Short communication
Synergistic Extraction of Europium and Americium into Nitrobenzene by Using Hydrogen Dicarbollylcobaltate
and Dodecaethylene Glycol
Emanuel Makrlik,1* Petr Vanura2 and Pavel Selucky3
1	Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamycka 129,
165	21 Prague 6, Czech Republic
2	Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technicka 5,
166	28 Prague 6, Czech Republic
3 Nuclear Research Institute, 250 68 Rez, Czech Republic * Corresponding author: E-mail: makrlik@centrum.cz Received: 17-10-2012
Abstract
Extraction of microamounts of europium and americium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of dodecaethylene glycol (DDEG, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, H2L2+, ML3+ and MH-1L2+ (M3+ = Eu3+, Am3+; L = DDEG) are extracted into the organic phase. The values of extraction and stability constants of the complex species in nitrobenzene saturated with water have been determined. It was found that in this nitrobenzene medium, the stability constant of the EuL3+ complex is comparable with that of AmL3+.
Keywords: Europium, americium, hydrogen dicarbollylcobaltate, dodecaethylene glycol, complexation, water-nitrobenzene system, extraction and stability constants
1. Introduction
The dicarbollylcobaltate anion1 and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+, Sr2+, Ba2+, Eu3+ and Am3+) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes,2-20 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.21-23
Solvent extraction of microamounts of Sr2+ and Ba2+ by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B) 1 in the presence of polyethylene glycols PEG 200, PEG 300 and PEG 400 has been studied. It has been found that the extraction of the protonated polyethylene glycol molecule HL+ and the extraction of the complex ML2+ (M2+ = Sr2+, Ba2+; L = PEG 200, PEG 300, PEG 400) are predominant reactions in this water-nitrobenzene system. The respective equilibrium constants have been
determined. The extraction and stability constants of the HL+ and ML2+ complex cations in the organic phase increase in the cation order H+ < Sr2+ < Ba2+, whereas the hydration numbers decrease in the same sequence.24
Recently, the extractive properties of a synergistic mixture of hydrogen dicarbollylcobaltate (H+B-) 1 and dodecaethylene glycol (DDEG, L) toward Cs+, Ca2+ and Sr2+ have been investigated in the water-nitrobenzene system.25,26 On the other hand, in the current work, the solvent extraction of microamounts of Eu3+ and Am3+ into nitrobenzene by means of this synergistic mixture was studied. We intended to find the composition of the species in the organic phase and to determine the corresponding equilibrium constants.
2. Experimental
Dodecaethylene glycol, HO(CH2CH2O)12H (abbrev. DDEG or L, respectively), was purchased from Fluka. Ce-
sium dicarbollylcobaltate (Cs+B-) was synthesized by means of the method published by Hawthorne et al.27 The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)1 was prepared from Cs+B- by the procedure described elsewhere.28 The radionuclides 152,154Eu3+ and 241Am3+ were supplied by Polatom, Poland; their radionuclidic purities were 99.9%.
The extraction experiments in the two-phase wa-ter-HCl-DDEG-M3+(microamounts; M3+ = Eu3+, Am3+) -nitrobenzene- H+B- systems were performed in 10 mL glass test-tubes with polyethylene stoppers, using 2 mL of each phase. The test-tubes filled with the solutions were shaken for 2 h at 25 ± 1 oC, using a laboratory shaker. Under these conditions, the equilibria in the systems under study were established after approximately 20 min of shaking. Then the phases were separated by centrifugation. Afterwards, 1 mL samples were taken from each phase and their /-activities were measured by means of a welltype NaI(TI) scintillation detector connected to a /-analy-zer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of europium and americium, D, were determined as the ratios of the corresponding measured radioactivities of 152,154Eu3+ and 241Am3+ in the nitrobenzene and aqueous samples.
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 ligand DDEG in the initial aqueous phase, log c(L), are given in Figures 1 and 2, respectively. The initial concentration of hydrogen dicar-bollylcobaltate in the organic phase, cB = 0.01 mol/L, as well as the initial concentration of HCl in the aqueous
Figure 1. Log D as a function of log c(L), where L is DDEG, for the water- HCl- DDEG-Eu3+(microamounts)-nitrobenzene-H+B-system; c(HCl) = 0.05 mol/L, cB = 0.01 mol/L. The curve was calculated using the constants given in Table 3.
Figure 2. Log D as a function of log c(L), where L is DDEG, for the water- HCl- DDEG-Am3+(microamounts)-nitrobenzene-H+B-system; c(HCl) = 0.05 mol/L, cB = 0.01 mol/L. The curve was calculated using the constants given in Table 4.
phase, c(HCl) = 0.05 mol/L, are always related to the volume of one phase.
With respect to previous results,1,4-6,13-20,29-33 the considered water-HCl- DDEG (L) - M3+(microamounts; M3+ = Eu3+, Am3+)-nitrobenzene-H+B- systems can be
described by the set of reactions:
Laq O Lore
2h:e + laq oh2y};m

(1) (2)
(3)
(4)
K + K +<r+	MHrL,:;31+ + 3H; (5)
to which the following equilibrium constants correspond:
[L0J

[LJ
K,,(HL+org) =

[hl;j Kjiuj
[hx=;j
[h:J-[L3 i
K fM3M POTO
[Mi;][H:rgr
K IMH l'r+3l+^ [MHrL':;3'+][H;]3
(6)
(7)
(8) (9)
aq JL iiqJ
-<W8 J
The subscripts "aq" and "org" denote the aqueous and organic phases, respectively.
A subroutine UBBE, based on the relations given above, the mass balance of the DDEG ligand and the elec-troneutrality conditions in both phases of the system under study, was formulated 24,34 and introduced into a more general least-squares minimizing program LETAGROP 35 used for determination of the "best" values of the extraction constants Kex(MHrLorrg3)+) (M3+ = Eu3+, Am3+; L = DDEG). The minimum of the sum of errors in log D, i.e., the minimum of the expression
U=I(log Dcalc - log D V-
(11)
was sought.
The values log KD = -3.0 (see Table 3, footnote a), log Kex(HL+rg) = 4.12 (see Table 3, footnote b), log Kex(H2L2o+g) = 7.67 (see Table 3, footnote b), log Kex(Eu3o+g)
1.33
and log Kex(Am3o+g)
1.5 were used for the res-
pective calculations. The results are given in Tables 1 and 2. From these tables it is evident that the extraction data can be best explained assuming the species ML3+ and
MH-1L2+ (M = Eu, Am ; L = DDEG) to be extracted into the nitrobenzene phase.
Knowing the values log KD = -3.0 (see Table 3, footnote a), log Kex(HL+rg) = 4.12 (see Table 3, footnote b), log Kex(H2L2o+regx) = 7or.g67 (see Table 3, footnote b), log Kex(Eu3o+g) = 1.338 and log Kex(Am3o+g) = 1.5,38 as well as
the extraction constants log Kex(Eu;+g) = 10.23 and log Kex(AmL3+g) = 10,59 determined here (see Tables 1 and 2), the stability constants of the complexes HL+org, H2L2o+rg and ML3o+g (M3+ = Eu3+, Am3+; L = DDEG) in the nitrobenzene phase defined as
P(HL;J =
[HL-^J [H:J[L(,J

[HIJ2[L 1
[Mt]

(12)
(13)
(14)
Table 1. Comparison of various models of europium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DDEG.
Europium complexes in the organic phase
log K
U '
EuL3+	11.17 (11.63)	11.41
EuHL4+	14.29 (15.01)	57.62
EUH-!L20^ EUL3+, EUHL4+	8.06 ± 0.22	0.82
	Transformed to EuL3+	
EUL3+, EUH-JL2O+s	10.23 ± 0.06, 7.78 ± 0.04	0.01
a The values of the extraction constants are given for each complex. The reliability interval of the constants is given as 3c(K), where c(K) is the standard deviation of the constant K.35 These values are given in the logarithmic scale using the approximate expression log K ± {log[K + 1.5c(K)] - log[K - 1.5c(K)]} . For c(K)>0,2K, the previous expression is not valid and then only the upper limit is given in the parentheses in the form of K(log [K + 3c(K)].35 b The error-square sum U = X(log Dcalc log K -
-l°g Dexp)2
Table 2. Comparison of various models of americium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DDEG.
Americium complexes in the organic phase
log K
» e?
U
AmL3+	11.46 (11.91)	1.04
AmHL4+	14.58 (15.29)	55.31
AmH^ AmL3+, AmHL4+	8.35 ± 0.26	1.13
	Transformed to AmL3+	
AmL3+, AmH-1L20++,	10.59 ± 0.08, 8.01 ± 0.06	0.01
a See Table 1, footnote a; b See Table 1, footnote b.
can be evaluated applying the simple relations:
log p (hl; J = log kra(hl;re) - log kd	(15)
log p (Ha L=;) = log Kex (H 2L;;) - log KD	(16) logp(ML^) = log KC!1(ML34)
Similarly, the protonation constant of the complex cation MH-1L2+ (M3+ = Eu3+, Am3+; L = DDEG) in nitrobenzene saturated with water, i. e., the equilibrium constant of the following general reaction

defined as
K(ML^) =
[ML3;
[mh,l;;][h:j
was detennined on the basis of Relation (20):
log K (ML3^) = log Kex(ML34)
-logK„(MH,L;;)
(18)
(19)
(20)
The respective equilibrium constants are summarized in Tables 3 and 4.
Moreover, Figure 3 depicts the contributions of the species H+rg, HL+rg and H2L2+ to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figures 4 and 5 show the contributions of the cations Eu3+, EuL3+, EuH ,L2+ and Am3+, AmL3+, AmH ,L2+,
org	org	-1 org	org	org	-1 org'
a
a
Table 3. Equilibrium constants in the water-HCl-DDEG-Eu3+(mi-croamounts)-nitrobenzene - H+B-system.
Table 4. Equilibrium constants in the water-HCl-DDEG-Am3+ (microamounts)-nitrobenzene -H+B- system.
Equilibrium
log K
Equilibrium
log K
Laq « Lorg
H org + Laq « HL+rg2
2H org + Laq « H2L2+g
Eu3a+ + 3H org « Eu3aq + 3H+q
Eu3a+ + Laq + 3H0rg « ^L+g + 3H0q
Eu3a+ + Laq + 2H+rg « EuH^ + 3H+q
H org + Lorg « HL:rg9
2H3+rg + Lorg « H2L3+g
Eu3o+g+2Lorg «EuL3+g 3 ^H^P HQrg « EuL3,+g_
-3.0 a 4.12 b 7.67 b
1.3 c
10.23 7.78 7.12 10.67 11.93 2.45
Laq « Lorg
H org + Laq « HL+rg2 2H+3g+ Laq« H2L2o+g3
Am3aq + 3H+rg « Am34 + 3H+q
Am3aq + Laq+ 3H+rg « AmL3+g + 3H++ Am3aq + Laq + 2H+rg « AmH-iL^^ + 3H+q H org + Lorg « HLora 2H++ + Lorg « H2L2o+3
Am3o+g+2Lorg « AmL3+g 3
AmH-iL2o^ + HQrg « AmL*_
-3.0 a 4.12 b 7.67 b 1.5 c
10.59 8.01 7.12 10.67 12.09 2.58
a Determined by the method of the concentration dependent distribution.36
b Determined by the method described in detail in Ref. 37. c Ref. 38.
a Determined by the method of the concentration dependent distri-bution.36
b Determined by the method described in detail in Ref. 37. c Ref. 38.
Figure 3. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-DDEG-Eu3+(microa-mounts)-nitrobenzene- H+B- extraction system in the forms of H+, HL+ and H2L2+;
c(HCl) = 0.05 mol/L, cB = 0.01 mol/L.
1	S(H+) = P+rgMff)
2	S(HL+) = [HL+r l/c(H+)0]
org
3 8(H2L2+) = 2[H2L2o+g]/c(H+)( where c(HXg = [HL+rg.
[HL+rg] + 2[H;L2o^]
The distribution curves were calculated using the constants given in Table 3.
Figure 4. Distribution diagram of europium in the equilibrium nitrobenzene phase of the water-HCl-DDEG-Eu3+(microa-mounts)-nitrobenzene-H+B- extraction system in the forms of Eu3+, EuL3+and EuH 1L2o^g; c(HCl) = 0.05 mol/L, cB = 0.01 mol/L.
1	8(Eu3+) = [Eu3o+g]/c(Eu3+)0rg,
2	8(EuL3+) = [EuL3o^g]/c(Eu3+)org,
3	5(EuH-iL3+g = ^H ^MI^g, 2+
where c^u3^ = [Eu3+g] + [EuL3+g] + [EuR^].
The distribution curves were calculated using the constants given in
Table 3.
respectively, to the total trivalent metal cation concentration in the corresponding equilibrium organic phase.
Finally, it should be noted that the stability constants of the complex species EuL3o+g and AmL^, where L is DDEG, in nitrobenzene saturated with water is log ß(EuL3o+g) = 11.93 and log ß(AmL3o+g) = 12.09, as given in Tables 3 and 4, respectively. Recently, the stability con-
stants of the complexes CsL+rg, CaL2+g and SrL2+g (L = DDEG) in water-saturated nitrobenzene were determined as log p(CsL+org) = 6.83,25 log p(CaL2o+g) = 11.8726 and log P(SrL2,+g) = 13.19.26 Thus, in this nitrobenzene medium, the stability of the considered cationic complex species CsL+rg, CaLo+g, SrLo+g, EuL3o+g and AmL3o+g increases in the series of Cs+ < Ca2+ «Eu3+ « Am3+ < Sr2+.
Figure 5. Distribution diagram of americium in the equilibrium nitrobenzene phase of the water-HCl-DDEG-Am3+(microa-mounts)-nitrobenzene-H+B- extraction system in the forms of Am3+, AmL3+ and AmH c(HCl) = 0.05 mol/L, cB = 0.01 mol/L.
1	8(Am3+) = [Am30+g]/c(Am3+)
2	8(AmL3+) = [AmLo+g]/c(Am3+)org
3	S^mHiL; = [AmH-3lL0^s]/c(An33+)org,
where c(Am3Xg = [Amo+g] + [AM*,] + [Am^^].
The distribution curves were calculated using the constants given
in Table 4.
4. Acknowledgements
This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No.: 42900/1312/3114 "Environmental Aspects of Sustainable Development of Society" and by the Czech Ministry of Education, Youth, and Sports, Project MSM 6046137307.
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Povzetek
Proučevali smo ekstrakcijo mikrokoličin evropija in americija z raztopino hidrogen dikarbolilkobaltata (H+B-) v ni-trobenzenu ob prisotnosti dodekaetilen glikola (DDEG, L). Ravnotežja smo pojasnili s pomočjo predpostavke, da se kompleksi HL+, H2L2+, ML3+ in MH_jL2+ (M3+ = Eu3+, Am3+; L = DDEG) ekstrahirajo v organsko fazo. Določili smo konstante ekstrakcije in konstante stabilnosti kompleksov v nitrobenzenu, nasičenem z vodo. Ugotovili smo, da so konstante stabilnosti kompleksov evropija, EuL3+, primerljive s konstantami stabilnosti kompleksov z americijem, AmL3+.