Improving the Permselectivity of Commercial Cation-Exchange

Chapter 21

Improving the Permselectivity of Commercial CationExchange Membranes for Electrodialysis Applications

Downloaded by RUTGERS UNIV on March 12, 2016 | http://pubs.acs.org Publication Date: April 20, 2004 | doi: 10.1021/bk-2004-0876.ch021

Sophie Tan, Alexis Laforgue, and Daniel Bélanger Département de Chimie, Université du Quebec àMontréal, C.P. 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada

Cation-exchange membranes bearing sulfonate groups (Neosepta C M X ) were modified by chemical polymerization of aniline at the surface of the membranes. The doped polyaniline (PANI) adsorbed at the surface of the membrane consists of a positively charged layer which acts as an electrostatic barrier for multivalent cations. The resulting composite membranes ( C M X - P A N I ) were characterized by electrodialysis, scanning electron microscopy (SEM), exchange capacity measurements (EC) and X-ray photoelectron spectroscopy (XPS). The presence the P A N I layer was shown to improve the membrane permselectivity for protons vs. bivalent cations ( Z n and Cu ) by a factor of at least 20 after electrodialysis in acidic solutions. Optimization of the anilinium exchange time as well as the polymerization time were performed. It was also demonstrated that the blocking efficiency of the PANI layer depended on the thickness and uniformity of this layer. 2+

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© 2004 American Chemical Society

Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction Ion-exchange membranes are widely used in several electrochemical technologies such as fuel cells, electrolysis, and electrodialysis. Anion- and cation-exchange membranes allow selective transport of anions and cations, respectively, based on electrostatic repulsions. For instance, cation-exchange membranes (CEM) typically possess carboxylate (-COO) or sulfonate (-S0 ) groups which are negatively charged in sufficiently acidic solutions. Anionexchange membranes (AEM) usually contain amine groups (-NR , R = H or alkyl chains) that are positively charged (7,2). Electrodialysis is a process in which several pairs of CEMs and AEMs are placed alternatively between two electrodes. A current is applied between these two electrodes which forces the displacement of anions towards the anode and cations towards the cathode. The presence of CEMs and AEMs allows the separation of compounds in different compartments. Examples of applications for electrodialysis include the reduction of electrolytes in the food industry, the recovery of valuable electrolytes (pure NaCl from seawater, amino acids from protein hydrolysates, acids from metal pickling and rinsing baths, etc.), salt splitting and more (7,5). The main limitations of electrodialysis are mostly related to the permselectivity of the membranes or to their stability after repetitive usage. In order to improve the permselectivity of cation-exchange membranes for specific ions, different approaches have been studied {4). One was based on a sieving effect by which smaller hydrated ions selectively cross the membranes (4). A second approach was to form a positively charged layer at the surface of a cation-exchange membrane to exclude transport of higher valence cations. Using the latter approach, Sata and co-workers have demonstrated that the presence of a polypyrrole (5) or polyaniline (PANI) (6) at the surface of a Neosepta CM-1 membrane improved its permselectivity for Na against Ca after electrodialysis in neutral solutions. In this study, commercial CEMs bearing sulfonate groups (Neosepta C M X from Tokuyama Soda) were modified by adsorbing a positively charged PANI layer on one side of the membrane. This modification is done in order to block the transport of bivalent metal cations in electrodialysis for the recovery of spent acids. We also report the characterization of the composite membrane. 3

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Experimental Materials and Chemicals Neosepta C M X membranes (Tokuyama Corporation) were stored in 0.5 M NaCl prior to modification. (NH ) S O (EM Science), HC1 (EM Science), 4

2

2

g


313 H N 0 ( E M Science), N a C l (BDH), H S 0 ( E M Science), Z n S 0 7 H 0 (Anachemia) and C u S 0 5 H 0 (Anachemia) were of A . C . S . reagent grade and used as received. Aniline (Aldrich) was distilled twice prior to use. Millipore water (18 M Q ) was used for the preparation of all solutions. The Neosepta C M X membrane properties are given in Table I. 3

2

4

4

4

2

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Table I. Characteristics of the Neosepta C M X membrane given by the manufacturer (7) Properties

Neosepta C M X

Composition

Poly(styrene-co-divinylbenzene)

Supporting Material

Polyvinyl chloride)

Thickness

170-190 ^ m

Exchange Capacity

1.5-1.8 meq/g Na-form dry membrane

Burst Strength

5-6 kg/cm

Water Content

0.25-0.30 g H 0 / g Na-form dry membrane (equilibrated in 0.5 M NaCl) 2.5-3.5 Q c m (equilibrated in 0.5 M NaCl,at25°C)

Electric Resistance

2

2

2

Modification of Cation-Exchange Membranes The protocol used for the preparation of C M X - P A N I composite membranes is based on a published procedure (6) and the detailed procedure used in this study is described elsewhere (5). Briefly, after exchanging the protons with anilinium species, an oxidant, 1 M aqueous (NH ) S O , is added to induce polymerization. The modification was performed in a Teflon two-compartment cell in order to modify only a single face. Before use, the composite membranes were conditioned in a 1 M HC1 aqueous solution to ensure complete protonation ofPANI. 4

2

2

g

X-ray Photoelectron Spectroscopy (XPS) After being conditioned in 1 M HC1, samples were rinsed with water and dried under vacuum at room temperature for 36 h. To compensate for charging effects, binding energies were corrected for covalent C12p of the P V C support found in C M X membranes at 200.6 eV after deconvolution. 3/2


314 Electrodialysis (ED) Electrodialyses were performed in a two-compartment cell containing, in die anodic compartment, a solution of 15 g Zn or Cu per liter of 0.5 M H S 0 and in the cathodic compartment, 0.5 M H S0 . The modified surface was placed facing the anolyte. A current density of 100 mA/cm was applied between two platinum plates for 3 h. The metal concentration in both compartments was determined by atomic absorption spectroscopy. The total mass balance for cations was not calculated because die variation of proton concentration was too small to be detected at 0.5 M H S0 . A smaller concentration needs to be used to allow detection of proton concentration variations. However, the percent metal leakage was corrected for the total mass of metal obtained after electrodialysis. It was calculated according to: % leakage = (mg metal^^) x 100 / (mg metal ) . The total mass of metal in the cathodic and anodic compartments obtained in the latter expression corresponded to the initial amount of metallic ions added at the beginning of ED assays. 2

2

4

4

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2

4

cathodic + m o d k

Exchange Capacity (EC) After conditioning in 1 M HC1, H 0 and 1 M NaCl alternatively, the membranes were soaked for 24 h in a 1 M NaCl solution to ion-exchange H with Na . The excess sodium chloride was then removed by immersion in water and the membranes were dried under vacuum for 48 h at room temperature in order to measure their weight in the Na form. The Na ions were released by immersion in a 1 M HC1 solution during 24 h. The sodium concentration in the latter solution was determined by atomic emission spectroscopy. 2

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Results and Discussion Chemical oxidation of aniline using aqueous ammonium peroxodisulfate or (NH ) S O leads to die formation of the emeraldine form of polyaniline under optimal reaction conditions. The general structure of PANI is given below (9,10); 4

2

2

g

where y = 1, 0.5 and 0 for leucoemeraldine, emeraldine base and pernigraniline, respectively, and A" * CI", !4 S0 " or CMX-S0 " in our experimental conditions. The emeraldine salt form of PANI contains amine and imine groups that can 2

4

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315 both be protonated in acidic solutions, their respective p K values being 2.5 and 5.5 (10). Therefore, after immersing the membrane in 1 M HC1, a protonated polyaniline layer is expected to be formed at the surface of the membrane. Indeed, it was demonstrated by X P S that the P A N I layer obtained under optimal polymerization conditions can be doped with chloride anions (8). a


Characterization of the PANI Layer Upon oxidation of aniline, the membrane presents a dark coloration associated with the presence of polyaniline. Due to the light brown color of the initial C M X membrane, it is difficult to determine whether the layer adsorbed at the surface is blue or green, characteristic colors of the emeraldine base and salt, respectively. The P A N I seemed to adhere well to the membrane and cannot be removed even by scraping the surface with a spatula. To determine i f P A N I is present at the surface or within the bulk of the C M X membrane, S E M , E C and X P S characterization were performed. The surface morphology was studied using S E M with micrographs taken at an angle of approximately 60°. Figure 1 shows that unmodified C M X membranes present a relatively smooth surface when compared to C M X - P A N I composite membrane prepared under optimal conditions (anilinium exchange time: 1 h, polymerization time: 1 h). This first observation already suggests the presence of P A N I at the surface of the membrane. In previous works (8,11), we have also deduced that there might be only a small amount of P A N I within the membrane. In fact, the S E M micrographs of the cross-section of these membranes did not allow us to distinguish the P A N I layer from the bulk of the membrane possibly because both layers are organic. It is actually believed that the P A N I layer is partially interpenetrated within the C M X membrane considering the method used for modification. A s for the thickness of the membrane, it was observed that after modification under optimal conditions, the thickness decreased by approximately 10 pm when compared to the C M X . This change in thickness was explained by the electrostatic interactions between the sulfonate groups and the positively charged P A N I chains (//). Figure 2 presents the X P S survey spectra of a bare C M X and a C M X - P A N I membrane. The presence of P A N I at the surface of the membrane is confirmed by the decrease in peak intensity corresponding to the fixed sulfonate groups, 978 eV ( C K K L ^ L ^ ) Auger peak), 531 eV (Ols), 228 eV (S2s) and 169 eV (S2p), and the enhancement of the N l s peak signal at 400 eV. It should be noted that the opposite surface of the modified membranes showed neither any presence of P A N I on the S E M micrographs nor on the X P S spectra (not shown). Both showed the same surface as the unmodified C M X membrane.



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Figure 1. Scanning electron micrographs of the surface of a (a) CMX and (b) CMX-PANI membrane (prepared under optimal conditions).


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CMX-PANI

w—JL

CO

C1s

O 100

01s


CMX °(

K L

N1s

2, 2,) L

1000

800

600

400

CI2p CI2s .

200

- Binding Energy (eV) Figure 2. XPS survey spectra of a CMX and a CMX-PANI membrane (prepared under optimal conditions).

Blocking Efficiency of the CMX-PANI Membranes A significant decrease in metal leakage is observed when comparing results obtained for the unmodified C M X and the modified membrane in Figure 3: the C M X membrane leads to metal leakage of over 10% whereas the modified membrane gives a value lower than 1%. Two modification parameters have been studied up to now: (1) the exchange time for incorporation of anilinium cations and (2) the polymerization time. Figure 3 shows the effect of immersion time (in a 10% aniline solution prepared in aqueous 1 M HC1) on the Zn(II) and Cu(II) leakage. Very little variation in blocking efficiency is observed with anilinium exchange time indicating that an optimal blocking efficiency is already attained after 1 h (with 1 h of polymerization). On the contrary, the polymerization time plays an important role in the P A N I layer blocking efficiency. A s observed in Figure 4, a polymerization time between 45 and 60 min leads to the lowest metal leakage. Interestingly, Sata and co-workers also observed a loss in permselectivity for N a vs. C a above 1 h of polymerization (6). They suggested that this could +

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318 be caused by the overoxidation o f P A N I to form pernigraniline, the most oxidized state of P A N I corresponding to the insulating form of P A N I . 10

•Zn -Cu

25


CO

_

10

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30

40

50

60

70

Anilinium Exchange Time (h) Figure 3. Zn(II) and Cu(II) leakage (%)for the unmodified and CMX-PANI composite membranes as a function of immersion time in aniline solution, t = 0 corresponds to the unmodified membrane. Polymerization time: 1 h. 12 , 1

Polymerization Time (h) Figure 4. Zn(II) and Cu(II) leakage (%)for the unmodified and CMX-PANI composite membranes as a function of polymerization time. Anilinium exchange time: 1 h. t = 0 corresponds to the unmodified membrane. (Adaptedfrom reference 8.)


319 Actually, the surface electronic conductivity of the membrane was found to decrease after 1 h of polymerization. Sata has demonstrated by doping PANI with bromine that no bromine was detected within the composite membrane after more than 10 h of polymerization.


Exchange Capacity In order to understand the factors affecting the blocking efficiency of the PANI layer, a thorough characterization of the membrane was performed. First, exchange capacity values were measured as a function of anilinium exchange time and of polymerization time (Figure 5). EC data give an indication of positively charged amine groups present on and within the membrane. A portion of the positive charges on the polyaniline chains will be compensated by the fixed sulfonate groups on the membrane. Only the "free" sulfonate groups will be compensated by small ions such as H or Na . Hence, the amount of PANI does not seem to change significantly when increasing the immersion time in the aniline solution over 1 h (Figure 5). This behavior could explain the electrodialysis data given in Figure 3. However, the EC results obtained as a function of polymerization time cannot directly explain the curve profile illustrated in Figure 4 for the metal leakage vs. polymerization time. +

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1.65 1.60 ^

1.55

O)

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1.50

O

1.45 1.40 1.35 1.30 0

10

20

30 40 50 Time (h)

60

70

80

Figure 5. Exchange capacity of the CMX and CMX-PANI membranes as a function of immersion time in aniline solution (O) and of polymerization time (


320 The EC values decrease continuously with increasing polymerization time whereas the percent metal leakage for both Zn and C u increases for polymerization times above 1 h. To further investigate the effect of polymerization time, an extensive characterization of the composite membrane has been completed and published elsewhere (8). 2+

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Evidence of PANI Degradation Figure 6 illustrates the variation in surface content of different elements found for the CMX-PANI membranes as a function of polymerization time. The surface composition was determined using the appropriate XPS core level spectra (8). As mentioned previously, the PANI content at the surface of the membrane can be studied by monitoring the sulfur and nitrogen peaks. Figure 6a shows that the largest amount of PANI adsorbed at the surface of the membrane is found at 1 h of polymerization. Interestingly, this corresponds to the optimal polymerization time obtained from ED tests shown in Figure 4 which indicates that the blocking efficiency of the PANI layer might be related to the amount of PANI present at the surface. In fact, the EC and complete XPS study led us to believe that to obtain an efficient and permselective composite membrane, it is necessary to have a sufficiently thick and uniform PANI (8). The evolution of the different chlorine components detected at the membrane surface was also studied and is depicted in Figure 6b. The unmodified C M X membrane is known to contain a large amount of polyvinyl chloride) (PVC) since this polymer acts as a supporting material (i, 12). In addition, since die PANI chains are positively charged, chloride ions, introduced after conditioning the membrane in HC1, behave as dopants. Therefore, in the presence of a uniform PANI layer, the PVC content should be relatively low and if PANI is in fact doped, chloride ions should be in higher proportions. Again, Figure 6b indicates that an optimal PANI layer is obtained after 1 h of polymerization. From die data given in Figure 6, we can conclude that as die polymerization time increases (above 1 h), a degradation of the PANI layer is observed. Cyclic voltammetry and UV-visible spectroscopy results are in agreement with this statement which demonstrated the presence of benzoquinone, a well-known degradation product of PANI (8, 9). The degradation of the PANI layer would explain the loss in permselectivity of the CMX-PANI composite membrane when these membranes are prepared at longer polymerization times. Therefore, this degradation, rather than the oxidation of aniline into its insulating form as suggested by Sata and co-workers (d), would explain why they observed the absence of the dopant, bromine, at long polymerization times and the loss of electronic conductivity at the surface of the composite membrane.



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Figure 6. Variation of membrane atomic composition of (a) S2p and Nls, (b) Cl2p as a function ofpolymerization time. Anilinium exchange time: 1 h. Moreover, S E M surface micrographs of a degraded membrane showed a smooth surface, very similar to the surface of an unmodified C M X membrane, and no cracks were observed (8). Partial delamination was not observed because (i) the modified membranes were pressed in a stainless steel grid for cyclic voltammograms analyses and (ii) no peaks associated with P A N I were observed


322 in the cyclic voltammograms indicating the absence of PANI in contact with the grid(*).


Conclusions In this work, we have demonstrated that the permselectivity of CMX membranes for monovalent against bivalent cations can be improved by a factor of at least 20 when modified with a PANI layer and used in an acidic solution. These modified CMX membranes did not seem to have a significantly higher resistivity in comparison to an unmodified membrane since the cell voltage (during ED experiments) were found to be similar. To confirm this observation, further work will need to include ionic conductivity measurements. The PANI layer was characterized by SEM and XPS. The latter showed the degradation of PANI when prepared at polymerization times longer than 1 h.

Acknowledgments The authors wish to thank Mr. Raymond Mineau (Departement Sciences de la Terre, UQAM) for the SEM micrographs. This research was funded by the Natural Science and Engineering Research Council of Canada through a strategic grant (234959-00) and an equipment grant for an XPS spectrometer (to D.B. and nine others). S.T. acknowledges the "Fonds Quebecois de Recherche sur la Nature et les Technologies" for a graduate student fellowship. The financial contribution of UQAM is also acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8.

Davis, T.A.; Genders, J.D.; Pletcher, D. A First Course in Ion Permeable Membranes; The Electrochemical Consultancy: England, 1997. Mulder, M . Basic Principles of Membrane Technolog; Kluwer Academic Publishers: Netherlands, 1996. About electrodialysis applications, http://www.electrosynthesis.com Sata, T. J.Membr. Sci. 1994, 93, 117 and references therein. Sata, T.; Funkoshi T.; Akai, K. Macromolecules 1996, 29, 4029. Sata, T.; Ishii, Y.; Kawamura, K.; Matsusaki K. J. Electrochem. Soc. 1999, 146, 585. Neosepta Ion Exchange Membranes, Product Brochure, Tokuyama Soda Inc., Japan. Tan, S; Laforgue, A.; Bélanger, D. Langmuir, 2003, in press.


323 MacDiarmid, A.G.; Epstein, A.J. Faraday Discuss. Chem. Soc. 1989, 88, 317. 10. Hatchett, D.W.; Josowicz, M.; Janata, J. J. Phys. Chem. B 1999, 103, 10992. 11. Tan, S.; Viau, V.; Bélanger, D. Electrochem. Solid-State Lett. 2002, 5, E55. 12. Mizutani, Y.; Tesima, W.; Akiyama, S.; Yamane, R.; Ihara, H. U.S. Patent 3,451,951, 1969.


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Improving the Permselectivity of Commercial Cation-Exchange

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