Introduction

Using chemical procedures to separate and recover the useful components from the spent fuel produced by nuclear reactor was playing a fatal role for the sustainable development of nuclear power [14]. Among the various separation methods at industrial scale, due to several outstanding advantages such as the ability for continuous operation, high throughput and solvent recycling, the solvent extraction is recognized as an essential technique in the concentration and purification of uranium in the nuclear fuel field [5, 6]. For treatment of high level radioactive waste, the extraction and separation of the useful components have extensively been studied using the novel extractants [7, 8]. For instance, originated from the early monoamides, many different structures of diamides have been developed for recovery of actinides from spent fuel, including oxalamide, succinamide, malonamide, glutalamide and diglycolamides [914]. The previous reports have revealed that the different structural backbone or substituent group of diamides has an impact to the actinide extraction abilities in liquid–liquid extraction [9, 15].

Based on our previous study in synthesis of amidic extractants [11], we attempted to attach two succinamide molecules to form new BisSCA backtone by several-step synthetic strategies. Due to these compounds offering the possibility of four oxygen donors that can chelated with the metal ion, these ligands were expected to improve affinity for actinides and lanthanides and enhance extraction efficiency compared to the previous single functional succinamides [16, 17]. The newly synthesized extractants, N 1,N 1,N 4,N 4-tetrabutyl-N 2,N 3-ethidene bis-succinamide (TBE-BisSCA), N 1,N 1,N 4,N 4-tetrahexyl-N 2,N 3-ethidene bis-succinamide (THE-BisSCA) and N 1,N 1,N 4,N 4-tetraoctyl-N 2,N 3-ethidene bis-succinamide (TOE-BisSCA) were summarized in Fig. 1. In this study, the detailed synthetic routes of the above BisSCA were described. The extraction behavior of uranium ions using the above ligands was investigated from nitric acid solution.

Fig. 1
figure 1

Newly synthesized BisSCA extractants

Experimental

Materials and equipments

All diluents and reagents were of analytical grades and used without any further purification. 1H NMR and 13C NMR spectra of extractants prepared were recorded with a Varian Mercury-400 spectrometer in CDCl3 by using TMS as the internal standard. Purification of all the extractants was further characterized by high-resolution mass spectra (HRMS) using a Bruker Esquire 6000 mass spectrometer. IR spectra were recorded with an FT-IR spectrometer and only major peaks were reported. Analytical grade U3O8 was used. The concentration of UO2 2+ in aqueous phase was determined by the Arsenazo-III spectrophotometric method using a 723 N model UV–Visible Spectrophotometer (Shanghai, China).

Synthesis of extractants

TBE-BisSCA and its two analogous THE-BisSCA and TOE-BisSCA were synthesized in our laboratory as described in Scheme 1. Commercially available succinic anhydride (20 mmol) was dissolved in CH2Cl2 (50 mL), and then the ethylenediamine (8 mmol) in CH2Cl2 was added dropwise to the above solution over 30 min. After stirred at room temperature for about 20 h, the mixture was filtered under vacuum and washed with acetone for several times. The obtained powder was dried overnight. Subsequently, the powder (5 mmol, no further purification) was poured into a round bottom flask with 50 mL of CH2Cl2, and then dicyclohexylcarbodiimide (DCC, 11 mmol) and 1-hydroxybenzotriazole (HOBT, 11 mmol) was added to the solution. After stirred for 30 min, an appropriate secondary amine (11 mmol) in CH2Cl2 was added to the reaction solution. After the further 20 h, the crude products were obtained by the appropriate procedure. The expected TBE-BisSCA, THE-BisSCA and TOE-BisSCA extractants were obtained in 70–85 % yields by the appropriate procedures and final silica gel column separation method.

Scheme 1
scheme 1

Synthesis of BisSCA extractants by starting succinic anhydride

Extraction procedure

In each extraction experiment, 3.00 mL pre-equilibrated organic phase with the desired amount of BisSCA extractants was contacted with 3.00 mL nitric acid solution of variable concentrations (1.0–6.0 mol/L) containing uranyl ions. The mixture was placed in a 10 mL plastic vial and shaken mechanically for 30 min (it had been found in our previous experiments that this time was adequate to obtain equilibrium) at 298 ± 1 K. After centrifugation (2 min and 2000 rpm/min), duplicate 2.00 ml of aliquot was taken from the aqueous phases and the concentration of uranyl ions in the aqueous phase was determined by the Arsenazo-III spectrophotometric method. The concentration of uranyl ions in the organic phase was calculated by mass balance. The distribution ratio D U(VI) was defined as the ratio of the concentration of uranyl ions in the organic phase to that in the aqueous phase. The data reported in this study are the averages of at least two experiments, and the errors are no more than 5 %.

Results and discussion

Effect of nitric acid concentration in various diluents

Based on the constant molar concentration of TBE-BisSCA (50 mmol/L), various popular diluents were employed to evaluate the extraction behavior of uranyl ions at the different concentration of nitric acid. As shown in Fig. 2, the distribution ratios of U(VI) (D U(IV)) were clearly distinct in the different diluents. It varied from 0.003 to 2.85 depending upon the nature of diluents used and the concentration of nitric acid. It is worthwhile to compare the data generated in the present study. Generally, the non-polar and weak polar solvents for extraction exhibited the better extractability compared to the polar lauryl alcohol. In additional, it was observed that D U(IV) increased with increase in the concentration of nitric acid for all present diluents, reached a maximum value at 4 or 5 mol/L depending on the diluents used, and followed by decreased in D U(IV) values. On the other hand, these BisSCA extractants in solvents such as lauryl alcohol was also studied considering a future industrial application. However, only disappointed results was observed with lauryl alcohol in the range of 1–6 M nitric acid, where distribution ratios were one order of magnitude lower than those obtained in other solvents. In comparison with all solvents employed, xylene exhibited the excellent extraction behavior in nitric acid solution of relatively high acidity. Therefore, xylene was considered as the optimum diluent for further extraction experiments.

Fig. 2
figure 2

Dependences of the distribution ratio (D) of U(VI) on various diluents using constant BisSCA from 1.0 to 6.0 M HNO3 solution. [TBE-BisSCA] = 0.05 M, [UO2(NO3)2] = 5.173 × 10−4 M, T = 298 ± 1 K

Optimization of extractant in various concentration of nitric acid

The extraction of uranyl ions with xylene as optimum diluent by the above extractants (0.05 mmol/L) was carried out under various concentration of nitric acid. As shown in Fig. 3 the distribution ratio of uranyl ions increased with increasing the concentration of nitric acid, reached the maximums at 4 M for TBE-BisSCA, at 5 M for THE-BisSCA and TOE-BisSCA, respectively. The further increase concentration of nitric acid to higher acidity resulted in a decrease of the distribution ratio of uranyl ions, which can be attributed to competition of U(VI) ions and HNO3 for the coordination sites of BisSCA. It is noteworthy that the distribution ratio of U(VI) ions obtained for all the present BisSCA extractants are higher than those determined for single succinamides including N,N,N′,N′-tetrabutylsuccinamide (TBSCA), N,N,N′,N′-tetrahexylsuccinamide (THSCA) and N,N,N′,N′-tetraoctylsuccinamide (TOSCA) under the same experimental conditions, even when they are used at double concentration to match the number of binding groups of BisSCA. It indicated that BisSCAs could be associated with the metal ion in other mode of complexation compared to single succinamides. Additionally, Fig. 3 also demonstrated that the extraction of uranyl ions using THE-BisSCA extractant gave the highest distribution ratio under the present condition compared to other two extractants. Therefore, the best extraction of uranyl ions from the aqueous phase can be proceeded at about 5.0 mol/L of nitric acid with THE-BisSCA as the optimum extractant, which was chosen for the following extraction experiments.

Fig. 3
figure 3

Dependences of the extraction of U(VI) on different extractants from 1.0 to 6.0 M HNO3 solution. Organic phase, [BisSCA] = 0.05 M in xylene; aqueous phase, [UO2(NO3)2] = 5.173 × 10−4 M, T = 298 ± 1 K

Effect of extractant and salting-out agent concentration

Based on the optimum THE-BisSCA extractant, the relations of logD versus. log[BisSCA] at the different ionic strengths are illustrated in Fig. 4. It is observed that the distribution ratio of uranyl ions increases with increasing the extractant loading and the salting-out agent (NaNO3) concentrations at 5.0 mol/L of nitric acid medium. The linear relationships of logD and log[BisSCA] were obtained in a slope of ca. 1.3 without or with NaNO3 in the present extraction system. It indicates that the stoichiometry of the extraction complex was one molecule UO2 2+ combining to one or two molecule BisSCA, and the major complex can be recognized as 1:1 complex. Due to the BisSCA extractants containing four oxygen atoms of amide, may show mutidentate features. As we all known, the U(VI) cation has a coordination numbers of 4–6 on the equatorial site. Therefore, the extraction reaction can be assumed to proceed by formation of the major UO2(NO3)2·(BisSCA)2 and minor UO2(NO3)2·(BisSCA)2 species as depicted in Fig. 5.

Fig. 4
figure 4

Effect of THE-BisSCA concentrations in the organic phase on the distribution ratios of UO2+ in the presence of salting-out agent; [HNO3] = 5.0 mol/L, [UO2(NO3)2] = 5.173 × 10−4 M, T = 298 ± 1 K, [NaNO3] = 0, 1, 2 M

Fig. 5
figure 5

The structure of the possible complexes of uranyl ion with BisSCA extractants

Figure 4 also show the effect of sodium nitrate concentrations (salting-out agent) on the distribution ratios of U(VI) while keeping acid concentration at 5.0 M. Although the above study exhibited that the distribution ratio of uranyl ions reached the maximums at 5M for THE-BisSCA, it is clear that the distribution ratios of U(VI) increase with loading of nitrate sodium. The lowering of D U(VI) when adding nitrate sodium into aqueous solution is not observed. This difference can be attributed to competition of U(VI) ions and HNO3 for the coordination sites of BisSCA, and the absence of NaNO3 competition for available BisSCA sites.

Infrared spectra

To detailed insight into the bonding of the extracted species of U(VI) by BisSCA, the Infrared spectra of the organic phase were investigated before and after U(VI) extraction from 5M HNO3. The free C=O stretching vibration of THE-BisSCA is found at 1641 cm−1. Two vibration bands appearing at 1464 and 1429 cm−1 are due to the C–N group of the extractant. The IR spectra of U(VI) extracted by THE-BisSCA showed that the relative intensity of the free C=O group at 1641 cm−1 decreased with the appearance of new bands at 1583 cm−1 of the complexed carbonyl vibration for U(VI). The shift of 58 cm−1 in the C=O frequency suggests that a strong binding of THE-BisSCA with U(VI) after the extraction of UO2 2+. The C–N band at 1464 cm−1 is shifted to a higher frequency at 1472. These data indicated that U(VI) are mainly coordinated to the oxygen of carbonyl group of BisSCA. The present results are in good agreement with the previous reports [18].

Spectroscopic data

After purification and identification, the purity of three BisSCA extractants was found to be >97 %. For THE-BisSCA, 1H NMR (CDCl3, 400 MHz): δ 7.18 (s, 2H), 3.29(t, J = 7.6 Hz, 8H), 3.23 (t, J = 7.6 Hz, 4H), 2.70 (t, J = 6.4 Hz, 4H), 2.50 (t, J = 6.4 Hz, 4H), 1.43–1.59 (m, 8H), 1.25–1.35 (m, 8H), 0.88–0.97 (m, 12H) ppm; 13C NMR (CDCl3, 100 MHz): δ 173.65, 171.50, 47.65, 45.82, 38.77, 31.41, 30.83, 29.82, 28.83, 20.17, 20.08, 13.78 ppm; IR (KBr): 3317, 2958, 2929, 2869, 2374, 1704, 1631, 1559, 1547, 1459, 1429, 1376, 1257, 1211, 1143, 1115 cm−1; HRMS (ESI, C26H50N4O4): Calcd. m/z: 505.3730 [M + Na]+; Found m/z: 505.3720[M + Na] + . For THE-BisSCA, 1H NMR (CDCl3, 400 MHz): δ 7.21 (s, 2H), 3.20–3.30 (m, 12H), 2.70 (t, J = 5.6 Hz, 4H), 2.50 (t, J = 6.0 Hz, 4H), 1.48–1.56 (m, 8H), 1.26–1.30 (m, 24H), 0.85–0.90 (m, 12H) ppm; 13C NMR (CDCl3, 100 MHz): δ 173.60, 171.40, 47.85, 46.07, 38.69, 31.50, 31.40, 28.80, 28.70, 27.64, 26.62, 26.52, 22.48, 13.91 ppm; IR (KBr): 3314, 2956, 2928, 2858, 1641, 1549, 1464, 1429, 1377, 1256, 1183, 1142 cm−1; HRMS (ESI, C34H66N4NaO4): Calcd. m/z: 595.5162 [M + H]+, 617.4982 [M + Na]+; Found m/z: 595.5156 [M + H]+, 617.4972 [M + Na]+. For TOE-BisSCA, 1H NMR (CDCl3, 400 MHz): δ 7.27 (s, 2H), 3.20–3.31 (m, 12H), 2.69 (t, J = 6.0 Hz, 4H), 2.50 (t, J = 5.6 Hz, 4H), 1.47–1.55 (m, 8H), 1.25–1.28 (m, 40H), 0.86–0.88 (m, 12H) ppm; 13C NMR (CDCl3, 100 MHz): δ 173.63, 171.55, 47.97, 46.18, 38.76, 31.72, 31.42, 29.30, 29.15, 28.78, 27.70, 27.01, 26.89, 22.54, 13.99 ppm; IR (KBr): 3315, 2926, 2856, 1704, 1634, 1547, 1463, 1430, 1377, 1354, 1258, 1175, 1141 cm−1; HRMS (ESI, C42H83N4O4): Calcd. m/z: 707.6414 [M + H]+, 729.6234 [M + Na]+; Found m/z: 707.6417 [M + H]+, 729.6234 [M + Na]+.

Conclusions

The three novel extractants based on the BisSCA structure have been synthesized and studied for U(VI) extraction from nitric acid medium. The present method for synthesis of BisSCA extractants has simple process and easy post-treatment procedures, and is especially suitable for production at large scale. The extraction experience demonstrated that compared to the other diluents employed, the best distribution ratio of uranyl ion was observed in xylene medium. The extraction efficiency of U(VI) using BisSCA increased with increase in the concentration of nitric acid, and the best D U(IV) values were obtained when THE-BisSCA was exploited as extractant at 5 M HNO3 solution. Unlike the single succinamides, BisSCA extractant exhibited the better extraction behavior, which could be attributed that BisSCAs associated with the UO2 2+ ion in different mode of complexation. Further extension of the present study must be of interest, and the results obtained will inspire us to synthesize more new extractants, and extract the other actinides and lanthanides.