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E L S E V I E R Desalination 147 (2002) 399--403

DESALINATION

www.elsevier.com/locate/desal

Combined effect of salt concentration and pressure gradients

across charged membranes

Juana Benavente a*, Gunnar Jonsson b

aDepartamento de F[sica Aplicada, Facultad de Ciencias, Universidad de Mcilaga, E-29071 M6laga, Spain Fax +34 (952) 132000; email: J_Benavente@uma.es

bChemical Engineering Department, Building 229, Technical University of Denmark, DK-2800 Lyngby, Denmark

Received 20 February 2002; accepted 5 March 2002

Abstract

The combined effect of both concentration and pressure differences on electrical potential (Ad~) for two ion-exchanger membranes, one positively charged (AE) and another negatively charged (CE), measured with the membranes in contact with NaCI solutions was studied. Results show a linear dependence between Adp and pressure, independently if AC and AP have the same or opposite directions. The ratio of the streaming potential for cation/anion exchange membranes is r = (2.1+0.4). A "bipolar" membrane (BM) was obtained by joining together both ion-exchanger membranes. In order to correlate the behaviour of the BP membrane with that corresponding to each sublayer, the same kind of measurements was carded out for both opposite external conditions, this means, applying the pressure on the cation exchanger (CABM) or on the anion exchanger membrane (ACBM), respectively, From values obtained at AP = 0, the counter-ion transport number in each ion-exchange membrane was obtained and the contribution of membrane potential on A0 values can be evaluated. Results show clear differences on both the membrane potential and the effect of pressure in the bipolar membrane depending on these two external situations, and in this case the ratios are: r ~c~a~vAcB~ = (2.2±0.4),/c~uJcE) = (5.0-a:1.2) and r tAcBM/AE) = (4.9-~0.3); higher influence of the anion exchange sublayer on the electrical potential difference values was obtained. A reduction around 75-85% in the transport number of the ions in the bipolar membrane with respect to that corresponding at each charged sublayer was estimated.

Keywords: Ion-exchange membranes; Bipolar membranes; Concentration and pressure-induced potentials; Ion transport numbers

1. I n t r o d u c t i o n properties are c o m m o n l y used in m a n y separation processes such as nanofiltration and reverse C o m p o s i t e m e m b r a n e s consisting in two osmosis [1]. The influence o f each sublayer in layers from different material and transport the performance o f composite membranes

*Corresponding author, depends on the structure and transport

Presented at the International Congress on Membranes and Membrane Processes (1COM), Toulouse, France, July 7-12, 2002.

0011-9164/02/$- See front matter © 2002 Elsevier Science B.V. All rights reserved

400 J. Benavente, G. Jonsson / Desalination 147 (2002) 399-403

characteristic of each layer and the external conditions [2,3]. In general, standard procedures

for in situ membrane characterisation cannot be

used for composite membranes without different extra assumptions, which usually suppose to neglect/minimise the contribution of one sub- layer. On the other hand, membrane asymmetry can affect the transport of solution and ions across these membranes due to the different concentration gradients across each sublayer, and differences in the value of some characteristic parameters (salt permeability, ion transport numbers,) depending on membrane orientation have already been reported in the literature [4,5]. Bipolar membranes consist of two oppositely charged layers, and thus they can be considered as a particular kind of composite membranes; in this case, the different transport properties of each layer are related to the high exclusion of the co-ions presented by each sublayer.

In a previous work, filtration streaming potential and membrane potential for two ion- exchanger membranes, one positively charged and the other negatively charged, and for a bipolar membrane obtained by joining the two single charged membranes were measured [6], and some parameters such as membrane perm- selectivity and streaming potential coefficient at different NaCI concentrations (0.005 _< C(M) < 0.1) were determined. In this work, the combined effect of both concentration (AC) and pressure (AP) gradients on the electrical potential differ- ence (A~) at both sides of the bipolar membrane for the two opposite orientations (CABM and ACBP), depending on the charged sublayer in contact with the high concentration and pressure side was considered. Measurements were carried out by keeping the concentration of the NaCI solution at one side of the membrane constant (Co = 0.05 M) and varying the pressure difference between 0.4 atm and 10 atm. A linear dependence between A~ and AP for both bipolar membrane orientations, independently of AC and AP having the same or opposite directions, was found. Fil-

tration streaming potential coefficient ~s, = (A~/AP)Ac= 0 for the bipolar membrane (BM) was compared with that corresponding to each single ion-exchanger membrane (SEM), and results indicate a ratio r = ~st(BM/SEM)= 5 for both BM orientations. On the other hand, a reduction in the transport number of the ions (t~) in the bipolar membrane with respect to that corresponding at each charged sublayer was estimated.

2. Experimental

Two ion-exchanger membranes o f the same thickness (300 ~tm), a negatively charged C-60 and a positively charged A-60, by American Machine & Foundry Co., and a bipolar membrane (BP) obtained by a series association of these two oppositely charged membranes were used. Membrane capacity is 1.6 and 2.0 meq/g dry for the cation and anion exchangers, respect-ively. Measurements were carried out with NaCI solutions at different concentrations, neutral pH and constant temperature t = (25.0 ~= 0.2)°C.

The experimental system used for measure- ments is similar to that indicated elsewhere [7]. It basically consists of a test cell made of PVC, with two holes in the centre o f each half-cell to place the reversible Ag/AgCl electrodes, which were connected to a high impedance voltmeter.

The electrical potential difference at both sides of the different membranes (AE) due to the action of both concentration and pressure gradients was measured. Experiments were carried out keeping the concentration of the solution at one side of the membrane constant (Co = 0.05 M) and changing gradually the con- centration of the solution at the other side (Cv), for 10 -3 _< Cv(M) ___ 10-~; for each pair of concen- trations (Cc,C~) a pressure difference was also applied across the membrane (0.4 <__ AP (atm) < 9). The speed of the circulating solution at the high pressure side was 115 cm/s approximately, while the pump output at the low pressure side was 136 cm3/min. Membrane contribution to the

electrical potential was obtained from experi- mental values by subtracting to AE values the electrode potential contribution: A o ~ ¢ =

- ( R T / z F ) In (C1/C2). Values corresponding to measurements carried out when there was only concentration or pressure gradients were already reported [6].

3. Results and discussion

E

Membrane potential and streaming potential were separately measured for ion-exchange membrane characterisation. When a membrane is separating to solutions of the same electrolyte but different concentration (C~ and C2), the electrical potential existing at both membrane sides is called membrane potential, A~b m (no pressure gradient across the membrane exists). Variation of membrane potential with external salt concentration ratio is shown in Fig. ! a for both ion-exchange membranes. For comparison, mem- brane potential values for an ideal cation- exchange membrane and an ideal anion-exchange membrane are also indicated in Fig. la as dotted lines; as can be seen, no differences exist for the anion-exchange membrane, but slight differences with respect to the ideal behaviour were obtained for the cation-exchanger.

Streaming potential, A~st, is the electrical potential difference at both sides of a channel or capillary caused by the application of a pressure on the solution existing into the capillary, assuming that AC = 0 [8]. In Fig. lb streaming potential as a function of the external applied pressure for both ion-exchange membranes is shown (C = 0.05 M). A linear relationship was obtained for both membranes, but the slope of the cation-exchange membrane (streaming potential coefficient) is about 1.5 times higher that for the anion-exchange membrane.

Once the effect o f both concentration and pressure on the charged membrane was sepa- rately evaluated, the combined effect of both on each ion-exchange membrane was studied.

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J. Benavente, G. Jonsson / Desalination 147 (2002) 399--403 401

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Fig. 2a shows a comparison o f the electrical potential, A ~ , at both sides o f the ion-exchange membranes as a function o f the applied pressure difference, for similar concentration gradients. As can be seen, a linear Aqb-Ap dependence, with negative slope for the cation-exchange membrane and positive for the anion one, was obtained in all cases, independently of the

4 0 2 J. Benavente, G. Jonsson / Desalination 147 (2002) 399-403

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F i g . 2 . V a r i a t i o n o f t h e e l e c t r i c a l p o t e n t i a l d i f f e r e n c e a t b o t h s i d e s o f t h e m e m b r a n e s (Ad~) w i t h a p p l i e d p r e s s u r e

(AP) for different concentration gradients (AC = C~- C2). a. Cation-exchange membrane: (0) AC<0 and AP>0 (C2~ 0.1 M); (A) AC and AP>0 (C2 ~ 0.03 M); Anion- exchanger membrane: (V) AC<0 and AP>0 (C2 ~ 0.1 M); (0) AC and AP>0 (C2 = 0.03 M). b. Bipolar membrane: CABM orientation: (o) AC<0 and A/~0 (C 2 ~ 0.1 M); (A) AC and AP>0 (C2 -~ 0.03 M); ACBM orientation: (0) AC<0 and AP>0 (C2 ~ 0.1 M); (T) AC and AP>0 (C2

0 . 0 3 M ) .

concentration gradient direction (C>Cv or

Cc<Cv). This kind o f dependence is similar to that

shown in Fig. lb for the corresponding mem- brane. However, since streaming potential values are determined with the same concentration at both sides o f the membrane (Cc = Cv), differences obtained for Adp and Adds t values correspond to the effect o f the concentration potential, which also depends on Cv values•

A BM was obtained by joining together both oppositely charged membranes. In this case two different orientations exist: CABM and ACBM. The CABM orientation means that the negatively charged layer is in contact with the high pressure and, consequently, in this orientation the cation- exchange membrane can be considered as the "active layer" o f this composite (bipolar) membrane; in the case o f ACBM orientation, the anion-exchange membrane will act as the active layer. In Fig. 2b a comparison o f the electrical potential, Ad~, as a function o f the applied pres- sure difference for the two opposite orientations of the bipolar membrane is shown. Differences observed in the experimental values shown in both pictures clearly indicate the different influence o f each charged sublayer and orien- tation in the total electrical potential difference measured with the bipolar membrane. Although the same type o f dependency between A ~ and AP was obtained, differences in both the values and the slopes o f the straight lines for both opposite orientations were found. In fact, the streaming potential coefficient for the CABM orientation (d~st) is around two times higher than for the opposite one. This fact indicates a higher control o f the cation-exchanger sublayer as active layer o f the bipolar membrane•

On the other hand, ion transport numbers in a membrane can be obtained from the membrane or concentration potential. Accordingly, membrane potentials for the two opposite orientations o f the bipolar membrane were determined by extrapo- lation at AP = 0 of the experimental values obtained at the different Cv measured. A comparison o f the transport numbers for the bipolar membrane with those found for the single

J. Benavente, G. Jonsson / Desalination 147 (2002) 399-403 403

charged membranes [6] indicates a reduction in the transport number o f the counter-ions in the active layer, which is around 85% for the cation- exchange membrane and 75% for the anion- exchange one. These results also agree with the higher barrier effect presented by the negatively charged sublayer in the bilayer membrane previously obtained from pressure-induced potentials.

4. Conclusions

Concentration and pressure effect on two ion- exchange membranes were separately measured by using NaCI solutions.

The combined effect o f both concentration and pressure gradients on two oppositely charged membranes and a bipolar one obtained by joining together both single charged membranes was studied. Streaming potential coefficient and ion transport numbers for the two opposite orien- tations o f the bipolar membrane have been correlated with those obtained for each single charged layer.

Results indicate a greater influence o f the negatively charged sublayer on the behaviour o f the bipolar membrane.

References

[1] M. Mulder, Basic Principles of Membrane Tech- nology, Kluwer Academic, Dordrecht, 1991. [2] G. Jonsson and J. Benavente, Determination of some

transport coefficients for the skin and porous layers of a composite membrane, J. Membr. Sci., 69 (1992) 29. [3] J. Benavente and G. Jonsson, Electrokinetic charac-

terization of composite membranes: estimation of different electrical contributions in pressure induced potential measured across reverse osmosis mem- branes, J. Membr. Sci., 172 (2000) 189.

[4] V. Kudela, H.-H. Schwarz and K. Richau, Measure- ment and interpretation of concentration potential and at-de resistances of annealed cellulose acetate mem- branes, J. Membr. Sci., 43 (1988) 39.

[5] A. Cafias and J. Benavente, Electrochemical charac- terisation of an asymmetric nanofiltration membrane with NaCI and KCI solutions: influence of membrane asymmetry on transport parameters, J. Colloid Interface Sci., 246 (2202) 328.

[6] J. Benavente and G. Jonsson, A comparison between streaming potential and membrane potential measured across single charged and bipolar membranes, Sep. Purl. Tech., 22-23 (2001) 637.

[7] J. Benavente and G. Jonsson, Effect of adsorbed protein on the hydraulic permeability, membrane and streaming potential values across a microporous membrane, Colloid Surf., 138 (1998) 255.

[8] R.J. Hunter, Zeta Potential in Colloid Science. Principles and Applications, Academic Press, London, 1988.