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All issues Volume 4 / No 1 (2025) Natl Sci Open, 4 1 (2025) 20240028 Full HTML
Issue
Natl Sci Open
Volume 4, Number 1, 2025
Special Topic: Nuclear Environment Advances
Article Number 20240028
Number of page(s) 35
Section Earth and Environmental Sciences
DOI https://doi.org/10.1360/nso/20240028
Published online 20 September 2024

© The Author(s) 2024. Published by Science Press and EDP Sciences.

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

INTRODUCTION

The ever-growing demand for clean energy in modern society has led to the exploration of new low-carbon technologies [1–3]. Nuclear power has demonstrated its efficiency in addressing the energy crisis [2,4,5]. Recent reports from World Nuclear Association claimed that by the year of 2024, nuclear power is providing about 10% of the world’s electricity from about 440 power reactors and about one quarter of the world’s low-carbon electricity is now from nuclear power [6]. The fast expansion of nuclear power accelerates humanity’s transition to a low-carbon energy society while also becomes essential to properly manage the nuclear waste produced by nuclear plants to minimize environmental impact and alleviate public concerns. It has been estimated that the annual worldwide generation of nuclear waste from the current nuclear plants could be in the range of 8800 to 13200 tons based on the 440 operational nuclear power reactors (by the year of 2021). This back-end streams from the nuclear fuel circle contain a large body of elements from the periodic table and include isotopes with their half-life ranging from microseconds to billions of years (Figure 1). Thus, separations of these radioactive isotopes from the SNF were believed to be key steps for the sustainable development of nuclear power.

thumbnail Figure 1

Spent nuclear fuel (SNF) constituent distribution from a typical light-water reactor (LWR) with parameters: 1 GWe, burn-up = 33 GWd/tHM, capacity factor of 0.8 and cooling at 150 days. The zircaloy weight is included.

Nuclear fuel cycles

The proper management of nuclear fuel could both benefit dealing with the current waste deposition and reducing the geometry volume. Also, the literally waste could be important sources for groups of strategy metals. For example, the unreacted uranium could be back-filled for fuel fabrication; plutonium recycling could, on one side, be used in mixed oxide fuel and, on the other side, reduce the risk for nuclear proliferation [7,8]; some of the fission isotopes could be used for cancer therapy and medical imaging (cecium-137, strontium-90, molybdenum-99 and gadolinium-153) [9,10]; americium and curium isotopes could be used to produce other man-made isotopes [11–15]. In order to properly deal with the SNF, multiple reprocessing processes have been developed and have been ever updating to meet the fast development of nuclear power.

First reported in 1949 and subsequently operated with irradiated nuclear fuel at a large scale in 1954, the PUREX (Plutonium Uranium Reduction EXtraction) process is, by far, the most applied SNF management process and serves as the foundation of most modern separation processes for SNF used at industrial levels [7,8,16]. The process is based on liquid-liquid solvent extraction by use of the well-known extractant, tri-n-butyl phosphate (TBP) diluted by normal paraffinic hydrocarbon (NPH) as organic phase. It should be noted that the PUREX process is developed and applied primarily to produce pure Pu stream for military (weapon) applications while later modifications of the process (for example, the adapted PUREX and simplified PUREX) aim to meet new and/or future policies regarding waste and product stream composition (Pu proliferation) [16]. The remaining raffinate (high-level liquid waste, HLLW) is highly radiotoxic, primarily due to the presence of minor actinides (MAs, such as Np, Am, and Cm). These actinides need to be separated from lanthanides (Lns) to enable an effective neutron-assisted transmutation process, as lanthanides have significantly larger neutron cross sections that can impede the transmutation efficiency. Moreover, lanthanides do not form solid solutions in metal alloys or mixed oxides and consequently they segregate into separate phases that tend to grow during thermal treatments. Since minor actinides (MAs) tend to concentrate in these phases, this results in an unacceptable non-uniform heat distribution in the fuel matrix during irradiation [17]. Current demonstrated strategies include co-extraction of both Lns(III) and Ans(III) through processes such as Transuranic Extraction (TRUEX, extractant used: octyl(phenyl)-N,N-diisobutylcarbamoylmethyl-phosphine oxide, CMPO) [18,19], diamide extraction (DIAMEX, extractant used: malonamide derivatives) [20,21], followed by processes such as Trivalent Actinide-Lanthanide Separations by Phosphorus-reagent Extraction from Aqueous Komplexes (TALSPEAK) [22] and selective actinide extraction (SANEX)/innovative-SANEX (i-SANEX) [23,24] to separate mainly Am(III) and Cm(III) from Lns(III) (Figure 2). For example, the TRPO process using the dithiophosphinic acid (Cyanex 301) gave high separation factors of Am(III) over Eu(III) of up to 5800 and displayed high selectivity for Am(III) over several light Lns(III) [25–27]. Eventually, the intragroup trivalent actinides of Am(III) and Cm(III) are further separated through processes exemplified as Selective Extraction and Separation of Americium by Means of Electrolysis (SESAME) [15] and Lanthaniden Und Curium Americium Trennung (LUCA) [28] as the presence of curium will complicate the fabrication and operation of nuclear fuel as well as its important role for production of californium-252 [29].

thumbnail Figure 2

Summaries of current advanced reprocessing processes. PUREX: plutonium uranium reduction extraction [7,8]; TRUSPEAK: a single process combines Transuranic Extraction (TRUEX) process and the Trivalent Actinide-Lanthanide Separations by Phosphorus-reagent Extraction from Aqueous Komplexes (TALSPEAK) process [30]; ALSEP: actinide lanthanide separation [31]; SANEX: selective actinide extraction [23,24]; GANEX: Grouped ActiNide Extraction [32]; EXAM: Extraction of Americium [33]; Super-DIREX: supercritical fluid direct extraction method [34]; TRUEX: transuranium extraction [18,19]; DIAMEX: diamide extraction [20,21]; TRPO: trialkyl phosphine oxides [35]; TODGA: N,N,N′,N′-tetraoctyl diglycolamide [36,37]; UNEX: universal extraction [38–40]; SREX: strontium extraction [41]; CSEX: cesium extraction [42]; SESAME: selective extraction and separation of americium by means of electrolysis [15]; LUCA: Lanthaniden Und Curium Americium Trennung [15,28].

Solvent extraction is the primary technology used in most of the aqueous spent nuclear fuel reprocessing processes for its ability to be operated in continuous, countercurrent manner with multiple contacting/separating stages to achieve high efficiency for the desired separations. In a typical solvent extraction process, the aqueous dissolver product is mixed with an organic phase containing an extractant (typically organic chelating ligand). The mixing facilitates the transfer of part of the components into the organic phase. The two immiscible phases are intimately mixed to allow for selective solute transfer, and then separation. The components of interest are subsequently removed from the organic phase and transferred back to an aqueous phase through back-extraction. For effective processing, the two phases must be immiscible, have sufficient density difference to enable rapid separation, possess appropriate viscosity for transport through processing equipment, and exhibit limited solubility of the organic phase in the aqueous phase to maintain extractant concentration over long-term use. Additionally, both the organic phase and the extractant must have adequate hydrolytic and radiolytic stability to ensure long-term reuse, minimizing organic waste volumes. The main determinant for solvent extraction is the coordination ligands used either in the organic and/or aqueous phases [29,43–56]. From a practical point of view, it has been suggested that the hydrophilic ligands used in the aqueous phase that could shield certain metal cations could be more promising as such processes could benefit from both the lipophilic ligands in the organic phase and the hydrophilic ligand in the aqueous phase [29,45,49,50,56–58]. The overall metal distribution ratios and the separation factors are boosted by the typical reverse extraction of the two ligands in both phases. While currently, most of the literature reports focused on the lipophilic ligands with far less attention being devoted to the hydrophilic ones (the reasons for this will be discussed in the later section) [50,56–58].

Ligand design for radio-active f-block elements coordination and separations

For efficient f-block elements coordination and separation, the ligand design is the game changer. In the pursuit of highly efficient actinides-selective ligand, the lipophilic ones definitively took the lion’s share—the majority of the reported ligands were designed to be used as organic extractant that formed lipophilic complexes once encountered with the aqueous metal cations and eventually led the transfer of the metal cations into the organic phase. The ligand selectivity towards different metals was determined by the relative ligand-metal affinities. Enormous efforts have been delivered to uncover the ligand design principles [43,45,49,50,52,55,56], synthetic approaches [44,47,59,60], radio stabilities [61–66] and structure-function relationships [46,49,50,52,53,56,67–70]. Thus, a short summary of the representative lipophilic ligands was demonstrated to warm-up the readers. Furthermore, in the following discussion, we would show that the knowledge gained in the design of efficient lipophilic ligand could be readily transferred to the design of novel hydrophilic counterparts.

Currently, the design of ligands for the separation of trivalent lanthanides and actinides is primarily based on Pearson’s Hard and Soft Acids and Bases (HSAB) theory. Early experimental results by Peppard et al. indicated that oxygen-containing (“hard base”, electron donor) ligands had good coordination abilities with lanthanides and actinides but could not distinguish between the two [49,50]. Until around 1980, Musikas et al. discovered that pyridine-containing ligands (such as TPTz, Figure 3) could selectively extract Am(III) from Eu(III), thus initiating the search for novel nitrogen-containing ligands. Subsequently, a series of nitrogen heterocyclic ligands, including pyridine, bipyridine, and phenanthroline, were reported [43,44,49,50,56]. Although trivalent lanthanides and actinides have similar physicochemical properties, the more diffuse 5f orbitals of actinides make them slightly “softer” as compared to lanthanides. This leads to a stronger covalent character in actinide complexes when coordinating with soft donor atoms like nitrogen and sulfur, facilitating the separation of lanthanides and actinides. Early nitrogen-containing ligands, represented by TPTz, could selectively separate Am(III) from Eu(III) at low acidity (≤ 0.1 M), but their separation factor (SFAm/Eu) was only about 10 [71]. Moreover, since their extraction mechanism was based on cation exchange, lipophilic co-extractants (such as α-brominated acids) were needed during the extraction process. To improve the acid resistance of the ligands, Kolarik et al. from the Karlsruhe Institute of Technology reported that introducing the 1,2,4-triazine moiety significantly increased the acid resistance of the ligands: BTPs derivatives could effectively separate Am(III) from 0.9 M nitric acid solutions, with separation factors ranging from 30 to 100 (depending on the diluent) [72,73]. Subsequent studies showed that the interaction between adjacent nitrogen atoms on the 1,2,4-triazine fragment reduced the basicity of the coordinating nitrogen atoms (enhancing acid resistance) and increased their affinity for “soft” metal ions (Figure 3, ligand 2, summarized as the α-effect). Further stability tests indicated that the presence of benzylic hydrogen atoms in BTPs reduced the chemical and radiation stability of the ligands [74]. To tackle this problem, Geist et al. replaced the benzylic hydrogen atoms with dimethylcyclohexane, which effectively increased the stability of the ligands by methylating the active sites on the nitrogen heterocycles. Compared to the tridentate BTPs ligands (with three coordinating nitrogen atoms), the tetradentate BTBPs ligands reduced the number of ligands in the first coordination sphere (three BTPs around the metal center to give 9-coordinated geometry while two BTBPs with one nitrate cation to fulfill 10-coordinated architecture, Figure 4), favoring metal cation back-extraction [75]. It is worthy to point out that CyMe4-BTBP is one of the most successful lipophilic ligands to date and has been used as the reference ligand in the SANEX process for minor actinide separation in Europe [49,50]. However, due to the high rotational freedom of the single bond in the bipyridine unit of BTBPs ligand, there is an energy barrier to overcome for the ligands to adopt a cis conformation for coordination with metal cations [76,77]. The conformation changes result in slow extraction kinetics for BTBPs-based ligand. This can be relieved by adding phase transfer agents (alkyl amides) to enhance the extraction rate. With the purpose to increase the extraction dynamics, Lewis et al. replaced the BTBPs backbone with more rigid BTPhen (Figure 3 and Figure 4). The rigidity of the phenanthroline backbone greatly improved the extraction kinetics with respect to bipyridine-based BTBPs ligand. CyMe4-Phen ligand reached extraction equilibrium within 15 minutes without phase transfer catalysts (Figure 4f) and the corresponding equilibrium constant was 5–10 times higher than that of CyMe4-BTBP [76]. Although CyMe4-BTBP and CyMe4-Phen have shown significant improvements in extraction performance, solubility, and stability as compared to previous generations of ligands, their strong metal extraction capabilities make the back-extraction difficult. Additionally, the complicated synthesis of CyMe4-substituted ligands leads to high ligand preparation cost, which is not conducive to large-scale applications [49,52]. In recent years, imide ligands containing both nitrogen and oxygen have gained widespread attention [25,56,70,78–84]. Representative molecules, such as the diamide phenanthroline derivatives (termed as DAPhen), are relatively easy to synthesize with readily available raw materials. They exhibit excellent extraction kinetics and easy back-extraction. However, their extraction distribution ratios and separation factors for actinides are relatively low (SFAm/Eu < 100), and they display poor solubility in conventional diluents (kerosene, alkanes, etc.) [78,85]. Currently, the design and synthesis of lipophilic ligands with excellent performance and balanced parameters (acidity tolerance, solubility, extraction kinetics, separation efficiency, back-extraction efficiency, etc.) remain at the forefront of related research [25,29,56,70].

thumbnail Figure 3

Benchmark lipophilic ligands with the advantages and existing problems to be solved. The numbers in BTPs (2) were for the easy discussion of α-effect on triazine rings. Red circles in CyMe4-BTBP (3) and CyMe4-BTPhen (4) showed the absence of benzylic H-atoms to enhance the ligands’ radiation stability. The blue circles in PhenDA (5) indicated the coordination sites of the ligand.
thumbnail Figure 4

Comparison of representative lipophilic ligands for their extraction dynamics and single crystal structures. (a–c) Ligand structures for CyMe4-BTP, CyMe4-BTBP and CyMe4-BTPhen. Solvent extraction dynamics (d–f, blue dots for Am(III) and blacks for Eu(III)) and single crystal structures (g–i) for the three ligands. Experimental conditions: 8.8 mM ligand with 0.5 mM of N,Nʹ-dimethyl-N,Nʹ-dioctyl-2-ethoxymalonamide (DMDOHEMA) as phase-transfer agent in n-octanol with 0.5 M HNO3 (d); 20 mM ligand in n-octanol with 0.5 M HNO3 (e); 10 mM ligand in n-octanol with 1.0 M HNO3 (e). Data in panel (d) and (g) were recreated according to Ref [86]. Extraction dynamics for CyMe4-BTBP in panel (e) was from Ref [75] and the crystal structure was from Ref [87]. Data in panel (f) and (i) were recreated according to Ref [76].

Another important approach for achieving effective separation of Lns(III)/Ans(III) is to first co-extract the chemically similar trivalent lanthanides and actinides into the organic phase (commonly using alkyl phosphates, such as bis(2-ethylhexyl) phosphate (HDEHP), or diamide ligands, such as TODGA). Then, by adding a selective stripping ligand in the aqueous phase, the separation of lanthanides and actinides is achieved as demonstrated in the TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorus-reagent Extraction from Aqueous Komplexes) process developed by the Oak Ridge National Laboratory in the USA and the i-SANEX (innovative SANEX) process developed by the French Alternative Energies and Atomic Energy Commission (CEA). In the TALSPEAK process, HDEHP is primarily used as the lanthanide extractant, while diethylenetriaminepentaacetic acid (DTPA) is used in the aqueous phase to selectively complex actinides, achieving a separation factor (SFEu/Am) of over 100. The main drawbacks of the TALSPEAK process include its complexity, significant influence of acidity (or pH) on separation efficiency, and slow phase separation kinetics [22]. The i-SANEX process utilizes the non-selective diamide ligand (TODGA) to co-extract trivalent lanthanides and actinides, and then employs a water-soluble ligand to selectively strip the actinides back into the aqueous phase. Essentially, it is a reverse-phase TALSPEAK process [24]. Although hydrophilic f-block element coordination ligands have long been used in the previous reprocessing processes and are believed to be efficient strategy to boost the separation and at the same time, minimize the environmental impact of the aqueous reprocessing processes by reducing the organic solvents used in the extraction, the rational design of the hydrophilic ligands was largely in the early trial-and-error stages, more attentions should be paid to reveal the ligand structure-function relationship and more specifically, the ligand selectivity of different metal cations and protons, the acid-resistance of the ligand to extract under high acidity and the synthetic accessibilities of the ligands, etc.

Current problems for hydrophilic ligands

Hydrophilic ligands could be generally used in two ways in the aqueous reprocessing processes of SNF. The water-soluble ligands could either be used in aqueous phase in combination with nonselective lipophilic ligands to selectively chelate one metal to prevent its extraction from another metal cation (termed as hold-back ligand or scrubbing ligand) or by stripping it from metal-loading organic phase (termed as stripping ligand). During the development of advanced aqueous reprocessing processes for SNF (as demonstrated in Figure 2), a large number of water-soluble ligands were actually used in the reported processes. For example, N,N,Nʹ,Nʹ-tetra-ethyl-diglycolamide (TEDGA) is used to increase the Am/Cm and Am/heavy Lns selectivity because of its preferential complexation of curium and heavy lanthanides in EXAM process [33]; the combination of DTPA (0.05 M) with 1.5 M citrate to selectively strip actinides from lanthanides at pH of 3.5 in TRUSPEAK process [30]; acetohydroxamic acid (AHA) was used to assist neptunium and plutonium stripping in a novel EURO-GANEX process [32] and the combined use of oxalic acid and N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA) for preventing the extraction of molybdenum, zirconium and palladium in DIAMEX process [20,21]. These reported hydrophilic aqueous chelators were typically simple carboxylic derivatives featured with small size allowing the high biphasic mass transfer rates [88]. While their low efficiencies required multiple processing circles to achieve satisfying separation. Furthermore, these small hydrophilic chelators were typically pH-sensitive, which resulted in narrow practical pH windows [15,56,58].

In the past two decades, most of the research on novel hydrophilic ligands were based on pyridine-derivatives by direct sulfonating the lipophilic ligands or by introducing the hydroxyl groups through click reaction between ethynyl-functionalized N-donor ligands and the corresponding azides [57,58,85,89–94]. Some of these ligands displayed outstanding Eu(III)/Am(III) separation abilities (SF approaching 1000) and even the potential for group Lns(III)/Ans(III) separation [85,89,90,95]. While the main drawback of these ligands, despite their harsh preparation approaches, was that the ligand could only function in low acidity. Thus, a bigger library of efficient ligands and the establishment of structure-function relationship for hydrophilic ligands are urgently needed. Typically, the design of highly efficient hydrophilic f-block coordination ligands is tricky mainly for the following reasons: (1) the synthetic and purification obstacles for water soluble ligands themselves with respect to the lipophilic ones: water soluble ligands are synthetically hard to access as the preparation typically requires polar or ionic functional groups, which can be chemically challenging to incorporate. Reaction conditions in water can destabilize the intermediates and lead to hydrolysis or degradation of the product. Additionally, purification and separation of these compounds from aqueous solutions can be complex, often require specialized techniques such as ion exchange, crystallization from water or chromatography on water-compatible media [96]; (2) the strong coordination ability of water molecules requires more desolvation energy for stronger ligand to give stable complexes while, on the other side, this somewhat reduces the ligand selectivity [97].

Aims and scope of this paper

In the current manuscript, a short summary of the development for hydrophilic ligands will be discussed. The hydrophilic ligands were divided into four main catalogues based on their structure similarities, namely, ligands with alkyl-chain skeleton; ligands with simple-aromatic-skeleton; ligands with multiple-aromatic-skeleton and ligands based on another topological skeleton. At last, a brief summary of our recent findings in the phenanthroline diimide systems would be discussed to show how the molecular structure alternation could alter the ligand acid-resistance, extracting properties and selectivity. Finally, based on our understanding and literature reports from closely-related fields, potential ligand design based on self-assembly/clusters/polymers (sharing the same concept of multivalent cooperativity), macrocyclic ligands, bioinspired and biobased ligands will be proposed and discussed. We should point out that we do not intend to give the readers a comprehensive overview on how to design efficient hydrophilic ligands for radioactive f-block elements, as it would be too early to draw any conclusion at this stage. More work and larger ligand inventory are needed to give any systematical principle to guide the future ligand design. Also, we have focused mostly on the liquid-liquid-based separation of Ln(III) and Am(III)/Cm(III). Other actinides such as U, Np and Pu were not included and oxidation-based separation approaches were also not mentioned. Interested readers are encouraged to explore the relevant reviews [25,46,56,58]. At last, effective f-block element coordination and extraction are complicated hydrometallurgical questions, any practical advantages would require efforts from researchers of multidiscipline such as organic chemistry, physical chemistry especially solution coordination analysis and monitoring, analytical chemistry, crystallographer and structural chemistry and chemical engineering.

HYDROPHILIC LIGANDS FOR F-BLOCK ELEMENTS SEPARATION

Weaver Boyd once said “Much fruitless effort was expended in searching for extractants which separated the two groups of elements (i.e., rare-earth fission products and transplutonium actinides) … the separation was finally accomplished only by considerably modifying the aqueous medium.” [98], which clearly emphasized the importance to search for novel hydrophilic ligands for the coordination and separation of f-block elements. In the following discussions, the developments of hydrophilic ligands used in the aqueous reprocessing processes of f-block elements both as masking or stripping agents would be summarized into four main catalogues according mainly to their structure backbones: hydrophilic ligands based on alkyl-chain-skeleton; simple-aromatic-skeleton; multiple-aromatic-skeleton and other topological skeleton. In each part, representative ligands would be introduced and their extracting performances, main advantages and disadvantages would be discussed and summarized. As noted in the previous section, we do not intend to give the readers a comprehensive developments overview of all the hydrophilic ligands while, to give only important examples based on our understandings to shed light on how the ligand structural modification would affect the f-block elements coordination and extraction.

Hydrophilic ligands based on alkyl-chain-skeleton

Aliphatic acid-based ligands especially carboxylate ligands, which contain the carboxylate functional group (R–COO–), play a significant role in the chemistry of f-block elements (lanthanides and actinides) [99]. These ligands are particularly useful in the separation, extraction, and stabilization of f-block elements due to their strong coordinating ability and the formation of stable complexes [100,101]. Simple carboxylic containing ligands such as glycolic acid and cysteine are among the most widely explored for their inexpensive and highly-soluble nature. These simple carboxylate ligands have been proved to be useful under high acidity without formation of third phases and their small sizes allow high biphasic mass transfer rates, which enable the feasibility of usage of centrifugal contactors (illustrated processing processes are shown in Figure 5) [58,88]. Generally, increasing the number of carboxylic groups on the ligands could potentially increase the complexes stability and offer more stable and structurally diverse complexes with f-block elements as compared to the mono carboxylic ligands [99–102]. Along this line, oxalate, nitrilotriacetic acid (NTA), citric acid and multi-carboxylic containing ligands (more than four carboxylic acids) such as DTPA, HEDTA, HEDTTA and CDTA (Figure 6) were explored for the coordination and separation of Cs+, Ln3+ and An3+ from other fission products. For example, oxalate complexes of actinides (also fluoride) were sufficiently insoluble, thus they facilitated the separation of actinides from most fission products by precipitation/coprecipitation techniques [98]. CDTA (0.05 M) was used as a masking agent for Zr and Pd in the i-SANEX process [24]. DTPA used in combination with concentrated lactate buffer with HDEHP in 1,4-diisopropylbenzene as organic phase was widely recognized benchmark for advanced aqueous partitioning of 4f/5f elements in conventional TALSPEAK process [22]. Also, selective Am stripping with buffered DTPA solution allowed for the separation of the MAs from lanthanides with only serval stages in a counter-count extraction process with separation factors exceeding 30 for Nd/Am in the ALSEP process [31].

thumbnail Figure 5

Flow sheet showing the 1-cycle SANEX processes with different carboxylic acids used during multiple stages of the process. Figure was recreated according to Ref [24].
thumbnail Figure 6

Hydrophilic ligand structures based-on-alkyl-chain-skeleton. NTA: nitrilotriacetic acid; CDTA: cyclohexane-1,2-diaminetetraacetic acid; DTPA: diethylenetriaminepentaacetic acid; HEDTA: N-(2-Hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid; HEDTTA: N-Hydroxyethyl-diethylenetriamine-N,N′,N′′,N′′-tetraacetic acid.

The operation of conventional TALSPEAK is characterized by numerous competitive reactions occurring in both the aqueous and organic phases. A particularly problematic behavior in the organic phase is the partitioning of significant amounts of water and lactate, which consumes the extractant and may lead to the formation of a third phase [103,104]. In the aqueous phase, slow phase transfer kinetics hinder equilibrium-based separations in a contactor system and require high concentrations of carboxylic acid to ensure acceptable performance [22,105]. These issues seem to stem from the use of complexants that, while well-matched, are probably stronger than necessary. A recent report suggested that these complex interactions in TALSPEAK could be avoided by using a weaker complexant and holdback reagent. Replacing HDEHP and DTPA in conventional TALSPEAK with the weaker 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[HEP]) extractant and HEDTA significantly reduced the system’s complexity. The modified TALSPEAK (termed as TALSQuEAK, Qu stands for quicker extractants) provides more rapid phase transfer kinetics and less reliance on carboxylic acids to mediate lanthanide extraction [106]. Similarly, to tackle the slow attainment of the liquid-liquid distribution equilibrium drawback of TALSPEAK, N-Hydroxyethyl-diethylenetriamine-N,N′,N′′,N′′-tetraacetic acid (HEDTTA) with less acetate pendant arms with respect to DTPA was developed and displayed reduced Lns/Ligand complex stability about 3 orders of magnitude smaller, thus HEDTTA demonstrated a significantly enhanced equilibration rate as compared to DTPA [88].

Despite the wide applications of aliphatic acid in multiple reprocessing processes, their pH-sensitive nature is the major problem for these kind of simple chelating ligands. Another important class of hydrophilic ligands based on alkyl-chain-skeleton is polyamide extractants [48,94,107–112]. As exampled in Figure 6 (structures displayed at the bottom line), polyamide extractants typically contain amides as the main coordination sites, which were linked through alkyl chain containing either oxygen or nitrogen atoms (see TODGA in Figure 6). A large body of polyamide extractants are used as lipophilic ligands among which the diglycolamides derivatives are the most investigated and used ones [48,110,111]. The lipophilic diglycolamides derivatives could be converted to hydrophilic ones if the proper alkyl chains were to be used or other hydrophilic groups were introduced. Ding and his coworkers demonstrated that the extraction selectivity for An3+ over Ln3+ could be significantly improved more than 2-3 times by introducing the complexing agent N,N,N′′′,N′′′-tetraethyl-N′′,N′′-ethidene bisdiglycolamide (TEE-BisDGA, Figure 6) to the aqueous phase [109]. Later countercurrent separation test was conducted on laboratory-scale mixer-settler unit using combination of NTAamide (used as extractant with hydrogenated kerosene as the diluent), TEE-BisDGA as the scrubbing agent and HNO3 as the stripping agent. Satisfying results with decontamination factor of 1320 for Am/Eu and 96% recovery rate for Am3+ after 12 stages of extraction, 4 stages of scrubbing and 6 stages of stripping were observed [113]. Furthermore, changing the alkyl diamine linkage into aromatic diamine could increase the selectivity of An3+ over Ln3+ as demonstrated in the case of TOX-BisDGA and SO3-TEX(m)-BisDGA [94].

Hydrophilic ligands based on simple-aromatic-skeleton

In the first part of the discussion, we showcased the representative aliphatic acid-based ligands and their application in radioactive f-block elements coordination and separation. These ligands mostly coordinated to lanthanides/actinides through hard oxygen atoms, thus high metal bindings (efficient stripping) were always observed considering the hard nature of both lanthanides and actinides as defined by the Pearson’s classification on Hard and Soft Acids and Bases (HSAB). While despite the narrow working pH range for aliphatic acid-based ligands, the selectivity for different metal cations was relatively low. Considering the fact that softer nitrogen and sulfur containing extractants could potentially offer better selectivity among 4f and 5f elements and also inspired by the pioneering work of Musikas et al. demonstrating that nitrogen donor ligands such as TPTZ were able to selectively extract Am(III) from Eu(III) [44,49,50,67], Pascale and his coworkers had demonstrated the impact of N-donor softness on the selectivity of aqueous ligands for Am(III)/Eu(III) separations [114,115]. N-heterocycles of pyridine and pyrazine with different softness (pyrazine softer than pyridine) were introduced onto tetradentate ethylenediaminetetraacetic acid (EDTA) ligand, offering two new tetrapodal N, O ligands (ligand 21, Figure 7). The ligands were so designed that the soft N-donors improved the ligands selectivity while hard O-donors guaranteed the strong affinity. The back-extraction experiments with both 20 and 21 validated the ligand design principles: both pyridine and pyrazine substituted ligands displayed weaker stripping abilities with respect to EDTA while the selectivity increase with the softness of the substituents (Figure 7b). To further elucidate the softness of the substituents on the ligand selectivity, stability constants (logβ) for ML species were measured through potentiometric titrations of the corresponding ligands with La(III), Nd(III), Eu(III), Dy(III) and Lu(III) cations. As shown in Figure 7c, increasing complex stability was observed when the ionic radius of the cation decreased, which obeyed the well-known electrostatic trends for lanthanides coordination. The smoother electrostatic evolution of the stability constants across the 4f series for pyrazine-substituted ligand 21 with respect to that containing pyridine could be attributed to the softer nature of pyrazine producing smaller electrostatic interactions. Overall, these results clearly indicated that proper compromise between ligand softness and hardness by combination of both soft and hard coordination atoms could provide potential selectivity enhancement for 4f and 5f elements.

thumbnail Figure 7

(a) Molecular structures for tetrapodal ligands EDTA (ethylenediaminetetraacetic acid) and its pyridine and pyrazine derivatives. (b) Percentage of stripped cations and separation factors for the back-extraction of Eu(III) and Am(III) by 0.5 M water solution of ligand 20 and 21 at pH of 3 comparing to HEDTA (ligand 14, Figure 6). (c) Evolution of stability constants logβ for ML complexes as an inverse function of the cation ionic radii for La(III), Nd(III), Eu(III), Dy(III) and Lu(III). Data in panel (b) and (c) are recreated from Ref [115].

Similarly, detailed structure-function relationships in DTPA systems were investigated by Peter et al. from Idaho National Laboratory [88,116–119]. One acetate arm of DTPA was systematically substituted with ethyl alcohol, amide and pyridinyl methyl groups at the same time. Thermodynamic impacts of these modifications towards f-element complexation were revealed. As exampled in Figure 8, the introduction of pyridine groups on one arm of DTPA gave the ligand of DTTA-PyM, which displayed enhanced preferences towards trivalent actinides with respect to DTPA. Fluorescence lifetime studies directly revealed the inner coordination environments for both Eu(III) and Cm(III) cations: the ligand preserved octadentate architecture throughout the measured pH range of 2.1 to 5.0 for Cm(III) (Figure 8b, right hand), while evidence of heptadentate complexation was observed for Eu(III) at lower pH and changed to octadentate architecture at higher pH (5.0) [117]. This suggested that the additional pyridyl nitrogen atom was protonated first and then participated in the metal binding as the pKa value of pyridyl nitrogen atom was approached. The observed suppressed protonation of the pyridyl nitrogen in the case of Cm(III) was attributed to the stronger trivalent actinide binding of DTTA-PyM and this was further explained by density functional theory calculations. Comparison of metal-to-ligand π-back-bonding interactions clearly indicated that stronger interactions existed for 5f Cm(III) than that for 4f Eu(III) cations (Figure 8c). These enhanced interactions were believed to be the origin of Eu(III)/Am(III) selectivity.

thumbnail Figure 8

(a) Molecular structures for DTPA and pyridinyl-substituted DTPA (termed as DTTA-PyM). (b) Luminescence lifetime decays for DTTA-PyM with Eu(III) and Cm(III) in aqueous solution with different pCH. For all solutions, CEu(III) = 2.0 mM, CCm(III) = 50 μM, CDTTA-PyM = 10.0 mM, I = 2.0 M Na(H)ClO4, and T = 25.0 ± 0.1 oC. (c) Comparison of π-back-bonding interactions between the representative singly occupied 5f orbital of Am (nAm, top) and 4f orbital of Eu (nEu, bottom) and the Rydberg orbitals on the amine (Ry*N2) and pyridine (Ry*N4) nitrogen atoms, respectively. Panel (b) and (c) were replotted according to Ref [117].

Hydroxypyridinones are another important family of aqueous f-block elements chelators with 3,4,3-LI-(1,2-HOPO) (abbreviated as HOPO hereafter, Figure 9a) as the mostly studied. HOPO was initially developed to replace DTPA for the decorporation of radionuclides from contaminated human bodies to solve the problems of oral administration inactivity and the low binding abilities towards certain trivalent actinides such as Am3+, Pu4+, UO22+ and NpO2+. Based on the similar coordination behavior of Fe3+ and Pu4+ (including their similar charge-to-ionic-radius of 4.6 and 4.3, respectively), HOPO family was developed as synthetic analogs of the microbial iron transporter siderophores and proved to be efficient actinide chelators among over 60 evaluated ligands [120,121]. Later, this bio-inspired chelator was studied with serval f-elements for their solution coordination behaviors including Ce4+/3+ [122], Th4+ [122,123], UO22+, Pu4+ [124], Am3+/Cm3+/Bk3+/Cf3+, and Es3+ [125,126] (Figure 9b and 9c). Although the solution thermal dynamic data of HOPO clearly indicated the preferences of Ans(III) over Lns(III) [122,123], it was not until recently that the thorough Ans(III)/Lns(III) separation abilities of this promising ligand was demonstrated. By combination of the water-soluble octadentate chelator of HOPO with several industry-relevant organic extractants, Rebecca and her coworkers revealed that efficient separation of Gd(III) from four transplutonium elements (Am(III), Cm(III), Bk(III) and Cf(III)) could be achieved, in which the combination of HDEHP/HOPO showing satisfying separation performances of SFGd/Am = 30, SFGd/Cm = 8.5, and SFGd/Cf = 773 [127]. Together with the water solubility, biocompatibility, rapid excretion and relatively high affinity towards multiple radioactive f-elements, HOPO stands for a promising f-block binding reagent.

thumbnail Figure 9

(a) Molecular structure for HOPO. (b) Computed DFT structure of Bk(IV)-HOPO and (c) complex structure of Es(III)-HOPO. The octadentate chelator of HOPO typically compose four 1-hydroxy-pyridin-2-one metal binding unites attached to a spermine scaffold (marked in blue in panel c). Eight metal-binding oxygen atoms (red) fulfill the first coordination sphere around metal center on ligand deprotonation and metal complexation. Panel (b) was adapted from Ref [125] with permission.

Hydrophilic ligands based on multiple-aromatic-skeleton

Except the oxygen dominated coordination ligands as mentioned in the previous two sections, ligands based on polypyridyl derivatives are currently the most efficient and widely studied class of the water-soluble f-elements coordination ligands [56,58]. It is worthy to point out that, to be differentiated from that discussed previously, the term “multiple-aromatic-skeleton” used in this section refers to the structures containing polyaromatic motifs linked directly by single bonds or fused together. Inspired by the successes in the design of highly efficient lipophilic Lns(III)/Ans(III) separation ligands, triazin-substituted polypyridyl ligands were functionalized with water solubilizing groups such as sulfonic acid with the purpose to increase the ligand acid resistances and at the same time, to further boost the ligand selectivity considering the softer nature of nitrogen atoms with respect to oxygen. Geist and Panak reported the synthesis of hydrophilic BTP ligands, 2,6-bis(5,6-di(sulfophenyl)-1,2,4-triazin-3-yl)pyridine (SO3-Ph-BTP, ligand 26, Figure 10) and demonstrated its ability for selectively masking the trivalent actinides in nitric acid solutions [89]. Used in combination with TODGA (0.2 M) in TPH (5% vol. n-octanol) as the organic phase, SO3-Ph-BTP displayed obvious suppression of Am(III) extraction. The best SFEu/Am of about 1000 was achieved for 18 mM SO3-Ph-BTP in 0.5 M HNO3. The Am(III)/Eu(III) selectivity remained active in nitric acid of up to 2 M and no buffering or salting-out agents were needed in the aqueous phase. Successive time-resolved laser fluorescence spectroscopy (TRLFS) of Cm(III) and Eu(III) indicated formation constants (logβ) of 12.2 and 10.2 for ML3 species and a more negative Gibbs free energy difference was observed for Cm(III), mirroring the selectivity observed in the extraction experiments [128]. Based on these experimental results, a new system named AmSel (Americium Selective Extraction) was proposed, which, in its first step, was similar to i-SANEX that both trivalent lanthanides and actinides were coextracted by TODGA, followed by selective Am(III) stripping with SO3-Ph-BTP [129]. Further ligand structure modification revealed that the number of sulfonate groups was more important for the separation of Am(III)/Eu(III) than the type of ligand used, the location of the sulfonated phenyl rings in the molecules or the counterion used (H+/Na+) [90]. While another work from the same author showed that when more rigid backbone of phenanthroline was used (TS-BTPhen2, ligand 27, Figure 10), higher intergroup actinides separation for Am(III)/Cm(III) could be observed, indicating the importance of ligand cavity size for the even trickier Am(III)/Cm(III) separation [130]. Recently, similar efficient Am(III)/Eu(III) separation was observed in sulfonated 2,9-diamide-1,10-phenanthroline ligand (DS-Ph-DAPhen, ligand 28, Figure 10) while with less mass fraction of sulfonic acid, thus it offers much greener processing processes [85].

thumbnail Figure 10

Chemical structures for hydrophilic ligands based on multiple-aromatic-skeleton.

The main problem for the sulfonated polypyridyl ligands was the second waste generation after incineration of the spent solvent streams. Also, the requirement of harsh preparation conditions [90], the decrease of ligand crystallinity [81,85] and the radiostability issues [63,65] were also largely unsolved. To this end, series of hydroxyl-group functionalized N-coordination and N, O-coordination ligands were reported, in which the 2,6-bis-triazolyl-pyridines (PyTri, ligand 29, Figure 10) was the benchmark ligand [95]. Developed by Casnati et al., hydrophilic PyTri ligands demonstrated the ability to strip actinides in all oxidation states from a TODGA-containing kerosene solution into an acidic aqueous phase. Their high selectivity for actinides, along with their efficiency, rapid extraction kinetics, and chemical and radiolytic stability, highlighted these ligands as outstanding candidates for advanced separation processes in future for the closed nuclear fuel cycles. Further radiolytic stability tests by ESI-MS revealed that PyTri displayed superior radiostability with respect to the sulfonated counterparts [65]. Furthermore, the identified by-products of degradation residue from PyTri preserved the hydrophilicity and maintained the MAs selectivity, which further emphasized the success of this kind of ligands. Although PyTri displayed overall balanced Lns(III)/Ans(III) separation, it was incapable for Am(III)/Cm(III) separation and the separation performances decreased dramatically with increasing solution acidity. To improve Am(III)/Cm(III) separation, Panak and Whitehead reported more rigid bipyridine and phenanthroline backbones while keeping the hydroxyl-functionalized triazolyl side groups, both ligands could distinguish Am(III) and Cm(III) with similar SFAm/Cm of around 2.5 [91,92]. Along this line, Jansone-Popova and her coworkers designed more preorganized N, O-ligands of bis-lactam-1,10-phenanthroline (BLPhen-aq, ligand 34, Figure 10) with the purpose of locking the ligand confirmation to compensate the entropy enhancement for metal coordination processes [79,131,132]. The lipophilic counterparts of ligand 34 gave high SFEu/Am of about 250 in 3 M HNO3 with 1 mM ligand concentration in 1,2-dichloroethane [79]. Introduction of hydrophilic substituents of ethylene glycol units onto BLPhen framework gave the water-soluble ligands of 34, which had been proved efficient for intragroup lanthanides separation while the less constrained phenanthroline diamide ligand (PhenDA-4EG, ligand 33, Figure 10) failed to perform comparable discrimination ability among trivalent lanthanides [132]. Unfortunately, as far as we know, no data is currently available for this hydrophilic ligand to show the selective actinides masking performances.

By combination of water-solubilizing groups with previously reported ligands capable of discriminating Lns(III)/Lns(III), Lns(III)/Ans(III) and Ans(III)/Ans(III), efficient and selective extraction of radioactive f-block elements could be fulfilled. While current problems such as sophisticated ligand preparation methods, inefficient masking/stripping of desired radionuclides under high acidity, and unclear solution coordination chemistry/dynamics because of either factor from the ligands side such as limited solubility, inferior crystallinity or from the rareness of the radionuclides or the constrained requirement of specific experimental facilities, had brought much uncertainty in the rational design of highly efficient element-specific hydrophilic ligands. Some of the literature results are even misleading or partially right under their very specific cases, these have posed tremendous obstacles for the rational design of efficient and practical ligands for real world applications as will be discussed in detail in extension parts.

Hydrophilic ligands based on other topological skeletons

Macrocyclic receptors have long been used to form stable cryptate inclusion complexes and displayed molecular recognition towards spherical, tetrahedral and linear substrates of various kinds, including metal cations, inorganic anions, organic/biological cations or anions [133,134]. Early research interests of macrocyclic ligands with f-block elements were to serve as a springboard to explore the coordination chemistry of these metal cations as their attracting applications in radiopharmaceuticals, radioimmunotherapy and other medical applications such as positron emission tomography, contrast-enhancing agents in magnetic resonances imaging and NMR shifting reagents [43]. While initial thermodynamic results indicated macrocyclic ligands could be used as effective metal cations chelators [135–137] and displayed the potential for the separation of lanthanides [138,139], which spurred the need for synthesizing stable macrocyclic complexes of f-block metal cations to explore the bonding parameters and to tap their potential uses.

In the first decade of the 21th century, a large body of research have been focusing on the design of macrocyclic lanthanides complexes for high efficiency luminescing complexes leveraging mainly the high affinity and shielding effect of the cyclic ligands [140–144]. Polyazamacrocycles represented one of the most attractive platforms for the formation of stable metal complexes, with some of which displayed high acid resistances [145–150]. The macrocyclic f-block element chelating ligand of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its derivatives were the most studied ligands of such kind, which had been widely investigated for medical imaging and therapeutic applications and with some being conducted on radionuclide separations [151]. Herein, in the following context, we mainly focused on macrocyclic ligands containing both O and N as the binding sites. Carlos and Teresa were among the pioneers to introduce rigid picolinate pendants onto the 4,13-diaza-18-crown-6 framework (well-known architecture for alkali metal and other anions binding) with the purpose to increase the selectivity of large Lns(III) cations [152]. They found that except for extremely high selectivity of light Lns(III) cations, the modified macrocyclic ligands also displayed a steady variation of complex stability and fast formation kinetics. While for real world industrial application, monotonous affinity changes were not ideal. By optimizing the position and number of picolinate substitutes on the macrocyclic ring, Wilson and his workers observed an unreported selectivity pattern among the lanthanides series—the “S” shaped binding curves where one minimum and two maxima of the stability across the whole lanthanides (Figure 11b), which was believed to be more appealing for intragroup lanthanides discrimination [153,154]. Single crystals of both largest and smallest La(III) and Lu(III) with the ligands were cultivated, displaying significant coordination conformation shift moving from large Lns(III) to smaller cations, which allowed the high thermodynamic stability for both light and heavy lanthanides (Figure 11c). Additional DFT calculations reinforced the existence of this conformational toggle, showing that ligand strain and metal-ligand binding energies were complementary factors driving the conformational switch. Considering the similarity of Lns(III) with Ans(III) cations and the fact that some of the Lns(III) cations were frequently used to resemble the rare and inaccessible radioactive actinides, it is reasonable to expect similar conformational changes might exist for some of the macrocyclic ligands that could serve for efficient intra-actinides separations.

thumbnail Figure 11

Water soluble macrocyclic ligands with an unprecedented size-selectivity pattern for lanthanides ions. (a) Molecular structures for macrotripa and macrodipa. (b) Stability constants for Ln(III) complexes formed with macrotripa and macrodipa plotted versus lanthanides cations radii. (c) Depictions of the conformational toggles present in Ln(III)-macrodipa complex system with the representative crystal structures of [La(macrodipa)]+ and [Lu(macrodipa(OH2)]+. Figure is recreated from Ref [153] by courtesy of the American Chemical Society.

EXTENSIONS AND FURTURE DIRECTIONS

Extended discussions

The separation of radioactive f-block elements, in the very specific cases, Lns(III)/Ans(III) and Ans(III)/Ans(III) separations, remains a significant hydrometallurgical challenge [25,29,44–46,49,50,52,53,56,58, 67,69,70,155–160]. Actinides, such as americium (a representative minor actinide), and lanthanides share both physical and chemical similarities. Both groups predominantly exhibit a stable +3 oxidation state in solution and have cations of comparable size. These similarities arise from their unique electronic structures: for 4f and 5f elements, additional electrons with increasing atomic number are accommodated in inner shells. In other words, the valence electrons in 4f orbitals, for instance, are shielded by the 5s and 5p electrons. This results in two main consequences: the poor shielding by these shell electrons does not offset the increasing positive nuclear charge, leading to greater-than-expected decreases in atomic radii, known as lanthanide contraction [54,161,162]. Moreover, the interactions between metal centers and coordination ligands are largely electrostatic, with coordination numbers and complex geometries primarily determined by steric factors [163]. Consequently, differentiating and isolating 4f and 5f elements is highly challenging, particularly when considering separations within the same group where the basic principle of HASB is mostly invalid [29].

To effectively separate trivalent lanthanides and minor actinides (MAs), two main approaches have been investigated: oxidation state control and HASB-based ligands design [46,164–170]. The former exploits the distinct coordination geometries of the oxidated radionuclide such as Am(V)/Am(VI) as compared to Lns(III), while the latter leverages the softer nature of the more dispersed 5f orbitals of actinides, which exhibits more covalent bonding with soft donor atoms like nitrogen and sulfur. Oxidation state control is theoretically more efficient than selective recognition by soft donor ligands, but the instability of the oxidized species, the introduction of corrosive reagents, and the incompatibility with current industrial settings have limited its widespread application [81,83,84]. On the other hand, selective separation by repeated redistribution of trivalent lanthanides and actinides using carefully designed soft donor ligands offers scalability and compatibility with various recovery methods. In this approach, ligand design is crucial. As summarized in the aforementioned parts, the pursuit of efficient ligands/methods for the separation of radioactive f-block elements is ever-growing. Significant successes have been achieved with desirable metal distribution and separation factors for target metal cations. Based on Hard-Soft Acid-Base (HSAB) theory, various ligands containing nitrogen, sulfur, and oxygen have been designed and utilized for Lns(III)/Ans(III) separation. Solution species evolutions have been investigated through multiple titration methods, and single-crystal X-ray diffraction along with theoretical calculations have elucidated the coordination modes and chemical driving forces of the separation process [50,56,171]. Furthermore, ligand design principles such as preorganization, the combination of hard and soft donors to enhance selectivity [56,76,79,83,131,132], the alpha-effect [49,50], and semirigid ligand architecture to increase acid resistance [78,79,172–174], and the elimination of benzyl solubilizing alkyl chains to improve radiostability [49,50,62] have been extensively reviewed and demonstrated. However, there remains a high demand for ideal ligands suitable for real-world applications. These ligands should provide high efficiency for both extraction and stripping, ease of preparation and purification, fast kinetics, and superior stability. Additionally, modern recycling processes require greener and more sustainable ligand design and processing procedures to minimize environmental impact.

Despite great efforts have been devoted to develop efficient and selective f-block binding ligands and some promising results are reported. Problems still exist for hydrophilic ligands: (1) Most of the ligands are still based on solubilizing the known lipophilic ligands, systematical ligand design is lack to guide rational structure construction and modification. One should be aware that the coordination chemistry for lipophilic ligands and hydrophilic ones must be different considering both the hydration of the metal cations and ligands in the aqueous phase especially in highly acidic conditions. Solvent-solute, solute-solute interactions could be more complicated in the case of hydrophilic ligands. (2) The competing of protons at high acid concentration cripples most of the hydrophilic ligands as summaries in Figure 12. It seems more severe for hydrophilic ligands with respect to the lipophilic counterparts for protonation, which leads to the dramatical loss of both extraction ability and the selectivity (shown in the case of ligand 27 and 28, Figure 12). (3) The synthetic obstacles for water soluble ligands as discussed in the introduction part. (4) Conflicting results are found when comparing data and conclusions from different groups, which further impede rational ligand design from the rather limited reports. For example, Macerata et al. claimed that the flat and rigid phenanthroline rings could aggregate by hydrophobic π-π interaction and this might be detrimental for hydrophilic ligands (in case of ligand 32, Figure 12) [175]. Later the same authors reported the benchmark ligand of PyTri (ligand 29, Figure 12) with single bonded N-containing mono-heterocyclics [95]. In the structure optimization of PyTri, the same group concluded that the existing of hard amide carbonyl groups might interfere with the actinides separation even the ligand displayed higher water solubility. While later the N, O-mixing ligands containing imide carbonyl groups were proved to be critical building blocks in phenanthroline-based hydrophilic ligand systems [81,82,84,85,132,176].

thumbnail Figure 12

Summary of representative hydrophilic ligands from their extraction performances and extraction experimental conditions. Data in blue and yellow shaded areas were based on hydrophilic ligands with alkyl-chain-skeleton and multiple-aromatic-skeleton, respectively. The rest were from hydrophilic ligands with simple-aromatic-skeleton. The dashed arrows in the yellow area indicated the decrease of the extraction performances of the same ligand at different acidity. The experimental conditions were summarized in Table 1.

Aiming at increasing the acid resistance of widely studied preorganized phenanthroline ligands, we have conducted a series of work on phenanthroline diimides trying to illustrate that the seemingly spectating flanking groups could be game changer for the sensitization and extraction performances through complicated crystal structure adjustments. We first conducted DFT-calculation guided structure optimization to compare the free energy changes for both phenanthroline diamide and phenanthroline diimide [83]. Detailed analysis of previous reports indicated that during metal complexation, the carbonyl oxygens rotated from pointing away from each other in the free ligand to facing the ligand binding cavity in the final complexes (Figure 13a) [56]. Thus, switching from amide to imide reduced the energy barrier and facilitated complexation thermodynamically, as further confirmed by extraction experiments. This concept proved effective for both lipophilic and hydrophilic ligands [81–84]. Specifically, we discovered that the additional hydrogen bond formation ability of imide may help establish a hydrogen bond network in the complexes, contributing to the ligand’s acid resistance. Alternation of the length of hydrophobic alkyl chain, on the other side, could help protection of the binding sites from protonation, further increasing the acid resistance of the ligand (Figure 13b) [84]. Additionally, we demonstrated that introducing a strong coordinating carboxylic group onto the phenanthroline diimide backbone resulted in the formation of dimeric Eu(III) complexes (Figure 13c) [81]. This alkyl-chain connected dimeric structure created a lipophilic microenvironment around the metal centers, preventing polar hydrated protons from approaching and enhancing the ligand’s extraction performance under high acidity. These reported ligands were easy to prepare, had a high tendency to crystallize, and were efficient under acidity of over 1 M HNO3, achieving Eu(III)/Am(III) separation factors of around 200. While the structural isomers with alternation of linking sequences of imide carbonyl groups with respect to phenanthroline displayed totally different solubility, photophysics and sensing selectivity. Unfortunately, the isomeric ligand was not soluble enough for extraction [177]. In fact, water solubility of both hydroxyl and carboxylic groups functionalized phenanthroline diimides were quite limited. For example, carboxylic acid functionalized phenanthroline diimide ligands were only soluble in acid concentrations over 0.5 M HNO3, with a maximum concentration of 10 mM, and ligands with hydroxyl groups barely dissolved in pure water at concentrations of 10 mM. To further enhance water solubility, we proposed introducing amine terminal groups onto the phenanthroline diimide backbone [82]. The protonation of amine groups produced highly water-soluble ammonium salts. However, recent findings from other groups suggested that positive terminal groups might hinder metal cations from approaching the ligand due to electrostatic repulsion [93]. Our results revealed that proper spacing of the positive ammonium parts from the binding cavity using nonconjugated alkyl chains could mitigate this repulsion, thus balancing water solubility and extraction performance. All these results added up together to show that small structural alternation could result in significant functional changes for multi-dentated small molecular ligands.

thumbnail Figure 13

Brief summary of our recent approaches towards hydrophilic ligand design under high acidity for efficient masking actinides from lanthanides.

Table 1

Summary of representative hydrophilic ligands from their extraction performances and extraction experimental conditions

Future directions

In the current review, we have briefly summarized the development of hydrophilic ligands for radioactive f-block elements coordination and separation. The literature results were cataloged into four groups based on the ligand backbones (Figure 14), namely ligands with alkyl-chain skeleton; ligands with simple-aromatic-skeleton; ligands with multiple-aromatic-skeleton and ligands based on other topological skeletons. For each part, the ligand design principles were discussed by comparison with the previous results and both advantages and existing problems were given. Afterwards, another aqueous solution-based f-block elements separation approach through oxidation state control was briefly introduced and compared to the coordination-based extraction processes. Together with our recent findings in the design of efficient hydrophilic masking agent for the separation of Lns(III) and Ans(III), the establishing of structure-function relationship to guide future ligands design was emphasized. In the last part of this contribution, we have proposed four more f-block elements chelating ligands with some of them are reported to be efficient for solid-liquid or membrane-based radionuclide separation with the aim to inspire the search for more efficient and robust systems for these important elements utilization and recycling.

thumbnail Figure 14

Summary of literature reported hydrophilic ligands discussed in this review (top-half) and the four potential future directions of ligand design based on our understanding. Figures for polymeric and biobased ligands are recreated from Refs [178] and [54] with permissions of the Springer Nature Limited and the American Association for the Advancement of Science (AAAS). Single crystals structures for macrocyclic and bioinspired ligands are plotted from CCDCs reported in Refs [152] and [179].

Polydentate hydrophilic ligands to construct self-assemble systems based on f-block elements

Multivalency is a key principle in nature for achieving strong yet reversible interactions [180]. Examples of these phenomena include the nature-inspired velcro from burr, which takes advantage of multiple hooks on the surfaces entangling with the loops on the other surfaces giving strong connect of the two surfaces (Figure 15a, top). Another example is the multivalent binding of a virus to a cell surface (Figure 15a, bottom), the sum of the multiple weak interaction adds up to give strong adhesion. Antivirus drugs are designed according to this mechanism by interrupting these weak interactions through the comparative binding from multiple dentate ligands. The properties of this multivalent cooperative interaction are appealing for the subtle differences-based f-block elements separation processes [44,144,159,181,182]. Representative systems from Sun [183] and Mei [184] demonstrated that the in-situ self-assembly formed between dedicatedly designed ligands with either Lns(III) and U(VI) gave distinct extraction performances with respect to the unassembled systems, emphasizing the potential application of these concept in the design of highly efficient f-element selective systems.

thumbnail Figure 15

Multivalent cooperative enhancement of f-block metal selectivity. (a) Demonstration of nature inspired multivalent cooperative binding enhancements in the case of burr (top) and antivirus drugs (bottom). (b) A supramolecular lanthanides separation reported by Sun et al. based on the concept of multivalent cooperative enhancement. (c) A supramolecular actinides separation reported by Mei et al. through in-situ formation of self-assembled metal-organic nanocages. Panel (a) is recreated with permission from Ref [180] by courtesy of the John Wiley & Sons, Ltd. Panel (b) is recreated with permission from Ref [183] by courtesy of the Springer Nature Limited. Panel (c) is recreated with permission from Ref [184] by courtesy of the American Chemical Society.

Decoration of cations-specific binding groups onto soft polymer backbones is another way to fulfill multivalent cooperative interactions to boost the metal selectivity. Representative example from Ma et al. had demonstrated that rational introduction of uranyl-specific diamidoxime “hooks” onto porous frameworks for efficient uranium extraction from seawater with high enrichment index (Figure 16) [185]. Their results indicated that manipulating the relative distances and angles of amidoxime moieties in the ligands could maximize the cooperative binding of adjacent amidoxime groups, thus giving new benchmarks for uranium absorbent materials. Except the manipulating of the metal binding moieties on the polymer chain, Berkland et al. from The University of Kansas showed that the vital role of the soft polymeric scaffold on the metal affinity [186]. They claimed that the polymer chain flexibility could spontaneously produce the highest possible metal affinities for the metal binding ligands, which could obviate the sophisticated synthesis of traditional rigid scaffolds. As a demonstration of this “conformational stability effect”, they showed that simple conjugation of small molecule catechol ligands to a polyallylamine chain resulted in more than 8–9 orders of magnitude enhancement of the iron-binding affinity, which was comparable to that of enterobactin, the strongest iron chelator ever known [187,188].

thumbnail Figure 16

Porous organic polymers decorated with amidoxime hooks for cooperative binding of uranyl. (a) Chemical structures of diamidoxime-functionalized POPs. The numbers in electronvolts were complexation energies calculated from DFT of the shaded units. (b) Single crystal structures of 3 with UO2. Hydrogen is omitted for clarity. (c) Illustration of POPs with diamidoxime for uranyl extraction. (d) The distribution coefficient values for uranium and vanadium over different sorbent materials (cyan, uranium; red, vanadium). Figure is recreated from Ref [185] by courtesy of the American Chemical Society.

Macrocyclic hydrophilic ligands to combine multiple coordination and structural selectivity

The applications of macrocyclic ligands for f-block elements binding have long been investigated. A number of comprehensive reviews are available [43,133,134,189]. Also, we have discussed the potential of the hydrophilic macrocyclic ligands in lanthanides separation previously. While, here we want to emphasize again these topological ligands marry the merits of both cooperative multidentate ligands with the possible structural adjustment to maximize the differentiation of almost identical metal cations. These could be very promising and efficient approaches for intra-group lanthanides/actinides separations.

Bioinspired hydrophilic ligands for f-element decorporation and radiopharmaceutics

Biomolecules typically comprise multiple highly polar motifs containing oxygen, sulfur, phosphorus, and nitrogen, such as those found in amino acids, RNA, and DNA. These elements have also found widely applications in metal binding, specifically for f-block elements [53,102,162,190–196]. The most successful transition of this kind is HOPO of which the core binding motif is hydroxypyridinone. The bidentate hydroxypyridinone units are frequently found in a few siderophores and plant products with its two isomers, 1,2-hydroxypyridinone and 3,2-hydroxypyridinone serving as structural and electronic analogs of hydroxamic acid and functionalized catechol. First developed as the synthetic analogs of the microbial iron transporter siderophores, HOPO family ligands are now used as therapeutics for treatment of individuals contaminated with selected actinides (U, Np, Pu, Am, and Cm) and also for selectively binding and separation of radioactive actinides [125,197–200]. Similar to HOPO, Sessler et al. recently reported that another FDA-approved agent of deferasirox for treatment of iron overload disease could be used as Lu(III) specific chelators (up to 80%) through selective precipitation in competition of La(III), Ce(III) and Eu(III) [179]. These results clearly show that bio-related small chelators with multiple O, S, N and P sites could serve as potential and safe chelators for radioactive f-block elements decorporation and radiopharmaceutics [53,102,190,191].

Biobased hydrophilic ligands for green and sustainable f-element coordination and separations

Biobased hydrophilic ligands discussed here specially refer to biomacromolecules that are at the frontiers for investigating the interaction and biological role of f-block elements with living systems and searching of green, efficient bioengineering approaches for the tedious chemical separation industry [53,54,162,195, 196,201]. While seemingly exotic at first glance, biological utilization of f-block elements, especially lanthanides are very logical from a chemical perspective considering the relative abundant nature of these elements in the earth crust. The discovery in 2011 that methylotrophic bacteria specifically incorporate certain lanthanides into pyrroloquinoline quinone (PQQ)-dependent alcohol dehydrogenases (ADHs) has opened new possibilities for more sustainable and efficient aqueous extraction and separation of these elements [162,195,196]. The recently discovered LanM family of lanthanide-binding proteins has shown that nature has evolved macromolecules with selectivity surpassing that of synthetic f-element chelators [202]. The prototypical LanM from M. extorquens AM1 (Mex-LanM) is a small, monomeric protein that undergoes a selective conformational response to picomolar concentrations of lanthanides [202,203] and actinides [204–207]. This discovery has advanced our understanding of lanthanide uptake in methylotrophs and has served as a technology platform for f-element detection [208], recovery [203,209], and separation [210]. Except for these natural biomacromolecules, manmade peptides and their mimics also showed specific binding affinity to certain f-block elements [211,212] as revealed in the recent work from Wang et al., which demonstrated that a biosafe omiganan peptide displayed high affinity and selectivity towards uranium in seawater and meanwhile it had remarkable resistance against biofouling [213].

CONCLUSIONS

Liquid-liquid separation of radioactive f-block elements is essentially a tug-of-war between lipophilic and hydrophilic chelators in the two immiscible phases. With most of the research efforts devoted in the lipophilic ligands design, hydrophilic ones are less explored but are believed to be important alternatives to relieve the environmental impact and further boost the separation efficiency. Aware of the lack in both fundamental understanding and enough ligands library to draw any constructive conclusions, we have in the current manuscript briefly summarized the developing of hydrophilic ligands from four different catalogs grouped according to the structural similarities and the chronological order. Some of our recent findings in the phenanthroline diimide systems are also discussed to show how the molecular structure alternation could alter the ligand acid-resistance, extracting properties and selectivity. In the outlook part, we have displayed potential ligand design based on self-assembly/clusters/polymers (sharing the same concept of multivalent cooperativity), macrocyclic ligands, bioinspired and biobased ligands based on our understanding and literature reports from closely-related field. This review does not aim to provide a comprehensive guide to design efficient hydrophilic ligands for radioactive f-block elements, as it would be too early to draw any conclusion at this stage. More work and larger ligand inventory are needed to give any systematical principle to guide the future ligand design. Effective coordination and extraction of f-block elements are complex, multidisciplinary challenges requiring contributions from researchers in organic and physical chemistry, analytical chemistry, crystallography, and chemical engineering. We could foresee the blooming futures of hydrophilic f-block elements chelating ligands in the cases of nuclear waste managing, clean energy production, environmental remediation, nuclear medicine, bioimaging and other disease diagnostics.

Funding

C. Xu would like to thank the National Natural Science Foundation of China (U2067213, 22325603), L. Wang would like to thank the National Natural Science Foundation of China (22105205) and the Beijing Natural Science Foundation (2232002).

Author contributions

L.W., B.L., M.-J.B. and Y.K. wrote the initial manuscript; L.W. L.-D.W. Y.-Y.L. and C.X. revised and formatted the final manuscript; L.W. and C.X. managed the funding acquisition. All authors have given their approval to the final version of the manuscript.

Conflict of interest

The authors declare no conflict of interest.

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All Tables

Table 1

Summary of representative hydrophilic ligands from their extraction performances and extraction experimental conditions

In the text

All Figures

thumbnail Figure 1

Spent nuclear fuel (SNF) constituent distribution from a typical light-water reactor (LWR) with parameters: 1 GWe, burn-up = 33 GWd/tHM, capacity factor of 0.8 and cooling at 150 days. The zircaloy weight is included.
In the text
thumbnail Figure 2

Summaries of current advanced reprocessing processes. PUREX: plutonium uranium reduction extraction [7,8]; TRUSPEAK: a single process combines Transuranic Extraction (TRUEX) process and the Trivalent Actinide-Lanthanide Separations by Phosphorus-reagent Extraction from Aqueous Komplexes (TALSPEAK) process [30]; ALSEP: actinide lanthanide separation [31]; SANEX: selective actinide extraction [23,24]; GANEX: Grouped ActiNide Extraction [32]; EXAM: Extraction of Americium [33]; Super-DIREX: supercritical fluid direct extraction method [34]; TRUEX: transuranium extraction [18,19]; DIAMEX: diamide extraction [20,21]; TRPO: trialkyl phosphine oxides [35]; TODGA: N,N,N′,N′-tetraoctyl diglycolamide [36,37]; UNEX: universal extraction [38–40]; SREX: strontium extraction [41]; CSEX: cesium extraction [42]; SESAME: selective extraction and separation of americium by means of electrolysis [15]; LUCA: Lanthaniden Und Curium Americium Trennung [15,28].
In the text
thumbnail Figure 3

Benchmark lipophilic ligands with the advantages and existing problems to be solved. The numbers in BTPs (2) were for the easy discussion of α-effect on triazine rings. Red circles in CyMe4-BTBP (3) and CyMe4-BTPhen (4) showed the absence of benzylic H-atoms to enhance the ligands’ radiation stability. The blue circles in PhenDA (5) indicated the coordination sites of the ligand.
In the text
thumbnail Figure 4

Comparison of representative lipophilic ligands for their extraction dynamics and single crystal structures. (a–c) Ligand structures for CyMe4-BTP, CyMe4-BTBP and CyMe4-BTPhen. Solvent extraction dynamics (d–f, blue dots for Am(III) and blacks for Eu(III)) and single crystal structures (g–i) for the three ligands. Experimental conditions: 8.8 mM ligand with 0.5 mM of N,Nʹ-dimethyl-N,Nʹ-dioctyl-2-ethoxymalonamide (DMDOHEMA) as phase-transfer agent in n-octanol with 0.5 M HNO3 (d); 20 mM ligand in n-octanol with 0.5 M HNO3 (e); 10 mM ligand in n-octanol with 1.0 M HNO3 (e). Data in panel (d) and (g) were recreated according to Ref [86]. Extraction dynamics for CyMe4-BTBP in panel (e) was from Ref [75] and the crystal structure was from Ref [87]. Data in panel (f) and (i) were recreated according to Ref [76].
In the text
thumbnail Figure 5

Flow sheet showing the 1-cycle SANEX processes with different carboxylic acids used during multiple stages of the process. Figure was recreated according to Ref [24].
In the text
thumbnail Figure 6

Hydrophilic ligand structures based-on-alkyl-chain-skeleton. NTA: nitrilotriacetic acid; CDTA: cyclohexane-1,2-diaminetetraacetic acid; DTPA: diethylenetriaminepentaacetic acid; HEDTA: N-(2-Hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid; HEDTTA: N-Hydroxyethyl-diethylenetriamine-N,N′,N′′,N′′-tetraacetic acid.
In the text
thumbnail Figure 7

(a) Molecular structures for tetrapodal ligands EDTA (ethylenediaminetetraacetic acid) and its pyridine and pyrazine derivatives. (b) Percentage of stripped cations and separation factors for the back-extraction of Eu(III) and Am(III) by 0.5 M water solution of ligand 20 and 21 at pH of 3 comparing to HEDTA (ligand 14, Figure 6). (c) Evolution of stability constants logβ for ML complexes as an inverse function of the cation ionic radii for La(III), Nd(III), Eu(III), Dy(III) and Lu(III). Data in panel (b) and (c) are recreated from Ref [115].
In the text
thumbnail Figure 8

(a) Molecular structures for DTPA and pyridinyl-substituted DTPA (termed as DTTA-PyM). (b) Luminescence lifetime decays for DTTA-PyM with Eu(III) and Cm(III) in aqueous solution with different pCH. For all solutions, CEu(III) = 2.0 mM, CCm(III) = 50 μM, CDTTA-PyM = 10.0 mM, I = 2.0 M Na(H)ClO4, and T = 25.0 ± 0.1 oC. (c) Comparison of π-back-bonding interactions between the representative singly occupied 5f orbital of Am (nAm, top) and 4f orbital of Eu (nEu, bottom) and the Rydberg orbitals on the amine (Ry*N2) and pyridine (Ry*N4) nitrogen atoms, respectively. Panel (b) and (c) were replotted according to Ref [117].
In the text
thumbnail Figure 9

(a) Molecular structure for HOPO. (b) Computed DFT structure of Bk(IV)-HOPO and (c) complex structure of Es(III)-HOPO. The octadentate chelator of HOPO typically compose four 1-hydroxy-pyridin-2-one metal binding unites attached to a spermine scaffold (marked in blue in panel c). Eight metal-binding oxygen atoms (red) fulfill the first coordination sphere around metal center on ligand deprotonation and metal complexation. Panel (b) was adapted from Ref [125] with permission.
In the text
thumbnail Figure 10

Chemical structures for hydrophilic ligands based on multiple-aromatic-skeleton.
In the text
thumbnail Figure 11

Water soluble macrocyclic ligands with an unprecedented size-selectivity pattern for lanthanides ions. (a) Molecular structures for macrotripa and macrodipa. (b) Stability constants for Ln(III) complexes formed with macrotripa and macrodipa plotted versus lanthanides cations radii. (c) Depictions of the conformational toggles present in Ln(III)-macrodipa complex system with the representative crystal structures of [La(macrodipa)]+ and [Lu(macrodipa(OH2)]+. Figure is recreated from Ref [153] by courtesy of the American Chemical Society.
In the text
thumbnail Figure 12

Summary of representative hydrophilic ligands from their extraction performances and extraction experimental conditions. Data in blue and yellow shaded areas were based on hydrophilic ligands with alkyl-chain-skeleton and multiple-aromatic-skeleton, respectively. The rest were from hydrophilic ligands with simple-aromatic-skeleton. The dashed arrows in the yellow area indicated the decrease of the extraction performances of the same ligand at different acidity. The experimental conditions were summarized in Table 1.
In the text
thumbnail Figure 13

Brief summary of our recent approaches towards hydrophilic ligand design under high acidity for efficient masking actinides from lanthanides.
In the text
thumbnail Figure 14

Summary of literature reported hydrophilic ligands discussed in this review (top-half) and the four potential future directions of ligand design based on our understanding. Figures for polymeric and biobased ligands are recreated from Refs [178] and [54] with permissions of the Springer Nature Limited and the American Association for the Advancement of Science (AAAS). Single crystals structures for macrocyclic and bioinspired ligands are plotted from CCDCs reported in Refs [152] and [179].
In the text
thumbnail Figure 15

Multivalent cooperative enhancement of f-block metal selectivity. (a) Demonstration of nature inspired multivalent cooperative binding enhancements in the case of burr (top) and antivirus drugs (bottom). (b) A supramolecular lanthanides separation reported by Sun et al. based on the concept of multivalent cooperative enhancement. (c) A supramolecular actinides separation reported by Mei et al. through in-situ formation of self-assembled metal-organic nanocages. Panel (a) is recreated with permission from Ref [180] by courtesy of the John Wiley & Sons, Ltd. Panel (b) is recreated with permission from Ref [183] by courtesy of the Springer Nature Limited. Panel (c) is recreated with permission from Ref [184] by courtesy of the American Chemical Society.
In the text
thumbnail Figure 16

Porous organic polymers decorated with amidoxime hooks for cooperative binding of uranyl. (a) Chemical structures of diamidoxime-functionalized POPs. The numbers in electronvolts were complexation energies calculated from DFT of the shaded units. (b) Single crystal structures of 3 with UO2. Hydrogen is omitted for clarity. (c) Illustration of POPs with diamidoxime for uranyl extraction. (d) The distribution coefficient values for uranium and vanadium over different sorbent materials (cyan, uranium; red, vanadium). Figure is recreated from Ref [185] by courtesy of the American Chemical Society.
In the text

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