Decoding the Three-Pronged Mechanism of NO3• Radical Formation

Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX

Decoding the Three-Pronged Mechanism of NO•3 Radical Formation in HNO3 Solutions at 22 and 80 °C Using Picosecond Pulse Radiolysis Raluca Musat,‡ Sergey A. Denisov,‡ Jean-Louis Marignier,‡ and Mehran Mostafavi*,‡ Laboratoire de Chimie Physique, CNRS/Université Paris-Sud, Bâtiment 349, Orsay 91405, France S Supporting Information *

ABSTRACT: With nitric acid (HNO3) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO3 solutions under ionizing radiation equates bringing efficiency and safety into their operation. In this work, the three mechanisms contributing to the formation of nitrate radicals (NO•3 ) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO•3 in HNO3 solutions (0.5−23.5 M) at room (22.5 °C) and elevated (80 °C) temperatures on time scales spanning from picosecond to microsecond by pulse radiolysis measurements. We conclude that the formation yield of NO•3 , just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO−3 and the water cation radical, H2O•+. The absolute −7 formation yield of NO•3 radicals due to the direct effect, Gdir mol·J−1, irrespective of the NO•3 , is found to be (3.4 ± 0.1) × 10 concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO•3 formation from the reaction between •OH radicals and undissociated HNO3 is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 107 M−1·s−1 for 22.5 °C, reaching a value of (1.1 ± 0.2) × 108 M−1·s−1 at 80 °C.

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INTRODUCTION The understanding of the processes involving nitric acid (HNO3) and nitrate radicals (NO•3 ) is of critical importance through the perspective of the numerous applications utilizing HNO3: in the civil nuclear industry, in the water treatment industry, and in the manufacturing industry, where HNO3 is used in the production of explosives, fertilizers, etc. Furthermore, this is underlined by the role played by NO•3 radicals in the night-time chemistry of the troposphere and stratosphere.1,2 In the context of the global energy paradigm, nuclear power production is being reevaluated due to the role it could play in securing energy production and carbon dioxide abatement. In the two past decades, we observed a decline of the global share of nuclear power generation. However, at the moment, 60 new reactors are under construction worldwide (not taking into account naval reactors) destined to replace old ones and provide additional power generation, which will lead to more nuclear waste materials, in addition to the >280 000 tons of heavy metals already accumulated in storage facilities worldwide.3 The constantly growing rate of nuclear waste production emphasizes the need for a full understanding of the radiation-induced phenomena in spent nuclear fuel during reprocessing, to recover more efficiently fissile and fertile materials for further use. France, the UK, Russia, Japan, and recently the USA have instated policies concerning the management of spent nuclear fuel either by storing it in repositories or by reprocessing it. The recycling of spent nuclear fuel contributes to the energy security through reduction of waste volume and its radioactivity © XXXX American Chemical Society

levels. Several processes have already been developed toward this end, aiming to separate uranium and plutonium and convert them into Mixed Oxide (MOx) material that can be reused as fuel. These commercial methods (PUREX, TRUEX, DIAMEX) for actinide separation from spent fuel are liquid− liquid extraction processes that allow the selective partitioning of the desired elements from the solution.4,5 On an industrial scale, the initial step of these processes is the dissolution of fuel elements in hot concentrated solutions of HNO3, followed by separation of plutonium and uranium from the fission stream and minor actinides.6 The framework in which low concentrations of HNO3 solutions radiolytically evolve is now fully understood and several studies have also been performed on solutions at high HNO3 concentrations by steady-state and pulse radiolysis.7−12 In HNO3 solutions under ionizing radiation, the NO−3 anions are reduced by H• atoms and solvated electrons, forming NO•2 radicals, which recombine to N2O4 that decays to HNO2 and HNO3 (Table 1 and Table S1). The radiolysis of highly concentrated HNO3 solutions on the contrary is still not well understood as many factors, negligible in dilute systems, come into play: the direct effect of ionizing radiation, a fast electron transfer toward the ionized solute/solvent from the surrounding solvent/solute, change of solution properties such as viscosity, affecting radical dynamics, the ionic strength influence Received: December 26, 2017 Revised: January 24, 2018 Published: January 24, 2018 A

DOI: 10.1021/acs.jpcb.7b12702 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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experiments at the end of the pulse (2.5 μs) were overestimated, mostly due to the use of incorrect values of extinction coefficients for this species.18,23,24 Through the debate over which mechanism is the dominant one, and how the pH of the solutions influences the formation of NO•3 , Katsumura et al.17 shed light on these questions. Using nanosecond pulse radiolysis, they have identified two main formation pathways for NO•3 radicals: a fast process due to the direct effect of ionizing radiation and a slow process resulting from the reaction of •OH radicals with undissociated HNO3 molecules. They were also able to estimate G values for NO•3 radicals: 1.35 × 10−7 mol·J−1 (direct yield) and 3.33 × 10−7 mol·J−1 (indirect yield) for a 6 M HNO3 solution. More recently, Balcerzyk et al.20,25 investigated the radiolysis of a few concentrated HNO3 aqueous solutions (99

0.00 0.03 0.06 0.08 0.11 0.16 0.21 0.30 0.38 0.46 0.54 0.62 0.68 0.89 >0.98

1.00 0.97 0.94 0.92 0.89 0.84 0.79 0.70 0.62 0.54 0.46 0.38 0.32 0.11 99 wt %) ultrapure grade (99.999%) were purchased from Alfa Aesar and HoneyWell, respectively, and used without further purifications. Each experiment was performed with a fresh HNO3 solution. All solutions were prepared at 22.5 °C using ultrapure Elga water with a resistivity of 18.2 MΩ and less than 10 ppb organic carbon. The concentrations of solutions at high temperature were recalculated with respect to the density change at 80 °C. The physicochemical properties of the investigated solutions are presented in Table 2. The formation and decay of the NO•3 was followed using the pulse radiolysis system of the ELYSE accelerator (Paris-Sud University), which utilizes short pulses of electrons to produce, and femtosecond broad supercontinuum, to examine transient species with high time resolution. Two experimental areas developed at the ELYSE platform allow conducting pulse radiolysis studies on the picosecond to low nanosecond time scale, using the transient absorption pulse-probe setup and nanosecond to millisecond time scale, using the streak camera setup.36 Laser (260 nm) driven Cs2Te photocathode allows production of short electron pulses with a typical half width of 7 ps and a charge of ∼6 nC, and energy of ∼7.8 MeV, at a repetition rate of 10 Hz.37 For the picosecond pump−probe experiments, the broadband supercontinuum (380−750 nm), generated by focusing a small part of the fundamental laser (780 nm) into a CaF2 crystal, is split 60/40: probe/reference paths. Both probe and reference are coupled into optical fibers, transmitted to a spectrometer, and dispersed onto a cooled CCD camera. The sample cells used for the pump−probe measurements are 0.5 cm optical path length synthetic fused silica cells through which the sample is continuously flowed (ca. 20 cm3/min). The cells’ optical windows are of 200 μm thickness to minimize the contributions to the signal from the transient species generated in quartz by the electron pulse. All kinetics presented in the work are calculated from averaging 15 transient maps with 10 electron pulses at each time delay step. For the nanosecond to millisecond pulse radiolysis experiments we use a highly dynamic Hamamatsu C7700-01 streak camera, coupled to a Chromex 250 IS spectrograph for detection, while the analyzing light is delivered by a home-built flash xenon lamp. An optical quartz cell with an optical path length of 1 cm is used for room temperature experiments, and 0.5 cm for the high-temperature experiments, is placed as close as possible to the output window of accelerator to minimize divergence of electron beam. The measurements are obtained as an average of 750 acquisitions per scan. A modified coil type condenser (Figure S1 of the Supporting Information) with an external heating cable, placed before the entrance of the sample cell, was used to bring the samples to the desired working temperature. A PID controller remotely regulated the temperature (with a precision of ±0.5 °C) with Pt-100 temperature sensors placed on the external side of the quartz cell and inside the heater. The measurements were performed at 22.5 and 80 °C. The measurements at both 22.5 and 80 °C required usage of special materials (e.g., MasterFlex Chem-Durance Bio tubing) and thorough verification of the setup’s connections reliability to ensure safety for both

A(λ ,t ) = ϵλ ·l·c(t ) = ϵλ ·l·F ·D·G(t )

(11)

where l is the optical path length and the dose factor F is given by:40 p× F = ρsol ×

Z HNO3 M HNO3

+ (100 − p) ×

Z H2O M H2O

100 × Z H2O M H2O

(12)

where ρsol is the density of the solution, p is the solute weight fraction per 100 g of solution, ZHNO3 and ZH2O are the numbers of electrons per molecule for HNO3 and H2O, respectively, and MHNO3 and MH2O are the molecular weights of HNO3 and H2O, respectively. The measurements were performed at 22.5 and 80 °C under the same experimental conditions, paying particular attention to the absorbed dose. For the transient absorption measurements at long time scales, the samples were continuously purged with argon gas during the measurements. The transient species reaction with O2 are too slow to be considered under the present conditions. All results presented herein were measured at an absorbed dose of 53 Gy per pulse.

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RESULTS AND DISCUSSION Results. As reported in the literature,10,17,22,23,41 the NO•3 radical exhibits a characteristic absorption spectrum in the visible region, with three maxima at 602, 638, and 676 nm (Figure 1), with all other transient species produced during pulse radiolysis of HNO3 not contributing to this absorption, except the solvated electron. For the solutions with concentration lower than 2 M, the solvated electron reaction with NO3− and H+ lasts over several hundred picoseconds. Figure 1A presents the measured absorption spectra just after the passage of the 7 ps electron pulse. In the case of HNO3 low concentrations (2 M. The transient spectra for 0.5, 1, 1.5, and 2 M are recorded after the decay of the solvated electron signal (300, 200, 200, and 100 ps, respectively, after the electron pulse). The spectra are shifted by 1.5 mOD for clarity. The inset presents the variation of the measured absorbance at 640 nm as a function of the solution concentration. (B) Transient absorption spectra at 80 °C of NO•3 recorded just after the passage of the 7 ps electron pulse in HNO3 solutions. The transient spectrum for the 2 M HNO3 solution is recorded 200 ps after the electron pulse. The spectra are shifted by 1.5 mOD for clarity. The inset shows the variation of the measured absorbance as a function of the solution concentration.

concentration of the HNO3 solutions is observed, as can be seen in Figure 1B.

Figure 2. Recorded kinetics at 640 nm in solutions containing different concentrations of HNO3 at 22.5 °C (left) and 80 °C (right). D

DOI: 10.1021/acs.jpcb.7b12702 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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where Gdir is the radiolytic yield of NO•3 radicals resulting from direct ionization of NO−3 , fs and f w are the electron fractions of the solute (HNO3) and water, respectively, Gindir is the radiolytic yield of NO•3 radicals resulting from indirect effects within the electron pulse due the reaction with H2O•+ (eq 7), and the coefficient α describes the fraction of the H2O•+ radical involved in electron transfer reaction: 0 < α < 1. The water cation radical is involved in two competitive reactions: electron transfer with NO−3 (eq 7) and proton transfer with another water molecule, forming hydroxyl radicals in less than 1 ps (eq 6). Therefore, only a part of the H2O•+ radical is contributing to the formation of NO•3 . From eq 14, applied at 7 ps, and by taking in account the yield of water ionization it is possible to deduce the α value (see below). The observed initial yields (just after the pulse) measured at 22.5 and 80 °C show an increase with the HNO 3 concentrations, as can be seen in Figure 4A. The yield after the 7 ps electron pulse in neat HNO3 is (3.4 ± 0.1) × 10−7 mol·J−1 at 22.5 and 80 °C.

the dissociation degree and the temperature (Figure S3). This point will be discussed below. The transient absorption measurements at longer times allow us to follow the formation of NO•3 radicals through reaction (eq 10) to completion, as can be seen in Figure 3. A faster formation of NO•3 radicals in higher concentrations of HNO3 solutions is observed, consistent with the presence of the larger amount of undissociated HNO3.

Figure 3. Time evolution of the absorbance at 640 nm, measured at 22.5 °C (blue) and 80 °C (red) in solutions containing different concentrations of HNO3.

Figure 4. (A) Observed radiolytic yields of NO•3 after the 7 ps electron pulse as a function of the solvent electron fraction in the investigated solutions at 22.5 °C (blue) and 80 °C (red), and the maximum yield of NO•3 measured at different concentrations. For the lowest concentrations the presented yields are measured after the decay of the solvated electron. Inset: Observed radiolytic yields of NO•3 after the 7 ps electron pulse as a function of the solvent fraction in the investigated solutions at 22.5 °C, displaying the contribution of direct effects (red) and indirect effects (blue) to the observed yield. (B) Contribution of the ultrafast indirect effect (value of α) to the observed NO•3 radiolytic yields as a function of the investigated solutions concentration at 22.5 °C (blue) and 80 °C (red).

Discussion. On the basis of the pulse-probe measurements, we can evaluate the radiolytic yield of NO•3 radical in the investigated HNO3 solutions according to the following equation: G (t ) =

A e−aq (640 nm,t ) ϵ640nm ·l·F ·D

(13)

where the extinction coefficient of NO•3 at 640 nm ϵ640 nm is 1300 M−1·cm−1.17 The radiation energy is absorbed proportionally to the electron number fraction of component, meaning that in lower concentration solutions, it is absorbed mainly by the solvent (indirect effects) and in concentrated solutions a significant part is absorbed by the solute (direct effects), so that the experimental yield just after the electron pulse is expressed as first approximation: 7ps G NO • = f · Gdir + f · α · Gindir s w 3

When considering the initial radiolytic yields of NO•3 radicals, we also need to consider the scavenging of the H2O•+ by NO−3 (eq 7). In the case of concentrated HNO3 solutions, this reaction becomes non-negligible. This mechanism is known to play a role in the formation of the radicals of HNO320 and has also been observed in concentrated sulfuric acid60,61 and phosphoric acid62 solutions. Consequently, assuming that the yield of the H2O•+ is the same as that of hydrated electrons measured in the picosecond range, i.e., 4.5 × 10−7 mol/J,38 we

(14) E

DOI: 10.1021/acs.jpcb.7b12702 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B can calculate the value of the coefficient α at different concentrations from eq 14. As can be seen from Figure 4B, the value of α is of 60% for the 15.7 M solution of HNO3, and of 20% for the 2 M solution. At the same time, within the experimental error, we can state that the temperature increase does not affect the contribution of the direct and indirect effects to the NO•3 radicals production at picosecond range, except for the case of highly concentrated solutions. For the highest concentration (21.1 and 23.5 M) solutions, the H2O quantity is low, and therefore the energy deposited in water is small, leading to considerable error in the evaluation of the α coefficient. The observed initial yield (at the end of the 7 ps electron pulse) of NO•3 is therefore the result of direct effect of ionizing radiation on NO−3 and electron transfer between NO•3 and H2O•+ (Figure 4A inset). At 7 ps, the reaction between •OH radicals and undissociated HNO3 is unlikely to occur. Figure 4 shows the contribution of the direct and indirect effects to the experimental yield at 22.5 °C measured after the electron pulse. As can be seen, at low concentrations (12 M) the contribution of the electron transfer reaction in NO•3 formation is less important than for the concentrations between 3 and 10 M. This is a result of the fact that at low concentrations the energy is absorbed mainly by water, but the amount of NO−3 in contact with a H2O•+ is low; therefore the reaction (eq 6) is the dominant one for H2O•+, whereas at high concentrations, the dose is mainly absorbed by the solute. On the basis of the measurements performed on different time scales, we can estimate the radiolytic yield of NO•3 radical at different times. Due to the reaction (eq 10), an increase of GNO•3 at longer times is observed, as well as an acceleration of this rise at 80 °C (Figure 3). The elevated temperature also expedites the NO•3 decay, which was already observed at room temperature on a microsecond range.21 At elevated temperatures the decay reactions are accelerated; thus at 80 °C and high concentrations, 100 ns after the passage of ionizing radiation, the NO•3 radicals already start to decay (Figure 3 top decay at 13.4 M at 80 °C). The total scavenging yield of oxidizing radicals (•OH and its precursor H2O•+), Gox, could be determined by subtraction the direct effect yield of NO•3 from its maximum yield, G(NO•3 )max and by taking into account the dose absorbed by water (Table 3): Gox =

To find Gox as mentioned above, we excluded the two solutions with the highest concentrations for which the value of f w is very low, inducing large errors in the calculations (Table 3). The maximum value of (4.7 ± 0.4) × 10−7 mol·J−1 for Gox indicates that almost all H2O•+ and •OH are scavenged by HNO3, forming NO•3 . This value is close to the yield of •OH radical measured recently at 7 ps in neat water, (5.0 ± 0.2) × 10−7 mol·J−1,63 a value higher than the corresponding one in homogeneous step, (2.8 ± 0.2) × 10−7 mol·J−1. The high yield of NO•3 formed from the reactions with •OH and its precursor H2O•+ in concentrated HNO3 solutions (8−15.7 M) indicates that the decay of NO•3 in the nonhomogeneous step (spur reactions) is negligible, contrary to the case of •OH radical and solvated electron in water. In fact, the only efficient reaction of NO•3 decay, during the first nanosecond, is NO•3 + •OH → NO•2 + HO•2

This reaction is in competition with the reaction (eq 10), which is dominant at the high concentration of undissociated HNO3. Other decays reactions (Table S1) are rather slow to occur on a nanosecond time-scale. Previous studies gave different values for the reaction rate for (eq 10, Table 1) at room temperature without an agreement on its exact value. To determine the rate constant of this reaction, it is necessary to estimate the amount of undissociated HNO3 in each solution for the two studied temperatures. Several studies reported the dissociation degree of HNO3,42−48,50−52,54−59 and for our calculations, we used the average of the published data, as presented in Figure S3. We estimated the rate constant value of this reaction from the different kinetics measured on long time scales (Figure 2 and Figure 3). On the basis of these measurements, by fitting the long-time increase to a pseudo-first order reaction and plotting the observed reaction rates as a function of the average concentration of undissociated HNO3, we determined the reaction rate of (eq 10), k10, as can be seen in Figure 5. At 22.5

max • − f ·G G NO s dir 3

fw

(15) Figure 5. Observed reaction rates as a function of the concentration of undissociated HNO3 at 22.5 °C (blue) and 80 °C (red). The corresponding concentration of HNO3 solutions are given on the top axis.

Table 3. Contribution in NO3• Formation Yield 7 max −1 c22.5°C HNO3 , M 10 GNO•3 , mol·J

2 4 8 12 15.7 21.1 23.5

(16)

3 3.6 3.8 3.9 3.8 3.5 3.5 Gdir NO•3

G7NOps•3 fs·Gdir f w·GH2O•+ NO•3 1.3 1.9 2.6 3 3.2 3.2 3.4 = 3.4 ×

0.4 0.9 0.7 1.2 1.3 1.3 1.8 1.2 2.2 1 3.1 ∼0.1 3.4 10−7 mol·J−1

f w· G•OH

Gox ± 10%

1.7 1.7 1.2 0.9 0.6 ∼0.3 ∼0.1

3 3.7 4 4.5 4.7

°C, k10 has a value of (5.3 ± 0.2) × 107 M−1·s−1, which among the different values in the literature, is in agreement with the one proposed by Jiang et al.33 of 5.3 × 107 M−1·s−1. At 80 °C, the reaction (eq 10) is accelerated, with k10 = (1.1 ± 0.2) × 108 M−1·s−1. Up to now, there is no value reported in the literature for the reaction rate (eq 10) at 80 °C in solution. However, several studies exist in the literature on the value of this reaction rate in F

DOI: 10.1021/acs.jpcb.7b12702 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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reprocessing solvents, further studies are required to comprehend the fate of the NO•3 radical and that of the excited states in these solutions.

the gas phase. The reported values for the rate constant are very dispersed. Most investigators indicate that the temperature increase slows down this reaction rate. The more recent investigation gives 7.9 × 107 and 5.8 × 107 M−1·s−1 at room temperature and 80 °C, respectively.64−66 These values are different from our determined reaction rates in solution showing that the reaction mechanism is affected by the solvent. We note that for our estimate of the rate constant of the •OH and undissociated HNO3 reaction, we limited the data to solutions with concentrations lower than 12 M. The reason for this limitation lies in the difficulty to evaluate the association/ dissociation in HNO3 at higher concentrations. Raman spectroscopic investigations and theoretical calculations suggest that different associated species exist in HNO3 solutions: neutral molecules (or contact pairs, CP) and ion pairs or solvent separated ion pairs (SSIP), basically formed from an anion and a cation in strong electrostatic interaction, separated by a water molecule (H3O+−H2O−NO−3 ).57,67 It was estimated that at higher concentration of HNO3 (>8 M), the associated species are composed of ∼80% CP and ∼20% SSIP. In addition, when the concentration of water is decreased other species, such as H2NO+3 are formed. With this compositions, a correct evaluation of the undissociated HNO3 molecules available for reaction with •OH is extremely difficult.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b12702. Main decay pathways of nitrate radicals, experimental details of the heating system, transient absorption spectra of nitrate radicals, and literature values of dissociation degree of nitric acid at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*M. Mostafavi. E-mail: [email protected]. Phone: +33 (0)169157887. Fax: +33 (0)169156188. ORCID

Sergey A. Denisov: 0000-0001-7881-1979 Mehran Mostafavi: 0000-0002-4510-8272 Author Contributions ‡

Contributed equally to this work

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Notes

The authors declare no competing financial interest.

CONCLUSIONS The importance of the concentration on the production of NO•3 radicals has been shown by performing a systematic radiolytic study of HNO3 solutions with concentrations ranging from 0.5 to 23.5 M at 22.5 and 80 °C. The formation of NO•3 occurs only in highly concentrated solutions via three different mechanisms: direct effect, electron transfer between the H2O•+ and NO3−, and the reaction between •OH radicals and undissociated HNO3 molecules. The absolute formation yield of NO•3 radicals due to the direct effect, Gdir NO•3 , is found to be −7 −1 (3.4 ± 0.1) × 10 mol·J , irrespective of the concentration and temperature. At low concentrations (lower than 0.1 M), the direct effect and the electron transfer reaction to H2O•+ are negligible as the concentration of NO−3 in solution is low. Moreover, as the reaction with the •ΟH radical occurs only with undissociated HNO3 molecule, this contribution is also negligible due the total dissociation of HNO3 in dilute solutions. The reaction rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 107 M−1·s−1 for 22.5 °C, reaching a value of (1.1 ± 0.2) × 108 M−1· s−1 at 80 °C. In the present work, we were able to quantify the different contributions by observing the production of NO•3 radicals in solutions. The importance of each contribution depends on the composition (concentration and dissociation degree) of the studied solutions. The electron fraction of the solvent controls the first mechanism, the presence of a NO−3 in contact of H2O•+ controls the second one, and the dissociation degree of HNO3 dictates the third mechanism of NO3• production. The combination of radiation effects and dissociation behavior of HNO3 affects the performance of this complex system and may have undesirable detrimental effects on the actinide separation and extraction efficiency, as well as the highlevel waste containers integrity. The behavior of HNO3 solutions under radiation is of extreme importance in the reprocessing of spent fuel, and although these investigations lay out the foundation for understanding the radiolysis of

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DOI: 10.1021/acs.jpcb.7b12702 J. Phys. Chem. B XXXX, XXX, XXX−XXX