Calcium ionophore I as very effective extraction agent for trivalent europium and americium

The solvent extraction of microamounts of Eu and Am from water into nitrobenzene by means of a mixture of hydrogen dicarbollylcobaltate (HB) and calcium ionophore I (marked as the ligand L) was investigated. The equilibrium data have been explained assuming that the species HL, HL2,ML2 and ML3 (M: Eu or Am) are extracted into the organic phase. The extraction and stability constants of the cationic complex species in nitrobenzene saturated with water have been determined and discussed. It was proven experimentally that the ligand L could be considered as a very strong receptor for Eu and Am.


INTRODUCTION
Organic complexing agents with the prevailing hydrophobic character form the inclusion complexes with inorganic metal cations.That's why they found use especially as components for the ion-selective electrodes involving liquid membranes.][3] The dicarbollylcobaltates 4 (Fig. 1) and some of its halogen derivatives have been employed often for the solvent extraction of various metal cations (e.g., Cs + , Sr 2+ , Ba 2+ , Eu 3+ and Am 3+ ) from aqueous solutions into a polar organic phase, both under laboratory conditions for theoretical or experimental purposes, [5][6][7][8][9] and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste. 10,11Generally, only the Cs + can be extracted into the organic phase as a simple cation.For the extraction of multivalent cations the mixture of dicarbollylcobaltate extractant with the above mentioned neutral organic complexing ligands must be used.In this context, we must state that the very bulky lipophilic dicarbollylcobaltate is present almost only in the equilibrium organic phase of the investigated two-phase extraction system 4 , so that this univalent hydrophobic anion significantly facilitates the extraction of the mentioned cations from the aqueous phase into the organic one.Furthermore, a process involving chlorinated cobalt dicarbollide, polyethylene glycol and diphenyl-N,N-dibutylcarbamoylmethyl phosphine oxide, also called UNEX, has been suggested for the simultaneous recovery of cesium, strontium, lanthanides, and actinides from highly acidic media into phenyltrifluoromethyl sulfone (FS-13). 10,11It is necessary to emphasize that the FS-13 diluent was developed for the UNEX process as an alternative organic diluent to the highly polar nitrobenzene, while in Russia trifluoronitrotoluene (abbrev.F-3) is used for the same purposes.Finally, FS-13 has the advantage of low viscosity and good solubility of metal solvates as well as the UNEX extractants. 11icarboxylic acid diamides are a subject of active research as potential extractants of actinides (in particular of minor actinides) from radioactive wastes.The important information concerning the substituted malonic diamides has been reported. 12,135][16] The ability of TODGA to extract many other metals has been discussed [17][18][19] and the very high extraction capacity of this agent was shown to allow its application as a solid extractant. 191][22] Besides, some of these functionalized calixarenes have been applied for the isolation of carrier-free 90 Y from 90 Sr. 23 Recently, the diamides of 1,10-phenanthroline-2,9-dicarboxylic acid have been intended as selective extractants for trivalent americium and curium.The mixture of N,N,N'N'-tetraoctyl-1,10-phenanthroline-2,9-dicarboxamide and brominated dicarbollylcobaltate effectively extracts americium with a separation factor (SF Am/Eu ) over forty. 24High SF Am/Eu values (up to 51) have been also demon-CALCIUM IONOPHORE I -EXTRACTION AGENT FOR EUROPIUM AND AMERICIUM 201 strated in metal extraction by 1,10-phenanthroline-2,9-dicarboxamides from perchloric media. 25][28] In addition, as follows from Fig. 2, the calcium ionophore I ((R,R)-N,N′-bis-- [11-(ethoxycarbonyl)undecyl]-N,N′,4,5-tetramethyl-3,6-dioxaoctane-diamide) contains two ethereal oxygens, two substituted amide moieties and two ethyl ester groups, so that this compound could be an efficient agent especially for the solvent extraction of lanthanides and actinides.It must be pointed out that this reagent is commercially available.Therefore, in the current work, the solvent extraction of microamounts of trivalent europium and americium from aqueous nitric acid solution into nitrobenzene by means of hydrogen dicarbollylcobaltate (H + B -) 4 and the calcium ionophore I was investigated.We intended to find the composition of the species in the organic phase of the water-nitrobenzene extraction system and to determine the corresponding equilibrium constants.It means that the main aim of our study is discovering the respective extraction mechanisms and the quantitative characterization of the investigated two-phase extraction system.

Chemicals
Calcium ionophore I (puriss., ≥ 99%; Fig. 1) was purchased from Fluka and it was employed as received.Cesium dicarbollylcobaltate, Cs + B -, was synthesized by the method published by Hawthorne et al. 29 Other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity.A nitrobenzene solution of hydrogen dicarbollylcobaltate (H + B -) 4 was prepared from Cs + B -by the procedure described elsewhere. 30The carrier-free radionuclides 152,154 Eu 3+ and 241 Am 3+ were obtained from Polatom, Poland; their radionuclidic purities were 99.9 %.

Extraction
The extraction experiments in the two-phase water-HNO 3 -152,154 Eu 3+ (ca.20 kBq)--nitrobenzene-L-H + B -and water-HNO 3 -241 Am 3+ (ca.20 kBq)-nitrobenzene-L-H + B -sys-tems were performed in 10 mL polypropylene test-tubes with polypropylene stoppers, using 2 mL of each phase.In these extraction systems, the respective initial aqueous phase additionally contained 1×10 -7 mol L -1 of Eu(NO 3 ) 3 .The test-tubes filled with the solutions were shaken for 1 h at 25±1 °C, using a laboratory shaker.However, under these conditions, the equilibria in the systems under study were established after approximately 15 min of shaking.Then the phases were separated by centrifugation.Finally, 1 mL samples were taken from each phase and their γ-activities were measured by means of a well-type NaI(Tl) scintillation detector, connected to a γ-analyzer Triathler (Hidex, Turku, Finland).
The equilibrium distribution ratios of europium and americium, D, were determined as the ratios of the corresponding measured radioactivities of 152,154 Eu 3+ and 241 Am 3+ in the nitrobenzene and aqueous samples (the uncertainties of these distribution ratios were always lower than 3 %).

RESULTS AND DISCUSSION
The dependences of the logarithm of the europium and americium distribution ratios (log D) on the logarithm of the total concentration of the electroneutral calcium ionophore I, denoted in the following figures (Figs. 3-7) as ligand L in the initial nitrobenzene phase, log c(L), are presented in Figs. 3 and 4. The initial concentrations of hydrogen dicarbollylcobaltate in the organic phase, c B = 0.0005 mol L -1 , as well as the initial concentrations of HNO 3 in the aqueous phase, c(HNO 3 ) of 0.02 and 0.05 mol L -1 , are always related to the volume of one phase.III.
Regarding the results of our previous papers, 4,7-9,31 the considered water--HNO 3 -M 3+ (microamounts; M 3+ : Eu 3+ or Am 3+ )-nitrobenzene-(L)- The subscripts "aq" and "org" denote the aqueous and organic phases, respectively.At this point we must add that Eq. ( 4) characterizes the investigated two-phase systems at [L org ] → 0. A subroutine UBBE, based on the relations given above, the mass balance of the ligand L and the electroneutrality conditions in both phases of the system under consideration, was formulated 32,33 and introduced into a more general least--squares minimizing program LETAGROP, 34 I and II).This value is the main criterion for the determination of the respective extraction mechanism.The other one is the visual fit of the experimental data with the calculated dependencies of log D on log c(L).TABLE I. Comparison of various models of europium extraction from aqueous solution of HNO 3 by nitrobenzene solution of H + B -in the presence of the ligand L; the values of the extraction constants are given for each complex.The reliability interval of the constants is given as 3σ(K), where σ(K) is the standard deviation of the constant K. 34 These values are given in the logarithmic scale using the approximate expression log K ± {log [K+1.5σ(K)]--log [K-1.5σ(K)]}.For σ(K) > 0.2K, the previous expression is not valid and then only the upper limit is given in the parentheses in the form of log K (log [K + 3σ(K)]) 34 Europium   inferred from literature 37 Moreover, Fig. 5 presents the contributions of the species H org + , HL org + and HL 2,org + to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figs 6 and 7 show the contributions of the cations M org 3+ , ML 2,org 3+ and ML 3,org 3+ to the total trivalent metal cation concentrations in the corresponding equilibrium organic phase.From Figs. 5-7 it follows that the species HL 2,org + , EuL 3,org 3+ and AmL 3,org 3+ are present in significant concentrations only at relatively high amounts of the ligand L in the systems under study.
The distribution curves were calculated using the constants given in Tables III and IV.].The distribution curves were calculated using the constants given in Table III.Synergistic factor defined as S = D(Me 3+ )/D 0 (Me 3+ ), where D(Me 3+ ) is the maximal distribution ratio of the extracted metals in the systems under study and D 0 (Me 3+ ) is the distribution ratio reached for the same concentrations of HNO 3 and dicarbollylcobaltate anion, but in the absence of calcium ionophore I, is very high, log S(Eu) = 6.0 and log S(Am) = 6.9.This means that the extraction system ________________________________________________________________________________________________________________________ (CC) 2019 SCS.
Available on line at www.shd.org.rs/JSCS/CALCIUM IONOPHORE I -EXTRACTION AGENT FOR EUROPIUM AND AMERICIUM 207 water-HNO 3 -calcium ionophore I-dicarbollylcobaltate-nitrobenzene can be used for the separation of Eu 3+ and Am 3+ from the PUREX process raffinate.Value of separation factor α(Am/Eu) ≈ 5 allows also mutual separation of Am/Eu by the same extraction agents.
Finally, Table V summarizes the stability constants of the complex species HL + , HL 2 + , ML 2,org 3+ and ML 3,org 3+ with the two electroneutral ligands (L = calcium ionophore I or magnesium ionophore II) in water-saturated nitrobenzene.From the data reviewed in this table it is apparent that in the considered nitrobenzene medium, the stabilities of the complexes HL org + , HL  3+ and Am 3+ of the calcium ionophore I ligand under study is also comparable with that of the magnesium ionophore II.On the other hand, no separation of Am from Eu has been found in the system with magnesium ionophore II (SF Am/Eu ≈ 1).

CONCLUSIONS
In the present work, the solvent extraction of trivalent europium and americium from nitric acid solutions into nitrobenzene was studied using hydrogen dicarbollylcobaltate (H + B -) and calcium ionophore I.It was evidenced that the cationic species HL + , HL 2 + , and ML 3+ , ML 2 3+ and ML 3 3+ (M 3+ : Eu 3+ or Am 3+ ) are extracted into the organic phase of the water-nitrobenzene system.The stability constants of the corresponding complexes EuL n,org 3+ and AmL n,org 3+ , where n = 2 or 3, were found to be comparable in nitrobenzene saturated with water.From the very high stabilities of the proven ML 2 3+ and ML 3 3+ cationic complexes in the organic phase of the studied water-nitrobenzene system it is obvious that this very efficient ligand L for Eu 3+ and Am 3+ could be considered as a potential extraction agent for nuclear waste treatment.
used for determination of the "best" values of the extraction constants K ex ML n,org 3+ (M 3+ : Eu 3+ or Am 3+ ).The present version of this program, written in Embarcadero Delphi, has graphic output allowing also the printing of the Figs.3-7.The minimum of the sum of squares of deviations in log D, i.e., the minimum of the expression: U = Σ(log D calc -log D exp ) 2 (11) was sought (Tables

TABLE II .
Comparison of various models of americium extraction from aqueous solution of HNO 3 by nitrobenzene solution of H + B -in the presence of the ligand L a The error-square sum, U = Σ(log D calc -log D exp ) 2 ________________________________________________________________________________________________________________________ (CC) 2019 SCS.Available on line at www.shd.org.rs/JSCS/