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Langmuir 1997, 13, 1446-1450
Removal of Copper Ions by Micelle-Based Separation Processes. Electrochemical Behavior of Copper Ions Trapped in Micellar Particles C. Tondre,*,† M. Hebrant,† M. Perdicakis,‡ and J. Bessiere‡ Laboratoire d’Etude des Syste` mes Organiques et Colloı¨daux (LESOC), Unite´ Associe´ e au CNRS No. 406,§ Faculte´ des Sciences, Universite´ Henri Poincare´ -Nancy I, B.P. No. 239, 54506 Vandoeuvre-le` s-Nancy Cedex, France, and Laboratoire de Chimie Physique pour l’EnvironnementsLCPE, CNRSsUnite´ mixte de recherchesUMR 9992,§ 405, rue de Vandoeuvre, 54600 Villers-le` s-Nancy, France Received July 23, 1996. In Final Form: December 16, 1996X Micellar-based separation processes have been shown in the past few years to be especially attractive for the removal of metal ions from dilute aqueous solutions, when micellar extraction is combined with ultrafiltration. In the present paper we consider two ways of removing copper ions from micellar processes: the first is based on ion-exchange (anionic SDS micelles), the second on metal ion complexation by a micelle-solubilized extractant (CTAB/butanol/water/Kelex 100 microemulsions). The influence of the relative concentrations of the species involved and of the pH on the yield of extraction is investigated, and in the first case the results are compared with a theoretical prediction. In a second part we compare for the first time the electrochemical behavior of copper ions whether they are simply bound to the micelle surface or more deeply imbedded in the core of a microemulsion as a hydrophobic complex. The electroreduction of the copper ions is easily obtained when the metal ions are simply bound to the micellar surface, offering a way of recycling the micellar phase after the metal ions have been removed. The situation is more complicated when the copper ions form stable complexes solubilized in microemulsions, since the electroreduction is possible only once the complex has been destroyed in acidic media.
Introduction The separation of metal ions is a problem encountered in many industrial activities going from hydrometallurgy to the treatment of industrial streams or liquid wastes. Many different processes have been used both at the industrial scale or at the laboratory level: liquid-liquid extraction,1,2 liquid membranes3-5 (bulk or supported), hollow fibers,5,6 double emulsions,7,8 etc. Most of these processes require an organic solvent which is undesirable from an environmental point of view because the transfer of small amounts of the organic phase into the aqueous phase cannot be avoided. During the past few years a growing interest has appeared for micelle-based separation processes,9,10 which permit the treatment of diluted solutions in conditions where the system remains essentially aqueous, since the aqueous phase may represent * Author to whom correspondence should be addressed. † Laboratoire d’Etude des Syste ` mes Organiques et Colloı¨daux. ‡ Laboratoire de Chimie Physique pour l’Environnement. § Institut Nance ´ ien de Chimie Mole´culaire (INCM)sFU CNRS No. 8. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Ritcey, G. M.; Ashbrook, A. W. Solvent Extraction Principles and Applications to Process Metallurgy, Part I; Elsevier: Amsterdam, 1984. (2) Tavlarides, L. L.; Bae, J. H.; Lee, C. K. Sep. Sci. Technol. 1987, 22, 581. (3) Schlosser, S.; Kossaczky, E. J. Radioanal. Nucl. Chem. 1986, 101, 115. (4) Danesi, P. R. Sep. Sci. Technol. 1984-85, 19, 857. (5) Izatt, R. M.; Lamb, J. D.; Bruening, R. L. Sep. Sci. Technol. 1988, 23, 1645. (6) D’Elia, N. A.; Dahuron, L.; Cussler, E. L. J. Membr. Sci. 1986, 27, 309. (7) Boyadzhiev, L.; Bezenshek, E. J. Membr. Sci. 1983, 14, 13. (8) Larson, K.; Raghuraman, B.; Wiencek, J. J. Membr. Sci. 1994, 91, 231. (9) Surfactant-Based Separation Processes; Surfactant Science Series, Vol. 33; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989. (10) Pramauro, E.; Bianco Prevot, A. Pure Appl. Chem. 1995, 67, 551.
S0743-7463(96)00730-5 CCC: $14.00
some 99% of the total volume. The metal ions themselves are too small to be retained by typical ultrafiltration membranes, but this becomes possible when they are trapped by larger colloidal particles such as micelles.11-22 It is now well established that metal ions can be removed by coupling micellar extraction with ultrafiltration, provided that the pores of the membrane have a diameter smaller than the micelle size. There are two different ways of considering the use of micelles for the present purpose: (i) a simple ion-exchange process involving anionic surfactants can be used if the metal ion species to be removed are preferentially bound on the micelle surface (case of divalent metal ions opposed to monovalent ones); (ii) the hydrophobic core of the micelles, which is hydrocarbon-like, can be used to solubilize lipophilic complexing agents. In this case there is a complete analogy between solvent extraction and micelle extraction, the classical organic phase being replaced by the micellar pseudophase.23,24 Contrary to (11) Scamehorn, J. F.; Christian, S. D.; Ellington, R. T. In ref 9, p 29. (12) Dharmawardana, U. R.; Christian, S. D.; Taylor, R. W.; Scamehorn, J. F. Langmuir 1992, 8, 414. (13) Scamehorn, J. F.; Christian, S. D.; El-Sayed, D. A.; Uchiyama, H.; Younis, S. S. Sep. Sci. Technol. 1994, 29, 809. (14) Pramauro, E.; Bianco Prevot, A.; Pelizzetti, E.; Marchelli, R.; Dossena, A.; Biancardi, A. Anal. Chim. Acta 1992, 264, 303. (15) Pramauro, E.; Bianco, A.; Barni, E.; Viscardi, G.; Hinze, W. L. Colloı¨ds Surf. 1992, 63, 291. (16) Reiller, P.; Lemordant, D.; Moulin, C.; Beaucaire, C. J. Colloid Interface Sci. 1994, 163, 81. (17) Hafiane, A.; Issid, I.; Lemordant, D. J. Colloid Interface Sci. 1991, 142, 167. (18) Ismael, M.; Tondre, C. Langmuir 1992, 8, 1039. (19) Tondre, C.; Son, S.-G.; Hebrant, M.; Scrimin, P.; Tecilla, P. Langmuir 1993, 9, 950. (20) Ismael, M.; Tondre, C. J. Colloid Interface Sci. 1993, 160, 252. (21) Hebrant, M.; Bouraine, A.; Tondre, C.; Brembilla, A.; Lochon, P. Langmuir 1994, 10, 3994. (22) Richmond, W.; Tondre, C.; Krzyzanowska, E.; Szymanowski, J. J. Chem. Soc., Faraday Trans. 1995, 91, 657. (23) Tondre, C.; Boumezioud, M. J. Phys. Chem. 1989, 93, 846.
© 1997 American Chemical Society
Micelle-Based Copper Ion Removal
the first case, the selectivity for some particular metal ion species can be controlled through the choice of the extractant and of the nature of the surfactant. For instance, a high copper rejection has been obtained with N-n-dodecyliminodiacetic acid solubilized in cationic micelles, with no rejection of calcium.25 With micelles containing 6-[(alkylamino)methyl]-2-(hydroxymethyl)pyridines, one can extract copper19 but not cobalt or nickel. In some particular cases the extractant was solubilized in surfactant/alcohol mixed micelles in a form similar to that of microemulsions.26 This was shown to be possible with 7-(4-ethyl-1-methyloctyl)-8-hydroxyquinoline (C11-HQ) in CTAB/butanol micelles, which allowed the kinetic separation of nickel and cobalt using ultrafiltration.18,20 A related problem which is far from being solved concerns the recycling of the micellar pseudophase after metal extraction. When complexing agents are used, in most instances the metal can be back-extracted in acid media.20,27 This is however not the case for the complexes of cobalt with C11-HQ which are so stable (at least kinetically speaking) that they cannot be destroyed whether they are solubilized in an organic solvent or in micelles.27 The electrochemical reduction of the metal ions could be another way to displace them from the micellar phase after the concentration step. Although electrochemistry has been extensively applied for the analysis of reactions involving micelles and microemulsions,28 only a few works have been carried out on the reduction of metal ions to their metallic form in such systems. The principle of the recovery of metals by associating ultrafiltration and electrolysis was recently proposed by Rumeau et al.,29 but to our knowledge no experimental results have been reported so far. The electrochemical behavior of lipophilic cobalt complexes in the presence of dodecyl sulfate micelles has been studied by Davies and Hussam,30 but it concerns the Co(III)/Co(II) transformation. Andriamanampisoa and Mackay31 have used voltammetry to specify the diffusion parameters of cadmium bound to dodecyl sulfate micelles using Cd(DS)2. Mackay et al.32 also investigated by polarography, the effect of the nature of the hydrocarbon on the behavior of quinoline-copper complexes in sodium cetyl sulfate (SCS)/ 1-pentanol/oil/water microemulsions when a mineral oil was replaced by benzene. The binding of copper ions to anionic micelles or to micelle-solubilized complexing agents will be first considered in the present work in order to draw a parallel between these two ways of effectively removing copper ions using ultrafiltration. In a second step we will examine the possibility of reducing the copper ions associated in these different ways to the micelles. The electrochemical behavior of copper ions in the presence of a large excess of sodium dodecyl sulfate at a concentration larger than the critical micelle concentration (cmc) will be considered in view to recover copper by electrolysis at a platinum or copper cathode, avoiding the mercury electrode whose use (24) Tondre, C.; Hebrant, M.; Ismael, M.; Son, S.-G. Proceedings of the First World Congress on Emulsions, Vol. 3, Comm. 4-30-047, Paris, 1993. (25) Klepac, J.; Simmons, D. L.; Taylor, R. W.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1991, 26, 165. (26) Kim, H. S.; Tondre, C. Sep. Sci. Technol. 1989, 24, 485. (27) Ismael, M.; Tondre, C. Sep. Sci. Technol. 1994, 29, 651. (28) Rusling, J. F. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, p 1. (29) Diawara, C. K.; Lo, S. M.; Diaw, M.; Rumeau, M. Informations Chimie no. 351-septembre 1993. (30) Davies, K.; Hussam, A. Langmuir 1993, 9, 3270. (31) Andriamanampisoa, R.; Mackay, R. A. Langmuir 1994, 10, 4339. (32) Mackay, R. A.; Dixit, N. S.; Agarwal, R. ACS Symp. Ser. 1982, 177, 179.
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is now prohibited for large scale processes. Then we will analyze the behavior of copper ions in copper-Kelex complexes solubilized in CTAB/butanol/water microemulsions to see if they can be reduced to metallic form at the same electrodes. Experimental Part Chemicals. The surfactants used had the following origins: sodium dodecyl sulfate (SDS) from Roth was used as received; cetyltrimethylammonium bromide (CTAB) from Fluka was twice recrystallized from methanol-diethyl ether. Sodium bromide and 1-butanol were obtained from Fluka, and copper chloride was from Prolabo. Copper dodecyl sulfate was prepared according to the literature.33 The elemental analysis indicated that the salt was crystallized with four water molecules (theory/ experiment: C, 43.3/43.2; H, 8.7/9.0; O, 28.9/29.3; S, 9.6/9.5; Cu, 9.6/9.8). The absence of sodium was checked by potentiometry using a sodium selective electrode. The cmc obtained from conductivity measurements was 1.15 × 10-3 M at 25 °C. The extractant 7-(4-ethyl-1-methyloctyl)-8-hydroxyquinoline (C11HQ) was obtained from chromatographic purification of Kelex 100 (Schering, Germany).34 It was solubilized in CTAB/1-butanol mixed micelles as reported before,26 except that 0.1 M NaBr was used instead of 0.1 M triethanolamine buffer. Some experiments were performed with the crude Kelex 100 instead of the purified sample. Techniques. The ultrafiltration experiments were carried out with an Amicon stirred cell of 10-mL volume at room temperature using cellulosic disk membranes YM10 (Millipore) with molecular weight cutoff 10 000. The applied pressure was 3.5 bar. The metal content of the filtrate [Cu2+]fil was analyzed from atomic absorption measurements (Varian AA-1275 apparatus). The extraction yield R was given by R ) 100 ([Cu2+]init - [(Cu2+]fil)/[Cu2+]init, assuming that the concentration of free species was the same in the filtrate and in the retentate. This approximation is neglecting the Donnan effect which may affect in some cases the distribution of the metal ions between the permeate and the retentate12 (this problem will be addressed in a forthcoming publication35). It is largely justified in the present case where most experiments were carried out in the presence of added electrolytes. The voltammograms were established at 25 °C with a potentiostat PRT 100-1X Tacussel, using a rotating platinum electrode EDI Tacussel (600 rpm) and an ECS Tacussel ERT34 as reference electrode. The potential rate was fixed at 100 mV/s.
Results and Discussion 1. Removal of Copper Bound to Micelles by Ultrafiltration. The two possibilities of copper separation using micelle-based processes, namely, the ionexchange method and the solubilized extractant method, will be first illustrated by relevant examples. In Figure 1 we have plotted the yield of extraction of Cu2+ ions when the concentration of SDS is varied. The copper ion concentration was fixed at 10-4 M and the ultrafiltration experiments were carried out at two different pH values (pH 2 and 5). In the first case we have added to the solution 10-2 M of hydrochloric acid. In the second case, pH 5 ((0.5) was the natural pH of the solution, but we have added 10-2 M NaBr in order to have an identical ionic strength in both cases. The yields obtained at pH 5 ((0.5) with the lowest concentration of SDS are significantly higher than those obtained at pH 2. This may be attributed to the retention of some hydrolyzed species, which are no longer present at pH 2. Indeed, due to the uncertainty on the value of the natural pH of the unbuffered solution, we cannot completely (33) Treiner, C.; Fromon, M.; Mannebach, M. H. Langmuir 1989, 5, 283. (34) Boumezioud, M.; Tondre, C.; Lagrange, P. Polyhedron 1988, 7, 513. (35) Hebrant, M.; Francois, N.; Tondre, C. Manuscript submitted.
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Tondre et al.
Figure 1. Yield of copper extraction (from ultrafiltration experiments) vs SDS concentration. [Cu2+] ) 10-4 M. Results obtained at pH 2 in the presence of 10-2 M HCl (9) and pH 5 in the presence of 10-2 NaBr (4). The full lines represent the theoretical predictions according to eqs 1-3: left curve assumes cmc ) 1.15 × 10-3 M; right curve assumes cmc ) 5.6 × 10-3 M.
exclude the formation of hydrolysis products when approaching pH 6.36 A very high rejection of copper from aqueous solutions using SDS micelles coupled with ultrafiltration, was already reported before,37 but only SDS concentrations at least five times larger than the cmc were considered. The present data indicate that there is a significant extraction of copper ions at SDS concentrations smaller than the expected cmc. In fact, the precise value of this cmc is not exactly known because of the presence of salts: cmc’s of 5.62 × 10-3 M have been reported for SDS in the presence of 10-2 M NaCl at 21 °C;38 it is likely to have a similar value in the presence of 10-2 M NaBr. On the other hand, the cmc of (DS)2Cu at 30 °C was reported to be 1.2 × 10-3 M39 (this value is very close to our own determination, 1.15 × 10-3 M). We can thus expect the actual cmc to stand between 5.62 × 10-3 and 1.2 × 10-3 M, but much closer to the former value, since the copper concentration is much smaller than the SDS concentration. We have tried to make a theoretical prediction of the yield of extraction on the basis of the ion-exchange model developed by Hafiane et al.17 Kex
2+ + 2+ 2Na+ m + Cuw S 2Naw + Cum
where the subscripts m and w refer to micellar bound ions and free ions, respectively. Neglecting the activity coefficients, the ion-exchange constant can be expressed as
Kex )
2+ 2+ 2+ 2 ([Cu2+ o ] - [Cuw ]) (2[Cuo ] - 2[Cuw ] + cmc) 2+ 2+ 2 [Cu2+ w ] (Cs - cmc - 2[Cuo ] - 2[Cuw ])
(1)
[Cu2+ o ] and Cs are the total copper and surfactant concentrations, respectively. This equation assumes that the copper ions bound to the micelles include the ions both in the Stern layer and in the diffuse layer. It was solved to obtain [Cu2+ w ] when considering the two cmc values indicated above. Kex was given a value deduced from the Ke value (0.42) obtained by Hafiane et al.17 at 0.05 M SDS (we checked that the contribution of the (36) The Hydrolysis of Cations; Baes, C. F., Mesmer, R. E., Eds.; John Wiley & Sons: New York, 1976; p 267. (37) Scamehorn, J. F.; Ellington, R. T.; Christian, S. D.; Penney, B. W.; Dunn, R. O.; Bhat, S. N. AIChE Symp. Ser. 1986, 250, 48. (38) Critical Micelle Concentrations of Aqueous Surfactant Systems; Mukerjee, P., Mysels, K. J., Eds.; National Standard Reference Data Series; National Bureau of Standards: Washington, DC, 1971. (39) Satake, I.; Iwamatsu, I.; Hosokawa, S.; Matuura, R. Bull. Chem. Soc. Jpn. 1963, 36, 204.
Figure 2. Yield of copper extraction (from ultrafiltration experiments) vs Cu2+ concentration. [SDS] ) 10-2 M (*) and [SDS] ) 5 × 10-2 M (4) (T ) 30° C). The solid lines are the smoothed experimental curves.
activity coefficients in the equation used by Hafiane et al. was negligible)
Kex )
Ke 2(Cs - cmc)
(2)
Kex was thus taken equal to 5. From [Cu2+ w ] and the mass balance equation for copper we can obtain the ratio 2+ [Cu2+ w ]/[Cum ], which is directly related to the extraction yield R, defined as
R)
2+ [Cu2+ o ] - [Cuw ]
[Cu2+ o ]
(
) 100/ 1 +
100
)
[Cu2+ w ] [Cu2+ m ]
(3)
The theoretical curves obtained are shown in Figure 1. It is worth noticing that the experimental data points are situated close to the theoretical curve which assumes the cmc of (DS)2Cu, when the SDS concentration is low, and progressively tend toward the second curve when the SDS concentration increases. This behavior is consistent with the fact that the copper concentration becomes negligible comparatively to the surfactant concentration. We have also measured the yield of extraction when the SDS concentration was fixed and the copper concentration was varied. The results obtained at 30 °C for two different SDS concentrations are reported in Figure 2. They show that, as expected, the amount of copper extracted increases with increasing the surfactant concentration: for instance at a copper concentration of 10-2 M, the extraction yields are around 50% and 95% for SDS concentrations 10-2 and 5 × 10-2 M, respectively. Note that the 50% yield obtained when equal concentrations of SDS and Cu2+ are present corresponds to the exchange of all the sodium ions by copper ions, the cmc being almost negligible. The exchange is not as complete when the surfactant concentration is 5 × 10-2 M since the yield is only around 40% when the copper concentration is equal to the surfactant one. This can be attributed to the screening effect of the copper salt itself on the electrostatic potential of the micelles. The method using micelle-solubilized extractants is illustrated in Figure 3. We have used in this case a copper concentration of 3.34 × 10-4 M and the composition of the micellar solution was 0.45% CTAB/0.45% 1-butanol/99.1% water, 0.1 M NaBr with the C11-HQ concentration varying
Micelle-Based Copper Ion Removal
Figure 3. Yield of copper extraction (from ultrafiltration experiments) vs extractant to metal ratio. CTAB 0.45%/1butanol 0.45 %/water (0.1 M NaBr) 99.1%; [Cu2+] ) 3.34 × 10-4 M; [C11-HQ] ) 3.34 to 16.7 × 10-4 M. pH 2.0 (0); 1.5 (O); 0.9 (*); 0.5 (4). The solid lines are the smoothed experimental curves.
from 3.34 × 10-4 to 1.67 × 10-3 (i.e., an extractant to metal ratio (L/M) varying from 1 to 5). Since it was known from previously reported experiments that the release of copper ion from its complex with C11-HQ takes place below pH 2.5,20 the ultrafiltration experiments were carried out at pH values 2.0, 1.5, 0.9, and 0.5. The yields of extraction depend both on the pH and on the L/M ratio. The copper removal is close to 100% at pH 2 when L/M ) 5, but it decreases significantly when the pH is decreased. These results are consistent with those obtained before with 6-[(alkylamino)methyl]-2-(hydroxymethyl)pyridine extractants.19 They further substantiate the possibility of obtaining high metal extraction yields with micellesolubilized extractants provided that the extractant is strongly partitioned in favor of the micellar pseudophase. 2. Electrochemical Behavior of Copper Ions Trapped in Micellar Particles. (a) Reduction of Copper Bound to Anionic Micelles. The relative concentrations of copper and SDS were chosen so as to satisfy two requirements, (i) most of the copper ions should be bound to the micelles, with a negligible concentration of free copper ions, and (ii) the voltammograms should be obtained without addition of electrolyte, since the latter could perturb the copper partitioning between the micelles and the solution. The conditions were similar to those used by Scamehorn et al.37 (see first lines of Table 1). The possibility to obtain voltammograms (Figure 4) without added electrolyte shows clearly that ionic species are present in a high enough concentration. A cathodic signal is recorded, corresponding to the reduction of copper bound to the dodecyl sulfate micelles. This is well demonstrated by the anodic dissolution of copper metal deposited on the platinum electrode. The curves obtained are close to those relative to copper sulfate: the electrochemical system is rapid, its E1/2 being close to the value relative to the Cu2+/ Cu system. The presence of an excess of sodium dodecyl sulfate does not perturb the voltammograms.40,41 A prior (40) Shinozuka, N.; Hayano, S. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum: New York, 1979; Vol. 2, p 599. (41) Besio, G. L.; Prud’homme, R. K.; Benzinger, J. B. Langmuir 1988, 4, 140.
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Figure 4. Voltammograms relative to copper reduction at a platinum electrode in the presence of SDS micelles: copper concentration (a) 0 mM, (b) 4.8 mM, (c) 8.0 mM; SDS concentration, 80 mM. Table 1. Limiting Currents Relative to Cu2+ Reduction at -0.400 V/ECS in the Presence or Absence of Dodecyl Sulfate Micelles and Electrolyte NaNO3 (10-2 M)
SDS (mM)
(Cu2+)0 (mM)a
i (µA)
0 0 0 0 5 5 5 5 10 10 10 10 10 10 10 10
80 80 80 80 80 80 80 80 80 80 80 80 0 0 0 0
0.0 1.2 (2.2 × 10-3) 4.0 (7.1 × 10-3) 8.0 (3.7 × 10-2) 0.0 1.4 4.0 8.0 0.0 1.4 4.0 8.0 0.0 1.2 4.0 8.0
0.3 7.5 23 35 0.3 11 33 50 0.5 22 58 120 0.5 22 58 120
a Values in parentheses refer to the free Cu2+ concentration in mM as reported by Scamehorn et al.37 in similar conditions.
deposit of copper onto the platinum electrode does not change the shape of the cathodic curves, which suggests that the reduction of copper micelles is also possible on copper cathodes. The reversibility of the Cu2+ m /Cu system and the fact that the potential has a usual value are in favor of a great availability of Cu(II) reduction due to Cu2+ m position outside the micelle. This result is similar to that observed by Andriamanampisoa and Mackay31 for the reduction of Cd(DS)2 by polarography. The limiting current increases (Table 1) with the copper ions concentration. If one considers that the SDS concentration is high enough to make negligible the migration effects, it is possible to have an evaluation of the difference between the diffusion coefficients of free copper ions and of copper bound to micelles. For a total concentration of copper of 4 × 10-3 M, the ratio between the limiting ) 2.6 (see Table 1). This value has to currents iCu2+/iCu2+ m be considered with caution because it may depend on the nature and concentration of the electrolyte chosen to determine iCu2+ (0.1 M NaNO3 in the present case). Indeed Andriamanampisoa and Mackay31 have shown that this ratio is equal to 4.4 when comparing the behaviors of pure
1450 Langmuir, Vol. 13, No. 6, 1997
Cd(DS)2 micelles and of free Cd2+ ions in the presence of NaNO3 as electrolyte, but the value relative to the free ion diffusion is very dependent on the nature of the electrolyte used and on its concentration. It can be divided by a factor close to 1.6 when the concentration of NaNO3 varies from 0.05 to 0.10 M. If the same variation is supposed for the diffusion of cupric ion, the ratio iCu2+/iCu2+ becomes 4.1, which is very close to the value m observed for cadmium species. The addition of NaNO3 electrolyte in the mixtures SDS-CuSO4 increases the cathodic current. This is compatible with the Cu2+/Na+ exchange reaction already mentioned. The precise voltammetric evaluation of free copper liberated by the copper ions associated with SDS micelles is dependent on the change of the diffusion coefficient of Cu2+ ions upon NaNO3 addition. The electroreduction of copper(II) bound to SDS micelles in the presence of a large excess of SDS is thus possible and it does not require a large overvoltage compared to the reduction of free Cu2+ ions. The recycling of the surfactant after the removal of metallic ions by micellar extraction can thus be achieved by electroregeneration. We have confirmed the chemical reduction of Cu(DS)2 by sodium borohydride, which was already used by Pileni et al.42 for obtaining monodispersed nanoparticles of copper. At pH 9, NaBH4 4 × 10-2 M reduces CuSO4 10-2 M in the presence of SDS 10-1 M. (b) Reduction of Copper Complexed with a Hydrophobic Extractant in Microemulsions. Whereas the complexation of cuprous ions is supposed to be weak, the strong complexation of cupric ion by Kelex 100 highly reduces its oxidative properties. Contrary to the mercury electrode which is able to explore the low potential range, it is a priori more difficult to detect the reduction of complexed Cu(II) than that of free Cu(II) at a platinum or at a copper electrode. The experiments reported in Figure 5 confirm the previous expectation: for an initial Kelex/Cu(II) ratio ) 2.0 and at pH ) 6.5, no cathodic reduction of Cu(II) is observed contrary to the case of Cu(DS)2 micelles. It is necessary to bring the system to very acidic media (H2SO4, pH ) 1.0) to be able to detect such a reduction. In that case the complex is destroyed and the copper ion can be released from the microemulsion particle, as is confirmed by the permeation experiments reported in the first part. The voltammograms are weakly reproducible, and the copper(II) reduction is only detected (42) Pileni, M. P.; Petit, C. J. Phys. Chem. 1988, 92, 2282.
Tondre et al.
Figure 5. Voltammograms relative to copper/Kelex 100 complex in CTAB/butanol/water microemulsions: (a) pH ) 6.5; (b) pH ) 1.0.
by the anodic dissolution of copper deposit and not by its cathodic wave. It can be recalled that complexation of Cu(II) by quinoline in the same kind of microemulsion can be detected with a mercury electrode because of the low stability of the complex and of the large overvoltage of proton reduction.32 Conclusion The two ways of extracting copper ions in micellar particles (namely, association through ion-exchange and complexation with micelle-solubilized hydrophobic extractants) have been examined from the points of view of ultrafiltration removal and of electrochemical reduction of the trapped metal ions. New data have been reported concerning the yields of copper extracted by ultrafiltration in both processes, varying conditions such as concentrations of species involved and pH. In addition we have shown that the electroreduction of copper ions associated to SDS micelles takes place easily, which can be explained by the surface location of the metal ions. The recycling of the micellar extracting phase through electrolysis is clearly possible in that case. On the other hand the stability of Cu2+-Kelex complexes is too high to recover copper metal by electrolysis from CTAB/ butanol/water microemulsions. LA960730D
