Influence of γ-radiation on the ionic liquid [C₄mim][PF₆] during extraction of strontium ions

Abstract

The preliminary results presented here show that γ-irradiation of ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]) markedly decreases Sr²⁺ partitioning in the crown ether/[C₄mim][PF₆] extraction phase due to the competition between radation-generated H⁺ and Sr²⁺ to interact with the crown ether, however, washing irradiated [C₄mim][PF₆] with water gives a simple way of recycling the ionic liquid.

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Room-temperature ionic liquids (RTILs), such as 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]), have recently received increasing attention as environmentally benign alternatives to volatile organic solvents for the separation of the highly radioactive nuclides, a result of their unique physical and chemical properties.¹ Thus, studies on the extraction of metal ions using RTILs and the effect of radiation on RTILs are becoming a ‘must’ to guarantee their practical applications. A preliminary study on the solvent extraction of strontium nitrate by RTILs in combination with crown ether demonstrated that highly efficient extraction of the strontium ion (Sr²⁺) from water can be achieved using RTILs such as [C₄mim][PF₆], instead of conventional organic solvents.² Likewise, when using crown ether as extractant, the metal ions partitioning from nitric acid media into [C_nmim][PF₆]³ and [C_nmim][NTf₂]⁴ (n = 4, 6, 8 and [NTf₂]⁻ is bis(trifluoromethylsulfonyl)imide), revealed very different extraction action from conventional solvent systems. This initiated an interest in the study of the mechanism of RTIL-based extraction. The solvent extractions of rare earth and actinide metals were also investigated using [C₄mim][PF₆]⁵ and [C₄mim][NTf₂],^5,6 where octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) was used as extractant. The results showed that the use of RTILs greatly enhances the extraction efficiency and selectivity of CMPO for metal ions compared with when n-dodecane was used as the extracting solvent. These works highlight the vast opportunities for RTIL-based extraction systems in the separation of highly radioactive nuclides from waste water. Full realization of this potential, however, requires a comprehensive knowledge of the stability and properties of the RTILs under radiation. The investigation on the radiation stability of hydrophobic RTILs [C₄mim][PF₆] and [C₄mim][NTf₂] showed that only less than 1% of the samples underwent radiolysis even for an absorbed dose exceeding 1200 kGy, suggesting a very high stability of these ILs.⁷Wu and coworkers further investigated in detail γ radiolysis⁸ and laser photolysis⁹ of neat [C₄mim][PF₆]. Recently, we carried out a radiation stability test of [C₄mim][PF₆] in the presence of nitric acid, and the results demonstrated that incorporating nitric acid into the [C₄mim][PF₆] system slightly accelerates the radiolysis of [C₄mim][PF₆] at high doses (550 kGy) but obviously inhibits the radiation-induced darkening of [C₄mim][PF₆].¹⁰ All of the above mentioned studies have dealt with the influence of radiation on RTILs themselves. The radiation effect on RTILs for the extraction of metal ions, which might be rather significant, has never been reported.

In this work, the solvent extraction of Sr²⁺ using irradiated [C₄mim][PF₆] in the presence of dicyclohexyl-18-crown-6 (DCH18C6) was performed. [C₄mim][PF₆] (> 97%) was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China), and purified according to a procedure described in the literature.¹¹ After purification, the water content in the [C₄mim][PF₆] sample was found to be less than 200 ppm, measured by Karl–Fischer titration, and NMR and element analysis also revealed the absence of impurities (< 0.1%) in the [C₄mim][PF₆] sample. DCH18C6 was obtained from TCI Chemical Co. and used after recrystallization from n-heptane.

Irradiation of the [C₄mim][PF₆] was carried out under a nitrogen atmosphere using a ⁶⁰Co source with a total absorbed dose ranging from 50 to 550 kGy. The extraction experiments were conducted by contacting 0.5 ml of irradiated [C₄mim][PF₆] with a 1.0 ml of Sr(NO₃)₂ aqueous solution for about 30 min in a vibrating mixer followed by centrifugal separation, where the concentrations of the DCH18C6 in the ionic liquid and Sr(NO₃)₂ in aqueous solution are 0.1 mol L⁻¹ and 0.01 mol L⁻¹, respectively. For comparison purposes, an identical extraction experiment was also conducted using unirradiated [C₄mim][PF₆]. Sr²⁺ partitioning in [C₄mim][PF₆] was measured by determining the remnant Sr²⁺ in the aqueous solution using an atomic absorption spectrophotometer (AAS). The distribution ratios were calculated as D_Sr = 2(C_i−C_f)/C_f, and the extraction efficiencies were calculated as E_Sr = (C_i−C_f)/C_i, where C_i and C_f designate the initial and final concentrations of Sr²⁺ in the aqueous phase, respectively.

Fig. 1 shows the experimental results of Sr²⁺ extraction. As can be seen, a high D_Sr of 30 for unirradiated [C₄mim][PF₆] containing DCH18C6 was obtained under the conditions that give negligible extraction with conventional organic solvents,² suggesting that the [C₄mim][PF₆]-based extraction system is indeed an effective medium for liquid/liquid extraction. However, a significant decline of the D_Sr was observed when [C₄mim][PF₆] was irradiated, and the decline enhanced with increasing dose (Fig. 1). After being irradiated at a dose of 550 kGy, the D_Sr of [C₄mim][PF₆] decreased to 11, a value 3 times lower than that of unirradiated [C₄mim][PF₆], while the E_Sr decreased from 93.8% to 84.1%. Obviously, the decline of D_Sr and E_Sr is related to the radiolysis of [C₄mim][PF₆].

Fig. 1 Influence of dose on Sr²⁺ extraction from aqueous solution by irradiated [C₄mim][PF₆] in combination with DCH18C6.

The radiolysis of [C₄mim][PF₆] has been reported in the literature.^7,8 The results suggested that the acids were formed during irradiation of [C₄mim][PF₆]. It was thought that the formation of acid during irradiation was the main factor that influenced the extraction of Sr²⁺. In this work, ¹H, ³¹P and ¹⁹F NMR spectra of [C₄mim][PF₆] before and after irradiation were compared. All the chemical shifts in the ¹H NMR spectra were assigned to appropriate H atoms (Fig. 2). The comparison showed no discernible changes in any of the NMR spectra, even those irradiated at a dose of 550 kGy, indicating that the amount of nonvolatile radiolysis product does not exceed 0.5% in irradiated [C₄mim][PF₆]. This is consistent with reports by Wuet al.,⁸ who observed by NMR that 0.5% of [C₄mim][PF₆] underwent radiolysis after 400 kGy irradiation, and Berthon et al.,⁷ who found by electrospray ionization mass spectrometry and NMR analysis the presence of nonvolatile radiolysis products of [C₄mim][PF₆] at concentrations below 1 mol% for a dose exceeding 1200 kGy. However, as seen from Fig. 2, when a little water was added as probe, the peak of water broadened and shifted towards low field in irradiated [C₄mim][PF₆] compared with that in unirradiated [C₄mim][PF₆]. Such an observation is an indication of the formation of acid products during the irradiation of [C₄mim][PF₆] since the addition of nitric acid in unirradiated [C₄mim][PF₆] leads to similar broadening and shifting of the water signal.

Fig. 2 NMR spectra of [C₄mim][PF₆] before (1) and after (2) γ-irradiation at 550 kGy under a nitrogen atmosphere, deuterated DMSO was used as solvent in the measurements.

FTIR spectroscopic probe analysis provided additional evidence for the formation of acid during irradiation of [C₄mim][PF₆]. Yang and Kou¹² reported that pyridine can be used as a molecular probe to measure the Brønsted acidities of ILs of the type [C₄mim]Cl/MCl_x (MCl_x designates metal chlorides) by monitoring the appearance of IR absorption bands at around 1540 cm⁻¹. According to this method, the pyridine probe was added into unirradiated and irradiated [C₄mim][PF₆], respectively, and the FTIR spectra were recorded immediately. As shown in Fig. 3, a distinct absorption band at 1522 cm⁻¹ for irradiated [C₄mim][PF₆] was observed while unirradiated [C₄mim][PF₆] showed no discernible absorption at the same wavenumber. The result further proved that the irradiation of [C₄mim][PF₆] led to the formation of acid. The difference of 1540 cm⁻¹ in Yang’s work and 1522 cm⁻¹ in our work is probably attributed to the different variety of the ionic liquids. What is more, washing irradiated [C₄mim][PF₆] with deionized water yielded an aqueous phase at pH ≈ 2–3 seems to be direct evidence that the acid was formed during the irradiation of [C₄mim][PF₆]. The acid is probably related to HF, formed as a result of PF₆⁻degradation, because [C₄mim]F·H₂O has been identified crystallographically as a hydrolysis product of [C₄mim][PF₆].¹³

Fig. 3 FTIR spectra of [C₄mim][PF₆] before (1) and after (2) γ-irradiation at 550 kGy with pyridine as probe, pyridine : RTIL= 1 : 5 by volume.

The studies on solvent extraction of Sr²⁺ from nitric acid media by DCH18C6 using [C_nmim][PF₆]³ and [C_nmim][NTf₂]⁴ (n = 4, 6, and 8) demonstrated that the efficiency of Sr²⁺ partitioning decreased as the acidity of the aqueous solution increased. Similarly, the irradiation of [C₄mim][PF₆] induced the formation of acid, and the acid accumulated with increasing dose, leading to the raising of the acidity of the aqueous solution when the extraction experiments were conducted. As the result, the Sr²⁺ partitioning in [C₄mim][PF₆] decreased. To prove this, a comparative experiment was performed, in which the [C₄mim][PF₆] irradiated at 550 kGy was washed several times with deionized water before extraction, and the removal of H⁺ was confirmed by making sure that a neutral upper aqueous phase was obtained. Expectably, the Sr²⁺ partitioning in [C₄mim][PF₆] recovered after thoroughly washing the irradiated [C₄mim][PF₆] with water. The high D_Sr of 27 and high E_Sr of 93% were regained as shown in Fig. 1. Hence, we unambiguously establish that H⁺ is responsible for the decrease of Sr²⁺ partitioning in irradiated [C₄mim][PF₆].

As is well known, Sr²⁺ partitioning from aqueous solution into RTILs involves a cation-exchange mode, which is very different from conventional solvents such as n-alkanol.⁴ Here the cationic constituent of RTILs is C₄mim⁺, and a related cation-exchange mode of Sr·CE²⁺ can be depicted as follows:

(Sr·CE)²⁺ + 2C₄mim⁺_org⇌ (Sr·CE)²⁺_org + 2C₄mim⁺ (1)

where CE represents crown ether, i.e. DCH18C6, and the subscript designates the organic phase. When H⁺ coexists in the extraction system, hydrates may influence the extraction of Sr²⁺ by interacting with the crown ether and then moving into the organic phase by a similar cation exchange:

(H₃O·CE)⁺ + C₄mim⁺_org⇌ (H₃O·CE)⁺_org + C₄mim⁺ (2)

the formation of the (H₃O·CE)⁺ adduct in ionic liquids has been identified by IR spectroscopy during acid extraction using crown ether.¹⁴

Based on the above discussions, we confirm that the influence of γ-radiation on [C₄mim][PF₆] for the extraction of Sr²⁺ will be diminished by increasing the acidity of the aqueous phase, because the high acidity of the aqueous phase reduces the difference in the amount of H⁺ formed during the irradiation of [C₄mim][PF₆] at different doses. Investigations on this aspect are still underway in our laboratory.

In summary, we report here the first study of solvent extraction of Sr²⁺ using irradiated [C₄mim][PF₆] in combination with DCH18C6. The results show that the irradiation of [C₄mim][PF₆] has a significant influence on the DCH18C6/[C₄mim][PF₆] system for the extraction of Sr²⁺. The Sr²⁺ partitioning in irradiated [C₄mim][PF₆] decreases as the absorption dose increases. Both NMR and FTIR spectroscopic probe analysis revealed the formation of acid during the irradiation of [C₄mim][PF₆]. The decrease of Sr²⁺ partitioning is attributed to the competition between H⁺ and Sr²⁺ to interact with DCH18C6, shown by the recovery of extraction of Sr²⁺ into [C₄mim][PF₆] after washing the irradiated [C₄mim][PF₆] with water. This work is directed primarily toward acquiring a further assessment of the feasibility of RTILs as alternative media for the separation of highly radioactive nuclides from waste water.

Notes and references

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