Desalination 271 (2011) 29–33

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Desalination

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Electrochemical impedance spectroscopy analysis of sulfonated polyethersulfonenanofiltration membrane

Ying Xu, Meng Wang ⁎, Zhun Ma, Congjie Gao ⁎Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, ChinaCollege of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, Shandong Province, China

⁎ Corresponding authors. Key Laboratory of Marine Cogy, Ministry of Education, China. Tel.: +86 532 6678 6

E-mail addresses: membranewm@yahoo.com.cn (M(C. Gao).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.12.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 August 2010Received in revised form 1 December 2010Accepted 2 December 2010Available online 8 January 2011

Keywords:Nanofiltration membraneElectrochemical impedance spectroscopyEquivalent circuit

In this study, the surface charged and structural information of sulfonated polyethersulfone nanofiltrationmembrane are acquired through the analysis on the relevant electrochemical impedance spectroscopies (EIS)expressed in terms of dispersions of the impedance with frequency (1 KHz–1 MHz). The measurements areperformed in the different electrolyte solutions (NaCl, MgCl2, LaCl3, KCl, K2SO4, and K3PO4) with differentconcentrations ranging from 1×10−4 mol/L to 6×10−3 mol/L. After that, the corresponding equivalentcircuit as a theoretical model is set up. Experimental results indicate that the resistance of active layer isinfluenced by the concentration and ion species of electrolytes, while the capacitance is independent of them.For different cations and anions the corresponding resistances of the active layer take on a sequence of SO4

2

−NCl−NPO43- and Na+NLa3+NMg2+ respectively, which is in well agreement with the variation trend of ion

retention, especially for monovalent and divalent ions. Furthermore, the thickness of the active layer in theworking state is estimated from the capacitance values and compared with the result obtained by SEM. On thebasis of the above, it is reasonable to think that EIS can be used for online analysis in membrane process.

hemistry Theory and Technol-513; fax: +86 532 6678 6513.. Wang), gaocjie@mail.hz.zj.cn

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

It is well known that the polymer membranes have been widelyapplied in desalination, drinking water production, wastewatertreatment and many other industrial scenario [1–3]. Thereinto, themembrane properties are determined by several aspects, includingsized effect due to the dimension and distribution of pore sizes,charged effect resulting from the dissociation of the functional groupsand/or the specific adsorption of the relevant ions on the membranesurface, and so on. Accordingly, it is necessary to grasp the charac-teristics of the employed membrane in depth. Besides the traditionalcharacterization method for polymer membranes, the exploration onnew technologies and methods is on-going.

As an on-line noninvasive analytical technique, electrochemicalimpedance spectroscopy has been proven to be a valuable tool for thestructural characterization of the polymer membranes. For examples,H.G.L. Coster et al. [4] used it to characterize the surface porousstructure and pore size distribution of ultrafiltration membrane. L.Gaedt et al. [5,6] confirmed that EIS measurements can quantifyperformance parameters of membrane process in situ, which is betterthan other techniques that cannot avoid the disruption of the mem-

brane separation process. J. Benavente et al. [7–9] also did someresearches on structural characterization of sub-layers of compositemembranes using EIS.

Besides, the method allows evaluating the kinetic parameters ofwater dissociation reaction and the structure of bipolar junction[10,11]. For an example, J.R Smith et al. [12] proved the existence ofintermediate layer of bipolar membrane according to the analysis ofthe impedance spectra in low frequency. In addition, EIS also providesvaluable information on ion transport process through membranes. Inparticular, S. Bason et al. [13,14] used redox-active ion pairs to obtaina comprehensive picture of ion transport through polyamide reverseosmosis membranes.

Moreover, the investigations on the fouling phenomena ofelectrodialysis processes by EIS had been also carried out. Amongthem, Watkins et al. [15] used capacitance spectroscopy to track thefouling process of anion exchange membrane by organic macro-molecules in real time. J.S. Park et al. [16] reported that the con-ductance and capacitance dispersions of the fouled ion exchangemembranes decreased alongwith the accumulation of the BSA foulinglayer on the membrane surface. Furthermore, they found that thechanges of negatively charged fouling layer from the loose state to thedense state on the membrane surface appear along with enhancedwater dissociation phenomena on the surface of the fouledmembraneat a higher current density [17]. Hong-Joo Lee et al. [18] tried toinvestigate the influence of the fouling on the performance of theelectrodialysis process and illustrate the sodium dodecylbenzenesul-fonate fouling mechanisms by means of the EIS.

30 Y. Xu et al. / Desalination 271 (2011) 29–33

As far as we know, EIS is scarcely applied to investigate the struc-tural characteristics and the charged behaviors of a nanofiltrationmembrane which has dissociated functional groups though somerelevant explorations mentioned above have been performed suc-cessfully. In the present contribution, the electrochemical impedancespectroscopies of the sulfonated polyethersulfone nanofiltrationexpressed in terms of dispersions of the impedance with frequency(1 Hz–1 MHz) are measured in the different electrolyte solutions(NaCl, MgCl2, LaCl3, KCl, K2SO4, and K3PO4) with different concentra-tion ranging from 1×10−4 mol/L to 6×10−3 mol/L. Furthermore, incombination with the relevant retention measurements, the surfacecharged and structural information are obtained through the analysison the equivalent circuit set up based on the corresponding EIS.

2. Experimental

2.1. Materials and solutions

The polymer membrane studied in this work is a kind of com-mercial nanofiltration membrane, NTR7450, manufactured by NittoDenko Co., Japan. The effective layer of the membrane is peeled fromthe supporting fabric, cleaned with ethanol and deionized waterseveral times in order to remove impurities on the membrane surface,and then conserved in deionized water under room temperature.Before each set of measurements, the membranes are equilibrated formore than 12 h in the test solution. Measurements are carried out indifferent electrolyte solutions, such as KCl, K2SO4, K3PO4, NaCl, MgCl2,and LaCl3 with different concentrations ranging from 1×10−4 mol/Lto 6×10−3 mol/L.

2.2. Electrochemical setup

As shown in Fig. 1, an Electrochemical Workstation (IM6exZahner, Germany) controlled by a computer is used for EIS measure-ments. The electrochemical experiments are carried out using an acpotentiostatic four-electrode mode, which is equipped with four self-made Ag/AgCl electrodes to minimize the influence of waterdissociation reactions near electrodes on the EIS measurements, inthe range of 1 Hz–1 MHz at a maximum voltage of 0.01 V. In order toeliminate concentration polarization effects, the test solution iscirculated in each cell. The membrane area is about 0.785 cm2. Allthe experiments are conducted at natural pH value (6.5±0.1) and25±1 °C.

Fig. 1. The experimental setup for determination of ac impedance spectra 1 impedance ana

2.3. Retention measurements

The experimental setup consisted of a circular disc membrane cellwith an effectivefiltration area of 19.625 cm2 and a stainless steel pistonpump, as illustrated in Fig. 2. The pressure is adjusted ranging from0.1 MPa to 0.45 MPa. The pump is used to circulate the aqueous solutionof the investigated salts at ambient temperature in a total re-circulationmode, i.e. both the concentrate and the permeate streams are recycledinto the feed tank in order to maintain a constant concentration. Thepermeate flux and rejection are determined by varying the appliedfeed pressure and feed concentration. The rejection rate (R) is acquiredfrom conductivity measurements which are carried out in both feedand permeate concentrations at the same time.

Just as mentioned above, the NTR-7450 nanofiltration membranesare immersed in the test solution overnight before measurements toequilibrate the membrane. Subsequently, the experiment is con-ducted with the electrolyte solution at a transmembrane pressuredifference of 0.4 MPa until both permeate pH and conductivity reachconstant values.

3. Results and discussion

3.1. Equivalent circuit

Fig. 3a and b shows the impedance plots of NTR7450 membranein different electrolyte solutions with the same concentration(3×10−3 mol/L) in two customary ways which are the Bode plot(|Z|versus frequency) (Fig. 3a) and Nyquist plot (-Zimg versus Zreal)(Fig. 3b), respectively. Similar curves can also be obtained in otherexperimental systems. It is worth noting that two different timerelaxations can be observed clearly in the two plots. Obviously, theyindicate that the membrane embraces an asymmetric structure,which is corresponding to the active layer and the supporting layer,respectively.

For the quantitative analysis of the impedance spectroscopy, thecorresponding equivalent circuits composed of several electricelements, including resistor, capacitor, inductor, etc. should be setup. In view of the existence of a significant deviation during thefitting process when a pure capacitor is introduced, a non-idealcapacitor called “constant phase elements” (CPE) is adopted instead.The CPE used to describe the dispersion of the capacitance isexpressed by ZQ ωð Þ = 1

Y0jωð Þ−n, where the admittance Y0(Ω−1s−n)

and n are two empirical parameters (0bnb1). In these cases, anequivalent capacitance, C, can be determined from EIS results, being

lyzer 2 work/counter electrode 3 reference/sense electrode 4 membrane 5 electrolyte.

Fig. 2. The schematic diagram of experimental apparatus 1 feed tank 2 piston pump 3 buffer 4 pressure gauges 5 flat membrane cell 6 flowmeter 7 graduated cylinder.

31Y. Xu et al. / Desalination 271 (2011) 29–33

C=[(RY0)(−n)]/R [7,8,19]. A special case can be considered whenn=0.5, then the circuit element is called a Warburg impedance, W,which is generally associated with the diffusion process [13,14].Therefore, the equivalent circuit used in this study contained a seriesassociation of two parts (Fig. 4), each branch includes a parallelassociation of a resistor (Ral) and a CPE (Qal) for the active layer, and aresistor (Rpl) and a capacitor (Cpl) for the supporting layer. Thefitting results of the elements based on the suggested equivalentcircuit in different electrolyte solution with the same concentration(3×10−3 mol/L) are shown in Table 1.

Fig. 3. The impedance plots of themembrane in the different electrolyte solutions whenthe concentration of K+ (for KCl, K2SO4 and K3PO4 solution) and Cl− (for NaCl, MgCl2and LaCl3 solution) is 3×10−3 mol/L, respectively. (a) The Bode plot, (b) the Nyquistplot.

3.2. Surface charged information of active layer in different electrolytes

The changes of Ral with the electrolyte solution concentration areshown in Fig. 5. From these figures, the strong dependence ofresistance values on electrolyte concentration is observed. That is, Ral

keeps declining with increasing electrolyte concentration. Thisindicates that the conductance of active layer becomes larger withthe increase of the electrolyte concentration and accordingly the ionspermeate into themembranemore easily. On the other hand, it can beseen clearly that Ral is also greatly influenced by ion species, especiallyat low concentrations. However, with the concentration increasing,the contribution of ion species to Ral could be ignored.

For different anion species, Ral decreases in the order ofSO2−

4 NCl−NPO43−, which is displayed in Fig. 5a. On the one hand,

the magnitude of hydration radii may attribute to this. In addition,another possible explanation can be formed according to theDonnan exclusion. Imaginably, the negatively charged membranesurface resulting from the dissociation of sulfonic acid groups andthe relevant specific adsorption of ions on the membrane surfacemake the transport of SO4

2− more difficult than that of Cl− due totheir difference in electrostatic repulsion. Moreover, it can beobserved from Figs. 5a and 6a that a good agreement exists betweenthe dispersion trend of Ral and ion retention for SO4

2− and Cl− at a certainconcentration, for an example, 0.001 mol/L. However, for PO4

3−, the caseis somewhat exceptional. The minimum resistance displayed in K3PO4

solution may be related to the hydrolysis effect of PO43−, whereas the

steric hindrance effect attributes to the lowest transportation capacityshown by PO4

3−.Donnan exclusion keeps co-ions from entering the membrane

phase, but attracts counterions. As can be seen from Figs. 5b and 6b, agood agreement between the dispersion trend of Ral and ion retentionfor Na+, Mg2+ is also observed. In the active layer, the polymer chainsmay keep as far away as possible from each other due to the electro-static repulsion effect [9] due to the existence of the dissociatedfunctional groups. Hence, the relatively big cations, such as La3+, canalso transport through the membrane relatively easily. Besides, theDonnon effect is also responsible for its easier transport through themembrane than Mg2+ and Na+ under the operating pressure. Inaddition, the shielding effect may attribute to the difference of Ral

Fig. 4. The equivalent circuit.

Table 1Fitting results of the elements using the suggested equivalent circuit shown in Fig. 3.

NTR-7450 NaCl MgCl2 LaCl3 KCl K2SO4 K3PO4

a.l. Ral(KΩ) 1.2 0.540 0.633 0.377 0.511 0.328Q al(nF) 8.859 10.22 9.204 12.01 10.94 11.14n 0.77 0.957 0.793 0.878 0.835 0.858

p.l. Rpl(KΩ) 3.332 3.446 3.01 2.407 2.33 2.198Cpl(pF) 67.69 64.73 70.27 61.65 66.81 68.53

32 Y. Xu et al. / Desalination 271 (2011) 29–33

displayed in the Mg2+ system and La3+ system, especially in theelectrolyte solution with low concentration.

3.3. Structural information of active layer in different electrolytes

Indeed, there are many methods which can be used for the char-acterization of the membrane structure, such as scanning electronmicroscopy (SEM), atomic force microscopy (AFM), etc. However,these methods will become powerless when the timely structuralinformation under working conditions is expected [20]. In view of thecharacteristics of EIS, it is necessary to attempt this technology in thecharacterization of membrane morphology in situ.

Generally speaking, the capacitance of a homogeneous system isonly related to the inherent properties of the material, but indepen-dent of testing conditions, for an example, the frequency of alter-nating current. However, for nanofiltration membrane/electrolyte

Fig. 5. The concentration dependence of Ral in different electrolyte solutions. (a) Differentanions, (b) different cations.

Fig. 6. The retention (R) as a function of the permeate flux (Jv) in the differentelectrolyte solutions when the concentration of K+ (for KCl, K2SO4 and K3PO4 solution)and Cl− (for NaCl, MgCl2 and LaCl3 solution) is 0.001 mol/L, respectively. (a) Differentanions, (b) different cations.

heterogeneous system, the capacitance C dispersed with frequency.The active layer is assumed as a plate capacitor whose capacitance canbe described by the equation:

Cm = ε0εmAδ

ð1Þ

where εm is the dielectric constant of the film. Here, for themembrane material (polyethersulfone), εm=3.5 is assumed [21]. ε0 isthe permittivity of free space, 8.85418×10−12F/m. A is themembranearea. Then the effective thickness of the active layer δ can be cal-culated from Eq. (1).

As shown in Fig. 7, the capacitance values of the active layer arepractically independent of electrolyte concentration and the averagevalues for the whole range of concentrations in different electrolytesolutions can be obtained. Furthermore, according to Eq. (1), thethickness of the active layer can be estimated, which is shown inTable 2. It is worth noting that the calculated thicknesses areinfluenced by the electrolyte type to a certain extent and largerthan the measuring result, about 0.150 μm, obtained by SEM [22]. Thismay be related to the swelling effect of the hydrophilic sulfonatedpolyethersulfone layer in electrolyte solutions. Anyway, it can beconfirmed that the thickness of the active layer under the workingstate can be obtained successfully through EIS through this work.This indicates that EIS can be used for online analysis in themembraneprocess.

Fig. 7. The concentration dependence of Cal in different electrolyte solutions. Theconcentrations here denote the concentration of K+ (for KCl, K2SO4 and K3PO4 solution)and Cl− (for NaCl, MgCl2 and LaCl3 solution), respectively.

Table 2The thickness of membrane active layer in different cation solutions.

NaCl MgCl2 LaCl3 KCl K2SO4 K3PO4

δ(μm) 0.2739 0.2218 0.2501 0.2035 0.2188 0.2096

33Y. Xu et al. / Desalination 271 (2011) 29–33

4. Conclusions

In this study, the measurements of the electrochemical impedancespectroscopies in the different electrolyte solutions (NaCl, MgCl2,LaCl3, KCl, K2SO4, and K3PO4) with different concentration rangingfrom 1×10−4 mol/L to 6×10−3 mol/L are carried out. After that, thecorresponding equivalent circuit as a theoretical model is set up. Twodifferent time relaxations can be observed clearly in EIS, whichindicate that the membrane embraces an asymmetric structure andare corresponding to the active layer and the supporting layer, res-pectively. Experimental results show that the resistance of the activelayer is influenced by the concentration and ion species of electro-lytes, while the capacitance is independent of them. For differentcations and anions the corresponding resistances of the active layertake on a sequence of SO4

2−NCl−NPO43− and Na+NLa3+NMg2+

respectively, which is in well agreement with the variation trend ofion retention, especially for monovalent and divalent ions. Further-more, the thickness of the active layer in the working state isestimated from the capacitance values. Compared with the measuringresult obtained by SEM, the thickness of the active layer under theworking state can be obtained successfully through EIS in this work.In a word, it is reasonable to think that there must be plenty ofroom for the applications of EIS in online analysis of membraneseparation process.

Acknowledgements

The authors are grateful to Dr Xu Ping and Dr Fu Kangbo of NittoDenko Co. for kindly supplying the NTR7450 membrane specimens.This research is supported in part by the National Science foundationof China. Nos. (20706050 and 20636050) and the National BasicResearch Program of China No. (2009CB623402).

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