Hydrazine Radiolysis by Gamma-Ray in the N₂H₄–Cu⁺–HNO₃ System

Abstract

Radiolysis of chemical agents occurs during the decontamination of nuclear power plants. The γ-ray irradiation tests of the N₂H₄–Cu⁺–HNO₃ solution, a decontamination agent, were performed to investigate the effect of Cu⁺ ion and HNO₃ on N₂H₄ decomposition using a Co-60 high-dose irradiator. After the irradiation, the residues of N₂H₄ decomposition were analyzed by Ultraviolet-visible (UV) spectroscopy. NH₄⁺ ions generated from N₂H₄ radiolysis were analyzed by ion chromatography. Based on the results, the decomposition mechanism of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ solution under γ-ray irradiation condition was derived. Cu⁺ ions form Cu⁺N₂H₄ complexes with N₂H₄, and then N₂H₄ is decomposed into intermediates. H⁺ ions and H^● radicals generated from the reaction between H⁺ ion and e_aq⁻ increased the N₂H₄ decomposition reaction. NO₃⁻ ions promoted the N₂H₄ decomposition by providing additional reaction paths: (1) the reaction between NO₃⁻ ions and N₂H₄^●+, and (2) the reaction between NO^● radical, which is the radiolysis product of NO₃⁻ ion, and N₂H₅⁺. Finally, the radiolytic decomposition mechanism of N₂H₄ obtained in the N₂H₄–Cu⁺–HNO₃ was schematically suggested.

1. Introduction

Hydrazine (N₂H₄) is commercially used to produce plastics, medicines, and textile dyes and to reduce the corrosion of a boiler in a thermal power plant [1,2]. In the nuclear field, N₂H₄ is added to the primary feed water to maintain the hydrogen concentration and to remove dissolved oxygen [3]. In addition, N₂H₄ can be applied as a chemical decontamination solution to remove radioactive nuclides in an oxide layer of a primary system in the nuclear power plant [4]. The decontamination solution containing N₂H₄ is developed to reduce the damage of the base metal and the secondary radioactive wastes compared with the decontamination using organic acid [5,6]. They are composed of N₂H₄ and inorganic acids such as HNO₃ and H₂SO₄ [5,6,7]. Furthermore, the metal ions can be added into the decontamination solution containing N₂H₄ for improving the decontamination performance [8]. For this reason, a N₂H₄–Cu⁺–HNO₃ decontamination solution was suggested as the effective chemical decontamination solution [7]. The decontamination solution, however, can be decomposed under the high radiation field [9]. The radiolysis of the decontamination solution occurs by radionuclides in the primary system, such as Co-60 and Co-58, during the application. The decomposition of major compositions of the decontamination solution affects the decontamination performance. Therefore, it is necessary to analyze the radiolysis of N₂H₄, which is the major component of the N₂H₄–Cu⁺–HNO₃ decontamination solution, during irradiation.

In this regard, a number of research studies concerning decomposition of N₂H₄ solution have been carried out. It is known that the decomposition of N₂H₄ under irradiation conditions can occur through a reaction with radiolysis products of water. Various radicals and products, such as e_aq⁻, ^•OH, and H₃O⁺, are generated by the radiolysis of water [10]. The radiolysis products of water as represented in Equation (1) [11].

$$\begin{array}{c}{\mathrm{H}}_2\mathrm{O}\stackrel{\mathsf{\gamma }-\mathrm{radiation}}{\to }0.27{\mathrm{e}}_{\mathrm{a}\mathrm{q}}{}^{-}, 0.06 {\mathrm{H}}^{•}, {0.26}^{•}\mathrm{O}\mathrm{H}, 0.045{\mathrm{H}}_2, 0.08 {\mathrm{H}}_2{\mathrm{O}}_2, 0.27{\mathrm{H}}_3{\mathrm{O}}^{+}\end{array}$$

(1)

The decomposition reaction mechanism of N₂H₄ in the aqueous solution by γ-ray irradiation can be found in the study of Buxton et al. [12]. They reported that radicals such as N₂H₄^●+, NH₂^● and N₂H₃^●, generated from N₂H₄, are decomposed into N₂ and NH₃. It is also shown that the NH₄⁺ ion was produced by a reaction between H^● radical and N₂H₅⁺. Garaix et al. studied the decay mechanism of the NO₃^● radical generated by the radiolysis of NO₃⁻ ions in the N₂H₄ solution using an electron beam [13]. They concluded that N₂H₄ exists mainly as N₂H₅⁺ or N₂H₆²⁺ in the acidic solution, both of which cause rapid consumption of NO₃^● radicals. Motooka et al. also reported that the deoxygenation reaction with radiolysis of N₂H₄ can be suppressed by salt in the water in the γ-radiation field [14]. In this way, the molecular structure of N₂H₄ after radiolysis and the decomposition mechanism of the N₂H₄ depend on the composition of the solution. However, there are few research studies about the N₂H₄ decomposition reaction in N₂H₄–Cu⁺–HNO₃ solution. Therefore, it is necessary to study the decomposition reaction mechanism of the N₂H₄ in N₂H₄–Cu⁺–HNO₃ solution during γ-ray irradiation to simulate the decontamination condition.

In this study, we evaluated the effects of Cu⁺ ions and HNO₃ on N₂H₄ decomposition under the γ-radiation field. The study was performed by analyzing the concentration of remaining N₂H₄ and the concentration of the products in N₂H₄–Cu⁺–HNO₃ solution after the irradiation. The decomposition mechanism of N₂H₄ in the solution containing Cu⁺ ions and HNO₃ was also suggested.

2. Theoretical Background

2.1. Radiolysis of Hydrazine in Acidic Solution

Hydrazine generally exists in the form of N₂H₅⁺ by its reaction with H⁺ ions in an acidic solution, as given in Equation (2) [13,15,16]. At pH 1, N₂H₆²⁺ coexists with N₂H₅⁺ through the reaction shown in Equation (3) [13,15,17]. Therefore, N₂H₄ exists in the forms of N₂H₅⁺ and N₂H₆²⁺ in the N₂H₄–HNO₃ system before irradiation.

N₂H₄ + H⁺ 2194 N₂H₅⁺, pK ≈ 7.9

(2)

N₂H₅⁺ + H⁺ ↔ N₂H₆²⁺, pK ≈ −1

(3)

In an acidic solution, the same as the condition of this study, e_aq⁻, H^●, OH^●, and H₂O₂ are generated as products after water radiolysis. It is possible that e_aq⁻ reacts with H⁺ ions in the acidic solution and generates H^● as in the following Equation (4) [18]. The reactions between the water radiolysis products and the chemical species of N₂H₄ lead to the decomposition of N₂H_4.

e_aq⁻ + H⁺ → H^●, k = 2.2 × 10¹⁰ M⁻¹s⁻¹

(4)

The principal decomposition reactions and rate constants of the chemical species of N₂H₄ in the irradiation condition are listed in Equations (5)–(23). As shown in Equation (5), N₂H₆²⁺ reacts with OH^● and produces N_2, the end product of N₂H₄ decomposition at pH 1 [19].

N₂H₆²⁺ + 4OH^● → N₂ + 4H₂O + 2H⁺

(5)

In addition, N₂H₅⁺ is the main species form of N₂H₄ in the acidic solution. N₂H₅⁺ reacts with the radiolysis products of water such as e_aq⁻, H^●, or OH^● as shown in Equations (6)–(9) [12,20]. NH₄⁺ ion, one of the end products of N₂H₄ decomposition, is produced by the reaction between N₂H₅⁺ and H^●, as indicated in Equation (7). The intermediates of N₂H₅⁺ decomposition, N₂H₄, NH₂^●_, and N₂H₄^●+, are generated by the reactions in Equations (6), (8) and (9). These intermediates cause the consecutive decomposition reactions of N₂H₄. In particular, N₂H₄ can also be hydrolyzed into N₂H₅⁺ and N₂H₆²⁺ as shown in Equations (2) and (3).

The consecutive decomposition reactions of N₂H₄ with OH^●, N₂H₄^●+, and H₂O₂ are listed in Equations (10)–(12) [12,20,21]. N₄H₈⁺ is formed by the reaction between N₂H₄ and N₂H₄^●+, as indicated in Equation (10). ^●N₂H₃ is generated by the reaction between N₂H₄ and OH^●, as shown in Equation (11). The intermediates, N₄H₈⁺ and ^●N₂H₃, participate in the other consecutive decomposition reactions of N₂H₄. However, N₂ is produced as the end product by the reaction between N₂H₄ and H₂O₂ (Equation (12)).

NH₂^● generated by the reaction in Equation (7) causes the reactions with N₂H₅⁺ or N₂H₄, as represented in Equations (13) and (14) [12]. N₂H₄^●+, ^●N₂H₃, and NH₃ are formed after the reactions shown in Equations (13) and (14). Among these products, N₂H₄^●+ and ^●N₂H₃ cause the consecutive reactions because they are the reactive intermediates.

N₂H₄^●+ produced by Equations (8), (9) and (13) participate in the consecutive reactions represented in Equations (15) and (16) [12]. As shown in Equation (15), N₄H₉^●2+ is generated after the reaction of N₂H₄^●+ with N₂H₅⁺. As indicated in Equation (16), N₄H₉^●2+ reacts with N₂H₄^●+, and the reaction products are N₄H₈²⁺ and N₂H₅⁺. The N₄H₈²⁺ is directly decomposed into N₂ and NH₃ in the ratio of 1 to 2. On the other hand, N₂H₅⁺ is repeatedly decomposed into other forms, as represented in Equations (6)–(9), (13) and (15).

^●N₂H₃ generated by Equations (11) and (14) is decomposed into various forms, as listed in Equations (17)–(22) [12]. The main end products of ^●N₂H₃ decomposition are N₂ and NH₃, as represented in Equations (19) and (20). The main intermediates, N₂H₄ and N₂H₂, are also generated from the decomposition reaction of ^●N₂H₃, as shown in Equations (17)–(22). N₂H₄ is hydrolyzed into N₂H₅⁺ and N₂H₆²⁺ in the acidic solution or causes consecutive decomposition reactions. N₂H₂ reacts with H^●, and ^●N₂H₃ is produced as shown in Equation (23).

As mentioned above, it is expected that various intermediates are generated during the decomposition of the chemical species of N₂H₄. Therefore, the intermediates can affect the reaction with Cu⁺ ions or NO₃⁻ ions in the N₂H₄–Cu⁺–HNO₃ system.

N₂H₅⁺ + e_aq⁻ → H^● + N₂H₄, k = 1.6 × 10⁸ M⁻¹s⁻¹

(6)

N₂H₅⁺ + H^● → NH₂^● + NH₄⁺, k = 1.0 × 10⁴ M⁻¹s⁻¹

(7)

N₂H₅⁺ + H^● → H₂ + N₂H₄^●+, k = 1.3 × 10⁵ M⁻¹s⁻¹

(8)

N₂H₅⁺ + OH^● → N₂H₄^●+ + H₂O, k = 8.2 × 10⁷ M⁻¹s⁻¹

(9)

N₂H₄ + N₂H₄^●+ ↔ N₄H₈^●+, k_f = 6.0 × 10⁷ M⁻¹s⁻¹, k_b = 4.0 × 10⁵ M⁻¹s⁻¹

(10)

N₂H₄ + OH^● → ^●N₂H₃ + H₂O, k = 5.4 × 10⁹ M⁻¹s⁻¹

(11)

N₂H₄ + 2H₂O₂ → N₂ + 4H₂O, k = 2.4 × 10⁸ M⁻¹s⁻¹

(12)

NH₂^● + N₂H₅⁺ → N₂H₄^●+ + NH₃, k = 1.0 × 10⁶ M⁻¹s⁻¹

(13)

NH₂^● + N₂H₄ → ^●N₂H₃ + NH₃, k = 1.0 × 10⁷ M⁻¹s⁻¹

(14)

N₂H₄^●+ + N₂H₅⁺ ↔ N₄H₉^●2+, k_f = 6.0 × 10⁷ M⁻¹s⁻¹, k_b = 4.0 × 10⁵ M⁻¹s⁻¹

(15)

N₂H₄^●+ + N₄H₉^●2+ → N₄H₈²⁺ + N₂H₅⁺ (N₄H₈²⁺ → N₂ + 2NH₃),
k = 4.0 × 10⁸ M⁻¹s⁻¹

(16)

^●N₂H₃ + e_aq⁻ + H₂O → N₂H₄ + OH⁻, k = 7.0 × 10⁹ M⁻¹s⁻¹

(17)

^●N₂H₃ + H^● → N₂H₄, k = 7.0 × 10⁹ M⁻¹s⁻¹

(18)

^●N₂H₃ + N₄H₈^●+ → N₂H₄ + N₄H₇⁺ (N₄H₇⁺ → N₂ + 2NH₃),
k = 1.0 × 10⁹ M⁻¹s⁻¹

(19)

^●N₂H₃ + N₄H₉^●2+ → N₂H₄ + N₄H₈²⁺ (N₄H₈²⁺ → N₂ + 2NH₃),
k = 1.0 × 10⁹ M⁻¹s⁻¹

(20)

^●N₂H₃ + N₂H₄^●+ → N₂H₂ + N₂H₅⁺, k = 7.0 × 10⁸ M⁻¹s⁻¹

(21)

^●N₂H₃ + ^●N₂H₃ → N₂H₂ + N₂H₄, k = 6.0 × 10⁸ M⁻¹s⁻¹

(22)

N₂H₂ + H^● → ^●N₂H₃, k = 3.0 × 10⁹ M⁻¹s⁻¹

(23)

2.2. Change of Copper Species during Irradiation

Copper ions in the solution would cause the decomposition of N₂H₄ during the irradiation. The redox reactions mainly occur between copper ions and radiolysis products of water such as e_aq⁻, H^●, and OH^●, as listed in Equations (24)–(28) [22,23,24,25]. The equations show that copper ions coexist in the forms of Cu⁰, Cu⁺ ions and Cu²⁺ ions regardless of initial chemical species. In addition, Fenton reaction occurs in an acidic condition, as represented in Equation (29) [26,27]. The above reactions can affect the decomposition of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ system.

Cu⁺ + e_aq⁻ → Cu⁰

(24)

Cu⁺ + H^● → Cu⁰ + H⁺, k = 5 × 10⁹ M⁻¹s⁻¹

(25)

Cu⁺ + OH^● → Cu²⁺ + OH⁻, k = (2±1) × 10¹⁰ M⁻¹s⁻¹

(26)

Cu²⁺ + e_aq⁻ → Cu⁺, k = 3.5 × 10¹⁰ M⁻¹s⁻¹

(27)

Cu²⁺ + H^● → Cu⁺ + H⁺, k < 1.0 × 10⁶ M⁻¹s⁻¹

(28)

H₂O₂ + Cu⁺ + H⁺ → OH^● + H₂O + Cu²⁺

(29)

2.3. Radiolysis of Nitrate Ion

The principal reactions of NO₃⁻ ions during the irradiation are listed in Equations (30)–(38) [13,28,29,30,31,32,33]. The reactions can be classified by direct and indirect decomposition reactions. As shown in Equation (30), the NO₃⁻ ion is directly changed into NO₃^● and electron due to the γ-ray irradiation [13,28]. The NO₃⁻ ion is also changed into NO₃²⁻ ion or NO₂^● through the reactions with e_aq⁻ or H^●, as can be seen in Equations (31) and (32) [29,30]. The NO₃^2−● reduces into NO₂^● in the water, as represented in Equation (33) [29,31]. During irradiation, NO₂^● reacts with the radiolysis products of water such as e_aq⁻, H^●, and OH^●, and H⁺, NO₂⁻ ion, and NO₃⁻ ions are produced as listed in Equations (34)–(36) [32]. On the other hand, NO₂^● reacts with water and generates NO₂⁻ and NO₃⁻ ions, as shown in Equation (37) [13,33]. As represented in Equation (38), NO₂⁻ ions generated from the reaction in Equations (34), (35) and (37) are changed into NO₃⁻ ions and NO₂^● through the reaction with NO₃^● [13,33]. The generated NO₂⁻ ions are directly consumed, and NO₂^● and NO₃⁻ ions are regenerated by the reaction indicated in Equation (38). As mentioned above, NO₃⁻ ions and their radicals generated from the radiolysis of NO₃⁻ can also participate in the decomposition reaction of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ system.

$$\begin{array}{c}\mathrm{N}{\mathrm{O}}_3{}^{-}\stackrel{\mathrm{radiation}}{\to }\mathrm{N}{\mathrm{O}}_3{}^{-}*\to \mathrm{N}{\mathrm{O}}_3{}^{●}+{\mathrm{e}}^{-}\end{array}$$

(30)

NO₃⁻ + e_aq⁻ → NO₃^2−●, k = 9.7 × 10⁹ M⁻¹s⁻¹

(31)

NO₃⁻ + H^● → NO₂^● + OH⁻, k = 1.0 × 10⁷ M⁻¹s⁻¹

(32)

NO₃^2−● + H₂O → NO₂^● + 2OH⁻, k = 1.0 × 10³ M⁻¹s⁻¹

(33)

NO₂^● + e_aq⁻ → NO₂⁻, k = 1.0 × 10¹⁰ M⁻¹s⁻¹

(34)

NO₂^● + H^● → H⁺ + NO₂⁻, k = 1.0 × 10⁹ M⁻¹s⁻¹

(35)

NO₂^● + ^●OH → H⁺ + NO₃⁻, k = 1.0 × 10¹⁰ M⁻¹s⁻¹

(36)

2NO₂^● + H₂O → NO₂⁻ + NO₃⁻ + 2H⁺, k = 4.5 × 10⁸ M⁻¹s⁻¹

(37)

NO₃^● + NO₂⁻ → NO₂^● + NO₃⁻, k = 4.4 × 10⁹ M⁻¹s⁻¹

(38)

3. Results

3.1. Effect of Copper Ions on Hydrazine Decomposition

In order to investigate the effect of copper ions on the N₂H₄ decomposition, γ-ray was irradiated to the N₂H₄–Cu⁺–HNO₃ solution and N₂H₄–HNO₃ solution at pH 3. The absorbed dose was varied from 0 to 20 kGy, and the [N₂H₄]₀ in the solutions was equal to 50 × [N2H4]0 in the solutions was equal to 50 × 10⁻³ mol dm⁻³. The pH of the solution was adjusted to 3using HNO₃. Figure 1 shows the change in the concentration of N₂H₄ as a result of the γ-irradiation. The decomposed portion of N₂H₄ increased with the increase in the absorbed dose regardless of the presence of the Cu⁺ ions. This result indicates that the amount of radiolysis products of water participating in the N₂H₄ decomposition was enhanced when the absorbed dose was increased. At the same absorbed dose, the decomposed portion of N₂H₄ was higher when the copper ions existed than that when the copper ions were absent, as indicated in Figure 1. In particular, 12.48 × 10⁻³ mol dm⁻³ of N₂H₄ in the solution containing Cu⁺ ions was decomposed after the 20 kGy of γ-irradiation. When the Cu⁺ ions were absent in the solution, 9.05 × 10⁻³ mol dm⁻³ of N₂H₄ was decomposed. Moreover, the G-values for the N₂H₄ decomposition were calculated for 20 kGy of absorbed dose and listed in

Table 1. G(–N₂H₄) values in the N₂H₄–HNO₃ and N₂H₄–Cu⁺–HNO₃ solutions of which [N₂H₄]₀ = 50 × 10⁻³ mol dm⁻³ at 20 kGy of absorbed dose.

Total Dose (kGy)	G(–N2H4) Value (10−7 mol/J)
	w/o Cu+ Ions	w/Cu+ Ions
20	4.52	6.24

. G(–N₂H₄), for the solution containing Cu⁺ ions, was higher than that of the solution not containing Cu⁺ ions.

There are several explanations for the effect of copper ions on the decomposition of N₂H₄: (1) a catalyzed reaction of H₂O₂ occurs [34], (2) copper ions lower the energy barrier of N-H bonds cleavage in the gas phase [35], and (3) the formation of Cu⁺N₂H₄ occurs [36,37]. The experimental condition of Zhong and Lim is similar to that of this study [36,37]. They suggested that Cu⁺N₂H₄ complex acts as a scavenger and it increases the decomposition reaction of N₂H₄. The predicted mechanism is given in Equations (39)–(41). N₂H₄ reacts with Cu⁺ ion and forms the Cu⁺N₂H₄ complex, as shown in Equation (39). This complex can react with H₂O₂ and produces N₂H₂, as indicated in Equation (40). The reaction between the Cu⁺N₂H₄ complex and N₂H₂ causes the generation of Cu(^●N₂H₃)₂ that decomposes into Cu⁺ ion and ^●N₂H₃, as represented in Equation (41).

Cu⁺ + N₂H₄ ↔ Cu⁺N₂H₄

(39)

Cu⁺N₂H₄ + H₂O₂ → Cu⁺ + N₂H₂ + 2H₂O, (slow)

(40)

Cu⁺N₂H₄ + N₂H₂ ↔ Cu⁺(^●N₂H₃)_{2, cage} → Cu⁺ + 2^●N₂H₃, (slow)

(41)

N₂H₄ was considered to be reproduced as the intermediate during radiolysis in this study; however, the hydrazine was hydrolyzed into N₂H₅⁺ or N₂H₆²⁺ in acidic conditions. The N₂H₄ could form the Cu⁺N₂H₄ complex with the Cu⁺ ion through the reaction, as indicated in Equation (39). As can be seen in Equations (40) and (41), it was possible that the Cu⁺N₂H₄ complex was separated into Cu⁺ ion, N₂H₂, and water after reacting with H₂O₂ generated from the radiolysis of water. The N₂H₂ could react with H^● and generate ^●N₂H₃ according to the Equation (23). Moreover, N₂H₂ could also cause a reaction with the Cu⁺N₂H₄ complex and form Cu⁺(N₂H₃^●)₂. Cu⁺(N₂H₃^●)₂ was directly decomposed into ^●N₂H₃ and Cu⁺ ion following the reaction in Equation (41). ^●N₂H₃ was decomposed into N₂ or NH₃ during the γ-ray irradiation, as listed in Equations (19) and (20). The Cu⁺ ion regenerated from the reactions shown in Equations (40) and (41) repeatedly formed the Cu⁺N₂H₄ complex and caused the decomposition reaction of N₂H₄ again. Therefore, these two reactions offered new decomposition reaction paths of N₂H₄ where the Cu⁺ ion acted as the catalyst.

The electrochemical characterization, under the same conditions as this experiment, was also performed by Yang et al. [38]. Figure 2 shows the cyclic voltammograms using an ITO (Indium-Tin Oxide) electrode in solutions of 3 mM N₂H₄, 0.3 mM Cu(NO₃)₂, 0.1 M NaNO₃, and 3 mM N₂H₄ + 0.3 mM Cu(NO₃)₂. All the test solutions were adjusted to pH 3 using HNO₃. The oxidation peak of the N₂H₄ near 0.3 V increased significantly with the addition of Cu(NO₃)₂. The peak is quite different from that of N₂H₄ alone or Cu(NO₃)₂ alone. This result implies that Cu⁺ ions make coordination compounds with N₂H₄, as listed in reaction Equation (39). Therefore, it is inferred that Cu⁺ ions affect the N₂H₄ decomposition by formation of the Cu⁺N₂H₄ complex in the N₂H₄–Cu⁺–HNO₃ system.

3.2. Effect of HNO₃ on Hydrazine Decomposition

HNO₃ affects the decomposition of N₂H₄ in two ways: (1) the effect of H⁺ ions and (2) the effect of NO₃⁻ ions. In order to investigate the effect of H⁺ ions and NO₃⁻ ions on N₂H₄ decomposition, the concentration of the chemical species of N₂H₄ was analyzed according to the pH change under the radiation field. The pH of each solution was adjusted to 1, 3, and 5 by adding HNO₃, respectively. Every sample solution included 50 × 10⁻³ mol dm⁻³ of N₂H₄ and 0.5 × 10⁻³ mol dm⁻³ of Cu⁺ ions. As a result, the concentration of chemical species of N₂H₄ decreased with a decreasing pH at the same absorbed dose, as shown in Figure 3. The concentrations of decomposed N₂H₄ were 29.96 × 10⁻³ mol dm⁻³ (pH = 1), 15.92 × 10⁻³ mol dm⁻³ (pH = 3), and 13.42 × 10⁻³ mol dm⁻³ (pH = 5) at 40 kGy, respectively. The decomposed portion of N₂H₄ significantly increased as the solution’s pH decreased from 3 to 1. At the same time, the G(–N₂H₄) at pH 1 was 7.49 × 10⁻⁷ mol J⁻¹ for 40 kGy of absorbed dose, as shown in

Table 2. G(–N₂H₄) values in the different pH of N₂H₄–Cu⁺–HNO₃ solutions of which [N₂H₄]₀ = 50 × 10⁻³ mol dm⁻³ at 40 kGy of absorbed dose.

Total Dose (kGy)	G(–N2H4) Value (10−7 mol/J)
	pH 1	pH 3	pH 5
40	7.49	3.98	3.35

. The G(–N₂H₄) at pH 3 and 5 were 3.98 × 10⁻⁷ mol J⁻¹ and 3.35 × 10⁻⁷ mol J⁻¹ for 40 kGy of absorbed dose, respectively. Based on this result, it was confirmed that the G-values for the decomposition of N₂H₄ increased with the decreasing of the pH.

Firstly, the above results can be explained by the effect of the H⁺ ion on N₂H₄ decomposition. As mentioned above, the reaction between the H⁺ ion and e_aq⁻ caused the generation of H^●, as represented in Equation (4). The increase in H^● promoted the reaction between H^● and the intermediates of N₂H₄ decomposition, such as N₂H₅⁺, N₂H₃^●, and N₂H₂, listed in Equations (7), (8), (18) and (23). When the reactions shown in Equations (7), (8) and (23) occurred, NH₄⁺ ions or the intermediates such as ^●NH₂, N₂H₄^●+_, and ^●N₂H₃ were produced. As represented in Equation (18), N₂H₄ is recovered through the reaction between ^●N₂H₃ and H^●_. This N₂H₄ could be decomposed after hydrolysis into N₂H₅⁺ or be decomposed directly through the reactions listed in Equations (10)–(12). On the other hand, it was possible to form the Cu⁺N₂H₄ complex with copper ions and cause a decomposition reaction of N₂H₄ using Equations (40) and (41). In particular, N₂H₄ and ^●N₂H₃ generated from the reactions shown in Equations (18) and (23) have a high reaction rate, which are 7.0 × 10⁹ M⁻¹s⁻¹ and 3.0 × 10⁹ M⁻¹s⁻¹, among the reactions concerned H^●. The N₂H₄ and ^●N₂H₃ are mostly decomposed into N₂ and NH₃, as mentioned above.

Secondly, the increase in the decomposition reaction of N₂H₄ with the lowering of the pH of the N₂H₄–Cu–HNO₃ solution could also be explained by the effect of the NO₃⁻ ion. When the NO₃⁻ ion reacts with NH₄^●+, which is the chemical species of N₂H₄, N₂H₂ and ^●NO₂ are produced due to the reaction, as represented in Equation (42) [39]. As listed in Equation (23), the N₂H₂ reacts with H^●, and ^●N₂H₃ is generated. As mentioned above, ^●N₂H₃ normally decomposes into N₂ and NH₃, leading to N₂H₄ decomposition. ^●NO₂ participates in the reaction with radiolysis products of water, and finally NO₃⁻ is formed by the reaction shown in Equations (34)–(38). On the other hand, NO₃^● generated during the radiolysis of NO₃⁻ ions also affects N₂H₄ decomposition. When NO₃^● reacts with N₂H₅⁺, N₂H₄^●+ and HNO₃ are produced, as shown in Equation (43) [13,39]. N₂H₄^●+ is consecutively decomposed not only by the reaction listed in Equations (15) and (16) but also by the reaction shown in Equation (42). NO₃⁻ ions recovered from the reaction shown in Equation (43) participate in the decomposition reaction of the chemical species of N₂H₄. Therefore, the increase in HNO₃ in the N₂H₄–Cu⁺ solution accelerated the decomposition of N₂H₄ by increasing the occurrence of reaction concerned with H^● and adding new decomposition reaction paths, including that of the NO₃⁻ ion.

N₂H₄^●+ + NO₃⁻ → N₂H₂ + H₂O + ^●NO₂, k = 2.5 × 10⁷ M⁻¹s⁻¹

(42)

N₂H₅⁺ + NO₃^● → N₂H₄^●+ + NO₃⁻ + H⁺, k = 1.3 × 10⁹ M⁻¹s⁻¹

(43)

Moreover, the decomposed portion of N₂H₄ increased rapidly at pH 1 compared to at pH 3 and 5, when the absorbed doses increased at the rates shown in Figure 3. This was caused by H^● and NO₃^● being generated by irradiation. Since the amount of HNO₃ added at pH 1 was larger than at pH 3 and 5, the amount of H^● and NO₃^● produced was larger at pH 1 than at pH 3 and 5. The increase in H^● and NO₃^● promoted the decomposition of N₂H₄ through the reactions, as mentioned above.

In order to investigate the anionic effect, the remaining concentration of N₂H₄ in the NO₃⁻ ion system was compared with that of the SO₄²⁻ ion system at pH 3. Quantities of 10, 20, and 40 kGy of the absorbed doses were irradiated to each solution. The initial concentration of N₂H₄ in each solution was 50 × 10⁻³ mol dm⁻³. As shown in Figure 4, the decomposed concentration of N₂H₄ increased when the absorbed dose increased regardless of the type of acid added. At the same absorbed dose, three times higher concentrations of N₂H₄ in the NO₃⁻ ion system were decomposed as compared to the SO₄²⁻ ion system. As indicated in

Table 3. G(–N₂H₄) values of N₂H₄–Cu⁺–HNO₃ and N₂H₄–Cu⁺–H₂SO₄ solutions of which [N₂H₄]₀ = 50 × 10⁻³ mol dm⁻³ at 40 kGy of absorbed dose.

Total Dose (kGy)	G(–N2H4) Value (10−7 mol/J)
	Containing HNO3	Containing H2SO4
40	3.98	1.25

, the G-value for the decomposition of N₂H₄ at 40 kGy was 3.98 × 10⁻⁷ mol J⁻¹ when the acid added in the solution was HNO₃. G(–N₂H₄) was 1.25 × 10⁻⁷ mol J⁻¹ when the acid injected in the solution was H₂SO₄. From these results, we found that the NO₃⁻ ions facilitated the decomposition of N₂H₄ in the solution.

3.3. Decomposition Mechanism of Hydrazine in N₂H₄–Cu⁺–HNO₃ System

The decomposition reactions of the N₂H₄ in N₂H₄–Cu⁺–HNO₃ system are schematically shown in Figure 5. N₂H₄ in the acidic solution is hydrolyzed and coexists as N₂H₅⁺ or N₂H₆²⁺. These species are decomposed into intermediates such as N₂H₄^●+, ^●N₂H₃, and NH₂^● under irradiation conditions. N₂H₄ can decompose into NH₄⁺ ion, N₂, or NH₃. However, it was verified that the end products were mainly formed with N₂ or NH_3. In addition, N₂H₄ decomposition was promoted by the influence of the Cu⁺ ion, H⁺ ion, and NO₃⁻ ion in the N₂H₄–Cu⁺–HNO₃ system, as explained in Section 3.1 and Section 3.2. As represented in green line in Figure 5, Cu⁺ ions form the Cu⁺N₂H₄ complex with N₂H₄. The Cu⁺N₂H₄ complex participates in the reactions, as shown in Equations (40) and (41). The complex decomposes into ^●N₂H₃ and further decomposes into the end products through the consecutive reactions, as listed in Equations (17)–(22). The recovered Cu⁺ ion from the complex repeatedly forms an N₂H₄ complex that acts as a catalyst to accelerate the decomposition of N₂H₄. H^● was produced through the reaction between the H⁺ ion and e_aq⁻. Therefore, the decomposition reaction of N₂H₄ by H^● was promoted as the concentration of the H⁺ ion increased. As shown in the orange line in Figure 5, NO₃⁻ ions or NO₃^● radicals accelerate the N₂H₄ decomposition by providing the additional reaction paths to change N₂H₅⁺ into N₂H₄^●+. They also cause N₂H₄^●+ to decompose into N₂H_2. NO₃⁻ ion and NO₃^● were regenerated by the radiolysis of NO₂^● and NO₃^●, as shown in Equations (34)–(38), and they participated in the N₂H₄ radiolysis reaction again. Consequently, N₂H₄ decomposition was promoted in the N₂H₄–Cu⁺–HNO₃ system through the mechanism shown in Figure 5.

In order to verify the effect of Cu⁺ ions on the decomposition mechanism of N₂H₄ in the N₂H₄–HNO₃ solution, the fraction of N₂H₄ and end products were analyzed after irradiation with 20 kGy of the absorbed dose. The initial concentrations of N₂H₄ in the solution were 50 × 10⁻³ mol dm⁻³, and the pH of the solution was adjusted to 3. The results were compared with and without Cu⁺ ions in the solution, as represented in Figure 6. Through the above reactions, it was verified that N₂H₄ in the N₂H₄–Cu⁺–HNO₃ solution was finally decomposed into N₂, NH₃, and NH₄⁺ ion under an irradiation condition by the reactions with radiolysis products of water or consecutive decomposition reactions. For this reason, the fraction of N₂ and NH₃ in the solution after γ-ray irradiation was calculated by subtracting the amount of remaining chemical species of N₂H₄ and NH₄⁺ ions produced after irradiation from the initial amount of N₂H₄.

As shown in Figure 6, N₂H₄ decomposed into N₂, NH₃, and NH₄⁺ ion. It is well known that most of NH₃ reacts with H⁺ ions in the acidic solution and exists in the form of NH₄⁺ [40]. Therefore, it was considered that most of the remaining gas phase end product was composed of N₂ after irradiation in this study. It was judged that the NH₃ was converted into NH₄⁺ ion after the irradiation. When the Cu⁺ ion is present in the N₂H₄–HNO₃ solution, N₂H₃^● is generated, as indicated in Figure 5 and Equation (41). N₂H₃^● participated in the reaction, generating N₂ or NH₃, as shown in Equations (19) and (20). Therefore, it was confirmed that the fraction of N₂ and NH₄⁺, the form of NH₃ in the acidic condition, increased when the Cu⁺ ions were present in the N₂H₄–HNO₃ solution, as represented in Figure 6.

To confirm the effect of HNO₃ on the decomposition mechanism of N₂H₄ in the N₂H₄–Cu⁺ solution, the fraction of remaining N₂H₄ and end products in each sample after 40 kGy of absorbed dose irradiation at pH 1, 3, and 5 was analyzed. The initial concentrations of N₂H₄ in the solutions were 50 × 10⁻³ mol dm⁻³. The result is represented in Figure 7. As mentioned above, NH₃ is converted into NH₄⁺ ion in the solution because of the acidic condition. As shown in Figure 7, NH₄⁺ ions were not generated after 40 kGy of absorbed dose irradiation at pH 1. Therefore, it was considered that most of N₂H₄ was decomposed into N₂. The obtained result at pH 1 can be explained as follows. At pH 1, N₂H₄ exists in the form of N₂H₆²⁺ as a result of hydrolysis, as shown in Equation (3). The N₂H₆²⁺ ion generated N₂ through the decomposition reaction, as indicated in Equation (5).

In addition, the amount of end products of N₂H₄ decomposition were decreased with increasing pH. This was the case because the large amount of H^● produced by the reaction between H⁺ ions and e_aq⁻ affected the N₂H₄ decomposition, as shown in Figure 5. For this reason, the decrease in the concentration of end products of N₂H₄ decomposition following an increase in the pH was caused by a decrease in the N₂H₄ decomposition reaction.

However, the concentration of NH₄⁺ ions generated after irradiation increased with increasing pH. This was the case because the N₂H₄ exists as a form of the N₂H₅⁺ ion rather than the N₂H₆²⁺ ion as the pH increases. As indicated in Equations (6)–(23), the end products of the reactions with a high reaction rate, among the consecutive reactions of N₂H₅⁺ ion decomposition in which H^● participated, were mainly N₂ and NH₃. The NH₃ reacted with the H⁺ ion in an acidic condition and existed in the form of NH₄⁺ ions, as mentioned above. NO₃⁻ ions were also related to the generation of N₂ and NH₃. The reaction between the N₂H₅⁺ ion and NO₃^● shown in Equation (43) might affect the generation of N₂H₄^●+, NO₃⁻ ion and H⁺ ion. As shown in Equations (15) and (16), the end products generated by the reactions of N₂H₄^●+ were mostly N₂ and NH₃. Otherwise, the N₂H₂ is produced when the N₂H₄^●+ reacts with NO₃⁻ ions, as shown in Equation (42). N₂H₂ is the intermediate of N₂H₄ decomposition and it generates ^●N₂H₃ after the reaction with H^●, as represented in Equation (23). The end reaction products involving ^●N₂H₃ are also N₂ and NH₃, as represented in Equations (17)–(22). The NH₃ generated by the reaction between N₂H₅⁺ and NO₃^● also existed in the form of NH₄⁺ ion in the acidic condition. Therefore, it is concluded that HNO₃ can affect the decomposition of N₂H₄ through the mechanisms listed in Equations (5)–(23) and Equations (42) and (43) by investigating the end product of the expected decomposition paths.

4. Materials and Methods

4.1. Chemicals and Sample Preparation

Hydrazine monohydrate (Junsei, Tokyo, Japan, 98.0%), nitric acid (EMSure, Darmstadt, Germany, 65.0%) and copper (I) chloride (SIGMA-ALDRICH, St. Louis, MO, USA, 97.0%) were used to prepare N₂H₄–Cu⁺–HNO₃ solution in this study. The conditions of each sample is listed in

Table 4. Sample preparation.

Sample Solution	Concentration (mM)
	N2H4	Cu+ Ions	HNO3
pH 1	50	0.5	144.7
pH 3 (without Cu+ ion)	50	-	50.8
pH 3 (with Cu+ ion)	50	0.5	50.8
pH 5	50	0.5	49.9

. All the solutions contain 50.0 mM of N₂H₄. In order to investigate the effects of Cu⁺ ions on N₂H₄ decomposition, the solutions were prepared according to the presence of 0.5 mM of copper ions. Each sample was adjusted to pH 3 by adding nitric acid. To analyze the effects of HNO₃ on N₂H₄ decomposition, each solution was adjusted to pH 1, 3, and 5 by adding 144.7 mM, 50.8 mM, and 49.9 mM of nitric acid, respectively. All the sample solutions included 0.5 mM of copper ions. The 30 mL of sample solutions were stored in the 50 mL vials. After storing the solution in the vial, the nitrogen purging was conducted for 10 min. during the γ-ray irradiation.

4.2. γ-rradiation

A high-dose γ-ray irradiator (Co-60 source) at the Korea Atomic Energy Research Institute was used for irradiation on the solutions. Quantities of 0, 5, 10, 20, and 40 kGy of absorbed doses were given to each sample to compare the dose effects on the decomposition of N₂H₄. All irradiation experiments were carried out with a dose rate of 10 kGy/hr at room temperature.

4.3. Analysis

The concentration of chemical species of N₂H₄ in the solutions was measured using a UV Spectrometer (DR 5000, Hach Co., Ames, IA, USA). The p-dimethylaminobenzaldehyde method was applied to detect the chemical species of N₂H₄.

5. Conclusions

The radiolysis of N₂H₄ in the N₂H₄–Cu⁺–HNO₃ solution during γ-ray irradiation was verified through the irradiation experiment and the analysis of a chemical species of N₂H₄ concentration. When copper ions were present in the N₂H₄–HNO₃ solution, the N₂H₄ decomposition, via the decomposition of the Cu⁺N₂H₄, complex was promoted by the catalytic reaction of Cu⁺ ions. HNO₃ also accelerated the N₂H₄ decomposition in the N₂H₄–Cu⁺–HNO₃ system through the influence of the H⁺ ion and NO₃⁻ ion. This is because H^● produced by the reaction between H⁺ ion and e_aq⁻ participated in the N₂H₄ decomposition reaction. Owing to the H⁺ ion effect, the N₂H₄ decomposition during irradiation was raised when the pH was decreased. NO₃⁻ ion and NO₃^● led to an increase in the N₂H₄ decomposition through the reaction with N₂H₄^●+ or the reaction with N₂H₅⁺. These additional paths, due to the existence of the Cu⁺ and NO₃⁻ ions, improved the N₂H₄ decomposition under irradiation condition. These findings can be applied, in accordance with the characteristics of radiolysis, to define the conditions of N₂H₄ concentration during chemical decontamination processes.