Introduction

In the past decades, extraction and separation of actinides and lanthanides using amide as an alternative to phosphorus-containing extractant has attracted interesting in the processes of extracting and refining nuclear fuel materials, the treatment of radioactive wastes and radioanalytical chemistry [1–6]. It is due to the fact that amide ligand represents several outstanding advantages compared to phosphorus-containing extractant, such as facile preparation, high chemical and radiolytic stability, complete incineration which dramatically reduces the amounts of waste [7, 8]. The amide as extractant for the extraction of actinides was pioneered by Siddall in 1963 [9]. In contrast with monoamides, diamides have attracted extra attention due to strong complexation and better extraction ability. For instance, malondiamides were widely developed for the extraction of actinides during the 1980s [10]. Therefore, N,N′-dimethyl-N,N′-dioctylhexylethoxy malonamide (DIAMEX) ligand was ultimately found to have the best extraction behavior by further structural optimizations in the family of malondiamides. In the early 1990s, Stephan et al. [11] introduced an oxygen atom between both amide groups of malondiamide to synthesize diglycolamides (DGAs). The DGAs were assumed to associates with metal ions to form two bridged five-membered rings based on three oxygen atoms of one ether and two amide groups, and expected to increase the affinity with metal ions as tridentate ligands [12]. However, some important studies revealed that DGAs still are bidentate or imperfect tridentate ligands due to intrinsic property of chemical structure [13, 14]. In fact, as we all known, the two bridged five-membered rings are not as stable as two bridged six-membered rings due to the larger ring tension. For instance, naphthalene and quinoline are extremely common compounds, but similar species containing two bridged five-membered rings is rare.

Based on the above reason, we envisioned to synthesize N,N,N′,N′-tetraoctyl-4-oxaheptanediamide(TOOHA) and it’s three analogous diamides N,N,N′,N′-tetrabutyl-4-oxaheptanediamide(TBOHA), N,N,N′,N′-tetrahexyl-4-oxaheptanediamide(THOHA) and N,N,N′,N′-tetra isooctyl-4-oxaheptanediamide(Ti−OOHA). These 4-oxaheptanediamide (OHA) ligands were assumed to be associated with the metal ion to form stable two bridged six-membered rings, and significantly increase extraction behavior to actinides. In this study, we described the synthesis and characterization of various substituted OHA derivatives, and their extraction properties for uranium(VI) and thorium(IV) ions from nitric acid solution.

Experimental

Materials and instrumentation

All diluents and reagents were of analytical grades and used without any further purification. Analytical grade Th(NO3)4·4H2O and U3O8 were used. 1H NMR(400 M) and 13C NMR(400 M) spectra of extractants prepared were recorded with a Varian Mercury-400 spectrometer in CDCl3 by using TMS as the internal standard. All the extractants were further characterized by Vario EL elemental analyzer. The concentration of UO2 2+ or Th4+ in aqueous phase was determined by a 723 N model UV–Vis Spectrophotometer(Shanghai Precision and Scientific Instruments Co., Ltd., Shanghai, China).

Synthesis of extractants

TOOHA and it’s three analogous TBOHA, THOHA, Ti−OOHA were synthesized in our laboratory as depicted in Scheme 1. First, commercially available 3,3′-oxydipropanenitrile 1 was treated with concentrated hydrochloric acid (HCl) gave the corresponding 3,3′-oxydipropanoic acid 2 in excellent yield for 24 h; subsequent reaction of 3,3′-oxydipropanoic acid 2 with an appropriate dialkylamine afforded the expected ligand TOOHDA, TBOHDA and THOHDA, Ti−OOHDA in the presence of dicyclohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBT) in dichloromethane.

Scheme 1
scheme 1

Synthesis of diamide extractants

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TOOHA, TBOHA, THOHA and Ti−OOHA solutions of 8.0 × 10−2 mol/mL were prepared by dissolving the required amount of the mentioned reagents in organic solvent, which were used for the distribution experiments after proper dilution. Sodium nitrate solution of 4.0 mol/mL was used for ionic strength adjustment. The solvents were pre-saturated before use in order to prevent volume changes of the phases during extraction. The extraction was carried out at room temperature.

For a sample in each extraction experiment, 3.00 mL of diluent containing 5–80 mM extractant (pre-equilibrated with an aqueous phase and no metal ion) was placed in a 10 mL plastic vial and mixed with 3.00 mL of UO2(NO3)2 or Th(NO3)4 aqueous solution at the desired pH at 293 ± 1 K. In preliminary experiments, it had been found that the mixture was shaken for 30 min, and adequate to reach equilibrium. After phase disengagement by centrifugation (2 min, 2,000 rpm/min), duplicate 2.00 mL aliquots were taken from the aqueous phase. The remaining concentration of UO2 2+ or Th4+ in the aqueous phase was determined by the Arsenazo-III spectrophotometric method. The concentration of UO2 2+ or Th4+ in the organic phase was calculated by mass balance.

Results and discussion

Effect of nitric acid concentration in various diluents

Based on constant molar TOOHA concentration, different aliphatic and aromatic diluents were evaluated for the extraction of U(VI) with the increase of nitric acid concentration. As shown in Fig. 1, for same concentration of nitric acid, the values of distribution ratios are different in various diluents although the tendency of the change in extraction distribution is similar for all diluents. In all the range of concentrations tested, as the concentration of nitric acid increases, the distribution ratio also increases. It indicates that no obvious competing extraction of nitric acid appeared in the range of 1.0–6.0 M of nitric acid in the proposed condition. Compared to other solvents investigated, n-octane demonstrated excellent extraction behavior in nitric acid solution of relatively high acidity. Therefore, n-octane was considered as preferable diluent for further extraction study.

Fig. 1
figure 1

Dependence of the distribution ratio (D) of U(VI) on various diluents using constant molar TOOHA from 1.0 to 6.0 M HNO3 solution. [TOOHA] = 0.05 M, [UO2(NO3)2] = 6.301 × 10−4 M, T = 293 ± 1 K

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Optimization of extractant in various HNO3 concentration

Figures 2 and 3 show the effect of nitric acid concentration on the distribution ratio (D) of U(VI) and Th(IV) between the two phases based on 0.05 mol/L of the above mentioned extractants. In general, the D U(VI) increases with increasing the concentration of HNO3 from 1.0 to 6.0 M in exception with TBOHA. In the case of TBOHA, the D U(VI) reaches a maximum value at about 5.0 M HNO3, after which D U(VI) slightly decreases. This decrease can be attributed to competition of U(VI) ions and HNO3 for the coordination sites of TBOHA. It is also noted that the distribution ratio of Th(IV) slightly increases with increasing nitric acid concentration up to 6.0 M. However, the Th(IV) extraction was always significantly lower than that of U(VI). In addition, although the extractant of TBOHA always show better characteristic extraction behavior in extraction of uranyl ion compared to other three extractants, unfortunately, TBOHA has a strong tendency to form a third phase in aliphatic solvents. In our experiments, to form a third phase was often observed in TBOHA extraction system. Therefore, as a compromise of distribution ratio (D) and extraction behavior, TOOHA as a preferable extractant was chosen for the following extraction experiments. In addition, based on the above large difference of distribution ratios, we must conclude that a better U–Th separation could be achieved for the extraction of U(VI) and Th(IV). To compare D U(VI) values in Fig. 2 with ones in Fig. 3, the best separation of U(VI) from Th(IV) can be proceeded from 4.0 M HNO3 with TOOHA as a preferable extractant.

Fig. 2
figure 2

The dependences of the extraction of U(VI) on different extractants from 1.0 to 6.0 M HNO3 solution. Organic phase, [OHA] = 0.05 M in n-octane; aqueous phase, [UO2(NO3)2] = 6.301 × 10−4 M, T = 293 ± 1 K

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Fig. 3
figure 3

The dependences of the extraction of Th(IV) on different extractant from 1.0 to 6.0 M HNO3 solution. Organic phase, [OHA] = 0.05 M in n-octane; aqueous phase, [Th(NO3)4] = 4.310 × 10−4 M, T = 293 ± 1 K

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Effect of extractant and salting-out agent concentrations

The extraction of uranium(VI) and thorium(IV) from 4.0 mol/L nitric acid solution with TOOHA increases with increasing extractant and salting-out agent(NaNO3) concentrations (Figs. 4, 5).

Fig. 4
figure 4

Effect of TOOHA concentrations in the organic phase on the distribution ratios of UO2 2+ in the presence of salting-out agent; [HNO3] = 4.0 M, [UO2(NO3)2] = 6.301 × 10−4 M, T = 293 ± 1 K, [NaNO3] = 0, 1, 2, 4 M

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Fig. 5
figure 5

Extraction dependence of Th4+ on TOOHA concentration in the presence of salting-out agent. [HNO3] = 4 M, [Th(NO3)4] = 4.310 × 10−4 M, T = 293 ± 1 K, [NaNO3] = 0, 1, 2, 4 M

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The extraction of UO2 2+ in Fig. 4 shows the linear relations of log D U versus log[TOOHA](o) at the different ionic strengths (the plots in figures are the averages of two experiments). The calculated slopes are about two in case of U(VI). As we known, the coordination number of U in UO2 2+ manner is 6 [11]. Thus, the present result suggests TOOHA functions as a tridentate ligand, which is in good agreement with our expectation (see “Introduction” section), and is distinct with diglycolamide extractants reported [9]. The plots of log D U versus log[TOOHA](o) also indicates that the extracted complex can be described as UO2(NO3)2(TOOHA)2, the extraction reaction can be expressed as follows:

$$ {\text{UO}}_{{2\,\,({\text{a}})}}^{2 + } + 2{\text{NO}}_{{3\,\,({\text{a}})}}^{ - } + 2{\text{TOOHA}}_{{({\text{o}})}} \rightleftharpoons {\text{UO}}_{2} ({\text{NO}}_{3} )_{2} ({\text{TOOHA}})_{{2({\text{o}})}} $$
(1)

For the above extraction reaction, the equilibrium constant K ex can be written as:

$$ K_{\text{ex}} { = }\frac{{ [ {\text{UO}}_{2} ({\text{NO}}_{3} )_{2} ({\text{TOOHA}})_{2} ]_{{({\text{o}})}} }}{{ [ {\text{UO}}_{2}^{2 + } ]_{{({\text{a}})}} [{\text{NO}}_{3}^{ - } ]_{{({\text{a}})}}^{2} [{\text{TOOHA}}]_{{({\text{o}})}}^{2} }}, $$
(2)

where subscripts (a) and (o) stand for the presence of the species in the aqueous and organic phase, respectively. In view of the tracer metal extracted, the distribution ratio D is described as:

$$ D{ = }\frac{{ [ {\text{UO}}_{2} ({\text{NO}}_{3} )_{2} ({\text{TOOHA}})_{2} ]_{{({\text{o}})}} }}{{ [ {\text{UO}}_{2}^{2 + } ]_{{({\text{a}})}} }} $$
(3)

Introducing distribution ratio to the equilibrium Eq. 2, which can be rewritten by taking the logarithm and rearranged as:

$$ { \log }\,D = { \log }\,K_{\text{ex}} + {\text{ 2log }}\left[ {{\text{NO}}_{ 3}^{ - } } \right]_{{({\text{a}})}} + {\text{ 2log }}\left[ {\text{TOOHDA}} \right]_{{({\text{o}})}} $$
(4)

Thus, instead of activities with molar concentrations, values of equilibrium constants of U(VI) can be easily determined by using the slope method according to Eq. 4.

The linear relations of log D Th with log[TOOHA] are illustrated in Fig. 5. The calculated slopes are within the range from 1.90 to 2.73. As we known, Th4+ has a coordination number of 8 or 9 [11]. Based on these non integral data, so we can assume at least two following reactions for Th4+ extraction:

$$ {\text{Th}}_{{({\text{a}})}}^{4 + } + 4{\text{NO}}_{{3\,\,({\text{a}})}}^{ - } + 2{\text{TOOHDA}}_{{({\text{o}})}} \rightleftharpoons {\text{Th}}({\text{NO}}_{3} )_{4} ({\text{TOOHA}})_{{2({\text{o}})}} $$
(5)
$$ {\text{Th}}_{{({\text{a}})}}^{4 + } + 4{\text{NO}}_{{3\,\,({\text{a}})}}^{ - } + 3{\text{TOOHDA}}_{{({\text{o}})}} \rightleftharpoons {\text{Th}}({\text{NO}}_{3} )_{4} ({\text{TOOHA}})_{{3({\text{o}})}} $$
(6)

The above data suggest that the dominating compositions of extracted species in organic phase is Th(NO3)4·2TOOHA in low nitrate ion; But, it has a tendency to form Th(NO3)4·3TOOHA species with increasing nitrate ion concentrations or stronger salting-out effect. Because Th4+ has a coordination number of 8 or 9, the present results suggested that Th(NO3)4·2TOOHA species must have inner sphere hydration under low ionic strength.

Figures 4 and 5 also show the effect of sodium nitrate concentrations (salting-out agent) on the distribution ratios of U(VI) and Th(IV) while keeping constant acid concentration at 4.0 M. It is clear that the distribution ratios of U(VI) dramatically increase as initial nitrate ion concentrations increase. Due to D Th only slightly increase accompanying with increasing concentrations of salting-out agent. Therefore, a maximum U–Th separation could be completed for the extraction of U(VI) and Th(IV). The separation factor for D U(VI)/D Th(VI) > 10 shows the potential value of TOOHA extractant in U(VI)/Th(IV) separations.

Spectroscopic data

After purification and identification, the yields of TOOHA, TBOHA, THOHA and Ti−OOHA are 75.8, 64.3, 73.0 and 74.2 %, respectively, and the purity of all the ligands was found to be >97 %. For TOOHA, 1H NMR(CDCl3, 400 MHz): δ 3.78 (t, J = 7.2 Hz, 4H), 3.28 (t, J = 7.6 Hz, 4H), 3.20 (t, J = 7.6 Hz, 4H), 2.59 (t, J = 6.8 Hz, 4H),1.50–1.59 (m,8H), 1.19–1.31 (m, 40H), 0.86–0.90 (q, 12H) ppm; 13C NMR(CDCl3, 100 MHz): δ 170.11, 67.28, 47.91, 45.80, 33.30, 31.66, 29.26, 29.14, 29.00, 27.66, 26.96, 26.83, 22.51, 13.96. C38H76N2O3(609.02): calcd. C 74.94 %, H 12.58 %, N 4.60 %, O 7.88 %; found C 74.63 %, H 12.54 %, N 4.61 %, O 7.90 %.

For TBOHA, 1H NMR(CDCl3, 400 MHz,): δ 3.77 (t, J = 7.2 Hz, 4H), 3.28 (t, J = 7.6 Hz, 4H), 3.21 (t, J = 7.6 Hz, 4H), 2.59 (t, J = 6.8 Hz, 4H), 1.44–1.55 (m, 8H), 1.24–1.33 (m, 8H), 0.88–0.96 (m, 12H) ppm; 13C NMR(CDCl3, 100 MHz): δ 170.45, 67.36, 47.82, 45.72, 33.40, 31.15, 29.86, 20.22, 20.10, 13.84. C22H44N2O3(384.60): calcd. C 68.70 %, H 11.53 %, N 7.28 %, O 12.48 %; found C 68.93 %, H 11.55 %, N 7.26 %, O 12.51 %.

For THOHA, 1H NMR(CDCl3, 400 MHz): δ 3.77 (t, J = 6.8 Hz, 4H), 3.28 (t, J = 7.6 Hz, 4H), 3.21 (t, J = 7.6 Hz, 4H), 2.59 (t, J = 6.4 Hz, 4H), 1.49–1.59 (m, 8H), 1.19–1.31 (m, 24H), 0.86–0.91 (q, 12H) ppm; 13C NMR(CDCl3, 100 MHz): δ 170.20, 67.30, 47.97, 45.85, 33.36, 31.49, 28.99, 27.64, 26.61, 26.48, 22.51, 13.91. C30H60N2O3(496.81): calcd. C 72.53 %, H 12.17 %, N 5.64 %, O 9.66 %; found C 75.79 %, H 12.20 %, N 4.61 %, O 9.68 %.

For T i−OOHA, 1H NMR(CDCl3, 400 MHz): δ 3.77 (t, J = 6.8 Hz, 4H), 3.23–3.33 (m, 4H), 3.14-3.21 (m, 4H), 2.51 (t, J = 6.8 Hz, 4H), 1.60–1.65 (m, 4H), 1.24–1.27 (m, 32H), 0.84–0.90 (q, 24H) ppm; 13C NMR(CDCl3, 100 MHz): δ 171.11, 67.47, 51.85, 48.55, 38.30, 36.86, 33.72, 30.45, 30.36, 28.66, 23.73, 23.66, 22.93, 13.93, 10.74, 10.57. C38H76N2O3(609.02): calcd. C 74.94 %, H 12.58 %, N 4.60 %, O 7.88 %; found C 75.14 %, H 12.63 %, N 4.60 %, O 7.89 %.

Conclusion

In conclusion, different types of structurally modified 4-oxaheptanediamide ligands have been prepared. TOOHA was chosen as a new chelating ligand for extracting U(VI) and Th(IV) in n-octane. UO2(NO3)2(TOOHA)2 was determined as predominant extracted species by comparing the distribution ratios (D U) of uranium(VI), in the concentration ranges of TOOHA and HNO3 between the aqueous and organic phases. The extraction of UO2 2+ into n-octane by strong complexation with TOOHA from nitric acid aqueous solution indicates 4-oxaheptanediamides probably functions as a true tridentate ligand in extraction process, rather than bidentate chelation like diglycolamide extractant. Excellent separation factors for D U/D Th > 10 show the potential value of present extractant in U–Th separation. Further work to extend the extraction of Eu3+, NpO2 +, Am3+ and other actinides and lanthanides using 4-oxaheptanediamide ligands is ongoing in our laboratory.