2.1 Reagent

AHA was supplied by Aldrich Chemical Company, and its purity was 98%. Two kinds of standard gas mixtures were supplied by Shanghai Institute of Measurement and Testing Technology: one was composed of 2.50% hydrogen, 1.00% carbon monoxide, 1.00% carbon dioxide, 0.30% methane, 0.01% ethane, 0.01% ethene, and 95.18% nitrogen; the other contained 3.00% nitrous oxide and 97.00% nitrogen.

2.2 Main equipment and accessories

A 3.7 × 10¹⁵ Bq ⁶⁰Co-γ ray irradiator was supplied by Shanghai Institute of Applied Physics, Chinese Academy of Sciences. A T90 UV-VIS spectrophotometer was supplied by Beijing Purkinje General Instrument Co. Ltd. An ion chromatograph (research-style) was obtained from Switzerland Metrohm Co. Ltd. A GC900A gas chromatograph and a packed column with 5 Å molsieve (2 m × 3 mm) were supplied by Shanghai Ke Chuang Chromatograph Instruments Co. Ltd. A capillary column with aluminum oxide (50 m × 0.53 mm) was supplied by Lanzhou Institute of Chemistry and Physics, Chinese Academy of Sciences.

2.3 Sample preparation, irradiation, pretreatment, and analysis

0.2 mol·L^-1 AHA solutions containing different concentrations of HNO₃ were prepared as follows. A certain amount of AHA was put in a beaker, which was then placed in an ice-water bath. A certain amount of 0.5 mol·L^-1 HNO₃ was slowly dropped into the beaker with stirring, and then, a certain amount of slightly diluted HNO₃ was added. The solution was transferred to a 100-mL measuring flask to a constant volume. Thus, solutions in which the AHA concentration was 0.2 mol·L^-1 and the HNO₃ concentration was 0.2, 0.5, 1.0, and 2.0 mol·L^-1 were prepared. Four milliliters of each solution was placed in 7-mL glass vials, which were sealed with a sealing machine. The samples were irradiated by ⁶⁰Co γ-ray to 5, 10, 15, 20, and 25 kGy, and the absorbed doses were monitored by dichromate dosimeters. Control samples were used for comparison of the radiation effect on the loss of AHA by hydrolysis as well as on the amount of radiolytic product. The gases evolved from the irradiated AHA solutions were tested by gas chromatography for the analysis of H₂, N₂ O, CH₄, and C₂ H₆. The irradiated AHA solutions and control samples were neutralized by KOH solution and then diluted to the suitable concentrations. Finally, these pretreated samples were tested by ultraviolet-visible spectrophotometry and ion chromatography for the analysis of AHA, HCOOH, and HNO₂.

2.4 Analysis of AHA and its radiolytic products

Quantitative analysis of AHA was performed by ultraviolet-visible spectrophotometry. 2.7, 5.3, 8.0, and 10.7 m mol L^-1 standard AHA solutions were prepared using high-purity water. 1.00 milliliter of each solution was placed in a 10 mL measuring flask, to which 3.00 mL FeCl₃ solution (10 wt %) and 4.00 mL CCl₃ COOH solution (2.5 wt %) were separately added. Then, water was added to constant volume, and the solutions were shaken well. The reference solution was prepared in the same manner as the standard AHA solutions but without AHA. These standard AHA solutions were tested at 502 nm by using an ultraviolet-visible spectrophotometer. The response curve for AHA was obtained from the AHA concentrations and the corresponding absorbance. The irradiated AHA solutions and control samples were neutralized by KOH solutions and diluted to suitable concentrations. These pretreated samples were tested by using the ultraviolet-visible spectrophotometer. From the response curve for AHA and the absorbance of the diluted samples, the residual AHA concentrations in the samples were calculated. H₂, N₂ O, CH₄, and C₂ H₆ were analyzed by gas chromatography. For H₂ analysis, a 5 Å molsieve packed column was used, and the column temperature was 80 ℃; a thermal conductivity detector (TCD) was employed, and its temperature was 110 ℃. The carrier gas was Ar, with a flow rate of 10 mL min^-1. N₂ O analysis was conducted using a packed shincarbon T column and a TCD detector. H₂ was used as the carrier gas, and its flow rate was 25 mL· min^-1. The column temperature was programmed as follows: initial temperature, 100℃; initial isothermal period, 7 min; programmed heating rate, 35 ℃·min^-1_; final temperature, 220 ℃; and final isothermal period, 1 min. The TCD temperature was 120 ℃. CH₄ and C₂ H₆ were analyzed using a PLOT Al₂ O₃ column and a flame ionization detector (FID). The carrier gas was N₂, and its flow rate was 19 mL·min^-1. The column temperature was 40 ℃, and the FID temperature was 110 ℃. CH₃ COOH and HNO₂ were analyzed by ion chromatography with an electric conductivity detector, using a METROSEP A SUPP 5-250 column. The eluent was a solution containing 3.2 mmol·L^-1Na₂ CO₃ and 1.0 mmol·L^-1NaHCO₃, and its flow rate was 0.7 mL·min^-1.

2.5 Formulae used in the paper

The hydrolysis rate of AHA is defined as follows: hydrolysis rate = (C₀ -C_c )/C₀ ×100%, where C₀ is the original AHA concentration and C_c is the AHA concentration in the control sample. The radiolysis rate of AHA is defined by the equation: radiolysis rate=(C_c -C_i )/C_c ×100%, where C_i is the AHA concentration in the irradiated sample. The volume fraction of the gas product is calculated as follows: if the response curve equation of a component is y = ax + b, the X axis is the injected volume of the standard gas mixture, and the Y axis is the corresponding peak area of the component. Then, the volume fraction of the component is calculated by the following formula: volume fraction= (A-b)c/ae, where A is the component peak area in the gas chromatogram of the gas sample, c is the volume fraction of the component in the standard gas mixture, and e is the injected volume of sample gas.

3.1 Radiolysis of AHA in HNO₃ solution

As AHA can complex with Fe(III) in an acidic solution to yield a colored product, it can be analyzed by ultraviolet-visible spectrophotometry [15]. The response curve for AHA is y=114.38x+0.0142 (concentration range: 2.70-10.7 m·mol L^-1), and the correlation coefficient (R²) is 0.9994. The AHA concentration in control samples decreases drastically with increasing HNO₃ concentration. When the HNO₃ concentration is 0, 0.2, 0.5, 1.0, and 2.0 mol·L^-1, the hydrolysis rate of AHA is 8.20%, 23.1%, 41.2%, 66.2%, and 85.9% respectively, indicating that the rate increases with the HNO₃ concentration. This result is consistent with that reported by Taylor [20] and Tkac [10]. In an acidic solution, AHA hydrolyzes to acetic acid and hydroxylamine:

As the time lag among sample preparation, irradiation, and neutralization is very long (about 9h), this observation cannot be used to elucidate the hydrolysis of AHA employed in the advanced Purex process. In the separation of Pu and Np from U by countercurrent liquid-liquid extraction, the residence time is about 30 min; if the separation occurs in the centrifugal contactor, the residence time is much shorter. We have studied the hydrolysis of AHA in HNO₃ at different times [21]. At 0.2 mol·L^-1AHA, with 0.2, 0.5, 1.0, and 2.0 mol·L^-1 HNO₃, the hydrolysis rate of AHA at 0.5h is 2.40, 5.30, 7.60, and 8.40 respectively; thus, the hydrolysis of AHA in 0.2-2.0 mol·L^-1HNO₃ would not be drastic. In addition, fast complexation of AHA with a metal leads to slower hydrolysis and stabilization of AHA in solution [10], so the hydrolysis of AHA will not affect its application in the separation of Pu and Np from U. Fig.1 illustrates the radiolysis rate of AHA as a function of dose.

Fig. 1Full-screen View

Radiolysis rate of AHA as a function of dose

Fig.1 shows that the radiolysis rate of AHA is 6.63%-77.5 % in AHA solution irradiated to 5-25 kGy, and it increases with the dose and HNO₃ concentration. Karraker [19] reported that AHA radiolysis shows weak dependence on both HNO₃ concentration and absorbed dose, and this differs from our experimental results. When the dose is increased fivefold, from 5 to 25 kGy, the radiolysis rate increases 1.3-2.8 times. When the HNO₃ concentration is increased fivefold, from 0.2 to 1.0 mol·L^-1, the radiolysis rates increases 2.3-4.2 times. Thus, the effect of HNO₃ concentration on the radiolysis of AHA is stronger than that of the dose. Water radiolyzes to produce ·OH, e^-1_aq,·H, and so on:

(2)

·OH may react with HNO₃ by H abstraction to form ·NO₃ [22]:

When using 0.2-2.0 mol·L^-1 HNO₃, most of the acid dissociates to form NO₃^-. γ-ray may react directly with HNO₃ and NO₃^- to produce ·NO₃ [23]:

(4)

(5)

·OH and ·NO₃ are oxidative radicals, and they may react with AHA as follows [24]:

Since the concentrations of ·OH and ·NO₃ increase with dose (Eqs.2, 4-5) and the ·NO₃ concentration increases with HNO₃ concentration (Eqs.3-5), the radiolysis rate increases with both dose and HNO₃ concentration.

3.2 Radiolytic product of AHA

HNO₃ ionizes to produce H⁺, and e^-1_aq can react with H⁺ to form H:

(8)

H· may react with AHA to form H₂:

e_aq^- and ·H· may also react with NO₃^- generated from the ionization of HNO₃, as follows [22]:

Two ·NO₂ radicals may react with each other to form N₂ O₄, which can react with water to produce HNO₂:

The C-C bond in the excited AHA molecule may be broken as follows [25]:

(13)

·CH₃ may react with AHA by H abstraction to form CH₄, or two ·CH₃ radicals may also react with each other to produce C₂ H₆:

Two CH₃ CONHO· radicals may react with each other to produce CH₃ CON=O [26]:

(16)

CH₃ CON=O may hydrolyze to form acetic acid and nitroxyl [27-28]:

(17)

Two nitroxyls can react with each other to generate N₂ O:

Thus, the main radiolytic product of AHA in HNO₃ may be H₂, N₂ O, CH₄, C₂ H₆, CH₃ COOH, and HNO₂.

3.2.1 H2 generated by the radiolysis of AHA in HNO3

H₂ was analyzed by gas chromatography using a packed 5 Å molsieve column and a TCD detector [29]. The response curve of H₂ is y=2297.8x+48.87 (volume range: 0.100-1.20 mL), and R² is 1.000. The volume fraction of H₂ as a function of dose is shown in Fig.2.

Fig. 2Full-screen View

Volume fraction of H₂ as a function of dose

Fig.2 shows that the volume fraction of H₂ evolved from the AHA solutions irradiated with a dose of 5-25 kGy is (1.30-11.8) × 10^-3. The H₂ volume fraction increases with the dose but decreases with increased HNO₃ concentration. H₂ is produced by the reaction of H· and AHA (Eq.9). H· is generated from the radiolysis of H₂ O (Eq.2), and it may also be produced by the reaction of H⁺ and e_aq^- (Eq.8). As the concentrations of H· and e_aq^- increase with the dose, the H₂ volume fraction also increases with increasing dose. Upon the addition of HNO₃, NO₃^- can react with ·H and e_aq^- (Eqs.10-11), so that the ·H concentration is reduced; hence, the volume fraction of H₂ decreases with increased HNO₃ concentration.

3.2.2 N2 O generated from the radiolysis of AHA in HNO3

We used gas chromatography with a packed shincarbon T column and a TCD detector to analyze N₂ O. The response curve of N₂ O is y=220.83x–17.67 (volume range: 0.020-3.00 mL), and R² is 0.9933. The volume fraction of N₂ O as a function of dose is shown in Fig.3.

Fig. 3Full-screen View

Volume fraction of N₂ O as a function of dose

Fig.3 shows that the volume fraction of N₂ O produced from AHA solutions irradiated up to a dose of 5-25 kGy is (3.29 -16.2) × 10^-2. The N₂ O volume fraction increases with the dose and with the HNO₃ concentration when the acid concentration is ≤ 0.5 mol·L^-1. The dependence of the N₂ O volume fraction on the HNO₃ concentration is not obvious when the acid concentration is higher than 0.5 mol·L^-1. ·OH, ·NO₃, ·H, and ·CH₃ react with AHA to form CH₃ CONHO· (Eqs. 6-7, 9, 14), and two CH₃ CONHO· radicals react to produce CH₃ CON=O (Eq.16). After hydrolysis, the HNO produced generates N₂ O (Eqs.17-18). Since the concentrations of ·OH, ·NO₃, ·H, and ·CH₃ increase with the dose (Eqs.2-5, 13), the volume fraction of N₂ O also increases with the dose. On the other hand, the higher the HNO₃ concentration, the higher is the concentration of ·NO₃, CH₃ CONHO·, CH₃ CON=O, and HNO (Eqs.3-5, 7, 16-17). N₂ O is produced by the reaction between two moieties of HNO (Eq.18), and thus, the volume fraction of N₂ O increases with HNO₃ concentration. However, as discussed above, the radiolysis rate of AHA increases with HNO₃ concentration, and the reduction of AHA concentration causes a decrease in the N₂ O content (Eqs. 7, 16-17). At higher HNO₃ concentration, The increase in N₂ O content resulting from the increased HNO₃ concentration is balanced by the decrease in N₂ O content due to the reduced AHA concentration; consequently, the volume fraction of N₂ O changes very slightly at high HNO₃ concentrations.

3.3.3 CH4 and C2 H6 generated from radiolysis of AHA in HNO3

CH₄ and C₂ H₆ were analyzed by gas chromatography with a PLOT Al₂ O₃ column and a FID detector [21]. The response curves of CH₄ and C₂ H₆ are y=242.7x+193.8 and y=25.56x-171.3, respectively. The volume range is 2.0 to 200.0 μL, and the corresponding R²values are 0.9998 and 0.9956. The volume fractions of CH₄ and C₂ H₆ evolved from AHA solutions irradiated up to 5-25 kGy are (1.90-47.4) × 10^-5 and (0.200-1.00) × 10^-5, respectively; the volume fraction of CH₄ is much higher than that of C₂ H₆. CH₄ is produced by the reaction of ·CH₃ and AHA (Eq.14), while C₂ H₆ is generated by the reaction of two ·CH₃ radicals (Eq.15). As the concentration of AHA exceeds that of ·CH₃, the volume fraction of CH₄ is much higher than that of C₂ H₆.

Fig.4 shows that the volume fraction of CH₄ increases with the dose and with the HNO3 concentration at HNO3≤1.0 mol·L-1. When the HNO3 concentration increases from 1.0 to 2.0 mol·L^-1, there is no obvious change in the CH₄ volume fraction. CH₄ is generated by the reaction of ·CH3 and AHA (Eq.14). The ·CH3 concentration increases with the dose, so the volume fraction of CH₄ also increases with the dose. ·CH3 may react with HNO₃ as below:

Fig. 4Full-screen View

Volume fraction of CH₄ as a function of dose

$\cdot {\text{CH}}_{\text{3}}{\text{+HNO}}_{\text{3}}\to {\text{CH}}_{\text{4}}\text{+}\cdot {\text{NO}}_{\text{3}}$ (19)

Thus, the volume fraction of CH₄ increases with the HNO₃ concentration. However, the radiolysis rate of AHA also increases with the HNO₃ concentration, as shown above, and the reduction of AHA concentration leads to a decrease in the CH₄ content (Eq.14). At higher HNO₃ concentration, the reduction in CH₄ content caused by the decrease of AHA concentration is balanced by the increase in CH₄ content resulting from the increase of HNO₃ concentration; hence, the volume fraction of CH₄ changes slightly at high HNO₃ concentrations. The volume fractions of N₂ O, H₂, CH₄, and C₂ H₆ have the following relationship: N₂ O ≈ 10 H_{2  .} ≈ 150 CH₄  ≈ 7500 C₂ H₆.

3.3.4 CH3 COOHand HNO2 generated by radiolysis of AHA in HNO3

Under alkaline conditions, CH₃ COOH and HNO₂ can be converted into CH₃ COO^- and NO₂^-, which can be analyzed by ion chromatography. The response curves of CH₃ COOH and HNO₂ are y=872.97x and y=3766.1x, respectively. The concentration range for CH₃ COOH is 0.169-1.70 m·mol L^-1, and that for HNO₂ is 0.0217-0.435 m·mol L^-1. The R² values for the response curves of CH₃ COOH and HNO₂ are 0.9998 and 0.9984, respectively.

Fig.5 shows that the CH₃ COOH concentration is (4.64-19.7) × 10^-2 mol·L^-1 in AHA solutions irradiated up to 5-25 kGy. The CH₃ COOH concentration increases markedly with HNO₃ concentration and slightly with the dose. ·OH, ·NO₃, ·H, and ·CH₃ react with AHA to form CH₃ CONHO· (Eqs.6-7, 9, 14); these radicals react with each other to produce CH₃ CON=O (Eq.16), which in turn hydrolyzes to produce CH₃ COOH (Eq.17). Since the concentration of ·OH, ·NO₃, ·H, and ·CH₃ increases with the dose (Eqs.2-5, 13), the CH₃ COOH concentration also increases with the dose. When the HNO₃ concentration is high, the concentration of ·NO₃, CH₃ CONHO·, and CH₃ CON=O (Eqs.3-5, 7, 16) also increases, as does the concentration of CH₃ COOH (Eq. 17). For this reason, the CH₃ COOH concentration increases with HNO₃ concentration.

Fig. 5Full-screen View

CH₃ COOH concentration as a function of dose

Fig.6 shows that the HNO₂ concentration is (0.321-4.80) × 10^-3 mol·L^-1 in AHA solutions irradiated at a dose of 5-25 kGy; the concentration of HNO₂ increases with increasing concentration of HNO₃. When the HNO₃ concentration is ≤1.0 mol·L^-1, the dependence of the HNO₂ concentration on the dose is not obvious. At a HNO₃ concentration of 2.0 mol·L^-1, the HNO₂ concentration increases with the dose at lower dose levels, reaching the maximum value at 15 kGy, and then decreases with a further increase in the dose but changes only slightly beyond 20 kGy. Some papers [27,29,30] reported that AHA can react with HNO₂ and can scavenge HNO₂ and stabilize Pu(III) and Np(V); thus an additional holding reductant is unnecessary[17]. The above mentioned results show that a significant concentration of HNO₂ exists in the irradiated AHA-HNO₃ solution; this means that AHA cannot effectively destroy HNO₂, so a holding reductant should be used when AHA is applied for the separation of Pu and Np from U.

Fig. 6Full-screen View

HNO₂ concentration as a function of dose