3994
Langmuir 1994,10, 3994-4000
Complexation Kinetics and Ultrafiltration Removal of Nickel(11) by Long-chain Alkoxypicolinic Acids and Alkoxypyridine Aldoximes in Micellar Media M. Hebrant, A. Bouraine, and C. Tondre* Laboratoire $Etude des Syst2mes Organiques et Colloidaux (LESOC), Unit6 Associte au CNRS no. 406, Universit6 de Nancy I, B.P. 239, 54506 Vandoeuvre-12s-Nancy Cedex, France
A. Brembilla and P. Lochon Laboratoire de Chimie-Physique MacromolCculaire (LCPM), Unit6 Associde au CNRS no. 494, ENSIC-INPL, BP 451, 1 rue Grandville, 54001 Nancy Cedex, France Received March 14, 1994. In Final Form: August 1, 1994@ In view of the development of new surfactant-based extraction processes avoiding the use of organic solvent, we have investigated the kinetics of complex formation between Ni2+ ions and long-chain 5-alkoxypicolinic acids (C,-PIC, with n = 12,15,18)and 5-alkoxypyridine aldoximes (C,-PAX, with n = 12,15)solubilized in different types of micelles. Stopped-flow kinetic experiments were performed in cetyltrimethylammoniumbromide (CTAB),hexaethyleneglycol dodecyl ether (C12E06),and CTAB/c1806 mixed micelles. For C,-PAX derivativesat neutral pH, the apparent rate constants for complex formation are some 2 orders of magnitude lower than for the low molecular weight analogue, pyridine aldoxime (PAX),with dividing factors of 100 and 350 in C12E06 and CTAB, respectively. In the case of CTAB, a strong effect of added ionic strength was observed, and its origin is discussed. Concerning C,-PIC, the strong ion-pairing interaction between the carboxylic function of the extractant and the polar head of CTAB molecules introduces a new feature responsible for a specific behavior: the apparent rate constant for complex formation is 2-3 orders of magnitude lower in CTAB micelles than in ClzEOs micelles. The rate in mixed micelles is extremely sensitive to the value of the mole fraction of each surfactant. This observation is discussed in relation to the variation of the electrostatic potential at the surface of mixed micelles, and a theoretical prediction of the variation of the observed rate constant is attempted. Finally, the practical removal of Ni2+ions was investigated using ultrafiltration. The yield of extraction at pH 7 is shown to be as high as 95-98% in CTAB micelles when the ligand to metal ratio (LIM)is high enough. For low LIMvalues the extractionyield increases with addition of C12EO6 and with increasing alkyl chain length of the extractant.
Introduction The field of surfactant-based separation processes has experienced constantly increasing importance during the past few years.l One of the promising applications in this domain concerns the removal of metal ions by ultrafiltration coupled with micellar e x t r a ~ t i o n . ~ The - l ~ trapping of metal ions in the micellar pseudophase can be achieved in two different ways: (i)exchange ofmultivalent metal ions with the monovalent counterions of anionic Abstract published inAdvance ACSAbstracts, October 1,1994. (1)Surfactant-Based Separation Processes;Surfactant Science Series, Vol. 33; Scamehorn, J. F., Harwell, J. H. Eds.; Marcel Dekker: New York, 1989. (2) Scamehom, J. F.; Christian, S. D.; Ellington, R. T. In ref 1, p 29. (3) Dunn, R. 0.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. 1989, 35, 49. (4) (a) Klepac, J.; Simmons, D. L.; Taylor, R. W.; Scamehorn, J. F.; Christian, S.D. Sep. Sci. Technol. 1991,26,165. (b)Dharmawardana, U.R.; Christian, S.D.; Taylor, R. W.; Scamehorn, J. F. Langmuir 1992, 8, 414. (5)Hafiane, A.;Issid, I.; Lemordant, D. J . Colloid Interface Sci. 1991, 142, 167. (6) Pramauro, E.; Prevot, A. B.; Pelizzetti, E.; Marchelli, R.; Dossena, A.; Biancardi, A. Anal. Chim. Acta 1992,264, 303. (7) Pramauro, E.; Bianca, A,; Barni, E.; Viscardi, G.; Hinze, W. L. Colloids Surf. 1992, 63, 291. (8)Lemordant, D.; Letellier, P.; Rumeau, M.; Soma, C. Patent Fr. 2,619,727. (9) Reiller, P. Thesis University of Paris VI - Pierre et Marie Curie, 1993. (10) Reiller, P.; Lemordant, D.; Moulin, C.; Beaucaire, C. J . Colloid Interface Sci. 1994,163, 81. (11)Ismael, M.; Tondre, C. Langmuir 1992, 8, 1039. (12)Tondre, C.; Son, S.-G.; Hebrant, M.; Scrimin, P.; Tecilla, P. Langmuir 1993, 9, 950. (13) Ismael, M.; Tondre, C. J . Colloid Interface Sci. 1993,160,252. (14) Ismael, M.; Tondre, C. Sep. Sci. Technol. 1994, 29, 651. @
m i ~ e l l e s ~ *and ~ , ~(ii) J ~complexation of metal ions by lipophiliccomplexing agents solubilizedin the hydrophobic core of the m i c e l l e ~ . ~ , ~ !In ' J ~the - ~first ~ case the extent ofbinding ofthe metal ions is controlled by the equilibrium relations governing ion exchange. In the second case it will depend on the stability constant of the metal/ extractant complex in a strong similarity with classical solvent extra~ti0n.l~ We are mainly interested in the latter situation, which is more favorable for selective removal of some specificmetal ions.11J3J4 From this point ofview, an infinite number of different chemical structures can be consideredfor the complexing agent accordingto specific goals: the hydrophobic part may be aromatic, aliphatic, linear, or branched, etc.; the complexing site may have different geometries and may involve different kinds of heteroatoms, ionizable or nonionizable groups, etc. Different successful attempts to achieve metal ion removal using this kind of process have been reported. They concern,for instance, the selectiveretention of copper in mixtures of copper and calcium salts? the extraction of ir0n(II1),~ uranium,6~ o p p e r ( 1 I ) , ~ ~~ ~o Jb~a Jl t~(JI ~I ) , ~ ~ J ~ J ~ and n i ~ k e l ( I I ) . ~ ' JMost ~ J ~of these extractionswere usually performed in equilibrium conditions, but in a few cases, selective separations on the same principle were based on kinetic criteria. This was clearly demonstrated for the separation of nickel(I1) and cobalt(I1)mixtures, in which the much slower rate of complexation of the Ni2+ions was exploited to selectively remove the Co2+ions.11J3 In this case one has to carefully control the time alloted between (15) Szymanowski,J.;Tondre, C. Soh. Extr. Ion Exch. 1994,12,873.
0743-746319412410-3994$04.50/0 0 1994 American Chemical Society
Ni(II) Removal in Micellar Media
Langmuir, Vol. 10, No. 11, 1994 3995
the addition of the metal ions to the extracting phase and the start of the ultrafiltration. The same kind of kinetic separation between copper(I1) and cobalt(I1) has been attempted with addition of small amounts of octane to the CTABhutanol micelles, in order to fbrther decrease the rate of complexation.ls The slower reacting species is in this case the cobalt ion, for which a significant enrichment in the filtrate was obtained. In order to improve these separation techniques on the basis of the rate at which the formation of the metal complexes proceeds, it is important that the reaction be as slow as possible. So far, only two different kinds of complexing agents have been considered for this purpose: and an alkylated derivative of hydroxyquin01ine~~J~J~J~ a series of 6-[(alkylamino)methyl]-2-(hydroxymethyl)pyridines with variable chain l e n g t h ~ . ~ ~ The J ~ Jpresent * work was aimed at testing new complexing agents having specific properties. These compounds are 5-alkoxypicolinic acids, which will be abbreviated C,PIC (formula I), and 5-alkoxypyridine aldoximes, which will be abbreviated C,PAX (formula 11):
N
'OH
(For compounds I1the E and Z isomers can be consideredl9 but only the E isomers were found to have good complexing abilities J In case of compounds I, we expect that, in cationic micelles like micelles of cetyltrimethylammonium bromide (CTAB), there will be a strong electrostatic interaction between the head group of the surfactant and the carboxylate function. This ion-pair formation should be further stabilized by the additional hydrophobic interactions, making metal complex formation more difficult than with neutral or negatively charged surfactants. This could lead to the low rate of complex formation we are looking for, since the extractant will be retained at the surface of the micelles. The second type of compounds is expected to be neutral in a larger range of pH than the previously studied complexing agents. This may be advantageous in comparison to hydroxyquinoline derivatives for instance, for which the anionic forms could never be totally neglected in the treatments of the data. In addition,both compounds are able to form complexes with Nia+ions, which can be considered as slow-reacting metal ions. Stopped-flow kinetic measurements are performed in cationic, nonionic, and mixed cationidnonionic micelles. Some ultrafiltration experiments demonstrate the effectiveness of the removal of nickel ions by such systems.
Experimental Part Chemicals. The synthesis of long-chain 5-alkoxypicolinic acids (Clz-PIC, Cia-PIC, and Cls-PIC) has been reported elsewhere.20 Concerning the preparation of S-alkoxypyridine aldoximes (C12-PAX and ClE-PAX), the synthesis was carried out (16)Tondre, C.;Boumezioud, M. J. Phys. Chem. 1989,93,846. (17)Son, 5.-G.;Hebrant, M.; Tecilla, P.; Scrimin, P.; Tondre, C. J. Phys. Chem. 1992,96,11072. (18)Tondre, C.;Hebrant, M.J. Phys. Chem. 1992,96,11079. (19)Ogino, K.;Shindo, K.; M i n d , T.; Tagaki, W.; Eiki, T. Bull. Chem. Soc. Jpn. IS=, 66,1101. (20)Faivre,V.;Roizard, D.;Brembilla, A,;Lochon,P. Bull. Soc. Chim. Fr. 1991,128,278.
as follows. A mixture of 3 x mol of aldehyde,z04.2 x 10-8 molofhydroxylaminehydrochloride,and 1.5 x 10-smolofbarium carbonate in 7 mL of methanol was heated at 100 "C for 12 h. Upon cooling, the solvent was removed and water was added to the residue. The product was extracted with diethyl ether. E and Z isomers were separated by chromatography on silica gel with a mixture chloroform-ethyl acetate (90/10 v/v) as eluent. For ClaPAX, melting points were, respectively, 113 and 91 "C for E and Z isomers. The low molecular weight analogues picolinic acid (PIC)and pyridine aldoxime (PAX)were obtained from Kodak and Aldrich, respectively. The purity of the long-chain compounds was checked by 1H NMR in CDCls. The low molecular weight analogues were used as received. Cetyltrimethylammonium bromide (CTAB) was purchased from Fluka, and hexaethylene glycol n-dodecyl ether (C12EO6)was obtained from Nikko Chemicals (Japan). CTAB was twice recrystallized from methanoVdiethy1ether. Nickel chloride (NiCla,GHzO)was from Fluka, and calcium chloride (CaClz,2H20) and triethanolamine (TEA) were from Merck. Techniques. The stopped-flow kinetic experiments were performed at 26 "C with a Durrum D-110 apparatus. The kinetic curves were recorded on a Gould 1602 storage oscilloscope interfaced with a Victor V286 PC. In a typical experiment, two identical micellar solutions were mixed, one of them containing the metal ions and the other one containing the complexingagent. Alarge excess ofthe metal ions over the extractant concentration M) was used. The final pH was systematically controlled on an aliquot sample collected directly afbr the solutions were mixed in the stopped-flow. All the concentrations refer to the overall analytical concentrations upon mixing. The observed rate constants (8-l)were computed from least squares fitting of the time-dependent absorbance change with exponential functions. It is thus equivalent to the reciprocal relaxation time 1/r characterizing the exponential change of absorbance. In most cases the kinetic curves were purely first order, but in some particular pH conditions a better least squares fitting was obtained by considering a biexponential function. The ultrafiltration experiments were carried out with an Amicon stirred cell of volume 10 mL, at room temperature. Millipore cellulosic disk membranes were used, with a molecular weight cutoff 10 000 Da, and the pressure applied was 3.6bar. Details of the procedures used can be found in a previous publication.la The metal content of the filtrate was analyzed by atomic absorption spedrometry. The apparatus was aVarian AA-1275. The quantity of metal ion extracted (extraction yield Y) was determined by assuming that the concentration of free species (Le. all metal species not retained in the micellar pseudophase) was the same in the filtrate and in the retentate. The measure of pH in micellar media was performed with a combined glass electrode, taking the usual precautions.
Results and Discussion The first part of this study was concerned with pyridine aldoxime (PAX) and its alkylated analogues. We have studied first the effect of pH on the rate of complex formation between Ni2+ions and PAX in the absence of micelles. This investigation was necessary to determine the best experimental conditions before we moved to more complicated systems. The variation of the observed rate constant with pH is represented in Figure 1. These data have been obtained either in pure water solutions or in the presence of an added ionic strength (0.1 M KC1) or an added buffer (0.1 M TEA). It turns out that the transmitted light increases upon complex formation for pH values smaller than 4.0,whereas the reverse is true at pH values larger than 4.0. Since the pK of the ring nitrogen is 3.42,2l this is very likely an indication that the protonated form of the ligand has a larger absorbance than the metal complex at the wavelength considered. The observed rate constant increases regularly from pH 2 to 5.7 almost independently of the change of sign in the absorbance curve. Above 5.7 the kinetic curve is no
-
(21)Hanania, G. I. H.;Irvine, D. H.J. Chem. SOC.1962,2,2745.
Hebrant et al.
3996 Langmuir, Vol. 10,No. 11, 1994
- 0.08 . -0.06
- 0.04 -0.02 0
e ++-y
4
a
4
f
pH 10'
,
0 ;
longer monoexponential in the absence of buffer. It can then be characterized by two relaxation times (the l/z values obtained in this case have been connected by a dashed line in the figure). With addition of the TEA buffer the curve becomes strictly monoexponential and the value of llz becomes equivalent to those obtained at pH from 4.4 to 5.7. We can thus conclude that, provided the pH does not change too much during the complex formation reaction, the reaction rate is almost independent of the pH value in the range 4.4-6.6. On the other hand, the results obtained in the presence of 0.1 M KC1 do not show any significant effect of the added ionic strength. We have selected a pH of 6.5 for the following experiments, which we intended to carry out in the presence of TEA buffer and with no added ionic strength. The kinetic parameters characterizing the formation of Ni2+complexes can usually be obtained from the variation of llz with the metal ion concentration:16J7
[hq(mol 1')
I
1
1
10
h
O d A /
,/
IO3[MI(mol I-')
+
l l z = kfaPP[M2+Io k t P P where [M2+]o is the analytical metal ion concentration assumed to be in large excess compared to the concentration of the complexing agent. k p p and kdaPP are the apparent rate constants for complex formation and dissociation, respectively. These rate constants are the only ones which are relevant for practical applications, and they are "apparent" rate constants because they include all the present metal or ligand species,which may exist in fast equilibria.16J8 As expected, a linear variation of l/z with the Ni2+ concentration is obtained (in Figure 2) for complexation with PAX in a micellar solution of CTAB (2.5x M). For the sake of comparison we have shown, on the same figure, the position of the data points relative to C12-PAX and CIS-PAX (the same results are presented.inFigure 3, with a more adequate scale). Unfortunately, the addition of TEA caused some problems with the long-chain extractants, which were poorly soluble, so in this case the pH was adjusted with the addition of sodium hydroxide to the solution. A reduction of k p p by a factor of about 350 is obtained upon going from PAXto C12-PAX(seeTable 1). Considering that the presence of TEA reduced the measured rate in water solutions (Figure l),we assume that it is not responsible for this large change of k p p .
water ~PPP
PAX(O.lM
(M-l s-l)
(8-l)
1280
0.06
C12E06 CTAB (2 x M) (2.5 x M) kpep @ee ~PPP kdaw (M-ls-l) (s-l) (M-l s-l) (9-l) 2010
0.3
1030
0.02
19.3
0.05
2.95
0.02
TEA buffer)
C12-PAX
not soluble
Very similar results, which are not given in the figures, have been obtained with the nonionic surfactant C12EO6. The corresponding kinetic parameters, which are given in Table 1, show a reduction of k p by 2 orders of magnitude from PAX to C12-PAX. The larger rate reduction effect obtained with CTAB can be attributed to the electrostatic repulsions between the nickel ions and the surface of the micelles. We have investigated in Figure 3 the effect of the alkyl chain length of the extractant on the kinetics of the reaction. Due to the low solubility of the CIS derivative in CTAB micelles, we tried to improve it by addition of a cosurfactant (butanol in a 1:lweight ratio with CTAB) or by addition of the same cosurfactant
Lungmuir, Vol. 10, No. 11,1994 3997
Ni(II) Removal in Micellar Media 410
1, k" 10; [MI (mol
r')
10' [MI (mol r')
0
0
0
1
2
3
4
5
6
'
1
8
9
Figure 4. Variation of the observed rate constant versus Niz+ concentrationfor complexation by C,-PAX at pH 6.5 in micellar solutions of CIZEO~:Clz-PAX (+I; cl6-PAX (A). Extractant M. concentrations, lo-' M; ClzEO6 concentration, 2 x kobSs.9
J
A
,'
Y'* /"
Om
IO3 [MI(mol f ' )
0 0
1
2
3
4
5
6
7
8
Figure 8. Effect of added salt on the kinetics of complexation of Niz+with C,-PIC in micellar solutionsof CTAB at pH 7 (f0.5). No added salt: CIZ-PIC(+, and curve drawn);cl6-PIc (8); Cl6PIC (0).Addition of CaClz to maintain the added ionic strength constant C1z-PIC (A,and straight line drawn). Insert: CIZPIC with addition of 0.1 M NaBr. Extractant and surfactant concentrations as in Figure 2. and traces of octane, which results in an oil-swollenmicelle (or an d w microemulsion). In any case the values of the observed rate constants obtained are not strongly changed in comparison to the case for C12-PAX. Figure 4 shows the same kind of results with the extractants solubilized in Cl2EOs micelles, where they are more soluble than in CTAB. Here again, the effect of increasing the chain length above 12 carbon atoms does not appear t o produce any further significant reduction of the reaction rate. The second part of this work was concerned with the series of compounds I (PIC and its alkylated analogues). We have attempted to measure the rate of complex formation betweenNi2+and PIC at pH 7.0,but the reaction was exceedingly fast with regard to the capabilities of the stopped-flow technique. Figure 5 shows the variation of the observed rate constant with the concentration of Ni2+ for C~Z-PIC in CTAB at pH 7 f 0.5. The plot obtained in the absence of added salt shows a strong curvature, which disappears completely to give a straight line when the ionic strength is maintained a t a constant value. This was done in two different ways: (i) Instead of simply adding the nickel salt (NiClz),we added a mixture ofNiCl2 and CaC12 so that their total concentration was always the same and equal to 7.0 x M, corresponding to an
0
1
2
3
4
5
6
7
8
Figure6. Effect of the alkyl chain length of the extractant on the kinetics of complexation of Ni2+with C,-PIC in micellar solutions of C12EO6 at pH 7.0 (&O.~):C~Z-PIC (0, and straight line drawn); Cla-PIC (A);CIS-PIC(A). added ionic strength 2.1 x lo+. We do not expect the Ca2+ions to compete in any manner with the Ni2+ions for complexation,since the stability constants measured with PIC are very different (Kcas+= 1.66 x loaM-l and KN$+ = 4.27 x 10' M-1).22 (ii)Another series of measurements was performed in the presence of 0.1 M NaBr. The straight line obtained in this case is represented in the insert of Figure 6. The value of K p P is 22.2M-l s-l in the first case and 208 M-l s-l in the second case. This important increase of k p with the added ionic strength cannot be simply attributed to the modification of the interactionsbetween the Ni2+ions and the negatively charged extractant (a decrease of the reaction rate would be expected in that case since the reagents have opposite electric charges). Conversely, the interaction between the negatively charged extractant and the polar heads of the surfactant molecules a t the surface of the micelles will become weaker, which will facilitate the diffusion of the extractant in the bulk water phase where it can more easily encounter the metal ions. For the sake of comparison we have also plotted in Figure 5 a few data relative to Cls-PIC and C1e-PIC. These data points were obtained without maintaining constant the ionic strength, and they must be compared with the curved plot. They do not show any clear effect ofthe chain length of the extractant since the results relative to the Cia-derivative lie very close to the curve measured for C12-PICbut below it, whereas the results for CIS-PIC are slightly above it. We carried out the same study in ClaEOs micelles a t pH 7 f 0.6 (Figure 6). A straight line was obtained with C12-PICwith no special caution regarding the preceding ionic strength problem. This observation further substantiates the above explanation for the curvature effect observed in Figure 6, especially since the value of K P P with the nonionic surfactant is so much larger than the values measured with CTAB:39.6 x los M-l s-l instead of 22.2-208. On the ather hand, the effect of the alkyl chain length of the extractant shows a similar behavior for Clz-PIC and CIS-PIC but a significant reduction of rate for the Cle-derivative. In a similar study involving positively charged extractants (6-[(alkylamino)methyl]-2-(hydroxymethyl)pyridines)," the apparent rate constants characterizing the (22) Critical Stability Constants;Martell,A. E.,Smith, R.M., Eds.; Plenum Press: New York,1974;vol. 1, p 367.
3998 Langmuir, Vol. 10, No. 11, 1994
Hebrant et al. I
90 _I
f
I
A 03
4 01
A
0
A
4
4
02
04
06
XNI
i
complexation of copper ions were found to have quite similar values in CTAB and C12E06, respectively. Here, the difference of k p in these two micellar systems is so large that we wanted to be sure that we were really observing the same chemical process. For this reason we performed some experiments in mixtures of the cationic and nonionic surfactants. The results obtained at two different pHs (4.5 and 7.0) are represented in Figure 7 where the observed rate constants are plotted versus the mole fraction of CI2EO6.They definitely confirm the effect of the electric charge of the micelle surface on the rate of complex formation. We have attempted to theoretically predict this charge effect by taking into account the expected variation of the electrostatic potential at the surface of the mixed micelles. The problem of counterion binding and surface charge density in mixtures of ionic and nonionic surfactants has been investigated using ion selective electrodes and generally speaking, e m f m e a s u r e m e n t ~ . ~The ~ - fraction ~~ of bound counterions is related to the net electrostatic potential VO. It is out of the question to develop a strictly quantitative model to evaluate the variation of wo with the mole fraction of CTAB in CTAB/C12E06 mixtures, because too many quantities are not precisely known, but we will try to roughly predict this variation, admitting a large number of approximations. An assumption frequently madez5in order to obtain an analytical expression of the surface charge density u is to consider the surface of micelles to be planar, which implies that the curvature is assumed to be very small. Within the framework of Guy-Chapman theory, u is given by the following expre~sion:~~
2nez
2kT 12nezo qo= sinh- ze ck TK The variation of ucan be replaced by a simple expression,26 assuming a linear dependence with the micellar mole fraction of the nonionic surfactant xNI: 0
= ao(l- XNI)
(3)
where uois the surface charge density of the pure CTAB micelles 00
OB
Figure 7. Variation of the observed rate constant versus mole fraction of C&06 in mixed micelles of CTAB and ClZEO6 containing C12-PIC: (e) pH 4.5 (f0.5);(A) pH 7 (f0.5).Nizf concentration, M; extractant concentration, M; overall surfactant concentration, 2 x M.
0=- 'kTK s
counterion and surfactant, and $JO is the electric potential, which can thus be expressed as
e =a
(4)
a being the surface occupied per polar head. Taking into account that the valency z is equal to unity, the final
expression for the electric potential is
We now have to explain the variation of the observed rate constant as a function of this potential. A very simple hypothesis is to assume that complex formation can only take place in the bulk aqueous phase. This is consistent with the large value of kppp measured in the nonionic micelles and with the fact that, if the negatively charged C12-PIC is in the vicinity of CTAB micelles, it is probably preferentially attached to the CTAB polar heads by its carboxylicfunction. We can thus consider that the reaction in the bulk water phase will be less and less favorable as the mole fraction of CTAB increases, since more and more C12-PIC molecules will bind to the mixed micelles. This view is quite analogous to the situation encountered before with hydroxyquinoline.28 In that case a decrease of k,bs was attributed to a stronger and stronger partitioning of the extractant as the volume fraction of the micellar pseudophase & increased. The data were shown to be consistent with the following expression, in which K p is the partition coefficient of the extractant between the micellar pseudophase and the bulk aqueous phase:
In the following we will focus on the first term, characterizing complex formation, and we will make a further approximationby assuming thatK,&/(l- &) >> 1,which is reasonable, provided that K p is large enough. With these simplifications,
i n hzeV0 (m)
(7)
where 6 is the dielectric constant, k is the Boltzmann constant, T i s temperature, K is the Debye parameter, e is the charge of an electron, z is the valency of the
w
kapp [M2f]o &O)
(8)
K'P
~~~~~
(23) Treiner, C.; Mannebach, M. H. Colloid Polym. Sci. 1990,268,
--.
RR
(24) Treiner, C.; Amar Khodja, A.; Fromon, M. J. Colloid ZnteTface Sci. 1989, 128, 416. (25)Rathman, J. F.; Scamehorn, J. F. J . Phys. Chem. 1984,88,5807. (26) Hall, D. G.;Price, T. J. J. Chem. SOC.Faraday Trans. 1 1984, 80, 1193. (27) Hiemenz, P.C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York,1977; Chapter 9.
Equation 8 considers that the volume fraction of the micellar pseudophase is constant when we change the composition of the mixed micelles, maintaining constant the overall surfactant concentration. This is not rigourous since the molar volumes are not exactly the same for CTAB (28) Kim, H. S.;Tondre, C. Langmuir 1989, 5 , 395.
Langmuir, Vol.10, No. 11, 1994 3999
Ni(II.. Removal in Micellar Media
43L
"t
I
I
a,
IO .. 6
[Ll/[Ml
XCTAB
(-0.365 dm%ol-l) and Cl~EOe(-0.45dm8?nol-l), but it is sufficient for the present purpose. ICp in eq 8 stands for the correcting term for C12E08 micelles relative to pure water and takes into account the hydrophobic contribution of the extractant molecule. The ratio o f k ~ , o p l K ' ,can be taken as the slope of the straight line drawn in Figure 6: 3.9 x lo4. According to classical ~ ~ n ~ ethepelectrostatic t s ~ ~ contribution ~ ~ ~ brought by the CTAB surfactant molecules to the partitioning of the extractant is expected to be represented by a multiplying term of the form exp(eI)dkll?, so that the observed rate constant becomes
where I),is given by eq 5. The results of the calculations carried out on this basis are represented in Figure 8. In the first step we have introduced into eq 5, after conversion to cgs units, the following commonly accepted values:27kTle = 25.7 mV; e21ckT= 7.15 A. The Debye screening length K - ~and the surface per polar head a were set equal to 50 A(considering the cmc of the surfactant and the concentration of NiCl2) and 60 &, respectively. The theoretical prediction obtained in this case is represented by the lower curve in Figure 8. These calculations lead to a I),value for pure CTAB micelles of 212 mV, i.e. much larger than usually accepted values (100-150 mV).sOFor this reason, we have calculated, in a second step, the kobs variations obtained when the value of the electric potential of pure CTAB micelles was fixed respectively at 100, 160,and 200 mV. It is clear from these calculations that the shape of the function used is in perfect agreement with the experimental results, which are very close to the curve obtained when a value of 200 mV was considered. However, due to the number of approximations involved, this value (29) Drummond, C. J.;Grieser,F.;Henry,T. W.J . Chem.SOC. Faraday Tram. I 1080,86,521. (30) Hebrant, M.;Tondre, C. J . Colloid Interface Sci. lsSa, 164,378.
[Ni2+l, - [Ni2+lfd
Y = 100
[Ni2+lo
where [Ni2+Iois the initial nickel ion concentration and [Nia+1mis the concentration found in the filtrate by atomic absorption analysis. This yield was studied with respect to two different parameters: the alkyl chain length of the extractant and the composition of the micellar systems, varying the mole fraction of surfactants in CTAB/C12EOe mixtures. Figure 9 shows the variation of Y versus the LIM molar ratio between the extractant and metal ion and for dSerent alkyl chain lengths of the extractant. The yield of extraction increases with the LIM ratio and reaches 95-99% for LIM > 4. The effect of the chain length of the extractant is significantonly for the lower LIMratios where the yield increases in the order Cia-PIC > CI5-PIC > (212-PIC. The differences are more difficult to distinguish for the larger values of LIM. Blank experiments, in the absence of extractants, have demonstrated only a very weak metal ion retention at pH 7 and even slightly negative retention at pH 5. This is likely to be due to ion expulsion effects similar to those reported by Christian et ~ 1 . 3However, ~ we have realized that one has to be very cautious about controlling the pH. Indeed the adjustment of the pH value with sodium hydroxide may in some cases induce the formation of hydrolyzed nickel species, which can be retained by the membrane and simulate a high extraction yield. The results obtained in mixed systems are represented in Figures 10 and 11. They show an improvement of the extraction yield for the same LIM ratio when pure CTAB micelles are replaced by a 40160 CTABlC12EOemole ratio. (31) Christian, S. D.;Tucker, E. E.; Scamehorn, J. F.; Lee, B.-H.; Sasaki,IC J. Langmuir 1080,6, 876.
4000 Langmuir, Vol. 10, No. 11, 1994
Hebrant et al.
I
/
";;
80 --
4)
a
1)
"ti
10
Llml
XNI 0
0
1
2
3
4
5
6
7
Figure 10. Yield of Ni2+ retention (from ultrafiltration experiments)versus extractant to metal ratio in mixed micelles CTAB/C12EO6(moleratio 40/60,overall concentration2 x M) at pH 7 (f0.5):C12-PIC (+); c16-PIc (A);ClS-PIc (A). In that case the yield reaches 95% for LIM = 3 for the three alkylated analogues of PIC. Figure 11 shows the effect of the mole fraction of C12EO6 a t LIM = 1. The yield
significantly improves when the alkyl chain length of the extractant is increased, especially for the lower mole fraction of C12EOs. This effect tends to decrease for the larger mole fractions. The yield goes from 10-30% in pure CTAB to 75-80% in mixed micelles containing 80% CizE06. In conclusion,the followingpoints can be stressed. The behavior of PAX and C,-PAX analogues in CTAB and ClzEO6 micelles is in many respects comparable to those observed with 8-hydroxyquinoline(HQ) and its alkylated analogue (C11-HQ). In particular, the rates of complex formation with Ni2+ ions are in the same range of magnitude, and for this reason PAX derivatives, which show serious solubility problems, do not appear to be a better choice than HQ derivatives, for achieving metal extraction in micellar systems. Neither would they facilitate kinetic separation of metal ion mixtures since the kobe values measured in CTAB are close to those obtained with C11HQ. Regarding C,-PIC compounds, the effect of ion pairing has been clearly shown to considerably affect the rate of
0
02
a4
0.6
0.8
1
Figure 11. Yield of Ni2+ retention (from ultrafiltration experiments)versus mole fraction of C12E06in mixed micelles of CTAB and ClzEOs containing C,-PIC at pH 7 (f0.5) and LIM = 1;overall surfactant concentration 2 x M. C12-PIC (+);
Cis-PIC (A);Cis-PIC (A).
complex formation, but unfortunately its value remains larger than with C,-PAX or with Cll-HQ. This is due to the fact that the dominating factor in determining the rate of reaction is the acceleration effect of the negative charge carried by the extractant. This makes complex formation much faster than with neutral (or a fortiori positively charged) extractants. However, these compounds have proved to be very efficient for the removal of Ni2+ ions in equilibrium conditions, using ultrafiltration methods to separate the micellar pseudophase from the surrounding bulk aqueous phase. They can thus be used for the purpose of decontamination of diluted aqueous effluents, in surfactant-based processes avoiding the use of organic solvents. The present work has shown that the yield of Ni2+ ion removal can be improved by increasingthe alkyl chain length of the extracting agent or by adapting the type of micellar systems used.
Acknowledgment. We thank S.-G. Son for preliminary stopped-flow experiments.
