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Transport mechanisms in liquid membranes with ion exchange carriers
journal of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 10g (1995) 231-244
Transport mechanisms in liquid membranes with ion exchange carriers I.M. Coelhoso a, T.F. Moura a,b, J.P.S.G. Crespo a,., M.J.T. Carrondo a,b "Dep. de Qufmica, Faculdade de Ci~ncias e Tecnologia da Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal b lnstituto de Tecnologia Qulmica Biolfgica (ITQB)/Instituto de Biologia Experimental e Tecnolfgica (IBET), Apartado 12, 2780 Oeiras, Portugal Received 28 March 1995; accepted 26 June 1995
Abstract The extraction of lactate by an ion-pairing mechanism using a quaternary ammonium salt ( Aliquat 336) was studied previously and a mathematical model for equilibrium was developed. The evaluation of the equilibrium constant (Ke) of the reaction between carrier and lactate allowed a good prediction of equilibrium for independent extraction and stripping, within a large range of experimental conditions. However, when extraction and stripping operations are carried out simultaneously using liquid membranes and different concentrations of the feed and stripping solutes are used, the resulting osmotic pressure difference between the two aqueous compartments (feed and stripping) has to be taken into account for equilibrium prediction, if the membrane is not totally impermeant to water. Model prediction of equilibrium and identification of the mechanisms involved on the transport of lactate and of the ion counter transported are presented. The influence of operating conditions on each mechanism and their relative contribution to the overall transport are evaluated. Prediction of equilibrium is accurate in the absence of an initial osmotic pressure difference between the two aqueous compartments but exhibit a large deviation for increasing initial chloride concentrations. To counterbalance the initial osmotic pressure difference an increasing hydrostatic pressure difference builds up leading to salt transport across the liquid membrane. Keywords: Liquid membranes; Liquid membrane stability; Ion-pairing transport; Facilitated transport; Lactate extraction
1. Introduction An immiscible organic liquid phase separating two aqueous phases (feed and stripping) may be considered as a liquid membrane. If a reactive extractant is added to the liquid membrane, solute transport can be * Corresponding author. Dep. de Qufmica, Faculdade de Ci~ncias e Tecnologia da Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal. Phone: 351 1 2954464, ext. 0919. FAX: 351 1 2954461. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(95)00174-3
enhanced by this agent, which may interact selectively and reversibly with the solute. The reversible reaction between solute and carrier simultaneously increases the flux of solute and improves selectivity [ 1 ]. Most applications of liquid membranes are concerned with recovery of metal ions [ 2 - 4 ] but studies on separation of carboxylic acids [ 5 - 1 2 ] and aminoacids [ 13-15] have increased recently due to the potential advantages of combining liquid-liquid extraction and membranes in only one operation. The
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
232
Feed Phase
I Organic Phase
Sthripping
IC"4N+a C--' a J IL-R4 Fig. 1. Mechanism of lactate extraction/stripping with a quaternary ammonium salt.
advantages of incorporating carriers in liquid membrane systems are. 1. Highly selective separations are possible, because the specificity of the complexation reaction improves separation. 2. Solutes, especially ions, can be concentrated. 3. Only small amounts of carrier are required due to the small amount of solvent used and its continuous regeneration associated with the reversible reaction [1]. Liquid membranes may contain only liquid phases, which is the case of bulk liquid membranes and emulsion liquid membranes patented two decades ago by Li [16-18] or may contain, additionally, a polymeric membrane to support the organic phase. In supported liquid membranes, a porous membrane impregnated with the organic phase separates the feed and the stripping phases [ 19-21 ]. In this work, the transport of lactate is accomplished by an ion-exchange process, using as carrier a quaternary ammonium salt. The transport of lactate ions from the feed phase to the stripping phase is counterbalanced by an equivalent transport of other anionic species from the stripping to the feed phase through a coupled transport mechanism (Fig. 1). In a previous study [ 22 ] the extraction of lactate by an ion-pairing mechanism using a quaternary ammonium salt (Aliquat 336) was studied and a mathematical model for equilibrium was developed. The evaluation of the equilibrium constant (Ke) of the reaction between carrier and lactate, allowed a good prediction of equilibrium for independent extraction and stripping, within a large range of experimental conditions. Since in liquid membranes extraction and stripping take place simultaneously, separation and concentra-
tion can only be achieved using a high stripping agent concentration, which may cause an osmotic pressure difference between the two aqueous compartments, if the membrane is not totally impermeable to water. The objectives of this work are: I. To predict the equilibrium concentrations in both feed and stripping compartments assuming an ionpairing mechanism for lactate and for the ion counter transported through the liquid membrane. 2. To verify if other transport mechanisms are involved. 3. To evaluate the relative contribution of each mechanism to the overall transport and assess how these contributions are affected by different operating conditions of extraction and stripping. Two different membrane configurations, bulk and supported liquid membranes are discussed.
2. Equilibrium modelling of lactate transport assuming an ion-pairing mechanism Lactate present in the feed phase is extracted by the quaternary amine (Aliquat 336) to the organic phase, while the chloride anion present in the stripping phase is carried in opposite direction (Fig. 1 ). The following reaction takes place in both interfaces: Ke
A-+RC1 ~ RA+CI-
(1)
with the equilibrium constant [RA] [C1-]f [RA] [CI- ]s K e - [ A - ] flRC1 ] - - [ A - ]s[RC1 ]
(2)
where IRA] and [RC1] are the lactate-amine complex and the chloride-amine complex concentrations in the organic phase, [ A - ] r and [ Cl- ] f are the lactate and chloride concentrations in the feed phase, and [ A - ] s and [C1- ]s are the lactate and chloride concentrations in the stripping phase. To evaluate the lactate equilibrium concentration in the feed and stripping phases the following assumptions were assumed: 1. The undissociated form of lactic acid is negligible at pH = 6.3 as the pk, = 3.86. 2. Transport of O H - is negligible compared with lactate and chloride transport, at pH = 6.3.
233
I.M. Coelhoso et al./Journal of Membrane Science 108 (1995) 231-244
3. The membrane is totally impermeable to water and so, water coextraction is insignificant and has no influence on the extraction process. 4. As lactate is very poorly soluble in the organic diluent there is no physical extraction (in agreement with experimental observation). 5. The ion exchange reaction between lactate and the quaternary ammonium salt occurs at the interface and the equilibrium constant, K¢, governs equilibrium at both interfaces. 6. Lactate and chloride transport is accomplished exclusively by an ion pairing mechanism. 7. The concentration of the lactate-amine complex, [RA], is the same in both interfaces. This is also valid for the chloride-amine complex, [RC1]. A discussion of these assumptions is presented elsewhere [ 22].
. [C1-]o-
[A-]~
(9)
and simplifying, a relationship between the lactate equilibrium concentration in the two compartments is obtained:
[A-Is
- -
-
[A-]f
[C1-]o
-
-
(10)
[A-]o
According to Eq. (10), the model predicts that the concentration effect, [ A - ] s / [ A - ] f increases with the increasing stripping agent concentration, [C1-]o, if lactate transport is exclusively accomplished by an ionpairing mechanism. An expression for the reaction equilibrium constant results from substitution of Eqs. (3), (4) and (6) in Eq. (2):
(iN_ ] o ~f - - - [ A - ] c - -uf- - - [ A - l s v~) ([A ] o - [ A - b )
2.1. Bulk liquid membrane
Uorg
Vorg
Uorg]
Ko -Uf [A-] [RC1]o---[A ] o + -Uf - [ A - ] r + [ A L vs t
The material balances for amine, lactate and chloride, using different volumes of feed, organic and stripping phases, respectively, vf, Vor~and vs, are:
Uorg'
Uorg
Uorg]
(11)
[RC1]o = [RC1] + [RA]
(3)
substituting [ A - ]s from Eq. (10), the lactate equilibrium concentration in the feed phase can be expressed
vf[A-]o=vf[A-]f+vs[A-]s+Vorg[RA ]
(4)
as:
Vorg[ RC1 ]o + vs[C1- ]o1 = Vorg[RC1 ] + v f [ C l - ] f + us[C1- ]s
[A-] -
b+ Ked- ~/(b+ Ked) 2-4ac( 1 -Ke) 2c( 1 - K~)
(5)
where [RC1]o, [ A - ] o and [C1-]o represent, respectively, the initial concentrations of Aliquat 336, lactate and chloride. As it was assumed that transport was exclusively accomplished by an ion-pairing mechanism then, at any time:
(12) where a = I A - ]o2 uf
(13)
Uorg
b = - 2 [ A - ] o vf _ [C1-]0 [ A - ]o = [ A - ]f--~ [C1- ]f
(6)
[Cl-]0 = [CI-]~+ [A-]~
(7)
From Eq. (2) the lactate equilibrium concentration in the stripping phase can be related to the lactate equilibrium concentration in the feed phase by: [A-ls= [A-lf~ Using Eqs. (6) and (7)
(8)
Vorg
Us
(14)
Uorg
c = vf + v~ [C1-]o Uorg ©org [ A - ] o
(15) Uf
d = [RC1]o- [ A - ] o - -
Uorg
(16)
This model allows for prediction of lactate equilibrium concentrations in the feed and stripping compartments [Eqs. (12) and (10), respectively], using defined volume phase ratios, vf/Vorgand Vs/Vorvinitial
234
L M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
lactate, chloride and amine concentration, [A-]o, [ C1- ] o and [ RCI ] o, if the equilibrium constant, Ke, is known.
2.2. Supported liquid membrane The liquid membrane is immobilized within the pores of the support porous membrane. The same considerations made for the bulk liquid membrane are valid and Eqs. ( 1) to (10) still apply. In this case, the organic phase volume is, however, much smaller than the feed and stripping phase volumes. Thus, lactate and chloride present in the organic phase are negligible when compared to the quantities of these two ions in the feed and stripping compartments. As a result, all terms that multiply by Vorgin Eqs. (4) and (5) can be neglected. Taking into account these simplifications, the chloride concentration in each compartment, the lactate concentration in the stripping phase and the equilibrium constant, Ke, may be obtained, expressed in terms of the initial lactate and chloride concentrations and volume phase ratio, as: [C1-]f = [A-]o-
[A-If
(17)
[C1- ]s= [C1- ] o - v f ( [ A - ] o - [ A - ]f)
(18)
Us Uf
[A-]s=--([A-]oUs
(19)
[A-]f
([A-]~vf-[A-]f(-2vf[A-]o-vs[Cl-]o)+a'[A-]~) [A ]f([RCl]ovo~g-vf[A-]o)+a'[A
]2
(20) where a'=
[C1-]o vf+vS[A-]o
(21)
and as Oorg[ RC1 ] o << vf[ A - ] o, lactate equilibrium concentration in the feed phase, [ A - ] f, becomes independent of the initial amine concentration, [RC1]o, beeing given by: [A-]f b' +Ke[A ] o - ~ / ( b ' + K e [ A - ] o ) 2 - 4 a ' [ A - ] ~ ( 1 - K e )
2a'(1 -Re) (22) where a' is defined by Eq. (21) and
b' = 2 [ A - ]o + [CI- ]o~ ve
(23)
As stated for the bulk membrane cell, the model predicts lactate equilibrium concentration in the feed and stripping compartments, using a defined volume phase ratio, vs/vf, initial lactate and chloride concentration, [ A - ]o and [C1- ]o, if the equilibrium constant, Ke, is known.
3. Experimental 3.1. Extraction system The bulk liquid membrane cell and the supported liquid membrane cell used in the experiments are represented in Fig. 2 and Fig. 3, respectively. The feed phase is a lactate solution (280 mM) obtained by dilution of sodium lactate 60% (Sigma, USA). The pH of this solution was adjusted to 6.3, which is the pH for lactic acid fermentation, with NaOH 0.1 M. The stripping phase is a sodium chloride solution of variable concentration ranging from 0.28 to 5 M. Three different compositions of the organic phase were used: 10% (w/w), 30% (w/w) and 50% (w/w) Aliquat 336 in Shellsol A. In the experiments using the bulk membrane cell the phase volume ratio employed was 3:3:1, that is, 180 ml of feed phase, 180 ml of organic phase and 60 ml of stripping phase. For the supported liquid membrane cell a polypropylene membrane with a pore diameter of 0.01/xm and, 55% porosity (Gelman Sciences, USA) was soaked in the organic phase and then placed between the feed and stripping reservoirs. In this case, the volumes of the feed and stripping phases were equal, 150 ml of each phase. For both cells, the stirring speed of the phases was adjusted to 100 rpm and the experiments performed at a constant temperature of 40°C during 8 days, until equilibrium was reached. This was assured by sampling once a day, for the last 3 days and obtaining a constant lactate concentration.
3.2. Analytical methods Lactate concentration was determined by HPLC.The column used was a Shodex SH 1011 (Showa Denko
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
235
-Q
--03
V
•- _ - - - - - - _ - 2 - - _ ___-_-
--_-
. . . .
z _ - _ - z _-_-_-.:
.
.
.
.
.
.
.
.
.
.
.
.
.
.
_ ~'.6
L---_-L:?_: _-_-_-
$'_-_-2-
-_
Fig. 2. Bulk liquid membranecell: 1, samplingpoint (feed phase); 2, sampling point (stripping phase); 3, sampling point (organic phase); 4, mechanical stirrer; 5, Teflon seal; 6, feed phase; 7, stripping phase; 8, organic phase; 9, magnetic stirrer. K.K., Japan) and the eluent was 0.01 N sulfuric acid. A refractive index detector ( Merck Hitachi, Japan) was employed. The pH of the aqueous solutions was measured using a combined electrode model U457-57/110 (Ingold) and an ion meter model 720A (Orion, USA). The water content of the organic phase was determined by Karl Fischer titration ( A Q U A P A L III, UK).
Chloride concentration was measured with a combined electrode model 96-17B (Orion, USA) and an ion meter model 720A (Orion, USA). In order to remove interferences of other ions and to adjust the ionic strength of samples and standards, to each ml of sample or standard, 1 ml of CISA solution (Orion, USA) and 0.04 ml of ISA solution (Orion, USA) were added, before measuring the chloride concentration.
236
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
for the reaction of lactate with the quaternary ammonium salt (Aliquat 336) was evaluated using equilibrium experiments. The value obtained was: Ke = 0.081 __+0.007
Fig. 3. Supported liquid membrane cell: 1, sampling point (feed phase); 2, sampling point (stripping phase); 3, connection for continuous operation; 4, mechanical stirrer; 5, Teflon seal; 6, feed phase; 7, stripping phase; 8, membrane; 9, magnetic stirrer; 10, porous glass support.
4. Results 4.1. Lactate transport assuming an ion-pairing mechanism Bulk liquid membrane In a previous study [22], the equilibrium constant
Using this value for the equilibrium constant and assuming that lactate transport is exclusively achieved by an ion-pairing mechanism, lactate equilibrium concentration in the feed and stripping compartments of the bulk membrane cell can be evaluated using Eqs. (12) and (10), respectively. Table 1 shows the predicted and experimental values obtained for the lactate equilibrium concentrations in both compartments of the cell, varying the concentration of the quaternary amine. The stripping agent concentration, [ C1- ] o is 1 M. The measured pH values remained constant in the range of the analytical error for pH detection. The model predicts that equilibrium concentrations of lactate in both phases decreases with increasing initial amine concentration as more [RA] is formed at equilibrium and remains in the liquid membrane. The experimental values follow the same pattern. The experimental and the predicted lactate concentrations are similar for each amine concentration tested, giving 10 to 16% deviation for lactate concentration in the feed compartment, and - 14 to - 26% deviation in the stripping compartment. Deviation is calculated as ( [ A - ] exp- [ A - ] model)/ [A-- ] e~p× 100. However, the deviation between experimental and predicted values of lactate equilibrium concentration, for the same amine concentration, increases with the initial chloride concentration present in the stripping compartment (Table 2). For equal concentrations of lactate and chloride (0.275 M), the deviation between model and experimental lactate concentration in both feed and stripping compartments is only - 4 % , which is in the range of
Table 1 Comparison of experimental and model lactate equilibrium concentration: Effect of amine concentration. Bulk membrane, phase ratio (feed:org:strip) 3:3:1 [A-]o (M)
[C1- ]0 (M)
Aliquat (%)
[ A - ] fexp (M)
[ A - ] fmoct~l (M)
Deviation (%)
[ A - ]~exp (M)
[A ] ~,,ooel (M)
Deviation (%)
0.281 0.281 0.267
1.000 0.998 1.000
10 30 50
0.137 0.135 0.109
0.123 0.113 0.097
10 16 11
0.383 0.318 0.298
0.436 0.402 0.364
- 14 - 26 - 22
1.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
237
Table 2 Comparison of experimental and model lactate equilibrium concentration: Effect of initial chloride concentration. Bulk membrane, phase ratio (feed:org:strip) 3:3:1
[A-]fmodel
[A-J0 (M)
[CI ]o A l i q u a t [ A - b e x p (M) (%) (M)
(M)
Deviation[A-]~exp (%) (M)
[A-]smode I (M)
Deviation [ A - L / [ A (%) exp
0.275 0.281 0.276 0.279
0.276 0.998 1.950 4.710
0.158 0.113 0.077 0.041
-4 16 35 67
0.158 0.402 0.540 0.688
-4 - 26 - 50 - 83
30 30 30 30
0.152 0.135 0.118 0.126
0.152 0.318 0.360 0.376
1.00 2.35 3.05 2.98
]f
[A ] ~ / [ A - ] f model 1.00 3.55 7.06 16.88
the analytical error for lactate detection. Moreover, the experimental values for lactate equilibrium concentration in both compartments are equal, as predicted by Eq. (10). For higher chloride concentration the deviations increase reaching 67% for the feed compartment and 83% for the stripping compartment, when a 4.71 M chloride concentration is used. Contrary to the predictions of Eq. (10), it is clearly demonstrated (Table 2) that it is not worthwhile to increase chloride concentration above 1 M. The concentration effect deviates further from the model predictions as the stripping agent concentration increases and the amount of lactate extracted using [C1-]o = 1.98 M and [C1-]o=4.71 M is not significantly different from the lactate extracted with [C1- ]o = 1 M. Similar results were obtained by Drioli and coworkers for phenylalanine extraction with Aliquat 336 [20].
the same initial amine concentration and different initial chloride concentrations. Prediction of equilibrium is accurate if both compartments (feed and stripping) have the same molar concentration of salts but is no longer valid for different salt concentrations. Higher deviations from model predictions are observed in the supported liquid membrane than in the bulk liquid membrane configuration. Indeed, a 92% deviation between the experimental and model lactate equilibrium concentration is obtained for the highest chloride concentration used. As stated before for the bulk membrane cell, in this system it is not worth to use [C1- ] o > 1 M. In fact, for the supported liquid membrane cell, the lactate
Supported liquid membrane
[ A - ]o (M)
Aliquat 336 (%)
[ A - ]f (M)
0.290 0.282 0.276
10 30 50
0.140 0.133 0.124
For the supported liquid membrane cell, since the organic phase volume is negligible when compared to the feed and stripping phase volumes, the predicted equilibrium concentration in the feed phase is independent of the initial amine concentration used, [RC1]o, according to Eq. (22). The experimental equilibrium values of [ A - ] f obtained for 10% ( w / w ) , 30% (w/w) and 50% (w/ w) of Aliquat 336 in Shellsol A for equal initial stripping agent concentration, [ C1- ] o = 1 M were not significantly different (Table 3). The measured pH values remained constant in the range of the analytical error for pH detection. Table 4 compares the experimental and predicted lactate equilibrium concentration, assuming that transport is exclusively accomplished by ion-pairing using
Table 3 Effect of amine concentration on the lactate extracted. Liquid supported membrane cell: polypropylene membrane 0.1/xm; [ C1 - ] o = 1 M
Table 4 Comparison of experimental and model lactate equilibrium concentrations: Effect of initial chloride concentration. Supported liquid membrane, polypropylene membrane 0.1 /zm [A-]o (M)
[Cl-]0 (M)
[A-]fexp
[A-]fm~e I
(M)
(M)
Deviation (%)
0.280 0.282 0.273 0.287
0.276 0.998 1.950 4.710
0.154 0.133 0.201 0.197
0.141 0.062 0.034 0.015
8 53 83 92
238
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
extracted with higher chloride concentration is even lower than with [C1- ]o = 1 M.
4.2. Chloride transport by alternative mechanisms The results presented in the previous section clearly indicate the presence of another type of transport beyond the ion-pairing mechanism assumed in the model. In fact, if the total amount of lactate transported is lower than the model prediction, as the difference of salt concentration between the feed and stripping compartments increases, that must be due to other mechanisms of transport of the chloride stripping ion. As the difference between the experimental and model prediction of equilibrium concentration of lactate increases the assumptions that: 1. the membrane is impermeable to water; 2. anion transport is exclusively accomplished by an ion-pairing mechanism; are no longer valid. Thus, there is a need to consider the contribution of the initial osmotic pressure difference between the two compartments of the cell and the resulting water and ion transport mechanisms occurring under these experimental conditions. Evidence of water equilibration is reflected by the observed hydration of the organic phase in the bulk liquid membrane. Analyzing the organic phase during W0(water m o l a r e o n c . / a m i n e
the time course of independent experiments an increase in water content was observed, from an initial value of Wo= 0.5 mol water/mol amine until a value of Wo= 5 mol water/mol amine within the first 20 h (Fig. 4). Indeed, when the system is first established and an initial osmotic pressure difference (A 17) exists, water will equilibrate between the two compartments. The ultimate consequence of water transport would be a volume change of both aqueous phases which would cause a variation of the concentrations of the species in both compartments. However, experimentally, a macroscopic volume change of either phase was not observed. As a result of the fixed volume of both aqueous compartments, an increase in hydrostatic pressure difference (AP) builds up until it balances the osmotic pressure difference and equilibrates water
A p = p s - pf= AH=RT~(C,s- C#)
(24)
i
where Ps, P f and Cis, fir represent the hydrostatic pressure and the concentrations of the i'th species in the stripping and feed compartments, respectively. Due to the hydrostatic pressure difference, the driving forces acting on chloride and sodium lead to their transport from the stripping to the feed compartment. Sodium is simultaneously transported in order to assure electroneutrality in both phases. The transport of chloride and sodium implies that less chloride is available to be counter transported by an ion-pairing mechanism. m o l a r cone.') O
t0~ ALIQUAT
0
30~ ALIQUAT
[]
50~ ALIQUAT
O 4
O
~0 []
0~00
00 0 ~D
O0
000
[]
[]
Q
© 2
0
i
I
I
I
50
I00
150
200
250
Time (h) Fig. 4. Evolution of the ratio water molar concentration/amine molar concentration in the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; [Cl-]o = 1 M).
L M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244 120
Under these circumstances, the chloride concentration determined from the model where transport by ionpairing is exclusively assumed is not correct; consequently, less lactate is extracted and reextracted. In order to verify this statement the salt concentration difference between the feed and stripping compartments
239
% Chloride T r a n s p o r t
1oo
[I~
Jmlt ion-pairing
89 84
80
86 .
.
.
.
60
was calculated at the begginning of the experiment and at equilibrium, using the measured chloride and lactate concentrations and keeping electroneutrality in the bulk aqueous solutions. Table 5 shows that this salt concentration difference at equilibrium is always lower than its initial value; thus, there was a net transfer of sodium chloride from the stripping to the feed compartment. This salt transport increases with increasing difference between initial salt concentration in the compartments, i.e. with increasing initial osmotic pressure difference. The relative contributions of the different mechanisms involved in chloride transport may be calculated from the molar balances for lactate and chloride in the feed compartment: [ A - ] o V f - [A ]fuf=nAi~.pai.ng
(25)
[C1 - ] ¢~f= ncJ~o._~a~..~+ nc,~a~,
(26)
Eqs. (25) and (26) show that lactate transported from the feed compartment is carried by an ion-pairing mechanism, while chloride arising in the feed compartment results from the addition of ion-pairing and other transport mechanisms. Since: nAion_pairing= ncl.~._p.in..
(27)
it is possible to calculate the amount of chloride that is transported coupled with sodium, ncl..a..
40
20
0
10% ALIQUAT
30% ALIQUAT
50% AL1QUAT
Fig. 5. Effect of amine concentration on chloride transport ( salt and ion-pairing) for the feed compartment of the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; [Cl- ]o = l M).
Bulk liquid membrane Fig. 5 shows that the contribution of other mechanisms of chloride transport is not affected by the amine concentration used and varies between 11 and 16% of the total transport, when using [ C1- ] o = 1 M. Fig. 6 shows an increasing contribution of the salt transport mechanism with increasing initial chloride concentration. This contribution is null (3% is within the experimental error of chloride detection) for equal salt concentration in both compartments (where (AH)o=0, ([A-]o =[C1-]o=280 mM) and increases with the difference between the initial salt concentrations [ C 1 - ] o - [ A - ] o , i.e. with increasing initial osmotic pressure difference.
Table 5 Difference of salt concentration between feed and stripping compartments. Bulk membrane, phase ratio (feed:org:strip) 3:3:1 [A-]o (M)
[Cl-]o (M)
Aliquat (%)
[A-]fex p (M)
[C1]-fexp (M)
[A-l~exo (%)
[Cl]-sexp (M)
(E~s--Cif)o
(E~s--~f)eq
(M)
(M)
0.275 0.281 0.276 0.279
0.276 0.998 1.950 4.710
30 30 30 30
0.152 0.135 0.118 0.126
0.127 0.164 0.182 0.237
0.152 0.318 0.360 0.376
0.122 0.597 1.230 3.215
0.002 1.434 3.348 8.862
0.01 1.232 2.580 6.456
240
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
Supported liquid membrane
% Chloride Transport
120
m
salt
ion-pairing
100
97 B9
B7
B0-
64 6 0 ~- ....
For the supported liquid membrane, the transport contribution resulting from the salt transport mechanism, calculated by Eqs. ( 2 5 ) - ( 2 7 ) , is more relevant than was obtained for the bulk membrane configuration, but decreases with increasing amine concentration (Fig. 7). This is due to the thinner thickness of the supported liquid membrane compared with the bulk membrane, where the resistance for salt (NaC1) transport is higher. However, the observed decrease of the contribution of this term with increasing amine concentration, may be explained by an increase in membrane resistance due to higher viscosity of the
40
Table 6 Variation of organic phase viscosity with amine concentration 20
!.
3 0 - m A0=C10=280 m M
CIO=I M
CI0--2 M
CI0=5 M
Fig. 6. Effect of the initial chloride concentration on chloride transport (salt and ion-pairing) for the feed compartment of the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; organic phase, 30% Aliquat + 70% Shellsol A). % Chloride
Aliquat 336 (%)
Viscosity (mPa s)
5 10 20 30 50
0.90 1.01 1.61 2.89 11.10
120
Transport
% Chloride T r a n s p o r t
B0
• m
m
71
salt len-pttr|~g
s~t I00
60
B0
60
40 40
20 20
0
10~. ALIQUAT
30% ALIQUAT
50~. ALIQUAT
Fig. 7. Effect of amine concentration on chloride transport (salt and ion-pairing) for the feed compartment of the supported liquid membrane cell (phase ratio, feed:strip l:l; [C1-]0 = 1 M).
A0=C10--280 mM
C]0=I M
ClO=2 M
C10=5 M
Fig. 8. Effect of amine concentration on chloride transport (salt and ion-pairing) for the feed compartment of the supported liquid membrane cell (phase ratio, feed:strip 1:1; organic phase, 30% Aliquat + 70% Shellsol A).
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
120
% Salt T r a n s p o r t
proceed via an ion-pairing mechanism is accurate in the absence of an initial osmotic pressure difference. However, for increasing initial osmotic pressure difference the model predictions show large deviations for both membrane configurations. Furthermore, our experimental results show that the assumption of water impermeability of the membrane was not valid and consequently the initial osmotic pressure difference affects the system performance. This effect accounts for the deviations observed from the model predicted values of (i) extracted lactate, (ii) concentration effect and (iii) salt (NaCI) transport. Although, two questions arise for discussion: 1. If the ion-pairing mechanism is not the sole responsible for ion transport, how can sodium and chloride be transported across the liquid membrane, since they are very poorly soluble in the organic phase? 2. If alternative mechanisms of transport are active, is it still possible to predict equilibrium?
Bulk m e m b r a n e -0-
Supported m e m b r a n e
I00
8O
60
0
40
20
Ol
t
i
I
i
2
4
6
8
241
I0
( a ~ /RT)O (M) Fig. 9. Variationof salt transportwith the initial differencebetween salt concentrations, (A I-I/RT)o, for both cells.
5. I. How are sodium and chloride transported across the liquid membrane ?
organic phase with increasing amine concentration (Table 6). The contribution of this non ion-pairing mechanism increases with the initial chloride concentration, i.e. with the initial osmotic pressure diference between both cell compartments. The results obtained are similar to the behaviour observed with the bulk liquid membrane, but the magnitude is higher (Fig. 8). For [ C1- ] 0 = 4.71 M, the ion-pairing mechanism is responsible for only 10% of the chloride transport. The contribution of the salt transport mechanism as a function of the initial difference between salt concentration, ( A H / R T ) o , is represented in Fig. 9 for both cells. An increase of this transport contribution as the difference between initial salt concentration for both cells increases can be observed. As mentioned before, the salt transport contribution is stronger for the supported liquid membrane than for the bulk liquid membrane.
Despite numerous experimental studies and modelling, the mechanism by which ions cross the interface between two immiscible liquids is not well understood. This results from a lack of direct knowledge about the structure of the interface and the difficulty of experimentally monitoring the transport process in real time. The common picture of a sharp interface cannot explain the mechanism of ion transport. Recently, it was shown that surface roughness and capillary distortion play important roles in the transfer process. These capillaries act as constantly moving "fingers" of water protruding into the organic phase, facilitating ion transport across the interface [ 23 ]. The hydrostatic pressure difference resulting from the initial osmotic pressure difference is the radial component of a tangential surface tension acting on the membrane, which eventually becomes high enough to cause membrane deformations. These deformations of equilibrated solution may originate aqueous "pockets" which can migrate through the membrane from one side to the other. This mechanism would increase the membrane permeability to sodium and chloride. This could explain why the transport resulting from other mechanisms than ion-pairing is more relevant in the supported liquid membrane. As a result of the thin-
5. Discussion
Model predictions assuming membrane impermeability to water and cations, where anion transport would
242
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
ner liquid membrane the formation of these aqueous protrusions is favoured and a larger number of ions can be transported by this alternative route, leading to a smaller contribution of the ion-pairing mechanism to the overall transport process. In the bulk liquid membrane, the existence of a very thick barrier hinders the development of these protuberances and the transport not accomplished by ionpairing becomes less favourable. However, the existence of these aqueous protrusions does not mean that the liquid membrane is unstable or even that it has collapsed. Studies on stability of supported liquid membranes indicate that they become increasingly unstable with high differences between the osmotic pressures of the feed and stripping compartments. The following mechanism of water transport through supported liquid membranes was proposed: (i) in the presence of an osmotic pressure gradient, water tends to flow through the organic-filled pores of the supported liquid membrane; (ii) when the amount of flowing water becomes massive, the organic phase is displaced from the support pores and replaced by water; at this stage the membrane behaves as a semipermeable diaphragm containing water-filled micropores [ 24]. If the supported liquid membrane had collapsed, the values of lactate concentration should be equal in both aqueous compartments. Definitely, this was not observed during these studies even for the highest chloride concentration tested. For the bulk liquid membrane a collapse is not possible due to the large thickness of the organic phase. Moreover, these results prove that salt transport by alternative mechanisms occurs in the presence of an initial osmotic pressure difference. From these results it may be concluded that salt transport resulting from the establishment of an initial osmotic pressure difference between the feed and stripping compartments leads to a reduction on the contribution of the ion-pairing mechanism to the overall transport process. However, this alternative mechanism proved to occur for both membrane configurations (bulk liquid membrane and supported liquid membrane) without membrane collapse.
ference. Additionally, the supported liquid membrane model predicts that equilibrium is independent of the initial amine concentration, which was experimentally confirmed. However, as the membrane is not totally impermeable to water, in the presence of an initial osmotic pressure difference salt transport occurs, decreasing the contribution of the ion-pairing mechanism. Equilibrium prediction requires the evaluation of the relative contribution of each mechanism for the overall transport. The fundamental relationship needed for equilibrium prediction is that the electrochemical potential,/zi, of any species i is the same in all phases to which the species has access. Thus, for the feed and stripping phases, it results in (28)
/.Lif = ].Zis
provided that species i can move between the two phases. For a specific solute of an ideal solution,/zi is given by Id~i = IZ~ O) -]- RTIn X i -~- PV~+ ziF0
where/~o) is the standard chemical potential (in the particular phase being considered) of the i'th species, X~ is the molar fraction of the i'th species, Vi is the partial molar volume of i'th species, z~is the valence of i'th species, P is the hydrostatic pressure of the phase, ~b is the electric potential of the phase, R is the gas constant, Tis absolute temperature and Fis the Faraday constant [ 25 ]. As a rough approximation the aqueous solutions will be considered dilute solutions and the molar fraction replaced by concentration. Applying Eqs. (28) and (29) to sodium, lactate and chloride: RTln[Na + ] f"[- Pf~'~Na + +F~bf=RTln[Na+]~+Ps~'Na++FO~ (30) RTIn[A ]f+PfVA_-FOf=RTln[A-]~+PsVA_-FOs (31)
5.2. Is it still possible to predict equilibrium? RTIn[ C1- ] f -~- P f V c I Model prediction of equilibrium concentrations assuming exclusively an ion-pairing mechanism proved to be accurate for a null osmotic pressure dif-
(29)
- Fl//f =
RTIn[ C1- ]s + PsVcl- -- F~b~ (32)
and for the solvent water we have Eq. (24) where
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
p -p¢= Ap= A H = R T ~ ( Ci - Ci,) = i
=RT[([A
]~+ [CI-]~+ [Na+]~) - ( [ A - If+ [CI- ]r+ [Na+ ]f)]
(33)
These expressions imply that, in order to predict the equilibrium concentration of permeant species, it is necessary to predict the terms for hydrostatic pressure and electric potential at equilibrium. However using these equations the concentrations of the species can be related. Subtracting Eqs. (31 ) and (32) and rearranging, the concentration effect is obtained [A ]~ [El ]~ [A-]f ~exp[
[(P~-P")((/c,--VA. ~-
)
]
(
34)
where ( P ~ - Pr) is given by Eq. (33). For a null osmotic pressure difference the exponential factor is 1 and Eq. (34) turns into Eq. (8), which was obtained assuming an ion-pairing mechanism. As this exponential factor is always < 1 for all other tested conditions, the concentration effect predicted by Eq. (34) will be always less than that predicted by Eq. (8) (Table 2) in accordance with the experimental results.
243
native mechanism occurs without membrane collapse. Salt transport by other mechanisms beyond ion-pairing may also explain the results obtained by other authors for phenylalanine extraction [20]. This work reports that extraction was not improved by increasing the counter ion concentration above a certain value, even though membrane did not collapse. Prediction of equilibrium was not possible to establish under an osmotic pressure difference. However, some conclusions and guidelines for operation can be pointed out: !. When using supported liquid membranes in coupled transport processes, the equilibrium concentration of the extracted solute is independent of the carrier concentration. This is advantageous from an economical point of view because the carrier concentration can be minimized and the same degree of extraction still being achieved. 2. Increasing counter ion concentration does not improve the extraction/stripping process because salt transport by other mechanisms increases and less stripping solute becomes available to be counter transported by an ion-pairing mechanism.
7. List of symbols 6. Conclusions Evaluation of the equilibrium constant, Ke, allows a good prediction of equilibrium concentrations for independent extraction and stripping operations and also for operation with bulk liquid membranes as well as for supported liquid membranes, if both aqueous compartments (feed and stripping) have the same initial molar concentration of salts. If these compartments have different molar concentration of salts it is necessary to account for other mechanisms of salt transport beyond the ion-pairing mechanism proposed. Salt transport occurs as a result of membrane permeability to water. The appearance of aqueous protrusions facilitates ion transport across the interface. This transport is shown to be more important when performing extraction/stripping using supported liquid membranes due to the much thinner thickness of the liquid membrane when compared to the bulk liquid membrane. However, for both membranes, this alter-
[A-] C [Cl-] F
K,~ P R [RA] [RCI] T V
lactate concentration (mol/1) concentration ( mol / 1) chloride concentration (mol/l) Faraday constant (96486.4 A s/mol) equilibrium constant (dimensionless) hydrostatic pressure (Pa) gas constant (8.314 J/mol K) lactate-amine complex (mol/1) chloride-amine complex (mol/l) absolute temperature (K) molar volume (l/mol) feed phase volume (1) stripping phase volume (1) organic phase volume (1) mole fraction (mol / mol)
Uorg X Greek letters chemical potential (J) tt electric potential (V) /7 osmotic pressure (Pa) Subscripts o initial
244
f s i if
ix
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
feed phase stripping phase i species i species in feed phase i species in stripping phase
References [ 1] J.J. Pellegrino and R.D. Noble, Enhanced transport and liquid membranes in bioseparations, Tibtechnol., 8 (1990) 216 [2] A.B. de Haan, P.V. Bartels and J. de Graauw, Extraction of metal ions from waste water. Modeling of the mass transfer in a supported liquid membrane process, J. Membrane Sci., 45 (1989) 281. [3] Z.M. Gu, D.T. Wasan and N.N. Li, Ligand accelerated liquid membrane extraction of metal ions, J. Membrane Sci., 26 (1986) 129. [4] H. Matasuyama, J. Boku and M. Teramoto, Separation and concentration of heavy metal ions by a spiral-type flowing liquid membrane module, Water Treat., 5 (1990) 237. [5] S.C. Boey, M.C. del Cerro and D.L. Pyle, Extraction of citric acid by liquid membrane extraction, Chem. Eng. Res. Dev., 65 (1973) 218. [6] R. Basu and K.K. Sirkar, Hollow fiber contained liquid membrane separation of citric acid, AIChE J., 37 ( 1991 ) 383. [7] J.B. Chaudhuri and D.L. Pyle, Emulsion liquid membrane extraction of organic acids. 1. A theoretical model for lactic acid extraction with emulsion swelling, Chem. Eng. Sci., 47( 1 ) (1992) 41. [8] D.T. Friesen, W.C. Babcock, D.J. Brose and A.R. Chambers, Recovery of citric acid from fermentation beer using supported liquid membranes, J. Membrane Sci., 56 ( 1991 ) 127. [9] P. Nuchnoi, T. Yano, N. Nishio and S. Nagai, Extraction of volatile fatty acids from diluted aqueous solution using a supported liquid membrane, J. Ferment. Technol., 65(3) (1987) 301. [10] J. Hartl and R. Marr, Extraction processes for bioproduct separation, Sep. Sci. Technol., 28( 1/3) (1993) 805. [ 11 ] Z. Lazarova and L. Peeva, Facilitated transport of lactic acid in a stirred transfer cell, Biotechnol, Bioeng., 43 (1994) 907.
[ 12] L.-H. Ju and A. Verma, Characteristics of lactic acid transport in supported liquid membranes, Sep. Sci. Technol., 29 (17) (1994) 2299. [13] P. Deblay, M. Minier and H. Renon, Separation of L-valine from fermentation broths using a supported liquid membrane, Biotechnol. Bioeng., 35 (1990) 123. [ 14] R. Haensel, W. Halwachs and K. Schtigerl, Reactive extraction of D,L-phenylalanine with trioctylmethylammonium chloride (TOMAC) as a carrier, i l l Equilibrium and mass transfer investigations, Chem. Eng. Sci., 47(7) (1986) 1811. [15] M.P. Thien, T.A Hatton and D.I.C. Wang, Separation and concentration of amino acids using liquid emulsion membranes, B iotechnol. Bioeng., 32 (1988) 604. [16] J.B. Chaudhuri and D.L. Pyle, Emulsion liquid membrane extraction of organic acids. II. Experimental, Chem. Eng. Sci., 47 ( I ) (1992) 49. [ 17] W.S. Ho, Emulsion liquid membranes: A review, Proc. 1990 Int. Congr. Membranes and Membrane Processes, Chicago, IL, Vol. I, 1990, p. 692. [ 18] M. Teramoto, T. Yamashiro, A. lnoue, H. Matsuyama and Y. Miyake, Extraction of amino acids by emulsion liquid membranes containing di(2-ethlhexyl) phosphoric acid as a carrier, J. Membrane Sci., 58 ( 1991 ) 11. [19] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Supported liquid membranes: instability elects, J. Membrane Sci., 67 (1992) 121. [20] R. Molinari, L. De Bartolo and E. Drioli, Coupled transport of amino acids through a supported liquid membrane, J. Membrane Sci., 73 (1992) 203. [21 ] O. Loiacono, E. Drioli and R. Molinari, Metal ion separation and concentration with supported liquid membranes, J. Membrane Sci., 28 (1986) 123. [22] I.M. Coelhoso, J.P.S.G. Crespo and M.J.T. Carrondo, Modelling ion-pairing extraction with quaternary amines, Sep. Sci. Technol., (1995) in press. [ 23 ] I. Benjamin, Mechanism and dynamics of ion transfer across a liquid-liquid interface, Science, 261 (1993) 1558, [ 24] P.R. Danesi, L. Reichley-Yinger and P.G. Rickert, Lifetime of supported liquid membranes: The influence of of interracial properties, chemical composition and water transport on the long term stability of the membranes, J. Membrane Sci., 31 (1987) 117. [25] A. Finkelstein and A. Mauro, Physical principles and formalisms of electrical excitability, in Handbook of Physiology, the Nervous System, Section I, vol 1, 1977, p. 161.
Journal of Membrane Science 10g (1995) 231-244
Transport mechanisms in liquid membranes with ion exchange carriers I.M. Coelhoso a, T.F. Moura a,b, J.P.S.G. Crespo a,., M.J.T. Carrondo a,b "Dep. de Qufmica, Faculdade de Ci~ncias e Tecnologia da Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal b lnstituto de Tecnologia Qulmica Biolfgica (ITQB)/Instituto de Biologia Experimental e Tecnolfgica (IBET), Apartado 12, 2780 Oeiras, Portugal Received 28 March 1995; accepted 26 June 1995
Abstract The extraction of lactate by an ion-pairing mechanism using a quaternary ammonium salt ( Aliquat 336) was studied previously and a mathematical model for equilibrium was developed. The evaluation of the equilibrium constant (Ke) of the reaction between carrier and lactate allowed a good prediction of equilibrium for independent extraction and stripping, within a large range of experimental conditions. However, when extraction and stripping operations are carried out simultaneously using liquid membranes and different concentrations of the feed and stripping solutes are used, the resulting osmotic pressure difference between the two aqueous compartments (feed and stripping) has to be taken into account for equilibrium prediction, if the membrane is not totally impermeant to water. Model prediction of equilibrium and identification of the mechanisms involved on the transport of lactate and of the ion counter transported are presented. The influence of operating conditions on each mechanism and their relative contribution to the overall transport are evaluated. Prediction of equilibrium is accurate in the absence of an initial osmotic pressure difference between the two aqueous compartments but exhibit a large deviation for increasing initial chloride concentrations. To counterbalance the initial osmotic pressure difference an increasing hydrostatic pressure difference builds up leading to salt transport across the liquid membrane. Keywords: Liquid membranes; Liquid membrane stability; Ion-pairing transport; Facilitated transport; Lactate extraction
1. Introduction An immiscible organic liquid phase separating two aqueous phases (feed and stripping) may be considered as a liquid membrane. If a reactive extractant is added to the liquid membrane, solute transport can be * Corresponding author. Dep. de Qufmica, Faculdade de Ci~ncias e Tecnologia da Universidade Nova de Lisboa, 2825 Monte de Caparica, Portugal. Phone: 351 1 2954464, ext. 0919. FAX: 351 1 2954461. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10376-7388(95)00174-3
enhanced by this agent, which may interact selectively and reversibly with the solute. The reversible reaction between solute and carrier simultaneously increases the flux of solute and improves selectivity [ 1 ]. Most applications of liquid membranes are concerned with recovery of metal ions [ 2 - 4 ] but studies on separation of carboxylic acids [ 5 - 1 2 ] and aminoacids [ 13-15] have increased recently due to the potential advantages of combining liquid-liquid extraction and membranes in only one operation. The
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
232
Feed Phase
I Organic Phase
Sthripping
IC"4N+a C--' a J IL-R4 Fig. 1. Mechanism of lactate extraction/stripping with a quaternary ammonium salt.
advantages of incorporating carriers in liquid membrane systems are. 1. Highly selective separations are possible, because the specificity of the complexation reaction improves separation. 2. Solutes, especially ions, can be concentrated. 3. Only small amounts of carrier are required due to the small amount of solvent used and its continuous regeneration associated with the reversible reaction [1]. Liquid membranes may contain only liquid phases, which is the case of bulk liquid membranes and emulsion liquid membranes patented two decades ago by Li [16-18] or may contain, additionally, a polymeric membrane to support the organic phase. In supported liquid membranes, a porous membrane impregnated with the organic phase separates the feed and the stripping phases [ 19-21 ]. In this work, the transport of lactate is accomplished by an ion-exchange process, using as carrier a quaternary ammonium salt. The transport of lactate ions from the feed phase to the stripping phase is counterbalanced by an equivalent transport of other anionic species from the stripping to the feed phase through a coupled transport mechanism (Fig. 1). In a previous study [ 22 ] the extraction of lactate by an ion-pairing mechanism using a quaternary ammonium salt (Aliquat 336) was studied and a mathematical model for equilibrium was developed. The evaluation of the equilibrium constant (Ke) of the reaction between carrier and lactate, allowed a good prediction of equilibrium for independent extraction and stripping, within a large range of experimental conditions. Since in liquid membranes extraction and stripping take place simultaneously, separation and concentra-
tion can only be achieved using a high stripping agent concentration, which may cause an osmotic pressure difference between the two aqueous compartments, if the membrane is not totally impermeable to water. The objectives of this work are: I. To predict the equilibrium concentrations in both feed and stripping compartments assuming an ionpairing mechanism for lactate and for the ion counter transported through the liquid membrane. 2. To verify if other transport mechanisms are involved. 3. To evaluate the relative contribution of each mechanism to the overall transport and assess how these contributions are affected by different operating conditions of extraction and stripping. Two different membrane configurations, bulk and supported liquid membranes are discussed.
2. Equilibrium modelling of lactate transport assuming an ion-pairing mechanism Lactate present in the feed phase is extracted by the quaternary amine (Aliquat 336) to the organic phase, while the chloride anion present in the stripping phase is carried in opposite direction (Fig. 1 ). The following reaction takes place in both interfaces: Ke
A-+RC1 ~ RA+CI-
(1)
with the equilibrium constant [RA] [C1-]f [RA] [CI- ]s K e - [ A - ] flRC1 ] - - [ A - ]s[RC1 ]
(2)
where IRA] and [RC1] are the lactate-amine complex and the chloride-amine complex concentrations in the organic phase, [ A - ] r and [ Cl- ] f are the lactate and chloride concentrations in the feed phase, and [ A - ] s and [C1- ]s are the lactate and chloride concentrations in the stripping phase. To evaluate the lactate equilibrium concentration in the feed and stripping phases the following assumptions were assumed: 1. The undissociated form of lactic acid is negligible at pH = 6.3 as the pk, = 3.86. 2. Transport of O H - is negligible compared with lactate and chloride transport, at pH = 6.3.
233
I.M. Coelhoso et al./Journal of Membrane Science 108 (1995) 231-244
3. The membrane is totally impermeable to water and so, water coextraction is insignificant and has no influence on the extraction process. 4. As lactate is very poorly soluble in the organic diluent there is no physical extraction (in agreement with experimental observation). 5. The ion exchange reaction between lactate and the quaternary ammonium salt occurs at the interface and the equilibrium constant, K¢, governs equilibrium at both interfaces. 6. Lactate and chloride transport is accomplished exclusively by an ion pairing mechanism. 7. The concentration of the lactate-amine complex, [RA], is the same in both interfaces. This is also valid for the chloride-amine complex, [RC1]. A discussion of these assumptions is presented elsewhere [ 22].
. [C1-]o-
[A-]~
(9)
and simplifying, a relationship between the lactate equilibrium concentration in the two compartments is obtained:
[A-Is
- -
-
[A-]f
[C1-]o
-
-
(10)
[A-]o
According to Eq. (10), the model predicts that the concentration effect, [ A - ] s / [ A - ] f increases with the increasing stripping agent concentration, [C1-]o, if lactate transport is exclusively accomplished by an ionpairing mechanism. An expression for the reaction equilibrium constant results from substitution of Eqs. (3), (4) and (6) in Eq. (2):
(iN_ ] o ~f - - - [ A - ] c - -uf- - - [ A - l s v~) ([A ] o - [ A - b )
2.1. Bulk liquid membrane
Uorg
Vorg
Uorg]
Ko -Uf [A-] [RC1]o---[A ] o + -Uf - [ A - ] r + [ A L vs t
The material balances for amine, lactate and chloride, using different volumes of feed, organic and stripping phases, respectively, vf, Vor~and vs, are:
Uorg'
Uorg
Uorg]
(11)
[RC1]o = [RC1] + [RA]
(3)
substituting [ A - ]s from Eq. (10), the lactate equilibrium concentration in the feed phase can be expressed
vf[A-]o=vf[A-]f+vs[A-]s+Vorg[RA ]
(4)
as:
Vorg[ RC1 ]o + vs[C1- ]o1 = Vorg[RC1 ] + v f [ C l - ] f + us[C1- ]s
[A-] -
b+ Ked- ~/(b+ Ked) 2-4ac( 1 -Ke) 2c( 1 - K~)
(5)
where [RC1]o, [ A - ] o and [C1-]o represent, respectively, the initial concentrations of Aliquat 336, lactate and chloride. As it was assumed that transport was exclusively accomplished by an ion-pairing mechanism then, at any time:
(12) where a = I A - ]o2 uf
(13)
Uorg
b = - 2 [ A - ] o vf _ [C1-]0 [ A - ]o = [ A - ]f--~ [C1- ]f
(6)
[Cl-]0 = [CI-]~+ [A-]~
(7)
From Eq. (2) the lactate equilibrium concentration in the stripping phase can be related to the lactate equilibrium concentration in the feed phase by: [A-ls= [A-lf~ Using Eqs. (6) and (7)
(8)
Vorg
Us
(14)
Uorg
c = vf + v~ [C1-]o Uorg ©org [ A - ] o
(15) Uf
d = [RC1]o- [ A - ] o - -
Uorg
(16)
This model allows for prediction of lactate equilibrium concentrations in the feed and stripping compartments [Eqs. (12) and (10), respectively], using defined volume phase ratios, vf/Vorgand Vs/Vorvinitial
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L M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
lactate, chloride and amine concentration, [A-]o, [ C1- ] o and [ RCI ] o, if the equilibrium constant, Ke, is known.
2.2. Supported liquid membrane The liquid membrane is immobilized within the pores of the support porous membrane. The same considerations made for the bulk liquid membrane are valid and Eqs. ( 1) to (10) still apply. In this case, the organic phase volume is, however, much smaller than the feed and stripping phase volumes. Thus, lactate and chloride present in the organic phase are negligible when compared to the quantities of these two ions in the feed and stripping compartments. As a result, all terms that multiply by Vorgin Eqs. (4) and (5) can be neglected. Taking into account these simplifications, the chloride concentration in each compartment, the lactate concentration in the stripping phase and the equilibrium constant, Ke, may be obtained, expressed in terms of the initial lactate and chloride concentrations and volume phase ratio, as: [C1-]f = [A-]o-
[A-If
(17)
[C1- ]s= [C1- ] o - v f ( [ A - ] o - [ A - ]f)
(18)
Us Uf
[A-]s=--([A-]oUs
(19)
[A-]f
([A-]~vf-[A-]f(-2vf[A-]o-vs[Cl-]o)+a'[A-]~) [A ]f([RCl]ovo~g-vf[A-]o)+a'[A
]2
(20) where a'=
[C1-]o vf+vS[A-]o
(21)
and as Oorg[ RC1 ] o << vf[ A - ] o, lactate equilibrium concentration in the feed phase, [ A - ] f, becomes independent of the initial amine concentration, [RC1]o, beeing given by: [A-]f b' +Ke[A ] o - ~ / ( b ' + K e [ A - ] o ) 2 - 4 a ' [ A - ] ~ ( 1 - K e )
2a'(1 -Re) (22) where a' is defined by Eq. (21) and
b' = 2 [ A - ]o + [CI- ]o~ ve
(23)
As stated for the bulk membrane cell, the model predicts lactate equilibrium concentration in the feed and stripping compartments, using a defined volume phase ratio, vs/vf, initial lactate and chloride concentration, [ A - ]o and [C1- ]o, if the equilibrium constant, Ke, is known.
3. Experimental 3.1. Extraction system The bulk liquid membrane cell and the supported liquid membrane cell used in the experiments are represented in Fig. 2 and Fig. 3, respectively. The feed phase is a lactate solution (280 mM) obtained by dilution of sodium lactate 60% (Sigma, USA). The pH of this solution was adjusted to 6.3, which is the pH for lactic acid fermentation, with NaOH 0.1 M. The stripping phase is a sodium chloride solution of variable concentration ranging from 0.28 to 5 M. Three different compositions of the organic phase were used: 10% (w/w), 30% (w/w) and 50% (w/w) Aliquat 336 in Shellsol A. In the experiments using the bulk membrane cell the phase volume ratio employed was 3:3:1, that is, 180 ml of feed phase, 180 ml of organic phase and 60 ml of stripping phase. For the supported liquid membrane cell a polypropylene membrane with a pore diameter of 0.01/xm and, 55% porosity (Gelman Sciences, USA) was soaked in the organic phase and then placed between the feed and stripping reservoirs. In this case, the volumes of the feed and stripping phases were equal, 150 ml of each phase. For both cells, the stirring speed of the phases was adjusted to 100 rpm and the experiments performed at a constant temperature of 40°C during 8 days, until equilibrium was reached. This was assured by sampling once a day, for the last 3 days and obtaining a constant lactate concentration.
3.2. Analytical methods Lactate concentration was determined by HPLC.The column used was a Shodex SH 1011 (Showa Denko
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
235
-Q
--03
V
•- _ - - - - - - _ - 2 - - _ ___-_-
--_-
. . . .
z _ - _ - z _-_-_-.:
.
.
.
.
.
.
.
.
.
.
.
.
.
.
_ ~'.6
L---_-L:?_: _-_-_-
$'_-_-2-
-_
Fig. 2. Bulk liquid membranecell: 1, samplingpoint (feed phase); 2, sampling point (stripping phase); 3, sampling point (organic phase); 4, mechanical stirrer; 5, Teflon seal; 6, feed phase; 7, stripping phase; 8, organic phase; 9, magnetic stirrer. K.K., Japan) and the eluent was 0.01 N sulfuric acid. A refractive index detector ( Merck Hitachi, Japan) was employed. The pH of the aqueous solutions was measured using a combined electrode model U457-57/110 (Ingold) and an ion meter model 720A (Orion, USA). The water content of the organic phase was determined by Karl Fischer titration ( A Q U A P A L III, UK).
Chloride concentration was measured with a combined electrode model 96-17B (Orion, USA) and an ion meter model 720A (Orion, USA). In order to remove interferences of other ions and to adjust the ionic strength of samples and standards, to each ml of sample or standard, 1 ml of CISA solution (Orion, USA) and 0.04 ml of ISA solution (Orion, USA) were added, before measuring the chloride concentration.
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for the reaction of lactate with the quaternary ammonium salt (Aliquat 336) was evaluated using equilibrium experiments. The value obtained was: Ke = 0.081 __+0.007
Fig. 3. Supported liquid membrane cell: 1, sampling point (feed phase); 2, sampling point (stripping phase); 3, connection for continuous operation; 4, mechanical stirrer; 5, Teflon seal; 6, feed phase; 7, stripping phase; 8, membrane; 9, magnetic stirrer; 10, porous glass support.
4. Results 4.1. Lactate transport assuming an ion-pairing mechanism Bulk liquid membrane In a previous study [22], the equilibrium constant
Using this value for the equilibrium constant and assuming that lactate transport is exclusively achieved by an ion-pairing mechanism, lactate equilibrium concentration in the feed and stripping compartments of the bulk membrane cell can be evaluated using Eqs. (12) and (10), respectively. Table 1 shows the predicted and experimental values obtained for the lactate equilibrium concentrations in both compartments of the cell, varying the concentration of the quaternary amine. The stripping agent concentration, [ C1- ] o is 1 M. The measured pH values remained constant in the range of the analytical error for pH detection. The model predicts that equilibrium concentrations of lactate in both phases decreases with increasing initial amine concentration as more [RA] is formed at equilibrium and remains in the liquid membrane. The experimental values follow the same pattern. The experimental and the predicted lactate concentrations are similar for each amine concentration tested, giving 10 to 16% deviation for lactate concentration in the feed compartment, and - 14 to - 26% deviation in the stripping compartment. Deviation is calculated as ( [ A - ] exp- [ A - ] model)/ [A-- ] e~p× 100. However, the deviation between experimental and predicted values of lactate equilibrium concentration, for the same amine concentration, increases with the initial chloride concentration present in the stripping compartment (Table 2). For equal concentrations of lactate and chloride (0.275 M), the deviation between model and experimental lactate concentration in both feed and stripping compartments is only - 4 % , which is in the range of
Table 1 Comparison of experimental and model lactate equilibrium concentration: Effect of amine concentration. Bulk membrane, phase ratio (feed:org:strip) 3:3:1 [A-]o (M)
[C1- ]0 (M)
Aliquat (%)
[ A - ] fexp (M)
[ A - ] fmoct~l (M)
Deviation (%)
[ A - ]~exp (M)
[A ] ~,,ooel (M)
Deviation (%)
0.281 0.281 0.267
1.000 0.998 1.000
10 30 50
0.137 0.135 0.109
0.123 0.113 0.097
10 16 11
0.383 0.318 0.298
0.436 0.402 0.364
- 14 - 26 - 22
1.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
237
Table 2 Comparison of experimental and model lactate equilibrium concentration: Effect of initial chloride concentration. Bulk membrane, phase ratio (feed:org:strip) 3:3:1
[A-]fmodel
[A-J0 (M)
[CI ]o A l i q u a t [ A - b e x p (M) (%) (M)
(M)
Deviation[A-]~exp (%) (M)
[A-]smode I (M)
Deviation [ A - L / [ A (%) exp
0.275 0.281 0.276 0.279
0.276 0.998 1.950 4.710
0.158 0.113 0.077 0.041
-4 16 35 67
0.158 0.402 0.540 0.688
-4 - 26 - 50 - 83
30 30 30 30
0.152 0.135 0.118 0.126
0.152 0.318 0.360 0.376
1.00 2.35 3.05 2.98
]f
[A ] ~ / [ A - ] f model 1.00 3.55 7.06 16.88
the analytical error for lactate detection. Moreover, the experimental values for lactate equilibrium concentration in both compartments are equal, as predicted by Eq. (10). For higher chloride concentration the deviations increase reaching 67% for the feed compartment and 83% for the stripping compartment, when a 4.71 M chloride concentration is used. Contrary to the predictions of Eq. (10), it is clearly demonstrated (Table 2) that it is not worthwhile to increase chloride concentration above 1 M. The concentration effect deviates further from the model predictions as the stripping agent concentration increases and the amount of lactate extracted using [C1-]o = 1.98 M and [C1-]o=4.71 M is not significantly different from the lactate extracted with [C1- ]o = 1 M. Similar results were obtained by Drioli and coworkers for phenylalanine extraction with Aliquat 336 [20].
the same initial amine concentration and different initial chloride concentrations. Prediction of equilibrium is accurate if both compartments (feed and stripping) have the same molar concentration of salts but is no longer valid for different salt concentrations. Higher deviations from model predictions are observed in the supported liquid membrane than in the bulk liquid membrane configuration. Indeed, a 92% deviation between the experimental and model lactate equilibrium concentration is obtained for the highest chloride concentration used. As stated before for the bulk membrane cell, in this system it is not worth to use [C1- ] o > 1 M. In fact, for the supported liquid membrane cell, the lactate
Supported liquid membrane
[ A - ]o (M)
Aliquat 336 (%)
[ A - ]f (M)
0.290 0.282 0.276
10 30 50
0.140 0.133 0.124
For the supported liquid membrane cell, since the organic phase volume is negligible when compared to the feed and stripping phase volumes, the predicted equilibrium concentration in the feed phase is independent of the initial amine concentration used, [RC1]o, according to Eq. (22). The experimental equilibrium values of [ A - ] f obtained for 10% ( w / w ) , 30% (w/w) and 50% (w/ w) of Aliquat 336 in Shellsol A for equal initial stripping agent concentration, [ C1- ] o = 1 M were not significantly different (Table 3). The measured pH values remained constant in the range of the analytical error for pH detection. Table 4 compares the experimental and predicted lactate equilibrium concentration, assuming that transport is exclusively accomplished by ion-pairing using
Table 3 Effect of amine concentration on the lactate extracted. Liquid supported membrane cell: polypropylene membrane 0.1/xm; [ C1 - ] o = 1 M
Table 4 Comparison of experimental and model lactate equilibrium concentrations: Effect of initial chloride concentration. Supported liquid membrane, polypropylene membrane 0.1 /zm [A-]o (M)
[Cl-]0 (M)
[A-]fexp
[A-]fm~e I
(M)
(M)
Deviation (%)
0.280 0.282 0.273 0.287
0.276 0.998 1.950 4.710
0.154 0.133 0.201 0.197
0.141 0.062 0.034 0.015
8 53 83 92
238
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
extracted with higher chloride concentration is even lower than with [C1- ]o = 1 M.
4.2. Chloride transport by alternative mechanisms The results presented in the previous section clearly indicate the presence of another type of transport beyond the ion-pairing mechanism assumed in the model. In fact, if the total amount of lactate transported is lower than the model prediction, as the difference of salt concentration between the feed and stripping compartments increases, that must be due to other mechanisms of transport of the chloride stripping ion. As the difference between the experimental and model prediction of equilibrium concentration of lactate increases the assumptions that: 1. the membrane is impermeable to water; 2. anion transport is exclusively accomplished by an ion-pairing mechanism; are no longer valid. Thus, there is a need to consider the contribution of the initial osmotic pressure difference between the two compartments of the cell and the resulting water and ion transport mechanisms occurring under these experimental conditions. Evidence of water equilibration is reflected by the observed hydration of the organic phase in the bulk liquid membrane. Analyzing the organic phase during W0(water m o l a r e o n c . / a m i n e
the time course of independent experiments an increase in water content was observed, from an initial value of Wo= 0.5 mol water/mol amine until a value of Wo= 5 mol water/mol amine within the first 20 h (Fig. 4). Indeed, when the system is first established and an initial osmotic pressure difference (A 17) exists, water will equilibrate between the two compartments. The ultimate consequence of water transport would be a volume change of both aqueous phases which would cause a variation of the concentrations of the species in both compartments. However, experimentally, a macroscopic volume change of either phase was not observed. As a result of the fixed volume of both aqueous compartments, an increase in hydrostatic pressure difference (AP) builds up until it balances the osmotic pressure difference and equilibrates water
A p = p s - pf= AH=RT~(C,s- C#)
(24)
i
where Ps, P f and Cis, fir represent the hydrostatic pressure and the concentrations of the i'th species in the stripping and feed compartments, respectively. Due to the hydrostatic pressure difference, the driving forces acting on chloride and sodium lead to their transport from the stripping to the feed compartment. Sodium is simultaneously transported in order to assure electroneutrality in both phases. The transport of chloride and sodium implies that less chloride is available to be counter transported by an ion-pairing mechanism. m o l a r cone.') O
t0~ ALIQUAT
0
30~ ALIQUAT
[]
50~ ALIQUAT
O 4
O
~0 []
0~00
00 0 ~D
O0
000
[]
[]
Q
© 2
0
i
I
I
I
50
I00
150
200
250
Time (h) Fig. 4. Evolution of the ratio water molar concentration/amine molar concentration in the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; [Cl-]o = 1 M).
L M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244 120
Under these circumstances, the chloride concentration determined from the model where transport by ionpairing is exclusively assumed is not correct; consequently, less lactate is extracted and reextracted. In order to verify this statement the salt concentration difference between the feed and stripping compartments
239
% Chloride T r a n s p o r t
1oo
[I~
Jmlt ion-pairing
89 84
80
86 .
.
.
.
60
was calculated at the begginning of the experiment and at equilibrium, using the measured chloride and lactate concentrations and keeping electroneutrality in the bulk aqueous solutions. Table 5 shows that this salt concentration difference at equilibrium is always lower than its initial value; thus, there was a net transfer of sodium chloride from the stripping to the feed compartment. This salt transport increases with increasing difference between initial salt concentration in the compartments, i.e. with increasing initial osmotic pressure difference. The relative contributions of the different mechanisms involved in chloride transport may be calculated from the molar balances for lactate and chloride in the feed compartment: [ A - ] o V f - [A ]fuf=nAi~.pai.ng
(25)
[C1 - ] ¢~f= ncJ~o._~a~..~+ nc,~a~,
(26)
Eqs. (25) and (26) show that lactate transported from the feed compartment is carried by an ion-pairing mechanism, while chloride arising in the feed compartment results from the addition of ion-pairing and other transport mechanisms. Since: nAion_pairing= ncl.~._p.in..
(27)
it is possible to calculate the amount of chloride that is transported coupled with sodium, ncl..a..
40
20
0
10% ALIQUAT
30% ALIQUAT
50% AL1QUAT
Fig. 5. Effect of amine concentration on chloride transport ( salt and ion-pairing) for the feed compartment of the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; [Cl- ]o = l M).
Bulk liquid membrane Fig. 5 shows that the contribution of other mechanisms of chloride transport is not affected by the amine concentration used and varies between 11 and 16% of the total transport, when using [ C1- ] o = 1 M. Fig. 6 shows an increasing contribution of the salt transport mechanism with increasing initial chloride concentration. This contribution is null (3% is within the experimental error of chloride detection) for equal salt concentration in both compartments (where (AH)o=0, ([A-]o =[C1-]o=280 mM) and increases with the difference between the initial salt concentrations [ C 1 - ] o - [ A - ] o , i.e. with increasing initial osmotic pressure difference.
Table 5 Difference of salt concentration between feed and stripping compartments. Bulk membrane, phase ratio (feed:org:strip) 3:3:1 [A-]o (M)
[Cl-]o (M)
Aliquat (%)
[A-]fex p (M)
[C1]-fexp (M)
[A-l~exo (%)
[Cl]-sexp (M)
(E~s--Cif)o
(E~s--~f)eq
(M)
(M)
0.275 0.281 0.276 0.279
0.276 0.998 1.950 4.710
30 30 30 30
0.152 0.135 0.118 0.126
0.127 0.164 0.182 0.237
0.152 0.318 0.360 0.376
0.122 0.597 1.230 3.215
0.002 1.434 3.348 8.862
0.01 1.232 2.580 6.456
240
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
Supported liquid membrane
% Chloride Transport
120
m
salt
ion-pairing
100
97 B9
B7
B0-
64 6 0 ~- ....
For the supported liquid membrane, the transport contribution resulting from the salt transport mechanism, calculated by Eqs. ( 2 5 ) - ( 2 7 ) , is more relevant than was obtained for the bulk membrane configuration, but decreases with increasing amine concentration (Fig. 7). This is due to the thinner thickness of the supported liquid membrane compared with the bulk membrane, where the resistance for salt (NaC1) transport is higher. However, the observed decrease of the contribution of this term with increasing amine concentration, may be explained by an increase in membrane resistance due to higher viscosity of the
40
Table 6 Variation of organic phase viscosity with amine concentration 20
!.
3 0 - m A0=C10=280 m M
CIO=I M
CI0--2 M
CI0=5 M
Fig. 6. Effect of the initial chloride concentration on chloride transport (salt and ion-pairing) for the feed compartment of the bulk liquid membrane cell (phase ratio, feed:org:strip 3:3:1; organic phase, 30% Aliquat + 70% Shellsol A). % Chloride
Aliquat 336 (%)
Viscosity (mPa s)
5 10 20 30 50
0.90 1.01 1.61 2.89 11.10
120
Transport
% Chloride T r a n s p o r t
B0
• m
m
71
salt len-pttr|~g
s~t I00
60
B0
60
40 40
20 20
0
10~. ALIQUAT
30% ALIQUAT
50~. ALIQUAT
Fig. 7. Effect of amine concentration on chloride transport (salt and ion-pairing) for the feed compartment of the supported liquid membrane cell (phase ratio, feed:strip l:l; [C1-]0 = 1 M).
A0=C10--280 mM
C]0=I M
ClO=2 M
C10=5 M
Fig. 8. Effect of amine concentration on chloride transport (salt and ion-pairing) for the feed compartment of the supported liquid membrane cell (phase ratio, feed:strip 1:1; organic phase, 30% Aliquat + 70% Shellsol A).
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
120
% Salt T r a n s p o r t
proceed via an ion-pairing mechanism is accurate in the absence of an initial osmotic pressure difference. However, for increasing initial osmotic pressure difference the model predictions show large deviations for both membrane configurations. Furthermore, our experimental results show that the assumption of water impermeability of the membrane was not valid and consequently the initial osmotic pressure difference affects the system performance. This effect accounts for the deviations observed from the model predicted values of (i) extracted lactate, (ii) concentration effect and (iii) salt (NaCI) transport. Although, two questions arise for discussion: 1. If the ion-pairing mechanism is not the sole responsible for ion transport, how can sodium and chloride be transported across the liquid membrane, since they are very poorly soluble in the organic phase? 2. If alternative mechanisms of transport are active, is it still possible to predict equilibrium?
Bulk m e m b r a n e -0-
Supported m e m b r a n e
I00
8O
60
0
40
20
Ol
t
i
I
i
2
4
6
8
241
I0
( a ~ /RT)O (M) Fig. 9. Variationof salt transportwith the initial differencebetween salt concentrations, (A I-I/RT)o, for both cells.
5. I. How are sodium and chloride transported across the liquid membrane ?
organic phase with increasing amine concentration (Table 6). The contribution of this non ion-pairing mechanism increases with the initial chloride concentration, i.e. with the initial osmotic pressure diference between both cell compartments. The results obtained are similar to the behaviour observed with the bulk liquid membrane, but the magnitude is higher (Fig. 8). For [ C1- ] 0 = 4.71 M, the ion-pairing mechanism is responsible for only 10% of the chloride transport. The contribution of the salt transport mechanism as a function of the initial difference between salt concentration, ( A H / R T ) o , is represented in Fig. 9 for both cells. An increase of this transport contribution as the difference between initial salt concentration for both cells increases can be observed. As mentioned before, the salt transport contribution is stronger for the supported liquid membrane than for the bulk liquid membrane.
Despite numerous experimental studies and modelling, the mechanism by which ions cross the interface between two immiscible liquids is not well understood. This results from a lack of direct knowledge about the structure of the interface and the difficulty of experimentally monitoring the transport process in real time. The common picture of a sharp interface cannot explain the mechanism of ion transport. Recently, it was shown that surface roughness and capillary distortion play important roles in the transfer process. These capillaries act as constantly moving "fingers" of water protruding into the organic phase, facilitating ion transport across the interface [ 23 ]. The hydrostatic pressure difference resulting from the initial osmotic pressure difference is the radial component of a tangential surface tension acting on the membrane, which eventually becomes high enough to cause membrane deformations. These deformations of equilibrated solution may originate aqueous "pockets" which can migrate through the membrane from one side to the other. This mechanism would increase the membrane permeability to sodium and chloride. This could explain why the transport resulting from other mechanisms than ion-pairing is more relevant in the supported liquid membrane. As a result of the thin-
5. Discussion
Model predictions assuming membrane impermeability to water and cations, where anion transport would
242
LM. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
ner liquid membrane the formation of these aqueous protrusions is favoured and a larger number of ions can be transported by this alternative route, leading to a smaller contribution of the ion-pairing mechanism to the overall transport process. In the bulk liquid membrane, the existence of a very thick barrier hinders the development of these protuberances and the transport not accomplished by ionpairing becomes less favourable. However, the existence of these aqueous protrusions does not mean that the liquid membrane is unstable or even that it has collapsed. Studies on stability of supported liquid membranes indicate that they become increasingly unstable with high differences between the osmotic pressures of the feed and stripping compartments. The following mechanism of water transport through supported liquid membranes was proposed: (i) in the presence of an osmotic pressure gradient, water tends to flow through the organic-filled pores of the supported liquid membrane; (ii) when the amount of flowing water becomes massive, the organic phase is displaced from the support pores and replaced by water; at this stage the membrane behaves as a semipermeable diaphragm containing water-filled micropores [ 24]. If the supported liquid membrane had collapsed, the values of lactate concentration should be equal in both aqueous compartments. Definitely, this was not observed during these studies even for the highest chloride concentration tested. For the bulk liquid membrane a collapse is not possible due to the large thickness of the organic phase. Moreover, these results prove that salt transport by alternative mechanisms occurs in the presence of an initial osmotic pressure difference. From these results it may be concluded that salt transport resulting from the establishment of an initial osmotic pressure difference between the feed and stripping compartments leads to a reduction on the contribution of the ion-pairing mechanism to the overall transport process. However, this alternative mechanism proved to occur for both membrane configurations (bulk liquid membrane and supported liquid membrane) without membrane collapse.
ference. Additionally, the supported liquid membrane model predicts that equilibrium is independent of the initial amine concentration, which was experimentally confirmed. However, as the membrane is not totally impermeable to water, in the presence of an initial osmotic pressure difference salt transport occurs, decreasing the contribution of the ion-pairing mechanism. Equilibrium prediction requires the evaluation of the relative contribution of each mechanism for the overall transport. The fundamental relationship needed for equilibrium prediction is that the electrochemical potential,/zi, of any species i is the same in all phases to which the species has access. Thus, for the feed and stripping phases, it results in (28)
/.Lif = ].Zis
provided that species i can move between the two phases. For a specific solute of an ideal solution,/zi is given by Id~i = IZ~ O) -]- RTIn X i -~- PV~+ ziF0
where/~o) is the standard chemical potential (in the particular phase being considered) of the i'th species, X~ is the molar fraction of the i'th species, Vi is the partial molar volume of i'th species, z~is the valence of i'th species, P is the hydrostatic pressure of the phase, ~b is the electric potential of the phase, R is the gas constant, Tis absolute temperature and Fis the Faraday constant [ 25 ]. As a rough approximation the aqueous solutions will be considered dilute solutions and the molar fraction replaced by concentration. Applying Eqs. (28) and (29) to sodium, lactate and chloride: RTln[Na + ] f"[- Pf~'~Na + +F~bf=RTln[Na+]~+Ps~'Na++FO~ (30) RTIn[A ]f+PfVA_-FOf=RTln[A-]~+PsVA_-FOs (31)
5.2. Is it still possible to predict equilibrium? RTIn[ C1- ] f -~- P f V c I Model prediction of equilibrium concentrations assuming exclusively an ion-pairing mechanism proved to be accurate for a null osmotic pressure dif-
(29)
- Fl//f =
RTIn[ C1- ]s + PsVcl- -- F~b~ (32)
and for the solvent water we have Eq. (24) where
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
p -p¢= Ap= A H = R T ~ ( Ci - Ci,) = i
=RT[([A
]~+ [CI-]~+ [Na+]~) - ( [ A - If+ [CI- ]r+ [Na+ ]f)]
(33)
These expressions imply that, in order to predict the equilibrium concentration of permeant species, it is necessary to predict the terms for hydrostatic pressure and electric potential at equilibrium. However using these equations the concentrations of the species can be related. Subtracting Eqs. (31 ) and (32) and rearranging, the concentration effect is obtained [A ]~ [El ]~ [A-]f ~exp[
[(P~-P")((/c,--VA. ~-
)
]
(
34)
where ( P ~ - Pr) is given by Eq. (33). For a null osmotic pressure difference the exponential factor is 1 and Eq. (34) turns into Eq. (8), which was obtained assuming an ion-pairing mechanism. As this exponential factor is always < 1 for all other tested conditions, the concentration effect predicted by Eq. (34) will be always less than that predicted by Eq. (8) (Table 2) in accordance with the experimental results.
243
native mechanism occurs without membrane collapse. Salt transport by other mechanisms beyond ion-pairing may also explain the results obtained by other authors for phenylalanine extraction [20]. This work reports that extraction was not improved by increasing the counter ion concentration above a certain value, even though membrane did not collapse. Prediction of equilibrium was not possible to establish under an osmotic pressure difference. However, some conclusions and guidelines for operation can be pointed out: !. When using supported liquid membranes in coupled transport processes, the equilibrium concentration of the extracted solute is independent of the carrier concentration. This is advantageous from an economical point of view because the carrier concentration can be minimized and the same degree of extraction still being achieved. 2. Increasing counter ion concentration does not improve the extraction/stripping process because salt transport by other mechanisms increases and less stripping solute becomes available to be counter transported by an ion-pairing mechanism.
7. List of symbols 6. Conclusions Evaluation of the equilibrium constant, Ke, allows a good prediction of equilibrium concentrations for independent extraction and stripping operations and also for operation with bulk liquid membranes as well as for supported liquid membranes, if both aqueous compartments (feed and stripping) have the same initial molar concentration of salts. If these compartments have different molar concentration of salts it is necessary to account for other mechanisms of salt transport beyond the ion-pairing mechanism proposed. Salt transport occurs as a result of membrane permeability to water. The appearance of aqueous protrusions facilitates ion transport across the interface. This transport is shown to be more important when performing extraction/stripping using supported liquid membranes due to the much thinner thickness of the liquid membrane when compared to the bulk liquid membrane. However, for both membranes, this alter-
[A-] C [Cl-] F
K,~ P R [RA] [RCI] T V
lactate concentration (mol/1) concentration ( mol / 1) chloride concentration (mol/l) Faraday constant (96486.4 A s/mol) equilibrium constant (dimensionless) hydrostatic pressure (Pa) gas constant (8.314 J/mol K) lactate-amine complex (mol/1) chloride-amine complex (mol/l) absolute temperature (K) molar volume (l/mol) feed phase volume (1) stripping phase volume (1) organic phase volume (1) mole fraction (mol / mol)
Uorg X Greek letters chemical potential (J) tt electric potential (V) /7 osmotic pressure (Pa) Subscripts o initial
244
f s i if
ix
I.M. Coelhoso et al. / Journal of Membrane Science 108 (1995) 231-244
feed phase stripping phase i species i species in feed phase i species in stripping phase
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[ 12] L.-H. Ju and A. Verma, Characteristics of lactic acid transport in supported liquid membranes, Sep. Sci. Technol., 29 (17) (1994) 2299. [13] P. Deblay, M. Minier and H. Renon, Separation of L-valine from fermentation broths using a supported liquid membrane, Biotechnol. Bioeng., 35 (1990) 123. [ 14] R. Haensel, W. Halwachs and K. Schtigerl, Reactive extraction of D,L-phenylalanine with trioctylmethylammonium chloride (TOMAC) as a carrier, i l l Equilibrium and mass transfer investigations, Chem. Eng. Sci., 47(7) (1986) 1811. [15] M.P. Thien, T.A Hatton and D.I.C. Wang, Separation and concentration of amino acids using liquid emulsion membranes, B iotechnol. Bioeng., 32 (1988) 604. [16] J.B. Chaudhuri and D.L. Pyle, Emulsion liquid membrane extraction of organic acids. II. Experimental, Chem. Eng. Sci., 47 ( I ) (1992) 49. [ 17] W.S. Ho, Emulsion liquid membranes: A review, Proc. 1990 Int. Congr. Membranes and Membrane Processes, Chicago, IL, Vol. I, 1990, p. 692. [ 18] M. Teramoto, T. Yamashiro, A. lnoue, H. Matsuyama and Y. Miyake, Extraction of amino acids by emulsion liquid membranes containing di(2-ethlhexyl) phosphoric acid as a carrier, J. Membrane Sci., 58 ( 1991 ) 11. [19] A.M. Neplenbroek, D. Bargeman and C.A. Smolders, Supported liquid membranes: instability elects, J. Membrane Sci., 67 (1992) 121. [20] R. Molinari, L. De Bartolo and E. Drioli, Coupled transport of amino acids through a supported liquid membrane, J. Membrane Sci., 73 (1992) 203. [21 ] O. Loiacono, E. Drioli and R. Molinari, Metal ion separation and concentration with supported liquid membranes, J. Membrane Sci., 28 (1986) 123. [22] I.M. Coelhoso, J.P.S.G. Crespo and M.J.T. Carrondo, Modelling ion-pairing extraction with quaternary amines, Sep. Sci. Technol., (1995) in press. [ 23 ] I. Benjamin, Mechanism and dynamics of ion transfer across a liquid-liquid interface, Science, 261 (1993) 1558, [ 24] P.R. Danesi, L. Reichley-Yinger and P.G. Rickert, Lifetime of supported liquid membranes: The influence of of interracial properties, chemical composition and water transport on the long term stability of the membranes, J. Membrane Sci., 31 (1987) 117. [25] A. Finkelstein and A. Mauro, Physical principles and formalisms of electrical excitability, in Handbook of Physiology, the Nervous System, Section I, vol 1, 1977, p. 161.