I. INTRODUCTION

The room-temperature ionic liquids (RTILs) are new-fashioned solvents and have too many attractive properties, especially in chemical stability and low vapour pressure, compared with traditional solvents. RTILs are considered promising solvents for the extraction of radioactive isotopes from spent nuclear fuel (SNF) [1-6]. This is an area of great significance to the nuclear industry, in which traditional volatile organic solvent extraction is currently used in SNF reprocessing and recycling. The solvent extractions of actinide metals were investigated using [C₄mim][PF₆], which showed that the use of [C₄mim][PF₆] greatly enhances metal ion partitioning, compared to using a traditional solvent [7]. A highly efficient extraction of Sr²⁺ from an aqueous solution can be achieved using [C₄mim][PF₆] in combination with crown ether [8, 9]. Sr²⁺ partitioning in the crown ether, combined with the [C₄mim][PF₆] extraction phase, decreased obviously after γ-irradiation [10]. The decline of the distribution ratio was attributed to the inhibition of the cation exchange mechanism and competition by radiation-formed hydrogen ions. However, γ-irradiation of [C₄mim][PF₆] showed no discernible influence on Sr²⁺ extraction from a nitric acid solution with high acidity [11]. This research show the feasibility of [C₄mim][PF₆] as an extracting solvent for the reprocessing of SNF.

In an extraction process involving SNF, there will be a requirement for extracting agents and solvents to be robust to high radiation doses [12-14]. Therefore, studies on the radiation effects of RTILs are of great importance before their practical application in SNF reprocessing and recycling. Micro-FTIR, ¹H NMR, and ¹⁹F NMR spectra of [C₄mim][PF₆] irradiated at 550 kGy suggested that no discernible changes were found in these spectra [14, 15]. Qi et al. reported that the radiolysis of [C₄mim][PF₆] led to an increase in UV-vis absorbance and a decrease in fluorescence intensity [12]. Radical generated species and different degradation pathways were proposed for imidazolium ionic liquids under electron irradiation and were confirmed by electron paramagnetic resonance [16-18]. The cation radical C₄mim⁺·, neutral radical C₄mim., and other potential species were observed by pulse radiolysis during the irradiation of dry [C₄mim][PF₆] [19]. However, as of now there are still few works focusing on the identification of radiolytic products of [C₄mim][PF₆].

In this work, the aim of the present study is to report the identification of water-soluble radiolytic products of [C₄mim][PF₆] by using Micro-FTIR, ¹⁹F NMR, and ³¹P NMR. ¹⁹F NMR was employed to provide a quantitative study for the radiolytic products of [C₄mim][PF₆] and their radiation chemical yields were obtained.

II. MATERIALS AND METHODS

III. RESULTS AND DISCUSSION

Micro-FTIR and ¹H NMR spectra of [C₄mim][PF₆] suggested that no discernible changes were found, even after irradiation at 550 kGy [14], which was an indication that the radiolytic species were very small in quantity. In order to separate water-soluble radiolytic products from the organic phase, the irradiated [C₄mim][PF₆] was washed by D₂O and then the aqueous phase (A-phase) was analysed by various spectroscopic methods. As shown in Fig. 1, the absorption of A-phase shows some changes, compared to that of the unirradiated sample. The absorption of the unirradiated sample is attributed to [C₄mim][PF₆] ionic liquid. For A-phase, an absorption band at 1521 cm^-1 was attributed to a Lewis acid, which has been identified using pyridine as a molecular probe [10]. The absorption bands at 1299 cm^-1 and 1140 cm^-1 corresponded to the vibration of the O=P^-O bonds [20, 21], which indicate that the P^-F bond was broken and an O=P^-O bond was formed during the irradiation of [C₄mim][PF₆]. The change of absorption band at 841 cm^-1 indicated the formation of a PF₂ group [21].

Fig. 1.Full-screen View

Micro-FTIR spectra of unirradiated and irradiated [C₄mim][PF₆] (The aqueous phase from the washing of unirradiated sample (a) and γ-irradiated sample (b)).

¹⁹F NMR and ³¹P NMR chemical shifts are highly sensitive to fluorine-containing and phosphorus-containing compounds, respectively. The A-phase was also analysed by ¹⁹F NMR and ³¹P NMR. As shown in Fig. 2, the ¹⁹F NMR of the unirradiated sample shows a duplicate peak at -71.70 ppm (J_F-P = 706.5 Hz) assigning to PF₆^-. The A-phase shows two new peaks at -82.60 (duplicate, J_F-P = 960.8 Hz) and -129.58 ppm, which were ascribed to the signals of the radiolytic products. The chemical shifts at -129.58 ppm can be assigned to the signal of HF, which has been identified as a main radiolytic product of [C₄mim][NTf₂] [22, 23]. HF fumes were also detected during the irradiation of [C₄mim][PF₆] [12], thus, HF was one of main radiolytic products of [C₄mim][PF₆].

Fig. 2.Full-screen View

¹⁹F NMR spectra of unirradiated and irradiated [C₄mim][PF₆] (The aqueous phase from the washing of unirradiated sample (a) and γ-irradiated sample (b)).

As shown in Fig. 3, the ³¹P NMR of an unirradiated sample shows a heptet at -145.01 ppm (J_F-P = 707.0 Hz) assigning to PF₆^-. A new triplet at -14.82 ppm is observed in the A-phase. Combined with the results of ¹⁹F NMR, the triplet at -14.82 ppm (J_F-P = 959.7 Hz) has the same coupling constants as the peak at -82.60 ppm in ¹⁹F NMR (J_F-P = 960.8 Hz), which indicates that the chemical structure of the radiolytic product contains a PF₂ group (PF₂-Gr). Lu et al. pointed out that [OP(O)F₂]^- was one of the hydrolysis products of [C₄mim][PF₆] [24]. HOP(O)F₂ is a possible radiolytic product when [C₄mim][PF₆] is irradiated by γ-radiation. A HOP(O)F₂ standard compound was obtained for further identification. The HOP(O)F₂ shows a duplicate peak at -82.64 ppm (J_F-P = 961.4 Hz) in ¹⁹F NMR and a triplet at -14.85 ppm (J_F-P = 961.4 Hz) in ³¹P NMR, so we can conclude that the radiolytic product PF₂-Gr can be definitely confirmed as HOP(O)F₂.

Fig. 3.Full-screen View

³¹P NMR spectra of unirradiated and irradiated [C₄mim][PF₆] (The aqueous phase from the washing of unirradiated sample (a) and γ-irradiated sample (b)).

According to the above results, HF and HOP(O)F₂ are confirmed as the main radiolytic products of [C₄mim][PF₆] under γ-irradiation. In order to realize a quantitative analysis of the radiolytic products of [C₄mim][PF₆] after γ-irradiation, herein, the irradiated sample was washed 4 times with D₂O before analysis to ensure that acidic radiolytic products were totally collected. After 4 washes, a neutral upper aqueous phase was obtained and analysed by ¹⁹F NMR. CF₃COONa was dissolved in deuteroxide (50 mmol L^-1) and used as an internal standard compound for the quantitative analysis. Compared to the unirradiated sample, the amounts of main radiolytic products increased obviously with dose increases (Fig. 4). The radiation chemical yields of radiolytic products (G = G(F^-) + G(HOP(O)F₂) = 0.14 μmol/J + 0.053 μmol/J = 0.19 μmol/J) are close to the radiation chemical yields of the anion (0.18 μmol/J) of [C₄mim][PF₆] determined by ¹⁹F NMR [25]. Compared to the radiation chemical yields of acidic radiolytic products of [C₄mim][NTf₂] (Table 1), the radiation stability of [C₄mim][PF₆] is better than that of [C₄mim][NTf₂] and is influenced by the chemical structure of the anion. HF and HOP(O)F₂ were identified as the main radiolytic products of PF₆^- of [C₄mim][PF₆], and their overall contents were less than 0.7% for [C₄mim][PF₆], even when irradiated at 500 kGy.

Fig. 4.Full-screen View

The relationship between the concentration of radiolytic products and doses.

IV. CONCLUSION

The trace water-soluble acidic radiolytic products of [C₄mim][PF₆] were confirmed by using various spectroscopic methods, including Micro-FTIR, ¹⁹F NMR, and ³¹P NMR. The main radiolytic products (HF and HOP(O)F₂) of [C₄mim][PF₆] were identified and their amount was quantified by ¹⁹F NMR. The overall concentration of non-volatile acidic radiolysis products was less than 0.7% for [C₄mim][PF₆], even when irradiated at 500 kGy, which shows that [C₄mim][PF₆] has excellent radiation stability and is promising for the application of extractions in nuclear fuel reprocessing.