Liquid-liquid extraction is the major technique being applied for the partitioning of f-elements from nuclear waste. In this review, the recent developments in ligand design, optimization and extraction properties are summarised for the main classes of extractants (organophosphorus ligands, diamides and N-heterocycles), with a focus on the separation of actinides and lanthanides. Structural modifications, pre-organisation and different solvent systems, as key factors for the fine-tuning of the extraction properties, are discussed. From this review, it appears that small modifications of the structure of the ligand, the pre-organising platform or the solvent can have significant impact on the extraction (and separation) of metal ions. Interest in the combinations of ligands for the extraction processes is growing, since they provide improvements over individual ligands. Similarly, unconventional approaches are being pursued to develop more efficient and greener processes.

Ligands for f-element extraction used in the nuclear fuel cycle

Data on the dependence of the extraction ability of the principal classes of organic compounds on their structure are surveyed. Mono- and bi-dentate neutral organophosphorus compounds (NOPC), sulfoxides, amines, quaternary ammonium, arsonium, and phosphonium bases, amine and arsine oxides, organophosphorus acids and their salts, and crown-ethers are examined. It is shown that the extraction ability of the NOPC in relation to any metal increases with increase in the basicity of the extractant and diminishes when electronegative substituents are introduced into its molecule. With increase in the basicity of neutral extractants, qualitative changes in the complex formation mechanisms are observed. The influence of diluents on the extraction ability of organic compounds is analysed. Methods for predicting extraction ability are considered. Examples of the use of the above rules for the development of new extractants are presented. The bibliography includes 157 references.

Dependence of the extraction ability of organic compounds on their structure

The extraction of microquantities of Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y from HNO3 solutions by selected diphosphine dioxides of various structures in dichlorethane has been studied. It has been found that the extraction of these elements decreases with an increase in the number of methylene units (n) in the molecule of dioxide Ph2P(O)(CH2)3P(O)Ph2. Extractability of rare-earth elements decreases from La to Lu when methylene dioxide is used. With ethylene dioxides, it changes in the reverse order. The effect of HNO3 concentration in the aqueous phase and that of the extractants in the organic phase on the extraction of Sc, rare earth elements, and Y are considered. The stoichiometry of the extracted complexes has been determined and the extraction constants calculated.

Extraction of scandium, rare-earth elements, and yttrium from nitric acid solutions by selected diphosphine dioxides

The extraction of microquantities of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y from HNO3 solutions by phosphoryl-containing podands of various structures has been studied. It has been found that the maximum extraction of rare earth elements is exhibited by reagents containing one ether oxygen atom in the molecule, bound to diphenylphosphoryl or ditolylphosphoryl containing podands towards REE varies in a wide range, depending on the number of oxygen atoms in the polyether chain and the structure of the fragment connecting the ether oxygen atom with the terminal PO group. The podand structure which ensures a polydentate coordination of the reagent molecule to yield chelate complexes is favourable for REE extraction. The replacement of phenyl radicals in the podand VII molecule for butyl ones (compound IX) drastically reduces REE extraction. REE extraction by podands VII and VII increase from La to Lu with a decrease in the radii of extracted ions

Extraction of rare-earth elements from nitric solutions by phosphoryl-containing podands

The trifunctional ligand 2,6-[(C6H5)2P(O)CH2]2C5H3NO forms stable complexes with trivalent and tetravalent f-element cations and it acts as an effective liquid–liquid extraction reagent when it is dissolved in CHCl3. In a search for more hydrocarbon solvent soluble extraction reagents, derivative ligands having alkyl substituents in the 4-pyridyl ring position, 4-R′-2,6-[(C6H5)2P(O)CH2]2C5H2NO (R′
= Et, Oct), have been prepared and characterized. A single crystal X-ray diffraction analysis for the Et derivative shows that the ligand molecular structure is closely related to the parent ligand, 2,6-[(C6H5)2P(O)CH2]2C5H3NO. The coordination chemistry of the ligands with Nd(NO3)3 has been examined, and the molecular structures of two complexes obtained with 1 ∶ 1 and 2 ∶ 1 ligand ∶ metal reactant ratios are reported.

Synthesis and coordination chemistry of arene soluble 4-alkyl-2,6-bis[(diphenylphosphino)methyl]pyridine N,P,P′-trioxide ligands

Extraction of Am ( III), Pu( IV), U( VI) from nitric acid solutions by dialkyl ( aryl) [ dialkylcarbamoylmethyl] phosphine oxides of different structures has been studied. The nature of substituents at phosphorus and nitrogen atoms and in methylene bridge affects extraction capacity, solubility and selectivity of reagents. The increase in extraction capacity of reagents with respect to Am, U, Pu and decrease in solubility and selectivity at consecutive replacement of alkyl-( or alkoxy-) radicals at phosphorus atom by phenyl ones was found. The advantage of reagents usage with two different radicals one of which is phenyl at phosphorus atom has been shown. The nature of substituent at nitrogen atom affect solubility but does not practically affect extraction capacity of reagent.

[dialkylcarbamoylmethyl] phosphine oxides on their extraction capacity and selectivity

Dialkyl(aryl)carbamoylmethylphosphine oxides (CMPO), prepared by interaction of tervalent phosphorus acid esters with chloracetic amides R1R2P(O)CHR4C(O)NR3 2, R1, R2, R3, R4 = Alkyl or Aryl, can be used successfully for concentrating the transpluto-nium elements.

G. A. Pribylova

Papers

[dialkylcarbamoylmethyl] phosphine oxides on their extraction capacity and selectivity

Dialkyl(Aryl) [Dialkylcarbamoylmethyl]Phosphine Oxides as Extractants for Concentrating of Transplutonium Elements from Acid Media

Dialkyl(aryl)(dialkylcarbamoylmethyl)phosphine Oxides as Extractants for Concentrating of Transplutonium Elements from Acid Media.