Short communication
Solvent Extraction of Calcium into Nitrobenzene
by Using an Anionic Ligand Based on Cobalt Bis(Dicarbollide) Anion with Covalently Bonded
CMPO Function
Emanuel Makrlik,1'* Pavel Selucky2 and Petr Vanura3
1 Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamycka 129,
165	21 Prague 6, Czech Republic
2 Nuclear Research Institute, 250 68 Rez, CzechRepublic
3 Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technicka 5,
166	28 Prague 6, Czech Republic
* Corresponding author: E-mail: makrlik@centrum.cz Received: 03-01-2012
Abstract
From extraction experiments and y-activity measurements, the exchange extraction constantcorresponding to the general equilibrium Ca2+(aq) + 2HL(nb) o CaL2(nb) + 2H+(aq) taking place in the two-phase water-nitrobenzene (L- = anionic ligand based on cobalt bis(dicarbollide) anion with covalently bonded CMPO function; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (Ca2+, 2HL) = 0.0 ± 0.1. Further, the stability constant of the electroneu-tral complex species CaL2 in water-saturated nitrobenzene was calculated for a temperature of 25 °C : log Pnb (CaL2) = 11.0 ± 0.2.
Keywords: Calcium, anionic ligand with CMPO function, complexation, 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 me-dia.1-13 Recently, also TODGA (N,N,NN-tetraoctyl diglycolamide),14-16 CMPO substituted calixarenes 17 and several other promising compounds have been investigated for this purpose. Moreover, 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.1819
The dicarbollylcobaltate anion20 and some of its halogen derivatives are very useful reagents for the extrac-
tion 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,21-36 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.18'19
The anionic ligand based on cobalt bis(dicarbollide) anion with covalently bonded CMPO function (abbrev. L-; see Scheme 1) has been synthesized with the aim to develop an efficient extraction agent for liquid-liquid extraction of lanthanides and actinides from high-level activity nuclear waste. The molecular structure of the sodium complex of this anionic ligand has been determined by single crystal X-ray diffraction analysis. The results on the separation of the fission products from the simulated PUREX feed by using the mentioned ligand have been presented with the procedure for
Eu3+ stripping.37 On the other hand, in the current work, the solvent extraction of Ca2+ into nitrobenzene by means of the considered ligand L- (in the form of the electroneutral HL species) was studied. Furthermore, the stability constant of the electroneutral complex CaL2 in the organic phase of the water-nitrobenzene extraction system was evaluated.
Scheme 1. Structural formula of an anionic ligand based on cobalt bis(dicarbollide) anion with covalently bonded CMPO function (abbrev. L-).
2. Experimental
A nitrobenzene solution of the species HL involving the anionic ligand L- based on cobalt bis(dicarbollide) anion with covalently bonded CMPO function (see Scheme 1) was supplied by Nuclear Research Institute, Rez, Czech Republic. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 45Ca2+ was purchased from DuPont, Belgium.
The extraction experiments were carried out in 10 mL glass test-tubes with polyethylene stoppers: 2 mL of an aqueous solution of Ca(NO3)2 of the concentration in the range from 1 x 10-3 to 5 x 10-3 mol/L and microa-mounts of 45Ca2+ were added to 2 mL of the nitrobenzene solution of HL, the initial concentration of which varied also from 1 x 10-3 to 5 x 10-3 mol/L. The test-tubes filled with the solutions were shaken for 12 h at 25 ±1 °C, using a laboratory shaker. Then the phases were separated by centrifugation. After evaporating aliquots (1 mL) of the respective phases on Al plates, their ¿¡-activities were measured by means of the apparatus NRB-213 (Tesla Premyslenf, Czech Republic).
The equilibrium distribution ratios of calcium, DCa, were determined as the ratios of the corresponding measured radioactivities of 45Ca2+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
Previous results38-42 indicated that the two-phase water-Ca(NO3)2-nitrobenzene-HL (L- = anionic ligand
based on cobalt bis(dicarbollide) anion with covalently bonded CMPO function) extraction system (see Experimental), chosen for determination of the stability constant of the electroneutral complex CaL2 in nitrobenzene saturated with water, can be characterized by the chemical equilibrium
Ca2+(aq) + 2HL(nb) « CaL2(nb) + 2H+(aq); K(Ca2+, 2HL)
(1)
with the equilibrium extraction constant Kex(Ca2+, 2HL):
K„(Ca^HL) JCaL2UH + ]f [Ca-+1[[[L];,
(2)
where aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively.
It is necessary to emphasize that L- is a considerably hydrophobic anionic ligand, practically present in the nitrobenzene phase only, where it forms the very stable electroneutral species HL and CaL2 .
Taking into account the mass balances of the H+ and Ca2+ cations and the anionic ligand L- studied at equal volumes of the nitrobenzene and aqueous phases
[CaJnB, + [CaLiU=C^0j)j [HL]nb + 2[CaLJnb=Cr
(3)
(4)
(5)
as well as the measured equilibrium distribution ratio of calcium
Dc, = [CaL:U/[Ca%
(6)
then combination of relationships (3) - (6) with Eq. (2) yielded the final expression for Kex(Ca2+, 2HL) in the form
(7)
where CCina,(aNqO3) is the initial concentration of Ca(NO3)2 in the aqueous phase and CiHn,Lnb denotes the initial concentration of HL in the nitrobenzene phase of the system under consideration.
In this study, from the extraction experiments and /-activity measurements (see Experimental) by means of Eq. (7), the logarithm of the constant Kex(Ca2+, 2HL) was determined: log Kex (Ca2+, 2HL) = 0.0 ± 0.1.
Moreover, with respect to References 38 - 42, for the exchange extraction constant Kex(Ca2+, 2H+) corresponding to the equilibrium Ca2+(aq) + 2H+(nb) «
Ca2+(nb) + 2H+(aq) and for the extraction constant Kex (Ca2+, 2HL) defined above, as well as for the stability constants of the electroneutral species HL and CaL2, denoted by Pnb (HL) and Pnb (CaL2), respectively, one gets
log ßnb (CaL2) = 2 log ßnb (HL) + log Kex (Ca2+, 2HL) - log Kex (Ca2+, 2H+)
(S)
Finally, using the constants log Kex(Ca2+, 2H+) = 0.243 and log Kex (Ca2+, 2HL) given above, the value log Pnb (HL) = 5.6 ± 0.1,44 and applying Eq. (8), we obtain the stability constant of the electroneutral complex species CaL2 in water-saturated nitrobenzene at 25 °C as log Pnb (CaL2) = 11.0 ± 0.2. In this context it should be noted that the stability constant of the cationic complex CaL2+, where L denotes the electroneutral 15-crown-5 ligand, determined previously in the same nitrobenzene medium, is log Pnb (CaL2+) = 11.57.45 This means that the stabilities of the mentioned complexes CaL2, where L- is the considered anionic ligand, and CaL^+ (L = 15-crown-5) are comparable in nitrobenzene saturated with water.
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
S pomočjo ekstrakcijskih eksperimentov in meritev y -aktivnosti smo v dvofaznem sistemu voda-nitrobenzen raziskovali ravnotežje Ca2+(aq) + 2HL(nb) o CaL2(nb) + 2H+(aq) (L-= anionski ligand na osnovi cobaltovega bisdikarbolida, aq = vodna faza, nb = faza nitrobenzena). Določili smo konstanto ekstrakcije, log Kex (Ca2+, 2HL) = 0.0 ± 0.1 in konstanto stabilnosti kompleksov, Pnb (CaL2), v vodni fazi, nasičeni z nitrobenzenom, log Pnb (CaL2, 25 °C) = 11.0 ± 0.2.