Separation of Am(III), Cm(III) and Eu(III) by electro-spun polystyrene-immobilized CyMe4-BTPhen

doi:10.1016/j.tet.2018.04.037

Tetrahedron

Volume 74, Issue 38, 20 September 2018, Pages 5258-5262

https://doi.org/10.1016/j.tet.2018.04.037 Get rights and content

Abstract

The synthesis of a novel 5-(4-vinylphenyl)-CyMe₄-BTPhen actinide selective ligand using selenium free synthetic procedures is reported. For the first time, we report the electrospinning of this actinide selective ligand into a polystyrene fiber and investigate its selective removal of Am(III) from Eu(III) and Am(III) from Cm(III). At 4 M HNO₃, the resulting fibrous solid extractant produced separation factors of SF_Am/Eu ≈ 57 and a small, but significant separation of SF_Am/Cm ≈ 2.9.

Graphical abstract

Introduction

The generation of used nuclear fuel (commonly known as “spent nuclear fuel”, SNF) has contributed to the global accumulation of actinides, where the separation of these radiotoxic elements is strongly affected by the presence of other elements, including transition metals and lanthanides, which compete for the binding sites in the ligands used for their separation [[1], [2], [3]]. After the removal of uranium (U) and plutonium (Pu) from the SNF by the currently employed PUREX process, most of the long-term radiotoxicity and heat-load of the waste arises from the presence of the minor actinides (americium, curium and neptunium), even though they only account for a small proportion of the waste (∼0.1%) [4]. Selective separation of the actinides Am(III) and Cm(III) from fission products and closely related lanthanides has been previously achieved using soft N-donor ligands such as (1), (2) and (3), which contain the 1,2,4-triazine moiety (Fig. 1) [[5], [6], [7], [8], [9], [10]].

It is generally accepted that one of the contributors to this selectivity is due to the more radially expanded nature of the 5f-obitals of the actinides compared to the 4f-orbitals of the lanthanides [[11], [12], [13]]. It is rationalized that this subtle difference means that soft N-donor extractants have increased ligand-actinide bond covalency and hence selectivity over the lanthanides. More recently, substitution at different positions of the 1,10-phenanthroline core in (3) has provided the ability to fine-tune the ligands electronically to be even more selective towards actinides over lanthanides. The efficiency for the extraction of the actinides over lanthanides by some di-amide and calixarene-based extractants has also been reported [[14], [15], [16]]. Furthermore, electronically modulated Br-CyMe₄-BTPhen (4) and 5-(4-hydroxyphenyl)-CyMe₄-BTPhen (5) (Fig. 2) have been shown to exhibit slight, but significant selectivity for Am(III) over Cm(III), elements that are adjacent to each other in the periodic table [17].

The ligands shown in Fig. 2 provide a means of amplifying the very small differences in the covalent interactions of Am(III) and Cm(III) with the ligands by subtle electronic modulation with 5-bromo- (4) and 5-(4-hydrxyphenyl)- (5) substituents, revealing separation factors for Am(III) over Cm(III) (SF_Am/Cm) as high as 7 [18,19].

There are several partitioning processes that have been proposed and studied to separate Am(III) from Eu(III), but most of these processes focus entirely on solvent extraction processes which possess certain disadvantages, including the need for large volumes of organic solvents and degradation of the solvents over time, resulting in reduced performance and efficiency. Often these liquid-liquid extraction systems require the use of phase modifiers to optimize extraction and third phase formation can be encountered. Extraction systems based on immobilized extractants would remove the need for excessive organic solvents. Synthesis of ligand (5) enabled immobilization of CyMe₄-BTPhen ligands onto solid supports, notably magnetic nano-particles (MNPs) and macroscopic silica gel, where their ability to separate Am(III) from Eu(III) has been previously demonstrated [20,21]. Related materials have also been prepared by cross-linking Me₄-BTPhen into PVB (polyvinyl benzyl) polymers and its Am(III) extraction ability was investigated [22]. The ability of these solid supports to be implemented in the extraction of Am(III) from Eu(III) in solutions of up to 4 M HNO₃ has opened up an area of research geared towards functionalizing solid materials for selective actinide separation. Moving towards solid supported processes, and in particular using column separation techniques, may ultimately help reduce the solvent waste generated by continuous solvent extraction processes.

In this work, we outline the synthesis of novel 5-(4-vinylphenyl)-CyMe₄-BTPhen (6) using a selenium-free synthetic protocol and report the ability of this ligand to separate Am(III) from Eu(III) and Am(III) from Cm(III) when immobilized into fibers of electro-spun polystyrene (8) (Scheme 1).

Section snippets

Synthesis of 5-(4-vinylphenyl)-CyMe₄-BTPhen (6)

Until recently, the synthesis of the core CyMe₄-BTPhen (3) unit required the use of stoichiometric amounts of toxic selenium dioxide to generate the phenanthroline bis-aldehyde, required for the one-pot conversion to the phenanthroline bis-nitrile, where nitrile functional groups are key precursors to produce many heterocyclic cores [23]. Edwards et al. demonstrated that the benzylic oxidation could be achieved by per-chlorination of the methyl groups in 2,9-dimethyl-1,10-phenanthroline and

Conclusions

We have reported the synthesis of electro-spun polystyrene-immobilized CyMe₄-BTPhen fibers that will selectively extract Am(III) from Eu(III) at 4 M HNO₃ (SF_Am/Eu > 57). The fibers also exhibit a small but significant selectivity for Am(III) over its actinide neighbor Cm(III), with a separation factor of SF_Am/Cm ≈ 3. The Am/Cm results are similar to separation factors achieved with previous solid supported ligands. The synthesis of novel 5-(4-vinylphenyl)-CyMe₄-BTPhen was achieved by adapting

Extraction studies

The aqueous solutions for the extraction experiments were prepared by spiking nitric acid solutions (0.001–4 M) with stock solutions of ²⁴¹Am, ¹⁵²Eu and ²⁴⁴Cm and then adding 1.000 μL of spiked aqueous solution to accurately weighed 19.8 mg of BTPhen-polystyrene (8) (V/m ratio: 30.3 mL/g). The mixture was sonicated for 10 min and shaken (Heidolph Reax) at 1800 rpm for 90 min. After centrifuging for 2 min, aliquots of the aqueous solutions (supernatant) were separated and taken for measurements.

Acknowledgments

Use of the Chemical Analysis Facility (CAF) and the Electron Microscopy Laboratory (EMLab) at the University of Reading is gratefully acknowledged. The research at the CTU was supported by the Grant Agency of the Czech Technical University in Prague (grant No. SGS15/216/OHK4/3T/14). We thank Mr Tahkur Singh Babra for running GPC analysis on the fibers and Mr Luyi Sun for providing some preliminary observations.

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Moreover, compared with P and S-donor ligands, N-donor ligands meet the CHON principle which can be entirely combusted to gases after use and will not create secondary contamination (Kolarik, 2008; Su et al., 2022). However, these extractants have their shortcomings, including poor solubility (Reilly et al., 2018), poor stability (Afsar et al., 2018) and weak resistance to radiation (Bhattacharyya and Mohapatra, 2019), slow extraction kinetics (Liu et al., 2020), so it is hard to be used in large-scale extraction system (Lewis et al., 2011). Selective extraction of trace amount of actinides using liquid-liquid extraction processes also has certain disadvantages, such as the need for multistage extraction for good decontamination effect, largescale of equipment (Ning et al., 2016a; Ning et al., 2020b), and the generation of large amounts of secondary waste (Ning et al., 2019; Zhang et al., 2016; Zhang et al., 2017).
2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe₄-BTPhen) is one of the most prospective ligands for selective separation of trivalent actinides from high level liquid waste (HLLW). To address its low solubility and irradiation resistance in liquid-liquid extraction systems, herein it was loaded into a porous silica-polymer carrier SiO₂-P by vacuum impregnation technique to obtain a high loading rate macro-porous adsorbent CyMe₄-BTPhen/SiO₂-P with CyMe₄-BTPhen as high as 30.48 wt%. BET, SEM, XRD, FT-IR and TOC analysis proved the stable macro-porous CyMe₄-BTPhen/SiO₂-P was successfully prepared. It exhibited high adsorption efficiency (>98 %) towards trace amount of radioactive ²⁴¹Am(III) in a broad acidity range of 0.01–4 M nitric acid solutions in less than 5 min, which was even up to 99.5 % in 1–2 M HNO₃ solution. Highly efficient separation between Am(III) and Eu(III) was achieved with the separation factor SF_Am/Eu up to 123 in 4 M HNO₃ solution. The kinetic and adsorption isotherm of Eu(III) are in good agreement with pseudo-second-order kinetic and Langmuir models, respectively. FT-IR and XPS analysis reveal that the complexation mechanism involves the immediate coordination of Eu(III) to N in phenanthroline group and triazine ring. Coordination behavior was further studied by UV titration that the ligand and Nd formed a 2:1 complex in acetonitrile solution. MBO analysis indicated that the Am-N bond in the complex has a shorter bond length and more covalent properties than the Eu-N bond. Meanwhile, thermodynamic results indicate that the ligand binding to Am(III) has a lower Gibbs free energy than that of Eu(III), which all reveal the reasons for the preferential bonding of the ligand to Am(III). In a word, CyMe₄-BTPhen/SiO₂-P proves to be a very promising adsorbent for separation minor actinides from HLLW.
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When applied to a complicated mixture, designed to simulate a nuclear forensic sample, americium recovery was unaffected and only cadmium and praseodymium co-extracted [26]. The BTPhen ligand has also been electrospun into polystyrene fibres [27] which showed reasonable distribution coefficients, with a maximum DAm=780 ± 50 in 0.1 M nitric acid and a maximum SFAm/Eu=57 ± 4 in 4 M nitric acid. The fibres also showed an ability to extract curium with a maximum DCm=440 ± 40 in 4 M nitric acid.
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Efficient adsorbents for U(VI) removal from aqueous solution is particularly important for energy strategy and environmental safety. Although dendritic fibrous nano-silica (DFNS) with large surface area, good stability and porous structure is attractive, modification of DFNS is an effective strategy to realize the efficient removal of U(VI). Hyper-branched poly(amidoamine)(PAMAM) with abundant amine/amide groups shows great potential in U(VI) adsorption. Herein, PAMAM was grown onto the rigid DFNS to obtain the nanocomposite. The nanocomposite served as a novel adsorbent for U(VI) removal and exhibited fast adsorption kinetics of less than 180 min and considerable adsorption capacity of 215.52 mg g^-1 owing to the open gully-like structure and functional groups on the surface of the absorbent. The adsorption mechanism was discovered to be complexation between amine/amide groups and U(VI) ions by EDS and XPS analysis. Furthermore, the adsorbent showed an excellent reusability.
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Separation of Am(III), Cm(III) and Eu(III) by electro-spun polystyrene-immobilized CyMe4-BTPhen

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of 5-(4-vinylphenyl)-CyMe4-BTPhen (6)

Conclusions

Extraction studies

Acknowledgments

Procedia. Chem.

Separ. Purif. Technol.

React. Funct. Polym.

Eur. Polym. J.

Chem. Rev.

Nucl. Energy. Environ.

Reprocessing and Recycling of Spent Nuclear Fuel

J. Am. Chem. Soc.

Chem. Soc. Rev.

Chem. Soc. Rev.

Dalton. Trans.

Chem. Commun. (J. Chem. Soc. Sect. D).

Inorg. Chem.

Dalton. Trans.

Inorg. Chem.

Separation of Am(III), Cm(III) and Eu(III) by electro-spun polystyrene-immobilized CyMe₄-BTPhen

Synthesis of 5-(4-vinylphenyl)-CyMe₄-BTPhen (6)