Th (IV) and U(VI) removal by TODGA in ionic liquids: Extraction behavior and mechanism, and radiation effect

NUCLEAR CHEMISTRY, RADIOCHEMISTRY, NUCLEAR MEDICINE

Th (IV) and U(VI) removal by TODGA in ionic liquids: Extraction behavior and mechanism, and radiation effect

Zhen Dong，

Wei-Jin Yuan，

Chao Liu，

Long Zhao，

Chao Zhao，

Fang-Dong Tang，

Lin-Feng He

Nuclear Science and Techniques

Vol.28, No.5

Article number 62

Published in print 01 May 2017

Available online 28 Mar 2017

DOI：10.1007/s41365-017-0214-y

41300

Extraction behavior of thorium(IV) and uranium(VI) from nitric acid (HNO₃) was studied using N,N,N’,N’-Tetraoctyldiglycolamide (TODGA) as extractant in different ionic liquids, and isooctane as comparison. Slope analyses with varying HNO₃ concentrations and diluents revealed the extraction mechanism. With increasing length of alkyl chain and HNO₃ concentration, the extraction mechanism of TODGA/IL system changed from cation exchange to neutral complex or/and anion exchange, and the molar ratio between TODGA and metal ions varied gradually from 2:1 to 1:1 for Th(IV) and 3:1 to 1:1 for U(VI). The kinetics and thermodynamic studies of Th(IV) and U(VI) by the best TODGA/[C₂mim][NTf₂] system showed that the exaction reached equilibrium within 2 h and extraction reactions were endothermic. Compared to TODGA/isooctane system, TODGA/[C₂mim][NTf₂] system presented higher radiation stability under γ-irradiation. Therefore, it would have a promising application in spent fuel reprocessing.

ExtractionTODGAIonic liquidsThorium(IV) and Uranium(VI)Radiation effect

1. Introduction

Nuclear energy is playing an important role as energy demands increase rapidly. Uranium, as the major commercial nuclear fuel, is a crucial nuclide in spent fuel reprocessing ^[1]. Thorium (Th) is three times more abundant than U and can be transformed to ²³³U by neutron irradiation. Thorium-based molten salt reactor is one of the six potential candidates for Generation-IV nuclear energy system ^[2,3]. Thus, it is important for the sustainable development of nuclear energy to recycle U and Th from spent fuel reprocessing.

Currently, solvent extraction is widely applied in separation and recovery of U and Th. Different extractants such as malonamide, tri-n-butyl phosphate (TBP), octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phoshine oxide (CMPO) ^[4] and diglycolamide (DGA) have been employed for partitioning of actinide ions. However, malonamide-based TBP are easy to form third-phase during extraction ^[5,6], while CMPO produces large amount of secondary waste ^[7,8]. Comparatively, N,N,N’,N’-tetraoctyldiglycolamide (TODGA), as representative of DGA, is a promising candidate extractant for recovery of actinide ions ^[9,10], in terms of its good solubility in solvents, good radiolytic stability ^[11,12] and extractability towards actinides from highly acidic solutions ^[13]. In addition, TODGA composes of C, H, O, and N atoms (the CHON principle), which makes it fully combustible to gaseous products, minimizing the generation of secondary waste ^[14]. However, TODGA is usually used in organic diluents, which resulting in health hazard and inflammability risk ^[15]. Therefore, it is essential to develop alternative diluents to overcome the drawbacks.

Ionic liquids (ILs) has received increasing attention as alternative diluents in nuclear fuel cycle, because of their low vapor press and no flammability, high thermal, chemical and irradiation stability, and high solubility ^[16,17]. Many authors demonstrated that the extraction system using ILs as diluents presented dramatically improved extraction performance for some radionuclides (eg. Sr, Cs, An and Ln) compared to organic solvent as diluents ^[14,18-21]. Therefore, application of IL to recover U and Th has been widely investigated ^[22]. Panja et al. [5] applied TODGA in [C_nmim][PF₆] (n=4, 6, 8) for actinide ions Am(III), Pu(IV) and U(VI), resulting in higher distribution ratios for all the actinides. Shen et al. reported the extraction of Th(IV) ^[15] and U(VI⁾ ^[23] from a HNO₃ solution into various [C_nmim][PF₆] (n=4,6,8) using DGA-based extractants, N,N,N’,N’-tetrabutyl-3-oxapentanediamide (TBDA) and N,N’-dimethyl-N,N’-dibutyl-3-oxapentanediamide (MBDA). Besides, extraction of Th(IV) and U(VI) from HNO₃ by TODGA in [C_nmim] [PF₆] (n=6, 8) was investigated ^[24]. Shen extracted Th(IV) using 2-thenoyltrifluoroacetone (HTTA) in [C_nmim][NTf₂] (n=2, 4, 6, 8) ^[25]. Although ILs are proved to be an alternative green solvent, its high cost and solubility limit its applications sometimes. The disadvantages can be solved with the further development and promotion of ILs. In addition, few investigations were conducted on related techniques for especially large-scale use of ILs. Therefore, studies are needed on extraction technology applying ILs towards radionuclides especially. To our knowledge, little attention has been given to the extraction of Th(IV) and U(VI) using TODGA in NTf₂-based ILs. Moreover, in spent nuclear fuel reprocessing, the extraction systems including extractants and diluents are used together to separate metal ions. Therefore, the radiation stability of extraction system is essential. However, investigations on the radiation effect were mainly focused on pure ILs and extractants themselves ^[26-28], radiation stability of extractants in ILs under irradiation was seldom reported.

The possibility to extract Th(IV) and U(VI) using TODGA as extractant in ionic liquid 1-alkyl-3- methylimidazolium bis(trifluoromethane sulfonyl)imide ([C_nmim][NTf₂]) was investigated in this work. The effect of the solvent type, contact time, HNO₃ concentration, extractant concentration and temperature were studied and the extraction mechanisms in various diluents were discussed in detail. Besides, the radiation effect of TODGA in ILs and isooctane was investigated. It is expected to offer a new extraction system for separating and recycling of Th(IV) and U(VI) in spent fuel reprocessing.

2. Experimental

2.1 Materials

N,N,N’,N’-Tetraoctyldiglycolamide (TODGA) (> 96%) was used as received. [C_nmim][NTf₂] (n=2, 12) (> 99%) were obtained from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). Their structures are shown in Fig. 1. The other organic and inorganic compounds were reagent grade commercial products and used without further purification.

Fig. 1

Molecular structures of [C_nmim][NTf₂] and TODGA.

2.2 Solvent extraction of U(VI) and Th(VI)

The extraction solution was prepared by dissolving TODGA in [C_nmim][NTf₂] (n=2, 12) or isooctane, and the aqueous solution was obtained by dissolving UO₂(NO₃)₂·6H₂O or Th(NO₃)₄·6H₂O in HNO₃ medium. The extraction experiments were carried out with equal volume of two phases (O/A=1) oscillating in a constant temperature incubator shaker. Then the mixtures were centrifuged for 3 min to ensure phase separation. After phase separation, the concentration of U(VI) or Th(VI) in aqueous solution were measured by Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICPS-7510, Shinadzu, Japan). The distribution ratios (D) and extraction efficiencies (E) were calculated as follows:

D = [(C_{i} - C_{f}) / C_{f}] (V_{aq} / V_{o}),

(1)

E = [(C_{i} - C_{f}) / C_{i}] \times 100 %,

(2)

where C_i and C_f are the initial and final concentrations of metal ions in aqueous phase, respectively; and V_aq and V_o are the volume of aqueous and organic phases, respectively.

2.3. Irradiation

Irradiation of TODGA/[C₂mim][NTf₂] and TODGA/isooctane solution were conducted in air at room temperature in a ⁶⁰Co source (Institute of Applied Chemistry of Peking University), where Fricke dosimetry system is used.

3. Results and Discussion

3.1 Effect of diluents

The extraction efficiency of Th(IV) and U(VI) ions with TODGA in the ILs system as function of HNO₃ concentration are shown in Fig.2, together with that in the isooctane. The extraction for Th(IV) and U(VI) ions in TODGA/[C_nmim][NTf₂] was much more efficient than that in TODGA/isooctane system. For TODGA/ [C₂mim][NTf₂] system, the extraction efficiencies of Th(IV) and U(VI) ions were always high, despite their decrease at 1 M HNO₃; while the extraction efficiencies of the TODGA/[C₁₂mim][NTf₂] system and TODGA/isooctane, which were low at low HNO₃ concentrations, increased with HNO₃ concentration. A similar phenomenon was reported by Cocalia et al. ^[29]. This implied similar extraction equilibrium for both diluent systems. Therefore, isooctane and [C₁₂mim][NTf₂] were used as diluents at 3 M HNO₃ in further experiments, due to their low D_Th and D_U at low HNO₃ concentrations.

Fig. 2

Influence of HNO₃ concentration on E_Th (A) and E_U (B) in different extraction systems. (Extraction conditions: TODGA =10 mM; U(VI)=Th(IV)=1 mM; T=25°C; t=120 min)

3.2 Effect of concentration of TODGA

The extraction efficiencies for Th(IV) and U(VI) as a function of TODGA concentration in [C_nmim][NTf₂] or isooctane are shown in Fig. 3. The amount of extracted Th(IV) and U(VI) ions increased with the TODGA concentration in different diluents. This indicates that extractant TODGA participated in the extraction process to form the extracted species ^[7].

Fig. 3

lgD as a function of lg[TODGA] for extracting thorium (IV) and uranium (VI) ions in different systems.

3.3 Extraction mechanism and stoichiometry determination for U(IV) and Th(VI) with TODGA

The variations trend of E for Th(IV) and U(VI) ions in TODGA/[C_nmim][NTf₂] system of different HNO₃ concentrations is accorded with the universal chemical mode proposed by Billard ^[30]: each of the three basic extraction mechanisms, i.e. cation exchange, anion exchange and neutral complex extraction, contributes depending on the extraction conditions such as HNO₃ concentration and IL types ^[6,30].

In order to further reveal the possible extraction mechanism of the three extraction systems, the relationship of lgD against lg[TODGA] in different diluents was investigated to determinate the stoichiometry of metal ion and TODGA.

For traditional TODGA/isooctane system, it has been found that neutral complex extraction mechanism dominated in organic solvent system according to Eqs. (3) and (4). The slopes in Fig. 3(D) are both 1, which indicates that the Th(IV) and U(VI) ions are extracted as mono-solvated species with TODGA in TODGA/isooctane system.

{UO}_{2} {^{2+}}_{(aq)} {+2NO}_{3} {^{-}}_{(aq)} + {[TODGA]}_{(org)} {↔[UO}_{2} {({NO}_{3})}_{2} • (TODGA)]_{(org)},

(3)

{Th}^{4+}_{(aq)} {+4NO}_{3} {^{-}}_{(aq)} + {[TODGA]}_{(org)} ↔[Th {({NO}_{3})}_{4} • (TODGA)]_{(org)},

(4)

where, (aq) and (org) denote the species in the aqueous phase and organic phase separately.

TODGA/[C₂mim][NTf₂] system is of excellent extraction performance with HNO₃ concentrations of 0.01–3 M. The distribution ratio of Th(IV) and U(VI) ions is always high, with a decrease at 1 M HNO₃ concentration, though. According to previous investigations, the extraction of metal ions happened at 0.01 M HNO₃ by cation exchange mechanism according to Eqs. (5) and (6). In this paper, the slopes at 0.01 M HNO₃ in Fig. 3(A) are 3.22 for U(VI) ion and 2.10 for Th(IV) ion. This implies that U(VI) ion and Th(IV) ion are in 1:3 complex and 1:2 complex with TODGA, respectively.

{UO}_{2} {^{2+}}_{(aq)} +3 {[TODGA]}_{(IL)} +2 {[C_{2} mim]}^{+}_{(IL)} {↔[UO}_{2} {(TODGA)}_{3}] {^{2+}}_{(IL)} +2 [C_{2} {mim]}^{+}_{(aq)}

(5)

{Th}^{4+}_{(aq)} +2 {[TODGA]}_{(IL)} +4 {[C_{2} mim]}^{+}_{(IL)} ↔[Th {(TODGA)}_{2}] {^{2+}}_{(IL)} +4 [C_{2} {mim]}^{+}_{(aq)}

(6)

where, (IL) refers to the species in ionic liquid phase.

The H⁺ concentration increases with HNO₃ concentration, which is a competitive cation with metal ions to coordinate with TODGA, as described by Eq. (7), resulting in the decrease in D value.

H^{+}_{(aq)} + {[TODGA]}_{(IL)} + {[C_{2} mim]}^{+}_{(IL)} ↔[H (TODGA)] {^{+}}_{(IL)} + [C_{2} {mim]}^{+}_{(aq)}

(7)

As HNO₃ concentration further increases, NO₃⁻ participates in extraction to coordinate with metal ions, leading to an increase in D values, just like isooctane system. Therefore, the neutral complex extraction mechanism is enhanced, as described by Eqs. (8) and (9), and in Fig. 3(B) at 3 M HNO₃ the slopes are 2.38 for U(VI) ion and 1.88 for Th(IV) ion.

{UO}_{2} {^{2+}}_{(aq)} {+mNO}_{3} {^{-}}_{(aq)} {+nH}^{+}_{(aq)} {+xTODGA}_{(IL)} + (2+n-m) {[C_{2} mim]}^{+}_{(IL)} → {[{UO}_{2} {({NO}_{3})}_{m} {(TODGA)}_{x}]}^{(2-m) +}_{(IL)} {+nH}^{+}_{(IL)} + (2+n-m) {[C_{2} mim]}^{+}_{(aq)}

(8)

{Th}^{4+}_{(aq)} {+mNO}_{3} {^{-}}_{(aq)} {+nH}^{+}_{(aq)} {+xTODGA}_{(IL)} + (4+n-m) {[C_{2} mim]}^{+}_{(IL)} → {[Th {(TODGA)}_{x} {({NO}_{3})}_{m}]}^{(4-m) +}_{(IL)} {+nH}^{+}_{(IL)} + (4+n-m) {[C_{2} mim]}^{+}_{(aq)}

(9)

At high HNO₃ concentration, Th(IV) and U(VI) ions exist as anion nitrate species [Th(NO₃)₅]⁻ and [UO₂(NO₃)₃]⁻, so the D_U increase may be attributed to anion exchange of NTf₂⁻with [Th(NO₃)₅]⁻ and [UO₂(NO₃)]⁻ ^[8,20], shown as follows:

{[{UO}_{2} {({NO}_{3})}_{3}]}^{-}_{(aq)} + {[TODGA]}_{(IL)} {+NTf}_{2} {^{-}}_{(IL)} {↔[UO}_{2} {({NO}_{3})}_{3} (TODGA)]^{-}_{(IL)} {+ NTf}_{2} {^{-}}_{(aq)}

(10)

{[Th {({NO}_{3})}_{5}]}^{-}_{(aq)} + {[TODGA]}_{(IL)} {+NTf}_{2} {^{-}}_{(IL)} ↔[Th {({NO}_{3})}_{5} (TODGA)]^{-}_{(IL)} {+ NTf}_{2} {^{-}}_{(aq)}

(11)

When the ionic liquid is [C₁₂mim][NTf₂], the alkyl chain length of 12 possesses higher hydrophobicity, which restrains cation exchange. The variation trend with the increasing HNO₃ concentration is similar to that in isooctane. The slopes in Fig. 3(C) at 3 M HNO₃ in TODGA/[C₁₂mim][NTf₂] system are 1.08 for U(VI) and 1.36 for Th(IV), being comparable to those in isooctane but much smaller than those in [C₂mim][NTf₂]. Thus, neutral complex mechanism dominants.

3.4 Effect of contact time

Equilibrium time, a key parameter in extraction experiment, is related to the extraction speed. The extraction kinetics of Th(IV) and U(VI) ions in TODGA/ [C₂mim][NTf₂] system at 25°C were investigated in 0.01 M HNO₃ aqueous. As shown in Fig. 4, both the processes were rapid, over 95% of the metal ions were extracted within 1 h. The Th(IV) and U(VI) ion extractions reached equilibrium within 2 h. At equilibrium, the Th(IV) ions were almost completely extracted into IL phase with E_Th=99.8%, while for U(VI) ions, the E_U=96%. The results indicate that the TODGA/[C₂mim][NTf₂] system performs better in extraction of Th(IV) ions. Therefore, to ensure to reach equilibrium, two hours of contact time was chosen for extraction of all samples.

Fig. 4

Effect of contact time on extraction of Th(IV) and U(VI) in TODGA/[C₂mim][NTf₂] system at 25°C in 0.01M HNO₃ aqueous.

3.5 Effect of extraction temperature

The dependency of extraction temperature on D by TODGA/[C₂mim][NTf₂] system in 0.01 M HNO₃ was investigated, as shown in Fig. 5. D values of both Th(IV) and U(VI) ions increase with the temperature, indicating that the extraction process in TODGA/[C₂mim][NTf₂] system is endothermic.

Fig. 5

Variation in distribution ratio of at different temperatures for 0.01 M HNO₃

The enthalpy change (∆H) and entropy change (∆S) for the extraction can be obtained by the Van’t Hoff equation:

{[\frac{\partial \lg D}{\partial (\frac{1}{T})}]}_{P} = \frac{- Δ H}{2.303 R} .

(12)

The plot of lgD vs 1/T is linear with slope of –∆H/(2.303R). The Gibb’s free energy (∆G) can be expressed as:

Δ G = - R T lnK',

(13)

where K' is the conditional extraction constant. At 0.01 M HNO_3, the extraction of Th(IV) and U(VI) ions with TODGA can be represented as:

M^{n} {^{+}}_{(aq)} + m {[TODGA]}_{(IL)} + n {[C_{2} mim]}^{+}_{(IL)} ↔[M {(TODGA)}_{m}] {^{n}}^{+}_{(IL)} + n [C_{2} {mim]}^{+}_{(aq)}

(14)

D = \frac{{[{M(TODGA)}_{m}^{n +}]}_{_{IL}}}{{[M^{n}^{+}]}_{aq}}

(15)

Equilibrium constant K for extraction reaction can be written as

K = \frac{{[{M(TODGA)}_{m}^{n +}]}_{_{IL}} {[C_{2} {mim}^{+}]}_{aq}^{n}}{{[M^{n}^{+}]}_{aq} {[TODGA]}_{IL}^{m} {[C_{2} {mim}^{+}]}_{IL}^{n}} = \frac{D}{{[TODGA]}_{IL}^{m}} \times \frac{{[C_{2} {mim}^{+}]}_{aq}^{n}}{{[C_{2} {mim}^{+}]}_{IL}^{n}},

(16)

\lg D = \lg K + \log (\frac{{[C_{2} {mim}^{+}]}_{IL}^{n}}{{[C_{2} {mim}^{+}]}_{aq}^{n}}) + n \lg [TODGA],

(17)

where [C₂mim⁺]_IL/[C₂mim⁺]_aq is regarded as constant. Therefore, lgD can be calculated by Eq. (18)

\lg D = \lg K' + n \lg [TODGA]

(18)

The K' value can be calculated from the intercept of a plot of lgD vs. lg[TODGA] at 0.01 M HNO₃ ^[7,23]. The ∆S at certain temperature can be calculated by Eq. (19)

Δ G = Δ H - T Δ S

(19)

Thermodynamics parameters of Th(IV) and U(VI) extraction from TODGA/[C₂mim][NTf₂] at 0.01 M HNO₃ at 25°C are given in Table 1. The negative value of ∆G indicates the spontaneous extraction reactions. The extraction of U(VI) ions (∆G = −25.8 kJ·mol⁻¹) by TODGA/[C₂mim][NTf₂] system is energetically more favourable than extraction of Th(IV) ions (∆G = −5.5 kJ·mol⁻¹). The extraction of Th(IV) and U(VI) ions, being endothermic process with a positive entropy change, might be attributed to the loss of rotational entropy of TODGA during complexation ^[7].

Thermodynamics parameters of Th(IV) and U(VI) extraction from TODGA/[C₂mim][NTf₂]-0.01 M HNO₃ at 25°C

Ions	∆H (kJ·mol⁻¹)	∆S (J·K⁻¹·mol⁻¹)	∆G (kJ·mol⁻¹)
Th(IV)	104.5	367	−5.50
U(VI)	68.7	315	−25.8

3.6 Radiation effect

In order to evaluate the applicability of TODGA in radiation conditions, it is essential to study the radiation stability of TODGA in different diluents ^[31]. In Fig. 6, the γ-irradiation effect on TODGA/[C₂mim][NTf₂] system for extracting Th(IV) and U(VI) ions is compared with that on TODGA/isooctane system. Obviously, the TODGA/[C₂mim][NTf₂] system is more radiation-resistant than the TODGA/isooctane system.

Fig. 6

Effect of γ-irradiation on extraction efficiency of (A) Th(IV) and (B) U(VI) ions in TODGA/isooctane at 3 M HNO₃ (a), and TODGA/ [C₂mim][NTf₂] at 0.01 M HNO₃ before (b) and after (c) washing with water.

Comparing the TODGA/isooctane systems before and after irradiation, one sees that the radiation affects greatly the extraction of Th(IV) and U(VI), and the E_U and E_Th decreased dramatically with increasing doses. Under irradiation, isooctane might decompose and generate active radicals, which could induce serious radiolysis of TODGA ^[6]. Sugo et al. demonstrated that charge transfer from radical cation of dodecane to TODGA caused the sensitization effect on radiolysis of TODGA ^[31]. Consequently, the extractability of irradiated TODGA/isooctane system remarkably decreased.

ILs have high radiation stability, with little radiolysis after 400 kGy irradiation ^[32]. For the TODGA/[C₂mim][NTf₂] system, the E_Th did not change virtually up to 500 kGy. It might be that the generated radiolysis products had extraction ability to Th(IV) ion. However, the irradiation affected greatly the E_U, being reduced significantly with increasing doses. Sun et al. ^[27] found that the radiolytic products of [C₄mim][NTf₂] were F⁻ and SO₄²⁻, which led to a sharp reduction in the extractability of U(VI) in TBP/irradiated-[C₄mim][NTf₂]. Yuan et al. ^[33] reported that the extractability declined using irradiated [C₄mim][NTf₂] as diluents. Irradiating [C₄mim][NTf₂] generates H⁺, which competes with metal ions to interact with extractant. SO₃²⁻ was observed in irradiated [C₂mim][NTf₂] by Zhou et al. ^[6] and Yuan et al.^[34].

In order to confirm whether the water-soluble radiolytic products of [C₂mim][NTf₂] led to the decreased extractability of irradiated TODGA/[C₂mim][NTf₂] system, the irradiated TODGA/[C₂mim][NTf₂] were washed with water for 3 times before extraction. We found that the extraction efficiency of irradiated TODGA/[C₂mim][NTf₂] for both Th(IV) and U(VI) were recovered completely, as shown in Fig.6 from histograms marked with (c). It can be concluded that the main reason for decease in extractability in TODGA/[C₂mim][NTf₂] system is the presence of water-soluble radiolytic products (F⁻, SO₄²⁻, SO₃²⁻, H⁺) from radiolysis of [C₂mim][NTf₂].

In one circle of the reprocessing of spent nuclear fuel, the adsorbed dose of extraction system is less than 10 kGy, which has litter impact on the extractability of TODGA/[C₂mim][NTf₂] ^[27]. Therefore, TODGA/[C₂mim][NTf₂] have a potential application in spent fuel reprocessing.

4. Conclusion

Th(IV) and U(VI) ions could be efficiently extracted from HNO₃ medium using TODGA as extratant in different diluents including [C₂mim][NTf₂], [C₁₂mim][NTf₂] and isooctane as comparison. The dominating extraction mechanism shifted from cation exchange to neutral complex/anion exchange with increasing HNO₃ concentration and alkyl chain length of [C_nmim]⁺. The TODGA/[C₂mim][NTf₂] presented the best extraction performance to both Th(IV) and U(VI) ions with E_U>75% and E_Th> 80% in HNO₃ medium ranged from 0.01 M to 6 M. The extraction of TODGA/[C₂mim][NTf₂] system against Th(IV) and U(VI) ions was endothermic reaction and the extraction equilibrium can reach within 2 h. TODGA/ [C₂mim][NTf₂] system showed higher radiation stability than TODGA/isooctane, and the decrease in extractability of irradiated TODGA/[C₂mim][NTf₂] system was mainly due to the presence of water-soluble radiolytic products, which could be easily eliminated by washing irradiated TODGA/[C₂mim][NTf₂] with water. Therefore, TODGA/[C₂mim][NTf₂] system would have a promising application in separation of Th(IV) and U(VI) ions in spent fuel reprocessing.

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