Synthesis and characterization of commercial cation exchange membranes modified electrochemically by polypyrrole: Effect of synthesis conditions on the transport properties

Desalination 416 (2017) 94–105

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Synthesis and characterization of commercial cation exchange membranes modified electrochemically by polypyrrole: Effect of synthesis conditions on the transport properties

MARK

Guadalupe Vázquez-Rodríguez, Luz María Torres-Rodríguez⁎, Antonio Montes-Rojas Laboratorio de Electroquímica, CIEP-Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Avenida Manuel Nava No.6, Zona Universitaria, C.P. 78210 San Luis Potosí, S.L.P., México

A R T I C L E I N F O

A B S T R A C T

Keywords: Transport number Chronopotentiometry Cation exchange membrane Polypyrrole Electropolymerization

Herein, we report the galvanostatic modification of a commercial cation exchange membrane CMX by polypyrrole (Ppy). The presence of Ppy in the cation exchange membrane (CEM) was confirmed by cyclic voltammetry, stereo microscopy, fourier transform infrared spectroscopy, scanning electron microscopy, and energy dispersive X-ray. Ppy was present both on the membrane surface and inside the pores. The quantity of Ppy in CEM, compactness of Ppy, and the charges on Ppy (negative or positive) were controlled effectively by changing the electrosynthesis conditions. The permeability and water content of the modified membranes obtained under different conditions were investigated. The transport properties of the modified CEMs were evaluated qualitatively using polarization curves and quantitatively by chronopotentiometry. NaCl and MgCl2 solutions ranging in concentrations from 5 mM to 0.1 M were used for testing the selectivity. The presence of Ppy in the membranes decreased the transport number of Na and Mg, and the reduction in transport number was more significant for Mg.

1. Introduction Concentration of electrolyte solutions by electrodialysisis a significant physicochemical wastewater remediation technique due to its high performance over separation rates and low demand for chemicals during operation [1–2]. Electrodialysis is generally used for treating solutions containing two or more ions, as in the case of desalination of water [3], or waste from metallurgy and surface treatments [4]. In some cases, a particular ion might need to be removed from the effluent. Therefore, the selectivity of the ion-exchange membrane used is very important, and several approaches have been developed to improve this aspect [5,6].Modification of ion-exchange membranes by intrinsically conducting polymers (ICPs) is one of the methods used to discriminate between species of different sizes and same charge present in an effluent [7–16]. In fact, the presence of ICPs in ion-exchange membranes decreases the permeation of divalent cations [16]. ICPs can improve selectivity for different reasons. The pores of the membrane modified with ICPs could be smaller than those of the untreated membrane, which makes the passage of bulkier ions difficult [9]. Additionally, ICPs can alter the hydrophilic character of the membrane [9], resulting in strong interaction of ions with the positive charges of

⁎

Corresponding author. E-mail address: [email protected] (L.M. Torres-Rodríguez).

http://dx.doi.org/10.1016/j.desal.2017.04.028 Received 1 February 2017; Received in revised form 24 March 2017; Accepted 29 April 2017 Available online 09 May 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.

the ICPs [7,8,10–12]. The porosity [17,18], hydrophilicity [19], and amount of charge [20] of the ICPs can be modulated by changing the conditions of polymerization, and this can be carried out chemically or electrochemically. Electrochemical polymerization presents significant advantages over chemical synthesis, as there is no need for an oxidizing agent, and fine control of the initiation and termination reaction can be achieved [20,21]. However, there have been few reports of electrochemical methods used [12,13] for the modification of ion-exchange membranes. On the other hand, modification has been carried out using two ICPs: polyaniline (PANI) [7–13] and, to a lesser extent, polypyrrole (Ppy) [14–16]. To the best of our knowledge, there have been no reports of commercial membranes modified electrochemically by Ppy. In this work, we present the electrochemical modification of a commercial cation exchange membrane by polypyrrole and its characterization. The porosity and charges were modulated by the electrosynthesis conditions and the selectivity of these membranes was evaluated by chronopotentiometry using NaCl and MgCl2 solutions of various concentrations. Films with the same amount of charge and different compactness were obtained using water and acetonitrile as solvents during electrosynthesis. The amount of Ppy was varied by changing the time of electrosynthesis. The charge was modulated by

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Table 1 Properties of the commercial membranes used in this work.

CMX AFN

Fixed charge

Electric resistance/Ωcm− 2

Pressure resistance/MPa

Thickness/mm

Exchange capacity/meqg− 1

Transport number

− SO3− − NR3+

1.80–3.80 0.3–1.0

≥ 0.40 ≥ 0.25

0.14–0.20 0.13–0.18

1.50–1.80 2.0–3.5

K+/0.96 NO3−/0.93

were mounted on a disk and sputter-coated with a thin layer of gold. For micrographs transversal of membranes, membranes were frozen using liquid nitrogen and divided by a cutter. Stereo microscope used was an Olympus SZX16. For water content test the membranes were immersed in distilled water for 24 h at room temperature (27 °C), the surface wiped with filter paper, and weighed with an analytical balance (Hinotek DHG9145A). The wet membranes were dried at a fixed temperature (50 °C) until a constant weight was obtained (approximately 4 h). The following equation was used for the calculation of water content:

changing the degree of oxidation of Ppy, as the functional groups of overoxidized Ppy [22,23] are different from those of untreated Ppy. 2. Materials and methods 2.1. Materials The CMX cation exchange and AFN anion exchange membranes used were supplied by ASTOM Corp., Japan. Table 1 lists the principal properties of these membranes.

⎛ Wwet − Wdry ⎞ water content % = ⎜ ⎟ × 100 ⎝ ⎠ Wwet

2.2. Electrochemical modification of membranes

(1)

where Wwet and Wdry are the weights of the membrane at the equilibrium swelling and dry state, respectively. The water vapor permeability test of the membranes was carried out using cylindrical penny cups of area 1.7671 cm2. The cups, each containing 5 g of water, were covered by the membranes (2.8353 cm2) and closely fixed by Parafilm. The water vapor permeability was determined by the loss of water/h through the membrane. Reflectance infrared spectra were recorded using a Nexus series 470 instrument. Typically, 2 scans were recorded in the range 5000–400 cm− 1, with a resolution of 1 cm− 1. Fourier transform infrared spectroscopy (FTIR) data were collected using the Bomem Grams/386 software for Windows (Version 3.01B, level II, 1991–1994).

Pyrrole (Sigma-Aldrich) was purified prior to use by passing through a micro-column constructed from a Pasteur pipette and silica. This procedure was repeated several times until a colorless liquid was obtained. The aqueous solutions were prepared using deionized water (18.8 MΩcm), and the solutions were deoxygenated by purging with nitrogen gas. Following this, a nitrogen atmosphere was maintained in the solutions during each run. The experimental montage for electrochemical polymerization was carried out according to Montes et al. [13]. Briefly, a typical threeelectrode cell was used, consisting of a commercial membrane adhered to a carbon paste electrode (2.8153 cm2) as working electrode, platinum spiral as counter electrode, and Ag/AgCl/NaCl 3 M (BASi) as the reference electrode. The carbon paste electrode was prepared by thoroughly mixing graphite powder (Alfa-Aesar) and Nujol (Alfa-Aesar) in proportion 60:40 w/w, and packing the resulting paste into a plastic 20 mL syringe in which a piece of copper wire was wound to produce the electrical contact. The membranes were pretreated by immersing in the support electrolyte solution for 1 h. To perform the electrochemical polymerization and characterization of Ppy, a potentiostat/galvanostat (BAS Epsilon) controlled by software (Epsilon-EC, version 1.31.65 NT) was used. Electrochemical deposition was carried out in the galvanostatic mode, and the constant current density used was 3.55 mAcm− 2. The synthesis time was varied between 30 and 840 s, resulting in deposits with varying amounts of Ppy. The working solution for electrodeposition was composed of pyrrole (0.01 M) and LiClO4 (Aldrich, 0.1 M), with water and/or acetonitrile (Fermont) as solvents. After deposition, the membrane was detached from the surface of the carbon paste electrode, and sonicated in chloroform to eliminate residues of carbon paste and oligomers of pyrrole. Following this, the membranes were used in chronopotentiometric tests or for the measurement of polarization curves and therefore the Ppy used was oxidized. For cyclic voltammetry studies, the modified membranes were attached to the surface of the carbon paste electrodes, and scanned from − 100 to 700 mV at 100 mVs− 1 in a solution containing only the support electrolyte. The charge of each film was obtained by the area under the curve of the voltammogram. The deposits of Ppy were overoxidized by imposing a constant potential of 1200 mV for 5 min in a working solution of LiClO4 (0.1 M).

2.4. Current-voltage polarization and chronopotentiometric curves Current-voltage and chronopotentiometric curves were recorded using an acrylic cell, fabricated in-house for the purpose. The cell contains four compartments (Fig. 1) with separating walls, and have a circular hole (diameter: 1 cm) in the geometric center, into which the membranes were clamped. The outer well contains the auxiliary membranes, CMX and AFN, while the membrane under study was placed in the central well. The face of membrane in contact with the solution during electrochemical modification was on the anode side. Two Pt spirals were placed in the outer compartments and used as the working electrodes. The chronopotentiometric curves were obtained using the same cell by the application of current between these electrodes using a galvanostat fabricated in-house. The resultant voltage was measured using a digital multimeter (LINI-T) connected to two Ag/AgCl/NaCl 3 M (BAS) electrodes, which were coupled with capillaries brought to the surface of the membrane under study. Data were collected using the program of the multimeter. The same configuration was used to obtain the polarization curves; however, in this case, a potential was applied between the Pt electrodes, and the resultant current and membrane potential were individually measured by a multimeter. The data were recorded manually. Solutions of NaCl (Fermont) and MgCl2 (J. T. Backer) were used for these studies. 3. Results and discussion

2.3. Characterization of the modified membranes

3.1. Synthesis and electrochemical characterization of the modified membranes

Scanning electron microscopy (SEM) studies were carried out using a Helios Nano Lab DualBeam 600 instrument, equipped with an energy dispersive X-ray spectroscopy (EDX) system operating at 5 kV. Samples

The membranes were modified electrochemically using the method reported earlier [13], in which the nonconducting membrane was adhered to a carbon paste electrode (CPE), and the ensemble formed 95

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 1. Schematic diagram of the four-compartment cell configuration for chronopotentiometric measurements.

behavior of unmodified membrane, electrochemical behavior of modified membrane was studied.Towards this, immediately after the Ppy electrosynthesis, the CMX/CPE ensemble modified with Ppy (Ppy/ CMX/CPE) was rinsed with water and transferred to a solution containing exclusively LiClO4. The cyclic voltammogram obtained presented a broad oxidation band and its cathodic counterpart (Fig. 2b).This curve is similar to that of Ppy obtained in metallic electrodes [17], but the response is more resistive due to the difficulty of charge transport across the membrane and the Ppy deposit. In fact, the responses of

was used as the working electrode in a three-electrode cell. Electropolymerization was carried out by the application of a constant current of 3.55 mAcm− 2. After electrosynthesis, the deposits on the membrane were analyzed by cyclic voltammetry. To distinguish the response of the CMX/CPE ensemble from that of the Ppy immobilized on the membrane, the responses of the CMX/CPE ensemble before polymerization were first evaluated. The curve obtained for CMX/CPE (Fig. 2a) did not show any Faradaic processes in the range of potential studied. After to define

Fig. 2. Cyclic voltammograms obtained in 0.1 M LiClO4 of:, (a) CMX/CPE, (b) Ppy/CMX/CPE after electropolymerization, (c) CPE after the removal of Ppy/CMX, (d) Ppy/CMX/CPE after the renovation of CPE surface and washing of Ppy/CMX, with the same membrane position as used during electrosynthesis, and (e) Ppy/CMX/CPE after the renovation of CPE surface, where the position of membrane is inverse to that used during the electrosynthesis. Scan rate: 100 mVs− 1. The Ppy was electrosynthesized by imposition of a current density of 3.55 mAcm− 2 for 220 s, using an aqueous solution containing pyrrole (0.01 M) and LiClO4 (0. 1 M). Inset: Magnification curve (a).

96

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

measured peak heights (Ip) scale linearly with the sweep rate (v) (Fig. 3 inset of left side), indicative of a surface localized oxidation/ reduction process. However, contrary to the established equation for an immobilized redox space, the y-intercept is not zero. Additionally, the correlation coefficient is lower than expected, implying that the experimental data are not completely adjusted to the model of immobilized electroactive species. This could be because the electronic transference is not carried out directly on the CPE surface, but through the modified membrane that can be considered as a thick film. In fact, in these cases, a semi-infinite electrochemical charge diffusion condition prevails, and the behavior of the voltammograms with scan rate resemble those in solution and diffusing to the electrode surface. Hence, a linear relation is expected between Ip and v½ [25]. The data were found to be a better fit for this model (inset right side of Fig. 3) than for that of the immobilized species. These experiments show that Ppy is attached effectively to the membrane, and semi-infinite diffusion prevails in the range of scan rate investigated. The reproducibility of the modification of CMX by Ppy was tested by repeating the experiments thrice. After this, the membranes were moved from the electrode surface and cleaned with chloroform, and finally adhered afresh to the electrode surface. The cyclic voltammograms of Ppy/CMX/CPE were almost the same for all experiments. This shows that the modification of membranes is reproducible. To evaluate if the Ppydeposits are deposited uniformly in the CMX, three circles of 0.1963 cm2 on a Ppy/CMX surface of 2.8153 cm2 were analyzed individually by cyclic voltammetry. The curves obtained were very similar, and therefore we could conclude that the Ppy is distributed homogeneously in the CMX. After verifying that the Ppy is effectively immobilized in CMX, the synthesis conditions were changed to modulate the characteristics of Ppy. First, the time of electrosynthesis was varied (between 30 and

thinner deposits of Ppy were less resistive. To determine, if the Ppy is deposited onto the CMX, CPE, or both, each part of the Ppy/CMX/CPE ensemble was analyzed separately. The CMX membrane modified with the Ppy was removed from the CPE, and the CPE was also examined by cyclic voltammetry (Fig. 2c). The curve obtained is similar to that of Ppy, but the current is smaller than that obtained for the Ppy/CMX/CPE ensemble (Fig. 2b). This result showed that CPE contains a part of the Ppy deposited. Finally, in order to study the presence of Ppy in the CMX membrane, the CMX membrane modified with Ppy was removed, washed, and sonicated in chloroform, with the objective of eliminating the residues of carbon paste and oligomers of pyrrole. Following this, the modified membrane was adhered to a CPE with a recently smoothed surface. The membrane was introduced in two different orientations. In the first experiment, the membrane was placed as it was during electrosynthesis, i.e., the face of the membrane that was exposed to solution during the electrosynthesis was placed in contact with the working solution. The curve obtained (Fig. 2d) also corresponded to Ppy, and therefore the Ppy is well deposited in the CMX. For the second experiment, the membrane was placed in the orientation, i.e., the face of the membrane exposed to the solution during electrosynthesis was in contact with the CPE. In this case, the response (Fig. 2e) was also similar to Ppy; however, the current value was very small in comparison with the curve obtained in Fig. 2d. Thus, both faces of the membrane contain Ppy, but in different amounts, implying that Ppy is present inside the membrane as well. This is in accord with other studies in which ICP has been immobilized inside cation-exchange membranes [12,24]. After electrosynthesis, the voltammetric behavior of the modified membranes was analyzed at different scan rates in a solution containing only the support electrolyte (Fig. 3). The Ppy curve showed that Ppy remained attached to the membrane. It can be observed that the

Fig. 3. Cyclic voltammograms of Ppy/CMX in an aqueous solution of 0.1 M LiClO4 at different scan rates: 25, 50, 75, 125, 150, 175, and 200 mVs− 1. Insets show the plots of peak current versus scan rate (top left) and peak current versus square root of scan rate (bottom right). Ppy was synthesized by imposing a current of 3.55 mAcm− 2 for 220 s. The working solution contained pyrrole (0.01 M) and LiClO4 (0.1 M). The arrow indicate the evolution of the current with increasing scan rate.

97

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 4. Voltammetry in a 0.1 M solution of LiClO4 of CMX membranes modified with varying amount of Ppy supported on CPE. Scan Rate: 100 mVs− 1. Membranes were modified according to the conditions given in Fig. 2, for different times (a) 30, (b) 60, (c) 220, (d) 420, and (e) 840 s. Inset: plot of Q versus time of polymerization.

material which corresponds to Ppy (Figs. 5b and c). Additionally, it can be seen that some part of the membrane valley is bare. This confirms the presence of Ppy on both sides of the membrane and that some parts of the surface are not fully covered by Ppy. The modified membranes were very similar in all the conditions studied: different amount of Ppy, solvent, and overoxidation (curves not shown).

840 s) in order to modulate the amount of Ppydeposited on the membrane. After electrosynthesis, the membranes were taken out of the CPE, rinsed with chloroform and adhered afresh to the CPE. As shown in Fig. 4, the current of the voltammograms increases with the time of deposition, indicating that Ppy increases with the time of deposition. In order to estimate the extent of Ppy fixed to the membranes, the charge (Q) of each film was obtained from the area under the curves of the voltammograms. In fact, the charge and number of moles of the electroactive species are directly proportional, according to Faraday's laws. Charges obtained were plotted as a function of time of electrosynthesis, and both parameters are directly proportional (inset Fig. 4), confirming that the amount of Ppy can be easily modulated by varying the time of electrodeposition. Another characteristic that can be modulated is the type of charges of Ppy. For this a modified membrane with Ppy was overoxidized after synthesis, and its voltammogram was found to be completely different from that of modified membrane with untreated Ppy. In fact, the response was very similar to that of the membrane before modification. This shows that the Ppy was overoxidized, and is no longer electronically conductive [26]. Finally, deposits of Ppy with different porosity were synthesized by using two different solvents: water and acetonitrile. When a current of 3.55 mAcm− 2 was applied, Ppy was fixed to the CMX membrane, but the electrodeposition was faster using acetonitrile, and the amount of Ppy obtained was controlled by changing the time of application of current.

3.2.2. SEM and EDX measurements The superficial morphology of the different modified membranes was analyzed by scanning electron microscopy. The surface of the unmodified CMX membrane (Fig. 6a) shows irregularly distributed pores of different sizes, indicating that the CMX membrane is microscopically heterogeneous. After defining the morphology of the unmodified membrane, the Ppy/CMX membranes were analyzed. Two Ppy/CMX membranes with the same charge, but synthesized in water medium (Ppy/CMX(wm)) and acetonitrile medium (Ppy/CMX(am)), (Fig. 6c, d, e, and f) were analyzed. The membranes were very similar and characterized by the presence of aggregates formed by flake-like units, where both the units and aggregates had variable form and size. The only difference was that the aggregates in Ppy/CMX(wm) were larger than those in Ppy/CMX(am). This result is analogous to those obtained in metallic surfaces when the films were thin, as in this case. In fact, the morphology of Ppy is composed of nodules, which were larger for Ppy synthesized in aqueous medium [27]. The fact that Ppy deposited in aqueous media had a more voluminous aspect than the depots obtained in acetonitrile, under similar amount of Ppy, indicated that probably the Ppy synthesized in water was more porous than that formed in acetonitrile solution, as in the case of deposition on metallic electrodes [17]. Finally, the appearance of both the faces of membranes is very similar, though a more significant amount of aggregates is present on the face in contact with the solution. This result corroborates the conclusion that Ppy was present on both faces of the membrane, and as a consequence inside the membrane pores. The morphology of the modified membranes was analyzed as a

3.2. Characterization of the modified membranes 3.2.1. Optical stereo microscope measurements The unmodified CMX membrane and Ppy/CMX membranes were analyzed using an optical stereo microscope, and the images are shown in Fig. 5. The unmodified membrane shows a pattern of fibers formed by valleys and ridges that form the membrane cross (Fig. 5a). Both surfaces of the modified membranes are partially covered by a sand-like 98

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 5. Stereo microscope overview of: (a) unmodified membrane, (b) modified membrane side in contact with solution during electrosynthesis, (c) opposite side of (b), and (d) magnified view of (c). Ppy was synthesized by imposing a current for 420 s, according to the conditions in Fig. 2.

present on all the surfaces of the membranes, but in some zones only. Therefore, it is not possible to refer to the thickness of Ppy, but only to the amount of Ppy fixed to the membrane.

function of the extent of Ppy, and it was observed that varying the time of polymerization with a view to controlling the amount of Ppy induced very small changes in the surface morphology (images not shown). In order to achieve greater control over the amount, the Ppy was synthesized in a solution of a mixture of water/acetonitrile (50% volume) and a higher concentration of pyrrole. The SEM micrograph (Fig. 6 g) obtained for this membrane showed that the membrane surface was completely covered by Ppy. The cross-section of this membrane (Fig. 6 h) is distinct from the untreated membrane (Fig. 6a) due to the presence of Ppy inside the membrane. The effect of overoxidation on the morphology was analyzed by comparing two membranes synthesized under identical conditions, where one of them was subjected to overoxidation. The SEM images of both membranes do not present any major differences, indicating that overoxidation did not significantly modify the superficial morphology of Ppy immobilized in membranes. This behavior agrees with that reported for Ppy overoxidation on metallic surfaces [26]. In order to determine the composition on the surface of the Ppy/ CMX membranes, EDX experiments were carried out. The analysis was performed in different zones of the membrane, and some zones presented EDX results identical to those of the unmodified membranes. This was because some zones are not fully covered by Ppy as observed in images from the stereo microscope. However, in other zones, the EDX spectra of the surface of the unmodified (inset Fig. 6a) and modified membranes (inset Fig. 6g) present evident differences. Firstly, N which is a component of Ppy and present in the Ppy/CMX membranes, is absent in the unmodified membrane. Secondly, the heights of C and Cl peaks in case of the CMX membrane were very similar, while in the Ppy/CMX membrane, the C peak was larger than the cl peak. This indicates that there was a major amount of C in the Ppy/CMX membrane, which was due to the immobilized Ppy on the membrane surface. Results of SEM and stereo microscopy show that under the electrosynthesis conditions used for the modification of membranes, Ppy is not

3.2.3. FTIR reflectance study FTIR reflectance studies of unmodified and the Ppy/CMX membrane revealed that the spectrum ofPpy/CMX (Fig. 7a) is different as those obtained from unmodified membranes (Fig. 7b). The spectrum of the modified membrane (Fig. 7a) reveals the characteristic absorption bands of Ppy [779 and 916 cm− 1 (CeH out of plane deformation), 1306 cm− 1 (CeN stretching vibration in the ring), 1554 cm− 1 (C]C stretching of Ppy ring), and 3440 cm− 1 (NeH stretching)] [28,29], in addition to the CMX bands. These results show that the Ppy was deposited onto the CMX membranes. In order to ensure the detection of Ppy, the amount of Ppy fixed to the membranes was more important than the rest of the membrane, and the concentration of pyrrole in the working solution was increased to 0.1 M to facilitate this. 3.2.4. Water content and permeability The water contents and permeabilities of all the membranes studied are shown in Fig. 8a, where it can be observed that the permeability of the Ppy/CMX membranes was lower than that of the uncoated membrane. This was probably caused by the polymerization of Ppy in the pores of the membranes which leads to a reduction of the pore diameter, or even blockage of some pores. In contrast, the humidity was found to be higher for the modified membranes. This was possibly because the modified membranes are not as smooth as the unmodified membranes, due to which there were more voids and cavities where the water could accumulate. Regarding the effect of the amount of Ppy, the results show that when amount of Ppy is increased, the humidity increased and permeability decreased. This increase in humidity could also be related to the presence of more cavities which retained water. The decrease of permeability was because Ppy operates as a barrier, and when its amount increased, it was more difficult for the water to pass 99

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 6. Scanning electron micrographs of unmodified CMX membrane, (a) surface and (b) cross section. (c) Ppy/CMX membrane synthesized in aqueous medium (side in contact with the CPE), and (d) side in contact with the solution. (e) Ppy/CMX membrane synthesized in acetonitrile medium (side in contact with the CPE), and (f) side in contact with the solution. (g) Ppy/CMX membrane synthesized in a solution (50:50) water: acetonitrile (side in contact with the solution), (h) cross section of a membrane modified in a solution of 50:50 water: acetonitrile. Inset shows the EDX analysis of the membranes. Synthesis conditions are the same as in Fig. 2, except for (g) and (h), which where synthesized in a water:acetonitrile medium and the concentration of pyrrole was 0.1 M (g) and 0.01 M (h), respectively.

changes in the hydrophobicity of the Ppy. In fact, overoxidized Ppy is present in hydrophilic micropores and hydrophobic polymer bulk [23], which explains the higher permeability and the decrease of humidity in comparison with the untreated membrane.

through the membrane. The influence of the solvent used in electrosynthesis on these parameters was also studied (Fig. 8b). The permeability of the membranes obtained in acetonitrile was considerably lower than that of the membrane modified in aqueous medium (same charge, Fig. 8a). As the amount of Ppy was the same in both membranes, this indicated that the Ppy deposited in organic medium (Ppy/CMX(am)) is more compact than that in aqueous medium (Ppy/CMX(wm)). The water content of the Ppy/CMX(am) membrane (Fig. 8b) was lower than that of bare CMX, while that of the Ppy/CMX(wm) membrane was higher (same charge, Fig. 8a). This behavior is also attributed to the increased roughness of the Ppy/CMX(am) membrane which presents more voids and cavities than the Ppy/CMX(am) membrane. These results showed that it was possible to modulate the porosity of the membranes. Finally, we studies two similarly modified membranes, where the Ppy in one was electrochemically overoxidized (Fig. 8c). The permeability of the overoxidized membrane was higher and its humidity lower than the untreated membrane (same charge, Fig. 8a). As overoxidation does not induce any gross changes in the morphology, the changes in water content and permeability can be attributed to the

3.3. Evaluation of transport properties 3.3.1. Polarization curves Current-voltage curves were recorded to study the polarization behavior of the CMX/Ppy composites (Fig. 9). The effects of the excent of Ppy, and overoxidation of Ppy were studied using NaCl and MgCl2 as the monovalent and bivalent ionic solutions, respectively. The three well-known regions of the current-voltage curve can be distinguished in all the plots: An ohmic region up to the limiting current (Section I), a plateau (Section II), and an overlimiting current region (Section III). It can be observed that the curves of the modified CMX membranes are different from those of the unmodified CMX membranes (Fig. 9). Also, the curves of the modified membranes are all not the same. These results confirm that Ppy alters the ion transport through the membrane and that the ion transport properties change with the synthesis 100

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 7. FTIR spectra of: (a) Side of Ppy/CMX membrane in contact with solution during electrosynthesis; (b) unmodified CMX membrane. Ppy was synthesized by the imposition of a current of 3.55 mAcm− 2 for 840 s. Working solution was pyrrole (0.1 M) and LiClO4 (0.1 M).

smaller than in the unmodified membranes. On the other hand, as the amount of Ppy increase, Ilim also increases. This indicates that ti decreases, and as a consequence, it is more difficult for the cations to pass through the membrane modified by Ppy. This effect is more evident in the case of Mg2 + than Na+ due to two possible reasons. Firstly, since Ppy acts as a physical barrier, larger amounts of Ppy make it more difficult for the cation to cross the membrane, and as a consequence, Ilim increases and ti decreases. In the case of Mg2 + the effect is more marked because the hydration radius of Mg2 + is much larger than that of Na+ [30], making the crossing of the membrane more difficult. Secondly, the positive charge of Ppy could neutralize the negative charge of the CMX membranes. In fact, the Ppy present in the membranes was in the oxidized state, so the positives charges present could be neutralized by the anions during electrosynthesis. Thus, as the amount of Ppy in the membrane increases, more charges are neutralized and selectivity decreases. The effect of overoxidation of Ppy on the transport properties was also compared. In the case of Na+, the Ilim of the overoxidized (Fig. 9d’)

conditions. The value of Ilim was analyzed obtained using the well-known equation for the limiting current density:

Ilim =

zDFCi0 δ (t − ti )

(2)

where δ is thickness of the diffusion boundary layer, is the concentration of the counter ion in the bulk solution, D is the electrolyte diffusion coefficient, z the charge of the counter ion, F the Faraday constant, and t and ti are the transport numbers of the counter ion in the membrane and solution, respectively. In the same solution D is the same for each cation, and as all curves were carried out in the same hydrodynamic conditions, D and δ were considered as constants. Fig. 9 show that for the modified membranes with different amounts of Ppy, the Ilim of Mg2 + is higher than that of Na+. This behavior can be understood from Eq. 2, where Ilim is directly proportional to the charge of the cation. In addition, the Ilim values of Na+ and Mg2 + are higher for the modified membranes, indicating that the transport number is Ci0

Fig. 8. Membrane water content ( ) and permeability ( ) as a function of: (a) charge of Ppy, (b) solvent used and (c) overoxidation of Ppy. The membrane was modified according to Fig. 2.

101

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 9. Current-voltage curves of the CMX and Ppy/CMX membranes in 0.01 M solutions of NaCl and MgCl2. Curves were modified according to the conditions listed in Fig. 2 for different times until get: (a) 0, (b) 4.14, (c) 10.60, (d) 69.94, (d´) 69.94 (overoxidated Ppy), (e) 120.41 and (e) 243.31 mC.

to the membrane, and the decrease of resistance is attributed to the presence of Ppy. It is also observed that the resistance decreases with an increase of Ppy. This was also observed for membranes modified chemically with Ppy [15,16]. The resistance for the overoxidized Ppy/CMX membranes is higher than for the untreated Ppy/CMX membranes in the MgCl2 solution, possibly because of its high ionic radius and the hydrophobicity of the bulk of overoxidized Ppy. This could induce a decrease of ions on the electrode surface and an increase in the resistance. In contrast, for Na+, Section I of polarization curves of the membranes with overoxidized and untreated Ppy have similar slopes, and the same resistance. This is because overoxidation changes Ppy from a conducting polymer to an ion-exchange polymer [31], and as a consequence, both types of Ppy are charged species, and the resistances offered by the untreated and treated Ppy are similar.

and unmodified (Fig. 9d) Ppy membranes are not very different. In contrast, the Ilim of the membrane containing overoxidized Ppy (Fig. 9d’) in the case of Mg2 + is markedly smaller than for the unmodified CMX membrane (Fig. 9d). This indicates that Mg2 + crossed the overoxidized Ppy membrane more easily than the untreated commercial membrane. This behavior is consistent with an increase of hydrophilicity of the pores of overoxidized Ppy due to the presence of alcohol, carboxylic, and carbonyl groups [22,23]. Thus, the negative charges of the functional groups of overoxidized Ppy increase its selectivity to cations. This result shows that modifying the chemical structure of Ppy can alter the cation transport through the membrane. The electrical resistance of the solution/membrane interface corresponds to the inverse of the slope of the ohmic part of the polarization curve. As shown in Fig. 9, the slopes (R, Fig. 9a) for the modified membranes are higher compared to those of the unmodified membranes. As the solution is the same, the changes in conductivity are due 102

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 10. (a) Chronopotentiometric curves at different applied current density measured with a CMX/Ppy membrane in 0.001 M NaCl (numbers next to the curve refer to part of curve), (b) dE/dt versus time for the curves, (c) dependence of Iτ½ on I, and (d) curve of τ versus I- ½. The membrane was modified according to the conditions in Fig. 2, and the current was applied for 220 s.

clarity, we determined the transition time from the chronopotentiometric curve derivative of potential (E) as a function of time (dE/dt) (Fig. 10b). The Sand equation implies that there is a linear relation between I and τ-½ and that the Iτ-½ vales are constant and independent of current. Figs. 10c and d shows that data fit to these requirements, and as a consequence, the Sand equation can be used. The value of tm(r) was calculated from the slope of the curves of I versus τ-½. For this, the D value was obtained from the literature [34] and ts was deduced from ionic and molecular conductance data.

3.3.2. Determination of transport number by chronopotentiometry The differences in the ion transport properties of unmodified and modified membranes were evident from their polarization curves. However, it was not possible to establish these precisely, as the differences are related to membrane selectivity. In order to determine the transport number of ions for different membranes, a chronopotentiometric study was carried out. The transport number can be obtained from the modified Sand equation [32]:

τ=

2 ⎛ πD ⎞ ⎛ ACi0 zi F ⎞ 1 ⎜ ⎟⎜ ⎟ 2 ⎝ 4 ⎠ ⎝ tm (r ) − ts ⎠ I

(3)

3.3.2.1. Effect of excent of Ppy and ion concentration. The transport number was analyzed as a function of different synthesis conditions. The results for the effect of quantity of Ppy in the membranes (Fig. 11) shows that for both Na+ and Mg2 + cations, the transport number decreases as the amount of Ppy increases. This is because Ppy acts as a physical barrier which obstructs the passage of cations [16]. In fact, Ppy grows inside the holes of membranes, implying that with increasing amount of Ppy the pores of the modified membranes become smaller. This behavior is congruent with the observed decrease of permeability with increase of Ppy in the membranes. Additionally, as the Ppy is oxidized, the positive charges reduce the passage of cations by electrostatic repulsion [35]; the amount of charge increases with the increasing extent of Ppy contained in the membrane. A similar relation between the amount of Ppy and relative transport number was reported for Ca2 + in commercial membranes that are modified chemically with Ppy [16]. Additionally, it was observed that the transport number of Mg2 + is

where I is the applied current, Ci is the concentration of the electrolyte, z is the valence of the ion, F is the Faraday constant, D is the diffusion coefficient, A is the membrane area, and tm(r) and ts are the transport numbers of ion in the membrane and solution phase, respectively, τ is the transition time, which corresponds to the time taken for a constant current to completely deplete the counter ions in the proximity of the membrane. Chronopotentiometric curves were obtained for modified and unmodified membranes in solutions of Na+ and Mg2 +. Fig. 10a shows the typical chronopotentiometric curves obtained for a modified membrane using a solution of 0.001 M NaCl. The curves obtained present a typical behavior and show four parts: (1) instantaneous increase in the voltage when current is applied, (2) slow increase in the voltage drop with time, (3) strong increase in the voltage drop, and (4) leveling of the voltage drop [32,33]. The transition time (τ) can be obtained from the intersection of the tangents of segments 2 and 3 of the curve. In order to obtain more 103

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

Fig. 11. Effect of concentration of cation on the transport number of Na+ and Mg2 + for (A and B): membranes modified with different amount of Ppy: (a) 0, (b) 4.14, (c) 10.60, (d) 69.94, (e) 120.41, and (f) 243.31mC; (C and D): membrane unmodified (a) and membranes modified with Ppy in water (b) and acetonitrile (c) by the imposition of potential for 220 s according to conditions listed in Fig. 2; (E and F): membrane unmodified (a) and membranes modified with overoxidized (b) and untreated (c) Ppy. Synthesis was carried out by the imposition of potential for 220 s according to conditions listed in Fig. 2.

lower than that of Na+ for all cases. Mg2 + is a bulkier ion and has a higher charge than Na+, so the hindrance and electrostatic repulsion are more significant. However, the selectivity for Na+ is lesser than expected for membranes modified with Ppy (Fig. 11A). This could be due to the large degree of impregnation of Ppy in the membrane, causing a reduction in the pore size, and a decrease of charges in the membranes by neutralization. Nevertheless, the difference between the transport number of Na+ and Mg2 + is generally higher for modified membranes than unmodified membranes, confirming the higher preference of the modified membranes for Na+. When amount of Ppy was low, (Fig. 11Ab), the selectivity of Na+ is high and similar to that of the unmodified membrane (Fig. 11Aa). In contrast, the transport number of Mg2 + is lesser than that of the unmodified membrane (Fig. 11Bb and Ba, respectively). This indicates that membranes modified with small quantities of Ppy are good candidates for a selective separation of

monovalent cations. As shown in Fig. 11, the transport number decreases when the concentration of the studied cation increases. This may be ascribed to the fact that at relatively high concentrations, Donnan exclusion is not very effective due to the increased interactions in counter ion membranes [36]. 3.3.2.2. Effect of compactness of Ppy. As indicated earlier, in order to obtain deposits of Ppy of different porosity, the modified membranes were synthesized with the same amount of Ppy, but using two different solvents: water and acetonitrile (Ppy/CMX(wm) and Ppy/CMX(am), respectively). For Na+, the values of transport number for all the concentrations studied was lower for Ppy/CMX(am) in comparison with CMX and Ppy/CMX(wm) (Fig. 11C). The transport number of Ppy/ CMX(wm) was very similar for untreated membranes at lower 104

Desalination 416 (2017) 94–105

G. Vázquez-Rodríguez et al.

electrodilaysis, Ind. Eng. Chem. Res. 32 (1993) 657–662. [5] T. Sata, W. Yang, Studies on cation-exchange membranes having permselectivity between cations in electrodialysis, J. Memb. Sci. 206 (2002) 31–60. [6] S. Mulyati, R. Takagi, A. Fujii, Y. Ohmukai, H. Matsuyama, Simultaneous improvement of the monovalent anion selectivity andantifouling properties of an anion exchange membrane in an electrodialysisprocess, using polyelectrolyte multilayer deposition, J. Membr. Sci. 431 (2013) 113–120. [7] S. Tan, A. Laforgue, D. Bélanger, Characterization of a cation-exchange/polyanilinecomposite membrane, Langmuir 19 (2003) 744–751. [8] T. Sata, Y. Ishii, K. Kawamura, K. Matsusaki, Composite membranes prepared from cationexchange membranes and polyaniline and their transport properties in electrodialysis, J. Electrochem. Soc. 146 (1999) 285–591. [9] R.K. Nagarale, G.S. Gohil, V.K. Shahi, G.S. Trivedi, R. Rangarajan, Preparation and electrochemical characterization of cation- and anion-exchange/polyaniline composite membranes, J. Colloid Interface Sci. 277 (2004) 162–171. [10] N.P. Berezina, N.A. Kononenko, A.A.R. Sytcheva, N.V. Loza, S.A. Shkirskaya, N. Hegman, A. Pungor, Perfluorinatednanocomposite membranes modified by polyaniline: Electrotransport phenomena and morphology, Electrochim. Acta 54 (2009) 2342–2352. [11] M. Kumar, M.A. Khan, Z.A. Alothman, M.R. Siddiqui, Polyaniline modified organicinorganic hybrid cation-exchange membranes for the separation of monovalent and multivalent ions, Desalination 325 (2013) 95–103. [12] P. Sivaraman, J.G. Chavan, A.P. Thakur, V.R. Hande, A.B. Samui, Electeochemical modification of cationexchange membrane with polyaniline for improvement in permselectivity, Electrochim. Acta 52 (2007) 5046–5052. [13] A. Montes-Rojas, Y. Olivares-Maldonado, L.M. Torres-Rodríguez, An easy method to modify the exchange membane of electrodialysis with electrosynthetized polyaniline, J. Membr. Sci. 300 (2007) 2–5. [14] T. Sata, T. Yamaguchi, K. Matsusaki, Preparation and properties of composite membranes composed of anion-exchange membranes and polypyrrole, J. Phys. Chem. 100 (1996) 16633–16640. [15] G.S. Gohil, V.V. Binsu, V.K. Shahi, Preparation and characterization of mono-valent ion selective polypyrrole composite ion-exchange membranes, J. Membr. Sci. 280 (2006) 210–218. [16] T. Sata, T. Funakoshi, K. Akai, Preparation and transport properties of composite membranes composed of cationexchange membranes and Ppy, Macromolecules 29 (1996) 4029–4035. [17] J.M. Ko, H.W. Rhee, S.M. Park, C.Y. Kim, Morphology and electrochemical properties of polypyrrole films prepared in aqueous and nonaqueous solvents, J. Electrochem. Soc. 137 (1990) 905–909. [18] G. Zotti, S. Cattarin, N. Comisso, Cyclic potential sweep electropolymerization of aniline the role of anions in the polymerization mechanism, J. Electroanal. Chem. 239 (1998) 387–396. [19] T. Darmanin, F. Guittard, Wettability of conducting polymers: from superhydrophilicity to superoleophobicity, Prog. Polym. Sci. 39 (2014) 656–682. [20] M.M. Gvozdenović, B.Z. Jugović, J.S. Stevanović, B.N. Grgur, Electrochemical synthesis of electroconducting polymers, Hem. Ind. 68 (2014) 673–684. [21] S. Bhadra, D. Khastgir, N.K. Sigha, J.H. Lee, Progress in preparation, processing and applications of polyaniline, Prog. Polym. Sci. 34 (2009) 783–810. [22] I. Rodríguez, B.R. Scharifker, J. Mostany, In situ FTIR study of redox and overoxidation processes in polypyrrole films, J. Electroanal. Chem. 491 (2000) 117–125. [23] F. Palmisano, C. Maltesta, D. Centonze, P.G. Zambonin, Correlation between permselectivity and chemical structure of overoxidized polypyrrole membranes used in electroproduced enzyme biosensor, Anal. Chem. 67 (1995) 2207–2221. [24] M.A. Smith, A.L. Ocampo, M.A. Espinosa Medina, A modified Nafion membrane with in situ polymerized polypyrrole for the direct methanol fuel cell, J. Power Sources 124 (2003) 59–64. [25] R.W. Murray, Chemicaly modified electrodes, in: A.J. Bard (Ed.), Electroanalytical Chemistry, A Serie of Advances, Marcel Dekker, Inc., N. Y., 1984, pp. 191–368. [26] A. Witkoski, M.S. Freud, A. Brajter-Toth, Effect of electrode substrate on the morphology and selectivity of overoxidizadPolypyrrolefilms, Anal. Chem. 63 (1991) 622–626. [27] L. Viau, J.Y. Hinn, S. Lakand, V. Flaud, B. Lakardh, Full characterization of polypyrrole thin film electrosynthesized in room temperature ions liquids, water or acetonitrile, Electrochim. Acta 137 (2014) 298–310. [28] M.A. Chougule, S.G. Pawar, P.R. Godse, R.N. Mulik, S. Sen, V.B. Patil, Synthesis and characterization of polypyrrole (Ppy) thin films, Soft Nanosc. Letter 1 (2011) 6–10. [29] H. Eisazadeh, Studying the characteristics of polypyrrole and its composites, World J. Chem. 2 (2007) 67–74. [30] J. Kielland, Individual activity coefficients of ions in aqueous solutions, J. Am. Chem. Soc. 59 (1937) 1675–1678. [31] Z. Gao, Z. Minxian, C. Beshen, The influence of overoxidation treatment on the permeability of polypyrrole films, J. Electroanal. Chem. 373 (1994) 141–148. [32] J.J. Krol, M. Wessling, H. Strathmann, Chronopotentiometric and overlimiting ion transport though monopolar ion exchange membranes, J. Membr. Sci. 162 (1999) 155–164. [33] N. Pismenskaia, P. Sisat, P. Hughet, V. Nikonenko, G. Pourcelly, Chronopotentiometry applied to the study of ion transfer through anion exchange membranes, J. Membr. Sci. 228 (2004) 65–76. [34] R.D. Lide, Handbook of Chemistry and Physics, 87th, Florida, USA. CRC Press Inc., Boca Raton, 2006. [35] H. Hamani, R. Bouamrane, M. Kameche, C. Innocent, Z. Derriche, Transport number and current-voltage of a cation exchange membrane equilibrated in aqueous and organic solutions, Phys. Chem. Liq. 51 (2013) 265–280. [36] F. Parvizian, S.M. Hosseini, A.R. Hamidi, S.S. Madaeni, A.R. Moghadassi, Electrochemical characterization of mixed matrix nanocomposites ion exchange membrane modified by ZnO nanoparticles at different electrolyte conditions “pH/ concentration”, J. Taiwan Inst. Chem. Eng. 45 (2014) 2878–2887.

concentrations and even lower at higher concentrations. This behavior is congruent with a decrease in the passage of ions due to a physical barrier of different porosity. The trend is similar for Mg2 +, except that for high concentrations the transport number of Ppy/CMX(wm) is slightly higher than that of Ppy/CMX(am). This could be a consequence of the Donnan effect [36]. Thus, the selectivity can be modulated by the compactness of Ppy. In the case of Mg2 +(Fig. 11D), it was observed that at low concentrations the selectivity was higher for membrane modified in aqueous medium, while at high concentrations the selectivity was higher in membranes synthesized in acetonitrile. This could be due to the increased hydrophilicity of the membranes obtained in aqueous media as compared to organic media. As the hydration radius of Mg2 + is large, it can cross the membrane obtained in aqueous medium more easily than that obtained in organic medium, despite the fact that the pores are smaller. The difference between the transport number of Mg2 + and Na+ is larger for modified membranes, showing that these cations can be separated more easily by the modified membranes than the unmodified membranes. 3.3.2.3. Effect of overoxidation of Ppy. Finally, we analyzed the effect of the chemical structure of Ppy on the selectivity of the membrane, using two CMX membranes that were electrochemically modified in identical conditions, of which one was overoxidized. The results (Fig. 11E and F) show that for Na+ and Mg2 + the transport numbers of the electrode modified by overoxidized Ppy are higher than the electrodemodified by Ppy, and lower than that for the untreated membrane. The difference in selectivity can be due to the negative charges on the functional groups present in overoxidized Ppy, C-OH, C]O [22], and COOH [23]. These ionized groups produce negative charges, in contrast to the positive charges present in Ppy, and as a consequence selectivity is improved. 4. Conclusions This work shows that cation exchange membranes can be easily modified galvanostatically with Ppy and their transport properties can be regulated by the electrosynthesis conditions. Ppy grows inside the membrane and on both sides, and depending on the electrosynthesis conditions, it covers some zones of all the surfaces of the membrane. The modulated properties were the extent of Ppy in the modified membrane, compactness of Ppy, and the identity of charges of Ppy (negative or positive). The transport numbers of the modified membranes were congruent with the desired characteristics during the design of electrosynthesis conditions. The modified membranes show a lower passage of divalent cations when comparison to the untreated membranes. Generally, the selectivity of the modified membranes decreases as the concentration of ions increases. 5. Acknowledgments We gratefully acknowledge the financial support from CONACYT (CB-2008-1-105875). G. Vázquez-Rodríguez thanks CONACYT for the scholarship (48994). We thank María Estela Nuñez Pastrana for the help in obtaining FTIR spectras. References [1] H. Strathmann, Electrodialysis and related processes, in: R.D. Noble, S. Stern (Eds.), Membrane Separations Technology-Principles and Applications, Elsevier, New York, NY, USA, 1995, p. 213. [2] R.K. Nagarle, G.S. Gohil, V.K. Shahi, Recent development on ion-exchange membranes and electro-membrane processes, Adv. Colloid Interf. Sci. 119 (2006) 97–130. [3] M. Sadrzadeh, T. Huhammad, Sea water desalination using electrodialysis, Desalination 221 (2008) 440–447. [4] G. Saracco, M.C. Zanetti, M. Onofrio, Novel application of monovalent-ionpermselective membranes to recovery treatment of an industrial wasterwater du

105