Complexation of Actinide(III) and Lanthanide(III) with H4TPAEN for a Separation of Americium from Curium and Lanthanides

Remarkable Improvement in Am3+ and Cm3+ Separation Using a Cooperative Counter Selectivity Strategy by a Combination of Branched Diglycolamides and Hydrophilic Polyaza-heterocycles

Separation of Am3+ and Cm3+ is one of the most challenging problems in the back-end of the nuclear fuel cycle. In the present work, we exploited the cooperative effect of the opposite selectivity of hydrophobic branched DGA derivatives and hydrophobic N-donor heterocyclic ligands taken in two different phases to achieve improved separation behavior. A systematic study was performed using a series of DGA derivatives to understand the effect and the position of branching in the alkyl chains on the separation behavior of Am3+ and Cm3+. A separation factor (S.F.) value as high as 10 for Cm3+ over Am3+ was obtained in the case of TiBDGA (N,N,N',N'-tetra-iso-butyl diglycolamide) using SO3PhBTPhen ((phenanthroline-2,9-diyl)-1,2,4-triazine-5,5,6,6-tetrayltetrabenzenesulfonic acid) as the aqueous complexant, which is the highest reported value so far for the ligand-based separation of Am3+ and Cm3+ without involving any oxidation or reduction step. The high selectivity favoring Cm3+ ion extraction in the case of this DGA derivative is also explained with the help of computational studies.

Theoretical investigation on the ligands constructed from phenanthroline and five-membered N-heterocyclic rings for bonding and separation properties of Am(III) and Eu(III)

The ligands, derived from the combination of phenanthroline and various five-membered N-heterocyclic rings, were subject to a comprehensive investigation for their potential in the extraction and separation of actinides and lanthanides. This study employed DFT methods to thoroughly explore the properties of both phenanthroline (Ph) and the diverse five-membered N-heterocyclic rings (R1-R8). Additionally, tridentate ligands RlPh (l = 1-8) and tetradentate ligands RlPhRr (l, r = 1-8) were analyzed in detail, encompassing their electrostatic potential (ESP), protonation energy, coordination bonding with the metals Am(III) and Eu(III), and the thermodynamics of extraction separation for Am(III) and Eu(III). The findings highlight that the electrostatic potential (ESP) and binding capabilities of the five-membered N-heterocyclic ring units serve as effective predictors for the properties of intricate tridentate and tetradentate ligands, as well as their coordination bonding affinity with metals. The ligands' binding energy is closely associated with their ESP, and notably, the binding energy of tridentate and tetradentate ligands correlates well with the binding energies of their constituent structural units. The computational results reveal that the R2 unit, along with its corresponding tridentate ligand R2Ph and tetradentate ligands R2PhRr, exhibits the highest ESP, superior binding energies, and the strongest coordination bonding affinity with the metals. The theoretical calculations further identify several promising extractants for the effective separation of Am(III) and Eu(III). The tridentate ligands R1Ph, R7Ph, and R4Ph, and the tetradentate ligands R4PhR4, R6PhR6, R2PhR2, R1PhR5 and R3PhR6 were identified as having excellent separation performance for Am(III) and Eu(III). This study would provide insights for the design of extractants for the separation of Am(III) and Eu(III) by use of five-membered N-heterocyclic rings as structural units.

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

Developing chelators that combine high affinity and selectivity for lanthanides and/or actinides is paramount for numerous industries, including rare earths mining, nuclear waste management, and cancer medicine. In particular, achieving selectivity between actinides and lanthanides is notoriously difficult. The protein lanmodulin (LanM) is one of Nature's most selective chelators for trivalent actinides and lanthanides. However, mechanistic understanding of LanM's affinity and selectivity for f-elements remains limited. In order to decipher, and possibly improve, the features of LanM's metal-binding sites that contribute to this actinide/lanthanide selectivity, we characterized five LanM variants, substituting the aspartate residue at the 9^th position of each metal-binding site with asparagine, histidine, alanine, methionine, and selenomethionine. Spectroscopic measurements with lanthanides (Nd³⁺ and Eu³⁺) and actinides (²⁴³Am³⁺ and ²⁴⁸Cm³⁺) reveal that, contrary to the behavior of small chelator complexes, metal-coordinated water molecules enhance LanM's affinity for f-elements and pH-stability of its complexes. Furthermore, the results show that the native aspartate does not coordinate the metal directly but rather hydrogen bonds to coordinated solvent. By tuning this first-sphere/second-sphere interaction, the asparagine variant nearly doubles LanM's selectivity for actinides versus lanthanides. This study not only clarifies the essential role of coordinated solvent for LanM's physiological function and separation applications, but it also demonstrates that LanM's preference for actinides over lanthanides can be further improved. More broadly, it demonstrates how biomolecular scaffolds possess an expanded repertoire of tunable interactions compared to most small-molecule ligands – providing an avenue for high-performance LanM-based actinide/lanthanide separation methods and bio-engineered chelators optimized for specific medical isotopes.

Nature’s most potent protein for f-elements, lanmodulin, relies on subtle first-sphere/second-sphere interactions to bind metal ions. Dissecting lanmodulin’s binding mechanism yielded variants with enhanced actinide/lanthanide selectivity.

Stability of Different BTBP and BTPhen Extracting or Masking Compounds against γ Radiation

The radiolytic stability of hydrophobic extracting compounds CyMe₄-BTBP and CyMe₄-BTPhen and a hydrophilic masking agent (PhSO₃H)₂-BTPhen, widely employed for trivalent minor actinoid and lanthanoid separation, against γ radiation was tested. Even though the solvent with a promising fluorinated diluent BK-1 provides better extraction properties compared to octan-1-ol, its radiation stability is much lower, and no extraction was observed already after an absorbed dose of 150 kGy (CyMe₄-BTBP) or 200 kGy (CyMe₄-BTPhen). For the (PhSO₃H)₂-BTPhen hydrophilic masking agent, the results showed that the rate of radiolytic degradation was significantly higher in 0.25 M HNO₃ than in 0.5 M HNO₃. For both the hydrophobic and hydrophilic agents, degradation was slower in the presence of both organic and aqueous phases during irradiation.

Effect of Oxygen-Donor Charge on Adjacent Nitrogen-Donor Interactions in Eu3+ Complexes of Mixed N,O-Donor Ligands Demonstrated on a 10-Fold [Eu(TPAMEN)]3+ Chelate Complex

To reduce high-level radiotoxic waste generated by nuclear power plants, highly selective separation agents for minor actinides are mandatory. The mixed N,O-donor ligand N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (H₄TPAEN; 1) has shown good performance as a masking agent in Am³⁺/Eu³⁺ separation studies. Adjustments on the pyridyl backbone to raise the hydrophilicity led to a decrease in selectivity and a decrease in M³⁺-N^am interactions. An enhanced basicity of the pyridyl N-donors was given as a cause. In this work, we examine whether a decrease in O-donor basicity can promote the M³⁺-N^am interactions. Therefore, we replace the deprotonated "charged" carboxylic acid groups of TPAEN^4- by neutral amide groups and introduce N,N,N',N'-tetrakis[(6-N″,N''-diethylcarbamoylpyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2) as a new ligand. TPAMEN was crystallized with Eu(OTf)₃ and Eu(NO₃)₃·6H₂O to form positively charged 1:1 [Eu(TPAMEN)]³⁺ complexes in the solid state. Alterations in the M-O/N bond distances are compared to [Eu(TPAEN)]^- and investigated by DFT calculations to expose the differences in charge/energy density distributions at europium(III) and the donor functionalities of the TPAEN^4- and TPAMEN. On the basis of estimations of the bond orders, atomic charges spin populations, and density of states in the Eu and potential Am and Cm complexes, the specific contributions of the donor-metal interaction are analyzed. The prediction of complex formation energy differences for the [M(TPAEN)]^- and [M(TPAMEN)]³⁺ (M³⁺ = Eu³⁺, Am³⁺) complexes provide an outlook on the potential performance of TPAMEN in Am³⁺/Eu³⁺ separation.

Magnetic Nanohydrometallurgy Applied to Lanthanide Separation

Lanthanides play an important role in modern technology because of their outstanding optical, electronic, and magnetic properties. Their current hydrometallurgical processing involves lixiviation, leading to concentrates of elements whose separation requires exhaustive procedures because of their similar chemical properties. In this sense, a new nanotechnological approach is here discussed, involving the use of iron oxide nanoparticles functionalized with complexing agents, such as diethylenetriaminepentaacetic acid (DTPA), for carrying out the magnetic extraction and separation of the lanthanide ions in aqueous solution. This strategy, also known as magnetic nanohydrometallurgy (MNHM), was first introduced in 2011 for dealing with transition metal recovery in the laboratory, and has been recently extended to the lanthanide series. This technology is based on lanthanide complexation and depends on the chemical equilibrium involved. It has been better described in terms of Langmuir isotherms, considering a uniform distribution of the metal ions over the nanoparticles surface, as evidenced by high angle annular dark field microscopy. The observed affinity parameters correlate with the lanthanide ion contraction series, and the process dynamics have been studied by monitoring the nanoparticles migration under an applied magnetic field (magnetophoresis). The elements can be reversibly captured and released from the magnetically confined nanoparticles, allowing their separation by a simple acid-base treatment. It can operate in a circular scheme, facilitated by the easy magnetic recovery of the extracting agents, without using organic solvents and ionic exchange columns. MNHM has been successfully tested for the separation of the lanthanide elements from monazite mineral, and seems a promising green nanotechnology, particularly suitable for urban mining.

Quantum chemical studies of selective back-extraction of Am(III) from Eu(III) and Cm(III) with two hydrophilic 1,10-phenanthroline-2,9-bis-triazolyl ligands

We theoretically investigated the selective back-extraction towards Am(III) over Eu(III) and Cm(III) with two water-soluble 2,9-bis-triazolyl-1,10-phenanthroline derivatives BTrzPhen1 (with two ethanol side chains) and BTrz-Phen2 (with two 1,2-butanediol side chains) by density functional theory (DFT). The molecular geometries and formation reactions of the metal-ligand complexes were modeled by using M(BTrzPhen)(NO3)3 and [M(BTrzPhen)2(NO3)]2+. Am(III) selectivity over Eu(III) and Cm(III) with BTrzPhen2 was successfully reproduced by back-extraction reaction free energy analysis. Moreover, bonding properties between the metal cations and coordinated ligands (model complexes) were studied in terms of Mayer bond order and quantum theory of atoms in molecule (QTAIM). The difference in covalency between An–N and Eu–N bonds were found to be the key factors for Am(III)/Eu(III) separation, while the Am(III) selectivity over Cm(III) of BTrzPhen2 might be attributed to the competition of donor atoms for cation binding preference toward Am(III) and Cm(III).

Influence of a Pre-organized N-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by an Aminopolycarboxylate Complexant

The thermodynamic influence of a pre-organized N-donor group on the coordination of trivalent actinides and lanthanides by an aqueous aminopolycarboxylate complexant has been investigated. The synthesized reagent, N-2-methylpicolinate-ethylenediamine-N,N',N'-triacetic acid (EDTA-Mpic), resembles ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA) with a single acetate pendant arm replaced by a 6-carboxypyridin-2-ylmethyl group. The rigid N-donor picolinate functionality has a profound impact on ligand protonation and trivalent f element complexation equilibria, as demonstrated by potentiometric, spectroscopic, and liquid/liquid metal-partitioning studies as well as by molecular dynamics calculations. Relative to diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA), the ability to preferentially bind trivalent actinides over trivalent lanthanides was moderately lowered due to the presence of the N-(6-carboxypyridin-2-ylmethyl) substituent. The structural modification substantially amplifies the total ligand acidity of EDTA-Mpic. As a result the complexant sustains the metal complexation and efficient An³⁺ /Ln³⁺ differentiation in aqueous mixtures of unprecedented acidity for this class of reagents.

An overview of eight- and nine-coordinate N-donor solvated lanthanoid(III) and actinoid(III) ions

The use of replacement lanthanoid ions in actinoid chemistry is commonplace, which requires a full understanding of the similarities and differences between the two series. This overview lists, compares and discusses the available crystallographic data for N-donors for the lanthanoids and the actinoids using their trivalent state as a natural starting point for comparison.

Understanding Am3+/Cm3+ separation with H4TPAEN and its hydrophilic derivatives: a quantum chemical study

Quantum chemical calculations have been used to help understand the back-extraction and separation of Am³⁺/Cm³⁺ with H₄TPAEN and its two hydrophilic derivatives.