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Inhibition of Radiolytic Molecular Hydrogen Formation by Quenching of Excited State Water Gregory P. Horne,*,†,‡ Simon M. Pimblott,§,∥ and Jay A. LaVerne‡,⊥ †
California State University at Long Beach, Long Beach, California 90804, United States Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States § Dalton Cumbrian Facility, The University of Manchester, Westlakes Science and Technology Park, Cumbria CA24 3HA, U.K. ∥ School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. ⊥ Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡
ABSTRACT: Comparison of experimental measurements of the yield of molecular hydrogen produced in the gamma radiolysis of water and aqueous nitrate solutions with predictions of a Monte Carlo track chemistry model shows that the nitrate anion scavenging of the hydrated electron, its precursor, and hydrogen atom cannot account for the observed decrease in the yield at high nitrate anion concentrations. Inclusion of the quenching of excited states of water (formed by either direct excitation or reaction of the water radical cation with the precursor to the hydrated electron) by the nitrate anion into the reaction scheme provides excellent agreement between the stochastic calculations and experiment demonstrating the existence of this short-lived species and its importance in water radiolysis. Energy transfer from the excited states of water to the nitrate anion producing an excited state provides an additional pathway for the production of nitrogen containing products not accounted for in traditional radiation chemistry scenarios. Such reactions are of central importance in predicting the behavior of liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.
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anion (NO3−) has been used historically for probing the radiation chemistry that occurs in the radiolysis of water, eq 1, as it is an effective scavenger of the hydrated electron (eaq−) and its precursor (epre−), and to a lesser extent the H• atom,10−12
INTRODUCTION The radiolysis of liquid water leads to a number of excited and ionized states of the medium that decay to the ground state or decompose to give water products.1 Excited states of water (H2O*) are thought to give hydrogen atoms (H•) and hydroxyl radicals (OH•) or oxygen atoms (O) and molecular hydrogen (H2), while the water radical cation (H2O•+) undergoes a proton transfer reaction with a water molecule to give the hydronium ion (H3O+) and the OH• radical.2,3 These simple dissociative reactions occur on the subpicosecond time scale and were long thought to be the only pathways for water decomposition. The advent of very fast pump−probe techniques using pulsed electron beams have demonstrated that reactions of H2O•+ occur in a wide variety of highly concentrated solutions.4−8 The water cation is highly oxidizing and can lead to new products or modifications to observed yields. There are no observations on similar reactions of the H2O*, although its existence is implied in track chemistry calculations.9 Scavenging techniques in radiation chemistry involve observation of the stable product of interest while increasing the concentration of a solute that will react with its precursors. The very first modeling studies on the radiolysis of water realized that simple scavenging of the precursors to H2 could not account for all of its production in γ-radiolysis. The nitrate © 2017 American Chemical Society
H 2O ⇝ eaq −, H3O+ , H•, OH•, H 2 , H 2O2 NO3− + e pre− → NO3•2 −
(1)
k 2 = 1 × 1013 dm 3mol−1s−1 (2)
NO3−
−
+ eaq →
NO3•2 −
9
3
−1 −1
k 3 = 9.7 × 10 dm mol s
(3)
NO3− + H• → HNO3•−
k4 = 1.0 × 107 dm 3mol−1s−1 (4)
Recent picosecond pulse radiolysis experiments have provided significant evidence for the reaction of H2O•+ with NO3−, eq 5, at sufficiently high NO3− concentrations.4 NO3− + H 2O•+ → NO3• + H 2O k5 = 1.0 × 1012 dm 3mol−1s−1
(5)
Received: March 23, 2017 Revised: May 4, 2017 Published: May 11, 2017 5385
DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
Article
The Journal of Physical Chemistry B This reaction of H2O•+ offers an additional pathway for NO3− decomposition to give a different product compared to normal radiolysis involving eqs 2 to 4. In this work, the scavenging of the precursors to H2 is examined with accurate and realistic stochastic radiation track calculations to elucidate the different contributions to the total yield of H2. Predictions of the change in the H2 yield with increasing NO3− concentration are used to show that a nonelectron precursor plays an important role.
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METHODS A combination of a stochastic radiation track structure model to determine energy deposition and water decomposition with an independent reaction times (IRT) diffusion-kinetic model to follow the subsequent nonhomogeneous kinetics was used to examine the γ-radiolysis of aqueous deaerated NO3− solutions over the concentration range 1 × 10−4 to 10 mol dm−3 in the presence of 1 × 10−3 mol dm−3 potassium bromide (KBr).13,14 The modeled systems correspond to the experimental conditions in which the measurements were made.15 Calculated radiolytic yields are reported for the evolution of the radiation track up to 10 μs, allowing for complete spatial relaxation of the radiation chemical track and its constituents. Radiation chemical yields are reported in the traditional G-value units of molecules/100 eV (1 molecule/100 eV = 0.104 μmol J−1). The stochastic track chemistry simulation approach has been developed by multiple groups,9,16−18 and the techniques used here have been described in depth previously. Radiation track chemistry calculations rely upon a variety of input data. The electron energy dependent collision cross sections employed in the track structure calculations are based on experimental data for liquid water (inelastic collisions, ionization efficiency) or density normalized gaseous water (elastic and vibrational collisions).14 The rate coefficients and diffusion coefficients used to model the chemical evolution of the radiation track are taken from the compilations of Buxton et al. and Elliot and Bartels.11,19 The rate coefficients for the scavenging of epre− were derived from experimentally measured C37 values.20−22 Calculations employing these inputs have been demonstrated to correctly predict the scavenging of electrons in the tracks of different types of radiations.10,23−26
Figure 1. G(H2) as a function of NO3− concentration (and prehydrated electron scavenging capacity): Peled and Czapski (black square),33 Draganic and Draganic (red circle),27 Mahlman (dark blue triangle),29−31 Pastina et al. (green triangle),15 Kazanjian et al. (light blue triangle),28 Nakagiri and Miyata (pink triangle),32 Rodenas et al. (under vacuum) (purple octagon),34 Rodenas et al. (aerated) (yellow star),34 Yoshida et al. (green pentagon).35 Solid curves are predicted values from stochastic modeling calculations for deaerated NO3− solutions containing 1 mM KBr: (A) without H2O* quenching, (B) quenching of H2O* produced by recombinant excitation eq 10, (C) quenching of H2O* produce by direct excitation, and (D) quenching of all H2O*.
H• + H• → H 2
H 2O* → H 2 + O(1D/3 P)
H 2O•+ + e pre− → H 2O*
k 8 = 5.5 × 10 dm mol s
or (iii) electron autodetachment from a transient negative anion formed by the attachment of an energetic electron (E > 6 eV) to a water molecule.1 H 2O + e−(ΔE) → H 2O*− → H 2O* + e−(ΔE′) ΔE > ΔE′
(11)
This transient anion might also dissociate (dissociative electron attachment, DEA) to produce H2 H 2O*− → H 2 + O−
(12)
H 2O*− → H− + OH
(13)
or followed by H− + H 2O → H 2 + OH−
(6)
(14)
All of these pathways to H2O* are possible in the radiolysis of water. The decrease of G(H2) with increasing NO3− concentration was originally explained in terms of the increased reaction of
eaq − + H• + H 2O → H 2 + OH− k 9 = 2.5 × 1010 dm 3mol−1s−1
k13 ∼ 4.3 × 1012 dm 3mol−1s−1 (10)
2eaq − + 2H 2O → H 2 + 2OH− −1 −1
(9)
where the excited state in eq 9 may be formed by (i) direct excitation, (ii) reaction of the water radical cation with the precursor to the hydrated electron, i.e. dissociative electron recombination,23
RESULTS AND DISCUSSION NO3− Scavenging of G(H2). Literature values for the experimental measurements of the observed radiolytic yield of H2 (G(H2)) as a function of NO3− concentration (1 × 10−5 to 10 mol dm−3) are presented in Figure 1.15,27−35 With a few exceptions, the agreement of the data is exceptional for such a wide range of concentrations collected over many decades of time. The data show a monotonic decrease in the yield of H2 with increasing NO3− concentration and that the formation of H2 is completely suppressed at very high NO3− concentration. Radiolytic formation of H2 from water is usually believed to originate from two types of processes: intratrack chemistry,11
3
(8)
or short-lived transient dissociation,
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9
k10 = 7.8 × 109 dm 3mol‐1s‐1
(7) 5386
DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
Article
The Journal of Physical Chemistry B NO3− with the precursors to H2 involved in intratrack eqs 2 to 4. However, the very first realistic modeling studies on the radiolysis of water showed that scavenging of the eaq− and H• precursors to H2 could not possibly account for all of its production in γ-radiolysis.26 More recent studies have shown that the complete scavenging of H2 is due to scavenging of a short-lived precursor, which was assumed to be epre−,15 as the loss of H2 could be correlated with the scavenging capacity (ks = k × [Scavenger]) for epre− for a wide variety of scavenger solutes.36 The upper axis of Figure 1 shows the values for the scavenging capacity for epre− at each corresponding NO3− concentration. The scavenging time of epre− is approximately equal to 1/ks so this gives an idea of the time scales involved in scavenging of the precursors to H2. Previous studies have shown that the radiolytic yield of H2 is strongly dependent on the linear energy transfer (LET) of the radiation, increasing with increasing LET, but that the time scale of formation was independent of LET.36 These results demonstrated that the dominant mechanism for H2 formation must involve more than one short-lived transient and this route was postulated as being dissociative electron recombination, i.e., eq 10 to form a water excited state followed by eq 9. This argument is supported in a recent radiolysis study of water at high temperatures (and pressures) where the authors postulated that the radiolytic H2 production is dominated by reactions involving epre− and that dissociative electron attachment reactions have a negligible role.37 They also suggested a correlation between epre− scavenging and the inhibition of H2 formation in positronium (Ps) lifetime studies. The positronium studies generally agree that (i) epre− is the precursor to positronium formation (β+ + epre− → Ps), and (ii) that by scavenging epre− positronium formation can be completely inhibited.38−42 While the correlation between epre− scavenging and the inhibition of H2 formation in Ps lifetime studies supports the conclusion that formation of H2 occurs primarily from epre− and not eaq−, it does not disprove a contribution to the total yield of H2 from excited states formed by eqs 9 and 11. However, experiments clearly demonstrate that all production of H2 can be scavenged so direct formation by dissociative electron attachment, eqs 11 to 14, is not likely as it would require scavenging of an energetic (>6 eV) electron. Curve (A) in Figure 1 shows the predicted dependence of the yield of H2 on NO3− concentration when calculated using eqs 2 to 4 for the scavenging of precursors to H2, but ignoring any scavenging of H2O*. Remember, calculation of the scavenging of the hydrated electron and its precursor using this reaction set accurately reproduces the experimental data for NO3− and a large variety of other scavengers.10 The discrepancy between experimental data and the predictions of the calculations demonstrates that the reduction of G(H2) at high NO3− concentrations (≥0.01 mol dm−3) cannot be solely explained by this set of electron scavenging reactions. The limiting value observed here at high NO3− concentration is 0.35 molecules/100 eV. This value corresponds to the yield of H2 highlighted previously as due to fast processes,15 but is somewhat higher than the value of 0.15 molecules/100 eV previously assumed for the “initial” yield of H2.26 Dependence of G(H2) on H2O* Quenching. Decomposition of the H2O* is expected to give H2 by eq 9; therefore, a decrease in G(H2) will be observed if the H2O* is quenched by NO3−, NO3− + H 2O* → NO3−* + H 2O
Inclusion of quenching of the H2O* into the reaction set gives curve (D) in Figure 1. The rate coefficient for the quenching eq 15 was obtained by a best fit to the data. The results are in excellent agreement with the experimental data for k15 ∼ 1 × 1013 dm3 mol−1 s−1. Lower values for the quenching reaction rate coefficient move the entire calculated curve to the left, while higher values move the curve to the right. The best fit rate coefficient obtained is (coincidentally) the same as that obtained for the scavenging of epre− from experimentally measured C37 values showing the process occurs on the same time scale. Clearly, inclusion of the H2O* quenching reaction gives a much better agreement with the experimental data than the conventional reaction set. The contribution of each precursor to G(H2) can be ascertained by examining the associated H2 formation pathways, eqs 6 to 9, and the individual scavenging and quenching eqs 2, 3, 4, and 15 output by the track chemistry calculations. The results are shown in Figures 2 and 3. Figure 2 shows the
Figure 2. (A) Predicted contributions of different reaction pathways to the formation of H2: (A) total H2, (B) direct excitation H2O*, (C) recombination to singlet excitation H2O*, (D) eaq− + H• reaction; (E) H• + H• reaction, (F) recombination to triplet excitation H2O*, and (G) eaq− + eaq− reaction.
Figure 3. Predicted effect of NO3− on the scavenged yields of eaq− (black line), epre− (red line), H• (blue line), and the quenching of H2O* (green line). The yields of ionization and direct excitation shown are those predicted by the track structure simulation, with the latter being an upper limit.
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DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
Article
The Journal of Physical Chemistry B effect of NO3− concentration on the yields of the different pathways. The majority (∼70% of the total 0.47 G(H2); derived from the sum of the respective H2 production processes) of the H2 formed during irradiation comes from decomposition of H2O* produced by either direct excitation or water radical cation−electron reaction with the remaining yield arising from intra track reactions involving eaq− and H• on a nanosecond to microsecond time scale. Reduction of G(H2) below about 0.35 molecules/100 eV necessitates the quenching of H2O* at short time scales. The nitrate concentration dependences of eqs 9 and 10 (curves (B), (C), and (F)) are determined by the competition between quenching and excited state dissociation, while the concentration dependences of eqs 6, 7, and 8 (curves (D), (E), and (G)) reflect the relative spatial distributions of the radiation induced radicals within the track. Consequently, the concentration dependences of the first group should be similar and different from the second group, and the second group of reaction processes should each have a different and characteristic concentration dependence. These differences are only clear for eq 7. The yields of the scavenging processes are shown in Figure 3. At the lowest NO3− concentrations scavenging of the eaq− is dominant and results in a small reduction in G(H2) of about 0.1 molecules/100 eV. As the NO3− concentration increases the scavenging of the epre− and the quenching H2O* become increasingly important. The former reaction does not prevent excited state formation. The total scavenged yield of electrons is less than the initial ionization yield: the reaction of water radical cation and the precursor to the hydrated electron takes place very rapidly and is determined by the spatial evolution of the track and not the scavenging of the two reactants. The scavenging reaction is in competition with the hydration of the electron. There is negligible scavenging of the H• atom by NO3−. Furthermore, although scavenging of the water radical cation by NO3− (eq 5) is included in the modeling reaction set (k5 = 1 × 1012 M−1 s−1, optimized through curve fitting to NO3• formation and decay data from refs 4, 6, and 48), it is not sufficiently fast to disrupt recombination of the water radical cation and the precursor to the hydrated electron, and thus does not influence G(H2). This is not to say that eq 5 is not an important pathway in the decomposition of NO3−, especially at high nitrate concentrations, as calculations presented by Garaix et al. demonstrate that H2O+ + NO3− plays a significant role in the formation of NO3•.6 The quenching of excited molecules will take place whatsoever the origin, i.e., by water radical cation-electron reaction, direct excitation or electron autodetachment. Assignment of the contributions of the different processes in γradiolysis is not possible; however, only the contribution of the water radical cation-electron reaction will be dependent on the radiation track structure while the contributions of the other two routes will be independent of radiation track structure. The decreasing yield of positronium with increasing concentration of epre− scavengers is not directly comparable to the effect on G(H2): positronium is solely formed by reaction of epre− with β+ and scavenging of epre− can therefore lead to complete inhibition of positronium formation; however, for complete inhibition of H2, all H2O* would have to be produced by reaction of epre− with H2O•+, eq 10. This assumption necessitates all electronic excitation events in the track would result in ionization giving (according to track structure simulation) an initial ionization yield, G(ionization),
of ∼6.5 ions/100 eV. This yield is significantly higher than the yield of epre− reported for picosecond pulse radiolysis studies using the silver ion as an electron probe, G(epre−) ∼ 4.5 ions/ 100 eV.43 This “precursor deficit” of ∼2.0 species/100 eV is not inconsequential. Excited states of water are readily accessible by direct excitation as there is a sizable oscillator strength distribution below the ionization potential of liquid water (∼9−10 eV).16,44 Previous water radiolysis models have also invoked the existence and contribution of directly produced water excited states to the formation of H2.16−18 The track structure simulations performed in this study give an ionization yield, G(ionization), of 4.5 ions/100 eV, and a maximum possible direct excitation yield, G(direct excitation), of 2.1 molecules/ 100 eV. This yield is significantly higher than is expected from experimental free radical yields and is based on the assumption that all electronic energy transfer events not resulting in ionization result in an excited state. In the calculations presented the rate of nonradiative decay to ground state is taken from previous calculations where it was fitted to match the short time OH• yield. Pulse-radiolysis studies with increasing selenate anion concentrations found that the conversion to the selenite anion resulted in a yield of about 6.5 molecules/100 eV at the highest selenate anion concentration, which gives the same results as observed here if it is assumed that the selenate anion reacts with eaq−, epre−, and H2O*.45 Formation of Excited State Nitrate Anion. To a first approximation, initial energy deposition by a radiation particle into each component of an aqueous medium can be divided according its electron density. Essentially all of the energy is deposited directly into the water in aqueous solutions at low solute concentrations and it is only at concentration levels ≥1 mol dm−3 that solutes directly absorb a significant amount of energy from the incident radiation. These direct effects have long been reported and contribute to the decomposition of the solute.31,46 Direct decomposition of the NO3− in aqueous and acidic solutions is believed to occur by way of eqs 17 and 18:47,48 NO3− ⇝ NO3−* → NO2− + O
(17)
NO3− ⇝ NO3−* → NO3• + e pre−
(18)
with recent pulse radiolysis studies strongly suggesting eq 18 is the main pathway for direct NO3− decomposition.4 Often the consequences of direct effects are inferred by the analysis of time-resolved measurements using chemistry models which can highlight the existence of pathways not normally expected in the radiolysis of aqueous solutions. At high solute concentration, radiation chemical mechanisms are complex and the existence of a particular pathway is difficult to ascertain. The quenching of water excited states by eq 15 provides a route to excited state nitrate anion (NO3−*) formation in addition to direct excitation by energy transfer from the radiation particle. In both cases, the yield of NO3−* will increase with increasing NO3− concentration. Distinguishing between the two pathways and reactions of NO3− with H2O•+ is a complex challenge that is still to be performed. The role of these processes that occur at very short time is not just important for fundamental understanding of the radiolysis of water and aqueous systems, it is impactful in understanding the radiolytic production of nitrite anion (NO2−) and nitrous acid (HNO2) in reprocessing liquor systems, which is a major 5388
DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
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The Journal of Physical Chemistry B
Science, United States Department of Energy through grant number DE-FC02-04ER15533. This is document NDRL-5170 of the Notre Dame Radiation Laboratory.
challenge in the nuclear industry both in spent fuel reprocessing and highly active liquor storage. The fate of the NO3−* is well-known from both gaseous and aqueous phases photochemistry.49 For aqueous solutions of 1.4 × 10 −2 mol dm −3 KNO 3 , approximately 56% of the photochemically generated NO3−* undergoes chemical transformation: 48% by rearrangement to peroxynitrate (ONOO−) NO3−* → ONOO−
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(1) Garrett, B. C.; Dixon, D. A.; Camaioni, D. M.; Chipman, D. M.; Johnson, M. A.; Jonah, C. D.; Kimmel, G. A.; Miller, J. H.; Rescigno, T. N.; Rossky, P. J.; et al. The Role of Water on Electron-Initiated Processes and Radical Chemistry: Issues and Scientific Advances. Chem. Rev. 2005, 105, 355−389. (2) Buxton, G. V. The Radiation Chemistry of Liquid Water: Principles and Applications. In Charged Particle and Photon Interactions with Matter; Mozumder, A., Hatano, Y., Eds.; Marcel Dekker: New York, 2004; pp 331−363. (3) Buxton, G. V. An Overview of the Radiation Chemistry of Liquids. In Radiation Chemistry from Basics to Applications in Material and Life Sciences; Spotheim-Maurizot, M., Mostafavi, M., Douki, T., Belloni, J., Eds.; EDP Sciences: Les Ulis Cedex A, France, 2008; pp 3− 16. (4) Balcerzyk, A.; El Omar, A. K.; Schmidhammer, U.; Pernot, P.; Mostafavi, M. Picosecond Pulse Radiolysis Study of Highly Concentrated Nitric Acid Solutions: Formation Mechanism of NO3 Radical. J. Phys. Chem. A 2012, 116, 7302−7307. (5) Balcerzyk, A.; Schmidhammer, U.; Wang, F. R.; de la Lande, A.; Mostafavi, M. Ultrafast Scavenging of the Precursor of H Atom, e−, H3O+, in Aqueous Solutions. J. Phys. Chem. B 2016, 120, 9060−9066. (6) Garaix, G.; Horne, G. P.; Venault, L.; Moisy, P.; Pimblott, S. M.; Marignier, J. L.; Mostafavi, M. Decay Mechanism of NO3 Radical in Highly Concentrated Nitrate and Nitric Acidic Solutions in the Absence and Presence of Hydrazine. J. Phys. Chem. B 2016, 120, 5008−5014. (7) Ma, J.; Schmidhammer, U.; Mostafavi, M. Picosecond Pulse Radiolysis of Highly Concentrated Phosphoric Acid Solutions: Mechanism of Phosphate Radical Formation. J. Phys. Chem. B 2015, 119, 7180−7185. (8) Mirdamadi-Esfahani, M.; Lampre, I.; Marignier, J.-L.; De Waele, V.; Mostafavi, M. Radiolytic Formation of Tribromide Ion Br3− in Aqueous Solutions, A System for Steady-State Dosimetry. Radiat. Phys. Chem. 2009, 78, 106−111. (9) Nikjoo, H.; Uehara, S.; Emfietzoglou, D. Interaction of Radiation with Matter; CRC Press: Boca Raton, 2012. (10) Pimblott, S. M.; LaVerne, J. A. On the Radiation Chemical Kinetics of the Precursor to the Hydrated Electron. J. Phys. Chem. A 1998, 102, 2967−2975. (11) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (OH/O-) in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17, 513−886. (12) Logager, T.; Sehested, K. Formation and Decay of Peroxynitric Acid - A Pulse Radiolysis Study. J. Phys. Chem. 1993, 97, 10047− 10052. (13) Clifford, P.; Green, N. J. B.; Oldfield, M. J.; Pilling, M. J.; Pimblott, S. M. Stochastic-Models of Multispecies Kinetics in Radiation-Induced Spurs. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2673−2689. (14) Pimblott, S. M.; LaVerne, J. A.; Mozumder, A. Monte Carlo Simulation of Range and Energy Deposition by Electrons in Gaseous and Liquid Water. J. Phys. Chem. 1996, 100, 8595−8606. (15) Pastina, B.; LaVerne, J. A.; Pimblott, S. M. Dependence of Molecular Hydrogen Formation in Water on Scavengers of the Precursor to the Hydrated Electron. J. Phys. Chem. A 1999, 103, 5841−5846. (16) Cobut, V.; Frongillo, Y.; Patau, J. P.; Goulet, T.; Fraser, M. J.; Jay-Gerin, J. P. Monte Carlo Simulation of Fast Electron and Proton Tracks in Liquid Water - I. Physical and Physicochemical Aspects. Radiat. Phys. Chem. 1998, 51, 229−243. (17) Hill, M. A.; Smith, F. A. Calculation of Initial and Primary yields in the Radioylsis of Water. Radiat. Phys. Chem. 1994, 43, 265−280.
(19)
and 8% by fragmentation into either NO and O2− (eq 20) or NO− and O2 (eq 21).49 NO3−* → NO + O2−
(20)
NO3−*
(21)
−
→ NO + O2
The remaining excited state molecules return to their electronic and vibrational ground-states within 2 ps. The major product of NO3−* decomposition ONOO− and its conjugate base ONOOH (pKa = 6.6) provide a route by which additional NO2− can be generated in concentrated NO3− solutions, thereby allowing more NO3− decomposition than is possible by scavenging of eaq−, epre−, and H2O•+.
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CONCLUSIONS Analysis of track chemistry modeling and experimental data clearly suggest the existence of H2O* production in the radiolysis of water and aqueous solutions and of a quenching process involving NO3− that leads to the production of NO3−*. Without the quenching process, the experimentally observed reduction in G(H2) with increasing NO3− concentration cannot be adequately explained, i.e., purely in terms of scavenging of the epre−, eaq−, and H• by NO3−. Incorporation of H2O* quenching by NO3− provides excellent agreement between experimental measurements and stochastic radiation track chemistry calculations over the investigated NO3− concentration range of 1 × 10−4 to 10 mol dm−3. Excitation energy transfer from H2O* to NO3− during quenching produces NO3−*, which provides potential pathways for the generation of NO2− and HNO2 not accounted for by traditional radiation chemistry reaction schemes. These findings have significant universal implications with regards to any aqueous system containing high solute concentrations.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Gregory P. Horne: 0000-0003-0596-0660 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research has been funded by the US-DOE Assistant Secretary for NE, under the FCR&D Radiation Chemistry program; DOE-Idaho Operations Office Contract DE-AC0705ID14517 and DE-NE0008406 grant, by the Dalton Cumbrian Facility Project, a joint initiative of the Nuclear Decommissioning Authority and The University of Manchester, by the UK Engineering and Physical Sciences Research Council (Grants EP/F013809/1, EP/I002855/1 and EP/ G037140/1), and by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of 5389
DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
Article
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DOI: 10.1021/acs.jpcb.7b02775 J. Phys. Chem. B 2017, 121, 5385−5390
