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High selective extraction for thorium(IV) with NTAamide in nitric acid solution: Synthesis, solvent extraction and structure studies
Separation and Purification Technology 138 (2014) 65–70
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High selective extraction for thorium(IV) with NTAamide in nitric acid solution: Synthesis, solvent extraction and structure studies Huang Huang a,b, Songdong Ding a,⇑, Dongping Su a, Ning Liu b, Jieru Wang a, Mengling Tan a, Jianen Fei a a b
College of Chemistry, Sichuan University, Chengdu 610064, China Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Accepted 13 October 2014 Available online 22 October 2014 Keywords: Extraction Thorium NTAamide Thermodynamic parameters Single-crystal analysis
a b s t r a c t N,N,N0 ,N0 ,N00 ,N00 -hexaoctylnitrilotriacetamide (NTAamide(C8)) and N,N,N0 ,N0 ,N00 ,N00 -hexabutylnitrilotriacetamide (NTAamide(C4)) were synthesized for selective solvent extraction of Th(IV) from U(VI) and lanthanides(III) (Lns(III)) and for coordination between Th(IV) and NTAamide, respectively. NTAamide(C8) exhibits excellent extraction power and selectivity toward Th(IV) over U(VI) and Lns(III) in a wide range of acidity from 0.1 to 10.0 M HNO3. Very high SFTh/M in the range between 2.5 102 and 10 103 was observed. Slope analysis shows that Th(IV), U(VI) and Lns(III) were all extracted as mono-solvated species at 3.0 M HNO3. The thermodynamic parameters describing the extraction of Th(IV) indicate that the process is endothermic and spontaneous at 298 K. The single-crystal X-ray diffraction observations also establish that 1:1 complex is formed between NTAamide and thorium nitrate, which are in good accordance with the solvent extraction results, and NTAamide is coordinated with Th(IV) in a tetradentate coordination mode. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thorium has immense potential for nuclear energy, because Th can be convertible to human-made fissile isotope 233U by absorbing slow neutrons. In nature, thorium, occurring entirely as 232Th isotope, is about three times more abundant than uranium. As a nuclear fuel, thorium does not require isotope enrichment. At the same time, much lesser plutonium and other transuranic elements would be generated by thorium fuel cycle than the traditional uranium fuel cycle [1,2]. These are good for reducing nuclear proliferation concerns and decreasing challenges in the spent fuel reprocessing. For the past few years, GenerationIV International Forum (GIF) has identified thorium molten salt reactor (TMSR) as one of the candidates for Generation IV nuclear energy system, and a lot of projects on the thorium fuel cycle have been implemented in many countries, especially in China and India [3–6]. Nuclear power generation program based on thorium fuel necessitates the effective separation and purification of thorium from metallic ores, which generally contain a sizeable fraction of uranium and rare earths [7,8]. These ore resources are extremely abundant in China. Industry reserves of thorium are estimated to be about 0.28 million tons. However, the thorium from rare earth 232
⇑ Corresponding author. Tel.: +86 28 85412329; fax: +86 28 85410259. E-mail address: [email protected] (S. Ding). http://dx.doi.org/10.1016/j.seppur.2014.10.008 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
ore is barely been utilized and treated as radioactive pollution so far, because it is still a big challenge to separate and recover thorium as completely as possible during the fabrication process of rare earths products [9]. In a conventional process, thorium is extracted by a solvent extraction process employing suitable organic extractants. Tri-n-butylphosphate (TBP) used as a representative extractant for the recovery and purification of thorium, however, suffers from some limitations. One limitation is that it produces a large amount of secondary waste in the form of P2O5 or H3PO4 during incineration of the spent solvent. Another obvious limitation is the separation factor between uranium and thorium is not high enough [10–14]. Furthermore, chemical and irradiation stability of TBP is not adequate and its degradation products could bring about interference in the extraction process [10,15–19]. So in these regards, it is very necessary to explore the new and efficient solvent extraction system for thorium separation. The amide extractant has many advantages. The first is that amide consists only of C, H, O and N elements and can be completely incinerated to gases, suggesting a significant reduction of the secondary solid wastes. Moreover, amides have good extraction performance and high chemical and irradiation stability [20–22]. In spite of this, their selectivities are not high enough for thorium separation over either uranium or rare earth [23,24]. In order to improve the selectivity, a kind of amides derived from aminopolycarboxylate ligands, such as ethylenendiamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and diethylenetriamine
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H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70
pentacetic acid (DTPA), have been explored for actinides separation [25–27]. Among them, those based on NTA with special tripod structure were paid more attention. Recently, NTAamide, i.e. amidated NTA, was found to have a good selectivity for trivalent actinides over lanthanides. The separation factor between Am(III) and Eu(III) can be achieved 52.6 in the case of 0.5 M NTAamide(C8) in n-dodecane at 0.2 M HNO3 [28,29]. In addition, it also has been well established that the stable complexes can be formed between Th(IV) and NTA in aqueous solution, whose stability constants of Th(IV) are much higher than those of U(VI) and Gd(III) [30–32]. From the view of this point, lipophilic NTAamide can be expected to obtain a high selective extraction for Th(IV). In the present paper, extraction and coordination of Th(IV) with NTAamide were investigated for the first time. The extraction behavior of NTAamide in kerosene for Th(IV) from nitric acid solution was described. The extraction mechanism for Th(IV) was discussed and the thermodynamic parameters were also presented. Moreover, thorium coordination complex was prepared and its structure was characterized through FT-IR, 1H NMR, 13C NMR and single-crystal X-ray diffraction analysis. 2. Experimental section 2.1. Chemicals and measurements All reagents employed were of AR grade and used without further purification. La(NO3)3, Ce(NO3)3, Nd(NO3)3, Sm(NO3)3, Eu(NO3)3, Gd(NO3)3 and Er(NO3)3 in HNO3 solution were prepared from the oxides (99.99%, Aldrich), whilst Th(NO3)4 and UO2(NO3)2 in HNO3 solution were obtained from the hydrates (99.9%, Aladdin, China). Sulfonated kerosene was produced by referring to the literature [33]. Nuclear magnetic resonance (NMR) spectra were measured on a Varian Inova 400 MHz NMR spectrometer (1H: 400 MHz; 13C: 100 MHz) with tetramethylsilane as an internal solvent resonances reference. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet Nexus 670 Model instrument. Electrospray ionization mass spectrometry (ESI-MS) data was obtained on a Bruker Amazon SL instrument in the form of positive model. Single-crystal X-ray diffraction measurements were carried out using a Xcalibur, Eos diffractometer. 2.2. Syntheses The two NTAamides were synthesized according to Scheme 1. 2.2.1. N,N,N0 ,N0 ,N00 ,N00 -hexaoctylnitrilotriacetamide (NTAamide(C8)) Oxalyl chloride (15 mL) was added dropwise to a solution of NTA (1.50 g, 7.85 mmol) with one drop of DMF as catalyst in 20 mL dichloromethane at 30 °C. The mixture was stirred under 40 °C for 6 h. After that, the dichloromethane and extra oxalyl chloride were removed by decompress distillation. The residue was diluted with 20 mL dichloromethane and then added dropwise into
a solution of dioctyl-amine (11.40 g, 47.29 mmol) in 30 mL dichloromethane. The temperature of the whole adding process was controlled below 5 °C. Then the mixture was refluxed for 12 h. The solvent was successively washed with water, 1 M HCl solution (2 50 mL), 1 M NaOH solution (2 50 mL), and water (2 50 mL). The organic layer was dried with anhydrous MgSO4 and concentrated under reduced pressure. The crude product was chromatographed on silica gel (petroleum/ethyl acetate, 10:1 (v/v)) to afford pure product as yellow oil (2.96 g, yield: 56%). 1H NMR (400 MHz, CDCl3) d 3.66 (6H, s, NCH2CON), 3.34–3.20 (12H, m, NCH2), 1.51 (12H, s, NCH2CH2), 1.28 (60H, s, (CH2)5CH3), 0.96– 0.82 (18H, m, CH3). 13C NMR (100 MHz, CDCl3) d 169.8, 55.3, 47.4, 46.0, 31.8 (d, J = 1.7 Hz), 29.4, 29.3 (d, J = 1.5 Hz), 29.1, 27.8, 27.2, 26.9, 22.7, 14.1. HRMS: m/z 861.8498 (M + H)+, calculated: 861.8500. 2.2.2. N,N,N0 ,N0 ,N00 ,N00 -hexabutylnitrilotriacetamide (NTAamide(C4)) NTAamide(C4) was synthesized through an analogous procedure as described for NTAamide(C8), which starting from oxalyl chloride (15 mL) and NTA (1.50 g, 7.85 mmol) in the presence of one drop of DMF as catalyst and then added into dibutyl-amine (6.10 g, 47.29 mmol). Purification by chromatographed on silica gel (petroleum/ethyl acetate, 5:1 (v/v)) gave pure product as yellow oil (3.79 g, yield: 72%). 1H NMR (400 MHz, CDCl3) d 3.67 (6H, s, NCH2CON), 3.27 (12H, t, J = 6.8 Hz, NCH2), 1.57–1.42 (12H, m, NCH2CH2), 1.37–1.20 (12H, m, CH2CH3), 0.92 (18H, q, J = 7.1 Hz, CH3). 13C NMR (100 MHz, CDCl3) d 169.8, 55.3, 46.36 (d, J = 148.4 Hz), 30.49 (d, J = 124.7 Hz), 20.18 (d, J = 19.5 Hz), 13.90 (d, J = 2.7 Hz). HRMS: m/z 525.4738 (M + H)+, calculated: 527.4744. IR (m/cm1): 2959(s), 2870(s), 1644(s), 1462(s), 1429(s), 1376(s), 1293(s), 1259(s), 1214(s), 1118(s), 1034(w), 964(w), 934(w), 739(w). 2.2.3. [(Th(NTAamide(C4))(NO3)4)] Th(NO3)45H2O (0.12 g, 0.20 mmol, in 10 mL of ethanol) was added dropwise to the solution of NTAamide(C4) (0.10 g, 0.20 mmol, in 10 mL of ethanol). The mixture was stirred at room temperature for 12 h. The white precipitated solid complex was filtered, washed with ethanol three times and dried in vacuo for 48 h to afford pure product as white powder (0.18 g, yield: 92%). 1H NMR (400 MHz, CDCl3) d 4.11 (6H, s, NCH2CON), 3.34 (12H, s, NCH2), 1.55 (12H, s, NCH2CH2), 1.42–1.21 (12H, m, CH2CH3), 1.02–0.83 (18H, m, CH3). 13C NMR (100 MHz, CDCl3) d 174.0 (s), 59.5 (s), 48.1 (d, J = 71.6 Hz), 29.6 (d, J = 117.0 Hz), 19.8 (d, J = 42.8 Hz), 13.7 (d, J = 12.7 Hz). IR (m/cm1): 2960(w), 2935(w), 2872(w), 1631(s), 1604(s), 1504(s), 1468(s), 1384(w), 1303(s), 1111(w), 1032(w), 899(w), 811(w), 744(w). 2.3. Extraction Extraction performance of metal ions between organic and aqueous phases by NTAamide(C8) were quantified by measuring the concentration of metal ions in each phase at equilibrium.
Scheme 1. Synthesis of NTAamides.
H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70
NTAamide(C8) dissolved in kerosene was employed as the organic phase. The mixed solution of Th(NO3)4, UO2(NO3)2, La(NO3)3, Ce(NO3)3, Nd(NO3)3, Sm(NO3)3, Eu(NO3)3, Gd(NO3)3 and Er(NO3)3 (50 ppm for each metal ion) in HNO3 solution was used as the aqueous phase. Before extraction, the organic phase was preequilibrated three times with equal volumes of HNO3 solutions of the counterpart concentration to ensure that the concentration of the aqueous electrolyte did not change during the equilibration with the metal ion present. Equal volumes (2.0 mL) of the organic phase and the aqueous phase were stirred in 10.0 mL stoppered glass tubes in water bath for 1 h at 25.0 ± 0.1 °C. After all extraction process, samples were centrifuged to separate phases. The concentrations of metal ions in the aqueous phase were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES, IRIS Advantage, Thermo Scientific). The metal nitrate distribution ratio is defined as the ratio of the total concentration of metal ion in each phase, D = [M]tot,org./[M]tot,aq.. The subscripts aq. and org. represent the aqueous phase and the organic phase, separately. The separation factor is expressed as SFTh/M = DTh/DM. The stripping and temperature experiments were using 100 ppm Th(IV) in 3.0 M HNO3 as aqueous phase. 0.02 M NTAamide(C8) dissolved in kerosene was used as the organic phase. Deionized water, 0.01 M HNO3, 2.0 M NaCl and 2.0 M Na2CO3 were selected as stripping agents for back extraction experiments. 2.5 mL organic phase was stirred with equal volume aqueous phase in 10.0 mL stoppered glass tube in water bath for 1 h at 25.0 ± 0.1 °C. Then, 2.0 mL loaded organic phase was taken out and stripped with equal volume stripping agent one to three times, respectively. After that, the aqueous phase was taken out to measure the concentrations of Th(IV). For the temperature study, the distribution experiments of Th(IV) as a function of temperature were conducted at 5.0° intervals between 20.0 and 40.0 °C. Extraction equilibrium was attained in 10.0 mL stoppered glass tubes of the two phases (2.0 mL of each) by stirring for 1 h in water bath capable of maintaining the temperature constant to within ±0.1 °C. The aqueous phases were taken out for measurements. The concentrations of Th(IV) were all determined by ICP-AES. Distribution ratios in the range of 102 < D < 102 are reproducible within ±20%. Distribution ratios outside this range may bear significantly larger errors and should be regarded with caution. 2.4. X-ray crystallographic measurements Crystals of [(Th(NTAamide(C4))(NO3)4)] were grown from methanol/dichloromethane by slow evaporation at room temperature. After about a week, transparent crystals formed from the solution. A suitable crystal was selected for crystallographic measurements. The crystal was kept at 143.00(10) K during data collection. Using Olex2 [34], the structure was solved with the Superflip [35] structure solution program using Charge Flipping and refined with the ShelXL-2012 [36] refinement package using Least Squares minimisation. Crystal data for Th(IV) complex [(Th(NTAamide(C4))(NO3)4)]: C30H60N8O15Th, M = 1004.90, monoclinic, space group P21/c (No. 14), a = 11.7963(2) Å, b = 15.8161(3) Å, c = 22.4960(4) Å, a = 90°, b = 96.0065(16)°, c = 90°, V = 4174.07(13) Å3, Z = 4, T = 143.00(10) K, l(Mo Ka) = 3.644 mm1, Dcalc = 1.599 g/mm3, 16,963 reflections measured (6.04 6 2H 6 52.74), 8533 unique (Rint = 0.0342) which were used in all calculations. The final R1 was 0.0331 (I > 2r(I)) and wR2 was 0.0634 (all data). The goodness of fit on F2 was 1.028. Positional parameters, hydrogen atom parameters, thermal parameters, bond distances and angles have been deposited as Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as a
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supplementary publication No. CCDC-971993. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax: (+44 1223 336 033; email: [email protected] or http://www.ccdc.cam.ac.uk). 3. Results and discussion 3.1. Extraction of thorium To ensure the complete equilibrium in the following extraction experiments, effect of contact time was firstly evaluated. Contact time was varied from 10 to 120 min at 25.0 ± 0.1 °C. The results were shown in Fig. 1. It can be seen that the system approaches to equilibrium very fast, and contact of two phases for 60 min is enough for complete equilibrium. The effect of HNO3 concentration on the extraction of Th(IV), U(VI) and Lns(III) by NTAamide(C8) was investigated and the results were illustrated in Fig. 2. It is clearly indicated that the extractant displays high extraction ability for Th(IV). With the increase of HNO3 concentration, DTh decreases from 55.1 to 15.1 until it levels on 1.0 M and then increases to 30.2 with a further increase of acidity until 10.0 M HNO3. The decrease of distribution ratio can be ascribed to protonation to N donor in the center of the backbone. The subsequently increase of distribution ratio with an increase of HNO3 concentration is owing to the ionpair extraction effect. In contrast, DU and DLns are much lower than DTh. The SFTh/M at 0.1, 3.0 and 10.0 M HNO3 is listed in Table 1. As the same as the expected results, NTAamide(C8) exhibits an excellent selectivity for Th(IV) over the other metal ions. Very high SFTh/M in the range between 2.5 102 and 10 103 can be observed. 3.2. Stripping of thorium Generally, in the back extraction process for thorium, deionized water, mineral acids, salt solutions and sodium carbonate solution are often employed as stripping agents [37,38]. Thus, the stripping of Th(IV) from the loaded organic phase was investigated by using deionized water, 0.01 M HNO3, 2.0 M NaCl and 2.0 M Na2CO3. The stripping effectiveness was given in Table 2. As observed, the stripping was incomplete with deionized water, 0.01 M HNO3 and 2.0 M NaCl. The stripping percentages of deionized water, 0.01 M HNO3 and 2.0 M NaCl were only 44.8%, 33.1% and 23.0% up to three stages. With 2.0 M Na2CO3, full stripping of Th(IV) can be achieved.
Fig. 1. Effect of contact time on the distribution ratios of various metal ions by NTAamide(C8) in kerosene at 3.0 M HNO3.
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3.3. Extraction mechanism The number of extractant molecules in the extracted species can be determined if the distribution ratio for the metal extraction at a constant acidity is plotted as a function of extractant concentration. The slope is approximately equal to the number of extractant molecules. Effect of extractant concentration on the distribution ratios at 3.0 M HNO3 were shown in Fig. 3. All D values show a linear relationship with NTAamide(C8) concentration, the slope values and y-intercept were listed in Table 3. These results indicate NTAamide(C8) extracted all metal ions as mono-solvated species. Thus, the main extraction reaction of NTAamide(C8) with metal ion from HNO3 solution can be expressed as: K ex
Mnþ aq þ nNO3aq: þ Lorg: ¢ MðNO3 Þn Lorg :
Fig. 2. Effect of the HNO3 concentration on the distribution ratios of various metal ions by NTAamide(C8) in kerosene.
where M and L mean metal ions and NTAamide(C8), severally. The extraction constant, Kex, is defined as:
K ex ¼
HNO3, M
0.1
3.0
10.0
SFTh/U SFTh/La SFTh/Ce SFTh/Nd SFTh/Sm SFTh/Eu SFTh/Gd SFTh/Er
1.6 103 2.2 103 7.1 102 3.8 102 6.8 102 7.0 102 9.3 102 1.2 103
4.1 102 4.0 102 2.7 102 2.5 102 7.4 102 8.1 102 1.2 103 1.6 103
9.7 102 6.0 103 4.3 103 3.8 103 6.0 103 7.5 103 10 103 10 103
Table 2 Effect of various stripping agents on the stripping of Th(IV).
1 2 3 a
½MðNO3 Þn Lorg:
ð2Þ
½Mnþ aq: ½NO3 naq: ½Lorg:
The distribution ratio can be expressed as below:
Table 1 The SFTh/M of Th(IV) over U(VI) and Lns(III).
Stages
ð1Þ
Stripping agent Deionized water Sa, %
0.01 M HNO3 S, %
2.0 M NaCl S, %
2.0 M Na2CO3 S, %
2.8 38.4 44.8
11.1 21.4 33.1
10.1 16.2 23.0
88.7 98.1 100
S: stripping percentage.
D¼
½MðNO3 Þn Lorg:
ð3Þ
½Mnþ aq:
Substituting the value of {[M(NO3)n.L]org./[Mn+]aq.} from Eq. (3) into Eq. (2) and rearranging as below
log K ex ¼ log D n log ½NO3 aq: log ½Lorg:
ð4Þ
In Eq. (4), the equilibrium concentration of extractant equals approximately its initial concentration. Therefore, the apparent extraction equilibrium constant (log Kex) can be calculated. The results were summarized in Table 4. The average log Kex of Th(IV), only positive one, is obviously higher than that of U(VI) and Lns(III). To obtain the thermodynamic parameters for the extraction of Th(IV) by NTAamide(C8) at 3.0 M HNO3, effect of temperature on the extraction were carried out. Plot of Log Kex values as a function of the inverse temperature in the range 293–313 K gave a straight line in Fig. 4. Thus, the enthalpy change (DH) and entropy change (DS) for the extraction can be calculated using the Van’t Hoff equation:
log K ex ¼
DH 1 DS : þ 2:303R T 2:303R
ð5Þ
where R is the gas constant. Based on the values of slope and intercept, DH and DS were calculated to be 75.2 kJ mol1 and 275.7 J mol1 K, respectively. In addition, the free-energy change (DG) for the extraction could be obtained from the following equation:
DG ¼ DH T DS ¼ 2:303RT log K
ð6Þ
At 298 K, DG was calculated to be 6.9 kJ mol1. Table 3 The slope values and y-intercepts of Th(IV), U(VI) and Lns(III).
Fig. 3. Effect of extractant concentration on the distribution ratios of various metal ions at 3.0 M HNO3.
Th(IV) U(VI) La(III) Ce(III) Nd(III) Sm(III) Eu(III) Gd(III) Er(III)
Slope values
Int, y
1.15 0.82 0.94 0.96 1.05 1.01 0.99 1.02 1.14
3.34 0.19 0.83 0.72 0.36 0.15 0.23 0.38 0.32
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H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70 Table 4 The values of the apparent extraction equilibrium constant (log Kex).
a
CLigand, M
0.002
0.005
0.01
0.02
0.05
0.1
0.2
Average
Th(IV) U(VI) La(III) Ce(III) Nd(III) Sm(III) Eu(III) Gd(III) Er(III)
1.25 n.d.a n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1.10 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1.18 0.35 0.43 0.65 1.20 1.59 1.65 1.86 2.03
1.19 0.48 0.56 0.59 1.14 1.62 1.62 1.85 1.97
1.46 0.65 0.56 0.67 1.13 1.65 1.65 1.83 1.93
n.d. 0.52 0.52 0.69 1.13 1.59 1.65 1.83 1.88
n.d. 0.62 0.55 0.66 1.12 1.58 1.65 1.84 1.86
1.24 0.52 0.52 0.65 1.14 1.60 1.64 1.84 1.93
n.d. = not determined.
Fig. 4. Effect of temperature on the apparent extraction equilibrium constants log Kex at 3.0 M HNO3.
The entropy and entropy change for the extraction of thorium are all positive, suggesting that the process is an endothermic process. These results can be explained as the high degree of hydration of Th(IV). More water molecules need to be released; hence more energy spent for this process and accordingly the enthalpy change is positive. Furthermore, the entropy gains due to water release and the entropy loss due to bonding of extractant and nitrate ions. In the case of tetrapositive thorium, the energy spent in releasing the larger number of water molecules results in a positive entropy change. The negative value of DG at room temperature indicates that the extraction reaction is spontaneous. 3.4. Structure of thorium complex Due to the presence of long alkyl chains in NTAamide(C8), it is very difficult to cultivate the single crystal of its thorium complex. Thus, the homologue NTAamide(C4) containing the shorter alkyl chains, was employed to investigate the complexation behavior. The structure of NTAamide(C4) and its Th(IV) complex were characterized by FT-IR, NMR and single-crystal X-ray analyses. The coordination structure of complex was shown in Fig. 5. The result of 1:1 coordination of NTAamide with Th(IV) is in good accord with the result of the slope analysis in solvent extraction studies. The coordination polyhedron of complex is formed by the tetradentate ligand and four NO-3 anions, resulting in a 12-fold coordinated Th(IV). Each Th(IV) center binds to one nitrogen atom and three oxygen atoms from the ligand as well as eight oxygen anions from nitrate groups. The same result was shown in the IR spectra (Figs. S7 and S8). The bands assignable to the C@O stretching vibration of the free NTAamide(C4) ligand originally appears at 1644 cm1, while the characteristic band of the complex shifts
Fig. 5. X-ray crystal structure of the complex [(Th(NTAamide(C4))(NO3)4)]. (a) View in front. (b) View from top. H atoms have been omitted for clarity. Blue, light purple, red and gray colors denote Th, N, O and C, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 5 Selected bond lengths (Å) and angles (degree) for [(Th(NTAamide(C4))(NO3)4)]. Th1AO1 Th1AO3 Th1AO5 Th1AO8 Th1AO11 Th1AO14 C1AN1ATh1 C21AN1ATh1 N1AC11AC12 O1AC2AC1
2.435(3) 2.396(2) 2.551(3) 2.610(3) 2.623(3) 2.606(3) 103.8(2) 112.4(2) 108.5(3) 117.7(3)
ThAO2 Th1AO4 Th1AO7 Th1AO10 Th1AO13 Th1AN1 C11AN1ATh1 N1AC1AC2 N1AC21AC22 O2AC12AC11
2.520(3) 2.660(3) 2.592(3) 2.579(3) 2.570(3) 2.886(3) 110.1(2) 109.2(3) 111.2(3) 118.3(3)
40 cm1 towards lower wave numbers. In the 13C NMR spectra (Figs. S4 and S6), the signal of C@O carbon shows a downfield shift of 4.2 ppm, also suggesting that the C@O groups participate in
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coordination. As shown in Table 5, the ThAO bond distances of the nitro groups range within the area of 2.551–2.660 Å. The value of the ThAN distance (2.886 Å) is longer than the average ThAOamide bond distance (2.450 Å), indicating that ThAOamide bond is stronger than the ThAN one. FT-IR and NMR spectrums of NTAamide(C4) and its Th(IV) complex are listed in Supporting Information. 4. Conclusions Two NTAamides were synthesized for investigating the extraction and coordination for Th(IV). NTAamide(C8) exhibits a high selectivity and extraction power for Th(IV) over U(VI) and Lns(III) in a wide range of HNO3 concentration. The thermodynamic parameters show that the extraction of Th(IV) is a process of endothermic and spontaneous at 298 K. Slope analysis and single crystal X-ray diffraction observations show that 1:1 complex Th(IV) is formed between NTAamide and thorium nitrate in a tetradentate coordination mode. The special tripod-type backbone of NTAamide is demonstrated to be an almost perfect match to Th(IV). High selective extraction ability of NTAamide(C8) indicates itself a good application prospect for separation and purification of Th(IV). Acknowledgment The authors are very grateful for the financial support by the National Science Foundation of China (Nos. 91126016, J1210004 and J1103315). Analytical & Testing Center of Key Laboratory of Green Chemistry & Technology (Sichuan University) of Education Ministry of China is acknowledged for NMR, MS and ICP-AES analyses. Analytical & Testing Center of Sichuan University is acknowledged for single-crystal X-ray diffraction measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur. 2014.10. 008. References [1] F. Sokolov, K. Fukuda, H.P. Nawada, Thorium fuel cycle – potential benefits and challenges, IAEA-TECDOC-1450, May 2005. [2] V. Arkhipov, Thorium based fuel options for the generation of electricity: Developments in the 1990s, IAEA-TECDOC-1155, May 2000. [3] J.S. Herring, P.E. Macdonald, K.D. Weaver, C. Kullberg, Low cost, proliferation resistant, uranium–thorium dioxide fuels for light water reactors, Nucl. Eng. Des. 203 (2001) 65–85. [4] G. Locatelli, M. Mancini, N. Todeschini, Generation IV nuclear reactors: current status and future prospects, Energ. Policy 61 (2013) 1503–1520. [5] T. Abram, S. Ion, Generation-IV nuclear power: a review of the state of the science, Energ. Policy 36 (2008) 4323–4330. [6] L. Mathieu, D. Heuer, R. Brissot, C. Garzenne, C. Le Brun, D. Lecarpentier, E. Liatard, J.M. Loiseaux, O. Méplan, E. Merle-Lucotte, A. Nuttin, E. Walle, J. Wilson, The thorium molten salt reactor: moving on from the MSBR, Prog. Nucl. Energ. 48 (2006) 664–679. [7] H. Tong, Y. Wang, W. Liao, D. Li, Synergistic extraction of Ce(IV) and Th(IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media, Sep. Purif. Technol. 118 (2013) 487–491. [8] Y. Zuo, J. Chen, D. Li, Reversed micellar solubilization extraction and separation of thorium(IV) from rare earth(III) by primary amine N1923 in ionic liquid, Sep. Purif. Technol. 63 (2008) 684–690. [9] Y. Wang, Y. Li, W. Liao, D. Li, Preparation of high-purity thorium by solvent extraction with di-(2-ethylhexyl) 2-ethylhexyl phosphonate, J. Radioanal. Nucl. Chem. 298 (2013) 1651–1657. [10] A. Suresh, T.G. Srinivasan, P.R.V. Rao, Parameters influencing third-phase formation in the extraction of Th(NO3)4 by some trialkyl phosphates, Solvent Extr. Ion Exc. 27 (2009) 132–158.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
High selective extraction for thorium(IV) with NTAamide in nitric acid solution: Synthesis, solvent extraction and structure studies Huang Huang a,b, Songdong Ding a,⇑, Dongping Su a, Ning Liu b, Jieru Wang a, Mengling Tan a, Jianen Fei a a b
College of Chemistry, Sichuan University, Chengdu 610064, China Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Accepted 13 October 2014 Available online 22 October 2014 Keywords: Extraction Thorium NTAamide Thermodynamic parameters Single-crystal analysis
a b s t r a c t N,N,N0 ,N0 ,N00 ,N00 -hexaoctylnitrilotriacetamide (NTAamide(C8)) and N,N,N0 ,N0 ,N00 ,N00 -hexabutylnitrilotriacetamide (NTAamide(C4)) were synthesized for selective solvent extraction of Th(IV) from U(VI) and lanthanides(III) (Lns(III)) and for coordination between Th(IV) and NTAamide, respectively. NTAamide(C8) exhibits excellent extraction power and selectivity toward Th(IV) over U(VI) and Lns(III) in a wide range of acidity from 0.1 to 10.0 M HNO3. Very high SFTh/M in the range between 2.5 102 and 10 103 was observed. Slope analysis shows that Th(IV), U(VI) and Lns(III) were all extracted as mono-solvated species at 3.0 M HNO3. The thermodynamic parameters describing the extraction of Th(IV) indicate that the process is endothermic and spontaneous at 298 K. The single-crystal X-ray diffraction observations also establish that 1:1 complex is formed between NTAamide and thorium nitrate, which are in good accordance with the solvent extraction results, and NTAamide is coordinated with Th(IV) in a tetradentate coordination mode. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thorium has immense potential for nuclear energy, because Th can be convertible to human-made fissile isotope 233U by absorbing slow neutrons. In nature, thorium, occurring entirely as 232Th isotope, is about three times more abundant than uranium. As a nuclear fuel, thorium does not require isotope enrichment. At the same time, much lesser plutonium and other transuranic elements would be generated by thorium fuel cycle than the traditional uranium fuel cycle [1,2]. These are good for reducing nuclear proliferation concerns and decreasing challenges in the spent fuel reprocessing. For the past few years, GenerationIV International Forum (GIF) has identified thorium molten salt reactor (TMSR) as one of the candidates for Generation IV nuclear energy system, and a lot of projects on the thorium fuel cycle have been implemented in many countries, especially in China and India [3–6]. Nuclear power generation program based on thorium fuel necessitates the effective separation and purification of thorium from metallic ores, which generally contain a sizeable fraction of uranium and rare earths [7,8]. These ore resources are extremely abundant in China. Industry reserves of thorium are estimated to be about 0.28 million tons. However, the thorium from rare earth 232
⇑ Corresponding author. Tel.: +86 28 85412329; fax: +86 28 85410259. E-mail address: [email protected] (S. Ding). http://dx.doi.org/10.1016/j.seppur.2014.10.008 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.
ore is barely been utilized and treated as radioactive pollution so far, because it is still a big challenge to separate and recover thorium as completely as possible during the fabrication process of rare earths products [9]. In a conventional process, thorium is extracted by a solvent extraction process employing suitable organic extractants. Tri-n-butylphosphate (TBP) used as a representative extractant for the recovery and purification of thorium, however, suffers from some limitations. One limitation is that it produces a large amount of secondary waste in the form of P2O5 or H3PO4 during incineration of the spent solvent. Another obvious limitation is the separation factor between uranium and thorium is not high enough [10–14]. Furthermore, chemical and irradiation stability of TBP is not adequate and its degradation products could bring about interference in the extraction process [10,15–19]. So in these regards, it is very necessary to explore the new and efficient solvent extraction system for thorium separation. The amide extractant has many advantages. The first is that amide consists only of C, H, O and N elements and can be completely incinerated to gases, suggesting a significant reduction of the secondary solid wastes. Moreover, amides have good extraction performance and high chemical and irradiation stability [20–22]. In spite of this, their selectivities are not high enough for thorium separation over either uranium or rare earth [23,24]. In order to improve the selectivity, a kind of amides derived from aminopolycarboxylate ligands, such as ethylenendiamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and diethylenetriamine
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pentacetic acid (DTPA), have been explored for actinides separation [25–27]. Among them, those based on NTA with special tripod structure were paid more attention. Recently, NTAamide, i.e. amidated NTA, was found to have a good selectivity for trivalent actinides over lanthanides. The separation factor between Am(III) and Eu(III) can be achieved 52.6 in the case of 0.5 M NTAamide(C8) in n-dodecane at 0.2 M HNO3 [28,29]. In addition, it also has been well established that the stable complexes can be formed between Th(IV) and NTA in aqueous solution, whose stability constants of Th(IV) are much higher than those of U(VI) and Gd(III) [30–32]. From the view of this point, lipophilic NTAamide can be expected to obtain a high selective extraction for Th(IV). In the present paper, extraction and coordination of Th(IV) with NTAamide were investigated for the first time. The extraction behavior of NTAamide in kerosene for Th(IV) from nitric acid solution was described. The extraction mechanism for Th(IV) was discussed and the thermodynamic parameters were also presented. Moreover, thorium coordination complex was prepared and its structure was characterized through FT-IR, 1H NMR, 13C NMR and single-crystal X-ray diffraction analysis. 2. Experimental section 2.1. Chemicals and measurements All reagents employed were of AR grade and used without further purification. La(NO3)3, Ce(NO3)3, Nd(NO3)3, Sm(NO3)3, Eu(NO3)3, Gd(NO3)3 and Er(NO3)3 in HNO3 solution were prepared from the oxides (99.99%, Aldrich), whilst Th(NO3)4 and UO2(NO3)2 in HNO3 solution were obtained from the hydrates (99.9%, Aladdin, China). Sulfonated kerosene was produced by referring to the literature [33]. Nuclear magnetic resonance (NMR) spectra were measured on a Varian Inova 400 MHz NMR spectrometer (1H: 400 MHz; 13C: 100 MHz) with tetramethylsilane as an internal solvent resonances reference. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet Nexus 670 Model instrument. Electrospray ionization mass spectrometry (ESI-MS) data was obtained on a Bruker Amazon SL instrument in the form of positive model. Single-crystal X-ray diffraction measurements were carried out using a Xcalibur, Eos diffractometer. 2.2. Syntheses The two NTAamides were synthesized according to Scheme 1. 2.2.1. N,N,N0 ,N0 ,N00 ,N00 -hexaoctylnitrilotriacetamide (NTAamide(C8)) Oxalyl chloride (15 mL) was added dropwise to a solution of NTA (1.50 g, 7.85 mmol) with one drop of DMF as catalyst in 20 mL dichloromethane at 30 °C. The mixture was stirred under 40 °C for 6 h. After that, the dichloromethane and extra oxalyl chloride were removed by decompress distillation. The residue was diluted with 20 mL dichloromethane and then added dropwise into
a solution of dioctyl-amine (11.40 g, 47.29 mmol) in 30 mL dichloromethane. The temperature of the whole adding process was controlled below 5 °C. Then the mixture was refluxed for 12 h. The solvent was successively washed with water, 1 M HCl solution (2 50 mL), 1 M NaOH solution (2 50 mL), and water (2 50 mL). The organic layer was dried with anhydrous MgSO4 and concentrated under reduced pressure. The crude product was chromatographed on silica gel (petroleum/ethyl acetate, 10:1 (v/v)) to afford pure product as yellow oil (2.96 g, yield: 56%). 1H NMR (400 MHz, CDCl3) d 3.66 (6H, s, NCH2CON), 3.34–3.20 (12H, m, NCH2), 1.51 (12H, s, NCH2CH2), 1.28 (60H, s, (CH2)5CH3), 0.96– 0.82 (18H, m, CH3). 13C NMR (100 MHz, CDCl3) d 169.8, 55.3, 47.4, 46.0, 31.8 (d, J = 1.7 Hz), 29.4, 29.3 (d, J = 1.5 Hz), 29.1, 27.8, 27.2, 26.9, 22.7, 14.1. HRMS: m/z 861.8498 (M + H)+, calculated: 861.8500. 2.2.2. N,N,N0 ,N0 ,N00 ,N00 -hexabutylnitrilotriacetamide (NTAamide(C4)) NTAamide(C4) was synthesized through an analogous procedure as described for NTAamide(C8), which starting from oxalyl chloride (15 mL) and NTA (1.50 g, 7.85 mmol) in the presence of one drop of DMF as catalyst and then added into dibutyl-amine (6.10 g, 47.29 mmol). Purification by chromatographed on silica gel (petroleum/ethyl acetate, 5:1 (v/v)) gave pure product as yellow oil (3.79 g, yield: 72%). 1H NMR (400 MHz, CDCl3) d 3.67 (6H, s, NCH2CON), 3.27 (12H, t, J = 6.8 Hz, NCH2), 1.57–1.42 (12H, m, NCH2CH2), 1.37–1.20 (12H, m, CH2CH3), 0.92 (18H, q, J = 7.1 Hz, CH3). 13C NMR (100 MHz, CDCl3) d 169.8, 55.3, 46.36 (d, J = 148.4 Hz), 30.49 (d, J = 124.7 Hz), 20.18 (d, J = 19.5 Hz), 13.90 (d, J = 2.7 Hz). HRMS: m/z 525.4738 (M + H)+, calculated: 527.4744. IR (m/cm1): 2959(s), 2870(s), 1644(s), 1462(s), 1429(s), 1376(s), 1293(s), 1259(s), 1214(s), 1118(s), 1034(w), 964(w), 934(w), 739(w). 2.2.3. [(Th(NTAamide(C4))(NO3)4)] Th(NO3)45H2O (0.12 g, 0.20 mmol, in 10 mL of ethanol) was added dropwise to the solution of NTAamide(C4) (0.10 g, 0.20 mmol, in 10 mL of ethanol). The mixture was stirred at room temperature for 12 h. The white precipitated solid complex was filtered, washed with ethanol three times and dried in vacuo for 48 h to afford pure product as white powder (0.18 g, yield: 92%). 1H NMR (400 MHz, CDCl3) d 4.11 (6H, s, NCH2CON), 3.34 (12H, s, NCH2), 1.55 (12H, s, NCH2CH2), 1.42–1.21 (12H, m, CH2CH3), 1.02–0.83 (18H, m, CH3). 13C NMR (100 MHz, CDCl3) d 174.0 (s), 59.5 (s), 48.1 (d, J = 71.6 Hz), 29.6 (d, J = 117.0 Hz), 19.8 (d, J = 42.8 Hz), 13.7 (d, J = 12.7 Hz). IR (m/cm1): 2960(w), 2935(w), 2872(w), 1631(s), 1604(s), 1504(s), 1468(s), 1384(w), 1303(s), 1111(w), 1032(w), 899(w), 811(w), 744(w). 2.3. Extraction Extraction performance of metal ions between organic and aqueous phases by NTAamide(C8) were quantified by measuring the concentration of metal ions in each phase at equilibrium.
Scheme 1. Synthesis of NTAamides.
H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70
NTAamide(C8) dissolved in kerosene was employed as the organic phase. The mixed solution of Th(NO3)4, UO2(NO3)2, La(NO3)3, Ce(NO3)3, Nd(NO3)3, Sm(NO3)3, Eu(NO3)3, Gd(NO3)3 and Er(NO3)3 (50 ppm for each metal ion) in HNO3 solution was used as the aqueous phase. Before extraction, the organic phase was preequilibrated three times with equal volumes of HNO3 solutions of the counterpart concentration to ensure that the concentration of the aqueous electrolyte did not change during the equilibration with the metal ion present. Equal volumes (2.0 mL) of the organic phase and the aqueous phase were stirred in 10.0 mL stoppered glass tubes in water bath for 1 h at 25.0 ± 0.1 °C. After all extraction process, samples were centrifuged to separate phases. The concentrations of metal ions in the aqueous phase were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES, IRIS Advantage, Thermo Scientific). The metal nitrate distribution ratio is defined as the ratio of the total concentration of metal ion in each phase, D = [M]tot,org./[M]tot,aq.. The subscripts aq. and org. represent the aqueous phase and the organic phase, separately. The separation factor is expressed as SFTh/M = DTh/DM. The stripping and temperature experiments were using 100 ppm Th(IV) in 3.0 M HNO3 as aqueous phase. 0.02 M NTAamide(C8) dissolved in kerosene was used as the organic phase. Deionized water, 0.01 M HNO3, 2.0 M NaCl and 2.0 M Na2CO3 were selected as stripping agents for back extraction experiments. 2.5 mL organic phase was stirred with equal volume aqueous phase in 10.0 mL stoppered glass tube in water bath for 1 h at 25.0 ± 0.1 °C. Then, 2.0 mL loaded organic phase was taken out and stripped with equal volume stripping agent one to three times, respectively. After that, the aqueous phase was taken out to measure the concentrations of Th(IV). For the temperature study, the distribution experiments of Th(IV) as a function of temperature were conducted at 5.0° intervals between 20.0 and 40.0 °C. Extraction equilibrium was attained in 10.0 mL stoppered glass tubes of the two phases (2.0 mL of each) by stirring for 1 h in water bath capable of maintaining the temperature constant to within ±0.1 °C. The aqueous phases were taken out for measurements. The concentrations of Th(IV) were all determined by ICP-AES. Distribution ratios in the range of 102 < D < 102 are reproducible within ±20%. Distribution ratios outside this range may bear significantly larger errors and should be regarded with caution. 2.4. X-ray crystallographic measurements Crystals of [(Th(NTAamide(C4))(NO3)4)] were grown from methanol/dichloromethane by slow evaporation at room temperature. After about a week, transparent crystals formed from the solution. A suitable crystal was selected for crystallographic measurements. The crystal was kept at 143.00(10) K during data collection. Using Olex2 [34], the structure was solved with the Superflip [35] structure solution program using Charge Flipping and refined with the ShelXL-2012 [36] refinement package using Least Squares minimisation. Crystal data for Th(IV) complex [(Th(NTAamide(C4))(NO3)4)]: C30H60N8O15Th, M = 1004.90, monoclinic, space group P21/c (No. 14), a = 11.7963(2) Å, b = 15.8161(3) Å, c = 22.4960(4) Å, a = 90°, b = 96.0065(16)°, c = 90°, V = 4174.07(13) Å3, Z = 4, T = 143.00(10) K, l(Mo Ka) = 3.644 mm1, Dcalc = 1.599 g/mm3, 16,963 reflections measured (6.04 6 2H 6 52.74), 8533 unique (Rint = 0.0342) which were used in all calculations. The final R1 was 0.0331 (I > 2r(I)) and wR2 was 0.0634 (all data). The goodness of fit on F2 was 1.028. Positional parameters, hydrogen atom parameters, thermal parameters, bond distances and angles have been deposited as Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as a
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supplementary publication No. CCDC-971993. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, U.K. (fax: (+44 1223 336 033; email: [email protected] or http://www.ccdc.cam.ac.uk). 3. Results and discussion 3.1. Extraction of thorium To ensure the complete equilibrium in the following extraction experiments, effect of contact time was firstly evaluated. Contact time was varied from 10 to 120 min at 25.0 ± 0.1 °C. The results were shown in Fig. 1. It can be seen that the system approaches to equilibrium very fast, and contact of two phases for 60 min is enough for complete equilibrium. The effect of HNO3 concentration on the extraction of Th(IV), U(VI) and Lns(III) by NTAamide(C8) was investigated and the results were illustrated in Fig. 2. It is clearly indicated that the extractant displays high extraction ability for Th(IV). With the increase of HNO3 concentration, DTh decreases from 55.1 to 15.1 until it levels on 1.0 M and then increases to 30.2 with a further increase of acidity until 10.0 M HNO3. The decrease of distribution ratio can be ascribed to protonation to N donor in the center of the backbone. The subsequently increase of distribution ratio with an increase of HNO3 concentration is owing to the ionpair extraction effect. In contrast, DU and DLns are much lower than DTh. The SFTh/M at 0.1, 3.0 and 10.0 M HNO3 is listed in Table 1. As the same as the expected results, NTAamide(C8) exhibits an excellent selectivity for Th(IV) over the other metal ions. Very high SFTh/M in the range between 2.5 102 and 10 103 can be observed. 3.2. Stripping of thorium Generally, in the back extraction process for thorium, deionized water, mineral acids, salt solutions and sodium carbonate solution are often employed as stripping agents [37,38]. Thus, the stripping of Th(IV) from the loaded organic phase was investigated by using deionized water, 0.01 M HNO3, 2.0 M NaCl and 2.0 M Na2CO3. The stripping effectiveness was given in Table 2. As observed, the stripping was incomplete with deionized water, 0.01 M HNO3 and 2.0 M NaCl. The stripping percentages of deionized water, 0.01 M HNO3 and 2.0 M NaCl were only 44.8%, 33.1% and 23.0% up to three stages. With 2.0 M Na2CO3, full stripping of Th(IV) can be achieved.
Fig. 1. Effect of contact time on the distribution ratios of various metal ions by NTAamide(C8) in kerosene at 3.0 M HNO3.
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3.3. Extraction mechanism The number of extractant molecules in the extracted species can be determined if the distribution ratio for the metal extraction at a constant acidity is plotted as a function of extractant concentration. The slope is approximately equal to the number of extractant molecules. Effect of extractant concentration on the distribution ratios at 3.0 M HNO3 were shown in Fig. 3. All D values show a linear relationship with NTAamide(C8) concentration, the slope values and y-intercept were listed in Table 3. These results indicate NTAamide(C8) extracted all metal ions as mono-solvated species. Thus, the main extraction reaction of NTAamide(C8) with metal ion from HNO3 solution can be expressed as: K ex
Mnþ aq þ nNO3aq: þ Lorg: ¢ MðNO3 Þn Lorg :
Fig. 2. Effect of the HNO3 concentration on the distribution ratios of various metal ions by NTAamide(C8) in kerosene.
where M and L mean metal ions and NTAamide(C8), severally. The extraction constant, Kex, is defined as:
K ex ¼
HNO3, M
0.1
3.0
10.0
SFTh/U SFTh/La SFTh/Ce SFTh/Nd SFTh/Sm SFTh/Eu SFTh/Gd SFTh/Er
1.6 103 2.2 103 7.1 102 3.8 102 6.8 102 7.0 102 9.3 102 1.2 103
4.1 102 4.0 102 2.7 102 2.5 102 7.4 102 8.1 102 1.2 103 1.6 103
9.7 102 6.0 103 4.3 103 3.8 103 6.0 103 7.5 103 10 103 10 103
Table 2 Effect of various stripping agents on the stripping of Th(IV).
1 2 3 a
½MðNO3 Þn Lorg:
ð2Þ
½Mnþ aq: ½NO3 naq: ½Lorg:
The distribution ratio can be expressed as below:
Table 1 The SFTh/M of Th(IV) over U(VI) and Lns(III).
Stages
ð1Þ
Stripping agent Deionized water Sa, %
0.01 M HNO3 S, %
2.0 M NaCl S, %
2.0 M Na2CO3 S, %
2.8 38.4 44.8
11.1 21.4 33.1
10.1 16.2 23.0
88.7 98.1 100
S: stripping percentage.
D¼
½MðNO3 Þn Lorg:
ð3Þ
½Mnþ aq:
Substituting the value of {[M(NO3)n.L]org./[Mn+]aq.} from Eq. (3) into Eq. (2) and rearranging as below
log K ex ¼ log D n log ½NO3 aq: log ½Lorg:
ð4Þ
In Eq. (4), the equilibrium concentration of extractant equals approximately its initial concentration. Therefore, the apparent extraction equilibrium constant (log Kex) can be calculated. The results were summarized in Table 4. The average log Kex of Th(IV), only positive one, is obviously higher than that of U(VI) and Lns(III). To obtain the thermodynamic parameters for the extraction of Th(IV) by NTAamide(C8) at 3.0 M HNO3, effect of temperature on the extraction were carried out. Plot of Log Kex values as a function of the inverse temperature in the range 293–313 K gave a straight line in Fig. 4. Thus, the enthalpy change (DH) and entropy change (DS) for the extraction can be calculated using the Van’t Hoff equation:
log K ex ¼
DH 1 DS : þ 2:303R T 2:303R
ð5Þ
where R is the gas constant. Based on the values of slope and intercept, DH and DS were calculated to be 75.2 kJ mol1 and 275.7 J mol1 K, respectively. In addition, the free-energy change (DG) for the extraction could be obtained from the following equation:
DG ¼ DH T DS ¼ 2:303RT log K
ð6Þ
At 298 K, DG was calculated to be 6.9 kJ mol1. Table 3 The slope values and y-intercepts of Th(IV), U(VI) and Lns(III).
Fig. 3. Effect of extractant concentration on the distribution ratios of various metal ions at 3.0 M HNO3.
Th(IV) U(VI) La(III) Ce(III) Nd(III) Sm(III) Eu(III) Gd(III) Er(III)
Slope values
Int, y
1.15 0.82 0.94 0.96 1.05 1.01 0.99 1.02 1.14
3.34 0.19 0.83 0.72 0.36 0.15 0.23 0.38 0.32
69
H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70 Table 4 The values of the apparent extraction equilibrium constant (log Kex).
a
CLigand, M
0.002
0.005
0.01
0.02
0.05
0.1
0.2
Average
Th(IV) U(VI) La(III) Ce(III) Nd(III) Sm(III) Eu(III) Gd(III) Er(III)
1.25 n.d.a n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1.10 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
1.18 0.35 0.43 0.65 1.20 1.59 1.65 1.86 2.03
1.19 0.48 0.56 0.59 1.14 1.62 1.62 1.85 1.97
1.46 0.65 0.56 0.67 1.13 1.65 1.65 1.83 1.93
n.d. 0.52 0.52 0.69 1.13 1.59 1.65 1.83 1.88
n.d. 0.62 0.55 0.66 1.12 1.58 1.65 1.84 1.86
1.24 0.52 0.52 0.65 1.14 1.60 1.64 1.84 1.93
n.d. = not determined.
Fig. 4. Effect of temperature on the apparent extraction equilibrium constants log Kex at 3.0 M HNO3.
The entropy and entropy change for the extraction of thorium are all positive, suggesting that the process is an endothermic process. These results can be explained as the high degree of hydration of Th(IV). More water molecules need to be released; hence more energy spent for this process and accordingly the enthalpy change is positive. Furthermore, the entropy gains due to water release and the entropy loss due to bonding of extractant and nitrate ions. In the case of tetrapositive thorium, the energy spent in releasing the larger number of water molecules results in a positive entropy change. The negative value of DG at room temperature indicates that the extraction reaction is spontaneous. 3.4. Structure of thorium complex Due to the presence of long alkyl chains in NTAamide(C8), it is very difficult to cultivate the single crystal of its thorium complex. Thus, the homologue NTAamide(C4) containing the shorter alkyl chains, was employed to investigate the complexation behavior. The structure of NTAamide(C4) and its Th(IV) complex were characterized by FT-IR, NMR and single-crystal X-ray analyses. The coordination structure of complex was shown in Fig. 5. The result of 1:1 coordination of NTAamide with Th(IV) is in good accord with the result of the slope analysis in solvent extraction studies. The coordination polyhedron of complex is formed by the tetradentate ligand and four NO-3 anions, resulting in a 12-fold coordinated Th(IV). Each Th(IV) center binds to one nitrogen atom and three oxygen atoms from the ligand as well as eight oxygen anions from nitrate groups. The same result was shown in the IR spectra (Figs. S7 and S8). The bands assignable to the C@O stretching vibration of the free NTAamide(C4) ligand originally appears at 1644 cm1, while the characteristic band of the complex shifts
Fig. 5. X-ray crystal structure of the complex [(Th(NTAamide(C4))(NO3)4)]. (a) View in front. (b) View from top. H atoms have been omitted for clarity. Blue, light purple, red and gray colors denote Th, N, O and C, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 5 Selected bond lengths (Å) and angles (degree) for [(Th(NTAamide(C4))(NO3)4)]. Th1AO1 Th1AO3 Th1AO5 Th1AO8 Th1AO11 Th1AO14 C1AN1ATh1 C21AN1ATh1 N1AC11AC12 O1AC2AC1
2.435(3) 2.396(2) 2.551(3) 2.610(3) 2.623(3) 2.606(3) 103.8(2) 112.4(2) 108.5(3) 117.7(3)
ThAO2 Th1AO4 Th1AO7 Th1AO10 Th1AO13 Th1AN1 C11AN1ATh1 N1AC1AC2 N1AC21AC22 O2AC12AC11
2.520(3) 2.660(3) 2.592(3) 2.579(3) 2.570(3) 2.886(3) 110.1(2) 109.2(3) 111.2(3) 118.3(3)
40 cm1 towards lower wave numbers. In the 13C NMR spectra (Figs. S4 and S6), the signal of C@O carbon shows a downfield shift of 4.2 ppm, also suggesting that the C@O groups participate in
70
H. Huang et al. / Separation and Purification Technology 138 (2014) 65–70
coordination. As shown in Table 5, the ThAO bond distances of the nitro groups range within the area of 2.551–2.660 Å. The value of the ThAN distance (2.886 Å) is longer than the average ThAOamide bond distance (2.450 Å), indicating that ThAOamide bond is stronger than the ThAN one. FT-IR and NMR spectrums of NTAamide(C4) and its Th(IV) complex are listed in Supporting Information. 4. Conclusions Two NTAamides were synthesized for investigating the extraction and coordination for Th(IV). NTAamide(C8) exhibits a high selectivity and extraction power for Th(IV) over U(VI) and Lns(III) in a wide range of HNO3 concentration. The thermodynamic parameters show that the extraction of Th(IV) is a process of endothermic and spontaneous at 298 K. Slope analysis and single crystal X-ray diffraction observations show that 1:1 complex Th(IV) is formed between NTAamide and thorium nitrate in a tetradentate coordination mode. The special tripod-type backbone of NTAamide is demonstrated to be an almost perfect match to Th(IV). High selective extraction ability of NTAamide(C8) indicates itself a good application prospect for separation and purification of Th(IV). Acknowledgment The authors are very grateful for the financial support by the National Science Foundation of China (Nos. 91126016, J1210004 and J1103315). Analytical & Testing Center of Key Laboratory of Green Chemistry & Technology (Sichuan University) of Education Ministry of China is acknowledged for NMR, MS and ICP-AES analyses. Analytical & Testing Center of Sichuan University is acknowledged for single-crystal X-ray diffraction measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur. 2014.10. 008. References [1] F. Sokolov, K. Fukuda, H.P. Nawada, Thorium fuel cycle – potential benefits and challenges, IAEA-TECDOC-1450, May 2005. [2] V. Arkhipov, Thorium based fuel options for the generation of electricity: Developments in the 1990s, IAEA-TECDOC-1155, May 2000. [3] J.S. Herring, P.E. Macdonald, K.D. Weaver, C. Kullberg, Low cost, proliferation resistant, uranium–thorium dioxide fuels for light water reactors, Nucl. Eng. Des. 203 (2001) 65–85. [4] G. Locatelli, M. Mancini, N. Todeschini, Generation IV nuclear reactors: current status and future prospects, Energ. Policy 61 (2013) 1503–1520. [5] T. Abram, S. Ion, Generation-IV nuclear power: a review of the state of the science, Energ. Policy 36 (2008) 4323–4330. [6] L. Mathieu, D. Heuer, R. Brissot, C. Garzenne, C. Le Brun, D. Lecarpentier, E. Liatard, J.M. Loiseaux, O. Méplan, E. Merle-Lucotte, A. Nuttin, E. Walle, J. Wilson, The thorium molten salt reactor: moving on from the MSBR, Prog. Nucl. Energ. 48 (2006) 664–679. [7] H. Tong, Y. Wang, W. Liao, D. Li, Synergistic extraction of Ce(IV) and Th(IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media, Sep. Purif. Technol. 118 (2013) 487–491. [8] Y. Zuo, J. Chen, D. Li, Reversed micellar solubilization extraction and separation of thorium(IV) from rare earth(III) by primary amine N1923 in ionic liquid, Sep. Purif. Technol. 63 (2008) 684–690. [9] Y. Wang, Y. Li, W. Liao, D. Li, Preparation of high-purity thorium by solvent extraction with di-(2-ethylhexyl) 2-ethylhexyl phosphonate, J. Radioanal. Nucl. Chem. 298 (2013) 1651–1657. [10] A. Suresh, T.G. Srinivasan, P.R.V. Rao, Parameters influencing third-phase formation in the extraction of Th(NO3)4 by some trialkyl phosphates, Solvent Extr. Ion Exc. 27 (2009) 132–158.
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