Short communication
Solvent Extraction of Calcium and Strontium into Nitrobenzene by Using a Synergistic Mixture of Hydrogen Dicarbollylcobaltate and Bis(Diphenylphosphino)Methane Dioxide
Emanuel Makrlik,1'* Zdenek Spichal,2 Petr Vanura3 and Pavel Selucky4
1 Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamyckâ 129,
165	21 Prague 6, Czech Republic
2 Department of Inorganic Chemistry, Faculty of Science, Masaryk University, Kotlârskâ 2,
611 37 Brno, Czech Republic
3 Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technickâ 5,
166	28 Prague 6, Czech Republic
4 Nuclear Research Institute, 250 68 Rez, Czech Republic * Corresponding author: E-mail: makrlik@centrum.cz Received: 22-08-2012
Abstract
Extraction of microamounts of calcium and strontium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-) in the presence of bis(diphenylphosphino)methane dioxide (DPPMDO, L) has been investigated. The equilibrium data have been explained assuming that the species HL+, HL2, ML2+, ML3+ and ML4+ (M2+ = Ca2+, Sr2+) are extracted into the organic phase. The values of extraction and stability constants of the cationic complexes in nitrobenzene saturated with water have been determined. In the considered nitrobenzene medium, it was found that the stability constants of the complexes CaL^+, CaL^+ and CaL^+, where L is DPPMDO, are somewhat higher than those of the corresponding complex species SrL^+, SrL^+ and SrL^+ with the same ligand L.
Keywords: Calcium, strontium, bis(diphenylphosphino)methane dioxide, hydrogen dicarbollylcobaltate, complexa-tion, extraction and stability constants, water-nitrobenzene system
1. Introduction
Bidentate phosphonates, phosphine oxides and ma-lonamides have been intensively studied for the extraction of trivalent lanthanides and actinides from acidic media.1-3 A process using octyl-phenyl-N,N-diisobutylcarba-moylmethyl phosphine oxide (i.e. "classical" CMPO) and called TRUEX was apparently used in the United States,1 while malonic diamides (RR'NCO)2CHR" (DIAMEX) were proposed in France.2 Furthermore, a process involving chlorinated cobalt dicarbollide, polyethylene glycol (PEG 400) and diphenyl-N,N-dibutylcarbamoylmethyl phosphine oxide (DPDBCMPO), also called UNEX, has been reported for the simultaneous recovery of cesium,
strontium, lanthanides and actinides from highly acidic media into phenyltrifluoromethyl sulfone (abbrev. FS 13).4'5 At this point it should be noted that the mentioned FS 13 diluent was developed for the UNEX process as an alternative organic diluent to the highly polar nitrobenzene. Besides this, FS 13 has the advantage of low viscosity and very good solubility of the UNEX extractants and metal solvates.5 On the other hand, nitrobenzene derivates have been successfully utilized as diluents for cobalt di-carbollide processes in Russia, however, they are deemed unsuitable for use in the United States due to the perceived hazards associated with nitrobenzene.
The dicarbollylcobaltate anion6 and some of its halogen derivatives are very useful reagents for the extrac-
tion of various metal cations (especially Cs+, Sr2+, Ba2
ratus NRB-213 (Tesla P?emysleni, Czech Republic). On
Eu and Am) from aqueous solutions into a polar orga- the other hand, in the case of the systems with Sr2
nic phase, both under laboratory conditions for purely theoretical or analytical purposes,7-27 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.4,5
Recently, extractive properties of a synergistic mixture of hydrogen dicarbollylcobaltate (H+B-)6 and bis(dip-henylphosphino)methane dioxide (DPPMDO, L; see Scheme 1) toward Eu3+ and Am3+ have been investigated in the water-nitrobenzene system.28 On the other hand, in the current work, the solvent extraction of microamounts of calcium and strontium by a nitrobenzene solution 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.
Scheme 1. Structural formula of bis(diphenylphosphino)methane dioxide (abbrev. DPPMDO or L, respectively).
2. Experimental
Bis(diphenylphosphino)methane dioxide (DPPMDO) was synthesized similarly as 1,2-bis(diphenylphosp-hino)ethane dioxide (DPPEtDO) by the method presented in Ref. 29. Cesium dicarbollylcobaltate, Cs+B-, was produced by means of the method published by Hawthorne et al.30 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B-)6 was prepared from Cs+B- by the procedure described elsewhere.31 The other chemicals used (Lac-hema, Brno, Czech Republic) were of reagent grade purity. The radionuclides 45Ca2+ and 85Sr2+ were supplied by DuPont, Belgium.
The extraction experiments in the two-phase wa-ter-HCl-M2+ (microamounts; M2+ = Ca2+, Sr2+)-nitroben-zene- DPPMDO- 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 appa-
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.
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 DPPMDO ligand in the initial nitrobenzene phase, log c(L), are given in Figures 1 and 2, respectively. The initial concentration of hydrogen dicarbollylcobaltate in the organic phase, cB = 0.001 mol/L, as well as the initial concentration of HCl in the aqueous phase, c(HCl) = 0.05 mol/L, are always related to the volume of one phase.
With respect to the results of previous papers,8,10,19,26,27 the considered water-HCl-M2+ (microa-mounts; M2+ = Ca2+, Sr2+)-nitrobenzene- DPPMDO(L)-H+B- systems can be described by the set of reactions:
H^L^ohl;

(1) (2) (3)
Figure 1. Log D as a function of log c(L), where L = DPPMDO, for the system water- HCl- Ca2+ (microamounts)- nitrobenzene -DPPMDO - H+B-; c(HCl) = 0.05 mol/L, cB = 0.001 mol/L. The curve was calculated using the constants given in Table3.
U = Klog DcaIc - log D)2
(11)
Figure 2. Log D as a function of log c(L), where L = DPPMDO, for the system water- HCl- Sr2+ (microamounts)- nitrobenzene -DPPMDO - H+B-; c(HCl) = 0.05 mol/L, cB = 0.001 mol/L. The curve was calculated using the constants given in Table 4.

M^ + nLorB+2H;reoMl/n;re+2H:;4
(4)
(5)
to which the following equilibrium constants correspond:
PkJ
Kd =
[Lm]

[m£][L î-pr]2
(6)
(7)
(8)
(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 DPPMDO ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated32'33 and introduced into a more general least-squares minimizing program LETAGROP34 used for determination of the "best" values of the extraction constants Kex(ML2+org) (M2+ = Ca2+, Sr2+; L = DPPMDO). The minimum of the sum of errors in log D, i.e., the minimum of the expression
6.56,28 and log
was sought.
The values log KD = 2.01,28 log $HL+rg) = log AHL+'OTg) = 9.48'28 log Kex(Ca2o+g) = 0.235 Kex(Sr2+g) = 0.732 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 explained assuming the complexes ML22+, ML23+ and ML24+ (M2+ = Ca2+, Sr2+; L = DPPMDO) to be2 extrac3ted into the4 nitrobenzene phase.
Table 1. Comparison of various models of calcium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DPPMDO.
Calcium complexes in the organic phase
log K
Ub
CaL2+	19.33 (19.97)	17.60
CaL3+	24.16 (24.64)	5.51
CaL4+	27.73 (28.73)	55.90
CaL2 , CaL3	17.81 (18.23), 23.33 (23.66)	0.61
CaL3 , CaL4	Transformed to CaL^+	
CaL2 , CaL3 , CaL4	17.81 (18.15), 23.22 (23.45), 25.17 (25.42)	0.03
a The values of the extraction constants are given for each complex. The reliability interval of the constants is given as 3 o(K), where c(K) is the standard deviation of the constant K.34 These values are expressed in the logarithmic scale using the approximate relation log K ±{log [K + 1.5c(K)] - log [K - 1.5c(K)]}. For c(K) > 0.2K, the previous relation is not valid and then only the upper limit is given in the parentheses in the form of log K (log [K + 3c(K)]).34 b The error-square sum U = X(log Dc.
- log Dexp)2.
Table 2. Comparison of various models of strontium extraction from aqueous solution of HCl by nitrobenzene solution of H+B- in the presence of DPPMDO.
Strontium complexes
in the organic phase	log Ke,
U4
SrL?+		18.18 (18.85)	20.60
SrL3+		22.87 (23.32)	3.83
SrL4+		25.58 (26.41)	24.40
SrL2+,	SrL23+	16.73 (17.04), 22.40 (22.67)	0.53
SrL3+,	SrL4+	Transformed to CaL^+	
SrL22+,	SrL23+, SrL2+	16.75 (17.01), 22.12 (22.40), 24.69 (25.02)	0.03
' See Table 1, footnote a.
See Table 1, footnote b.
K
t
log Kex
log Kex (CaL24+org) = 25.17, log log Kex (SrL23+0rg) = 22.12 and log K
0.235 and log as well as the extraction constants
Knowing the values log Kex(Cao+g) (Sr2o+g) = 0.7,32
(CaL22+org) = 17.81, log Ke K
(CaL23+org) = 23.22,
x(SrL22+org) (SrL24+,org
16.75, ) = 24.69 de-
a
a
b
termined here (see Tables 1 and 2), the stability constants of the complexes ML22+, ML23+and ML2+ (M2+ = Ca2+, Sr2+; L = DPPMDO) in the organic phase defined as
[MLLJ

[MLtJ

(12)
(13)
(14)
[Ml^Kj
can be evaluated applying the following simple relations:
log p	) = log K„{ML^, ) - log(15)
log ¿(MLt,) = log Kk(ML2^)- log Kw(M2; ) (16)
logp(ML2;rB) = log (ML^) - log K„ (M£) (17)
The respective equilibrium constants are summarized in Tables 3 and 4.
Table 3. Equilibrium constants in the water- HCl- Ca2+ (microa-mounts)- nitrobenzene - DPPMDO - H+B- system.
Equilibrium
log K
Laq « Lorg +
H+rg + Lorg « H+r+
H+n, + 2Lorg « H+,org
Ca2+ + 2Horg « Ca2+g + 2H+q
Ca2a+ + 2Lorg + 2Horg « ^org + 2H+q
Ca2a+ + 3Lorg + 2Horg « CaL3+org + 2H+q
Ca?a+ + 4Lorg + 2Horg « CaL4+org + 2H+q
Ca2o+g + 2Lorg « CaL+org
Ca2o+g + 3Lorg « CaL2+org
CaQg + 4Lorg « CaL24+org_
2.01 a 6.56 a 9.48 a 0.2 b 17.81 23.22 25.17 17.61 23.02 24.97
Moreover, Figure 3 depicts the contributions of the species H+rg, HL+rg and H+org to the total hydrogen cation concentration in the equilibrium nitrobenzene phase, whereas Figures 4 and 5 show the contributions of the cations Cao+g, CaL2+org, CaL3+rg, CaL4+,rg and S^, SrL^ SrL*^, SrL4+org, 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, CaL4+org and SrL4+org are present in significant concentrations only at relatively high amounts of the DPPMDO ligand in the systems under consideration.
Figure 3. Distribution diagram of hydrogen cation in the equilibrium nitrobenzene phase of the water-HCl-Ca2+(microa-mounts)-nitrobenzene-DPPMDO-H+B- extraction system in the forms of HL+, HL+ and HL+; c(HCl) = 0.05 mol/L, cB = 0.001 mol/L.
1 S(H+) = [H+ng]/c(H+)olg, 2 S(HL+) = [HL+J/c(H+) 3 S(HL+) = [HLy/c(H+)0[s, where c(H+)ore = [Hi] + [HL+1 +
a Ref. 28. b Ref. 32.
[HL2,org]'
The distribution curves were calculated using the constants given in Table 3.
Table 4. Equilibrium constants in the water- HCl- Sr2+ (microa-mounts)- nitrobenzene - DPPMDO - H+B- system.
Equilibrium
La+q « Lorg	+
H++org + Lorg « H+or+g
H+rg + 2Lorg « H+.org
S< + 2H+rg « S4 + 2H+
S*2+ + 2Lorg + 2Horg « ^rg + 2H+q S*2+ + 3Lorg + 2Horg « SrLtrg + 2H+q Sr3++ 4Lorg + 2Horg « SrL4^rg + 2H+q
Sr2+ + 2L___« SrL'
2+
"°rg ' " org ' ' """2,org Sr?o+g + 3Lorg « SrL3+org SC + 4Lorg « ^rg
Ref. 28. b Ref. 32.
log K
2.01 a 6.56 a 9.48 a 0.7 b
16.75 22.12 24.69 16.05 21.42 23.99
Finally, it should be noted that the stability constants of the complex species ML2+org, ML3+org and ML4+org (M2+ = Ca2+, Sr2+; L = DPPMDO) in nitrobenzene saturated with
17.61, log j8 (SrL2+_) = 16.05,
2+
water are log ß (CaL2+org)
log p (CaL23+org) = 23.02, log p (SrL^) = 2L42, log p (CaL4+org) = 24.97 and log p (SrL^) = 23.99, as given in Tables 3 and 4. Thus, in the considered nitrobenzene medium, the stability constants of the complexes CaL2+, CaL2+ and CaL4+ , where L is DPPMDO, are somewhat
3,org	4,org'	'
higher than those of the corresponding complex species SrL2+org, SrL23+org and SrL2+org with the same ligand L.
In conclusion, Table 5 summarizes the stability constants of the complexes HL+, HL+, ML2+, ML23+ and ML24+ (M2+ = Ca2+, Sr2+) with two electroneutral ligands L, denoted by the symbols DPPEtDO and DPPMDO (see
1.0
CO
o.s
	1
\	y
, —	— i
-4
-3
log c(L)
-2
Figure 4. Distribution diagram of calcium in the equilibrium nitrobenzene phase of the water- HCl- Ca2+ (microamounts)- nitrobenzene - DPPMDO - H+B- extraction system in the forms of Ca2+, CaL22+, CaL23+and CaL24+; c(HCl) = 0.05 mol/L, cB = 0.001 mol/L.
[Ca2+,]/c(Ca-= [CaL2+
3,org-
0Ig ]/c(Ca2
J/c(Ca2+)o,
1 8(Ca2+) = 3 8(CaL2+)
where c(Ca2+)ols = [CaJ+g] + [CaL-J + [CaL^ + [CaL^, The distribution curves were calculated using the constants given in Table 3.
2 6(CaL2+) = [CaL2+org]/c(Ca2+), )0Ig, 4 S(CaL2+) = [Ca^
2+ 2+ 2+
where L = DPPMDO, are substantially higher than those of the corresponding complex species HL+
HL+,
ML;
and ML2+ (M2+ = Ca2+, Sr2+) involving the DPPEtDO li-gand.
Table 5. Stability constants of the complex species HL+, HL+ and ML2+ [n = 2, 3, 4; M2+ = Ca2+, Sr2+; L = 1,2-bis(diphenylphosphi-no)ethane dioxide (abbrev. DPPEtDO), bis(diphenylphosphi-no)methane dioxide (DPPMDO)] in nitrobenzene saturated with water at 25 °C.
Quantity	DPPEtDO a	L DPPMDO
log P (HL+rg) log P (HL+,OTg) log P(CaLf+0rg) log P (CaL23+org) log P (CaL24+org) log P (SrL2+org) log P (SrL23+org) log P(SrL24+org)	4.88 7.33 12.34 17.03 10.29 14.12	6.56 c 9.48 c 17.61 23.02 24.97 16.05 21.42 23.99
' Ref. 36.
' This work.
c Ref. 28.
Scheme 2. Structural formula of 1,2-bis(diphenylphosphino)ethane dioxide (abbrev. DPPEtDO).
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.
5. References
Figure 5. Distribution diagram of strontium in the equilibrium nitrobenzene phase of the water- HCl- Sr2+ (microamounts)- nitrobenzene - DPPMDO - H+B- extraction system in the forms of Sr2+, SrL|+, SrL23+and SrL^+; c(HCl) = 0.05 mol/L, cB = 0.001 mol/L. 1 5(Sr2+) = [Sro+g]/c(Sr2+)0lg, 2 6(SrL22+) = [S^JMSr2^
3 5(SrL2+) = where c(Sr2+)ol
The distribution curves were calculated using the constants given in Table 4.
[SrL2+,rg]/c(Sr )org, 4 S(SrL4+) = [SrL^MSr2 : [Sro+J + [SrL22+„rg] + [SrL^] + [SrLj+J.
Schemes 1 and 2), in nitrobenzene saturated with water. From the data reviewed in this table it follows that in the mentioned medium, the stability constants of the cationic species HL+
HL+,
ML22+ and ML23+ (M2+ = Ca2+, Sr2+),
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Povzetek
Proučevali smo ekstrakcijo mikrokoličin kalcija in stroncija z raztopino hidrogendikarbolilkobaltata (H+B-) v nitroben-zenu v prisotnosti bis(difenilfosfino)metan dioksida (DPPMDO, L). Dobljena ravnotežja smo razložili s pomočjo predpostavke, da se kompleksi HL+, HL^, ML^+, ML^+ in ML^+ (M2+ = Ca2+, Sr2+) ekstrahirajo v organsko fazo. Določili smo konstante porazdelitve in konstante stabilnosti kationskih kompleksov v nitrobenzenu, nasičenem z vodo. Izkazalo se je, da so konstante stabilnosti kompleksov CaL2+, CaL^+ in CaL4+ (L je DPPMDO) višje kot pa so vrednosti konstant kompleksov SrL22+, SrL^+ in SrL^+.