Influence of Nitric Acid on the Helium Ion Radiolysis of Aqueous

Article pubs.acs.org/JPCA

Influence of Nitric Acid on the Helium Ion Radiolysis of Aqueous Butanal Oxime Solutions A. Costagliola,†,‡ L. Venault,*,‡ A. Deroche,‡ J. Vermeulen,‡ F. Duval,§ G. Blain,† J. Vandenborre,† M. Fattahi-Vanani,† and N. Vigier∥ †

SUBATECH, UMR 6457, Ecole des Mines de Nantes, CNRS/IN2P3, Université de Nantes, 4, Rue Alfred Kastler, La chantrerie BP 20722, 44307 Nantes cedex 3, France ‡ CEA, Nuclear Energy Division, RadioChemistry and Process Department, Marcoule Center, 30207 Bagnols Sur Cèze, France § CEMHTI Site Cyclotron, CNRS, 3A rue de la Férollerie, 45071 Orléans Cédex 2, France ∥ AREVA NC, BG aval/BO recyclage/RDP, Tour AREVA, 1 place Jean Milier, 92084 Paris La Défense Cedex, France ABSTRACT: Samples of butanal oxime in aqueous nitric acid solutions have been irradiated with the helium ion (4He2+) beam of the CEMHTI (Orléans, France) cyclotron. The consumption yield of butanal oxime has been measured by gas chromatography coupled with mass spectrometry. Gaseous products (mainly H2 and N2O) have also been monitored by micro-gas chromatography. Yields of liquid phase products (hydrogen peroxide and nitrous acid) have been determined by colorimetric methods. The influence of nitric acid on the radiation chemical behavior of butanal oxime depends on the nitric acid concentration. For a low concentration (≤0.5 mol L−1) butanal oxime is protected by the nitrate ions, which can efficiently scavenge the water radiolysis radicals. For higher concentrations, nitrous acid can accumulate in the medium, therefore leading to a strong increase of the butanal oxime degradation. The associated mechanism is an autocatalytic oxidation of butanal oxime by HNO2.

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INTRODUCTION In the PUREX process, a uranium and plutonium liquid−liquid extraction separation method,1 hydrazinium nitrate is used to avoid nitrous acid accumulation in the aqueous nitric acid phase.2−7 Without hydrazinium nitrate, plutonium cannot indeed be stabilized under its trivalent Pu(+III) species in nitric acid media due to an autocatalytic oxidation of Pu(+III) to Pu(+IV) by HNO2. Nevertheless, even with an aqueousbased antinitrous reactant such as hydrazinium nitrate, Pu(+III) reoxidation can still occur in the organic phase.8 Then, it is difficult to keep Pu(+III) in solution under this oxidation state for a long time. Nitrous acid is indeed well extracted in the organic phase by TBP.9 The use of butanal oxime as a potential substitute to hydrazinium nitrate in the PUREX process has been stated in a previous article.10 This compound can actually react with nitrous acid11 (eq 1), and some studies show that it can stabilize Pu(+III) in extraction systems.12,13

activity of such radionuclides. The radiolysis of binary butanal oxime−water systems has already been discussed in a previous article.10 First of all, water undergoes ionization/excitation phenomena,14,15 which further leads to the generation of free radicals and molecular species by a series of recombination reactions (eq 2). The degradation of butanal oxime in low concentration ( 0.5 mol L−1),30 direct effect of the ionizing radiation on the nitrate ion decomposition has to be taken into account.31−35 In this case, either nitrate NO3• radical (eq 13) or oxygen atom (eq 14) is formed:

H 2O−• → H 2 + O−•

(19)

1 d[Fe3 +] · dt G(Fe )ρFricke 3+

(20)

Ḋ Fricke (J kg min ) is the dose rate in the Fricke dosimeter, G(Fe3+) = 5.9 × 10−7 mol J−1 is the radiolytic yield of ferric ions,44 ρFricke (kg L−1) is the density of the Fricke solution, [Fe3+] (mol L−1) is the ferric ion concentration, and t (min) is the irradiation time. However, it appears more convenient to express the absorbed dose in J L−1 (eq 21) in order to measure the radiolytic yields G(X) (mol J−1), which represent the amount of species X formed by unit of energy absorbed by the solution.

(13) (14)

These new species formed by nitric acid radiolysis tend to modify the last steps of the radiolysis of water. As an example, nitrous acid can react with hydrogen peroxide (eq 15). Under the influence of an ionizing radiation in nitric acid, only the major species in solution between both can thus accumulate. A previous study31 showed that for a nitric acid concentration below 0.5 mol L−1 only hydrogen peroxide is observed at the equilibrium, whereas at higher nitric acid concentration only nitrous acid can accumulate. H 2O2 + HNO2 → H+ + NO3− + H 2O

(18)

−1

ionizing radiation

NO−3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NO2− + •O

H− + H 2O → H 2 + OH−

Ḋ Fricke =

ionizing radiation

NO3− ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NO3• + eaq −

(17)

EXPERIMENTAL SECTION Solutions described in this section are prepared with commercial chemical products as received with no further purification. All reactants are analytical grade and the aqueous samples are prepared with ultrapure (Milli-Q) water (resistivity at 25 °C, 18.2 MΩ cm; total organic carbon, ≤5 ppb). The experimental methods are the same as those reported in our previous work.10 As a summary, aqueous solutions of butanal oxime in dilute nitric acid are irradiated with the helium ion beams of the CEMHTI (Orléans, France) cyclotron. The initial energy of particles in the solution was 9.4 ± 1.3 MeV. During each set of experiment, the dose rate delivered by the helium ion beam in solution is monitored by ex situ Fricke dosimetry.43,44 The Fricke dosimeter is basically an iron (+II) solution. The dose rate received by the Fricke dosimeter is then calculated by eq 20.

(8)

HNO3−• → NO2• + OH−

→ H + HO

(16)

■

H+

NO32 −• ↔ HNO3−• ↔ H 2NO3•

•

The aim of this work is then to understand and describe the influence of the nitric acid on the radiation chemical behavior of butanal oxime in aqueous solution. Helium ion beams are indeed used in this study to mimic the alpha radiolysis without using actinide ions in solution which would have complicated the chemical system due to some redox reactions of actinide with oximes.38−42 The first part of this work is focused on the measurement of butanal oxime consumption yields in ternary (water−nitric acid−butanal oxime) solutions. Then, the impact of the irradiation on the formation yields of some gaseous (H2, N2O) and liquid phase (H2O2, HNO2) products is discussed. These results are also extensively compared with our previous work focused on binary water−butanal oxime systems.10

(5)

H + NO3− → HNO3−•

−

Ḋ = ρḊ Fricke

−1

(21)

Ḋ (J L−1) is the dose rate in the solution, and ρ (kg L−1) is the density of the solution. The procedure for the radiolytic yield determination is as follows. Each sample consists of a 20 mL solution irradiated in a closed cell at ambient temperature and pressure. A first sample is introduced in the Poly Ether Ether Ketone (PEEK) irradiation cell. It is then irradiated with the helium ion beam during a given time, corresponding to a precise deposited dose. The sample is then removed from the beamline, and several chemical compounds are identified and

(15)

However, a particularity of the radiolysis of nitric acid is that it can scavenge precursors of the hydrated electron at high concentration ([HNO3] > 1.5 mol L−1). In this case, nitric acid has a strong influence on the previously called nonscavengeable B

DOI: 10.1021/acs.jpca.6b12630 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A quantified. A newly prepared sample is then irradiated at another dose. Generally, four to six samples from the same chemical environment are used to study the variation of the concentration of the radiolytic products as a function of deposited dose. The resulting G-value is called the “zero-dose extrapolated yield” and is then obtained from the slope at the origin of the curve representing the dose dependency of the concentration of the products.10,31 Butanal oxime was quantified using an Agilent 7890A gas chromatograph with a 5975C electronic impact mass spectrometry detector. The separation was managed by a high polarity DB-WAX polyethylene glycol column. The butanal oxime consumption yield in nitric acid medium is harder to determinate due to several factors. First of all, butanal oxime is known to react by acid hydrolysis (eq 22).45,46 This reaction is then in competition with those induced by radiolysis. Consequently, for each irradiated solution, a blank sample containing the same chemical system is prepared and kept out of irradiation, and then analyzed as a reference system. Moreover, GC−MS columns are sensitive to mineral acids. The aqueous phase specific DB-WAX (poly ethylene glycol) column used in this study begins to deteriorate when the pH of the solution is lower than 2. It is then necessary to dilute the sample before it is injected in the chromatograph, leading to a decrease in the sensitivity of the technique.

Figure 1. Variation of the butanal oxime concentration consumed in water/0.5 mol L−1 nitric acid/0.1 mol L−1 butanal oxime medium as a function of the deposited dose (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

Table 1. Radiolytic Consumption Yields of Butanal Oxime Obtained by Irradiation of Aqueous Butanal Oxime Samples at Several Concentrations in the CEMHTI Cyclotron (Eα = 9.4 MeV, D = 2000 J L−1 min−1) G(-butanal oxime) (10−7 mol J−1) initial butanal oxime concentration (mol L−1) −3

1.04 × 10 1.04 × 10−2 1.04 × 10−1

C3H 7CHNOH + H3O+ → C3H 7CHO + NH3OH+ (22)

Gaseous species analysis was performed by micro-gas chromatography (μGC) using a SRA instruments μGC3000 coupled with a 5975C electronic impact mass spectrometer. Four columns were used simultaneously for specific analysis of the different compounds: a 5 Å molecular sieve, a Poraplot U, a Poraplot Q, and an OV-1. Quantification was done using a thermal conductivity detector on each column. Spectrophotometric analyses were performed with a Cary-60 (VARIAN) spectrophotometer. The hydrogen peroxide concentration was determined by complexation with Ti+IV ions (λmax = 407 nm, ε = 700 L mol−1 cm−1).47 The reactant was prepared by dissolving titanium(+IV) oxysulfate (TiOSO4, Sigma-Aldrich) in 0.5 mol L−1 sulfuric acid solution. The nitrous acid concentration was also quantified by spectrophotometry (λmax = 530 nm, ε = 42000 L mol−1 cm−1), using Griess− Illosvay’s reagent for nitrite analysis (Chemlab),48 the latter being added after the end of the irradiation.

HNO3 0.5 mol L−1

water10

3.5 ± 0.6 1.9 ± 0.4 13 ± 2

2.6 ± 0.4 3.9 ± 0.5 18 ± 2

10−7 mol J−1, respectively, for an initial concentration of 1.04 × 10−3 and 1.04 × 10−2 mol L−1. However, for a higher butanal oxime concentration (1.04 × 10−1 mol L−1), this consumption yield reaches G(-butanal oxime) = (13 ± 2) × 10−7 mol J−1. Such a high yield can be due to chain reactions issued by a radical attack. The effect of the variation of the nitric acid concentration on the alpha radiolysis of butanal oxime has been characterized (Figure 2). For a nitric acid concentration below 0.5 mol L−1, the butanal oxime degradation yield is lower than the one determined for the water−butanal oxime system at an equivalent concentration.10 G(-butanal oxime) diminishes indeed from 18.4 × 10−7 to 10.3 × 10−7 mol J−1, respectively,

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RESULTS AND DISCUSSION Butanal Oxime Radiolytic Consumption. Various butanal oxime solutions in nitric acid have been irradiated, using the helium ion beams of the CEMHTI cyclotron. Figure 1 presents the variation of the butanal oxime concentration consumed as a function of the deposited dose in a 0.5 mol L−1 nitric acid solution and for an initial 0.1 mol L−1 butanal oxime concentration. The butanal oxime consumption yield determined using this curve is equal to (13 ± 2) 10−7 mol J−1. At a constant nitric acid concentration ([HNO3] = 0.5 mol L−1), the effect of a variation of butanal oxime initial concentration on the radiolytic butanal oxime consumption yield is similar to the one observed in water.10 The values of the yields obtained at different butanal oxime concentrations are compiled in Table 1. For low initial butanal oxime concentrations, G(-butanal oxime) stays in the same order of magnitude between (3.5 ± 0.6) × 10−7 and (1.9 ± 0.4) ×

Figure 2. Variation of the radiolytic butanal oxime consumption yield as a function of the nitric acid concentration. The initial butanal oxime concentration is 1.04 × 10−1 mol L−1. Samples irradiated at the CEMHTI cyclotron (Eα = 9.4 MeV, D = 2000 J L−1 min−1). C


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The Journal of Physical Chemistry A in water and in 0.1 mol L−1 nitric acid. It is thus clear that nitrate ions play a protective role toward butanal oxime alpha radiolysis. For a nitric acid concentration higher than 0.5 mol L−1, the butanal oxime radiolytic degradation yield rapidly increases to reach 150 × 10−7 mol J−1 in the case where [HNO3] = 1.5 mol L−1. Finally, for the more concentrated nitric acid samples ([HNO3] > 1 mol L−1) and for an initial butanal oxime concentration below 10−2 mol L−1, it was not possible to detect any butanal oxime after irradiation. This implies that butanal oxime is in this case totally consumed after the irradiation of the solution. The brutal increase of the degradation yield with increasing nitric acid concentration from 0.5 to 1.5 mol L−1 would be due to an autocatalytic process, which will be discussed in detail later. Liquid Phase Products. In aqueous phases containing nitric acid, two major radiolysis products are formed in solution: hydrogen peroxide (H2O2) and nitrous acid (HNO2). The H2O2 formation results mainly from water radiolysis, whereas nitrate ions are known to form nitrous acid either by indirect17−19,21−24 or direct32−35 radiolysis. Hydrogen Peroxide. Figure 3 shows the variation of the hydrogen peroxide radiolytic yield as a function of butanal

butanal oxime concentration increases. This means that in nitric acid media butanal oxime inhibits the H2O2 production in the same way as it does in binary butanal oxime−water mixtures.10 Indeed, butanal oxime scavenges OH• radicals, thus leading to a decrease in H2O2 production. Moreover, the steady-state H2O2 production yield at high butanal oxime concentration is probably due to radiation chemistry of the butanal oxime degradation products. For nitric acid concentration higher than 0.5 mol L−1, no hydrogen peroxide accumulation has been observed. Nitrous Acid. The nitrous acid radiolytic formation yields following the helium ion beam irradiation of aqueous nitric solutions without butanal oxime are compiled on Table 2. The HNO2 yields obtained in this study are in good agreement with those obtained in previous works.30 Table 2. HNO2 Radiolytic Formation Yields in Nitric Acid Media Irradiated with Helium Ion Beams (Eα ≈ 10 MeV, D ≈ 2000 J L−1 min−1) G(HNO2) (10−7 mol J−1) [HNO3] (mol L−1) 0.50 0.75 1 1.5 2.0

Garaix30 0 0.05 ± 0.30 0.37 ± 0.04 0.93 ± 0.14

this work 0 0.32 ± 0.03 0.91 ± 0.15 1.10 ± 0.17

[HNO2] formed at 1000 J L−1 (10−4 mol L−1) 0 0.05 0.35 0.91 1.02

± ± ± ±

0.3 0.04 0.15 0.17

The effect of the variation of the butanal oxime concentration on the nitrous acid production following the helium ion beam irradiation of aqueous nitric acid−butanal oxime solutions has been investigated on a wide range of nitric acid concentration (from 10−2 to 2 mol L−1). For nitric acid concentrations below 0.5 mol L−1, no nitrous acid accumulation has been observed. Likewise, for highly butanal oximeconcentrated solutions (>10−2 mol L−1), the presence of butanal oxime inhibits the accumulation of HNO2. Butanal oxime is in this case in excess regarding the nitrous acid formed by nitric acid radiolysis. Indeed, for a total dose of 1000 J L−1 in a 2 mol L−1 nitric acid solution, the radiolysis of nitric acid generates 1.02 × 10−4 mol L−1. However, the situation is more complex in the case of solutions where nitric acid concentration stays between 1 and 2 mol L−1, and the oxime concentrations is below 10−3 mol L−1. Figure 4 presents the production of nitrous acid by irradiation

Figure 3. Influence of butanal oxime concentration on the hydrogen peroxide yield obtained by aqueous butanal oxime solutions irradiation for various nitric acid concentrations. Sample irradiated at the CEMHTI cyclotron (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

oxime concentration resulting from the irradiation of solutions containing various concentrations of nitric acid. In the absence of butanal oxime, Figure 3 shows that, as the nitric acid concentration increases, G(H2O2) decreases from G(H2O2) = (1.37 ± 0.20) × 10−7 mol J−1 for [HNO3] = 0.01 mol L−1 to G(H2O2) = (0.40 ± 0.05) × 10−7 mol J−1 for [HNO3] = 0.5 mol L−1. This tendency is in good agreement with a previous work.30,31 For nitric acid concentrations higher than 0.5 mol L−1, hydrogen peroxide cannot accumulate in solution due to its reaction with nitrous acid (eq 15). Indeed, as it has been shown that for a nitric acid concentration higher than 0.5 mol L−1 with comparable irradiation conditions (Eα = 10 MeV and D ≈ 2000 J L−1 min−1), nitrous acid becomes the major radiolysis product. Besides, Figure 3 shows that at a given nitric acid concentration, the hydrogen peroxide yield decreases as the butanal oxime concentration increases. For instance, at a nitric acid concentration [HNO3] = 0.1 mol L−1, the hydrogen peroxide yield strongly diminishes from (0.89 ± 0.09) × 10−7 mol J−1 for butanal oxime-free solutions, to (0.35 ± 0.08) × 10−7 mol J−1 for solutions containing 1 mmol L−1 butanal oxime. The H2O2 yield then stabilizes around this value as the

Figure 4. HNO2 production and evolution following the helium ion beam irradiation of butanal oxime (1.04 × 10−3 mol L−1) in nitric acid (1 mol L−1) media (Eα = 9.4 MeV, D = 2000 J L−1 min−1). D


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The Journal of Physical Chemistry A of a 10−3 mol L−1 butanal oxime solution in nitric acid medium (1 mol L−1) as a function of the deposited dose. The nitrous acid accumulation is only observed after an induction period, corresponding to a cumulated dose around 700 J L−1. For doses higher than 700 J L−1, the nitrous acid concentration rapidly increases. From a cumulated dose around 1400 J L−1, the nitrous acid concentration reaches 2.3 × 10−4 mol L−1 and thereafter rises linearly with the dose. It is then possible to determine a nitrous acid radiolytic production yield G*(HNO2)1M = (0.07 ± 0.03) × 10−7 mol J−1. This yield is recorded as “G*” because it does not refer to the classically used null dose extrapolated yield, but it refers to the HNO2 production yield at the pseudostationary state after the 1400 J L−1 dose threshold. However, this value is smaller than the HNO2 radiolytic formation yield in 1 mol L−1 nitric acid medium (Table 2). The latter is indeed extrapolated at zero dose,30,31 and it has been shown in these works that the HNO2 production kinetics slows down as the nitrous acid accumulates in solution. An example is displayed on Figure 5.

Figure 6. HNO2 production following the helium ion irradiation of butanal oxime 1.04 × 10−3 mol L−1 in aqueous 2 mol L−1 nitric acid solutions (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

curve exhibits a threshold or not in this particular case. The apparent nitrous acid radiolytic yield is determined: G*(HNO2)2M = (1.08 ± 0.12) × 10−7 mol J−1. This value is close from the value of HNO2 radiolytic yield in 2 mol L−1 nitric acid (Table 2). This confirms that butanal oxime is totally consumed and that HNO2 accumulation is only a consequence of nitric acid radiolysis. Gaseous Products. Other pieces of information concerning the butanal oxime degradation mechanisms in aqueous nitric acid phases can be derived from the analysis of the gaseous radiolytic products. Dihydrogen. Dihydrogen is one of the major products of the radiolysis of aqueous phases. Figure 7 shows the variation of

Figure 5. Determination of apparent HNO2 radiolytic yield (G*) for several doses and zero dose extrapolated yield (G) for a nitric acid (3 mol L−1) solution irradiated with a helium ion beam (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

In the present case, a certain quantity of nitrous acid has already been formed in solution, thus explaining the difference of yield with the literature. This indicates that after 1400 J L−1, the HNO2 formation is exclusively related to the nitric acid radiolysis. Other analyses have been carried out 24 and 36 h after the irradiations to measure the quantity of nitrous acid remaining in solution. It can be noted that the nitrous acid concentration progressively decreases with increasing time after irradiation until analysis (Figure 4). This observation confirms preliminary experiments showing that nitrous acid slowly decomposes in nitric acid media. HNO2 decomposition kinetics depends on the nitric acid concentration. Nitrous acid is indeed unstable in nitric acid media (eq 23),49 and its decomposition produces nitrogen monoxide (NO). The following of this species would be of particular interest. Unfortunately, it cannot be monitored by μGC. An attempt to determine both NO and NO2 using FTIR-spectroscopy has been made, but the method is still under development. 3HNO2 → HNO3 + 2NO + H 2O

Figure 7. Variation of H2 radiolytic production in aqueous nitric acid phases as a function of the deposited dose for several nitric acid concentrations (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

the hydrogen concentration produced by the irradiation of aqueous nitric acid solutions containing no butanal oxime as a function of the dose. In nitric acid media, and without butanal oxime, the hydrogen radiolytic formation yield decreases in comparison to the one obtained in pure water. For instance, for [HNO3] = 0.5 mol L−1, G(H2) = (0.65 ± 0.07) × 10−7 mol J−1, whereas G(H2) = (1.04 ± 0.10) × 10−7 mol J−1 in pure water. G values are compiled in Table 3. It is reported in the literature that nitrate ions act as hydrogen radical scavengers during the alpha radiolysis (eqs 24 and 25):23,25,26,28,29

(23)

Similar experiments have been carried out for a nitric acid concentration of 2 mol L−1 (Figure 6). In this case, the maximum nitrous acid concentration reaches 5.9 × 10−4 mol L−1. However, the experimental data collected do not allow to state undoubtedly if the nitrous acid production

NO3− +• H →• NO2 + OH− •

E

•

+

NO2 + NO2 + H2O → H +

(24)

NO3−

+ HNO2

(25)


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The Journal of Physical Chemistry A Table 3. Dihydrogen Radiolytic Formation Yield (10−7 mol J−1) Following the Irradiation of Butanal Oxime Samples in Aqueous Nitric Acid at the CEMHTI Facility (Eα = 9.4 MeV, D = 2000 J L−1 min−1)

the dihydrogen yield strongly decreases as long as there is a low butanal oxime concentration (between 10−3 and 10−2 mol L−1) in the solution compared to systems that do not contain any butanal oxime. This effect is opposed as the one observed after the irradiation of butanal oxime solutions in water.10 Indeed, in the irradiated samples without nitric acid, the presence of butanal oxime, even in small amounts, led to a significant rise of the dihydrogen radiolytic formation yield. Furthermore, as the butanal oxime concentration in nitric acid solutions increases (>10−2 mol L−1), the dihydrogen radiolytic formation yield rises until it reaches a value close to the system containing no oxime. Nitrous Oxide. Figure 10 presents the variation of the nitrous oxide radiolytic formation yield when aqueous butanal

[butyraldoxime] (mol L−1) [HNO3] (mol L−1) 0 0.01 0.1 0.5 1.0 1.5

0 1.04 0.81 0.74 0.65 0.37 0.23

± ± ± ± ± ±

0.001 0.10 0.08 0.08 0.07 0.04 0.03

1.13 0.80 0.59 0.46 0.24

± ± ± ± ±

0.15 0.09 0.06 0.05 0.03

0.1 1.28 1.04 1.03 0.70 0.42 0.21

± ± ± ± ± ±

0.08 0.11 0.11 0.07 0.05 0.03

The dihydrogen radiolytic production yield therefore decreases as the nitric acid concentration increases. The values of these yields are in good agreement with the data of the literature (Figure 8).

Figure 10. Variation of the nitrous oxide radiolytic formation yield following the helium ion beam irradiation of aqueous butanal oxime solutions as a function of the nitric acid concentration (Eα = 9.4 MeV, D = 2000 J L−1 min−1).

Figure 8. Variation of the dihydrogen radiolytic formation yield after irradiation of nitric acid (0−1.5 mol L−1) samples with a helium ion beam (Eα = 9.4 MeV, D = 2000 J L−1 min−1), and comparison with the data found in the literature.23,25,26,28,29

Table 3 compiles the G values of dihydrogen production from the helium ion beam irradiation of aqueous nitric acid− butanal oxime samples as a function of the nitric acid concentration, and considering several butanal oxime concentrations. For a given butanal oxime concentration, the H2 radiolytic formation yield decreases as the nitric acid concentration in the solution increases, which is the same trend as the systems without oxime. The variation of the dihydrogen radiolytic formation yield in nitric acid media as a function of the butanal oxime concentration is plotted on Figure 9. These results show that

oxime−nitric acid solutions are irradiated with a helium ion beam. For low nitric acid concentrations ([HNO3] < 1 mol L−1), the nitrous oxide formation is not detected following the irradiation of solutions containing no butanal oxime. However, when the nitric acid concentration rises ([HNO3] ≥ 1 mol L−1), a small amount of radiolytically generated N2O is measured. For instance, in 1.5 mol L−1 nitric acid, G(N2O) = (0.12 ± 0.03) × 10−7 mol J−1. This means that N2O is a nitric acid radiolysis product. The associated mechanism is probably resulting from direct effects of the radiation because the N2O formation is only observed at high nitric acid concentration. The presence of butanal oxime seems nevertheless to strongly enhance the formation of nitrous oxide. N2O is indeed observed after the irradiation of nitric acid solutions for which no N2O was measured in the absence of butanal oxime ([HNO3] ≤ 0.5 mol L−1). For instance, G(N2O) = (1.41 ± 0.10) × 10−7 mol J−1 for [butanal oxime] = 10−1 mol L−1 and [HNO3] = 0.5 mol L−1, whereas G(N2O) = 0 for [butanal oxime] = 0 and [HNO3] = 0.5 mol L−1. It has been shown previously that nitrous acid is formed by butanal oxime radiolysis in water,10 but the corresponding yields are much lower than in nitric acid media: from (0.004 ± 0.002) × 10−7 mol J−1 for 10−3 mol L−1 butanal oxime to (0.010 ± 0.003) × 10−7 mol J−1 for 10−1 mol L−1 butanal oxime in water. The mechanisms associated with the nitrous oxide production in nitric acid are then more complex than the ones in water and involving multiple processes.

Figure 9. Influence of the butanal oxime concentration on the dihydrogen radiolytic formation yield following the helium ion irradiation (Eα = 9.4 MeV, D = 2000 J L−1 min−1) of aqueous nitric acid samples. F


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The Journal of Physical Chemistry A Other Gases. Finally, the propene radiolytic formation yield due to the helium ion beam irradiation of butanal oxime in 0.5 mol L−1 nitric acid has been determined for several initial butanal oxime concentrations (Figure 11). The propene formation in this medium indicates that the C−C scission mechanisms that readily happen in water−butanal oxime systems10 also take place in nitric acid media.

In nitric acid media, the hydroxyl radical scavenging involves several reactions (eqs 27-29):27,50 NO3− + •OH + H+ → •NO3 + H 2O •

NO32 − + •OH → NO3− + OH−

HNO3 + •OH → •NO3 + H 2O

(27) (28) (29)

The first reaction (eq 27) involves the nitrate ions, which are present in high quantity in the medium. However, this reaction is relatively slow, with a rate constant comprised between 104 and 105 L mol−1 s−1.51,52 For low acidity solutions, another reaction between OH• and •NO32− has to be taken into account (eq 28). •NO32− is a reaction intermediate during the conversion of nitrate ion to nitrogen dioxide by solvated electron scavenging (eqs 5 and 8).30 Its rate constant k = 3 × 109 L mol−1 s−1 is clearly higher than the one corresponding to the reaction between nitrate ion and hydroxyl radical. Finally, the scavenging of the OH• radicals by undissociated nitric acid (eq 29) only takes place in highly concentrated nitric acid solutions ([HNO3] > 3 mol L−1). This reaction is fast: k = 1.4 × 108 L mol−1 s−1.34 The decrease of H2O2 radiolytic production in the presence of butanal oxime (Figure 3) can be translated by the fact that • OH radical scavenging by butanal oxime (eq 30) is more efficient than by nitrate ions (eq 27) and •NO32− (eq 28).

Figure 11. Influence of the initial butanal oxime concentration on the propene radiolytic formation yield produced by the helium ion beam (Eα = 9.4 MeV, D = 2000 J L−1 min−1) irradiation of butanal oxime in 0.5 mol L−1 nitric acid solutions.

C3H 7CHNOH + •OH → (CH3CH 2CH 2CHNO•) + H 2O

Besides, some other radiolysis products readily identified from the irradiation of butanal oxime in water, butene, and butanal were observed during the irradiation of butanal oxime in nitric acid solutions. However, these compounds could not be quantified. It is likely that in the case of the butene formation, the same mechanism as observed in water takes place. The semiquantitative analysis of the μGC chromatograms indicating that butanal is produced in larger amounts after the irradiation of butanal oxime in nitric acid solutions than in water. As an example, for a 7500 J L−1 irradiation of a 0.1 mol L−1 butanal oxime sample, the butanal peak area is 400 times higher in 1.5 mol L−1 nitric acid than in water.

(30)

The case of H2 production (Figure 9) can be explained by the following mechanism: H• + H• → H 2

(31)

NH3OH+ + H• → •NH3+ + H 2O

(32)

C3H 7CHNOH + H• → C3H 7CHNO• + H 2

(33)

C3H 7CHNOH + H• → CH3CH 2CH•CHNOH + H 2 (34)

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In acidic media, butanal oxime undergoes acidic hydrolysis thus forming hydroxylammonium ion and butanal (eq 22).45,46 The hydroxylammonium ion can then scavenge H• radical (eq 32),53 inducing a dihydrogen production diminution because H• radical can then not recombine to form H2 (eq 31). At low butanal oxime concentration, the major part of the butanal oxime in the medium is hydrolyzed. Therefore, the H• scavenging reactions by butanal oxime, leading to H 2 production (eqs 33 and 34), are disadvantaged compared to the H• scavenging by hydroxylammonium (eq 32). Thus, the radiolytic yield G(H2) decreases in comparison to solutions that do not contain any butanal oxime. However, with higher butanal oxime concentrations, there is a smaller part of hydrolyzed butanal oxime. This implies that the H• scavenging by butanal oxime becomes more and more effective versus the scavenging by hydroxylammonium when butanal oxime concentration increases. Finally, G(H2) rises as the butanal oxime concentration increases. Oxidation by HNO2. The two major liquid phase products formed during the radiolysis of nitric acid solutions are hydrogen peroxide and nitrous acid. However, both butanal oxime (eq 1)11,41,42 and hydrogen peroxide (eq 15)23,54−57 can react with nitrous acid.

DISCUSSION Scavenging of H and OH Radicals. In low concentration nitric acid solutions ([HNO3] < 0.5 mol L−1), butanal oxime is less degraded by ionizing radiations than in water (Figure 2). This can be explained by a competition between nitrate ions and butanal oxime for the scavenging of the radicals formed by water radiolysis. In particular, it is well-known that nitrate ions act as scavengers of H• radicals (eqs 24and 25).23,25,26,28,29 The rate constant of the reaction between nitrate ions and H• radical (eq 24) is equal to ik = 2.0 × 107 L mol−1 s−1.27 Thus, the nitrate ions in the medium play a protective role on butanal oxime by scavenging H• radicals. These results show that the reaction of the H• radicals formed by water radiolysis with butanal oxime (eq 26) is slower than their reaction with nitrate ions. For a given butanal oxime concentration, the H2 radiolytic formation yield decreases as the nitric acid concentration in the solution increases (Table 3). This result confirms the competition mechanism for the H• radical scavenging between butanal oxime and nitrate ions. C3H 7CHNOH + •H → (CH3CH 2CH 2CHNO)• + H 2 (26) G


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The Journal of Physical Chemistry A

nitric acid concentration. Several reactions concur in 1 mol L−1 nitric acid media containing butanal oxime under irradiation (eqs 1, 15, and 37).

As a consequence, there is a competition between these two reactions. The rate constant of the reaction between nitrous acid and acetaldoxime, another linear oxime, is k = 14 L mol−1 s−1 for [HNO3] = 1.0 mol L−1.41 Besides, acetaldoxime is a more efficient stabilizer of Pu(+III) than butanal oxime in aqueous phase.13 As the main Pu(+III) oxidant in nitric acid medium is HNO2, this indicates that butanal oxime reacts slower than acetaldoxime with nitrous acid. The rate constant of the reaction between nitrous acid and butanal oxime should thus verify k < 14 L mol−1 s−1 for [HNO3] = 1.0 mol L−1. The kinetics of the reaction between nitrous acid and hydrogen peroxide has been reported in the literature. In 1.0 mol L −1 nitric acid, k = 2.5 × 103 L mol−1 s−1.57 The rate constant of the reaction between H2O2 and HNO2 in 1.0 mol L−1 nitric acid is then 200 times higher than the rate constant of the reaction between butanal oxime and nitrous acid. Therefore, the nitrous acid formed by nitric acid radiolysis preferably reacts with H2O2 rather than with butanal oxime. This is coherent with the fact that the butanal oxime radiolytic consumption yield does not rise in low concentration nitric acid medium in comparison with water (Figure 2). The competition mechanism between butanal oxime and hydrogen peroxide reactivity toward nitrous acid can also explain the results on H2O2 production following the alpha radiolysis of butanal oxime in nitric acid aqueous solution. Figure 3 indicates that butanal oxime inhibits the accumulation of H2O2 in nitric acid media. A previous study with hydrazinium nitrate30 showed the opposite behavior. Hydrazinium nitrate favors the accumulation of hydrogen peroxide in nitric acid media due to radiolysis. Hydrazinium nitrate rapidly reacts with nitrous acid (eqs 35 and 36): N2H+5 + HNO2 → HN3 + 2H 2O + H+

(35)

HN3 + HNO2 → N2O + N2 + H 2O

(36)

2C3H 7CHNOH + HNO3 → 2C3H 7CHO + 2HNO2 + N2O + H 2O

(37)

In a first step, the hydrogen peroxide from the water radiolysis reacts with the nitrous acid induced by nitric acid radiolysis (eq 15). This reaction is faster than the butanal oxime−HNO2 reaction (eq 1). However, nitrous acid is formed in excess compared to the hydrogen peroxide under the effect of the irradiation. As a consequence, the excess of nitrous acid is then progressively consumed by butanal oxime (eq 1). The quick increase of the nitrous acid accumulated in solution from a certain dose indicates that it is mandatory to bring a certain amount of energy to the system to destroy all the butanal oxime in the medium. This is a typical feature of an autocatalytic mechanism with an induction period necessary to begin the reaction. The reaction between nitrous acid and acetaldoxime shows similar features. After an induction period, the reaction begins and forms HNO2 in an autocatalytic way.58 In the case of butanal oxime, the induction period corresponds to the first part of the mechanism involving oxidation of butanal oxime by nitrous acid (eq 1). In this case, one cannot detect any HNO2 accumulation in the system. On the second step, a reaction between butanal oxime and nitric acid, leading to an excess of nitrous acid, is launched (eq 37). In the case of the 2 mol L−1 nitric acid system (Figure 6), it is possible that HNO2 can accumulate fast enough to launch the butanal oxime oxidation by nitric acid for a very low dose. The difference of the curves between Figures 4 and 6 indicates that, as the nitric acid concentration increases, the dose threshold of the reaction (eq 37) decreases. When the whole butanal oxime is consumed, the HNO2 radiolytic formation yield resulting from the nitric acid (2 mol L−1) radiolysis is then G*(HNO2)2M = (1.08 ± 0.12) × 10−7 mol J−1. This value is close to the nitrous acid radiolytic yield in 2 mol L−1 nitric acid media without butanal oxime (Table 2). This confirms that, as soon as the butanal oxime is totally consumed, the HNO2 accumulation results from the nitric acid radiolysis. The evidence about the formation of nitrous oxide (Figure 10) and butanal (see Other Gases section) during the radiolysis of butanal oxime in nitric acid solutions are also in good agreement with a two-step oxidation mechanism of butanal oxime by nitrous and nitric acid, thus autocatalytically generating important amounts of nitrous acid, as stated before. The main products of the butanal oxime oxidation by HNO2 are indeed N2O and butanal (eq 1). To summarize this mechanism, below a dose threshold depending on the nitric acid concentration and the butanal oxime concentration, HNO2 is formed by nitric acid radiolysis and destroyed by butanal oxime (eq 1). As soon as this dose threshold is overlapped, the ratio between the butanal oxime concentration and the nitrous acid concentration allows the oxidation of butanal oxime by nitric acid, following an autocatalytic formation of nitrous acid (eq 37). The butanal oxime is then quickly and totally consumed, and the nitrous acid production is then controlled by the nitric acid radiolysis.

Hydrazinium nitrate consumes the radiolytically generated HNO2. The consumption reaction of H2O2 by HNO2 (eq 15) is then inhibited. Butanal oxime is also reactive toward nitrous acid (eq 1). It can then potentially react with HNO2 as soon as the latter is formed in solution by radiolysis. However, a H2O2 yield decrease is observed in nitric acid media when butanal oxime is present in solution. As the HNO2 consumption by butanal oxime (eq 1, k < 14 L mol−1 s−1) is much slower than the one involving hydrogen peroxide (eq 15, k = 2.5 × 1 03 L mol−1 s−1), HNO2 primarily reacts with H2O2 as long as the latter is present in solution. Moreover, butanal oxime inhibits the formation of H2O2 because of the scavenging of the hydroxyl radicals (eq 30). Thus, for [HNO3] = 0.5 mol L−1, no H2O2 accumulation is observed in the presence of butanal oxime. In highly concentrated nitric acid ([HNO3] > 0.5 mol L−1), an important increase of the butanal oxime radiolytic consumption yield is observed (Figure 2). As the nitrate ion concentration becomes higher, more nitrous acid is formed due to nitric acid radiolysis.31 When the nitrate ion concentration is higher than [NO3−] = 0.5 mol L−1, enough HNO2 is radiolytically created to totally consume H2O2 (eq 15). In this case, when only considering the nitric acid radiolysis: G(HNO2) > G(H2O2).30,31 In a second step, the remaining HNO2 can then oxidize butanal oxime (eq 1). The investigation of the HNO2 production (Figure 4) is necessary to fully understand the mechanisms involved at high H


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The Journal of Physical Chemistry A

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(4) Doherty, A. M. M.; Howes, K.; Stedman, G.; Naji, M. Q. Is Hydrazoic Acid (HN3) an Intermediate in the Destruction of Hydrazine by Excess Nitrous Acid? J. Chem. Soc., Dalton Trans. 1995, No. 19, 3103−3107. (5) Koltunov, V. S.; Marchenko, V. I. Kinetics of Hydrazine Oxidation by Nitrous Acid. J. Catal. 1967, 7, 97. (6) Perron, J. R.; Stedman, G.; Uysal, N. Kinetic and Product Study of the Reaction between Nitrous Acid and Hydrazine. J. Chem. Soc., Dalton Trans. 1976, 20, 2058−2064. (7) Perrott, J. R.; Stedman, G. The Kinetics of Nitrite Scavenging by Hydrazine and Hydrazoic Acids at High Acidities. J. Inorg. Nucl. Chem. 1977, 39, 325−327. (8) Sze, Y.-K.; Clegg, L. J.; Gerwing, A. F.; Grant, G. R. Oxidation of Pu (III) by Nitric Acid in Tri-n-Butyl Phosphate Solutions. Part I. Kinetics of the Reaction and Its Effect on Plutonium Losses in Countercurrent Liquid-Liquid Extraction. Nucl. Technol. 1982, 56, 527−534. (9) Gourisse, D.; Gautier, A. Comportement de l’acide nitreux dans les extractions par le Phosphate de Tributyle. J. Inorg. Nucl. Chem. 1969, 31, 839−850. (10) Costagliola, A.; Venault, L.; Deroche, A.; Garaix, G.; Vermeulen, J.; Omnee, R.; Duval, F.; Blain, G.; Vandenborre, J.; Fattahi-Vanani, M.; et al. Radiation Chemical Behavior of Aqueous Butanal Oxime Solutions Irradiated with Helium Ion Beams. Radiat. Phys. Chem. 2016, 119, 186−193. (11) Kliegman, J. M.; Barnes, R. K. Reaction of Nitrous Acid with Oximes. J. Org. Chem. 1972, 37, 4223−4225. (12) Dinh, B.; Baron, P.; Moisy, P.; Venault, L.; Bernier, G.; Pochon, P. Utilisation de l’oxime de butyraldéhyde en tant qu’agent anti-nitreux dans le retraitement de combustibles nucléaires usés. FR 2 917 227 A1. (13) Sze, Y.-K.; Gosselin, J. A. Oxidation of Pu (III) by Nitric Acid in Tri-n-Butyl Phosphate Solutions. Part II. Chemical Methods for the Suppression of Oxidation to Improve Plutonium Separation in Contactor Operation. Nucl. Technol. 1983, 63, 431−441. (14) Allen, A. O. The Radiation Chemistry of Water and Aqueous Solutions; Van Nostrand Company: Princeton, 1961. (15) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 3rd Ed; John Wiley and Sons Inc.: New York, 1990. (16) Dainton, F. S.; Logan, S. R. Radiolysis of Aqueous Solutions Containing Nitrite Ions and Nitrous Oxide. Trans. Faraday Soc. 1965, 61, 715−722. (17) Daniels, M.; Wigg, E. E. Radiation Chemistry of the Aqueous Nitrate System. I. γ-Radiolysis of Dilute Solutions. J. Phys. Chem. 1967, 71, 1024−1033. (18) Kazanjian, A. R.; Miner, F. J.; Brown, A. K.; Hagan, P. G.; Berry, J. W. Radiolysis of Nitric Acid Solution: LET Effects. Trans. Faraday Soc. 1970, 66, 2192−2198. (19) Kulikov, I. A.; Vladimirova, M. V. Gamma-Radiolysis of Nitrate Solutions. 1. Neutral NaNO3 Solutions. High Energy Chem. 1975, 9, 199−202. (20) Kulikov, I. A.; Vladimirova, M. V. Gamma-Radiolysis of Nitrate Solutions. 2. Acidic NaNO3 Solutions. High Energy Chem. 1975, 9, 465−468. (21) Miner, F. J.; Kazanjian, A. R.; Brown, A. K.; Hagan, P. G.; Berry, J. W. Radiation Chemistry of Nitric Acid Solutions; RFP-1299; Dow Chemical Co.: Golden, CO, 1969. (22) Miner, F. J.; Seed, J. R. Radiation Chemistry of Plutonium Nitrate Solutions. Chem. Rev. 1967, 67, 299−315. (23) Savel’ev, Y. I.; Ershova, Z. V.; Vladimirova, M. V. Alpha Radiolysis of Aqueous Solutions of Nitric Acid. Radiochimie 1967, 9, 244−249. (24) Vladimirova, M. V. Alpha Radiolysis of Aqueous Solutions. Russ. Chem. Rev. 1964, 33, 212−220. (25) Kazanjian, A. R.; Horrell, D. R. Radiolytically Generated Gases in Plutonium-Nitric Acid Solutions. Radiat. Eff. 1972, 13, 277−280. (26) Sheppard, J. C. Alpha Radiolysis of Plutonium (IV): Nitric Acid Solutions; BNWL-751; Pacific Northwest Lab.: Richland, WA, 1968. (27) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons,

CONCLUSION The influence of nitric acid on the behavior of aqueous butanal oxime solutions under helium ion irradiation has been studied by investigating butanal oxime radiolytic degradation yield and the formation yield of some radiolysis products. For a low nitric acid concentration ([HNO3] ≤ 0.5 mol L−1), butanal oxime is less degraded than in an aqueous phase containing no nitric acid. The nitrate ions in the medium act as butanal oxime protector. In particular, the nitrate ions are H• and •OH radicals scavengers. The variation of the dihydrogen and the hydrogen peroxide yields with the butanal oxime translates a competition mechanism between butanal oxime and the nitrate ions toward the scavenging of hydrogen and hydroxyl radicals. Besides, the nitrous acid, mostly issued from the nitric acid radiolysis, plays a key role in the butanal oxime radiolytic consumption. At low nitric acid concentration (≤0.5 mol L−1), butanal oxime is in competition with hydrogen peroxide to reduce HNO2. Butanal oxime is then protected by H2O2. However, for higher nitric acid concentrations (>0.5 mol L−1), H2O2 cannot accumulate anymore in contrary to HNO2. The latter can then react with butanal oxime, leading to an autocatalytic oxidation of butanal oxime. These phenomena explain the brutal increase of the butanal oxime radiolytic degradation at high nitric acid concentration. In order to better apprehend the butanal oxime radiolytic degradation mechanisms in aqueous phase, the quantitative analysis of some liquid phase products such as butanal, butyronitrile, or butanal oxime dimers would be important. Moreover, the behavior of these species under alpha irradiation has then to be considered in order to fully describe the system. Some gases such as COx−NOx could also not be monitored at this moment and can bring some interesting information on the mechanisms. Finally, the radiation chemical behavior of butanal oxime in organic media, and in particular the effect of the extracted nitric acid, is a crucial point to describe the impact of radiolysis on a PUREX-type system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

L. Venault: 0000-0003-2946-3730 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the AREVA group and by the CEA/Nuclear Energy Division/RadioChemistry and Processes Department. The authors would like to express their gratitude for the CEMHTI facility teams for their technical support in this study.

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REFERENCES

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Influence of Nitric Acid on the Helium Ion Radiolysis of Aqueous

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