N , N , N ’ , N ’-Tetrabutyl-1 , 10-phenanthroline-2 , 9-dicarboxamide as Very Effective Extraction Agent for Trivalent Europium and Americium

Solvent extraction of microamounts of Eu and Am from water into nitrobenzene by means of a mixture of hydrogen dicarbollylcobaltate (HB) and N,N,N’,N’-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide (L) was studied. The equilibrium data were explained assuming that the species HL, H2L , HL2, ML2 , and ML3 3+ (M = Eu, Am; L = N,N,N’,N’-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide) are extracted into the nitrobenzene phase. Extraction and stability constants of the cationic complex species in nitrobenzene saturated with water were determined and discussed. From the experimental results it is evident that this effective N,N,N’,N’-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide receptor for the Eu and Am cations could be considered as a potential extraction agent for nuclear waste treatment.


Introduction
Removal of heavy metals from wastes and soils is a very urgent environmental and technological problem.Solvent extraction is one of the most popular methods for separation of hazardous metals and for radioactive waste processing.One of the most challenging tasks in high-level waste (HLW) processing is the separation of americium and curium from lanthanides.][3][4][5][6][7][8][9] However, in spite of very high separation factors for americium/europium separation, some ligands proposed so far have demonstrated some disadvantages (e. g., low chemical stability, slow kinetics, and limited solubility in diluents).
Dicarboxylic acid diamides are a subject of active research as potential extractants of actinides (in particular of minor actinides) from radioactive wastes.Important information, concerning substituted malonic diamides has been reported. 10,113][14] The ability of TODGA to extract many other metals has been discussed 15,16 and the very high extractive capacity of this agent was shown to allow its application as a solid extractant. 179][20] Besides, some of these functionalized calixarenes have been applied for the isolation of carrier-free 90 Y from 90 Sr. 21 The dicarbollylcobaltate anion 22 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 analytical purposes, [23][24][25][26][27] and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste. 28,29Furthermore, 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 (abbrev.FS-13). 28,29It 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.Finally, FS-13 has the advantage of low viscosity and good solubility of metal solvates as well as the UNEX extractants. 29However, in Russia, nitrobenzene derivatives (e.g., 3-nitro-α,α,α-trifluorotoluene, also denoted by F-3) have been successfully utilized as diluents for cobalt dicarbollide processes. 28ecently, diamides of 1,10-phenanthroline-2,9-dicarboxylic acid have been proposed as selective extractants for trivalent americium and curium.The mixture of N,N,N'N'-tetraoctyl-1,10-phenanthroline-2,9-dicarboxamide and Br-cosan effectively extracts americium with a separation factor (SF Am/Eu ) over forty. 30High SF Am/Eu values (up to 51) have been also demonstrated for metal extraction by 1,10-phenanthroline-2,9-dicarboxamides from perchloric media. 31n the current work, the solvent extraction of microamounts of trivalent europium and americium into nitrobenzene by using hydrogen dicarbollylcobaltate (H + B -) 22 and N,N,N',N'-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide (abbrev.L; see Scheme 1) was investigated.In this context we must add that the solvent extraction of these trivalent cations into nitrobenzene by means of the mentioned electroneutral N,N,N',N'-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide ligand (L) is nearly negligible; therefore, the mixture of H + B -and L was employed.Moreover, 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.
The equilibrium distribution ratios of europium and americium, D, were determined as the ratios of the corres- ponding 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 numerical value of the total (analytical) concentration of the N,N,N',N' -tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide) ligand in the initial nitrobenzene phase, log c(L), are presented in Figures 1 and 2, as well as in Tables 1 and 2, respectively.The initial concentrations of hydrogen dicarbollylcobaltate (H + B -) in the organic phase, c B = 0.0025 and 0.005 mol/L, as well as the initial concentration of HNO 3 in the aqueous phase, c(HNO 3 ) = 0.05 mol/L, are always related to the volume of one phase.The occurrence of the characteristic maxima on these dependences can be explained qualitatively in terms of the competition between the charged trivalent complexes ML 3+ n,org (M 3+ = Eu 3+ , Am 3+ ) and the protonized ligand L (i.e., HL + org , H 2 L 2+ org , and HL + 2,org ; in detail, see the text below) during the balancing of the dicarbollylcobaltate electrostatic charge in the organic phase, analogously as in our previous work. 34egarding the results of our previous papers, 22,[25][26][27]35 the considered water-HNO 3 -M 3+ (microamounts; M 3+ = Eu 3+ , Am 3+ )-nitrobenzene-N,N,N',N'-tetrabutyl-1,10-   6. Thsubscripts "aq" and "org" denote the aqueous and organic phases, respectively.At this point we must add that Eq. ( 5) characterizes the investigated two-phase systems for [L org ] → 0.
In conclusion, we must state that the separation factors SF Am/Eu , reached in the studied two-phase water-nitrobenzene extraction system, defined by means of the corresponding equilibrium distribution ratios, D(Am 3+ ) / D(Eu 3+ ), are in the range from 12 to 38.This fact follows from the results presented in Figures 1 and 2 or in Tables 1 and 2, respectively.

Table 7. Stability constants of the complexes ML 3+
2 and ML 3+ 3 (M 3+ = Eu 3+ , Am 3+ ; L = N,N,N',N'-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide, bis(diphenylphosphino)methane dioxide (DPPMDO)see Scheme 2) in nitrobenzene saturated with water at 25 °C.trobenzene system.It was found that in nitrobenzene saturated with water, the stability constants of the complexes AmL 3+ 2,org and AmL 3+ 3,org are somewhat higher than those of the corresponding cationic species EuL 3+ 2,org and EuL 3+ 3,org .Finally, it was evidenced experimentally that the N,N,N',N'-tetrabutyl-1,10-phenanthroline-2,9-dicarboxamide ligand can be considered in the nitrobenzene medium as the very strong receptor for the Eu 3+ and Am 3+ cations.On the basis of the previous facts it is obvious that this investigated electroneutral ligand L could be also considered as a potential extraction agent for nuclear waste treatment.

30 . 74 a
Determined by the method of the concentration dependent distribution.39b Determined by the method described in Ref.34.c  Inferred from Ref.40.

Table 3 .
Comparison of various models of europium extraction from aqueous solution of HNO 3 by nitrobenzene solution of H + B - in the presence of N,N,N',N'-tetrabutyl-1,10-phenanthroline-2,9dicarboxamide (L). a The values of the extraction constants are given for each complex.

phase logK ex a U b
a See Table3, footnote a. b See Table 3, footnote b.Knowing the values K ex (Eu 3+ org ) = 1.3 and log K ex (Am 3+ org ), which were inferred from Ref. 40, as well as the extraction constants log K ex (EuL 3+ 2,org ) = 23.76,log K ex (EuL 3+