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Comparison of transport properties of monovalent anions through anion-exchange membranes
Journal of Membrane Science 143 (1998) 249±261
Comparison of transport properties of monovalent anions through anion-exchange membranes Abdulah Elattara, Azzedine Elmidaouia, Natalia Pismenskaiab, Claude Gavachc, Gerald Pourcellyc,* a Laboratoire des Techniques SeÂparatives, Universite Ibn Tofail, Kenitra, Morocco Department of Physical Chemistry, University of Kuban, Kuban, Russian Federation c Laboratoire des MateÂriaux et ProceÂdeÂs Membranaires, UMR 5635 CNRS, 1919 Route de Mende, 34293 Montpellier CeÂdex 5, France b
Received 27 October 1997; received in revised form 7 January 1998; accepted 8 January 1998
Abstract Transport properties of chloride, ¯uoride and nitrate ions through AFN, AMX, ACS, ACM anion-exchange membranes (from Tokuyama Soda) and MA-40 membrane (from the Institute of Plastic Materials, Moscow) have been investigated by studying electroconductivity, transport numbers and current±voltage characteristics. On the basis of the micro-heterogeneous model proposed by Nikonenko and Zabolotsky describing the microstructure of the membrane material, the volume fractions of the different phases and the electroconductivity of the joint-gel phase have been determined. Good correlations have been founded between the electroconductivity of the anion-exchange membranes (AEMs) and their structural properties. Recommendations are provided to determine which kind of membrane has to be selected for a given application. For all the membranes, increase of the counter-ion mass transfer is observed above the limiting current. # 1998 Elsevier Science B.V. Keywords: Ion-exchange membranes; Electroconductivity; Transport number; Convection
1. Introduction Electrically driven membrane processes (EDMP) involving ion-exchange membranes (IEMs) are not only a part of applied electrochemistry (membrane electrolysis, fuel cells, storage batteries...) but they also belong to the ®eld of separation techniques through electrodialysis, electro-electrodialysis, electrodeionization. Electrodialysis has been developed for several years, mainly for desalting brackish waters *Corresponding author. Fax: 33 (0) 467042820; e-Mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00013-1
and reconcentrating brine from sea water. Nowadays, the previous separation techniques are applied to the environment protection (depolluting and recycling of chemicals), to bioindustries (food, pharmacy and biotechnology) and to the treatment of drinking water. Some of these new applications need special membranes with adapted selectivity in ion transport, such as a very low membrane permeability to divalent ions with respect to monovalent, a very low permeability to hydroxyl ions (membranes for the chlor-alkali electrolysis), a very low permeability to protons (membranes for the treatment of used acids). Moreover, when applied to the treatment of ¯uids containing
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organic ions, EDMP necessitates membranes highly resistant to poisoning by hydrophobic ions. These speci®c properties are related to the nature of ionogenic groups and of the polymer matrix, to the degree of cross-linking, to the micro-heterogeneous structure of the membrane, to the surface layer which in some cases has a composition different from the bulk. To manufacture IEMs well-suited for a given application, it is necessary not only to know the main transport characteristics of the membranes, but also to predict the behavior of these membranes in relation with their structural properties. Zabolotsky and Nikonenko [1] have investigated the correlation of the microheterogeneous structure with the membrane transport properties. They proposed a micro-heterogeneous model of the membrane phase. The inhomogeneities of the microphase derived from their model appeared to be the main factor for relating the membrane transport properties ± such as electrical conductivity, diffusion permeability and transport number ± to the ionic concentration of the external aqueous solution. The aim of this paper is an investigation of transport characteristics of AFN, AMX, ACS, ACM and MA-40 anion-exchange membranes in contact with solutions containing monovalent anions. The results are analysed on the basis of the micro-heterogeneous model. 2. Experimental The main characteristics of the AEMs studied are collected in Table 1. The values are that of the sup-
pliers, checked in our laboratory. The AFN, AMX, ACS and ACM membranes are produced by Tokuyama Soda, the MA-40 is from the Institute of Plastic Materials of Moscow. The speci®c weight of AEMs was obtained by measuring both their dimensions and weights after drying at 458C for three days under the chloride form. For all these membranes, electrical conductivities, transport numbers of the counter-ions and current±voltage curves (CVC) have been obtained when the solutions in contact with the membranes were NaCl, NaF and NaNO3 with concentrations up to 0.1 mol/l. Moreover, dialysis ¯ux has been carried out for the AFN membrane. All the measurements were carried out at 258C. The electrical conductivity measurements were obtained from the clip-cell already used and described elsewhere [2]. They were carried out for solutions ranging from 0.005 to 0.1 M. Current±voltage curves were obtained using the two-compartment cell already described in several papers [3±5] and illustrated in Fig. 1. This measurement cell was composed of two symmetrical 100 cm3 half-cells (wide part of the cell). In the geometric center there was a cylindrical hole (narrow part of the cell) between which the membrane is clamped. Two Ag/AgCl electrodes immersed into Luggin capillaries allowed the measurement of the potential difference between two equipotential planes near the two membrane±solution interfaces. Mechanical strirrers were placed in each compartment. The electrical current was supplied by means of two Ag/AgCl or platinizedtitanium plane electrodes. The electrical circuit was
Table 1 Main characteristics of the AEMs studied (from the suppliers)
E.C. meq./g Cl-form of dry membrane W.C. g.H2O/g.Cl-form of dry membrane E.R. /cm2 in 0.5 M NaCl Thickness, mm Specifications
AFN
AMX
ACS
ACM
MA-40a
2.0±3.5
1.4±1.7
1.4±2.0
1.4±1.7
2.7
0.40±0.55
0.25±0.30
0.20±0.30
0.13±0.18
0.37
0.4±1.5 150±200 Resistant against organic fouling
2.5±3.5 160±180 High chemical strength
2.0±2.5 150±200 Mono-anion permselective
4.0±5.0 110±130 Low proton transport
6.5±8 400±500
AFN, AMX, ACS, ACM from Tokuyama Soda are homogeneous and contain quaternary ammonium groups [15]. MA-40 from the Institute of Plastic Material of Moscow is heterogeneous (mixture of ion-exchangers and polyethylene) and contains secondary, ternary and quanternary ammonium groups [8,20]. a Laktionov's data (State University of Kuban in Krasnodar).
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
jlim
Fig. 1. The polarization experimental cell: (a): Ag/AgCl probes; (b): pH electrodes; (c): stirrers; M: membrane.
composed of a Tacussel PRT-40-1XT intensiostatic generator monitored by a PIL-101T signal generator. Two types of measurements were carried out: the ®rst one recording the response to a 200 s wide square impulse of current (the static method), the second one by drawing the I±V curves directly on the X±Y plotter during the low rate current scanning (lower than 0.01 mA/s, the dynamic method). The two methods gave identical results. Determination of the transport number of counterions were achieved in the previous CVC cell but with a third compartment open on both sides. The transport number was obtained from the Hittorf method after a 15 min-duration electrodialysis. For ¯uoride and nitrate ions the con®guration of the cell was: ÿ Cathode j NaCl 0:1 M jCEMj Na F or NO3 0:1 M jAEMj NaCl 0:1 M j Anode For the chloride ion the con®guration of the cell was: ÿ Cathode j NaCl 0:1 M jAEMj Na2 SO4 0:1 M jCEMj NaCl 0:1 M j Anode The working area of the AEM was 0.5 cm2, the different current densities applied were 5, 10, 15, 20 and 40 mA/cm2. Titration of the solution in the anodic side of the AEM was achieved at the end of the electrodialysis. Fluoride and nitrate ions were titrated using speci®c electrodes. Chloride ions were analysed using a chlorimeter from Radiometer. Calculation of transport numbers of anions can also be achieved from values of the limiting current determined from CVC through the relation:
jzi jCFD tim ÿ ti
251
(1)
where jlim is the limiting current density, C and D the concentration and the diffusion coef®cient of the counter-ion `i' in the solution, F the Faraday constant, the thickness of the diffusion layer, tim and ti the transport numbers of the counter-ion in the membrane and in the solution, respectively. According to [6], tim is a maximum value corresponding to a minimum concentration at the solution±membrane interface. Dialysis ¯ux measurements were carried out for the AFN membrane using the previous cell without electrodes. 3. Results 3.1. Electrical conductivity The conductivity m of the membrane is given by the relation: e (2) m AR where e is the thickness of the wet membrane, A its area and R its electrical resistance. The variation of these conductivities with the electrolyte concentration are plotted in Fig. 2(a)±(c) for AEMs equilibrated with NaCl, NaF and NaNO3, respectively. The dotted lines represent the conductivity of the solution in contact with the membrane. 3.2. Current±voltage curves CVC recorded either by the static or the dynamic method are identical for low rate current scanning. The Ag/AgCl electrodes avoid the production of protons and hydroxyl ions. The cathode must be regularly controlled because of the AgCl reduction. To overcome this problem, we have also used platinized titanium electrodes and checked if CVC were identical as that obtained with Ag/AgCl electrodes. CVC are illustrated in Fig. 3(a)±(c) for 0.1 M NaCl, NaF and NaNO3 solutions, respectively. In Fig. 3(a), CVC were obtained for a NaCl solution using either Ag/ AgCl electrodes or platinized titanium electrodes as current suppliers. The two sets of CVC were quite
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Fig. 2. Electroconductivity of the AEMs in contact with salt solutions. (a): NaCl; (b): NaF; (c): NaNO3. Solid curves: membrane conductivity; dotted line: conductivity of the solution
Fig. 3. Current±voltage curves for different AEMs in contact with salt solutions in the conditions of natural convection. (a): NaCl; (b): NaF; (c): NaNO3.
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
similar for the four membranes. For NaF and NaNO3 solutions, only Ti(Pt) electrodes have therefore been used. However, in these two latter cases, a very small amount of NaCl has been added in both compartments to stabilise the initial reference potential of the two Ag/AgCl probes in the Luggin capillaries. 3.3. Transport numbers of the counter-ions Transport numbers were determined from both the Hittorf method and CVC through Eq. (1). From the Hittorf method: The values of the transport numbers of Clÿ, Fÿ and NO3ÿ ions are reported in Table 2 for different current densities lower than or close to the limiting one. For the AFN membrane, a diffusion dialysis process occurs. Its magnitude was evaluated and the corresponding amount of anions transferred by dialysis was deduced to the total amount determined with current. For all the other membranes, diffusion dialysis was not observed. From CVC: Limiting currents were determined from the intercept of the tangents to the ohmic part and to the in¯exion point (Fig. 3(a)). Values of D and ti for 0.1 M NaCl were found in the literature [7]. From
Table 2 Transport numbers of anions through AEMs in contact with 0.1 M salt solutions and for different current densities 10 mA/cm2
15 mA/cm2
20 mA/cm2
Clÿ ion AFN AMX ACS ACM
1.0 1.0 1.0 1.0
0.99 0.99 0.99 0.99
0.96 0.96 0.96 0.96
Fÿ ion AFN AMX ACS ACM
0.98 0.98 0.98 0.98
0.95 0.95 0.96 0.95
0.94 0.94 0.95 0.94
NO3ÿ ion AFN AMX ACS ACM
1.0 1.0 1.0 1.0
0.98 0.98 0.98 0.98
0.95 0.95 0.95 0.95
The limiting current density ranges from 15 to 20 mA/cm2 for the different ions and membranes. Values determined from the Hittorf method. The accuracy is estimated at 0.03.
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Table 3 Transport numbers through AEMs in contact with different 0.1 M solutions
NaCl NaF NaNO3
AFN
AMX
ACS
ACM
MA-40
0.92 0.90 0.93
0.91 0.89 0.95
0.96 0.95 0.93
0.93 0.92 0.93
0.98a 0.95 0.95
Values calculated from CVC at the limiting current [Eq. (1)]. The accuracy is estimated as 0.03. a Transport number of Clÿ ions through MA-40 in 0.1 M NaCl solution was taken from [8,13].
these values, that for NaF and NaNO3 are calculated from classical relations used in electrolyte solutions. m The value of tCl ÿ for MA-40 in 0.1 M NaCl was deduced from the Hittorf method [8]. That of was calculated from Eq. (1). We found 270 mm. Taking into account that, under natural convection [9]: Clÿ DClÿ 1=4 (3) A ÿ DAÿ where Aÿ is Fÿ or NO3ÿ, changes in the thickness of the diffusion layer are lower than the experimental error on the determination of jlim from CVC. So, we took the same value for regardless of the nature of the electrolyte at the concentration 0.1 M. The calculated values of these transport numbers are collected in Table 3. The values of transport numbers obtained from both methods are in good agreement when the applied current is close to the limiting. 4. Discussion 4.1. Electrical conductivity An ion-exchange membrane has to be considered as a microheterogeneous system. Among the models described in the literature, we have chosen one which has been previously proposed by Gnusin et al. [14] and more recently developed by Zabolotsky and Nikonenko [1]. The membrane may be considered as a combination of a `gel-phase' with a relatively uniform distribution of ionogenic groups and hydrophilic parts of the matrix polymer chains impregnated with the charged solution compensating the charge of the ®xed groups, on one hand, and an electroneutral solution
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®lling the interstices between the elements of the gelphase named the `inter-gel phase', on the other. When the ion-exchange membrane contains a volume fraction fin of inert phase, Zabolotsky and Nikonenko used again the two-phase model by combining the gelphase and the inert phase into a coherent region which can be considered as quasi-homogeneous and named `the joint-gel phase' which is characterised by a volume fraction f1
f10
fin
(4)
and an ion concentration ci
f10 0 c f1 1
(5)
f10 is the volume fraction of the pure gel phase free of inert inclusions. Let f1 and f21ÿf1, the volume fraction of the joint-gel phase (the active region) and the inter-gel phase (the interstitial region), respectively. Starting from this microheterogeneous model, the following relation for the membrane conductivity m may be deduced: m f1 f2 1=
(6a)
where is a structural parameter, which re¯ects the reciprocal arrangement of the phase elements in the membrane [1], and are the conductivities of the joint-gel phase and the solution, respectively. Eq. (6a) gives: m f1 f2
(6b)
For | | 1: m 1 ln m
(6c)
From Eq. (6c), Eq. (6b) gives: 1 ln m f1 1 ln f2 1 ln
(6d)
Dividing by and with f1f21, Eq. (6d) gives: m f1 f2
(7) m
From this equation, ln varies linearly with ln . Zabolotsky et al. [1] showed also that, near the isoconductance point Ciso, where m ; m , depends slightly on and, in the range 0.1Ciso
theoretical approach agrees with the experiment. The slopes of ln m±ln dependence give the volume fraction f2 of the inter-gel phase whose values are collected in Table 4. The value of the conductivity of the joint-gel phase may be derived from that of m at the isoconductance point m iso with: m iso
(8)
The isoconductance point is obtained from the intercept of the curves drawn in m±C, ±C coordinates (Fig. 2) or from ln m±ln dependence (Fig. 4). The ion-exchange capacity Q of the joint-gel phase can be calculated from the magnitude of the membrane capacity Qm through the relation: Q
Qm f1
(9)
At last, the diffusion coef®cients D of counter-ions in the joint-gel phase can also be calculated from the magnitude of the membrane conductivity at the isoconductance point through: D
RT m iso F2 Q
(10)
Analysis of concentration dependence of the conductivities shows that the AFN membrane has the highest conductivity regardless of the nature of the counter-ion. For a given counter-ion, the membrane conductivity m follows the sequence AFN>AMX>ACS>ACM (Fig. 2(a)±(c)). Comparison of the isoconductivity values m iso (Table 4) shows the following sequences of ion mobilities through joint-gel phase of the membranes: (i): for the AFN: Fÿ>NO3ÿ>Clÿ; (ii): for the AMX: ClÿFÿ>NO3ÿ; (iii): for the ACS: Clÿ>FÿNO3ÿ; (iv): for the ACM: Clÿ>Fÿ>NO3ÿ. The sequence of ion mobilities in water is Clÿ>NO3ÿ>Fÿ, which is the inverse of that for the AFN membrane. It is also interesting to note that for the AFN membrane m iso for the chloride ion is signi®cantly less than for the two other counter-ions. As shown in Table 4, the volume fraction f2 of the intergel phase follows the sequence: AFN AMX ACS>ACM which is that of the water contents reported in Table 1. f2, which is directly related to the structure of the membrane, only depends on the nature of the membrane and not on that of the counter-
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
255
Fig. 4. Variation of the membrane conductivity versus the external solution conductivity (ln±ln plots). Determination of the volume fraction f2 of the `inter-gel phase'. (a): NaCl; (b): NaF; (c): NaNO3.
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Fig. 4. (Continued)
Table 4 Values of conductivities, concentrations and volume fraction of the inter-gel phase at the isoconductance point membrane/solution NaCl
AFN AMX ACS ACM MA-40a
NaF
Ciso
m iso
f2
0.098 0.042 0.018 0.005 0.033
10.7 4.3 2.1 0.6 3.5
0.46 0.28 0.29 0.21 0.30
NaNO3
Ciso
m iso
f2
Ciso
m iso
f2
0.60 0.048 0.012 0.006
50 4.2 1.28 0.5
0.45 0.24 0.26 0.20
0.29 0.027 0.013 0.003
31 3.1 1.32 0.34
0.45 0.24 0.27 0.16
Ciso: expressed in mol/l; m iso : expressed in mS/cm; f2: dimensionless. a Laktionov's data (State University of Kuban in Krasnodar).
ion. For the AFN membrane f2 data are also in good correlation with that observed in other macroporous ion-exchange membranes [10]. This membrane has a macroporous structure, a low amount of cross-linking agent and a large concentration of ionogenic groups. The AMX and ACS membranes have close exchange capacities and water contents. Their volume fractions of inter-gel phase are similar while the isoconductivity is lower for the ACS than for the AMX. This is probably due to the effect of the surface modi®cation
of the ACS membrane which is designed to be monoanion permselective. The most cross-linked membrane ACM with the lowest water content has the worst electroconductance characteristics (Fig. 2, Table 4). The obtained values of the ion-exchange capacity for the MA-40 membrane and the volume fraction of inter-gel phase are close to the values determined for the ACS membrane (Tables 1 and 4). Though the MA-40 membrane contains an inert component (polyethylene), its ionogenic groups pro-
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257
Table 5 Diffusion coefficients of anions through the joint-gel phase of AEMs Membrane
AFN AMX ACS ACM MA-40a
Specific weight (kg/m3)
EC of the membrane Qm (mol.eq./m3)
1230
3075
1140
1710
1260
2142
1140
1824 1600
Anion
m iso ( ÿ1mÿ1)
f11ÿf2
EC of the joint-gel phase Qm (mol.eq./m3)
D (m2/s)
Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Clÿ
5.0 1.07 3.1 0.42 0.43 0.31 0.13 0.21 0.13 0.05 0.06 0.03 0.35
0.55 0.54 0.55 0.72 0.76 0.76 0.71 0.74 0.73 0.80 0.79 0.84 0.70
5591 5694 5590 2375 2250 2250 3016 2894 2934 2280 2309 2171 2285
2.410ÿ10 5.010ÿ11 1.510ÿ10 4.710ÿ11 5.110ÿ11 3.710ÿ11 1.210ÿ11 1.910ÿ11 1.210ÿ11 5.810ÿ12 6.910ÿ12 3.710ÿ12 4.110ÿ11
Exchange capacities of AEMs in the Clÿ form (in meq./kg): AFN: 2.5; AMX: 1.5; ACS: 1.7; ACM: 1.6. For MA-40: 1.6 meq./cm3. a Laktionov's data (State University of Kuban in Krasnodar).
vide higher mobility of Clÿ ions in comparison with ionogenic groups of the ACS membrane [11]. The calculated values of diffusion coef®cients of counter-ions in the joint-gel phase of AEMs Eq. (10) are reported in Table 5. Overall, and as expected, the AFN membrane presents the higher values of ionic diffusion coef®cients, regardless of the nature of the counter-ion. Moreover, its diffusion coef®cient for ¯uoride is at least ®ve-times higher than those observed with the other AEMs. In this section, the dependence of membrane electroconductivity with the concentration of the external solution was analysed on the basis of the microheterogeneous model. The comparison between the obtained data and the results from literature allows to predict transport characteristics of ion-exchange membranes. The application of this model may considerably reduce the number of experimental investigations necessary for such a prediction. Note that accuracy of transport characteristic obtained by applying this model may be increased by multiplying the number of measurements in the vicinity of the isoconductivity point. 4.2. Current±voltage curves and transport numbers For all ions, CVC have classical shapes for AMX, AFN, ACS and ACM homogeneous membranes, with
a clear-cut plateau corresponding to the limiting current. For the MA-40 membrane which is heterogeneous, the plateau is much less well-de®ned. The CVC are composed of three regions: an ohmic one up to the limiting current jlim, a plateau and an overlimiting current region. For values of j lower than jlim, the slope of the ohmic variation which gives the ohmic resistance between the extremities of the two capillaries, does not exactly correspond to the ohmic resistance of the membrane deduced from conductivity measurements. Taking into consideration that the distance between capillaries remains constant, the thickness of all membranes except MA-40, being approximately equal, this gap mainly depends on the contribution of the membrane. Comparing the concentration of solution (C0.1 M) with those at the isoconductivity point Ciso, we can conclude that contributions of membrane and solution to the resistance deduced from CVC are not equal, not only for different membranes and a given ion but also for different ions and a given membrane. For example, when the ACM membrane is in contact with 0.1 M NaCl, Ciso0.005 M 0.1 M. The membrane resistance is therefore much higher than that of the solution (for j jlim, the contribution of diffusion layer resistance is negligible). For the AFN membrane in NaCl, Ciso0.1 M and the contribution of solution and membrane to the ohmic potential difference (pd) are equal. For AFN in NaNO3,
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Ciso0.3 M, and for NaF Ciso0.6 M. The contribution of the solution to the ohmic pd therefore follows the sequence NaF>NaNO3>NaCl. The previous considerations explain why the slopes of the initial parts of the CVC cannot be directly correlated with the values of the conductivity. In Fig. 3(a), the intercept of the tangent to the origin with the tangent to the in¯exion point of the plateau (point A) gives a value of pd on the abscissa axis. This value is always lower for the AFN membrane (small electrical resistance due to large exchange capacity and high water content). It is always greater for the ACM membrane (high resistance due to small water content and modi®ed surface). For a given ion, the values of jlim are close and, within the accuracy method, they can be considered as equal. According to Eq.(1), for different ions but similar hydrodynamic conditions, jlim depends on the anion mobility in solution and follows the sequence Clÿ>NO3ÿ>Fÿ. Let us de®ne a second point as the intercept of the tangent to the in¯exion point with the asymptote corresponding to the overlimiting region (point B). The projection of AB on the abscissa axis gives the plateau length which ranges from 300 to 650 mV and depends on the nature of both the anion and the membrane. For Clÿ and NO3ÿ ions the sequence is AFNAMXMA-40
(ii): effect of the water dissociation phenomenon resulting in additional current of water dissociation products (H and OHÿ ions) and in exaltation of salt ions current [12±16]; (iii): co-ion transfer [12]; (iv): electroconvection and gravitational convection near the membrane solution interface [12,15,17±19]. (v): creation of space charges (Gouy±Chapman double layer) Phenomena (i), (iv) and (v) mainly depend on physico-chemical and hydrodynamic conditions, but not basically on the nature of the membrane. Their study was not the aim of the present paper. Let us consider the origins for a current increase above the limiting value, depending on phenomena (ii) and (iii). For an AEM in contact with NaA salts and under electrodialysis conditions, the current I crossing the membrane is given by: I IAÿ INa IOHÿ
(11)
and the different transport numbers are related by: tAÿ tNa tOHÿ 1
(12)
Values of transport numbers of counter-ions obtained from Hittorf method and from jlim (Tables 2 and 3) do not differ signi®cantly for the different salts (tAÿ 0:95 0:05). Considering that for symmetrical 0.1 M solutions, the co-ion transport number does not strongly increase with pd after the limiting current [16], a maximum contribution of co-ions and OHÿ ions to overlimiting transfer can be estimated as: INa IOHÿ I 1 ÿ tAÿ ;lim 0:1I
(13) ÿ
Actually, the contribution of co-ions and OH ions to the mass transfer under overlimiting modes may be expected less than 0.1I. With monovalent ions, water dissociation is more typical for AEMs than for CEMs. The studied Neosepta AEMs bear quaternary ammonium groups with a very weak catalytic activity [16,18,20]. Investigations on MA-40 membranes which bear secondary, ternary and quaternary ammonium groups, with a catalytic effect on water splitting, have shown that the magnitude of this water splitting depends on experimental conditions [8,15,19,20]. In 0.1 M NaCl without or with a low stirring, 2% of the current is carried by H and OHÿ generated by water dissociation in the range jlim±2jlim [8,12]. We can
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
therefore expect negligible water dissociation and exaltation effects for the systems investigated. In order to study the occurence of water splitting, pH of solutions on both sides of the membrane are measured for j2jlim for the two following con®gurations of the cell: Ag=AgClj 0:1 M NaCl jAEMj 0:1 M NaCl j AgCl=Ag I Ti Ptj 0:1 M NaCl jAEMj 0:1 M NaCl j Ti Pt II For system (I), where electrode reactions do not generate protons and hydroxyl ions, pH did not change during CVC recording, regardless of the nature of the membrane. For system (II), and for NaCl as well as for NaF or NaNO3, changes in pH were not more important than that estimated from electrode reactions. Moreover, these pH variations did not in¯uence the CVC shape. We can therefore conclude that water splitting due to the membrane is a phenomenon negligible for our system and has no in¯uence on the overlimiting current enhancement. Thus, enhancement of current in overlimiting mode is due to an
259
increase of the counter-ion ¯ux with pd. Dependence of partial current densities upon total current density ji/jlim±j/jlim (values divided by jlim) is illustrated in Fig. 5. Values of the partial current density ji are obtained from in which is deduced from Hittorf measurements. Three regions can be distinguished. The range [0.6jlim±jlim] exhibits a slight reduction of mass transfer through the membranes, an in¯exion point is observed in the range [1.2 jlim±1.5 jlim], while above 1.5 jlim, ji/jlim increases again and j/jlim±j/jlim dependence can be once again ®tted by a linear relationship. Analysis of curves allows to conclude that near jlim, another phenomenon than electrodiffusion mass transfer takes place. In the third region, for example at j2jlim, partial current ji is a little bit less than j for all membranes. Note that for the AFN membrane, which is macroporous and designed for dialysis, the contribution of diffusion to the mass transfer is larger and exceeds contribution of electromigration for j jlim. At these currents, values of partial densities of anion currents through AFN exceed
Fig. 5. Dependence of partial current densities of Clÿ, Fÿ, NO3ÿ-ions upon total current densities normalised on limiting current densities (ji/jlim±j/jlim.) for the different AEMs in salt solutions.
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this value determined for other membranes. For currents j>jlim contribution of this diffusion component to the current through AFN becomes negligible. If the ¯uxes of ions generated by water splitting represent less than 5% of the total current, contribution of exaltation effects to the mass transfer does not exceed 1% [12,13,17,20]. Therefore, the main reason for the enhancement of the overlimiting current is a coupling with convection of the solution near the interface. To estimate which kind of the convection dominates in the studied system, a speci®c study, overtaking the frame of the present study would be necessary.
6. List of symbols
5. Conclusion
R t T z AEM CVC pd
For all the anions studied: Clÿ, Fÿ, NOÿ 3 , electrical conductivity of the AEMs is in good correlation with their structure and thermodynamic properties. The macroporous AFN membrane with the highest concentration of ionogenic groups has the best transfer characteristics. The ACM membrane with large degree of crosslinking presents the lowest electroconductivity. The AMX and ACS membranes have similar characteristics. However, due to its modi®ed surface, the ACS membrane has a lower conductivity than the AMX. In dilute solutions and for currents close to the limiting where the contribution of back diffusion is negligible, the use of AFN is recommended, especially for Fÿ and NOÿ 3 ion removing. For concentrated electrolytes, the use of AMX is preferable regardless of the nature of anions, if solutions do not contain bivalent ions. The ion transfer through this membrane follows the sequence ClÿFÿ>NO3ÿ. If the solutions treated contain a mixture of monovalent and divalent ions, the ACM membrane is preferable. In this case, the electroconductivity follows the sequence Clÿ>FÿNO3ÿ. The ACM membrane is recommended to be used for removing anions from acid solutions. Use of the microheterogeneous model facilitates the research of the correlation between the structure and the membranes properties and makes possible the determination of some structural and kinetic membrane parameters from the concentration dependences of the membrane conductivity and that of solutions.
A C D e E f1, f2, fin, f10 F I, Ilim j, jlim Q, Qm
membrane area (m2) concentration (mol mÿ3) diffusion coefficient (m sÿ1) membrane thickness (m) electric field (V mÿ1) volume fractions of `joint' gel, solution, inert and `pure' gel phases Faraday constant current, limiting current (A) current density, limiting current density (A mÿ1) ion exchange capacities of gel phase and membrane (eq mÿ3) gas constant (J molÿ1 Kÿ1) transport number temperature (K) valency structural parameter thickness of the diffusion layer (m) specific conductivity (S mÿ1) anion-exchange membrane current±voltage curve potential difference
These parameters may have been used in another unit system than that above mentioned.
Acknowledgements The authors are grateful to Prof. V. Nikonenko from the State University of Kuban for helpful discussions and to Dr E. Laktionov for complementary experiments concerning the characterisation of the MA-40 membrane.
References [1] V.I. Zabolotsky, V.V. Nikonenko, Effect of structural membrane inhomogeneity on transport properties, J. Membr. Sci. 79 (1993) 181±198. [2] A. Chapotot, Membranes eÂchangeuses de cations aÁ haute permselectivite au proton. Elaboration, caracteÂrisation, etude de la compeÂtition proton-cation bivalent, Ph.D. Thesis, University of Montpellier II, France, 1994.
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261 [3] M. Taky, G. Pourcelly, F. Lebon, C. Gavach, Polarisation phenomena at the interfaces between an electrolyte solution and an ion-exchange membrane. Part I. Ion transfer with a cation-exchange membrane, J. Electroanal. Chem. 336 (1992) 171±194. [4] M. Taky, G. Pourcelly, A. Elmidaoui, Transport properties of a commercial cation-exchange membrane in contact with divalent cations or proton-divalent cation solutions during electrodialysis, Hydrometallurgy 43 (1996) 63±78. [5] Ph. Sistat, G. Pourcelly, Chronopotentiometric response of an ion-exchange membrane in the underlimiting current-range. Transport phenomena within the diffusion layers, J. Membr. Sci. 123 (1997) 121±131. [6] J. Tanaka, M. Seno, Concentration polarisation and water dissociation in ion-exchange membrane electrodialysis, J. Chem. Soc., Faraday Trans. 82 (1986) 2065±2077. [7] R.C. Weast (Ed.), Handbook of Chemistry and Physics, The Chemical Rubber Company, 53rd edn., Cleaveland. [8] M.V. Pevnistkaya, Mass-transport enhancement in the electrodialysis of dilute solutions, Electrokhimiya 28 (1992) 1708±1715; Russian J, Electrochem. 28(1992) (1992) 1690± 1696. [9] S.T. Hwang, K. Kammermeyer, Membranes in Separations, Wiley, New York 1975, p. 464. [10] Neosepta Ion-Exchange Membranes, Tokuyama Soda Co. Ltd. [11] G.A. Dvorkina, Influence of the structure of ion exchange membranes on its electoconductance properties, Ph.D. Thesis, Kuban State University, Russia, 1988. [12] V.I. Zabolotsky, V.I. Nikonenko, Ion Transport in Membranes, Moscow Nauka (Science), 1996, p. 392.
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[13] N.V. Sheldeshov, V.V. Ganych, V.I. Zabolotsky, Transport numbers of salt ions and water dissociation products in cation and anion-exchange membranes, Electrokhimiya 27 (1991) 15±19; Russian J, Electrochem. 23(1991) (1991) 11±15. [14] N.P. Gnusin, V.I. Zabolotsky, V.V. Nikonenko, M.R. Urtenov, Convective-diffusion model of electrodialytic desalination. Limiting current and diffusion layer, Electrokhimiya 22 (1986) 298±502; Russian J, Electrochem. 22(1986) (1986) 273±277. [15] V.I. Zabolotsky, V.I. Nikonenko, N.D. Pismenskaya, On the role of gravitational convection in the transfer enhancement of salt ions in the course of dilute solution analysis, J. Membr. Sci. 119 (1996) 171±181. [16] R. Simons, The origin and elimination of water-splitting during water demineralisation by electrodialysis, Desalination 28 (1978) 41±42. [17] J.J. Khakats, The mechanism of supralimiting currents at ion exchange membranes electrolyte interfaces, Electrokhimiya 21 (1985) 974±977; Russian J, Electrochem. 21(1985) (1985) 917±920. [18] I. Rubinstein, F. Maletsky, Electroconvection at an electrically inhomogeneous permselective membrane surface, J. Chem. Soc., Faraday Trans. 87 (1991) 2079±2113. [19] M. Taky, A. Elmidaoui, G. Pourcelly, C. Gavach, Influence of the nature of cation on water splitting at the interface of a cation-exchange membrane during electrodialysis, J. Chim. Phys. 93 (1996) 386±401. [20] N.D. Pismenskaya, Electrodialysis of dilute solutions. The influence of pH on the transport of salt ions, Electrokhimiya 32 (1996) 277±284; Russian J, Electrochem 32(1996) (1996) 252±258.
Comparison of transport properties of monovalent anions through anion-exchange membranes Abdulah Elattara, Azzedine Elmidaouia, Natalia Pismenskaiab, Claude Gavachc, Gerald Pourcellyc,* a Laboratoire des Techniques SeÂparatives, Universite Ibn Tofail, Kenitra, Morocco Department of Physical Chemistry, University of Kuban, Kuban, Russian Federation c Laboratoire des MateÂriaux et ProceÂdeÂs Membranaires, UMR 5635 CNRS, 1919 Route de Mende, 34293 Montpellier CeÂdex 5, France b
Received 27 October 1997; received in revised form 7 January 1998; accepted 8 January 1998
Abstract Transport properties of chloride, ¯uoride and nitrate ions through AFN, AMX, ACS, ACM anion-exchange membranes (from Tokuyama Soda) and MA-40 membrane (from the Institute of Plastic Materials, Moscow) have been investigated by studying electroconductivity, transport numbers and current±voltage characteristics. On the basis of the micro-heterogeneous model proposed by Nikonenko and Zabolotsky describing the microstructure of the membrane material, the volume fractions of the different phases and the electroconductivity of the joint-gel phase have been determined. Good correlations have been founded between the electroconductivity of the anion-exchange membranes (AEMs) and their structural properties. Recommendations are provided to determine which kind of membrane has to be selected for a given application. For all the membranes, increase of the counter-ion mass transfer is observed above the limiting current. # 1998 Elsevier Science B.V. Keywords: Ion-exchange membranes; Electroconductivity; Transport number; Convection
1. Introduction Electrically driven membrane processes (EDMP) involving ion-exchange membranes (IEMs) are not only a part of applied electrochemistry (membrane electrolysis, fuel cells, storage batteries...) but they also belong to the ®eld of separation techniques through electrodialysis, electro-electrodialysis, electrodeionization. Electrodialysis has been developed for several years, mainly for desalting brackish waters *Corresponding author. Fax: 33 (0) 467042820; e-Mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(98)00013-1
and reconcentrating brine from sea water. Nowadays, the previous separation techniques are applied to the environment protection (depolluting and recycling of chemicals), to bioindustries (food, pharmacy and biotechnology) and to the treatment of drinking water. Some of these new applications need special membranes with adapted selectivity in ion transport, such as a very low membrane permeability to divalent ions with respect to monovalent, a very low permeability to hydroxyl ions (membranes for the chlor-alkali electrolysis), a very low permeability to protons (membranes for the treatment of used acids). Moreover, when applied to the treatment of ¯uids containing
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organic ions, EDMP necessitates membranes highly resistant to poisoning by hydrophobic ions. These speci®c properties are related to the nature of ionogenic groups and of the polymer matrix, to the degree of cross-linking, to the micro-heterogeneous structure of the membrane, to the surface layer which in some cases has a composition different from the bulk. To manufacture IEMs well-suited for a given application, it is necessary not only to know the main transport characteristics of the membranes, but also to predict the behavior of these membranes in relation with their structural properties. Zabolotsky and Nikonenko [1] have investigated the correlation of the microheterogeneous structure with the membrane transport properties. They proposed a micro-heterogeneous model of the membrane phase. The inhomogeneities of the microphase derived from their model appeared to be the main factor for relating the membrane transport properties ± such as electrical conductivity, diffusion permeability and transport number ± to the ionic concentration of the external aqueous solution. The aim of this paper is an investigation of transport characteristics of AFN, AMX, ACS, ACM and MA-40 anion-exchange membranes in contact with solutions containing monovalent anions. The results are analysed on the basis of the micro-heterogeneous model. 2. Experimental The main characteristics of the AEMs studied are collected in Table 1. The values are that of the sup-
pliers, checked in our laboratory. The AFN, AMX, ACS and ACM membranes are produced by Tokuyama Soda, the MA-40 is from the Institute of Plastic Materials of Moscow. The speci®c weight of AEMs was obtained by measuring both their dimensions and weights after drying at 458C for three days under the chloride form. For all these membranes, electrical conductivities, transport numbers of the counter-ions and current±voltage curves (CVC) have been obtained when the solutions in contact with the membranes were NaCl, NaF and NaNO3 with concentrations up to 0.1 mol/l. Moreover, dialysis ¯ux has been carried out for the AFN membrane. All the measurements were carried out at 258C. The electrical conductivity measurements were obtained from the clip-cell already used and described elsewhere [2]. They were carried out for solutions ranging from 0.005 to 0.1 M. Current±voltage curves were obtained using the two-compartment cell already described in several papers [3±5] and illustrated in Fig. 1. This measurement cell was composed of two symmetrical 100 cm3 half-cells (wide part of the cell). In the geometric center there was a cylindrical hole (narrow part of the cell) between which the membrane is clamped. Two Ag/AgCl electrodes immersed into Luggin capillaries allowed the measurement of the potential difference between two equipotential planes near the two membrane±solution interfaces. Mechanical strirrers were placed in each compartment. The electrical current was supplied by means of two Ag/AgCl or platinizedtitanium plane electrodes. The electrical circuit was
Table 1 Main characteristics of the AEMs studied (from the suppliers)
E.C. meq./g Cl-form of dry membrane W.C. g.H2O/g.Cl-form of dry membrane E.R. /cm2 in 0.5 M NaCl Thickness, mm Specifications
AFN
AMX
ACS
ACM
MA-40a
2.0±3.5
1.4±1.7
1.4±2.0
1.4±1.7
2.7
0.40±0.55
0.25±0.30
0.20±0.30
0.13±0.18
0.37
0.4±1.5 150±200 Resistant against organic fouling
2.5±3.5 160±180 High chemical strength
2.0±2.5 150±200 Mono-anion permselective
4.0±5.0 110±130 Low proton transport
6.5±8 400±500
AFN, AMX, ACS, ACM from Tokuyama Soda are homogeneous and contain quaternary ammonium groups [15]. MA-40 from the Institute of Plastic Material of Moscow is heterogeneous (mixture of ion-exchangers and polyethylene) and contains secondary, ternary and quanternary ammonium groups [8,20]. a Laktionov's data (State University of Kuban in Krasnodar).
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
jlim
Fig. 1. The polarization experimental cell: (a): Ag/AgCl probes; (b): pH electrodes; (c): stirrers; M: membrane.
composed of a Tacussel PRT-40-1XT intensiostatic generator monitored by a PIL-101T signal generator. Two types of measurements were carried out: the ®rst one recording the response to a 200 s wide square impulse of current (the static method), the second one by drawing the I±V curves directly on the X±Y plotter during the low rate current scanning (lower than 0.01 mA/s, the dynamic method). The two methods gave identical results. Determination of the transport number of counterions were achieved in the previous CVC cell but with a third compartment open on both sides. The transport number was obtained from the Hittorf method after a 15 min-duration electrodialysis. For ¯uoride and nitrate ions the con®guration of the cell was: ÿ Cathode j NaCl 0:1 M jCEMj Na F or NO3 0:1 M jAEMj NaCl 0:1 M j Anode For the chloride ion the con®guration of the cell was: ÿ Cathode j NaCl 0:1 M jAEMj Na2 SO4 0:1 M jCEMj NaCl 0:1 M j Anode The working area of the AEM was 0.5 cm2, the different current densities applied were 5, 10, 15, 20 and 40 mA/cm2. Titration of the solution in the anodic side of the AEM was achieved at the end of the electrodialysis. Fluoride and nitrate ions were titrated using speci®c electrodes. Chloride ions were analysed using a chlorimeter from Radiometer. Calculation of transport numbers of anions can also be achieved from values of the limiting current determined from CVC through the relation:
jzi jCFD tim ÿ ti
251
(1)
where jlim is the limiting current density, C and D the concentration and the diffusion coef®cient of the counter-ion `i' in the solution, F the Faraday constant, the thickness of the diffusion layer, tim and ti the transport numbers of the counter-ion in the membrane and in the solution, respectively. According to [6], tim is a maximum value corresponding to a minimum concentration at the solution±membrane interface. Dialysis ¯ux measurements were carried out for the AFN membrane using the previous cell without electrodes. 3. Results 3.1. Electrical conductivity The conductivity m of the membrane is given by the relation: e (2) m AR where e is the thickness of the wet membrane, A its area and R its electrical resistance. The variation of these conductivities with the electrolyte concentration are plotted in Fig. 2(a)±(c) for AEMs equilibrated with NaCl, NaF and NaNO3, respectively. The dotted lines represent the conductivity of the solution in contact with the membrane. 3.2. Current±voltage curves CVC recorded either by the static or the dynamic method are identical for low rate current scanning. The Ag/AgCl electrodes avoid the production of protons and hydroxyl ions. The cathode must be regularly controlled because of the AgCl reduction. To overcome this problem, we have also used platinized titanium electrodes and checked if CVC were identical as that obtained with Ag/AgCl electrodes. CVC are illustrated in Fig. 3(a)±(c) for 0.1 M NaCl, NaF and NaNO3 solutions, respectively. In Fig. 3(a), CVC were obtained for a NaCl solution using either Ag/ AgCl electrodes or platinized titanium electrodes as current suppliers. The two sets of CVC were quite
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Fig. 2. Electroconductivity of the AEMs in contact with salt solutions. (a): NaCl; (b): NaF; (c): NaNO3. Solid curves: membrane conductivity; dotted line: conductivity of the solution
Fig. 3. Current±voltage curves for different AEMs in contact with salt solutions in the conditions of natural convection. (a): NaCl; (b): NaF; (c): NaNO3.
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
similar for the four membranes. For NaF and NaNO3 solutions, only Ti(Pt) electrodes have therefore been used. However, in these two latter cases, a very small amount of NaCl has been added in both compartments to stabilise the initial reference potential of the two Ag/AgCl probes in the Luggin capillaries. 3.3. Transport numbers of the counter-ions Transport numbers were determined from both the Hittorf method and CVC through Eq. (1). From the Hittorf method: The values of the transport numbers of Clÿ, Fÿ and NO3ÿ ions are reported in Table 2 for different current densities lower than or close to the limiting one. For the AFN membrane, a diffusion dialysis process occurs. Its magnitude was evaluated and the corresponding amount of anions transferred by dialysis was deduced to the total amount determined with current. For all the other membranes, diffusion dialysis was not observed. From CVC: Limiting currents were determined from the intercept of the tangents to the ohmic part and to the in¯exion point (Fig. 3(a)). Values of D and ti for 0.1 M NaCl were found in the literature [7]. From
Table 2 Transport numbers of anions through AEMs in contact with 0.1 M salt solutions and for different current densities 10 mA/cm2
15 mA/cm2
20 mA/cm2
Clÿ ion AFN AMX ACS ACM
1.0 1.0 1.0 1.0
0.99 0.99 0.99 0.99
0.96 0.96 0.96 0.96
Fÿ ion AFN AMX ACS ACM
0.98 0.98 0.98 0.98
0.95 0.95 0.96 0.95
0.94 0.94 0.95 0.94
NO3ÿ ion AFN AMX ACS ACM
1.0 1.0 1.0 1.0
0.98 0.98 0.98 0.98
0.95 0.95 0.95 0.95
The limiting current density ranges from 15 to 20 mA/cm2 for the different ions and membranes. Values determined from the Hittorf method. The accuracy is estimated at 0.03.
253
Table 3 Transport numbers through AEMs in contact with different 0.1 M solutions
NaCl NaF NaNO3
AFN
AMX
ACS
ACM
MA-40
0.92 0.90 0.93
0.91 0.89 0.95
0.96 0.95 0.93
0.93 0.92 0.93
0.98a 0.95 0.95
Values calculated from CVC at the limiting current [Eq. (1)]. The accuracy is estimated as 0.03. a Transport number of Clÿ ions through MA-40 in 0.1 M NaCl solution was taken from [8,13].
these values, that for NaF and NaNO3 are calculated from classical relations used in electrolyte solutions. m The value of tCl ÿ for MA-40 in 0.1 M NaCl was deduced from the Hittorf method [8]. That of was calculated from Eq. (1). We found 270 mm. Taking into account that, under natural convection [9]: Clÿ DClÿ 1=4 (3) A ÿ DAÿ where Aÿ is Fÿ or NO3ÿ, changes in the thickness of the diffusion layer are lower than the experimental error on the determination of jlim from CVC. So, we took the same value for regardless of the nature of the electrolyte at the concentration 0.1 M. The calculated values of these transport numbers are collected in Table 3. The values of transport numbers obtained from both methods are in good agreement when the applied current is close to the limiting. 4. Discussion 4.1. Electrical conductivity An ion-exchange membrane has to be considered as a microheterogeneous system. Among the models described in the literature, we have chosen one which has been previously proposed by Gnusin et al. [14] and more recently developed by Zabolotsky and Nikonenko [1]. The membrane may be considered as a combination of a `gel-phase' with a relatively uniform distribution of ionogenic groups and hydrophilic parts of the matrix polymer chains impregnated with the charged solution compensating the charge of the ®xed groups, on one hand, and an electroneutral solution
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®lling the interstices between the elements of the gelphase named the `inter-gel phase', on the other. When the ion-exchange membrane contains a volume fraction fin of inert phase, Zabolotsky and Nikonenko used again the two-phase model by combining the gelphase and the inert phase into a coherent region which can be considered as quasi-homogeneous and named `the joint-gel phase' which is characterised by a volume fraction f1
f10
fin
(4)
and an ion concentration ci
f10 0 c f1 1
(5)
f10 is the volume fraction of the pure gel phase free of inert inclusions. Let f1 and f21ÿf1, the volume fraction of the joint-gel phase (the active region) and the inter-gel phase (the interstitial region), respectively. Starting from this microheterogeneous model, the following relation for the membrane conductivity m may be deduced: m f1 f2 1=
(6a)
where is a structural parameter, which re¯ects the reciprocal arrangement of the phase elements in the membrane [1], and are the conductivities of the joint-gel phase and the solution, respectively. Eq. (6a) gives: m f1 f2
(6b)
For | | 1: m 1 ln m
(6c)
From Eq. (6c), Eq. (6b) gives: 1 ln m f1 1 ln f2 1 ln
(6d)
Dividing by and with f1f21, Eq. (6d) gives: m f1 f2
(7) m
From this equation, ln varies linearly with ln . Zabolotsky et al. [1] showed also that, near the isoconductance point Ciso, where m ; m , depends slightly on and, in the range 0.1Ciso
theoretical approach agrees with the experiment. The slopes of ln m±ln dependence give the volume fraction f2 of the inter-gel phase whose values are collected in Table 4. The value of the conductivity of the joint-gel phase may be derived from that of m at the isoconductance point m iso with: m iso
(8)
The isoconductance point is obtained from the intercept of the curves drawn in m±C, ±C coordinates (Fig. 2) or from ln m±ln dependence (Fig. 4). The ion-exchange capacity Q of the joint-gel phase can be calculated from the magnitude of the membrane capacity Qm through the relation: Q
Qm f1
(9)
At last, the diffusion coef®cients D of counter-ions in the joint-gel phase can also be calculated from the magnitude of the membrane conductivity at the isoconductance point through: D
RT m iso F2 Q
(10)
Analysis of concentration dependence of the conductivities shows that the AFN membrane has the highest conductivity regardless of the nature of the counter-ion. For a given counter-ion, the membrane conductivity m follows the sequence AFN>AMX>ACS>ACM (Fig. 2(a)±(c)). Comparison of the isoconductivity values m iso (Table 4) shows the following sequences of ion mobilities through joint-gel phase of the membranes: (i): for the AFN: Fÿ>NO3ÿ>Clÿ; (ii): for the AMX: ClÿFÿ>NO3ÿ; (iii): for the ACS: Clÿ>FÿNO3ÿ; (iv): for the ACM: Clÿ>Fÿ>NO3ÿ. The sequence of ion mobilities in water is Clÿ>NO3ÿ>Fÿ, which is the inverse of that for the AFN membrane. It is also interesting to note that for the AFN membrane m iso for the chloride ion is signi®cantly less than for the two other counter-ions. As shown in Table 4, the volume fraction f2 of the intergel phase follows the sequence: AFN AMX ACS>ACM which is that of the water contents reported in Table 1. f2, which is directly related to the structure of the membrane, only depends on the nature of the membrane and not on that of the counter-
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
255
Fig. 4. Variation of the membrane conductivity versus the external solution conductivity (ln±ln plots). Determination of the volume fraction f2 of the `inter-gel phase'. (a): NaCl; (b): NaF; (c): NaNO3.
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Fig. 4. (Continued)
Table 4 Values of conductivities, concentrations and volume fraction of the inter-gel phase at the isoconductance point membrane/solution NaCl
AFN AMX ACS ACM MA-40a
NaF
Ciso
m iso
f2
0.098 0.042 0.018 0.005 0.033
10.7 4.3 2.1 0.6 3.5
0.46 0.28 0.29 0.21 0.30
NaNO3
Ciso
m iso
f2
Ciso
m iso
f2
0.60 0.048 0.012 0.006
50 4.2 1.28 0.5
0.45 0.24 0.26 0.20
0.29 0.027 0.013 0.003
31 3.1 1.32 0.34
0.45 0.24 0.27 0.16
Ciso: expressed in mol/l; m iso : expressed in mS/cm; f2: dimensionless. a Laktionov's data (State University of Kuban in Krasnodar).
ion. For the AFN membrane f2 data are also in good correlation with that observed in other macroporous ion-exchange membranes [10]. This membrane has a macroporous structure, a low amount of cross-linking agent and a large concentration of ionogenic groups. The AMX and ACS membranes have close exchange capacities and water contents. Their volume fractions of inter-gel phase are similar while the isoconductivity is lower for the ACS than for the AMX. This is probably due to the effect of the surface modi®cation
of the ACS membrane which is designed to be monoanion permselective. The most cross-linked membrane ACM with the lowest water content has the worst electroconductance characteristics (Fig. 2, Table 4). The obtained values of the ion-exchange capacity for the MA-40 membrane and the volume fraction of inter-gel phase are close to the values determined for the ACS membrane (Tables 1 and 4). Though the MA-40 membrane contains an inert component (polyethylene), its ionogenic groups pro-
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257
Table 5 Diffusion coefficients of anions through the joint-gel phase of AEMs Membrane
AFN AMX ACS ACM MA-40a
Specific weight (kg/m3)
EC of the membrane Qm (mol.eq./m3)
1230
3075
1140
1710
1260
2142
1140
1824 1600
Anion
m iso ( ÿ1mÿ1)
f11ÿf2
EC of the joint-gel phase Qm (mol.eq./m3)
D (m2/s)
Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Fÿ Clÿ NO3ÿ Clÿ
5.0 1.07 3.1 0.42 0.43 0.31 0.13 0.21 0.13 0.05 0.06 0.03 0.35
0.55 0.54 0.55 0.72 0.76 0.76 0.71 0.74 0.73 0.80 0.79 0.84 0.70
5591 5694 5590 2375 2250 2250 3016 2894 2934 2280 2309 2171 2285
2.410ÿ10 5.010ÿ11 1.510ÿ10 4.710ÿ11 5.110ÿ11 3.710ÿ11 1.210ÿ11 1.910ÿ11 1.210ÿ11 5.810ÿ12 6.910ÿ12 3.710ÿ12 4.110ÿ11
Exchange capacities of AEMs in the Clÿ form (in meq./kg): AFN: 2.5; AMX: 1.5; ACS: 1.7; ACM: 1.6. For MA-40: 1.6 meq./cm3. a Laktionov's data (State University of Kuban in Krasnodar).
vide higher mobility of Clÿ ions in comparison with ionogenic groups of the ACS membrane [11]. The calculated values of diffusion coef®cients of counter-ions in the joint-gel phase of AEMs Eq. (10) are reported in Table 5. Overall, and as expected, the AFN membrane presents the higher values of ionic diffusion coef®cients, regardless of the nature of the counter-ion. Moreover, its diffusion coef®cient for ¯uoride is at least ®ve-times higher than those observed with the other AEMs. In this section, the dependence of membrane electroconductivity with the concentration of the external solution was analysed on the basis of the microheterogeneous model. The comparison between the obtained data and the results from literature allows to predict transport characteristics of ion-exchange membranes. The application of this model may considerably reduce the number of experimental investigations necessary for such a prediction. Note that accuracy of transport characteristic obtained by applying this model may be increased by multiplying the number of measurements in the vicinity of the isoconductivity point. 4.2. Current±voltage curves and transport numbers For all ions, CVC have classical shapes for AMX, AFN, ACS and ACM homogeneous membranes, with
a clear-cut plateau corresponding to the limiting current. For the MA-40 membrane which is heterogeneous, the plateau is much less well-de®ned. The CVC are composed of three regions: an ohmic one up to the limiting current jlim, a plateau and an overlimiting current region. For values of j lower than jlim, the slope of the ohmic variation which gives the ohmic resistance between the extremities of the two capillaries, does not exactly correspond to the ohmic resistance of the membrane deduced from conductivity measurements. Taking into consideration that the distance between capillaries remains constant, the thickness of all membranes except MA-40, being approximately equal, this gap mainly depends on the contribution of the membrane. Comparing the concentration of solution (C0.1 M) with those at the isoconductivity point Ciso, we can conclude that contributions of membrane and solution to the resistance deduced from CVC are not equal, not only for different membranes and a given ion but also for different ions and a given membrane. For example, when the ACM membrane is in contact with 0.1 M NaCl, Ciso0.005 M 0.1 M. The membrane resistance is therefore much higher than that of the solution (for j jlim, the contribution of diffusion layer resistance is negligible). For the AFN membrane in NaCl, Ciso0.1 M and the contribution of solution and membrane to the ohmic potential difference (pd) are equal. For AFN in NaNO3,
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Ciso0.3 M, and for NaF Ciso0.6 M. The contribution of the solution to the ohmic pd therefore follows the sequence NaF>NaNO3>NaCl. The previous considerations explain why the slopes of the initial parts of the CVC cannot be directly correlated with the values of the conductivity. In Fig. 3(a), the intercept of the tangent to the origin with the tangent to the in¯exion point of the plateau (point A) gives a value of pd on the abscissa axis. This value is always lower for the AFN membrane (small electrical resistance due to large exchange capacity and high water content). It is always greater for the ACM membrane (high resistance due to small water content and modi®ed surface). For a given ion, the values of jlim are close and, within the accuracy method, they can be considered as equal. According to Eq.(1), for different ions but similar hydrodynamic conditions, jlim depends on the anion mobility in solution and follows the sequence Clÿ>NO3ÿ>Fÿ. Let us de®ne a second point as the intercept of the tangent to the in¯exion point with the asymptote corresponding to the overlimiting region (point B). The projection of AB on the abscissa axis gives the plateau length which ranges from 300 to 650 mV and depends on the nature of both the anion and the membrane. For Clÿ and NO3ÿ ions the sequence is AFNAMXMA-40
(ii): effect of the water dissociation phenomenon resulting in additional current of water dissociation products (H and OHÿ ions) and in exaltation of salt ions current [12±16]; (iii): co-ion transfer [12]; (iv): electroconvection and gravitational convection near the membrane solution interface [12,15,17±19]. (v): creation of space charges (Gouy±Chapman double layer) Phenomena (i), (iv) and (v) mainly depend on physico-chemical and hydrodynamic conditions, but not basically on the nature of the membrane. Their study was not the aim of the present paper. Let us consider the origins for a current increase above the limiting value, depending on phenomena (ii) and (iii). For an AEM in contact with NaA salts and under electrodialysis conditions, the current I crossing the membrane is given by: I IAÿ INa IOHÿ
(11)
and the different transport numbers are related by: tAÿ tNa tOHÿ 1
(12)
Values of transport numbers of counter-ions obtained from Hittorf method and from jlim (Tables 2 and 3) do not differ signi®cantly for the different salts (tAÿ 0:95 0:05). Considering that for symmetrical 0.1 M solutions, the co-ion transport number does not strongly increase with pd after the limiting current [16], a maximum contribution of co-ions and OHÿ ions to overlimiting transfer can be estimated as: INa IOHÿ I 1 ÿ tAÿ ;lim 0:1I
(13) ÿ
Actually, the contribution of co-ions and OH ions to the mass transfer under overlimiting modes may be expected less than 0.1I. With monovalent ions, water dissociation is more typical for AEMs than for CEMs. The studied Neosepta AEMs bear quaternary ammonium groups with a very weak catalytic activity [16,18,20]. Investigations on MA-40 membranes which bear secondary, ternary and quaternary ammonium groups, with a catalytic effect on water splitting, have shown that the magnitude of this water splitting depends on experimental conditions [8,15,19,20]. In 0.1 M NaCl without or with a low stirring, 2% of the current is carried by H and OHÿ generated by water dissociation in the range jlim±2jlim [8,12]. We can
A. Elattar et al. / Journal of Membrane Science 143 (1998) 249±261
therefore expect negligible water dissociation and exaltation effects for the systems investigated. In order to study the occurence of water splitting, pH of solutions on both sides of the membrane are measured for j2jlim for the two following con®gurations of the cell: Ag=AgClj 0:1 M NaCl jAEMj 0:1 M NaCl j AgCl=Ag I Ti Ptj 0:1 M NaCl jAEMj 0:1 M NaCl j Ti Pt II For system (I), where electrode reactions do not generate protons and hydroxyl ions, pH did not change during CVC recording, regardless of the nature of the membrane. For system (II), and for NaCl as well as for NaF or NaNO3, changes in pH were not more important than that estimated from electrode reactions. Moreover, these pH variations did not in¯uence the CVC shape. We can therefore conclude that water splitting due to the membrane is a phenomenon negligible for our system and has no in¯uence on the overlimiting current enhancement. Thus, enhancement of current in overlimiting mode is due to an
259
increase of the counter-ion ¯ux with pd. Dependence of partial current densities upon total current density ji/jlim±j/jlim (values divided by jlim) is illustrated in Fig. 5. Values of the partial current density ji are obtained from in which is deduced from Hittorf measurements. Three regions can be distinguished. The range [0.6jlim±jlim] exhibits a slight reduction of mass transfer through the membranes, an in¯exion point is observed in the range [1.2 jlim±1.5 jlim], while above 1.5 jlim, ji/jlim increases again and j/jlim±j/jlim dependence can be once again ®tted by a linear relationship. Analysis of curves allows to conclude that near jlim, another phenomenon than electrodiffusion mass transfer takes place. In the third region, for example at j2jlim, partial current ji is a little bit less than j for all membranes. Note that for the AFN membrane, which is macroporous and designed for dialysis, the contribution of diffusion to the mass transfer is larger and exceeds contribution of electromigration for j jlim. At these currents, values of partial densities of anion currents through AFN exceed
Fig. 5. Dependence of partial current densities of Clÿ, Fÿ, NO3ÿ-ions upon total current densities normalised on limiting current densities (ji/jlim±j/jlim.) for the different AEMs in salt solutions.
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this value determined for other membranes. For currents j>jlim contribution of this diffusion component to the current through AFN becomes negligible. If the ¯uxes of ions generated by water splitting represent less than 5% of the total current, contribution of exaltation effects to the mass transfer does not exceed 1% [12,13,17,20]. Therefore, the main reason for the enhancement of the overlimiting current is a coupling with convection of the solution near the interface. To estimate which kind of the convection dominates in the studied system, a speci®c study, overtaking the frame of the present study would be necessary.
6. List of symbols
5. Conclusion
R t T z AEM CVC pd
For all the anions studied: Clÿ, Fÿ, NOÿ 3 , electrical conductivity of the AEMs is in good correlation with their structure and thermodynamic properties. The macroporous AFN membrane with the highest concentration of ionogenic groups has the best transfer characteristics. The ACM membrane with large degree of crosslinking presents the lowest electroconductivity. The AMX and ACS membranes have similar characteristics. However, due to its modi®ed surface, the ACS membrane has a lower conductivity than the AMX. In dilute solutions and for currents close to the limiting where the contribution of back diffusion is negligible, the use of AFN is recommended, especially for Fÿ and NOÿ 3 ion removing. For concentrated electrolytes, the use of AMX is preferable regardless of the nature of anions, if solutions do not contain bivalent ions. The ion transfer through this membrane follows the sequence ClÿFÿ>NO3ÿ. If the solutions treated contain a mixture of monovalent and divalent ions, the ACM membrane is preferable. In this case, the electroconductivity follows the sequence Clÿ>FÿNO3ÿ. The ACM membrane is recommended to be used for removing anions from acid solutions. Use of the microheterogeneous model facilitates the research of the correlation between the structure and the membranes properties and makes possible the determination of some structural and kinetic membrane parameters from the concentration dependences of the membrane conductivity and that of solutions.
A C D e E f1, f2, fin, f10 F I, Ilim j, jlim Q, Qm
membrane area (m2) concentration (mol mÿ3) diffusion coefficient (m sÿ1) membrane thickness (m) electric field (V mÿ1) volume fractions of `joint' gel, solution, inert and `pure' gel phases Faraday constant current, limiting current (A) current density, limiting current density (A mÿ1) ion exchange capacities of gel phase and membrane (eq mÿ3) gas constant (J molÿ1 Kÿ1) transport number temperature (K) valency structural parameter thickness of the diffusion layer (m) specific conductivity (S mÿ1) anion-exchange membrane current±voltage curve potential difference
These parameters may have been used in another unit system than that above mentioned.
Acknowledgements The authors are grateful to Prof. V. Nikonenko from the State University of Kuban for helpful discussions and to Dr E. Laktionov for complementary experiments concerning the characterisation of the MA-40 membrane.
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