INTRODUCTION

Extraction methods are widely used for recovery, preconcentration, and separation of actinides and rare earth elements (REE) from solutions obtained on the processing of spent nuclear fuel [1]. High extraction ability toward these elements is shown by polydentate organophosphorus compounds [2–9], diamides of malonic [10], diglycolic [11] and N-heterocyclic carboxylic acids [12], organophosphorus acids [13], salts of quaternary ammonium bases (QAB) [14], as well as binary extractants: salts of amines and QAB with organic acid anions [15–21]. The latter can be classified as functionalized ionic liquids (FIL) whose anionic moiety participates in complexation with metal ion. In recent time, there is growing interest in the use of FIL in extraction practice for preconcentration and separation of actinides and REE(III) [22].

Extraction properties were studied for FIL synthesized by introducing thioether [23], monoaza crown ether [24], phosphoryl [25], calixarene–phosphine oxide [26], malonamide [27], diglycolamide [28], carbamoylphosphone oxide (CMPO) [29–31] and other coordinating groups into cationic (1-alkyl-3-methylimidazolium) moiety of ionic liquid molecule. The anionic moiety of these FIL were mainly hexafluorophosphate or bis[(trifluoromethyl)sulfonyl]imide anions.

The aim of this work is to study extraction of REE(III) ions from nitric acid solutions with solutions of binary extractants based on cations of dialkylamino derivatives of CMPO L1 and L2 and anions of dinonylnaphthalenesulfonic acid.

Ph2P(O)CH2C(O)NH(CH2)3N(C2H5)2

L1

Ph2P(O)CH2C(O)NH(CH2)2N(CH3)2

L2

Ph2P(O)CH2C(O)NHC9H19

L

EXPERIMENTAL

Compound L previously described by us was obtained by reaction of ethyl diphenylphosphorylacetate with n-nonylamine on heating in ethanol solution [32]. Compounds L1 and L2 were obtained in similar manner using 3-(diethylamino)propylamine-1 and 2-(dimethylamino)ethylamine-1, respectively. NMR spectra were recorded on a Bruker AV 300 spectrometer.

N-(3-(diethylamino)propyl)diphenylphosphorylacetamide (L1 ). Yield 60%, mp = 140–141°C (toluene). 1H NMR (300 MHz, CDCl3), δ, ppm, J, Hz: 7.85–7.75 m, 4H (m-CH in Ph); 7.75–7.45 m, 7H (o- and p-CH in Ph + NH); 3.34 d, 2JP–H = 12.0, 2H, P–CH2; 3.28 t, 3JH–H = 6.0, NHCH2; 2.48 q, 3JH–H = 6.0, 4H, N–CH2; 2.41 t, 3JH–H = 6.0, 2H, CH2–NEt2; 1.59 quin, 3JH–H = 6.0, 2H, CH2–CH2–CH2; 1.01 t, 3JH–H = 6.0, 6H, CH3. 31P NMR (121 MHz, CDCl3), δ, ppm: 29.43 s.

For C21H29N2O2P anal. calcd. (%): C, 67.72; H, 7.85; N, 7.52.

Found (%): C, 67.74; H, 7.77; N, 7.52.

N-(2-(dimethylamino)ethyl)diphenylphosphorylacetamide (L2 ). Yield 61%, mp = 151–152°C (acetonitrile). 1H NMR (300 MHz, CDCl3), δ, ppm, J, Hz: 7.90–7.75 m, 4H (m-CH in Ph); 7.75–7.55 m, 7H (o- and p-CH in Ph + NH); 3.38 d, 2JP–H = 12.0, 2H, P–CH2; 3.33 t, 3JH–H = 6.0, NHCH2; 2.38 q, 3JH–H = 6.0, 2H, N–CH2; 2.25 s, 6H, NMe2. 31P NMR (121 MHz, CDCl3), δ, ppm: 29.21 s. IR, ν, cm–1: 3269 (NH), 3081 (NH), 1663 (C=O), 1564 (C–N), 1185 (P=O).

For C18H23N2O2P anal. calcd., %: C, 65.44; H, 7.02; N, 8.48; P, 9.38.

Found, %: C, 65.35; H, 7.05; N, 8.44; P, 9.39.

Dinonylnaphthalenesulfonic acid (DNNSA, Sigma-Aldrich) was purified by procedure [33]. Solutions of binary extractants were prepared by dissolution of equimolar amounts of L1 or L2 and DNNSA in organic solvent followed by washing with water. Organic solvents used were 1,2-dichloroethane, o-xylene, and chloroform of reagent grade without additional purification. Initial aqueous solutions containing 2 × 10–6 mol/L of each REE(III) were obtained by dissolution of the corresponding nitrates in water followed by addition of HNO3 solution to required concentration. Chemicals used were of reagent grade.

Extraction experiments were conducted in test tubes with ground stoppers at 21 ± 2°С and volume ratio of organic and aqueous phase of 1 : 1. Phases were contacted on a rotary apparatus for mixing with rotation speed of 60 rpm for 1 h. It was found preliminary that this time is sufficient to reach constant values of distribution ratios for REE(III) (DLn). After extraction, the phases were separated by centrifugation.

Rare earth element concentration in initial and equilibrium aqueous solutions was determined by mass spectral method with inductively coupled plasma ionization of sample (ICP-MS) using a Thermo Scientific X-7 mass spectrometer (USA). The concentration of elements in organic phase was determined by material balance equation. The value of DLn was calculated as the ratio of element concentrations in equilibrium organic and aqueous phases. Determination error for DLn was not larger 5%. Concentration of HNO3 in equilibrium aqueous phases was determined by potentiometric titration with NaOH solution.

RESULTS AND DISCUSSION

Compounds L1 and L2 contain in molecules bidentate coordinating fragment Ph2P(O)CH2C(O)NH– and amino group attached to the amide nitrogen atom through alkylene bridge as distinct from CMPO L. To assess the possibility of amino group to participate in complexation with Ln3+ ions, we studied extraction of REE(III) from ammonium nitrate solutions with solutions of compounds L, L1, and L2 in dichloroethane. Compounds L1 and L2 were found to extract REE(III) ions more efficiently than CMPO L (Fig. 1). It may be due to participation of the nitrogen atom of the amino group in complexation with Ln3+ ions. The efficiency of REE(III) extraction with compounds L1 and L2 increases from La(III) to Lu(III) as the atomic number (Z) of element increases. The opposite character of log DLn–Z dependence is observed for REE(III) extraction with CMPO L solutions, which can be due to the different character of coordination of Ln3+ ions to CMPO L and ligands L1 and L2. The difference in the extraction ability of CMPO L and L1 or L2 increases with Z.

Fig. 1.
figure 1

Extraction of REE(III) and Y(III) from 4 M NH4NO3 solution with 0.05 M solutions of compounds L, L1, and L2 in dichloroethane.

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The extraction ability of compounds L1 and L2 sharply decreases in the system with nitric acid, which is associated with protonation of nitrogen atom of the amino groups and transition of salts L1HNO3 and L2HNO3 into aqueous phase. However, L1 and L2 react with DNNSA in organic phase to form binary extractants L1HA and L2HA according to equations:

$${\text{L}}_{{{\text{(org)}}}}^{1} + {\text{H}}{{{\text{A}}}_{{\left( {{\text{org}}} \right)}}} \leftrightarrows {\text{ }}{{{\text{L}}}^{{\text{1}}}}{\text{H}}{{{\text{A}}}_{{{\text{(org)}}}}},$$
(1)
$${\text{L}}_{{{\text{(org)}}}}^{2} + {\text{H}}{{{\text{A}}}_{{\left( {{\text{org}}} \right)}}} \leftrightarrows {\text{ }}{{{\text{L}}}^{{\text{2}}}}{\text{H}}{{{\text{A}}}_{{{\text{(org)}}}}},$$
(2)

where A– is DNNSA anion. The high hydrophobicity of DNNSA anion determines the small transition of components of these binary extractants into aqueous phase.

The binary extractants L1HA and L2HA efficiently extract REE(III) ions from nitric acid solutions of low and moderate concentration. Under comparable conditions, DLn values on extraction with L1HA solution are slightly higher than on extraction with L2HA (Fig. 2), which can be due to higher hydrophobicity of compound L1, the component of binary extractant. Under these conditions, DNNSA extracts REE(III) less efficiently than the binary extractants on its basis, while compounds L1 and L2 do not extract REE(III) (DLn < 10–2) because they completely come into aqueous phase. The efficiency of REE(III) extraction with binary extractants decreases from La(III) to Lu(III) as Z rises. The position of Y(III) in the series of REE(III) between Ho(III) and Er(III) corresponds to the proximity of their ionic radii [34]. Similar character of log DLn–Z dependence was observed on REE(III) extraction from nitric acid solutions with CMPO solutions [35, 36]. One can suppose that, in contrast to REE(III) extraction with compounds L1 and L2 from ammonium nitrate solutions, the extraction with binary extractants L1HA and L2HA from HNO3 solution proceeds via complexation with Ln3+ ions of only bidentate complexing fragment Ph2P(O)CH2C(O)NH– of the binary extractants. On formation of extractable REE(III) complexes, the cationic moiety of binary extractant provides coordination solvation of Ln3+ ions, while anionic moiety imparts them high hydrophobicity, which favors to efficient transfer of metal ions into organic phase. Binary extractants L1HA and L2HA exceeds considerably CMPO L in the extraction ability of REE(III) ions (Fig. 2).

Fig. 2.
figure 2

Extraction of REE(III) and Y(III) from 3 M HNO3 solutions with 0.05 M solutions of CMPO L, DNNSA, and binary extractants L1HA and L2HA in dichloroethane.

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The separation factor of La(III) and Lu(III) βLa/Lu = DLa/DLu on extraction with binary extractant solutions in dichloroethane (βLa/Lu = 32.3 and 28.2 for L1HA and L2HA, respectively) exceeds that for DNNSA–dichloroethane system (βLa/Lu = 0.64). This fact indicates the considerable enhancement for the selectivity of REE(III) extraction from nitric acid solutions with solutions of binary extractants.

At constant initial concentration of binary extractant in organic phase, increase in HNO3 concentration in equilibrium aqueous phase is accompanied by decrease of DLn (Fig. S1). In the studied range of HNO3 concentrations, the slope of log DLn–log[HNO3] dependence is close to –2 for all REE(III), which indicates the transition of two H+ ions into aqueous phase during extraction.

The stoichiometric ratio REE(III) : binary extractant in extracted complexes was determined by equilibrium shift method. At constant HNO3 concentration in aqueous phase, the slope of log DLn–log[L1HA] dependences is close to 2 (Fig. S2). Solutions of binary extractant L2HA extract REE(III) ions as complexes of the same stoichiometry (Fig. S3).

The process of REE(III) extraction from nitric acid solutions with binary extractants can be described by the equation:

$$\begin{gathered} {\text{Ln}}_{{{\text{(aq)}}}}^{{{\text{3}} + }} + {\text{NO}}_{{{\text{3(aq)}}}}^{-} + {\text{2LH}}{{{\text{A}}}_{{{\text{(org)}}}}}{\text{ }} \\ \leftrightarrows {\text{ Ln}}{{{\text{L}}}_{{\text{2}}}}{{{\text{A}}}_{{\text{2}}}}{\text{N}}{{{\text{O}}}_{{{\text{3}}\left( {{\text{org}}} \right)}}} + {\text{2H}}_{{{\text{(aq)}}}}^{ + }, \\ \end{gathered} $$
(3)

where symbols (aq) and (org) refer to the components of aqueous and organic phase, respectively

Organic solvent nature has considerable effect on REE(III) extraction with CMPO solutions [37, 38]. On extraction with 0.05 M L2HA solutions in organic solvents from 1 M HNO3 solution, DLn values increase in the series chloroform < o-xylene < 1,2-dichloroethane (Fig. 3). Let us note that the change in polarity and solvating ability of organic diluent in the system with binary extractant shows considerably lower effect on REE(III) extraction efficiency than in systems with neutral CMPO. In the system with L2HA, the transition from o-xylene to dichloroethane is accompanied by the four-fold growth of DEu, whereas DEu rises by 50 times on extraction with neutral CMPO [39]. This effect may be caused by higher hydrophobicity of LnL2A2NO3 complexes as compared with that for coordination-solvated REE(III) nitrates extracted with neutral CMPO. The suppression of extraction on the use of chloroform as solvent seems to result from the solvation of donor centers of extractant molecule due to hydrogen bonding between P=O and HCCl3 to decrease activity of binary extractant in organic phase.

Fig. 3.
figure 3

Extraction of REE(III) and Y(III) from 1 M HNO3 solutions with 0.05 M solutions of binary extractants L1HA in (1) dichloroethane, (2) o-xylene, and (3) chloroform.

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The presented data show that the binary extractants obtained from equimolar amounts of dialkylamino derivatives of carbamoylmethylphosphine oxides and dinonylnaphthalenesulfonic acid in organic solvents efficiently extract REE(III) ions from nitric acid solutions. On the formation of extractable REE(III) complexes, the cationic moiety of the binary extractant provides coordination solvation of Ln3+ ions, while anionic moiety imparts their high hydrophobicity, which favors efficient transition of metal ions into organic phase.