Langmuir 1993,9, 95Q-955
Micellar Extraction: Removal of Copper(I1) by Micelle-SolubilizedComplexing Agents of Varying HLB Using U1trafiltration C. Tondre,' S.-G.Son, and M. Hebrant Laboratoire d'Etude des Syst8mes Organiques et Collozdaux (L.E.S.O.C.), Unit&Associke au CNRS Y o . 406, Universitk de Nancy I, B.P. 239, 54506 Vandoeuvre-18s-Nancy Cedex, France
P. Scrimin and P. Tecilla Dipartimento di Chimica Organica and Centro CNR, Meccanismi di Reazioni Organiche, Universita di Padova, Via Marzolo 1, 35131 Padova, Italy Received August 5, 1992. In Final Form: December 24,1992 The hydrophobic core of micellar systems can solubilize hydrophobic extractants in a way similar to the organic solventin a classicalsolventextraction. In this paper it is demonstrated that micellar extraction can be as effective as solvent extraction and shows the same characteristicfeatures (effect of pH, effect of the hydrophilic/lipophilic balance (HLB) of the extractant, extractant concentration, etc.). A series of 6-[(alkylamino)methyl]-2-(hydroxymethyl)pyridines (CnNHMePyr with n = 4,8,10,12,14,and 16) are used for this purpose in combination with CTAB and ClzEOs micelles. The removal of copper(I1) is achieved by ultrafiltration. A simplemodel is developed on the basisof equilibriumrelationsto theoretically predict the yield of copper extraction.
Introduction In the past few years there has been a growing interest for the potentialities of micellar and microemulsion systems in the field of metal extraction and separation. This interest has concerned different aspects of this problem. In the first place it has been recognized that microemulsions form spontaneously in the organic phase of some solvent extraction systems,l with a correlative improvement of extraction r a t e ~ . ~This 3 kind of observation, added to the fact that (i) the amphiphilic membranes of microemulsion droplets can bind metal ions4 and (ii) reversed micelles have interesting solubilizing properties: has induced a certain amount of work dealing with the possible advantages of using a microemulsion in place of the classical organic phase for the purpose of practical metal ion extraction.61s A second domain where active research has been conducted concerns the use of micellar systems as model systems to investigate the kinetics and mechanisms of reactions usually occurring in biphasic media.leZ0 This approach rests on the analogy existing between the (1) Uauer, D.; Fourre, P.; Lemerle, J. C. R.Acad. Sci. Ser. 2 1981,292, 1019. (2) Fourre, P.; Bauer, D. C. R. Acad. Sci. Ser. 2 1981,292, 1077. (3) Bauer, D.; Komomicki, J. Proc. Int. Solvent Extr. Conf. 1983,315. (4) Ovejero-hudero, F. J.;Angelino, H.; Casamatta, G. J. Dispersion Sci. Technol. 1987, 8, 89. (5) Ossao-Asare, K.; Keeney, M. E. Sep. Sci. Technol. 1980,15,999. (6) Vijayalakehmi, C. S.; Annapragada, A. V.; Gulari, E. Sep. Sci. Technol. 1990,25, 711. (7) Savastano, C. A.; Ortiz, E.S. Chem. Eng. Sci. 1991, 46, 741. (8) Paatsro, E.; Sj6blom, J. Hydrometallurgy, 1990,25, 231. (9) Paatero,E.; Sjtiblom, J.; Datta, S. J. Colloid Interface Sci. 1990, 138,388. (10) Oseeo-Asare, K.; Zheng, Y. Colloids Surf. 1991,53, 339. (11) Neuman, R. D.; Jones, M. A,; Wou, N.F. Colloids Surf. 1990,46, 45. (12) Tondre, C.; Xenakis, A. Faraday Discuss. Chem. SOC.1984, 77, 115. (13) Derouiche, A.; Tondre, C. Colloids Surf. 1990, 48,243. (14) Tondre, C.; Boumezioud, M. J. Phys. Chem. 1989,93, 846. (15) Miyake,Y.;Naketa,Y.;Suzuki,T.;Teramoto, M.Proc.Int.Soluent Extr. Conf. 1992, 823. (16) Boumezioud, M.; Kim, H. S.;Tondre, C. Colloids Surf. 1989,41, 255.
hydrophobiccore of micelles and solventa of hydrocarbon type. Both can solubilize strongly hydrophobic extractanta, but contrary to macroscopic organic/water phases under stirring conditions, the microscopic micellar pseudophases are perfectly transparent. In recent p u b lications from our laboratories this property was used to investigate by stopped-flow kinetics the "extraction" of copper(I1) by micelle-solubilized complexing agents of varying HLB and to examine the conditionsin which bulk or interfacial complexation reactions are likely to prevail.18319 Finally there is a third applied aspect which is closely related to the previous point. This refers to the use of micellar systems for practical separation of metal ions or for complete removal of undesirable metal ions in water Such operations imply some sort of phase separation, in which the micellar pseudophase can be physically separated from the aqueouspseudophase. This can be achieved by ultrafiltration, using semipermeable membranes with low molecular weight cut off. In particular circumstancesa selective separation of metal ions has been shown to be possible provided that the time parameter is con~idered~~ (kinetic separation). The present paper is concerned with this third aspect, which can be defined as "micellar extraction". It is a direct application of the results described in previouslypubliahed It is intended to demonstrate that extraction ~
(17) Muralidharan, S.; Yu, W.; Tagashira, 5.;Freiser, H. Langmuir 1990,6, 1190.
(18) Son, S.-G.; Hebrant, M.; Tecilla, P.; Scrimin, P.; Tondre, C. J. Phys. Chem. 1992, W,11072. (19) Tondre, C.; Hebrant, M. J . Phys. Chem. 1992, W,11079. (20) Miyake, Y.; Yamada, M.; Kikuchi, T.; Teramoto, M. J. Chem. Soc., Faraday Trans. 1992,88, 1285. (21) Kim, H. S.; Tondre, C. Sep. Sci. Technol. 1989,24, 485. (22) Scamehom,J. F.;Ellington, R. T.;Christian, S. D.; Penney, B. W.; Dunn,R. 0.; Bhat, 5.N. Recent Advances in Separation Techniques 111, no 250 1986, 82, 48. (23)Christian,S.D.;Tucker,E.E.;Scamehom,J.F.;Lee,B.-H.;Saeaki, K. J. Longmuir 1989,6876. Dunn, R. 0.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. 1989, 35,49. (24) Pramauro, E.;Bianco, A,;Barni, E.;V i d i , G.; Hinze, W. Colloids Surf. 1992, 63, 291. (25) Ismael, M.; Tondre, C. Langmuir 1992,8, 1039.
QN3-7463/93/ 2409-Q95Q$Q4.QQ/Q0 1993 American Chemical Society
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Micellar Extraction
by micelles ie very similar to liquid-liquid extraction in the aeme that the yield of metal recovered is affected by the Bame parameters (HLB of the extractant, pH, extractant concentration). These new techniques of metal ion removal may prove useful in the future as they completely avoid the use of an organic solvent, which is required in classical biphasic extraction. A process involvinga 99% aqueous solution is indeed quite attractive from the environmental point of view.
t i
/
Experimental Section Chemicals. The synthesis of 6-[(alkylamino)methyl]-2(hydroxymethy1)pyridines(C,NHMePyr with n = 4,8,10,12, 14,and 16)has been reported in previous publications.26-28 Their purity was checked by 'H NMR in CDC13and elemental analysis. Cetyltrimethylammoniumbromide (CTAB)was purchased from Fluka and hexaethylene glycol n-dodecyl ether (C12EOs) was obtained from Nikko Chemicals (Japan). The former one was twice recrystallized in methanoVdiethy1ether. Copper chloride (CuC12.2H20) was from Prolabo (France). The pH of the solution was adjusted with drops of diluted HC1or NaOH (MerckTitrisol). The measure of pH in micellar media was performed with a combined glass electrode, following recommendations given in previous works.28,30 Technique. Ultrafiltration was performed with an Amicon stirred cell of 10-mL volume at room temperature. We used cellulosic diek membranes (Millipore) with nominal molecular weight limit 10 OOO and the pressure applied was 3.5 to 4 bar. The mixing of equal volumes of solutions A and B was performed directly in the ultrafiltration cell, before fixing the cap (solution A was a micellar solution containing the metal ions, and solution B was a similar micellar solution containingthe extractant, except for blank experiments). According to a previously published work the complexation is quite fast.'* We waited usually 5-10 min before starting the filtration and we checked that the result was not depending on the time alloted. The filtration was stopped when half of the initial volume (either 10 or 5 mL depending on the amount of products available) was collected. Atomic absorption measurements were carried out in order to analyze the metal content of the filtrate. The apparatus was a Varian AA-1275 atomic absorption spectrophotometer. The copper concentration found in the filtrate was assumed to include both the free metal ions and the free complexes (i.e. the part of the copper complexes not partitioned into the micellar pseudophase). The quantity of metal ion extracted was obtained by subtraction of the copper concentration in the filtrate from the analytical initial concentration. This assumes that the concentration of free species is the same in the filtrate and in the retentate.
Results and Discussion In these experimentswe have considered only two kinds of micellar systems in which the extractant moleculeshave been solubilized cationic micelles made of CTAB and nonionic micelles made of C12EO6. Anionic micelles, like those obtained with SDS,were avoided in this study because they can bind by themselves to metal ions. This maybe consideredas an advantagein specificapplications, but for the purpose of the present work it would have resulted in a more complicated analysis of the data, implying (i) to be able to distinguish between the metal ions bound to the extractant molecules and those bound to the micellea and (ii) to take into account the exchange equilibriumbetween the sodiumcounterionsof the micelles and the metal ions to be extracted. (26) Scrimin, P.; Tecilla, P.; Tonellato, U. J . Org. Chem. 1991,56,161. (27) Tondre, C.;Claude-Montigny,B.; Ismael, M.; Scrimin,P.;Tecilla, P.Polyhedron 1991, 10, 1791.
(28) Hebrant, M.; Son, S.-C.;Tondre, C.; Tecilla, P.; Scrimin, P. J. Chem. SOC.,Faraday Trans. 1992,88,209. (29) Berthod, A.; Saljba, C. Analusis 1986,13,437; 1986, 14,44. (30)Bahri, H.; Letelher, P. J . Chim. Phya. Phya. Chim. Biol. 1985,82, 1009.
0 0
1
2
3
4
5
8
tL1, / tCl?+I, Figure 1. Yield of copper(I1) extraction R versus the ligand to metal ratio for CuNHMePyr in CTAB micelles at pH 2.4 (A), 3.5 (*), and 5.0 (0):[CU*+]O = lo-" M; [CTAB] 2.5 X 1 0 - 2 M.
In the followingfigures, the yield of micellar extraction
R has been defined as R=
[CU2+IO- [Cu2+1a1 [CU2+l0
where [Cu2+]0and [Cu2+1fi1represent respectively the initial concentration of copper and ita total concentration in the fiitrate (in free or complexed form). A first set of experiments was performed with the more hydrophobic extractant of the series at our disposal, c16' NHMePyr. In Figure 1we have represented the variations of R versus the ratio [Lld[Cu2+I0at three different pH values ([Ll, is the analytical concentration of the extractant). At pH 5.0, as soon as we have a 2-fold excess of extractant molecules over the metal ions, the yield of micellar extraction is larger than 90% and becomes very close to 100% when the concentration of extractant is furthermore increased. When the pH is decreased to 3.5, a drastic decrease of R is observed, but the extraction yield is still significant at [Lld[Cu2+10= 5. At pH 2.4 there is practically no extractionat all. These observations are consistent with our previous stopped-flow studies27128 in which we found the largest absorbance change due to complexationat pH 2 4. The rate of complexationbecame smaller at pH 3.5, where the amino group of the extractant molecule is expected to be totally protonated, but complexation was still possible. This was no longer the case at even more acidic pH values, where the protonation of the pyridine nitrogen is taking place. Figure 2 represents the same kind of results in nonionic C12E06 micelles. In this case, the extraction remains significant at p H 3.5 and it is far from being negligible at pH 2.3, attaining a yield of 50% at [Lld[Cu2+1,= 5. This difference of behavior between the two surfactants cannot come from the effect of the local pH existing in the immediate vicinity of the micelles, which is known to be ~~ effect different from the measured 0 1 1 8 . ~A~ ~reverse would be expected in that case, since the local pH is more basic for CTAB micelles than for C12E06 micelles (the (31) Berthod, A.; Georges, J. N o w . J. Chim. 1986, 9, 101. (32) Mackay, R. A.; Jacobson, K.; Tourian, J. J . Colloid Interface Sei. 1980, 76, 515.
Tondre et al.
952 Langmuir, Vol. 9, No. 4, 1993
t 4
i
/
75
I c
11 t
-
P
0
0
1
2
3
4
6
tL1, / t % 6 I 0 Figure 2. Same legend as in Figure 1 in CI2EO6micelles at pH 2.3 (A), 3.5 (*), and 5.7 ( 0 ) : [cuz+]o = 1O-l M, [C&06] = 2 X
M.
positively charged surface of the former is concentrating OH- ions rather than H+ ions, which are repelled from the micellar surface). The change of pK of the extractants due to this effect was shown to be of 1.75 pH units, from partitioning measurements and thermodynamic considerations.33 In fact, the local pH effect on the ionization state of the extractant is, in that case, overcompensated by the effect of partitioning of the metallextractant complex: the association constant of the latter to the micelles, &cU, is 1 to 2 orders of magnitude larger for the C12EO6 micelles than for CTAB micelles.lg In Figure 3, we have fixed the [Lld[Cu2+10ratio equal to 1 and we have studied the effect of the alkyl chain length of the extractant C,NHMePp, at different pH values and varying the nature of the surfactant. As could be expected from Figure 1,the extraction yield remains quite low with CTAB at pH 3.6 whatever the value of n, although a slight increase is observed between n = 4 and n = 14. The effect of the value of n is much more spectacular for CTAB at pH 5.0 and for CIPEO~ whatever the pH. In these cases the value of R is continuously increasing when n increases from 4 to 16, with no real saturation for the larger values. This last point may be taken as an indication that the extraction yield could still be improved by increasing furthermore the alkyl chain length of the extractant. Note that the experimentalpoints shown in Figure 3 at n = 0 do not refer to the extractant with no alkyl chain but to the situation in the absence of extractant. The values obtained have prompted us to perform more systematic blank experiments,including in the absence of micelles. The results of blank experiments are given in Figure 4. Theyindicatethatanonnegligible'extraction" (upto 10%) can be measured in some instances, even in pure water solutions. For this reason it does not seem to be due to binding of the metal ions to the micelles themselves. In fact this secondary effect appears to be more important when the pH becomes more basic. This suggests that it could be related with the interaction of the filtration membrane with the hydrolyzed species of copper.34 The membrane could retain microscopic precipitates that (33) Hebrant, M.; Tondre, C. J . Colloid Interface Sci. 1992, 254,378.
8 10 12 14 16
4
0
n
Figure 3. Yield of copper(I1) extraction R versus number of carbon atoms n of the alkyl chain in C,NHMePyr. Ligand to M): pH 3.5 (0);pH 5.0 (A). metal ratio = 1. CTAB (2.5 X ClzEO,3(2 X M): pH 3.5 (0);pH 5-6 (A). The experimental points on the Y-axis (n = 0) refer to the situation in the absence of extractant and not to the extractant with no alkyl chain.
i
15 -
t
5 : 0
i"
m
t ' * . oL----
i
i
c
0
1
2
3
4
!
"
5
6 PH
Figure 4. Blank experiments in the absence of extractant; yield of copper(I1) 'extracted" versus pH in different media: water micellar solution of CTAB (2.5 X M) with no surfactant (0); (*); micellar solution of C12E06(2 X 10-2 M) (m).
cannot be seen with the naked eye. These blank experiments explain why the origin of the extraction curves is sometimes different from zero, expecially at pH 2 5. This explains in turn why R values larger than 50% have been obtained in some instances even though the extractant to metal ratio was equal to 0.5. The results of a more exhaustive investigation of the influence of the extractant hydrophilic/hydrophobiccharacter are shown in Figures 5 and 6 for CTAB and C12E06, respectively. The values of R have been measured at pH 5 in function of the ratio [L]d[Cu2+10for the homologous series of extractant molecules C,NHMePp, with n being (34)Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976.
Langmuir, Vol. 9,No.4, 1993 953
Micellar Extraction
R (%I
We will attempt in the following to give a quantitative treatment able to predict the yield of extractionby micellar systems. For this purpose we will make use of the information obtained from preceding w0rks,~8J9*33 especially as regards the partitioning of the different species between the micellar and aqueous pseudophases. We will only consider 1:l complex formation although it is clear that higher complexes may form, particularly when [L], is in significant excess comparatively to [Cu2+1o. The validity of this assumption, that we were led to use in a first approach, will be dicussed at a later stage. Within this simplifying assumption the following equilibria have to be consideredlg
100
76
50
25
L,+
cu:
= Lcu:
L,+
cu,2+
=LCU,2*
0
kim
ki
k-1
0
1
2
3
4
5
6
tu0 / C U I0 Figure 5. Extraction in CTAB micelles, yield of copper(I1) extraction R versus the ligand to metal ratio for extractante with varying alkyl chain length n = 4 (*); n = 8 (m); n = 10 (A);n = 12 (0);n = 14 (0);n = 16 (0). [CTAB] = 2.5 X M [Cu2+]0 = 10-4 M;PH = 5.0.
R ($1 100
11
76
where L and LH+refer to the neutral and monoprotonated extractant, respectively, LCu2+to the complex, and the subscripts w and m to the aqueous and micellar pseudophases. Most of the equilibrium constants have been experimentally determined in previous works. Considering that the amount of copper extracted is equivalent to the amount of complex solubilized in the micelles, it follows that
50
[Lcum2+l 100 [CU2+1, in which the concentration of the micellized complex LCum2+ is
R=
- 0 0
1
2
3
4
tu,/
6
8
r&Io
Figure 6. Same legend as in Figure 5 for extraction in M. micelles. [C12EOBI= 2 X equal to 4, 8, 10, 12, 14, and 16. As already discussed
before, for one particular value of n, the extraction is stronger in &E06 than in CTAB. On the other hand, the effect of varying the value of n is extremelysensitive. This is clearly related to the displacement of the partitioning equilibrum of the extractant/copper complex more and more in favor of the micellar pseudophase,when it becomes more hydrophobic.l8J9.33. The shape of the preceding curves is tremendously resembling that obtained in a classical liquid-liquid extraction when similar conditionsare con~idered.~~ This observation strongly supporta the idea that "micellar extraction" can potentially be developed in a way similar to "solvent extraction". (35) Rcdehuser, L.; Rubini, P. R.; Bokolo, K.; Laakel, N.; Delpuech, G. J. Soluent Extr. Ion Exch. 1992, IO, 559.
[LCum2+l= KLc,[LCs2+lC (2) where C is the concentration of micellized surfactant and KLC,the binding constant of the complex to the micelles. Solving the mass balance equations [Ll, = [L,]
+ [LH;l+
[LH2l:
+ [L,l+
[LHm+l+ [LCyl2+1+ [Lcum2+l(3)
[CU2+IO= [Ck2+1+ tCU,2+1+ [LCLl$&?+I+ [LCum2+1(4) in addition to the equilibrium equations, it can be readily is the solution of a quadratic demonstrated that [LCuW2+] equation [LCI&Z+l = with
-B - (B2- 4AC)"' 2A
(5)
Tondre et al.
954 Langmuir, Vol. 9,No. 4, 1993
I i t
tl / 25
ti / 0
0
1
2
3
4
5
Figure7. Theoreticalsimulationsof R versus the ligand to metal ratio, according to eqs 1 and 5-9 effect of the alkyl chain length of the extractant in ClzEOs micellesat pH 5. Values of parameters ~ , KLH,K1: introduced in the calculation^^^^^^ in the order K L CKL, (n = 8) 5.4,299,13.2,1.26X los; ( n = 10) 100,6.31 X 103,80,1.26 X lo8;(n= 12) 1.59 X lo3,2.2 X lo4,620,1.26 X 108, (n = 14) 3.2 x 104,i.6 x 105,5 x i03,1.26 x 106;(n = 16) 5.6 x 105,i.i x 106, 3.5 x 104,1.26 x 108.
(6)
c = Kl[Llo[CU2+10
(8) where K1 is the stability constant of the complex in water (K1= [LCuw2+1/[Lw1[Cuw2+l; KC,,KL,and KLHare the associationconstants to the micelle in the form Kx = [X,1/ KWIC, and
+
a = 1 Hw+/Kalw + Hw+2/KalxJ(a2w
2
3
4
5
8
Figure 8. Theoreticalsimulationsof R versus the ligand to metal ratio, according to eqs 1 and 5-9: effect of pH for C16NHMePyr in ClzE06micelles. See legend of Figure 7 for the values introduced in the calculations.
8
A = Kl(l + KLCuO2
1
(9)
where Kalw and Kazware respectively the first and the second ionization constants of the extractanta in water. As reported before,28these constants are expected to be practically independent of the length of the alkyl chain. This approach is neglecting the fact that the filtration step is not instantaneous. In fact it takes a few minutes during which the micellar solution is concentrated and a shift in the equilibria involved can occur. In spite of this, the simplemodel developedhere is perfectly able to predict both the dependence of R with the hydrophobicity of the extractant and the effect of pH. This is illustrated in Figures 7 and 8 in which we have used previously published values of the different parameters needed to make computer calculations of expression 1, using eqs 5-9. The qualitative agreement between experiment (Figure6) and theory (Figure 7) is quite good, at least for the more hydrophobic extractants in the series. This particular point deserves to be discussed in relation with the fact that we have only considered 1:l complex formation. This
assumption may be correct, when the complex is strongly attached to the micelles, due to steric hindrances. Nevertheless it may be incorrect for short chain extractant because they are less imbedded in the amphiphilepalisade of the micelles and thus they are moving more freely. This could explain why there is a larger discrepancy between experiment and theory when n I 10. A further refinement of the theory, taking into account extractankmetal complexes of higher stoichiometry,will be useful in this respect, but it will also mean the introduction of new unknown quantities in the calculations. Finally, we recall that both the way we have calculated the yields of micellar extraction from the experimental data and the theoretical modeling rest on the assumption that the concentrations of free species are equal in the fiitrate and in the retentate. This assumption is absolutely safe for the case of the nonionic surfactant but it is somewhat in contradistinction with results of micellarenhanced ultrafiltration previously reported in the literature when cationicsurfactants are concerned. According to Klepac et al.,36 an ion-expulsion effect can be observed due to the fact that the positively charged micelle rejects the positively charged metal ions. This effect which is well-known in polyelectrolyte dialysis as the Donnan equilibrium effect, was also reported in the case of semiequilibrium dialysis experiments involving micellar systems.37 Would it occur in the present case, the concentration of copper would be higher in the permeate than in the retentate when no extractant moleculeis added to the micellar solution. With our approach this would have led in some instances to obtaining negative values of R, which we never observed. Such an effect is also definitely rejected by the results of the blank experiments reported in Figure 4. The difference between the observations of Klepac et al. and ours may be found in the range of concentrations used for both the surfactant and the metal ions. We can say, in conclusion, that micellar extraction is a very effective process presenting the same characteristic (36)Klepac, J.; Simmons, D. L.; Taylor, R. W.; Scamehom, J. F.; Christian, S. D. Sep. Sci. Technol. 1991, 26, 165. (37) Dhannawardana, U. R.; Christian,S. D.;Taylor, R. W.;Scamehorn, J. F.Langmuir 1992,8, 414.
Micellar Extraction
features as solvent extraction. The theoretical prediction of copper removal, based on previous equilibrium dialysis measurements33as well as stapped-flow kinetic measurements,'*J9is in quite good agreementwith the experimental observations. This demonstrates a large consistency
Langmuir, Vol. 9, No.4, 1993 955
between data obtained from so many different techniques. The results reported here are expected to encourage new research in the field of micellar extraction, which may prove very promising for the developmentof new processes avoiding the use of organic solvents.
