate ions are capable of effectively protecting Ce(IV) ions during ~-irradiation.
465
M. F. BARAKAT, M. M. ABDEL-HAMID: VALENCY STABILIZATION, II.
E
"
DC BA
1.0-
Io D .5OO
I 1000 1500 Irradiation time, h
Fig. 3. Change of Ce(IV) concentration with y-irradiation time of 10-3N solutions containing different bromate ion concentrations. Curves: A - 0.015N, B - 0.03N, C - 0.12N, D - 0.18N; dose rate: 336 Gy/h
The reduction of Ce(IV) by hydrogen radicals is relatively a slow reaction with K value at pH 1 of 5.5 9 107 M -t 9 s-1.13 The competing reaction, leading to the formation of tetravalent bromine, 14 H + BrO3--cH§ + BrO 2or H- + BrO3--->BrO2 + OHin acidic solutions is probably much slower with K value 2.0- 107 M -1 9s-1 at pH 7.15 It could be assumed that in presence of an excess of BrO~ ions, the hydrogen atoms reaction with Ce(IV) becomes largely inhibited in favour of their reaction with bromate ions. In order to understand the nature of the valency stabilization process and to identify the prevailing mechanism, it was tried to study the protection process at lower bromate concentrations ielative to the concentration of Ce(IV) used. Thus, 10-3N Ce(IV) solutions containing increasing bromate concentrations within the range 15.10-3-120 9 10-3N in 5N sulphuric acid, were irradiated at a dose rate 336 Gy/h and the concentration of Ce(IV) was followed during irradiation. The results obtained are shown graphically in Fig. 3. From these results it could be 466
M. F. BARAKAT,M. M. ABDEL-HAMID:VALENCYSTABILIZATION.II.
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observed that the quadrivalent state of cerium ions was retained for increasing periods of time depending on the amounts of bromate used. The recorded protection time is actually directly proportional to the bromate concentration used as could be observed from the data shown in Fig. 4. By linear regression analysis of these results the correlation coefficient was found to be 0.999 indicating the existence of a strong linear positive relationship with a slope 1.09 - 10-4 for the best straight line. Using the dose rate value, 336 Gy/h it is possible to note that one equivalent of bromate is capable of stabilizing the valency of Ce(IV) for about 9120 hours, involving the absorption of about 3080 kGy. Due to the fact that bromates interact with the reducing species in the medium at any concentration, as shown by the data in Fig. 4, it is clear that H atoms, formed as a result of water radiolysis, preferentially react with bromates during the occurrence of the protection process until the latter is completely exhausted then they start to react with Ce(IV). This conclusion is also conforming with the foregoing results that bromate is capable of oxidising Ce(III) only when present at very high concentrations and at high sulphuric acid concentration and even under such conditions a rather long reaction time is needed. The effect of acidity of the medium on the prevailing protection process has been studied. The concentration of Ce(IV) was followed during "t-radiolysis of 10-aN Ce(IV) solutions containing 15.10-3N bromate and different sulphuric acid concentrations, within the range 1-5 N. The results obtained are shown graphically in Fig. 5. 467
M. F. BARAKAT, M. M. ABDEL-HAMID: VALENCY STABILIZATION, II.
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Fig. 5. Behavior of Ce(IV) ions in 10-3N solutions containing 15.10-3N bromate ions at different H2SO4 concentrations. Curves: 1 - 1N, 2 - 2N, 2 - 3.5N, 4 - 5N; dose rate= 336 Gy/h
It could be observed that at lower acidities partial reduction of Ce(IV) took place while at 5 N sulphuric acid this did not occur. These results can find some explanation from the data shown in Fig. 6, which indicate that Ce0V) is more readily reduced at lower acidities than at higher acidities. It could probably be assumed that the reducing species, responsible of Ce(IV) reduction, most probably react more effectively at higher acidities with bromate ions while at relatively lower acidities they react more effectively with Ce(IV) ions. This implies that at lower acidities br0mate is consumed relatively slowly and consequently its overall reaction would last longer than at higher acidities. This view is in agreement with the data given in Fig. 5, showing that the bromate protection time, i.e., bromate survival time, extended to about 168 hours at lower acidities and only to 144 hours at higher acidities. The fate of bromate during radiolysis was also investigated. In this series of experiments the concentration of residual bromate was followed during irradiation of bromate solutions, within the concentration range 2 9 10-3-6.7 9 10-2N in 5N sulphuric acid in the presence and absence of Ce(IV). The obtained results are shown graphically in Fig.7. From these results it could be observed that bromate decays at a lower rate when irradiated in presence of Ce(IV). For example, 0.067N bromate solution decays 468
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completely in about 380 hours while the same solution in presence of 10-3N Ce(IV) will decay in 510 hours of 1,-irradiation. This shows that Ce(IV) has a clear protective effect against bromate decomposition during mdi0-1~sis:-Therefore, it could be assumed that while bromate has a protective effect on Ce(IV) during y-radiolysis, Ce(IV) in turn has got a parallel protective effect on the radiolytic decomposition of bromate. 469
M. F. BARAKAT, M. M. ABDEL-HAMID: VALENCY STABILIZATION, II.
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.
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The effect of acidity of the medium on the mutual protection process has been also studied. Thus, a series of experiments were carried out in which 0.067N bromate solutions containing different Ce(IV) and sulphuric acid concentrations were irradiated. The results obtained are shown in Fig. 8. From these results it is clear that by increasing the Ce(IV) concentration for four times, the decay rate and survival time of bromate only slightly changed, while on increasing the sulphuric acid concentration for five times the rate of bromate decay and survival time changed markedly. The higher the acidity of the medium, the higher will be the decay rate of bromate and the less will be its survival time. This is in agreement with the other findings quoted in the present work. Finally, the oxidation kinetics of radiolytically produced Ce(III) using 15 fold excess of bromate has been studied and compared with the results obtained on using pure Ce(III) solutions. The obtained results, shown in Fig. 8, indicate that in both cases there is a rather rapid oxidation step that takes place in the first 24 hours followed by a more slower step that extends for prolonged time intervals. In presence of 5N sulphuric acid effective oxidation takes place, with 93% yield, after the first four hours and with - 100% yield, after 48 hours. This shows indirectly that the protection of irradiated Ce(IV) by bromate very unlikely occurs by the oxidation of radiolytically formed Ce(IU) during gamma irradiation. From all the foregoing discussions it is probably possible to conclude that the protection of irradiated Ce(IV) by bromate takes place mainly by the preferential interaction of bromate ions present at any concentration with the reducing species formed during radiolysis of Ce(IV) aqueous solutions at high sulphuric acid concentrations. After complete exhaustion of the used bromate, depending on the magnitude of the adsorbed dose, Ce(IV) starts to be reduced. 470
M. F. BARAKAT, M. M. ABDEL-HAMID: VALENCY STABILIZATION, IL
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