γ-ray induced radiolysis of [C2mim][NTf2] and its effects on Dy3+ extraction

Special Section on the Second China-Japan Academic Symposium on Nuclear Fuel Cycle (ASNFC-2013)

γ-ray induced radiolysis of [C₂mim][NTf₂] and its effects on Dy³⁺ extraction

YUAN Wei-Jin，

AO Yin-Yong，

ZHAO Long，

WEI Yue-Zhou，

ZHAI Mao-Lin ，

LI Jiu-Qiang

Nuclear Science and Techniques

Vol.26, No.1

Article number S10306

Published in print 20 Feb 2015

Available online 20 Dec 2014

DOI：10.13538/j.1001-8042/nst.26.S10306

33400

The water-soluble radiolytic products of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ionic liquid ([C₂mim][NTf₂]) under γ-radiation, such as CF₃SOOH, CF₃SO₂NH₂, HF, and H₂SO₃, were identified by using ¹H NMR, ¹⁹F NMR, and ion chromatography. The extraction behavior of Dy³⁺ using irradiated [C₂mim][NTf₂] in combination with 2,6-di(5,6-diisobutyl-1,2,4-triazin-3-yl)pyridine (isobutyl-BTP) was studied and the abnormal increase of Dy³⁺ partitioning after irradiation is mainly attributed to the precipitation formed between Dy³⁺ and radiolytic products of [C₂mim][NTf₂] (F^- and SO₃^2-). Washing irradiated [C₂mim][NTf₂] with water provides a simple method for ionic liquid recycling.

γ-Radiation[C2mim][NTf2]Isobutyl-BTPRadiolytic productDy3+extraction

I. INTRODUCTION

Minor actinides-lanthanides (MA-Ln) separation is a huge challenge due to their similar chemical properties [1-3]. Bistriazinylpyridines (BTPs), such as heterocyclic nitrogen donor ligands, presents attractive selectivity for MA, compared to Ln that could make them excellent candidates for separating minor actinides [4-6]. Kolarik et al. reported that 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine in kerosene/2-etylhexanol extracted Am³⁺ with a distribution ratio of 23 from an aqueous phase [7]. Trumm et al. reported that a solution of 50 mmol/L BTPs in kerosene/1-octanol extracted Am³⁺ from 1 mmol/L HNO₃ with SF_Am/Eu at ca. 100 [8]. However, the BTPs in combination with a traditional solvent caused a new problem relating to radiation resistance. For example, a solution of 0.005 mmol/L C5-BTBP in cyclohexanone exhibited about an 80% decrease in the distribution ratio of Am³⁺ at 17 kGy [9]. It was also reported that an absorbed dose of 100 kGy resulted in 80% decomposition of BTPs in n-octanol [10]. It was thought that the reaction between BTPs and the radicals of diluents resulted in subsequent degradation, which was mainly responsible for the decrease in metal ion partitioning.

Due to a number of unique properties, such as non-volatility, good solubility, and chemical stability [11-13], room temperature ionic liquids (RTILs) have been highly studied for their potential application in the reprocessing of spent nuclear fuel [14-17]. Dai et al. first reported that the solvation environment offered by RTILs could enhance the extraction efficiency of metal ions from an aqueous solution [18]. Moreover, RTILs have demonstrated a notable radiation resistance under γ-radiation [19-24]. Berthon et al. noted that less than 1% underwent radiolysis when RTILs were exposed to a dose of 1200 kGy [25]. The main radiolytic products of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][NTf₂]) were identified definitely through various spectroscopic methods in our previous work, which greatly improved the radiolysis research on ionic liquids [26, 27]. The overall concentration of non-volatile acidic radiolysis products was less than 1% for [C₄mim][NTf₂], even at 500 kGy. However, [C₂mim][NTf₂] was considered as a better solvent rather than [C₄mim][NTf₂] in the separation of MA-Ln due to its larger dissolving capacity and its liquidity [28, 29]. Accordingly, it is necessary to access the radiation stability of [C₂mim][NTf₂] and extraction ability of extractant in combination with [C₂mim][NTf₂] under γ-radiation.

In this paper, an extraction system consisted of isobutyl-BTP (Fig. 1) as extractant and [C₂mim][NTf₂] as diluents is designed in our lab. The radiation effect on extraction behavior of the diluent [C₂mim][NTf₂] is mainly investigated. Dy³⁺ is a non-redox-active trivalent ion and has similar properties to the trivalent actinides, thus Dy³⁺ is chosen for assessing the extractability and the radiation stability of the isobutyl-BTP/[C₂mim][NTf₂] system.

Fig. 1.

The chemical structure of isobutyl-BTP.

II. EXPERIMENTAL SECTION

A. Materials

[C₂mim][NTf₂] (with a purity > 99%) was purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). No impurities were detected by ¹H NMR spectrometry. The isobutyl-BTP(> 95%) was synthesized according to [30]. All other solvents were analytical-grade reagent and used without further purification.

B. Irradiation

The irradiation of [C₂mim][NTf₂] was carried out in air (298 (4) K) using a ⁶⁰Co source, with an average dose rate of ca. 240 Gy/min (Institute of Applied Chemistry of Peking University). The absorbed dose was traced by a Fricke dosimeter.

C. Extraction of Dy³⁺

The organic phase (0.5 mL) contained 20 mmol/L of isobutyl-BTP dissolved in [C₂mim][NTf₂], and the aqueous phase (0.5 mL) contained 8 mmol/L of Dy³⁺. The extraction experiments were oscillated in a constant temperature incubator shaker, which maintained a thermo-stated water bath at 25 ℃ with a rotating speed of 120 rpm, then were centrifuged for 2 min to ensure that the two phases were completely separated. After phase separation, the aqueous solution was diluted with deionized water and the concentration of Dy³⁺ in the diluted aqueous solution was measured by the Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICPS-7510, SHIMADZU, JPN). The distribution ratios (D_Dy) were calculated by D_Dy = (C_i - C_f)/C_f and the calculation of extraction efficiencies (E_Dy) was based on D_Dy = (C_i - C_f)/_i, where C_i and C_f designate the initial and final concentrations of Dy³⁺ in the diluted aqueous solution, respectively.

D. Characterization

Micro-FTIR. The Micro Fourier transform infrared spectroscopies (Micro-FTIR) were recorded on a Thermo Scientific Micro Fourier transform infrared spectrometry.

¹H and ¹⁹F NMR. NMR experiments were carried out with a Bruker AV-500. The chemical shift scale was calibrated with tetramethylsilane at 0 ppm and NTf₂^- at -78.87 ppm for ¹H NMR and ¹⁹F NMR, respectively.

Ion chromatography analysis. The water-soluble products from the irradiated samples were analyzed using a MIC ion chromatography (IC) System (Metrohm Swiss). A Metrosep A SUPP 5-250 column (4 mm×250 mm) was used to provide a quantitative study on the radiolytic products of [C₂mim][NTf₂] ionic liquids. The 819 IC conductivity detector was applied and the injection volume was set at 10 μL. The eluent was 3.2 mmol/L Na₂CO₃/1.0 mmol/L NaHCO₃ solution at a constant flow rate of 0.7 mL/min. The operating back pressure was 11.6 MPa.

XPS analysis. The X-ray photoelectron spectra (XPS) of the samples were recorded by an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al K_α radiation and low energy electron flooding for charge compensation.

III. RESULTS AND DISCUSSION

A. Identification of water-soluble radiolytic products of [C₂mim][NTf₂] under γ-radiation

The water-washed sample of irradiated [C₂mim][NTf₂] was analyzed by Micro-FTIR based on previous work [26]. As illustrated in Fig. 2, the Micro-FTIR spectrum of irradiated sample changes obviously in comparison with that of unirradiated sample. The absorption is band at 1136 cm^-1 (1350 cm^-1), corresponding to the vibration of the C^-F bonds (S=O) of NTf₂^-, dropping obviously after γ-radiation. This indicated that C^-F and S=O bonds were broken during irradiation. In addition, OH groups (NH₂ groups) are observed at 1655 cm^-1 and 3450 cm^-1 (960 cm^-1) in Fig. 2b. These results indicate that the radiolytic products containing OH and NH₂ groups were formed after irradiation. These results are similar to that of [C₄mim][NTf₂] during irradiation [26] because [C₂mim][NTf₂] has analogous anion ions (NTf₂^-) with [C₄mim][NTf₂]. Hence, the observed results in Micro-FTIR are mainly attributed to the radiolysis of [NTf₂] anions.

Fig. 2.

(Color online) Micro-FTIR spectra of [C₂mim][NTf₂] in D₂O before (a) and after irradiation at 500 kGy (b).

¹H NMR and ¹⁹F NMR were employed to identify the radiolytic products of [C₂mim][NTf₂] after irradiation. As shown in Fig. 3, no discernible changes was observed in ¹H NMR at 500 kGy, suggesting that the nonvolatile radiolysis products of [C₂mim][NTf₂] do not exceed 1%. However, the peak of water broadened and shifted toward the low field after irradiation. This is consistent with information reported by Yuan et al., who found that the change of water peak shape was attributed to the acidic radiolytic products [28]. Therefore, acidic radiolytic products were formed during the irradiation of [C₂mim][NTf₂].

Fig. 3.

(Color online) ¹H NMR spectra of [C₂mim][NTf₂] before (a) and after irradiation at 500 kGy (b).

The ¹⁹F NMR spectra of [C₂mim][NTf₂] before and after irradiation are shown in Fig. 4. A single peak at -78.78 ppm, which is assigned to NTf₂^- was observed. Similar to the radiolysis of [C₄mim][NTf₂] [26], several fluorine-containing compounds in irradiated [C₂mim][NTf₂] have been identified as CF₃SOONH₂ (-79.33 ppm), CF₃SOOH (-87.08 ppm), HF (-164.55 ppm), and SiF₆^2- (-129.56 ppm) [26]. However, a new radiolytic product, which was not observed in irradiated [C₄mim][NTf₂], is shown at -76.75 ppm. The difference in alkyl chain between C₂mim⁺ and C₄mim⁺ leads to the formation of different radiolytic products.

Fig. 4.

(Color online) ¹⁹F NMR spectra of [C₂mim][NTf₂] in D₂O before (a) and after irradiation at 500 kGy (b).

B. A quantitative analysis of trace water-soluble radiolytic products of [C₂mim][NTf₂] under γ-irradiation

Ion chromatography was employed to provide a quantitative analysis of these water-soluble radiolytic products (HF, CF₃SOOH, and H₂SO₃) and the experimental details are given in a previous paper [26]. As indicated in Fig. 5, the amounts of radiolytic products increased obviously with the increase of each dose. For instance, the concentration of F^- increased from 0.030 mmol/L at 100 kGy to 0.087 mmol/L at 500 kGy. The G value of HF, CF₃SOOH, and H₂SO₃ was calculated at 0.22 μmol/L, 0.035 μmol/L, and 0.029 μmol/L, respectively. Compared with the G value of acidic radiolytic products of [C₄mim][NTf₂] (Table 1), the radiolysis of [C₂mim][NTf₂] is close to that of [C₄mim][NTf₂]. The quantitative study using ion chromatography indicated that the overall concentration of nonvolatile acidic radiolysis products was less than 1% for [C₂mim][NTf₂] even at 500 kGy, in which the results show that [C₂mim][NTf₂] still has excellent radiation stability under γ-irradiation.

Fig. 5.

(Color online) The relationship between the concentration of main radiolytic products and doses.

Radiation chemical yields of acidic radiolytic products of [C₂mim][NTf₂] and [C₄mim][NTf₂]

RTILs	G(F^-) (μmol/J)	G(CF₃SOO^-) (μmol/J)	G(SO₃^2-) (μmol/J)
[C₂mim][NTf₂]	0.22	0.035	0.029
[C₄mim][NTf₂] ^a	0.20	0.010	0.031

a Data was obtained from Ref. [26].

C. The influence of acidic radiolytic products of [C₂mim][NTf₂] on the extraction of Dy³⁺

The influence of dose on Dy³⁺ extraction was shown in Fig. 6. An abnormal increase of Dy³⁺ partitioning is obtained when irradiated [C₂mim][NTf₂] is used as extracting solvent. Some water-insoluble compounds were observed at the interface between irradiated [C₂mim][NTf₂] and the aqueous solution. After the irradiated [C₂mim][NTf₂] was washed by deionized water 3 times, Dy³⁺ partitioning recovered to the unirradiated level. These results suggest that the abnormal increase of D_Dy in irradiated [C₂mim][NTf₂] is ascribed to the influence of water-soluble radiolytic products. Based on our previous work [27], Dy³⁺ is precipitated with the radiolytic products generated from ionic liquid (F, SO₃^2-), resulting in the increase of Dy³⁺ partitioning in irradiated [C₂mim][NTf₂].

Fig. 6:

Influence of dose on Dy³⁺ extraction from aqueous solution by irradiated [C₂mim][NTf₂] in combination with isobutyl-BTP. *The irradiated sample was washed by water for 3 times before extraction experiment.

After centrifuging and washing and drying, the white sediment was obtained and analyzed by XPS. The binding energies were calibrated using the C1s hydrocarbon peak at 284.80 eV. The composition of sediment was determined to consist of F, O, S and Dy elements according to the XPS record (Fig. 7), indicating the main components of the sediment are DyF₃ and Dy₂(SO₃)₃. Similar results have been reported in the α-radiolysis of [C₄mim][NTf₂] ionic liquid irradiated with the helium ion beam [27]. Consequently, the increase of Dy³⁺ partitioning in irradiated [C₂mim][NTf₂] is due to the precipitation between the water-soluble radiolytic products of [C₂mim][NTf₂] with Dy³⁺.

Fig. 7.

(Color online) XPS spectra of the precipitate (a), DyF₃ (b), and Dy₂(SO₃)₃ (c).

IV. CONCLUSION

The main water-soluble radiolytic products of [C₂mim][NTf₂] (CF₃SOOH, CF₃SO₂NH₂, HF and H₂SO₃) under γ-irradiation were systematically investigated by using micro-FTIR, ¹H NMR, ¹⁹F NMR, and ion chromatography. The radiolytic products of [C₂mim][NTf₂] are similar to that of [C₄mim][NTf₂], because the two ionic liquids have a similar chemical structure and the same anion ions. The radiolytic behavior of [C₂mim][NTf₂] changes slightly compared to [C₄mim][NTf₂] as the difference of the alkyl chain length. The extracting behavior of Dy³⁺ using irradiated [C₂mim][NTf₂] in combination with isobutyl-BTP showed an abnormal increase of Dy³⁺ partitioning due to the precipitation between the acidic radiolytic products of the anion with Dy³⁺. The water-washing is a very effective and easy method to avoid the influence of radiolytic products of [C₂mim][NTf₂]. This work provides an assessment about the feasibility of [C₂mim][NTf₂] as alternative medium for the separations of MA-Ln from spent nuclear fuel.

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