Langmuir 1992,8, 1039-1041
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Kinetically Controlled Separation of Nickel(I1) and Cobalt(I1) Using Micelle-Solubilized Extractant in Membrane Processes Moutaoukel Ismael and Christian Tondre* Laboratoire d'Etude des Syst2mes Organiques et Colloidaux (L.E.S.O.C.), Unit&Associee au C.N.R.S.No. 406, University of Nancy I, B.P. 239, 54506 Vandoeuvre-12s-Nancy Cedex, France Received November 7, 1991 Micelle-solubilizedextractants have large possibilities in the field of metal recovery. A new application, based on the very slow rates of complexation observed between hydrophobic extractants (7-(4-ethyl-lmethylocty1)-&hydroxyquinoline) solubilized in cationic micelles (CTAB) and transition-metal ions (especially Ni2+ and Co2+),is suggested. It concerns the achievement of a selective separation of metal ions on the basis of their kinetics of complexation. The effectivenessof dialysisand ultrafiltration membrane processes for Ni2+/Co2+separation is compared. They can lead to a total removing of Co2+ ions. The potentialities of organized systems like micelles or microemulsions in the field of hydrometallurgy and metal recovery in general have been amply discussed in the recent Without trying to give an exhaustive view, the following applications can be mentioned. Surface active agents are, for instance, often added in solvent extraction systems to achieve increased extraction rates?JO This may result in a microemulsification of the aqueous phase in the organic phase,ll which brings about an increase of the interfacial area. Microemulsions have been used in equilibrium with an external aqueous phase (the so-called Winsor I1systems) to concentrate metal ions to be removed from aqueous stream@ or to transport metal ions in liquid membrane pr0cesses.~3J~Micellar-enhanced ultrafiltration techniques were also successfully employed for the same kind of purpose.1517 A last example can be found in the use of microemulsions or micelles as model systems to mimic in some way the aqueous/organic interfaces found in liquid-liquid extraction.1*J9 Such model systems may help elucidating the reaction mechanisms involved and precising the locale of the complexation step.20 On the other hand there is an obvious need of development of highly selective processes of metal ion separation. In a previous paper of this laboratory it was
suggested that the separation of metal ions from their mixtures could be achieved on a kinetic basis provided that the rates of complexation could be decreased to a large extent.21 Although a great amount of work has been devoted in the literature to micellar catalysis, comparatively only very few papers have been dealing with retardation effects on chemical reactions. The rate of formation of a metal complex has been shown to be usually well correlated with the rate of substitution of water molecules of the inner coordination sphere of metal ions.22*23These rates are changing by orders of magnitude from one metal ion to another, but unfortunately the time scale involved is always too short for a kinetic separation to be possible in usual conditions. Typically, complex formation in homogeneous solutions occurs on the time scale of milliseconds for Co2+ and of tenths of seconds for Ni2+at the millimolar concentration level. It has been previously demonstrated that the solubilization of strongly hydrophobic extractants in cationic micelles offers a way of shifting these reaction times to much longer time scales. Experimental conditions were found such that the characteristic times for complexation of Co2+and Ni2+were respectively of minutes and hours.21 (1) Osseo-Asare, K.; Zheng, Y. Colloids Surf. 1991,53, 339. This was attributed to the coupled effects of (i) the strong (2) Savastano, C. A,; Ortiz, E. S. Chem. Eng. Sci. 1991,46, 741. partition of the extractant in favor of the micelles and (ii) ( 3 ) Ganguly, B. (Nandi) J. Photochem. Photobiol., A 1990, 51, 401. the repulsive forces between the reacting metal ions and (4) Paatero, E.; Sj6blom, J. Hydrometallurgy 1990,25, 231. (5) Paatero, E.; Sj6blom,J.; Datta, S. K. J. Colloid Interface Sci. 1990, the micellar surface. These results were obtained in 138,388. systems which can be qualified as microheterogeneous, (6) Vijayalakshmi,C. S.;Annapragada, A. V.; Gulari, E. Sep. Sci. Techalthough the reaction of complexation takes place in nol. 1990,25, 711. (7) Vijayalakshmi, C. S.; Gulari, E. Sep. Sci. Technol. 1991,26,291. perfectly isotropic solutions. (8)Neuman, R. D.; Jones, M. A.; Zhou, N.-F. Colloids Surf. 1990,46, The transposition of these interesting kinetic observa45. (9) Fourre, P.; Bauer, D. C. R. Acad. Sci., Ser. 2 1981,292, 1077. tions to an effective separation of metal ions requires some (10) Osseo-Asare, K.; Keeney, M. E. Sep. Sci. Technol. 1980,15,999. sort of phase separation. The present paper is intended (11) Bauer, D.; Fourre, P.; Lemerle, J. C. R. Acad. Sci., Ser. 2 1981, to demonstrate that this can be achieved using membrane 292.1019. (12) Ovejero-Eacudero,F. J.; Angelino, H.; Caeamatta, G. J.Dispersion processes. Both dialysis membranes and ultrafiltration Sci. Technol. 1987, 8,89. membranes have been tested for this purpose, leading for (13) Tondre, C.; Xenakis, A. Faraday Discuss. Chem. SOC.1984, 77, the first time to a significant separation of Ni2+and Co2+ 115. (14) Derouiche, A.; Tondre, C. Colloids Surf. 1990,48,243. from their mixtures, within this new concept. (15) Dum, R. 0.;Scamehorn, J. F.; Christian, S. D. Colloids Surf. To our knowledge there has been so far only very few 1989,36,49. (16) Christian,S.D.;Tucker,E.E.;Scamehom,J.F.;Lee,B.-H.;Sasaki,reports of works aimed at performing kinetic separations ~~
K. J. Langmuir 1989,5, 876. (17) Klepac, J.; Simmons, D. L.; Taylor, R. W.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1991,26,165. (18) Tondre, C.; Boumezioud, M. J. Phys. Chem. 1989,93, 846. (19) Boumezioud, M.; Kim, H. S.; Tondre, C. Colloids Surf. 1989,41, 255. (20) Tondre, C.; Canet, D. J. Phys. Chem. 1991,95, 4810.
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(21) Kim, H. S.; Tondre, C. Sep. Sci. Technol. 1989, 24, 485. (22) Eigen, M. Pure Appl. Chem. 1963, 6, 97. (23) Eigen, M.; Wilkins, R. G. In Mechanism of Inorganic Reactions; Adv. Chem. Ser. 49; American Chemical Society: Washington, DC, 1965; p 55.
0143-1463/92/2408-1039$03.00/00 1992 American Chemical Society
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1040 Langmuir, Vol. 8, No. 4, 1992
of metal ions (see Kokusen et al.24and references therein). The authors just cited have demonstrated the possibility of separating Ni2+ from Co2+ on this basis, in chloroform/ water systems, but only at a very low concentration level (3 X mol~dm-~).
Experimental Section Chemicals. The extractant 7-(4-ethyl-l-methyloctyl)-8-hydroxyquinoline (Cll-HQ) was obtained from chromatographic purification of Kelex 100 (Schering, FRG).18 It was solubilized in cetyltrimethylammonium bromide (CTAB)/l-butanolmixed micelles (the solubilization was shown to be equivalent in this case to a microemulsification2*)so as to obtain a final composition (in wt % ): CTAB (4.5)/butanol(4.5)/Cll-HQ(l.O)/water (90.0). Dilutions of this stock solution by water were used (up to 50 times). The water component of the micellar solution was buffered at pH 6.5 with 0.1 M triethanolamine/HCl (Merck, pro analysis). CTAB from Fluka wm twice recrystallized in methanol/ diethyl ether. Techniques. Two different kinds of membrane techniques were used: dialysis techniques and ultrafiltration. The dialysis cells were made in our machine shop. They involved either two or three compartments separated by cellulose membranes (Visking) with an average pore diameter of 24 A. Each compartment was connected to a solution reservoir and a continuous circulation was ensured by peristaltic pumps. The ultrafiltration cell was a stirred cell (Amicon) equipped with Millipore membranes (PLGC cellulosic disks, 10.000 NMWL). A pressure of 3 bar was applied. The samples were analyzed for metal ion content with a Varian AA-1275 atomic absorption spectrophotometer. Stopped-flow kinetic experiments have been described in great details in previous papers.l*JS
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Figure 1. Kinetics of complexation: light transmission change versus time a t 400 nm during complexation of metal ions by C11-HQ solubilized in CTAB/butanol micelles at pH 6.5. The concentration of each metal ion was 1.7 X 104 mol-dm-3 and the microemulsion contained 0.1 7% extractant (adapted from ref 21). I
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Results and Discussion Our first concern was to check whether or not the rate of substitution of water molecules in the inner solvation shell of the metal ions still appears to be rate limiting in conditions where the complexation is strongly retarded. To verify this particular point we have used Cu2+ and Co2+mixtures rather than Ni2+and eo2+mixtures because the difference in the rate constants was expected to be L O much larger and thus even more demonstrative in the 0 5 10 15 20 former case. We have thus carried out stopped-flow kinetic Figure 2. Dialysis experiment in a two-compartment cell. experiments to measure the apparent rate constant for Proportion of metal ions found in the receiving phase R after a complex formation k p p from the slopes of the plots giving 2-h dialysis, in function of the concentration of Cll-HQ in the the reciprocal relaxation time versus the concentration of micellar phase. The values in the figure indicate the percent of C11-HQ. The values obtained were of 3.2 X lo3 and 8.6 metal ions transferred relatively to the initial concentration in the source phase S. M-' s-l for Cu2+ and Co2+,respectively, giving a ratio kfcu/kfco = 372. This value is in the same order of brane was in contact with a receiving aqueous solution (R) magnitude as that of the ratio of the rates of exchange of water molecules according to Eigen's ~ o r k . It~can ~ *be~ ~ at the same pH. We expected that during the transport from S to R driven by a concentration gradient, the faster deduced from this observation that the reaction is indeed reacting ion (cobalt in the present case) would be retained still controlled by water substitution around the metal in the micellar-extractant solution, contrary to the slower ion, which was a requirement for kinetic-based separations. reacting one. These experiments were unsuccessful probFigure 1shows typical kinetic curves for complexation ably due to both the fact that the transfer of ions was very of Co2+and Ni2+ by CII-HQ in CTAB/butanol micelles slow and that their concentrations in the micellar comdetected at 400 nm. In the experimental conditions used, partment in the initial times were very small comparatively the complete complexation of Co2+takes 2 min, whereas to the extractant concentration. These conditions were the complexation of Ni2+requires more than 1h to reach probably favoring the complexation of both types of ions. completion. (2) In order to avoid the preceding drawbacks we have Different membrane processes have been considered removed one of the two dialysis membranes in such a by us in order to achieve an effective separation of Ni2+ manner that the source phase and the micellar-extractand Co2+. ant solution constitute only one compartment. Practically (1)We have tested a three-compartment system in which we used a two-compartment cell and the metal ions were the micellar solution containing the extractant was placed injected directly into the micellar phase. The analysis of between, two dialysis membranes. The first dialysis the receiving phase R after a 2-h dialysis demonstrated membrane was in contact with a source solution (S) that a very significant enrichment in Ni2+can be obtained. containing the metal ions and the second dialysis memThis is shown in Figure 2 in which the initial concentrations of NiClz and CoCl2 were identical and equal to 3.34 X (24) Kokusen, H.; Suzaki, K.; Ohashi, K.; Yamamoto, K. Anal. Scc. mol~dm-~. Blank experiments in the absence of extractant1988, 4, 617.
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o 20 40 60 a0 100 120 140 Figure 3. Ultrafiltration experiment: percent of metal ion concentration in filtrate relative to initial concentration versus time between the start of reaction and ultrafiltration.
containing micelles, indicated a slightly larger transfer of Co2+ions, which is probably to be attributed to specific interactions between the dialysis membrane and the metal ions. According to Figure 2, when the concentration of C11-HQ in the micellar phase increases from 0.01 to 0.05% the proportion of Ni2+ to total metal ions in R increases from 58 to 95 % ,with a corresponding decrease of Co2+ions. The best separation was thus obtained when the concentration of C11-HQ was 2.5 times as large as the total concentration of metal ions (0.1% of C11-HQ repmol-dm-3). In terms resents a concentration of 3.34 X of yield, the amount of Ni2+recovered in R after 2 h was of the order of 12% of the initial one. (3) Finally, we have performed ultrafiltration experiments with the aim of improving this poor yield. A different device was used as described in the experimental part. The principle of these experiments was the same as in the preceding section, but the time parameter was more strictly taken into consideration. Figure 3 shows the variations of the metal ion concentrations in the filtrate (expressed as a percent of the initial metal concentration) versus the time elapsed between the start of the reaction and the ultrafiltration. The concentration of C11-HQ was 0.1 % and two different metal ion concentrations have been considered. As expected, after
0 4 a 12 16 20 Figure 4. Ultrafiltration experiment: proportion of metal ions found in the filtrate versus the total initial metal ion concentration in a 1/1 ratio. The ultrafiltration occurred 5 min after the start of reaction.
a time of 2 min, the concentration of Co2+in the filtrate remains undetected. The Ni2+ recovered decreases in function of the time alloted for reaction with the micellesolubilized C11-HQ. It can be totally recovered if the ultrafiltration follows reaction within 1 min or so. The percent recovered decreases when the initial concentration is increased. Figure 4 represents the proportion of metal ions in the filtrate versus the initial concentration of metal ions (always in a 1/1ratio), when the ultrafiltration takes place 5 min after the start of the reaction. The separation appears to be excellent up to an initial metal ion concentration close to 1mM. For larger concentrations the capacity of complexation of the micellar phase is exceeded. This is a preliminary report of the potentialities of organized systems to achieve separation of metal ions on a purely kinetic basis. The results are very promising and might be still improved and extended to other metal ions in further developments.
Acknowledgment. We thank Dr.M. HBbrant for his help in performing atomic absorption measurements. Registry No. CTAB, 57-09-0; Cll-HQ, 73545-11-6; Ni, 744002-0;CO, 7440-48-4; 1-butanol, 71-36-3.
