Complexation between uranyl(Ⅵ) and CMPO in a hydroxyl-functionalized ionic liquid: An extraction, spectrophotography, and calorimetry study

Complexation between uranyl(Ⅵ) and CMPO in a hydroxyl-functionalized ionic liquid: An extraction, spectrophotography, and calorimetry study

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Baihua Chen, Jun Liu, Hongyuan Wei, Yuchuan Yang, Xingliang Li, Shuming Peng^*, Yanqiu Yang^*

Chinese Chemical Letters | 2022, 33(7) : 3451 - 3455

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Chinese Chemical Letters | 2022, 33(7): 3451-3455

• Communication •

Complexation between uranyl(Ⅵ) and CMPO in a hydroxyl-functionalized ionic liquid: An extraction, spectrophotography, and calorimetry study

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Baihua Chen, Jun Liu, Hongyuan Wei, Yuchuan Yang, Xingliang Li, Shuming Peng^*, Yanqiu Yang^*

Affiliations

Institute of Nuclear Physics and Chemistry, CAEP, Mianyang 621900, China

Published: 2022-07-15 doi: 10.1016/j.cclet.2021.12.066

Outline

Abstract

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The extraction complexes of uranyl(Ⅵ) in HNO₃ to a hydroxyl-functionalized ionic liquid (IL) phase, HOEtmimNTf₂ bearing CMPO, were investigated. Three possibly successive extraction complexes, UO₂L²⁺ (L = CMPO), UO₂L₂²⁺ and UO₂L₃²⁺, were detected based on variable U/L ratios. Uranyl(Ⅵ) prefers to be extracted as complex UO₂L₃²⁺, combining with the ions from HOEtmimNTf₂ to construct a solid material through self-assembly. The thermodynamics of complexes, UO₂L²⁺ (j = 1-3), were studied by spectrophotometry and microcalorimetry. All the formation reactions are principally driven by entropy, although a small part of the driving force of complexes UO₂L₂²⁺ and UO₂L₃²⁺ comes from enthalpy. Based on the thermodynamic properties for complex UO₂L₃²⁺, we provide a possible coordination mode in HOEtmimNTf₂: the first CMPO molecule coordinates with UO₂²⁺ in a bidentate fashion while the others do in a monodentate fashion. The results offer a thermodynamic insight into the formation behaviors of the uranyl(Ⅵ)/CMPO complexes involving the special IL HOEtmimNTf₂, which is of significance to advance the novel IL extraction strategy.

Key words

Uranyl(Ⅵ) / CMPO / Ionic liquid / Solvent extraction / Complexation / Thermodynamics

Cite this Article

Baihua Chen, Jun Liu, Hongyuan Wei, Yuchuan Yang, Xingliang Li, Shuming Peng, Yanqiu Yang. Complexation between uranyl(Ⅵ) and CMPO in a hydroxyl-functionalized ionic liquid: An extraction, spectrophotography, and calorimetry study[J]. Chinese Chemical Letters, 2022 , 33 (7) : 3451 -3455 . DOI: 10.1016/j.cclet.2021.12.066

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Liquid-liquid extraction is the most maturely popular separation technique for the reprocessing of spent nuclear fuels. Due to its unique physicochemical and solvation properties [1, 2], ionic liquid (IL) is promising as an efficient and eco-friendly solvent to recover actinides from spent nuclear fuels. Extensive studies have been conducted over years on the extraction and complexation of actinides and lanthanides, replacing volatile molecular solvent dissolving the common ligands with ILs. Positive results on efficiency and selectivity have been achieved [2-6]. For example, uranyl(Ⅵ) in HNO₃ was more effectively extracted by trioctylphosphine oxide (TOPO) in butylmethylimidazolium bis(trifluoromethylsulfonyl)imide (C₄mimNTf₂) greatly elevated the extraction efficiency compared with the same extraction conducted with TOPO in dichloromethane [6].

Octylphenyl(N, N-diisobutylcarbamoylmethyl)phosphine oxide (CMPO) has received attention as one of the most widely studied phosphorous-based extractants to separate tri-, hexa- and tetravalent actinides from acidic solutions [7-9]. Schemes of actinides extraction from HNO₃ solutions by CMPO in ILs have been achieved recently [3, 10-12]. Comparison studies illustrated the extraction efficiency enhances largely with ILs replacing molecular solvents as the diluent of ligands. Visser et al. using the mixture extractant of CMPO and TBP (tributyl phosphate), found that the distribution ratios for Am(Ⅲ), Th(Ⅳ), Pu(Ⅳ) and U(Ⅵ) were all at least an order of magnitude higher when butylmethylimidazolium hexafluorophosphate (C₄mimPF₆) as the diluent of extracting phase than those when n-dodecane as diluent [10]. Rout et al. observed much larger D_Am values using TBP and CMPO in C₄mimNTf₂ than in n-dodecane [13].

Most recently, we provided a novel strategy for efficient capture of uranium from nitric acid solutions with CMPO in (1-hydroxyethyl-3-methyl)imidazolium bis(trifluoromethylsulfonyl)imide (HOEtmimNTf₂), a hydroxyl-functionalized ionic liquid [14]. The specific property of HOEtmimNTf₂ triggers the self-assembly of the uranyl(Ⅵ)/CMPO complex at the aqueous-IL interface to form a solid material, with the composition of 3/9/1/7 for UO₂²⁺/CMPO/HOEtmim⁺/NTf₂^-. Almost all aqueous uranium (95%) was transferred to the solid phase. However, the extraction species as well as the complexation of uranyl(Ⅵ) with CMPO involving HOEtmimNTf₂ still keep unknown. Herein, we describe the thermodynamic behaviors of complexation between uranyl(Ⅵ) and CMPO in HOEtmimNTf₂, investigated by biphasic extraction and monophasic titration. Based on the determined thermodynamics for complexes, we further discuss the chelation fashions of CMPO to uranyl(Ⅵ). The results and discussion are of significance in understanding the corresponding extraction behaviors and then optimizing the extraction systems.

It is generally accepted that ILs do not behave as inert diluents toward nitric acid [15, 16]. As depicted in Fig. S1 (Supporting information), the HNO₃ concentration in HOEtmimNTf₂ increased linearly with the increase of the aqueous HNO₃ concentration. HNO₃ distribution ratio was calculated to be D_HNO₃=0.31 by the linear slope, suggesting that strong interaction between HOEtmimNTf₂ and HNO₃ occurred. CMPO is of weak basicity and interacts with HNO₃. It has been demonstrated that HNO₃ can be extracted into the organic phase, using molecular solvents as the diluent of CMPO [17-19]. For example, equilibrating CMPO dissolved in NPHE (nitrophenylhexyl ether) with aqueous HNO₃ solution, HNO₃·CMPO was formed in the organic phase with the equilibrium constant K = 3.1 × 10^-5 [18]. Given this, we conducted an additional experiment, equilibrating HOEtmimNTf₂ bearing CMPO with 0.1 mol/L HNO₃. As shown in Table S1 (Supporting information), in the concentration range of CMPO (0.024–0.064 mol/L), the equilibrium HNO₃ concentrations in the IL phases (0.0283–0.0274 mol/L) changed around 0.0277 mol/L (the equilibrium HNO₃ concentration in the IL phases without CMPO). Taking experimental uncertainty into account, we believe that the CMPO concentration in the IL phase affects the HNO₃ distribution little. In other words, the interaction of HNO₃ with CMPO is incomparable with that with HOEtmimNTf₂. It is the right reason for our previous observation that the HNO₃ concentration in the aqueous phase did not affect the uranium extraction efficiency [14]. Consequently, to simplify the experiment process and data treatment, the experimental solutions were prepared with the equal-volume equilibrated HOEtmimNTf₂ and 1.0 mol/L HNO₃, while the protonation of ligand CMPO was not considered.

The formation of extraction complexes in the biphasic extraction system is essential to produce solid assembly material. Firstly we conducted extractions on a constant-temperature water bath shaker, with the aqueous phase with UO₂(NO₃)₂ (no more than 1.0 mmol/L) in 1.0 mol/L HON₃. Pre-experiments showed that no observably solid assembly material appeared at the inter-surface of the biphasic system in a day time when the initial uranyl(Ⅵ) concentration in 1.0 mol/L HNO₃ solution was less than 1.0 mmol/L. Cation exchange has been presumed to be the partitioning mechanism for the biphasic metal extraction equilibrium with hydrophobic IL as diluent [1, 20-22]. Considering the stoichiometric ratio of the solid material obtained at the interface of the biphasic system [14], we hypothesized three successive extraction complexes formed possibly in the IL phase, i.e., UO₂L²⁺, UO₂L₂²⁺ and UO₂L₃²⁺. The uranium distribution ratio (D_U) should function as a cubic polynomial equation along with the CMPO concentration ([L]) in the IL phase (Eq. S5 in Supporting information). Fig. 1 depicts the UO₂(NO₃)₂ extraction results by CMPO in HOEtmimNTf₂ at 25 ℃. Fitting the extraction data with Eq. S5 based on the least square method, as shown the short dash line in Fig. 1a, the apparent equilibrium constants (K_ex^app) for the three successive extraction complexes were obtained to be K_ex,1^app = 2.16 ± 0.24, K_ex,2^app = 4.97 ± 0.34, and K_ex,3^app = 8.53 ± 0.03, respectively. Interestingly that (K_ex,3^app-K_ex,2^app) > (K_ex,2^app-K_ex,1^app) > K_ex,1^app, suggests that, for this extraction system, uranium prefers to be extracted by the complexes of higher uranyl(Ⅵ) stoichiometric ratio, especially UO₂L₃²⁺. It can be interpreted that along with the uranyl(Ⅵ) stoichiometric ratio increasing, the extraction complexes become more hydrophobic, which facilitates their transfer into the organic phase [21]. As illustrated in Fig. 1b, in a dual logarithmic coordinate system a fine linearship we observed between D_U and [L] (2.6-4.5 mmol/L) with a slope of 2.93, suggesting complex UO₂L₃²⁺ being the predominant extraction species [6].

As a result of the Laporte forbidden electronic transitions from ligand orbitals to empty 5f orbitals of central uranium, uranyl(Ⅵ) in the range of 485–380 nm behaves weak absorption bands which are mainly governed by the geometry of the equatorial coordination and only to a less extent affected by the chemical nature of the ligands [23, 24]. In the present work, identification of the complex species between UO₂²⁺ and CMPO, as well as the determination of the stability constants of the complexes, was achieved by spectrophotometric titrations of UO₂²⁺ with CMPO in HOEtmimNTf₂ [25, 26]. As shown in Fig. 2, additions of CMPO caused significant changes in the shape and location of the absorption bands. As the L/U ratio (R = C_L/C_U) increased to about 2.32, the series of absorption bands became sharper, well-defined, continuously red-shifted, and intensified. However, different changes were observed after R was higher than 2.32. The main absorption bands at about 402, 413, 425.5, and 438 nm were continued to red-shifted, accompanied by decreases in intensities, while the absorption band at about 390 nm changed little. For example, as shown in the insert picture of Fig. 3a, when R increased from 2.32 to 3.49, the peak center of the most intensive absorption band red-shifted from 425.5 to 426.0 nm, while its intensity reduced from 0.0432 to 0.0419; the absorption bands at about 452, 464, 481, and 498 nm increased slightly in intensity. The continuous changes of the absorption spectra in titration indicate the successive complexation of UO₂²⁺ with CMPO.

Factor analysis by HypSpec program [27] suggested the presence of four absorption species in the system, i.e., free UO₂²⁺ and three successive complexes like UO₂L²⁺, UO₂L₂²⁺, and UO₂L₃²⁺. Fit converged well at the stability constants (logβ) of 4.82 (5), 9.39(12), and 12.98(5) for complex UO₂L²⁺, UO₂L₂²⁺, and UO₂L₃²⁺, respectively. The calculated molar absorption spectra of the species are depicted in Fig. 3b. The better-defined vibronic fine structure and sharper absorption bands clued that uranyl(Ⅵ) complexed with CMPO in an ordered ligand field [25, 28]. The spectrum changes of the titration system (Fig. 3a) can be interpreted with the quite different spectra of uranyl(Ⅵ) species and their concentration changes in the solution. For instance, Absorbancy of complex UO₂L₃²⁺ is weaker than that of complex UO₂L₂²⁺, responsible for the spectrum intensity decreased in the titration system when R > 2.32.

In Fig. 3, we compared the absorption spectra of the IL phases after extraction with those of complexes UO₂L₂²⁺ and UO₂L₃²⁺ in HOEtmimNTf₂. The spectra of UO₂L₂²⁺ and UO₂L₃²⁺, as depicted in Fig. 2b, were obtained in the monophasic titration experiments. As shown in Fig. 3a, for the spectra of the post-extraction IL phases, along with the increase of L/U ratio, the position of absorption bands red-shifted continuously (depicted the short-dash line), and the relative intensity of the absorption band between 447 nm and 459 nm (marked with ◊) strengthened slowly. No matter in terms of the peak positions or the relative intensities among the peaks, the post-extraction spectrum of the IL phase when R = 2.0 was almost identical to that of complex UO₂L₂²⁺ (Fig. 3b), and that of the IL phase when R = 6.0 nearly overlapped that of complex UO₂L₃²⁺ (Fig. 3c). Such observations suggest that with the increase of the L/U ratio, the uranyl(Ⅵ) species in the post-extraction IL phase successively change in the order of UO₂L²⁺, UO₂L₂²⁺, and UO₂L₃²⁺, consistent with the trend of uranium distribution ratio with the CMPO concentration in the IL phase (Fig. 1). When CMPO extracts aqueous uranyl(Ⅵ) to the IL phase as complex UO₂L₂²⁺, and similarly when R = 2.0 the extraction complex is UO₂L₃²⁺ [24, 25, 29]. With the experimental techniques of our previous work [14], we collected some self-assembly material and determined its absorption spectrum (Fig. 3d). Compared with the absorption spectrum of complex UO₂(CMPO)₃²⁺ in HOEtmimNTf₂ (Fig. 3c), significant redshifts occurred for the corresponding bands in the absorption spectrum of the assembly material. This is a common optical character resulting from the assembly of mono-molecules [30]. Typically two reasons can explain this phenomenon: one is related to an edge-to-edge (J-type) aggregation of complex monomers in self-assembly material, and the other is related to the state transition of complex monomers from the disordered dispersion in solution to the orderly tight arrangement the self-assembly material [30-32].

To check the effect of nitrate anions on the complexes formation, spectrophotometric titrations were also conducted with UO₂(NTf₂)₂ instead of UO₂(NO₃)₂, as shown in Fig. 4. Three successive uranyl(Ⅵ) complexes were also observed, which are of the same stability constants as the corresponding complexes of UO₂(NO₃)₂ (Table 1). Moreover, these species are of almost identical spectrum profiles (Fig. S2 in Supporting information). So, we believe that the species of uranyl(Ⅵ) are of the same coordination environments, no matter whether UO₂(NO₃)₂ or UO₂(NTf₂)₂ has been used as the initial substance. All the aforementioned results possibly explain why the counter ions detected in the solid material collected from the extraction system are NTf₂^- rather than NO₃^-.

Fig. 5 shows a representative microcalorimetric titration of U(Ⅵ)/CMPO complexation in HOEtmimNTf₂. At the beginning of titration, the heat-rate curve consisted of negative peaks; with the addition of ligand solution, the negative peaks decreased rapidly in turn and then changed to positive peaks (Fig. 5a). This phenomenon indicated that the complexation system was endothermic in the early stage with lower L/U ratios, while rapidly changing to be exothermic with the increase of L/U ratio. In conjunction with the stability constants determined by spectrophotometry, the enthalpies for the U(Ⅵ)/CMPO complexes were calculated with HypDeltaH [33] and summarized in Table 1. The formation reactions of complexes UO₂L²⁺, UO₂L₂²⁺ and UO₂L₃²⁺ are of small enthalpy and large positive entropy, in agreement with the normal characters of the inner-sphere complexation between a hard Lewis acid and hard Lewis base [34, 35]. The formation of UO₂L²⁺ is endothermic and entirely driven by entropy; the formations of UO₂L₂²⁺ and UO₂L₃²⁺ are weakly exothermic and driven by both enthalpy and entropy, whereas enthalpy only accounts for a small portion of the driving force. Positive enthalpy for complex UO₂L²⁺ indicates that the consumed energy in uranyl(Ⅵ) desolvation overrides the release in uranyl(Ⅵ)/CMPO coordination, whereas the opposite situation for complexes UO₂L₂²⁺ and UO₂L₃²⁺ [28, 36, 37].

In molecular solvents, the coordination number of linear UO₂²⁺ is five or six (CN = 5 or 6), forming pentagonal or hexagonal bipyramidal complexes with ligands [26, 38-42]. However, in ILs such as C₄mimPF₆ and C₈mimNTf₂, extended X-ray absorption fine structure (EXAFS) measurements showed that UO₂²⁺ behaves lower CN (CN = 4.0-4.5) than in molecular solvent [43, 44]. The entropy for complex UO₂L²⁺ in HOEtmimNTf₂ (120(7) J mol^-1K^-1) is equivalent to that in wet C₄mimNTf₂ (129(4) J mol^-1K^-1) [25], indicating that in HOEtmimNTf₂, the first CMPO molecule chelates to UO₂²⁺ in the same bidentate fashion as in C₄mimNTf₂ [25]. Such coordination mode has been confirmed by the solid crystallographic data of the corresponding complex [38, 39]. The stepwise enthalpy and entropy for complex UO₂L₂²⁺ (reaction UO₂L²⁺ + L → UO₂L₂²⁺, ΔH₂ = −10.9 ± 2.0 kJ/mol, ΔS₂ = 48 ± 8 J mol^-1K^-1) and for complex UO₂L₃²⁺ (reaction UO₂L₂²⁺ + L → UO₂L₃²⁺, ΔH₃ = −9.8 ± 2.0 kJ/mol, ΔS₃ = 33 ± 9 J mol^-1K^-1) are of equivalent values in error ranges, suggesting the second and third CMPO molecule most likely chelate to UO₂²⁺ in the same fashion. Consequently, we herein hypothesize that the second and the third CMPO would act as a monodentate ligand to coordinate with UO₂²⁺ [45]. The thermodynamic results can be interpreted perfectly with such a complexation assumption. One donor of CMPO (possibly the O-donor of P=O moiety because of its stronger basicity than that of the N-O moiety [46]) coordinates with UO₂²⁺, which is convenient to disperse the steric hindrances and coordination tensions. As consequence, less difference occurs in the energy needed to remove the inner-sphere solvent molecules of UO₂²⁺ in the last two complexations.

In summary, the complexation of uranyl(Ⅵ) with CMPO in a hydroxyl-functionalized ionic liquid HOEtmimNTf₂ has been investigated by solvent extraction, spectrophotometry, and calorimetry. Three successive complexes UO₂L²⁺, UO₂L₂²⁺ and UO₂L₃²⁺ form in HOEtmimNTf₂. Though UO₂L²⁺ and UO₂L₂²⁺ are the possible extraction species under a lower L/U ratio, complex UO₂L₃²⁺ is the preferable extraction species of uranyl(Ⅵ) in the IL phase, assembling its neutral molecules at the inter-surface of the biphasic system to yield a solid material [14]. In homogeneous HOEtmimNTf₂ solution, the stability constants of complexes UO₂L²⁺, UO₂L₂²⁺ and UO₂L₃²⁺ are 4.82(5), 9.39(12) and 12.98(5), respectively. All formation reactions are predominantly driven by entropy, although a small part of the driving force of complexes UO₂L₂²⁺ and UO₂L₃²⁺ comes from enthalpy. The thermodynamics of the UO₂²⁺/CMPO complexes is interpreted by hypothesizing that the first CMPO molecule coordinates with UO₂²⁺ in a bidentate fashion, and the others in a monodentate fashion.

Declaration of competing interest

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The authors declare no conflict of interest.

Acknowledgment

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This work was supported by the National Science Foundation of China (Nos. 22076175, 11675156, U1830202 and 21976165).

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.12.066

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Year 2022 volume 33 Issue 7

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doi: 10.1016/j.cclet.2021.12.066

Receive Date：2021-11-04
Online Date：2025-12-19
Published：2022-07-15

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Received：2021-11-04
Revised：2021-12-22
Accepted：2021-12-24

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Institute of Nuclear Physics and Chemistry, CAEP, Mianyang 621900, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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TxT

Fig. 1 UO₂²⁺ in nitricacid extraction with CMPO in HOEtmimNTf₂ at ambient temperature. Aqueous phase: 0.91 mmol/L UO₂(NO₃)₂ in 1.0 mol/L HNO₃; organic phase: CMPO (0.5~4.5 mmol/L) in HOEtmimNTf₂ (equilibrated by 1.0 mol/L HNO₃).

Fig. 2 Spectrophotometric titration of UO₂(NO₃)₂ with CMPO in HOEtmimNTf₂. Initial solution: V⁰ = 2.0 mL, C_U⁰ = 1.5 mmol/L, titrant: C_L = 69.7 mmol/L, 0.15 mL added. (a) Spectra of the titration system normalized to the initial concentration of U(Ⅵ); (b) molarabsorptivity of the species found in the titration system.

Fig. 3 (a) Absorption spectra of the IL phases after extraction; (b) Spectrum comparison of the IL phases after extraction (R = 2.0) with complex [UO₂(CMPO)₂]²⁺; (c) Spectrum comparison of the IL phases after extraction (R = 6.0) with complex [UO₂(CMPO)₃]²⁺; (d) Absorption spectrum of the solid assembly material.

Fig. 4 Spectrophotometric titration of UO₂(NTf₂)₂ with CMPO in HOEtmimNTf₂. Initial solution: V⁰ = 2.0 mL, C_U⁰ = 1.5 mmol/L, titrant: C_L = 97.4 mmol/L, 0.096 mL added. (a) Spectra of the titration system normalized to the initial concentration of uranyl(Ⅵ); (b) Molarabsorptivity of the species found in the titration system.

Fig. 5 Representative microcalorimetric titration of uranyl(Ⅵ)/CMPO complexation in HOEtmimNTf₂. (a) Titration thermograms; (b) cumulative reaction heat (black ◊ for experimental data (left y-axis), and: the red short dash line for the calculated results) and the uranyl(Ⅵ) species (right y-axis, black line: UO₂²⁺, red line: UO₂L²⁺, green line: UO₂L₂²⁺, and blue line: UO₂L₃²⁺) as a function of the titrant volume (V_titrant, µL). Initial solution in the cup: 0.800 mL of UO₂(NO₃)₂ solution (2.19 mmol/L) in HOEtmimNTf₂, titrant: 40.0 mmol/L CMPO in HOEtmimNTf₂.

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