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J. Phys. Chem. A 2010, 114, 2080–2085
Radiolytic Yield of UIV Oxidation into UVI: A New Mechanism for UV Reactivity in Acidic Solution E. Atinault,†,‡ V. De Waele,† J. Belloni,† C. Le Naour,§ M. Fattahi,‡ and M. Mostafavi*,† Laboratoire de Chimie Physique/ELYSE, CNRS-UniVersite´ Paris-Sud, Baˆt. 349, 91405 Orsay, France, SUBATECH, Ecole des Mines de Nantes, UniVersite´ de Nantes, 44307 Nantes, France, and Institut de Physique Nucle´aire, CNRS-UniVersite´ Paris-Sud, Baˆt. 100, 91406 Orsay, France ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: December 9, 2009
The yields of the radiolytic oxidation of UIV and of the UVI formation, measured by spectrophotometry, are found to be the same (G(-UIV)N2O ) G(UVI)N2O ) 8.4 × 10-7 mol J-1) and almost double the H2 formation yield (G(H2) ) 4.4 × 10-7 mol J-1) in the 60Co γ radiolysis of N2O-aqueous solutions in the presence of 2 mol L-1 Cl- at pH ) 0 (HCl). According to the mechanism of UIV radiolytic oxidation, we show that under the conditions of our experiments the UV ions do not disproportionate, but undergo a stoichiometric oxidation into UVI by H+ with forming H2. Introduction Aqueous solutions of uranium are widely used at different steps in nuclear power engineering. Uranium is present under different redox states, and for each of them various complexations are possible. Because the knowledge on redox and solubility properties of each state constitutes a key point for the treatment and the storage of uranium, the chemistry of the different valence states of uranium has been extensively studied under various conditions.1 As uranium is radioactive and usually coexists with other radioactive elements, the effect of irradiation on the uranium systems was also considered. In particular, the radiolysis of aqueous solutions of uranium ions and complexes and also of solid UO2 was widely studied.2 Specially, the oxidation of UIV has been extensively considered under acidic conditions (H2SO4 or HCl).3–6 The product of oxidation of UIV is UVI, which is stable and very well characterized. It is considered that UV is a transient valence state, which is unstable in solution.7 Photochemical reduction of UO2(CO3)34- in deoxygenated aqueous carbonate solutions at pH g 11.2 containing ethanol or sodium formate showed that the molecular formula of the uranium(V) complex under basic condition is [(UO2)4(OH)6(H2O9]2-.8 However, because of its short lifetime and its very low absorbency, the knowledge on this transient valence state UV is much less rich than on UIV or UVI. Although there is considerable discrepancy in the value of the reported rate constants, there is a general agreement that UV undergoes in these acidic solutions a disproportionation reaction:
2UV f UIV+ UVI
(1)
The kinetics and mechanism of the disproportionation of UV (initially at millimolar concentration) have been investigated more in detail by Ekstrom using the stop-flow method.9 He reported two other different second-order mechanisms catalyzed by protons involving either the formation of a complex between H+ and UO2+: * Corresponding author. E-mail:
[email protected]. † Laboratoire de Chimie Physique/ELYSE, CNRS-Universite´ Paris-Sud. ‡ Universite´ de Nantes. § Institut de Physique Nucle´aire, CNRS-Universite´ Paris-Sud.
+ V 2+ UVO+ 2 + H f U O2H
(2)
UVO2H2++ UVO+ 2 f products
(3)
or the formation of a dimer of UV reacting with H+:
2UV f [UV]2
(4)
[UV]2+ H+ f products
(5)
The decay of UV (initial concentration at a few millimolar, HClO4 acidic medium) was found to obey a second-order law with a rate constant of (1 to 4) × 102 L mol-1 s-1, depending on the [H+] conditions. The lifetime of millimolar UV was found to be in the range of seconds. The fundamental understanding of the mechanisms of these reactions remains obscure.10 More recently, a theoretical study reported that a binuclear complex is formed between two UO2+, and that this binuclear complex can accept two protons by forming [(UO2H)2]4+. According to the authors, this binuclear complex undergoes an inner-sphere disproportionation reaction.11 Recently, we measured the radiolytic yield of UIV oxidation in O2-saturated solutions.12 The oxidation yield of UIV was found to be 8.7 × 10-7 mol J-1 in the 60Co γ radiolysis of aqueous solutions containing 4.4 × 10-3 mol L-1 UIV in the presence of 2 mol L-1 Cl- at pH ) 0 (HCl). We presented a mechanism to explain the yield of oxidation of UIV, and we have shown that this rather high value suggested that all primary radicals formed by water radiolysis were used to eventually oxidize UIV in these solutions. However, in O2-saturated solution, it was difficult to draw a strict conclusion on the yields and mechanism of oxidation of UIV into UVI, because under these conditions the radical HO2 · is formed and the mechanism of oxidation of UIV by HO2 · is complex. Generally, the reactivity of HO2 · toward the metal ions needs a deep study because it may react through ligandation as an oxidizing or a reducing species. To avoid this difficulty, we study in the present work the results obtained in N2O-
10.1021/jp9051177 2010 American Chemical Society Published on Web 01/15/2010
Radiolytic Yield of UIV Oxidation into UVI saturated solutions. In fact, under N2O, the radical HO2 · is not formed. Under this condition, we compare the yield of radiolytic UIV oxidation with the yields of radiolytic formation of UVI and gases. On the basis of the quantitative results obtained on these yields, we revisit the mechanism of the oxidation process and discuss also the occurrence of the UV disproportionation reaction. Experimental Section The solutions were prepared using ultrapure water from a Millipore system (18 MΩ cm) and reagent grade chemicals used without further purification: HCl, 37%, from Normapur; NaCl, ACS reagent, with g99.8% purity from Sigma-Aldrich; N2 and N2O with 99.99% purity from Air Liquide. To study the radiolytic oxidation of an aqueous solution of UIV, it is first necessary to synthesize a pure solution of this oxidation state of uranium. For that purpose, an aqueous solution of hexavalent uranyl nitrate, UO2(NO3)2(H2O)6, which has been obtained by uranium metal oxidation with nitric acid, is reduced under N2 by applying a constant electric intensity between two electrodes. Solutions containing 100 mL of 5 × 10-3 mol L-1 uranium salt are electrolyzed using a potentiostat (Radiometer Voltalab 21) and applying a current of -1 mA during 27 h. The completion of the process, that is, total reduction of UVI into UIV, is checked by using spectrophotometrical monitoring. The pH of the medium, in which is dissolved the uranyl salt, is adjusted to 0 with 1 mol L-1 HCl and 1 mol L-1 NaCl to stabilize UIV under atmosphere containing O2.12 Actually, the nitrate anions coming from the uranium salt have disappeared to form N2 during the coulometric reduction of UVI into UIII, which is then oxidized into UIV as explained by Kolthoff et al.13,14 To minimize the possible reactions under irradiation, no other molecule is added, for example, as stabilizer. The complex forms of UIV and UVI are not clearly established under our conditions. It was reported that UVI in acetonitrile is complexed by 6 Cl- forming UCl62-,15,16 but a recent EXAFS study in water reported that UIV and UVI in the presence of 3 mol L-1 Cl- are under the forms of U(H2O)8Cl3+ and UO2(H2O)4Cl+, respectively.17 For dose-dependent absorbance measurements, when the coulometric preparation of UIV is over, the UIV solution (5 mM at pH ) 0 and 2 mol L-1 Cl-) is inserted in a special cell divided into two parts: the first one is a reservoir in which the sample is stored during the irradiation, and the second one with a rubber plastic septum is a spectrophotometric suprasil cell (1 cm). Both parts are connected by a bridge in quartz, which allows the system to be deaerated before irradiation and filled with the selected atmosphere, the nitrous oxide (N2O) in the present work. The concentration of N2O in pure water is 2.4 × 10-2 mol L-1, but at high ionic strength, such as in the solutions prepared in this work, it is only 1.5 × 10-2 mol L-1.18 The γ-irradiation was performed using the panoramic 60Co source at Chimie Physique laboratory, Orsay, with an activity of 2400 Ci. The dose rate depends on the distance of the sample to the 60Co source. In this method, the energy is absorbed by the solvent where radiolytic oxidizing radicals are generated with a controlled dose rate. The dose rate was measured using the Fricke dosimeter with a radiolytic yield G(Fe3+) equal to 16.2 × 10-7 mol J-1 and a molar extinction coefficient of Fe3+ at 304 nm of 2160 L mol-1 cm-1. The dose rate was corrected by taking into account the density of the UIV solutions, which is 1.04 kg L-1. The dose rate was equal to 45 ( 2 Gy min-1. After each irradiation dose absorbed by the UIV solution, an absorption spectrum of the sample was registered. On the basis of the calibration of the absorbencies of UIV and UVI, the dosedependent concentrations of the ions are obtained.
J. Phys. Chem. A, Vol. 114, No. 5, 2010 2081 To analyze the gases produced during irradiation, another series of samples were studied. The UIV solutions were irradiated in another specific polypropylene cell equipped with a valve, after being filled with nitrous oxide at a pressure equal to 1.5 ( 0.15 bar. After each irradiation, the molecular hydrogen and other gases were measured using an original connection device between the irradiated sample cell and the mass spectrometer (Minilab Residual Gas Analyzer from MKS). Before calibration and gas analysis, a cleaning of the setup is performed with pure argon to remove any trace of atmospheric gases. The calibration of the gas analysis involves two steps. During the first one, the signals of the relative gas pressures are calibrated in atmospheric air by using a Faraday detector and assuming 1% for the relative pressure of argon. In the second step, high-purity argon is injected to determine the different proportions of the isotopes: 40 Ar, 36Ar, and 38Ar. Three electron-multipliers are used for measuring separately the three argon isotopes. For the sample gas analysis, argon was used as the carrier gas with a flow rate of about 5 mL min-1. Two measurements were performed for each sample. The analysis uncertainty is 10%, and the detection threshold is 20 ppm. The results obtained are expressed as ratios between partial pressures of H2 and N2O detected. They permit one to calculate the amount of gas molecules from the partial pressure, and from the solution and dead volumes of samples. The N2O pressure is in large excess and does not change notably during irradiation (variation less than 5% within 3 kGy). Results Measurements of the Molar Extinction Coefficients of UIV and UVI. The radiolytic oxidation of UIV into UVI is measured by observing the dose-dependent absorption spectra. As shown in Figure 1a, UIV in NaCl/HCl is characterized by four peaks at 430, 495, 549, and 648 nm. As the uranium ions are highly sensitive to complexation, the extinction coefficients of these spectra should be accurately calibrated under the used conditions of high Cl- concentration. For each absorption peak, the values of the molar extinction coefficients of UIV in this medium were determined. Experiments were done by using a set of solutions of UIV at various concentrations in 1 mol L-1 NaCl and 1 mol L-1 HCl. In Figure 1b, the absorbance of these solutions at each selected wavelength is reported versus the concentration. The absorbance is strictly proportional to the concentration in the range 0.5-10 mmol L-1 (Figure 1b and inset). The molar optical absorptivity was measured at different wavelengths corresponding to the four maxima. The values are equal to (16 ( 1) L mol-1 cm-1 at 430 nm, (29 ( 1) L mol-1 cm-1 at 495 nm, (21 ( 1) L mol-1 cm-1 at 549 nm, and (62 ( 1) L mol-1 cm-1 at 648 nm. During the irradiation, the concentration of UIV was deduced from the absorbance at 648 nm where UVI does not absorb. It is interesting to compare the corresponding extinction coefficient to other ones, found in the literature. The value of the extinction coefficient (62 L mol-1 cm-1 at 648 nm) deduced from Figure 1 is higher than the value 53 L mol-1 cm-1 as obtained, for example, by spectrophotometry in the medium 1 mol L-1 HBr aqueous solution.19 The difference is explained by the specific complexation, because in the present solution the ions are complexed by Cl- (U(H2O)8Cl3+) instead of Br(UBr3+) in the reported reference.17,20 The intensity of the absorption band of UVI is known to be very low. Because of the difficulty to estimate accurately the extinction coefficient of UVI in such a low-concentration uranium solution, it was evaluated by using the isosbestic point at 423 nm between UIV and UVI spectra (see below).
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Figure 1. Absorption spectra of aqueous solutions containing 1 mol L-1 of NaCl and 1 mol L-1 of HCl with (a) 4.3 × 10-3 mol L-1 of UIV. Graph (b) represents the measurements of the molar absorptivity of UIV at different wavelengths. The correspondence with the symbols is as follows: (9) 430 nm, (b) 495 nm, (2) 549 nm, and ([) 648 nm. The results for the different molar extinction coefficients are (16 ( 1) L mol-1 cm-1, (29 ( 1) L mol-1 cm-1, (21 ( 1) L mol-1 cm-1, and (62 ( 1) L mol-1 cm-1, respectively. Inset of (b): Linear variation of the absorbance at low concentrations.
Figure 2. Absorption spectrum of N2O-saturated aqueous solutions (4.3 × 10-3 mol L-1 of UIV + 1 mol L-1 NaCl + 1 mol L-1 HCl) during the 60Co γ-irradiation. The arrows show the decrease of the peaks at 430, 495, 549, and 648 nm and the increase at 420 nm, with increasing irradiation dose (0-5.5 kGy).
Determination of the Radiolytic Oxidation Yield G(-UIV) and of the Formation Yields G(UVI) and G(H2). The disappearance of UIV and the formation of UVI were followed using the absorption spectrum scanned after each irradiation of N2Osaturated solutions (Figure 2). With increasing irradiation dose, the intensities of the four peaks at 430, 495, 549, and 648 nm, corresponding to UIV, decrease, whereas the intensity of UV band centered at 416 nm increases, characterizing the formation of UVI. It is worth noting that two isosbestic points independent of the irradiation dose are present at 423 and 448 nm, so indicating a stoichiometric oxidation of each UIV into UVI during the radiolysis and excluding the presence of other valence states as stable products on the time scale of analysis (Figure 2). In particular, the concentration of transient UV is negligible. The extinction coefficient found at 423 nm of UIV or UVI is 8 L mol-1 cm-1, and it is close to that reported in the literature. We deduce that the extinction coefficient of UVI at the maximum 416 nm is 10 ( 1 L mol-1 cm-1. According to literature data, the extinction coefficient of perchloric acidic UVI solutions at 416 nm is also 10 L mol-1 cm-1. The concentration of UIV is calculated for each irradiation dose from the absorption spectra at 648 nm. Because of the
Figure 3. Concentration of UIV (closed symbols) and UVI (open symbols) as a function of irradiation dose obtained for N2O-saturated aqueous solution (9) and O2-saturated aqueous solution (b). Average values of three series of experimental data.
observation of the isosbestic points, the UVI concentration is derived from the difference with initial [UIV]. Figure 3 includes the average values of three different series of experiments performed with initial concentrations of UIV at around 5 mM. The concentration follows a linear variation versus the irradiation dose up to the exhaustion of UIV and complete formation of UVI. The total irradiation time to achieve the oxidation of all UIV ions is around 3 h. The radiolytic yields of disappearance of UIV, G(-UIV), and of formation of UVI, G(UVI), which are equal, were obtained by linear fitting of the experimental data. In N2O-saturated solutions, G(-UIV)N2O ) G(UVI)N2O ) 8.4 × 10-7 mol J-1. It is worth noting that we checked that no postirradiation process occurred. Therefore, at the end of irradiation the system is in equilibrium. In addition, the H2 evolution for each irradiation dose was measured (Figure 4). Unexpectedly, as compared to water radiolysis, we found large H2 volumes under irradiation. Like the concentrations of UIV or UVI, the pressure of H2 versus irradiation dose presents also a linear evolution. The yield of H2 is G(H2)N2O ) (4.4 ( 0.4) × 10-7 mol J-1 (Figure 4).
Radiolytic Yield of UIV Oxidation into UVI
J. Phys. Chem. A, Vol. 114, No. 5, 2010 2083 Next, H · radicals produced by the water radiolysis and by reaction 7 are scavenged by nitrous oxide at the saturated concentration of 1.5 × 10-2 mol L-1 and form the radical OH · .18
H.+ N2O f N2+ HO. 2.1 × 106 mol-1 L s-1 22
(8)
According to the low rate constant of reaction 8 between H · and N2O, the scavenging half-time is about 30 µs. As Cl- is present in solutions at 2 mol L-1, the radicals OH · react predominantly with Cl- to form ClOH · -.
HO · + Cl- f ClOH · - 4.3 × 109 mol-1 L s-1
Figure 4. Average of two series of measurements of the concentration of H2 versus irradiation dose obtained for N2O-saturated aqueous solution. Conditions of Figure 2.
(9)
Next, H+ reacts with ClOH · - and produces Cl2 · - as transient species via the following reactions:23
ClOH · -+ H+ f Cl · + H2O 2.1 × 1010 mol-1 L s-1 (10) Cl · + Cl- f Cl2· - 2.2 × 1010 mol-1 L s-1
(11)
Nevertheless, in the spurs a very small quantity of hydrogen peroxide H2O2 can also be formed in competition with reaction 9:
OH · + OH · f H2O2 4.2 × 109 mol-1 L s-1
(12)
Consequently, in the N2O-saturated solutions, after a few tens of microseconds, the oxidizing species are Cl2 · - and at a much lower extent H2O2. The transient Cl2 · - is partially replaced by Cl3-: · 2Cl2 × 1010 mol-1 L s-1 2 f Cl3 + Cl
(13)
Cl3- can also oxidize UIV with 2 equivalents. According to the oxidation mechanism, the Cl2 · - species reacts with UIV with a rate constant of 8.3 × 103 L mol-1 s-1:6 Figure 5. Schematic process during the radiolysis of UIV under the same conditions as in Figure 2. The uranium ions are complexed by Cl-.
· IV Clf UV+ 2Cl2 + U
(14)
The Cl2 · - radical is also able to oxidize UV with a higher rate constant of 6.5 × 109 L mol-1 s-1.24
Discussion Under γ-irradiation, water molecules are either excited or ionized to produce at very short times the primary radiolytic products, that is, hydrated electron es-, hydroxyl radical OH · , hydrogen radical H · , proton H+, and very small amounts of molecular products H2 and H2O2.
H2O vv f OH · , es-, H · , H+, H2O2, H2
(6)
At pH ) 0, the hydrated electron reacts with H+ to form H · atoms within 100 ps (Figure 5):
es-+ H+ f H · 1.3 × 1010 mol-1 L s-1 21
(7)
· V VI Cl2 + U f U + 2Cl
(15)
Photochemical studies of solutions containing UO22+ and inorganic anions showed indeed that halide radical anions such as Br2- and I2- oxidize UV to UVI.25,26 However, because of the much lower concentration of UV as compared to the UIV concentration during the γ-radiolysis, this oxidation pathway (15), in competition with reaction 14, is negligible. During the radiolysis process, H2O2 can be formed through reaction 12. The products of the slow reaction between H2O2 and UIV are UV and the radical OH · :
UIV+ H2O2 f UV+ OH ·
(16)
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The latter is scavenged by Cl- and H+ of the solution to produce the radical Cl2 · -, which can oxidize a second UIV ion, or OH · may, in a second step, oxidize UV and form UVI directly. Consequently, one molecule of hydrogen peroxide can oxidize two UIV ions into two UV or more probably oxidize directly one UIV ion into UVI. However, this contribution is negligible because in our case the solutions contain 2 M Cl-, and almost all OH · radicals are scavenged by Cl- at around 100 ps and replaced by Cl2 · - (reaction 10, solid line in Figure 5). Hence, a very low amount of OH · contributes to form H2O2 (dotted line in Figure 5). At this point, it is important to know the values of the radiolytic yields of secondary oxidizing species under the conditions of the experiments and to express them versus the radiolytic yields of the primary radicals arising from the water radiolysis, that is, es-, H · , and OH · . In fact, Cl2 · - is issued from the OH · scavenging issued from reaction 6 or indirectly from reactions 7 and 8. Recent picosecond pulse radiolysis studies showed that G100ps(OH · ) is (4.4 ( 0.2) × 10-7 mol J-1.21 At pH 0, all hydrated electrons are replaced by H · atoms and after reactions 7-11 by Cl2 · -. G(es-) has been measured at picosecond range by Muroya et al. in 2005, and it was found to be (4.2 ( 0.2) × 10-7 mol J-1.27 The simulations showed that at picosecond range the yields of H2O2 and H · are very low (G(H2O2) ) 0.2 × 10-7 and G(H · ) ) 0.4 × 10-7 mol J-1).10 In the present experiments, HO2 · is not formed in N2O-saturated solutions; the single oxidizing species present is finally Cl2 · (or Cl3-). Consequently, we can consider that, at most, the total radiolytic yield of oxidizing species should be equal to G(Ox)total. · · G(Cl2 ) ) G(es ) + G(H ) +
G(OH · ) ≈ G(Ox)total ) 9 × 10-7 mol J-1
(17)
The earliest work on the radiolytic oxidation of UIV in acidic media was published by Haı¨ssinsky et al.4 They found that, in sulfuric acid solutions, G(-UIV) depends on the dose rate. At high dose rate, close to that used in the present work (45 Gy min-1), they found in O2-saturated solutions that G(-UIV) ) 9.4 × 10-7 mol J-1, and we found recently a close value in hydrochloric O2-saturated solutions. When the dose rate is 100 times lower, Haı¨ssinsky et al.4 found that G(-UIV) ) 18.6 × 10-7 mol J-1. This very high value of the oxidation yield was explained not only by a mechanism in which each H · atom gives 3 equiv of oxidizing species (through the formation of HO2 · , like in the Fricke dosimeter system), but also by the occurrence of a chain reaction favored at very low dose rate. Later, Bhattacharyya et al. revisited similar experiments in O2-saturated solution in the presence of 0.8 mol L-1 HCl. The chain mechanism was excluded, and it was found that G(-UIV) ) 7.8 × 10-7 mol J-1.6 The authors reported a mechanism in which H · atoms react with O2, giving HO2 · , and then HO2 · oxidizes UIV according to the Fricke mechanism with 3 equivalents. Both Haı¨ssinsky et al. and Bhattacharyya et al. considered that UV ions disproportionate. It is currently admitted in the literature that the uranium ions UV, which are produced from the scavenging of oxidizing radicals, disproportionate into UIV and UVI. This mechanism was suggested by earlier electrochemical or stopped-flow experiments, where UV is first produced very quickly via UVI reduction.7,9 However, in this respect, it is important to note that according to Haı¨ssinsky et al. and Pikaev et al., who were alone in measuring the amount of hydrogen produced during the radiolytic oxidation of UIV in acidic media, G(H2) was exceptionally
high as compared to the yield expected in the radiolysis of aqueous solutions (the primary yield of H2 formation in water is only 0.45 × 10-7 mol J-1), but the authors did not explain the origin of this high H2 evolution.2,4 It is also worthy to note that in studies where the disproportionation of UV was suggested, a possible formation of H2 was not checked.9 Under the present conditions, all of the uranium ions are expected to be complexed by Cl- anions, as was also suggested at least by the optical absorption spectrum of UIV. From literature data, the reduction potential of UO22+/UO2+ is +0.1 VNHE, which is close to that of H+/1/2H2.1 However, the complexation process has an influence on the reduction potential of the ions, which depends on the ligand nature. The reduction value of the uranium ions complexed by Cl- is, however, unknown. As explained above, the oxidation of UIV results from an efficient scavenging of primary radicals. However, quantitatively the oxidation results correspond to the formation of UVI with a yield of G(UVI) ) G(-UIV) ) 8.4 × 10-7 mol J-1, which implies an oxidation yield of G(Ox)exp ) 2G(-UIV) ) 16.8 × 10-7 equiv J-1, being almost twice as high as G(Ox)total of the radiolysis in the N2O system. Actually, if UVI were formed exclusively by the scavenging of the oxidizing species issued from the radiolysis of (N2O and Cl-) water solutions, followed by the disproportionation of the transient UV state, the yields G(-UVI) ) G(UVI) would be only G(Ox)total/2 ) 4.5 × 10-7 mol J-1, and no molecular hydrogen would be found. Let us consider other possible reactions of H · radicals producing H2, although H · atoms are in principle scavenged by N2O. (i) With the hypothesis of a possible reduction of UIV into III U by H · (the rate constant of reaction is known to be lower than 106 L mol-1 s-1), the overall uranium oxidation and the H2 yields would be lower than half of the experimental data. (ii) If H · were scavenged by UVI and UV disproportionate, the H2 yield would be zero and G(UVI) would be less than G(Ox)total. Therefore, our results preclude a possible reduction of uranium ions by H · in competition with reaction 8. (iii) Moreover, H2 production by H · dimerization in the absence of any other reaction would yield half of the uranium oxidation and H2 yields. The comparison between our experimental data and the total yield of oxidizing species thus raises the important point of the origin of the supplementary oxidation into UVI. The bieletronic UIV oxidation yield corresponding to 16.8 × 10-7 mol J-1 and H2 evolution G(H2) ) 4.4 × 10-7mol J-1 are clearly not achieved simply by the scavenging yield of the radiolytic oxidizing species (G(Ox)total ) 9 × 10-7 mol J-1). The experimental results exclude the hypothesis of the disproportionation under our conditions. The high H2 and uranium oxidation yields should then originate from an additional chemical reaction involving H+. Because UIV and UVI are stable in acidic medium, the species that is the most probable direct reductant of 2H+ into H2 is the dimer ion UV in the form of (UO2+)2 or of the complex (UO2H2+)2. We are compelled to conclude that, at least under hydrochloric conditions, after complexation of UV with H+ (reaction 2) and dimerization (reaction 3) the complex dimer [UVO2+,H+]2 produces directly H2 and UVI via inner-sphere electron transfer. This integral oxidation process of UV into UVI markedly differs from a disproportionation reaction. Indeed, a small shift of the reduction potential of UV due to the complexation by Cl- would suffice to favor, instead of a disproportionation reaction, a stoichiometric oxidation reaction
Radiolytic Yield of UIV Oxidation into UVI
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by H+ of a UV dimer into 2UVI. Therefore, the overall reaction that we propose is: + VI 2+ 2[UVO+ 2 ,H ]Cl- f H2+ 2[U O2 ]Cl-
(18)
Both experimental observations of the radiolytic oxidation of UIV, that is, the equality of the high yields G(-UIV) and G(UVI) with G(Cl2 · -) (and not half as with disproportionation), and the high yield of H2 formation, strongly support this mechanism. Hence, the formation and oxidation yields of transient UV are:
G(-UIV) ) G(UV) ) G(Cl2· -)
(19)
G(-UV) ) G(UVI) ) 2G(H2)
(20)
and
The mechanism would also account for the high hydrogen yields observed in N2-saturated aqueous solutions (HCl) found in previous studies.2,4 In Figure 3, the dose dependence of the concentrations is also represented for comparison for a sample saturated by O2.12 Actually, the yields of G(-UIV)O2 and G(UVI)O2 are close to those in N2O-saturated solution. The whole mechanism, including (2) and (18), so explains that the experimental results on G(-UIV) ) G(UVI) are identical in the presence of N2O or O2 and indirectly supports the previous hypothesis on the role of HO2 · . In both cases, the transient UV complex dimer is oxidized by H+ (reaction 18) and yields molecular hydrogen. The only difference between both systems is that the oxidation yield of UIV in O2-saturated solutions occurs via OH · and HO2 · due to the H · scavenging by O2 instead of via only Cl2 · - due to OH · scavenging in N2O-saturated solutions (Figure 5). The mechanism of the oxidation of UV has also an important consequence on the chemistry of UIV in more general cases. A possible H2 evolution during uranium oxidation should be examined. Obviously, UV ions would constitute as well an intermediate in the slow chemical oxidation of UIV by air in acidic media, where the oxidation of a dimeric complex [UVO2+,H+]2 should also be considered. In conclusion, new experiments on chemical oxidation of UIV with a complete product analysis, in particular of the gases as
performed in the present work, would be helpful to check the oxidation mechanism in different conditions of acidity and ligandation. Acknowledgment. We are grateful to Prof. J. Laverne from Notre Dame University for helpful discussions. Furthermore, we thank G. Blain and F. Crumie`re for their contribution in the experiments of gas measurements. References and Notes (1) Morss, L. R.; Edelstein, N. M.; Fuger, J. The Chemistry of the Actinide & Transactinide Elements; 2006; Vol. 1. (2) Pikaev, A. K.; Shilov, V. P.; Gogolev, A. V. Russ. Chem. ReV. 1997, 66, 763. (3) Halpern, J.; Smith, J. G. Can. J. Chem. 1956, 34, 1419. (4) Haissinsky, M.; Duflo, M. J. Chim.-Phys. 1956, 53, 970. (5) Sobkowski, J.; Zalkind, T. I. Zh. Fiz. Khim. 1965, 39, 1388. (6) Bhattacharyya, B. K.; Saini, R. D. Radiat. Phys. Chem. 1979, 13, 57. (7) Kern, D. M. H.; Orlemann, E. F. Inorg. Chem. 1949, 71, 2102. (8) Saini, R. D.; Bhattacharyya, P. K.; Iyer, R. M. J. Photochem. Photobiol., A 1989, 47, ????. (9) Ekstrom, A. Inorg. Chem. 1974, 13, 2237. (10) Weigl, F. The Chemistry of the Actinide Elements; 1986; Vol. 1. (11) Steele, H.; Taylor, R. J. Inorg. Chem. 2007, 46, 6311. (12) Atinault, E.; Waele, V. D.; Fattahi, M.; LaVerne, J. A.; Pimblott, S. M.; Mostafavi, M. J. Phys. Chem. A 2009, 113, 949. (13) Kolthoff, I. M.; Harris, W. E.; Matsuyama, G. J. Am. Chem. Soc. 1944, 66, 1782. (14) The Analyst 1945, 71, 91. (15) Beattie, W. H.; W. B. M. II; Holland, R. F. Spectrochim. Acta 1984, 40A, 897. (16) Preez, J. G. H. D.; Rohwer, H. E.; Morris, K. B. Inorg. Chim. Acta 1992, 191, 203. (17) Hennig, C.; Tutschku, J.; Rossberg, A.; Bernhard, G.; Scheinost, A. C. Inorg. Chem. 2005, 44, 6655. (18) Janata, E.; Kelm, M.; Ershov, B. G. Radiat. Phys. Chem. 2002, 63, 157. (19) Auzel, F.; Hubert, S.; Delamoye, P. J. Lumin. 1982, 26, 251. (20) Grenthe, I.; Fuger, J.; Konings, R. J. M. Chemical Thermodynamics of Uranium; 2004; Vol. 1. (21) Atinault, E.; Waele, V. D.; Schmidhammer, U.; Fattahi, M.; Mostafavi, M. Chem. Phys. Lett. 2008, 460, 461. (22) Czapski, G.; Peled, E. Isr. J. Chem. 1968, 6, 421. (23) Jayson, G. G.; Parsons, B. J.; Swallow, A. J. J. Chem. Soc., Faraday Trans. 1973, 69, 1597. (24) Lierse, C.; Sullivan, J. C.; Schmidt, K. H. Inorg. Chem. 1987, 26, 1408. (25) Condorelli, G.; Costanzo, L. L.; Pistara`, S.; Tondello, E. Inorg. Chim. Acta 1974, 10, 115. (26) Burrows, H. D.; Jesus, J. D. P. D. J. Photochem. 1976, 5, 265. (27) Muroya, Y.; Lin, M.; Wu, G.; Iijima, H.; Yoshii, K.; Ueda, T.; Kudo, H.; Katsumura, Y. Radiat. Phys. Chem. 2005, 72, 169.
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