Acta Chim. Slov. 2008, 55, 223–227 223 Short communication Solvent Extraction of Microamounts of Europium and Americium into Nitrobenzene by Using Hydrogen Dicarbollylcobaltate in the Presence of “classical” CMPO Emanuel Makrlíka,*, Petr Van¡urab and Pavel Seluckýc a Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic. b Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague, Czech Republic c Nuclear Research Institute, 250 68 Re`, Czech Republic ¡ * Corresponding author: E-mail:makrlik@centrum.cz Received: 24-07-2007 Abstract Extraction of microamounts of europium and americium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B–) in the presence of octyl-phenyl – N,N – diisobutylcarbamoylmethyl phosphine oxide (“classical” CMPO, L) has been investigated. The equilibrium data have been explained assuming that the complexes HL+, HL+2, ML3 +2, ML3 +3 and ML3 +4, (M3+ = Eu3+, Am3+) are extracted into the organic phase. The values of extraction and stability constants of the species in nitrobenzene saturated with water have been determined. Keywords: Europium, americium, hydrogen dicarbollylcobaltate, CMPO, extraction and stability constants, water-nitrobenzene system 1. Introduction Partitioning of transuranium elements, such as ame-ricium and curium, from high-level liquid radioactive waste (HLLW) and subsequent transmutation of these nucli-des to shorter-lived nuclides largely facilitates safety and low cost of nuclear waste management. A new class of extractants (bidentate phosphonates, phosphine oxides and malonamides) for the extraction of tervalent lanthanides and actinides from acidic media has been intensively studied.1–3 A process using octyl-phenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (i.e. “classical” CMPO) and called TRUEX was apparently used in the United States,1 while N, N-tetraalkyl alkyl propane diamides (RR´NCO)2CHR´´ (DIAMEX) were proposed in France.2 In the present work, the solvent extraction of mi-croamounts of europium and americium by a nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B–)4 in the presence of octyl-phenyl-N,N-diisobutylcarbamoyl-methyl phosphine oxide (“classical” CMPO, L) was stu- died. We intended to find the composition of the species in the nitrobenzene phase and to determine the corresponding equilibrium constants. 2. Experimental Octyl-phenyl-N,N-diisobutylcarbamoylmethyl phosphine oxide (abbrev. “classical” CMPO) was supplied by Alpha – Ventron. Cesium dicarbollylcobaltate, Cs+B–, was synthesized in the Institute of Inorganic Chemistry, R¡ e`, Czech Republic, using the method published by Hawthorne et al.5 A nitrobenzene solution of hydrogen dicarbollylcobaltate (H+B–) was prepared from Cs+B– by the procedure described in Reference 6. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclides241Am3+ and152,154Eu3+ (Polatom, Poland) were of standard radiochemical purity. The extraction experiments in the two–phase wa-ter–HCl–M3+(microamounts; M3+ = Eu3+, Am3+)–nitro-benzene–“classical” CMPO–H+B– systems were performed in 10 mL glass test-tubes covered with polyethylene Makrlík et al.: Solvent Extraction of Microamounts of Europium and Americium ... 224 Acta Chim. Slov. 2008, 55, 223–227 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 20 min of shaking. Then the phases were separated by centrifuga-tion. Afterwards, 1 mL samples were taken from each phase and their ?-activities were measured using a welltype NaI(T1) scintillation detector connected to a ?-analyzer NK 350 (Gamma, Budapest, Hungary). The equilibrium distribution ratios of europium and americium, D, were determined as the ratios of the corresponding measured radioactivities of152,154Eu3+ and241Am3+ in the nitrobenzene and aqueous samples. M;++nLürs + 3H:goML^+3H; (3) (4) (5) to which the following equilibrium constants correspond: ¦z 11-J ,(HKn.) = [HL+„] (6) (7) 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 “classical” CMPO ligand in the initial nitrobenzene phase, log c(L), are given in Figures 1 and 2, respectively. The initial concentration of hydrogen dicar-bollylcobaltate in the nitrobenzene phase, cB = 0.001 mol L–1, as well as the initial concentration of HCl in the aqueous phase, c(HCl) = 0.20 mol L–1, are always related to the volume of one phase. Regarding the results of previous papers,7–14 the considered water–HCl–M3+ (microamounts; M3+ = Eu3+, Am3+)–nitrobenzene–“classical” CMPO (L)–H+B– systems can be described by the set of reactions (1) (2) ,(HViJm) = K fMM [M^][H^]3 •V« Uvl™. ) —-----71------1—T -"' [m;+][h;j3 .«(ML«!<.rf!) = (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 “classical” CMPO ligand and the electroneutrality conditions in both phases of the system under consideration, was formulated15,16 and introduced into a more general least-squares minimizing program LETAGROP17 used for determination of the “best” Figure 1. Log D as a function of log c(L) for the system wa-ter–HCl– Eu3+ (microamounts)–nitrobenzene–“classical”CMPO– H+B–. c(HCl) = 0.20 mol L–1, cB = 0.001 mol L–1. The curve was calculated using the constants given in Table 3. Figure 2. Log D as a function of log c(L) for the system wa-ter–HCl–Am3+(microamounts)–nitrobenzene–”classical” CMPO– H+B–. c(HCl) = 0.20 mol L–1, cB = 0.001mol L–1. The curve was calculated using the constants given in Table 4. Makrlík et al.: Solvent Extraction of Microamounts of Europium and Americium ... Acta Chim. Slov. 2008, 55, 223–227 225 values of the extraction constants Kex(ML3n+,org) (M3+ = Eu3+, Am3+). The minimum of the sum of errors in log D, i.e., the minimum of the expression (11) was sought. The values log KD = 3.14,13 log ß(HL+r ) = 6.16,13 log ß(HL+org ) = 9.29,13 log Kex(Eu3o+) = 1.314 and log Kex(Am3+) = 1.514 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 ML3+2, ML33 and ML3+4 (M3+ = Eu3+, Am3+; L = “classical” CMPO) to be extracted into the nitrobenzene phase. Figure 3 presents the contributions of the species Eu3+, EuL3/, EuL3/ and EuL34+, to the total europium org 2,org 3,org org concentration in the equilibrium nitrobenzene phase, whereas Figure 4 shows the contributions of the cations Am3+ AmL , AmL 2,org 3,or and AmL34+,org to the total americium concentration in the equilibrium organic phase. From both of these figures it follows that the complexes EuL34+,org and AmL34+,org are present in significant concentrations only at relatively high amounts of the “classical” CMPO ligand in the systems under consideration. Figure 3. Distribution diagram of europium in the equilibrium nitrobenzene phase of the water-HCl-Eu3+(microamounts)-nitro-benzene–“classical” CMPO-H+B– extraction system in the forms of Eu3+, EuL32, EuL33, and EuL3+4. c(HCl) = 0.20 mol L-1, cB = 0.001 mol L-1. 1 8(Eu3+) = [Eu3+ ]/c(Eu3+) , 2 8(EuL2+) = [EuL32,+ ]/c(Eu3+) , 3 8(EuL3+) = [EuL33+, ]/c(Eu3+) , 4 8(EuL34+) = [EuL3,+o ]/c(Eu3+)o where c(Eu3+) = [Eu3o+r] + [EuL2,o ] + [EuL3,+ ] + [EuL33+,or ]. The distribution curves were calculated using the constants given in Table 3. Table 1. Comparison of various models of europium extraction from aqueous solution of HCl by nitrobenzene solution of H+B– in the presence of "classical" CMPO. Europium complexes in the organic phase log K ex a U b EuL3+ 22.21 (23.09) 65.80 EuL3+ 26.87 (27.37) 7.28 EuL43 + 31.52 (32.13) 14.10 EL 3+ EuL 3+ transformed to EuL33 + EuL3+, EuL43 + 26.14 (26.41), 30.40 (30.69) 0.52 EuL3+, EuL3+, EuL43 + 19.43 ± 0.24, 25.56 (25.87), 30.43 ± 0.14 0.05 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.17 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 log K(log [K + 3c(K)]).17 b The error-square sum U = ~L (logDcalc - log D )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 "classical" CMPO. Americium complexes in the organic phase log Kex a U b AmL3+ 22.79 (23.70) 61.90 AmL3+ 27.14 (27.71) 8.55 AmL43 + 31.49 (32.04) 7.55 AmL3+,AmL3+ transformed to AmL33 + AmL3+, AmL43 + 26.16 (26.41), 30.73 (30.96) 0.28 AmL3+, AmL3+, AmL43 + 19.48 (19.93), 25.82 (26.31), 30.73 (30.94) 0.07 a See Table 1, footnote a. b See Table 1, footnote b. Makrlík et al.: Solvent Extraction of Microamounts of Europium and Americium ... 226 Acta Chim. Slov. 2008, 55, 223–227 Figure 4. Distribution diagram of americium in the equilibrium nitrobenzene phase of the water-HCl-Am3+(microamounts)-nitro-benzene–“classical” CMPO-H+B– extraction system in the forms of Am3+, AmL3+, AmL3+, and AmL3+4. c(HCl) = 0.20 mol L-1, cB = 0.001 mol L-1. 1 8(Am3+) = [Am3+ ]/c(Am3+)o , 2 8(AmL32+) = [AmL32,+ ]/c(Am3+) , 3 8(AmL3+) = [AmL33+, ]/c(Am3+) , 4 8(AmL34+) = [AmL3,+ ]/c(Am3+) where c(Am3+) = [Am3+] + [AmL32+ ] + [AmL33,+ ] + [AmL33+, ]. The distribution curves were calculated using the constants given in Table 4. Knowing the values log Kex(Eu3o+rg) = 1.314 and log Kex(An3o+rg) = 1.514, as well as the extraction constants log Kex(EuL32+,org) = 19.43, log Kex(EuL33+,org) = 25.56, log Kex (EuL34+,org) = 30.43, log Kex(AmL32+,org) = 19.48, log Kex (AmL33+,org) = 25.82 and log Kex(AmL34+,org) = 30.73 determined here (Tables 1 and 2), the stability constants of the complexes ML32 +, ML33 +and ML34 + (M3+ = Eu3+, Am3+; L = “classical” CMPO) in the nitrobenzene phase defined as ,ML3* ) = [MLLj [MIJtL l3 ,ML'* ): [Mf;j[L„rj ,ML* ) = • (12) (13) (14) [r<][L„rj4 can be evaluated applying the simple relations: logP(ML':*oii ) = log K[s (ML3,+oti ) - log Kn (M ), (15) logPf,L^) = log Kcx (ML"^ ) - log Kra (M JK ), (16) logß(.L^OI]t) = log K„(ML4*orc)-log Kei(M0*e). (17) The respective equilibrium constants are summarized in Tables 3 and 4. Table 3. Equilibrium constants in the water–HCl–Eu3+ (microamounts)–nitrobenzene–"classical" CMPO–H+B– system. Equilibrium log K aq org H org + L org^HL org H+ org + 2L org <=> HL 2 Eu3+ + 3H+ Eu3+ + 3H+ Eu3+ + 2L + 3H+ EuL32+ + 3H+ aq org org Aorg a Eu aq + 3L + 3H+ EuL33+, + 3H+ Eu aq + 4Lorg + 3H+ EuL34,+ + 3H+ Eu3+ + 2L EuL3 2 + Eu3+ + 3L EuL 3 3+ Eu3+ + 4L EuL3 4 + 3.14 a 6.16 a 9.29 a 1.3 b 19.43 25.56 30.43 18.13 24.26 29.13 Ref. 13. Ref. 14. Table 4. Equilibrium constants in the water–HCl–Am3+ (microamounts)–nitrobenzene–"classical" CMPO–H+B– system. Equilibrium log K aq org Horg + Lorg^HLorg Horg + 2L <=> HL2, Am^ + 3Horg ** Am3org + 3Haq Am3+ + 2L + 3H+ AmL32+ + 3H+ aq org org Aorg a Am3+ + 3L + 3H+ AmL33+ + 3H+ Am3+ + 4L + 3H+ AmL34+ + 3H+ Am3+ + 2L AmL2 3+ 3L ^AmL33+, Am3+ + 4L AmL34+ Am 3.14 a 6.16 a 9.29 a 1.5 b 19.48 25.82 30.73 17.98 24.32 29.23 Ref. 13. Ref. 14. In conclusion, it should be noted that the stability constants of the complex species ML3+, ML3+ and ML3+ (M3+ = Eu3+, Am3+; L = “classical” CMPO) in nitrobenzene saturated with water are log /^EuL32+ ) = 18.13, log (8(AmL32+ ) = 17.98, log /?(EuL33+ ) = 24.26, log j3(AmL3+or ) = 24.32, log /i(EuL3+or ) = 29.13 and log j3(AmL3+or ) = 29.23 as follows from Tables 3 and 4. It means that the stability constants of the corresponding complexes EuL3+ and AmL3+, where n = 2, 3, 4 and L is “classical” CMPO, in the mentioned medium are practically the same. 4. Acknowledgements The present work was supported by the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6076137307. b Makrlík et al.: Solvent Extraction of Microamounts of Europium and Americium ... Acta Chim. Slov. 2008, 55, 223–227 227 5. References 1. W. W. Schulz, E. P. Horwitz, Sep. Sci. Technol. 1988, 23, 1191–1210. 2. C. Cuillerdier, C. Musikas, P. Hoel, L. Nigond, X. Vitart, Sep. Sci. 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Makrlík, J. Rais, M. Kyr{, Collect. Czech. Chem. Commun. 1982, 47, 1444–1464. 16. P. Van¡ura, E. Makrlík, Collect. Czech. Chem. Commun. 1993, 58, 1324–1336. 17. L. G. Sillén, B. Warnqvist, Arkiv Kemi. 1969, 31, 315–339. Povzetek Raziskovali smo ekstrakcijo mikrokoli~in evropija in americija z raztopino vodikovega dikarbolilkobaltata (H+B–) v ni-trobenzen ob prisotnosti oktil-fenil-N,N-diizobutilkarbamoilmetil fosfin oksida (CMPO, L). Dobljene rezultate smo obravnavali s predpostavko, da v organsko fazo preidejo kompleksi HL+, HL+2, ML3 +2, ML3 +3, in ML3 +4, (M3+ = Eu3+, Am3+) in dolo~ili konstante ekstakcije in stabilnosti kompleksov v nitrobenzenu, nasi~enem z vodo. Makrlík et al.: Solvent Extraction of Microamounts of Europium and Americium ...