Short communication
Synergistic Extraction of Calcium and Strontium into Nitrobenzene by Using Hydrogen Dicarbollylcobaltate and N,N '-Dimethyl-N,N '-Diphenyl-2,6-Dipicolinamide
Emanuel Makrlik,1'* Petr Vanura,2 Pavel Selucky,3 Vasily A. Babain4
and Igor V. Smirnov4
1	Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamyckâ 129,
165	21 Prague 6, Czech Republic
2	Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technickâ 5,
166	28 Prague 6, Czech Republic
3 Nuclear Research Institute, 250 68 Rez, Czech Republic 4 Khlopin Radium Institute, Research and Production Association, St. Petersburg, Russia * Corresponding author: E-mail: makrlik@centrum.cz Received: 18-01-2012
Abstract
Extraction of microamounts of calcium and strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of N,N'-dimethyl-N,N'-diphenyl-2,6-dipicolinamide (MePhDPA, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, HL+, ML2+and ML^+(M2+ = Ca2+, Sr2 +) are extracted into the organic phase. The values of extraction and stability constants of the cationic complex species in nitrobenzene saturated with water have been determined.
Keywords: Calcium, strontium, hydrogen dicarbollylcobaltate, N,N'-dimethyl-N,N'-diphenyl-2,6-dipicolinamide, 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,219 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.20,21
Dicarboxylic acid diamides are a subject of active research as potential extractants of actinides (in particular of minor actinides) from radioactive wastes. Important informations concerning substituted malonic diamides have been reported.22,23 Lately, interest has shifted to the properties of tetraalkyl-diglycolamides,24-27 with emphasis
on tetraoctyl-diglycolamide (TODGA) suggested as an extractant of Pu(IV), Np(IV), Am(III) and Cm(III) in solutions with hydrocarbon diluents.24-26 The ability of TODGA to extract many other metals was discussed27,28 and the very high extractive capacity of this agent was shown to allow its application as a solid extractant.29
Recently, the extractive properties of some 2,6-di-picolinamides have been investigated.30-34 In the current work, the solvent extraction of microamounts of cal-
Scheme 1. Structural formula of N,N'-dimethyl-N,N'-diphenyl-2,6-dipicolinamide (abbrev. MePhDPA or L, respectively).
cium and strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)1 in the presence of
N,N'-dimethyl-MN'-diphenyl-2,6-dipicolinamide (Me-
PhDPA) (see Scheme 1) was studied. We intended to find the composition of the species in the nitrobenzene phase and to determine the corresponding equilibrium constants.
2. Experimental
N,N'-Dimethyl-N,N'-diphenyl-2,6-dipicolinamide (MePhDPA) was produced as described in References 35 and 36. Cesium dicarbollylcobaltate, Cs+B-, was synthesized by means of the method published by Hawthorne et al.37 A nitrobenzene solution of hydrogen dicarbollylco-baltate (H+B-) 1 was prepared from Cs+B- by the procedure described elsewhere.38 The other chemicals used (Lac-hema, Brno, Czech Republic) were of reagent grade purity. The radionuclides 45Ca2+ and 85Sr2+ (DuPont, Belgium) were of standard radiochemical purity.
The extraction experiments in the two-phase wa-ter-HCl-M2+(microamounts; M2+ = Ca2+, Sr2+)-nitroben-zene- MePhDPA-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 °C, 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 centrifuga-tion. In the case of the systems involving 45Ca2+, after evaporating aliquots (1 mL) of the respective phases on Al plates, their ¿¡-activities were measured by using the apparatus NRB-213 (Tesla Premysleni, Czech Republic). On the other hand, in the case of the systems with 85Sr2+, 1 mL samples were taken from each phase and their y-ac-tivities were measured by means of a well-type NaI(Tl) scintillation detector connected to a y-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of calcium and strontium, D, were determined as the ratios of the corresponding measured radioactivities of 45Ca2+ and 85Sr2+ in the nitrobenzene and aqueous samples.
0.5 -
log d
o -
-o.s -
-1.0
log c(L)
Figure 1. Log D as a function of log c(L), where L = MePhDPA, for the system water- HCl-Ca2+ (microamounts) - nitrobenzene -MePhDPA - H+B-; c(HCl) = 0.01 mol/L, cB = 0.003 mol/L. The curve was calculated using the constants given in Table 3.
J_I_L
-4	-3	-2
log c(L)
Figure 2. Log D as a function of log c(L), where L = MePhDPA, for the system water- HCl-Sr2+ (microamounts) - nitrobenzene -MePhDPA - H+B-; c(HCl) = 0.01 mol/L, cB = 0.003 mol/L. The curve was calculated using the constants given in Table 4.
With respect to the results of previous pa-pers,3,5,12,34,39,40 the considered water-HCl-M2+ (microa-mounts; M2+ = Ca2+, Sr2+)-nitrobenzene-MePhDPA(L)-H+B- systems can be described by the set of reactions:
3. Results and Discussion
The dependences of the logarithm of the calcium and strontium distribution ratios (log D) on the logarithm of the numerical value of the total (analytical) concentration of the MePhDPA ligand in the initial nitrobenzene phase, log c(L), are given in Figures 1 and 2, respectively. The initial concentration of hydrogen dicarbolylcobaltate in the organic phase, cB = 0.003 mol/L, as well as the initial concentration of HCl in the aqueous phase, c(HCl) = 0.01 mol/L, are always related to the volume of one phase.
Horg + Lorg <=> H L
H;+2LnoHL^
K+nL«*+» ml1+oc8+2H;
(i) (2)
(3)
(4)
(5)
to which the following equilibrium constants correspond:
Table 2. Comparison of three different models of strontium extrac-(6)	tion from aqueous solution of HCl by nitrobenzene solution of
H+B- in the presence of MePhDPA.
/7(HL;J =
(7)	Strontium complexes in the organic phase	log Kex a	U b
(8)	SrL2+ SrL3+ SrL2+, SrL2+	20.01 (20.36) 22.57 (22.86) 19.33 ± 0.15, 22.39 ± 0.10	1.87 0.96 0.03
(9)
(10)
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 MePhDPA ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated 41,42 and introduced into a more general least-squares minimizing program LETAGROP 43 used for determination of the "best" values of the extraction constants Kex(ML2+org) (M2+ = Ca2+, Sr2+; L = MePhDPA). The minimum of the sum of errors in log D, i.e., the minimum of the expression
U - X(log Dtal - log D )2
was sought.
The values log KD
ß(HL+ ) KJSrOg) = 0.7'
1.29,34 log ß(HL+rg) = 10.7,34 log KeX(Ca2+g) = 0.244
(11)
9.3,34 log and log
41 were used for the respective calculations. The results are listed in Tables 1 and 2. From these tables it is evident that the extraction data can be best ex-
plained assuming the complexes ML22+ and ML23+ (M2+ = Ca2+, Sr2+; L = MePhDPA) to be extracted into the nitrobenzene phase.
a See Table 1, footnote a; b See Table 1, footnote b.
and log
Knowing the values log Kex(Ca2+g) = 0.244 Kex(Sr2o+g) = 0.7,41 log Kex(CaL22+org) = 19.49 as well as the extraction constants log Kex(CrL2,"forg) = 19.33, log
Kex(CaL23+org) = 22.26 and log KJSrL^) = 22.39 determined here (see Tables 1 and 2), the stability constants of the complexes ML^+ and ML23+ (M2+ = Ca2+, Sr2+; L = Me-PhDPA) in the organic phase defined as
(12)
(13)
can be evaluated applying the following simple relations:
log p (MLVors) = log K,s (ML;;rg) - log (M*) (14)
log m^) = log K .s(ML^) - log (M;;) (15)
The respective equilibrium constants are summarized in Tables 3 and 4.
Table 3. Equilibrium constants in the water - HCl- Ca2+(microa-mounts) - nitrobenzene - MePhDPA - H+B- system.
Equilibrium
log K
Table 1. Comparison of three different models of calcium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of MePhDPA.
Calcium complexes in the organic phase	log Kex a	Ub
CaL22+ CaL^+ CaL^ , CaL3	19.99 (20.30) 22.54 (22.95) 19.49 ± 0.17, 22.26 ± 0.15	1.36 2.10 0.04
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.43 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) > 0.2 K, 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 + 3c(K)]).43 b The error-square sum U =
= ^g Dcalc" log Dexp)2.
Laq « Lorg	+
H+rg + Lorg « HL+org
H+r,+ 2Lo+g « HL+.+rg
CaÎ++ 2H+rg « C4+ 2H+q
Ca2+ + 2Lorg + 2H+rg « CaL2+org+ 2H+q
Ca2+ + 3Lorg + 2H+rg « CaL3+org+ 2H+q
Cao+g+ 2Lorg « CaL2+org
CC+ 3Lorg « CaL3+org_
1.29 a 9.3 a 10.7 a 0.2 b 19.49 22.26 19.29 22.06
' Ref. 34;b Ref. 44.
Moreover, Figure 3 depicts the contributions of the species H+rg, HL+rg and HL+org to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figures 4 and 5 show the contributions of the cations Ca20+g, CaL22+Drg, CaL^g and S^, SrL^g, SrL^g,
Table 4. Equilibrium constants in the water - HCl -Sr2+(microa-mounts) - nitrobenzene - MePhDPA - H+B- system.
Equilibrium
log K
Laq O Lorg	+
H+rg + Lorg O HL+org H0rg+ 2Lorg O HL+,org
Sr2a++ 2H+rg O Sro+g+ 2H+
Sr2a+ + 2Lorg + 2H+org O ^L^ 2H+q Sr2a+ + 3Lorg + 2H+orgO	2H+q
Sro+g + 2Lorg O S4+org
Sr2+ + 3L O SrL:
org
org
2+
3,org
1.29 a 9.3 a 10.7 a 0.7 b 19.33 22.39 18.63 21.69
a Ref. 34; b Ref. 41.
respectively, to the total divalent metal cation concentration in the corresponding equilibrium organic phase. From Figures 3, 4 and 5 it follows that the cationic complex species HL+ org, CaL3+org and SrL3+org are present in significant concentrations only at relatively high amounts of the MePhDPA ligand in the systems under consideration.
1.0
0.5 -
-3	-2
log c(L)
Figure 3. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Ca2+ (microa-mounts)-nitrobenzene-MePhDPA-H+B- extraction system in the forms of H+, HL+ and HL+ ; c(HCl) = 0.01 mol/L, cB = 0.003 mol/L.
1	S(H+) = [H+rg]/c(H+)0rg,
2	S(HL+) = [HL+rg]/c(H+)0rg,
3	S(HL+) = [HL+,0rg]/c(H+)0rg
where c(H+)ols = [H J + [HLy + [HL+,0J.
The distribution curves were calculated using the constants given in Table 3.
Finally, it should be noted that the stability constants
2+
2,org'
SrL2+ ,
2,org
CaL
3,org
and
of the complex species CaL
SrL23,+org, where L = MePhDPA, in nitrobenzene saturated with 'water are log £(CaL2+org) = 19.29, log $SrL2+org) =
18.63, log ß(CaL3+Drg)
Figure 4. Distribution diagram of calcium in the equilibrium nitrobenzene phase of the water- HCl - Ca2+ (microamounts) - nitrobenzene - MePhDPA - H+B- extraction system in the forms of CaL2+, CaL2+and CaL2+; c(HCl) = 0.01 mol/L, cB = 0.003 mol/L.
1	S(CaL2+) = [CaL2o++,]/c(Ca2+)olg,
2	S(CaL22+) = [CaL2+0rg]/c(Ca2+)0rg,
3	S(CaL2+) = [CaL23+]/c(Ca2+)
where c(Ca2+)ols = [Cafj + [CaLfj + [CaL|+J.
-J3,oigJ'
The distribution curves were calculated using the constants given in Table 3.
22.06 and log ß(SrL23+OTg)
Figure 5. Distribution diagram of strontium in the equilibrium nitrobenzene phase of the water- HCl - Sr2+ (microamounts) - nitrobenzene - MePhDPA - H+B- extraction system in the forms of SrL2+, SrLfand SrLf+; c(HCl) = 0.01 mol/L, cB = 0.003 mol/L.
1	S(SrL2+) = [SrL20Jg]/c(Sr2+)0rg,
2	S(SrL|+) = [SrL22+0rg]/c(Sr2+)0l!!,
3	S(SrL2+) = [SrL|+0rg]/c(Sr2+)0rg
where c(Sr2+)ols = [Sr^] + [SrL^+J + [SrL^].
The distribution curves were calculated using the constants given in Table 4.
21.69, as given in Tables 3 and 4. Thus, in the considered nitrobenzene medium, the stability constants of corresponding complexes ML2+org and ML3+org (M2+ = Ca2+, Sr2+; L = MePhDPA) are comparable. ,
In conclusion, it is necessary to emphasize that the stability constants of the complexes CaL2+, SrL2+ and EuL3+ with the MePhDPA ligand in water-saturated nitrobenzene are log £(CaL23+org) = 22.06 (Table 3), log ASrL3+OTg) = 21.69 (Table 4) and log £^33+^) = 33.28.34 This means that the stability of these three complexes in the mentioned medium increases in the series of Ca2+ Sr2+ << Eu3+.
4. Acknowledgements
This work was supported by the Czech Ministry of Education, Youth and Sports, Project MSM 6046137307, and 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."
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Povzetek
Raziskovali smo ekstrakcijo mikrokoličin kalcija in stroncija z raztopino vodikovega dikarbolilkobaltata (H+B-) v nitrobenzenu in v prisotnosti N,W-dimetil-N,W-difenil-2,6-dipikolinamida (MePhDPA, L). Dobljene podatke za dosežena ravnotežja smo razložili ob predpostavki, da se ionske zvrsti HL+, HL+, ML2+in ML2+ (M2+ = Ca2+, Sr2+) prenesejo v organsko fazo. Za nitrobenzen, nasičen z vodo, smo določili konstante ekstrakcije in konstante stabilnosti vseh zvrsti kationskih kompleksov.