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A model for radiolysis of nitric acid and its application to the radiation chemistry of uranium ion in nitric acid medium
Radiation Physics and Chemistry 60 (2001) 369–375
A model for radiolysis of nitric acid and its application to the radiation chemistry of uranium ion in nitric acid medium Ryuji Nagaishi* Analytical Chemistry Research Group, Department of Environmental Science, Japan Atomic Energy Research Institute, Tokai Establishment, Tokai, Naka, Ibaraki 319-1195, Japan
Abstract Primary yields of radiolysis products of water and nitrate in concentrated nitric acid solutions for low LET radiation were evaluated from the previous studies on radiolysis while those for high LET radiation by using a diffusion kinetics model for the intra-track processes. Radiolytic reactions of NOx and uranium ions in the solutions were proposed mainly from the reported data, and then applied together with the yields to the radiolytic formation of HNO2 and oxidation of U4+ to demonstrate the reaction model. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radiolysis; Radiolytic reactions; Primary yields; LET; Concentrated nitric acid solutions; Uranium ions
1. Introduction Radiation chemistry of actinides in aqueous solution has been investigated and reviewed (Pikaev et al., 1986; Bhattacharyya and Natarajan, 1991) for the sake of nuclear engineering. A lot of investigations have been made in concentrated acid solutions, where radiolysis of acid solutes, change in activity of water and ions, and complexation of actinides take place. A subject of the radiation chemistry is to propose radiolytic reaction models and to estimate redox behavior of actinide ions in practical systems. A few models for uranium, neptunium, plutonium and americium ions in concentrated nitric and perchloric acid solutions (Vladimirova, 1998) have been proposed. However, they seem to be limited to the use of specified cases, the models of which were reproduced, because their proposed radiolysis processes of solutions have not been confirmed and rate constants for proposed reactions have been determined on the basis of the processes. In the present study, the primary yields of radiolysis products of water and nitrate in concentrated nitric acid solutions for low LET radiation were evaluated from the *Tel.: +81-29-282-5527; fax: +81-29-282-6806. E-mail address: [email protected] (R. Nagaishi).
previous electron pulse- and 60Co g-radiolysis studies (Jiang et al., 1991, 1994; Nagaishi et al., 1994) while those for high LET radiation by using a diffusion kinetics model for the intra-track processes. Then the model for radiolytic reactions of uranium ions was proposed mainly from the g-radiolysis studies (Jiang, 1992; Yotsuyanagi, 1989) on the radiolysis of aqueous diluted nitrogen compounds and uranium nitrate solutions. Rate constants for proposed reactions were examined by comparison of the reported and simulated results.
2. Calculation A computer code of FACSIMILE (Chance et al., 1977), which is widely used to solve differential equations for problems in chemistry and engineering, was used for the present simulations with homogeneous reaction models or a diffusion kinetic model. The compiled program is composed of chemical species involved in reactions as variables, initial conditions (solution and radiation) as parameters and a series of reactions with its rate constants. Density of nitric acid solution and dissociation of nitric acid as the initial parameters are dependent on total concentration of nitrate ½NO 3 total . The
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dissociation constant, Kd , of nitric acid (1) provides concentration ratio of the molecular form (HNO3) to the ion form (NO 3 ). It has been determined as ca. 20 for the diluted solution while its variation with ½NO 3 total for the concentrated solution can be determined from the degree of dissociation measured by using Raman and NMR (Redlich et al., 1968). HNO3 Ð Hþ þNO 3;
ð1aÞ
Kd :
The primary yields (unit: ions or molecules/100 eV) of radiolysis of water (gW ) and nitrate (gS ) as parameters were varied with ½NO 3 total as discussed below. It was assumed that energy of radiation should be deposited into the solution in proportion to its electron density and distributed into water and nitrate in accordance with the respective electron fractions ( fW ; fS ). The observable yield, GðiÞ, of product i can be expressed by using gW and gS . GðiÞ ¼ fW gW ðiÞ þ fS gS ðiÞ;
ð1bÞ ½NO 3 total
where fS is proportional to and a sum of fW and fS is equal to unity. Most of the reactions with its rate constants were taken from the reported literatures. For the proposed reactions, optimum rate constants were estimated thermodynamically to reproduce various experimental results.
3. Results and discussion 3.1. Primary yields of radiolysis products of water and nitrate
have been evaluated from the radiolytic reduction of cerric ion and the material balance (Nagaishi et al., 1994) as shown in Fig. 1. Maximum yield, GW ðH2 OÞ (¼ fW gW ), for water decomposition observable in nitric acid solutions was expected to exist at 05fS 51: 0 and calculated as 6.7 at fS ¼ 0:068 (½NO 3 total ¼ 1:34 mol dm3 ) while the maximum primary yield gW (H2O) was 7.9 at fS ¼ 0:13, close to that for water vapor. The fW gW of e aq þH and OH in Fig. 1 indicate that e and H are partly converted to NO2 3 to form aq NO2, and that OH to NO3. The conversions would be significant for the sequential homogeneous reactions. 2 The conversion of e aq to NO3 as an electron scavenger (S) has been defined as [S]37 in picosecond pulseradiolysis studies (Bronskill et al., 1970). On the other hand, the yield of OH converted to NO3 could be corresponding to that of ‘‘slow process’’ in NO3 formation (Jiang et al., 1991). Fig. 2 shows the experimental yields of OH, NO3 and the conversion of OH to NO3 in nitric acid solutions. It was found that OH was completely converted to NO3 in the range of 3 ½NO 3 total > 1:0 mol dm . A lot of investigations on radiolytic reactions of actinide ions have been made with high LET radiation, especially, a-ray while few data on the primary yields of radiolysis products of water and nitrate for high LET radiation has been obtained in concentrated nitric acid solutions. In the present study, the yields of water radiolysis were obtained by using a deterministic diffusion kinetic model for the intra-track processes described by Chitose (1998). This kinetic model is basically the same as reported by Burns et al. (1984)
Dissociation into e aq and NO3 (2) from pulseradiolysis studies on NO3 radical (Jiang et al., 1991) and that into O(3P) and NO 2 (3) from g-radiolysis studies on HNO2 formation (Jiang et al., 1994) have been determined as the radiolysis processes of nitrate: NO 3
e aq þNO3 ; gS1 ;
ð2Þ
NO 3
O 3 P þNO 2 ; gS2 :
ð3Þ
The primary yields have been evaluated as gS1 ðNO gS2 ðNO 3 Þ ¼ gS1 ðHNO3 Þ ¼ 4:8, 3 Þ ¼ 1:6 and gS2 ðHNO3 Þ ¼ 2:2 for the low LET radiation. Maximum yield, GS ðNO 3 Þ (¼ fS gS ), for the decomposition of nitrate would be 7.0 ideally at fS ¼ 1:0. It suggests that their yields were useful for high LET radiation in the present study. Since NO 3 ion is well known as a strong scavenger of not only hydrated but also pre-hydrated electrons (Aldrich et al., 1971) and furthermore HNO3 molecule is a scavenger of OH radical, the primary yields, gW , of water radiolysis may change with ½NO 3 total . The variations of fW gW in nitric acid and nitrate solutions
Fig. 1. The experimental yields of radiolysis products of water in nitric acid solutions for low LET radiation (Nagaishi, 1995) : H2O(*), e aq+H(n), H2(&), OH( ) and H2O2(*). The solid lines are fitting curves taken for simulations.
R. Nagaishi / Radiation Physics and Chemistry 60 (2001) 369–375
Fig. 2. The experimental yields of OH, NO3 and the conversion of OH to NO3 in nitric acid solutions for low LET radiation : total NO3 observed in nitric acid (*, Jiang et al., 1991), OH converted to NO3(n) and OH(solid line, Fig. 1).
and LaVerne and Pimblott (1991). For concentrated nitric acid solutions, pre-solvated electron with decay constant of 6.0 1011 s1 (lifetime510 ps) and [S] dependent rate constans for the reactions of e aq with scavengers (S) of H+ and NO 3 (Bronskill et al., 1970; Aldrich et al., 1971) were introduced into the model. The initial yield of water decomposition for the model was taken as 7.9 from the maximum gW ðH2 OÞ estimated for the low LET radiation as mentioned above. In preliminary simulations, the spur reaction model with the same initial yields could reproduce Fig. 1 especially 3 in the range of ½NO 3 total > 1 mol dm . Fig. 3 shows the simulated yields of radiolysis products of water at LET=100 eV nm1, corresponding to that for 5.8 MeV a-ray (244Cm).
371
Fig. 3. The simulated yields of radiolysis products of water in nitric acid solutions for high LET radiation. : H2O(*), e aq+H(n), H2(&), OH( ) and H2O2(*). The yields at 0.2 ms after irradiation were taken as the simulated yields. The LET was assumed to be 100 eV nm1. The solid lines are fitting curves taken for simulations.
In order to confirm the NOx reaction model and the simulated yields of water radiolysis for high LET radiation, the experimental kinetics of HNO2 formed in 244Cm a-radiolysis of nitric acid solutions (Frolov et al., 1991) was simulated. The concentration of HNO2 increased with time and then reached the steady state one as shown in Fig. 4. In the simulations, the first-order decay of HNO2, which has been indicated by Andreichuk et al. (1984), was necessary to be added to the NOx model. The decay constant was dependent on ½NO 3 total and the dose rate of the irradiation. Fig. 4 shows that the simulated results were in good agreement with the experimental ones in the whole range of ½NO 3 total . 3.3. Compiled reactions of uranium ions
3.2. Compiled reactions of nitrogen oxides Reaction model of water radiolysis has been well established, where the reactions and their rate constants have been summarized (Buxton et al., 1988). The NOx reactions of NO3 to N2 together with their rate constants have been determined (Jiang, 1992) and summarized (Katsumura, 1998) mainly from reported data in aqueous diluted NO 3 (Faraggi et al., 1971), NO2 (Schwarz and Allen, 1955) and NO (Knight and Sutton, 1967) solutions. Main reactants involving redox of uranium ions would be NO3, NO2 and HNO2, main reactions of which are shown in Table 1. It can be pointed out that the molecular products of HNO2 and H2O2 would not co-exist in nitrate solutions.
to U4+ including UO+ in Reactions of UO2+ 2 2 concentrated nitrate solutions should be compiled since U3+ is ready to be oxidized to U4+ in the solutions. Main reactants generated from water are H (e aq ) as a reductant and OH as an oxidant while those of NO3, NO2 and HNO2 from nitrate as oxidants. The rate constants for reactions of actinide ions (U, Np, Pu and Am) with radicals (OH, NO3, SO 4 , Cl2 , CO3 , etc.) have been summarized (Pikaev et al., 1990) from the pulseradiolysis studies. On the other hand, for the proposed reactions, the rate constants should be estimated thermodynamically from those obtained by pulse-radiolysis studies, and the reduction potentials of oneelectron couples involving inorganic radicals (Wardman,
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Table 1 Main NOx reactions of NO3 to NO in nitric acid solutionsa Reaction 2 NO 3 +eaq ! NO3 NO3 +H ! HNO 3
HNO3+OH Ð NO3+H2O NO3 þHO2 ! H þ þNO 3 þO2 2NO3 ! N2O4+O2 NO3+NO2 ! N2O5 NO3+NO 2 ! NO3 +NO2 NO3+HNO2 ! HNO3+NO2 + 2 HNO 3 Ð H +NO3 2 NO3 +H2O ! NO2+OH NO2 3 +O2 ! NO3 +O2 HNO3 ! NO2+OH 2NO2 Ð N2O4 N2O4+H2O Ð 2H++NO 3 +NO2 HNO2 Ð H++NO 2 + HNO2+H2O2 ! NO 3 +H +H2O 2HNO2 Ð NO+NO2+H2O NO+NO2 Ð N2O3 N2O3+H2O ! 2H++2NO 2
kforward =kback 9
9.7 10 2.0 107 5.3 107/5.3 105 3.0 109 4.0 106 1.7 109 4.4 109 8.0 106 pKd 7.5 1.0 103 5.0 106 2.3 109 4.5 108/6.8 103 18/1.7 102 pKd 3.2 4.6 103 [H+] 13.4/2.9 106 1.1 109/2.2 104 36
Reference c e d d b,f d d d g g e g h,f h,i
b,j k g,l l
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
The units of k are s1 for unimolecular and dm3 mol1 s1 for bimolecular reactions. The overall reaction. c Buxton et al. (1988). d Jiang et al. (1991). e Jiang (1992). f Pikaev et al. (1990). g Gra¨tzel et al. (1970). h Gra¨tzel et al. (1969). i Stedman (1979). j Bhattacharyya and Veeraraghavan (1977). k Park and Lee (1988). l Treinin and Hayon (1970). a
b
1989) and actinide ions (Bratsch, 1989). The main reactions in nitric acid solutions are listed in Table 2. Rate constants for the proposed reactions were detailed below. + Rate constants for UO+ 2 +OH and UO2 +NO3 in reactions (27, 28) were estimated from that for 9 3 1 1 UO+ s in acid solutions) 2 +Cl2 (4 10 dm mol and the reduction potentials of (H+, OH)/H2O (2.72 V vs. NHE), NO3/NO 3 (2.67) and Cl2 /2Cl (2.30). Rate + + constants for UO2 +NO2 and UO2 +HNO2 in reactions (29, 30) have been estimated in the following studies. Radiolytic reduction of UO2+ with ethanol has been 2 studied (Yotsuyanagi, 1989) in diluted nitrate solutions, in which radiolysis of nitrate would be negligible. The yields of U4+ and HNO2 were obtained as a function of 2þ ½NO 3 total and ½UO2 0 as shown in Fig. 5. Jiang (1992) has indicated that the reactions of UO+ 2 +NO2 and UO+ 2 +HNO2 should be important during the reduction of UO2+ 2 , and determined the respective rate constants
of k29 ¼ 2 104 and k30 ¼ 1 103 dm3 mol1 s1 to reproduce the experimental yields. It might be mentioned that rate constants for the oxidation of UO+ 2 to UO2+ would be influenced by complexation rather than 2 transformation since both the ions are oxygen-containing actinyl ones. In concentrated nitrate solutions, oxidation of U4+ to UO2+ would be more important than the reduction of 2 UO2+ to U4+. Formal potentials of U4+/UO+ 2 2 (0.38 V 2+ vs. NHE) and UO+ (0.17) indicate that for the 2 /UO2 oxidation by a specified radical, rate constant for U4+ would be lower than that for UO+ 2 . Rate constant for U4++NO3 in reaction (32) was estimated from those for U4++OH (8.6 109 dm3 mol1 s1) and U4++SO 4 (1.2 107) in acid solutions while that for U4++NO2 in reaction (33) was estimated by comparison with that for + UO+ would be 2 +NO2. Furthermore H2O and H responsible for the disproportionation of UO+ 2 and the oxidation of U4+ by radical (R) (U4++R+ + 2H2O Ð UO+ 2 +R +4H ) in reactions (23, 31–34),
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leading to ½NO 3 total dependence of the rate constants as reported by Vladimirova (1993). In the present study, the rate constants for the disproportionation and oxidation were assumed to be proportional to [H+] and the reciprocal of [H+], respectively.
Fig. 6 shows the experimental (Andreichuk et al., 1987) and simulated yileds of the oxidation of U4+ in 244 Cm a-radiolysis of nitric acid solutions. The simulated yields were found to agree well with the experimental ones in the whole range of ½NO 3 total .
Fig. 4. The experimental (n, Frolov, 1991) and simulated (*) steady-state concentrations of HNO2 formed in 244Cm aradiolysis of nitric acid solutions. The dose rate was 3.1 Gy s1. The constant, k, for the first-order decay of HNO2 was taken as 2 1 k¼ 2:5 103 ½NO (see text) for the simulations to 3 total h reproduce the experimental concentrations of HNO2.
Fig. 5. The observed yields of U4+ and HNO2 formed in gradiolysis of acidic uranyl nitrate solutions (Yotsuyanagi, 1989; Jiang, 1992) : experimental U4+(*), HNO2 without (n) and 4+ ( ) and with (m) 5.0 mmol dm3 UO2+ 2 , and simulated U HNO2(+).
Table 2 Main reactions of uranium ions in nitric acid solutions Reaction + 4+ 2+ +2H2O 2UO+ 2 +4H ! UO2 +U 2+ + UO2 +eaq ! UO2 + + UO2+ 2 +H ! UO2 +H 4+ 3+ +
U +H ! U +H 2+ UO+ 2 +OH ! UO2 +OH + 2+ UO2 +NO3 ! UO2 +NO 3 2+ UO+ 2 +NO2 ! UO2 +NO2 + 2+ UO2 +HNO2 ! UO2 +HNO 2 U4++OH ! UO+ 2 +OH U4++NO3 ! UO+ 2 +NO3 U4++NO2 ! UO+ 2 +NO2 4+ + U +H2O2 ! UO2 +OH+OH
k/dm3 mol1 s1 3
3.00 10 1.70 1010 4.10 107 1.00 106 4.00 109 4.00 109 2.00 104 1.00 103 8.60 108 2.00 107 1.00 102 11
Reference a b c c
See text See text d d a a a a,e
23 24 25 26 27 28 29 30 31 32 33 34
a H2O, H+ and OH are involved in the reactions mainly due to transformation between U4+ and UO+ 2 . The rate constants were estimated thermodynamically at [H+]=1.0 mol dm3. Their [H+] dependence was discussed in text. b Buxton et al. (1988). c Pikaev et al. (1990). d Jiang (1992). e [29]Shilov et al. (1994).
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Fig. 6. The experimental (n, Andreichuk et al., 1987) and simulated (*) yileds of the oxidation of U4+ in 244Cm aradiolysis of nitric acid solutions. The dose rate was 2.8 Gy s1. The initial concentration of U4+ was 6.6 mmol dm3.
Acknowledgements The author would like to thank Dr. Norihisa Chitose for encouraging me to utilize a deterministic diffusion kinetic model for the intra-track processes.
References Aldrich, J.E., Bronskill, M.J., Wolff, R.K., Hunt, J.W., 1971. Picosecond pulse radiolysis III. Reaction rates and reduction in yields of hydrated electrons. J. Chem. Phys. 55 (2), 530. Andreichuk, N.N., Frolov, A.A., Vasil’ev, V.Ya., 1987. Steadystate concentrations of nitrogen oxides formed in the a-radiolysis of nitric acid solution. Radiokhimiya 29 (3), 417. Andreichuk, N.N., Rotmanov, K.V., Frolov, A.A., Vasil’ev, V.Ya., 1984. Effect of alpha-irradiation of the valence states of actinides IV. Kinetics of HNO2 formation in nitric acid solutions. Radiokhimiya 26 (6), 740. Bhattacharyya, P.K., Natarajan, P.R., 1991. Radiation chemistry of actinide solutions. In: Freeman, A.J., Keller, C. (Eds.), Handbook on the Physics Chemistry of the Actinides. Elsevier Science Publishers B.V, Amsterdam, pp. 597 (Chapter 13). Bhattacharyya, P.K., Veeraraghavan, R., 1977. Reaction between nitrous acid and hydrogen peroxide in perchloric acid medium. Int. J. Chem. Kin 4, 629. Bratsch, S.G., 1989. Standard electrode potentials and temperature coefficients in water at 298.15 K. J. Phys. Chem. Ref. Data 18 (1), 1. Bronskill, M.J., Wolff, R.K., Hunt, J.W., 1970. Picosecond pulse radiolysis. I. The solvated electron in aqueous and alcohol solutions. J. Chem. Phys. 53 (11), 4201.
Burns, W.G., Sims, H.E., Goodall, J.A.B., 1984. Radiation chemical diffusion kinetic calculations with prescribed and non-prescribed diffusion } I. Radiat. Phys. Chem. 23, 143. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. 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 17, 513. Chance, E.M., Curtis, A.R., Jones, I.P., Kirby. C.R., 1977. FACSIMILE: a computer program for flow and chemistry simulation, and general initial value problems. Report AERE-R.8775, AERE Harwell. Chitose, N., 1998. Ion beam pulse radiolysis study on intratrack processes in radiolysis of aqueous solutions. Ph.D. Thesis, the University of Tokyo. Faraggi, M., Zehavi, D., Anbar, M., 1971. Radiolysis of aqueous nitrate solutions. Trans. Faraday Soc. 67, 701. Frolov, A.A., Andreichuk, N.N., Rotmanov, K.V., Kornilov, A.S., Kopytov, V.V., Frolova, L.M., Vasil’ev, V.Ya., 1991. Kinetics of radiation-chemical transformations of U, Np, Pu, Am, and Bk in HNO3 solutions. Radiokhimiya 33 (3), 104. Gra¨tzel, M., Henglein, A., Lilie, J., Beck, G., 1969. Pulsradiolytische untersuchung einiger elementarprozesse der oxydation und reduktion des nitritions. Ber. Bunsenges. Phys. Chem. 73, 646. Gra¨tzel, M., Henglein, A., Taniguchi, S., 1970. Pulsradiolytische beobachtungen u¨ber die reduktion des NO 3 -ions und u¨ber bildung und zerfall der persalpetrigen sau¨re n wariger lo¨sung. Ber. Bunsenges. Phys. Chem. 74, 292. Jiang, P.Y., 1992. The radiation chemistry of inorganic acids and nitrogen compounds in aqueous solutions, Ph.D. Thesis, the University of Tokyo. Jiang, P.Y., Katsumura, Y., Nagaishi, R., Oishi, T., Ishigure, K., Yoshida, Y., 1991. Pulse radiolysis study of aqueous nitric acid solutions. Formation mechanism, yields and reactivity of NO3 radical. J. Phys. Chem. 95, 4435. Jiang, P.Y., Nagaishi, R., Yotsuyanagi, T., Katsumura, Y., Ishigure, K., 1994. g-radiolysis study of concentrated nitric acid solutions. J. Chem. Soc. Faraday Trans. 90 (1), 93. Katsumura, Y., 1998. NO2 and NO3 radicals in radiolysis of nitric acid solutions. In: Alfassi, Z.B. (Ed.), N Centered Radical. Wiley, Chichester, England, pp. 393. Knight, R.J., Sutton, H.C., 1967. Radiolysis of aqueous solutions of nitric oxide. Trans. Faraday Soc. 63, 2628. LaVerne, J.A., Pimblott, S.M., 1991. Scavenger and time dependences of radicals and molecular products in the electron radiolysis of water: examination of experiments and models. J. Phys. Chem. 95, 3196. Nagaishi, R., Jiang, P.Y., Katsumura, Y., Ishigure, K., 1994. Primary yields of water radiolysis in concen trated nitric acid solution. J. Chem. Soc. Faraday Trans. 90 (4), 591. Park, J.Y., Lee, Y.N., 1988. Solubility and decomposition kinetics of nitrous acid in aqueous solution. J. Phys. Chem. 92, 6294. Pikaev, A.K., Gogolev, A.V., Shilov, V.P., Fedoseev, A.M., 1990. Reactivity of ions of actinides towards inorganic free radicals in irradiated aqueous solutions. Isotopenpraxis. 26 (10), 465.
R. Nagaishi / Radiation Physics and Chemistry 60 (2001) 369–375 Pikaev, A.K., Shilov, V.P., Spitsyn, V.I., 1986. Radiolysis of Aqueous Solutions of Lanthanides and Actinides. Nauka, Moscow. Redlich, O., Duerst, R.W., Merbach, A., 1968. Ionization of strong electrolytes XI. The molecular states of nitric acid and perchloric acid. J. Chem. Phys. 49 (7), 2986. Schwarz, H.A., Allen, A.O., 1955. The radiation chemistry of nitrite ion in aqueous solution. J. Am. Chem. Soc. 77, 1324. Shilov, V.P., Gogolev, A.V., Fedoseev, A.M., Pikaev, A.K., 1994. Rate constant evaluation for the reaction of H atoms with U(V) upon radiolysis of aqueous acid solutions of uranium ions. High Energy Chem. 28 (2), 92. Stedman, G., 1979. Reaction mechanisms of inorganic nitrogen compounds. Adv. Inorg. Chem. Radiochem. 22, 157.
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Treinin, A., Hayon, E., 1970. Absorption spectra and reaction kinetics of NO2, N2O3, and N2O4 in aqueous solution. J. Am. Chem. Soc. 92, 5821. Vladimirova, M.V., 1993. Mathematical modeling of the radiation-chemical oxidation of U(IV) in HNO3 solutions. Radiokhimiya 35 (6), 112. Vladimirova, M.V., 1998. Recent advances in radiation chemistry of actinides. Radiokhimiya 40 (5), 385. Wardman, P., 1989. Reduction potentials of one-electron couples involving free radicals in aqueous solution. J. Phys. Chem. Ref. Data 18 (4), 1637. Yotsuyanagi, T., 1989. Radiolytical reduction behavior of uranyl ion in aqueous nitric acid solution. M.Sc. Thesis, the University of Tokyo.
A model for radiolysis of nitric acid and its application to the radiation chemistry of uranium ion in nitric acid medium Ryuji Nagaishi* Analytical Chemistry Research Group, Department of Environmental Science, Japan Atomic Energy Research Institute, Tokai Establishment, Tokai, Naka, Ibaraki 319-1195, Japan
Abstract Primary yields of radiolysis products of water and nitrate in concentrated nitric acid solutions for low LET radiation were evaluated from the previous studies on radiolysis while those for high LET radiation by using a diffusion kinetics model for the intra-track processes. Radiolytic reactions of NOx and uranium ions in the solutions were proposed mainly from the reported data, and then applied together with the yields to the radiolytic formation of HNO2 and oxidation of U4+ to demonstrate the reaction model. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Radiolysis; Radiolytic reactions; Primary yields; LET; Concentrated nitric acid solutions; Uranium ions
1. Introduction Radiation chemistry of actinides in aqueous solution has been investigated and reviewed (Pikaev et al., 1986; Bhattacharyya and Natarajan, 1991) for the sake of nuclear engineering. A lot of investigations have been made in concentrated acid solutions, where radiolysis of acid solutes, change in activity of water and ions, and complexation of actinides take place. A subject of the radiation chemistry is to propose radiolytic reaction models and to estimate redox behavior of actinide ions in practical systems. A few models for uranium, neptunium, plutonium and americium ions in concentrated nitric and perchloric acid solutions (Vladimirova, 1998) have been proposed. However, they seem to be limited to the use of specified cases, the models of which were reproduced, because their proposed radiolysis processes of solutions have not been confirmed and rate constants for proposed reactions have been determined on the basis of the processes. In the present study, the primary yields of radiolysis products of water and nitrate in concentrated nitric acid solutions for low LET radiation were evaluated from the *Tel.: +81-29-282-5527; fax: +81-29-282-6806. E-mail address: [email protected] (R. Nagaishi).
previous electron pulse- and 60Co g-radiolysis studies (Jiang et al., 1991, 1994; Nagaishi et al., 1994) while those for high LET radiation by using a diffusion kinetics model for the intra-track processes. Then the model for radiolytic reactions of uranium ions was proposed mainly from the g-radiolysis studies (Jiang, 1992; Yotsuyanagi, 1989) on the radiolysis of aqueous diluted nitrogen compounds and uranium nitrate solutions. Rate constants for proposed reactions were examined by comparison of the reported and simulated results.
2. Calculation A computer code of FACSIMILE (Chance et al., 1977), which is widely used to solve differential equations for problems in chemistry and engineering, was used for the present simulations with homogeneous reaction models or a diffusion kinetic model. The compiled program is composed of chemical species involved in reactions as variables, initial conditions (solution and radiation) as parameters and a series of reactions with its rate constants. Density of nitric acid solution and dissociation of nitric acid as the initial parameters are dependent on total concentration of nitrate ½NO 3 total . The
0969-806X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 4 1 0 - 2
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R. Nagaishi / Radiation Physics and Chemistry 60 (2001) 369–375
dissociation constant, Kd , of nitric acid (1) provides concentration ratio of the molecular form (HNO3) to the ion form (NO 3 ). It has been determined as ca. 20 for the diluted solution while its variation with ½NO 3 total for the concentrated solution can be determined from the degree of dissociation measured by using Raman and NMR (Redlich et al., 1968). HNO3 Ð Hþ þNO 3;
ð1aÞ
Kd :
The primary yields (unit: ions or molecules/100 eV) of radiolysis of water (gW ) and nitrate (gS ) as parameters were varied with ½NO 3 total as discussed below. It was assumed that energy of radiation should be deposited into the solution in proportion to its electron density and distributed into water and nitrate in accordance with the respective electron fractions ( fW ; fS ). The observable yield, GðiÞ, of product i can be expressed by using gW and gS . GðiÞ ¼ fW gW ðiÞ þ fS gS ðiÞ;
ð1bÞ ½NO 3 total
where fS is proportional to and a sum of fW and fS is equal to unity. Most of the reactions with its rate constants were taken from the reported literatures. For the proposed reactions, optimum rate constants were estimated thermodynamically to reproduce various experimental results.
3. Results and discussion 3.1. Primary yields of radiolysis products of water and nitrate
have been evaluated from the radiolytic reduction of cerric ion and the material balance (Nagaishi et al., 1994) as shown in Fig. 1. Maximum yield, GW ðH2 OÞ (¼ fW gW ), for water decomposition observable in nitric acid solutions was expected to exist at 05fS 51: 0 and calculated as 6.7 at fS ¼ 0:068 (½NO 3 total ¼ 1:34 mol dm3 ) while the maximum primary yield gW (H2O) was 7.9 at fS ¼ 0:13, close to that for water vapor. The fW gW of e aq þH and OH in Fig. 1 indicate that e and H are partly converted to NO2 3 to form aq NO2, and that OH to NO3. The conversions would be significant for the sequential homogeneous reactions. 2 The conversion of e aq to NO3 as an electron scavenger (S) has been defined as [S]37 in picosecond pulseradiolysis studies (Bronskill et al., 1970). On the other hand, the yield of OH converted to NO3 could be corresponding to that of ‘‘slow process’’ in NO3 formation (Jiang et al., 1991). Fig. 2 shows the experimental yields of OH, NO3 and the conversion of OH to NO3 in nitric acid solutions. It was found that OH was completely converted to NO3 in the range of 3 ½NO 3 total > 1:0 mol dm . A lot of investigations on radiolytic reactions of actinide ions have been made with high LET radiation, especially, a-ray while few data on the primary yields of radiolysis products of water and nitrate for high LET radiation has been obtained in concentrated nitric acid solutions. In the present study, the yields of water radiolysis were obtained by using a deterministic diffusion kinetic model for the intra-track processes described by Chitose (1998). This kinetic model is basically the same as reported by Burns et al. (1984)
Dissociation into e aq and NO3 (2) from pulseradiolysis studies on NO3 radical (Jiang et al., 1991) and that into O(3P) and NO 2 (3) from g-radiolysis studies on HNO2 formation (Jiang et al., 1994) have been determined as the radiolysis processes of nitrate: NO 3
e aq þNO3 ; gS1 ;
ð2Þ
NO 3
O 3 P þNO 2 ; gS2 :
ð3Þ
The primary yields have been evaluated as gS1 ðNO gS2 ðNO 3 Þ ¼ gS1 ðHNO3 Þ ¼ 4:8, 3 Þ ¼ 1:6 and gS2 ðHNO3 Þ ¼ 2:2 for the low LET radiation. Maximum yield, GS ðNO 3 Þ (¼ fS gS ), for the decomposition of nitrate would be 7.0 ideally at fS ¼ 1:0. It suggests that their yields were useful for high LET radiation in the present study. Since NO 3 ion is well known as a strong scavenger of not only hydrated but also pre-hydrated electrons (Aldrich et al., 1971) and furthermore HNO3 molecule is a scavenger of OH radical, the primary yields, gW , of water radiolysis may change with ½NO 3 total . The variations of fW gW in nitric acid and nitrate solutions
Fig. 1. The experimental yields of radiolysis products of water in nitric acid solutions for low LET radiation (Nagaishi, 1995) : H2O(*), e aq+H(n), H2(&), OH( ) and H2O2(*). The solid lines are fitting curves taken for simulations.
R. Nagaishi / Radiation Physics and Chemistry 60 (2001) 369–375
Fig. 2. The experimental yields of OH, NO3 and the conversion of OH to NO3 in nitric acid solutions for low LET radiation : total NO3 observed in nitric acid (*, Jiang et al., 1991), OH converted to NO3(n) and OH(solid line, Fig. 1).
and LaVerne and Pimblott (1991). For concentrated nitric acid solutions, pre-solvated electron with decay constant of 6.0 1011 s1 (lifetime510 ps) and [S] dependent rate constans for the reactions of e aq with scavengers (S) of H+ and NO 3 (Bronskill et al., 1970; Aldrich et al., 1971) were introduced into the model. The initial yield of water decomposition for the model was taken as 7.9 from the maximum gW ðH2 OÞ estimated for the low LET radiation as mentioned above. In preliminary simulations, the spur reaction model with the same initial yields could reproduce Fig. 1 especially 3 in the range of ½NO 3 total > 1 mol dm . Fig. 3 shows the simulated yields of radiolysis products of water at LET=100 eV nm1, corresponding to that for 5.8 MeV a-ray (244Cm).
371
Fig. 3. The simulated yields of radiolysis products of water in nitric acid solutions for high LET radiation. : H2O(*), e aq+H(n), H2(&), OH( ) and H2O2(*). The yields at 0.2 ms after irradiation were taken as the simulated yields. The LET was assumed to be 100 eV nm1. The solid lines are fitting curves taken for simulations.
In order to confirm the NOx reaction model and the simulated yields of water radiolysis for high LET radiation, the experimental kinetics of HNO2 formed in 244Cm a-radiolysis of nitric acid solutions (Frolov et al., 1991) was simulated. The concentration of HNO2 increased with time and then reached the steady state one as shown in Fig. 4. In the simulations, the first-order decay of HNO2, which has been indicated by Andreichuk et al. (1984), was necessary to be added to the NOx model. The decay constant was dependent on ½NO 3 total and the dose rate of the irradiation. Fig. 4 shows that the simulated results were in good agreement with the experimental ones in the whole range of ½NO 3 total . 3.3. Compiled reactions of uranium ions
3.2. Compiled reactions of nitrogen oxides Reaction model of water radiolysis has been well established, where the reactions and their rate constants have been summarized (Buxton et al., 1988). The NOx reactions of NO3 to N2 together with their rate constants have been determined (Jiang, 1992) and summarized (Katsumura, 1998) mainly from reported data in aqueous diluted NO 3 (Faraggi et al., 1971), NO2 (Schwarz and Allen, 1955) and NO (Knight and Sutton, 1967) solutions. Main reactants involving redox of uranium ions would be NO3, NO2 and HNO2, main reactions of which are shown in Table 1. It can be pointed out that the molecular products of HNO2 and H2O2 would not co-exist in nitrate solutions.
to U4+ including UO+ in Reactions of UO2+ 2 2 concentrated nitrate solutions should be compiled since U3+ is ready to be oxidized to U4+ in the solutions. Main reactants generated from water are H (e aq ) as a reductant and OH as an oxidant while those of NO3, NO2 and HNO2 from nitrate as oxidants. The rate constants for reactions of actinide ions (U, Np, Pu and Am) with radicals (OH, NO3, SO 4 , Cl2 , CO3 , etc.) have been summarized (Pikaev et al., 1990) from the pulseradiolysis studies. On the other hand, for the proposed reactions, the rate constants should be estimated thermodynamically from those obtained by pulse-radiolysis studies, and the reduction potentials of oneelectron couples involving inorganic radicals (Wardman,
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Table 1 Main NOx reactions of NO3 to NO in nitric acid solutionsa Reaction 2 NO 3 +eaq ! NO3 NO3 +H ! HNO 3
HNO3+OH Ð NO3+H2O NO3 þHO2 ! H þ þNO 3 þO2 2NO3 ! N2O4+O2 NO3+NO2 ! N2O5 NO3+NO 2 ! NO3 +NO2 NO3+HNO2 ! HNO3+NO2 + 2 HNO 3 Ð H +NO3 2 NO3 +H2O ! NO2+OH NO2 3 +O2 ! NO3 +O2 HNO3 ! NO2+OH 2NO2 Ð N2O4 N2O4+H2O Ð 2H++NO 3 +NO2 HNO2 Ð H++NO 2 + HNO2+H2O2 ! NO 3 +H +H2O 2HNO2 Ð NO+NO2+H2O NO+NO2 Ð N2O3 N2O3+H2O ! 2H++2NO 2
kforward =kback 9
9.7 10 2.0 107 5.3 107/5.3 105 3.0 109 4.0 106 1.7 109 4.4 109 8.0 106 pKd 7.5 1.0 103 5.0 106 2.3 109 4.5 108/6.8 103 18/1.7 102 pKd 3.2 4.6 103 [H+] 13.4/2.9 106 1.1 109/2.2 104 36
Reference c e d d b,f d d d g g e g h,f h,i
b,j k g,l l
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
The units of k are s1 for unimolecular and dm3 mol1 s1 for bimolecular reactions. The overall reaction. c Buxton et al. (1988). d Jiang et al. (1991). e Jiang (1992). f Pikaev et al. (1990). g Gra¨tzel et al. (1970). h Gra¨tzel et al. (1969). i Stedman (1979). j Bhattacharyya and Veeraraghavan (1977). k Park and Lee (1988). l Treinin and Hayon (1970). a
b
1989) and actinide ions (Bratsch, 1989). The main reactions in nitric acid solutions are listed in Table 2. Rate constants for the proposed reactions were detailed below. + Rate constants for UO+ 2 +OH and UO2 +NO3 in reactions (27, 28) were estimated from that for 9 3 1 1 UO+ s in acid solutions) 2 +Cl2 (4 10 dm mol and the reduction potentials of (H+, OH)/H2O (2.72 V vs. NHE), NO3/NO 3 (2.67) and Cl2 /2Cl (2.30). Rate + + constants for UO2 +NO2 and UO2 +HNO2 in reactions (29, 30) have been estimated in the following studies. Radiolytic reduction of UO2+ with ethanol has been 2 studied (Yotsuyanagi, 1989) in diluted nitrate solutions, in which radiolysis of nitrate would be negligible. The yields of U4+ and HNO2 were obtained as a function of 2þ ½NO 3 total and ½UO2 0 as shown in Fig. 5. Jiang (1992) has indicated that the reactions of UO+ 2 +NO2 and UO+ 2 +HNO2 should be important during the reduction of UO2+ 2 , and determined the respective rate constants
of k29 ¼ 2 104 and k30 ¼ 1 103 dm3 mol1 s1 to reproduce the experimental yields. It might be mentioned that rate constants for the oxidation of UO+ 2 to UO2+ would be influenced by complexation rather than 2 transformation since both the ions are oxygen-containing actinyl ones. In concentrated nitrate solutions, oxidation of U4+ to UO2+ would be more important than the reduction of 2 UO2+ to U4+. Formal potentials of U4+/UO+ 2 2 (0.38 V 2+ vs. NHE) and UO+ (0.17) indicate that for the 2 /UO2 oxidation by a specified radical, rate constant for U4+ would be lower than that for UO+ 2 . Rate constant for U4++NO3 in reaction (32) was estimated from those for U4++OH (8.6 109 dm3 mol1 s1) and U4++SO 4 (1.2 107) in acid solutions while that for U4++NO2 in reaction (33) was estimated by comparison with that for + UO+ would be 2 +NO2. Furthermore H2O and H responsible for the disproportionation of UO+ 2 and the oxidation of U4+ by radical (R) (U4++R+ + 2H2O Ð UO+ 2 +R +4H ) in reactions (23, 31–34),
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leading to ½NO 3 total dependence of the rate constants as reported by Vladimirova (1993). In the present study, the rate constants for the disproportionation and oxidation were assumed to be proportional to [H+] and the reciprocal of [H+], respectively.
Fig. 6 shows the experimental (Andreichuk et al., 1987) and simulated yileds of the oxidation of U4+ in 244 Cm a-radiolysis of nitric acid solutions. The simulated yields were found to agree well with the experimental ones in the whole range of ½NO 3 total .
Fig. 4. The experimental (n, Frolov, 1991) and simulated (*) steady-state concentrations of HNO2 formed in 244Cm aradiolysis of nitric acid solutions. The dose rate was 3.1 Gy s1. The constant, k, for the first-order decay of HNO2 was taken as 2 1 k¼ 2:5 103 ½NO (see text) for the simulations to 3 total h reproduce the experimental concentrations of HNO2.
Fig. 5. The observed yields of U4+ and HNO2 formed in gradiolysis of acidic uranyl nitrate solutions (Yotsuyanagi, 1989; Jiang, 1992) : experimental U4+(*), HNO2 without (n) and 4+ ( ) and with (m) 5.0 mmol dm3 UO2+ 2 , and simulated U HNO2(+).
Table 2 Main reactions of uranium ions in nitric acid solutions Reaction + 4+ 2+ +2H2O 2UO+ 2 +4H ! UO2 +U 2+ + UO2 +eaq ! UO2 + + UO2+ 2 +H ! UO2 +H 4+ 3+ +
U +H ! U +H 2+ UO+ 2 +OH ! UO2 +OH + 2+ UO2 +NO3 ! UO2 +NO 3 2+ UO+ 2 +NO2 ! UO2 +NO2 + 2+ UO2 +HNO2 ! UO2 +HNO 2 U4++OH ! UO+ 2 +OH U4++NO3 ! UO+ 2 +NO3 U4++NO2 ! UO+ 2 +NO2 4+ + U +H2O2 ! UO2 +OH+OH
k/dm3 mol1 s1 3
3.00 10 1.70 1010 4.10 107 1.00 106 4.00 109 4.00 109 2.00 104 1.00 103 8.60 108 2.00 107 1.00 102 11
Reference a b c c
See text See text d d a a a a,e
23 24 25 26 27 28 29 30 31 32 33 34
a H2O, H+ and OH are involved in the reactions mainly due to transformation between U4+ and UO+ 2 . The rate constants were estimated thermodynamically at [H+]=1.0 mol dm3. Their [H+] dependence was discussed in text. b Buxton et al. (1988). c Pikaev et al. (1990). d Jiang (1992). e [29]Shilov et al. (1994).
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Fig. 6. The experimental (n, Andreichuk et al., 1987) and simulated (*) yileds of the oxidation of U4+ in 244Cm aradiolysis of nitric acid solutions. The dose rate was 2.8 Gy s1. The initial concentration of U4+ was 6.6 mmol dm3.
Acknowledgements The author would like to thank Dr. Norihisa Chitose for encouraging me to utilize a deterministic diffusion kinetic model for the intra-track processes.
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