Characterising nanostructure functionality of a cellulose triacetateforward osmosis membrane using electrical impedance spectroscopy

Siao Yien Yeo a, Yuan Wang a, Terry Chilcott a,b, Alice Antony a, Hans Coster b, Greg Leslie a,n

a UNESCO Centre for Membrane Science & Technology, School of Chemical Engineering, University of New South Wales, Sydney 2052, Australiab School of Chemical and Biomolecular Engineering, University of Sydney, NSW 2006, Australia

a r t i c l e i n f o

Article history:Received 19 December 2013Received in revised form2 May 2014Accepted 5 May 2014Available online 6 June 2014

Keywords:Forward osmosisElectrical impedance spectroscopyMaxwell–Wagner modelConcentration polarisationIn-situ monitoring

a b s t r a c t

Electrical Impedance spectra generated in situ, in real time for cellulose triacetate forward osmosis (CTA-ES) FO membranes was resolved with the Maxwell–Wagner theory to reveal distinct structures includingthe active separation layer and porous support. Two distinct structural elements with capacitance of7.7�10�6 (F/m2) and 7.8�10�4 (F/m2) and a corresponding thickness of 43(713) μm and 80(711) nmrepresenting the porous support and active separation layer respectively were determined from spectraacquired on membranes operating in Active Layer Draw Side (ALDS) mode on 0.5 M potassium chloridedraw solutions. Overall membrane thickness of 33–61 μm determined using Electrical ImpedanceSpectroscopy (EIS) compared favourably to a thickness range of 50–90 μm measured by ScanningElectron Microscopy. A stationary ion layer with a capacitance of 7.7�10�6 (F/m2) was visible on theporous support at draw solutions of 0.5 M KCl in the ALDS mode. However, as the concentration of thedraw solution increased, thereby increasing the conductivity of the region, EIS was unable to interpretthe interactions between stationary ion layer and the porous support. Reversing the membraneorientation to Active Layer Feed Side (ALFS) increased the amount of internal concentration polarisation(ICP) in the porous support compared with ALDS mode resulting in a decrease in FO flux from 3.9(70.2)L/m2 h to 2.5(70.2) L/m2 h. The presence of ICP and its subsequent impact on flux decline may berevealed from EIS spectra by observing an overall increase in conductance. While EIS remains a viabletechnique to characterise membrane structure and thickness, identification of coupled effects of internaland external concentration polarisation in situ remains elusive and requires further improvement ofsignal to noise ratio at higher concentrations and improvement in Maxwell–Wagner fitting algorithms.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Forward Osmosis (FO) is a liquid phase separation process inwhich a semi-permeable membrane is positioned between twosolutions with different osmotic pressures so that water moleculesare selectively transferred in the direction of the osmotic gradient.FO potentially may use less energy than other semi-permeablemembrane processes such as reverse osmosis (RO) and electro-dialysis (ED) currently used to desalinate seawater and brackishwater. However, membranes used for FO have not been optimised,or evolved, to the same extent as those used in RO and EDapplications. Improving the efficiency of FO systems is contingentupon development of membrane materials that can deliver highwater flux with high salt rejection but low fouling potential as wellas the porous support structure that can minimise internal con-centration polarisation (ICP) [1–4]. Some operational parameters,

such as the orientation of membranes, the type and concentrationsof draw solution also affect concentration polarisation profiles andsubsequently permeate flux and fouling behaviour [5–9].

The current methods of identifying behaviour of FO mem-branes are based on monitoring the flux decline or the change inconductivity of feed/draw solution, which is a reflection of themixed effects of external and internal concentration polarisation,reverse solute diffusion, membrane structure. Many mathematicalmodels have been built to predict external and internal concen-tration polarisation [10,11] and reverse solute permeation[6,7,12,13]. However, the development of these models relies onthe measurement of membrane water permeability and saltrejection, which is normally performed in reverse osmosis mode[6,12] and is significantly affected by the operating conditions.Moreover, monitoring the flux decline and change in conductivitycannot provide information on the underlying transport phenom-enon within the membrane structure in real time. Characterisationof the nanostructure of FO membranes reported in literature arebased on electron microscopy such as Scanning Electron Micro-scopy (SEM) and Transmission Electron Microscopy (TEM) and

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Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2014.05.0350376-7388/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ61 2 9385 6092; fax: þ61 2 9385 5966.E-mail address: g.leslie@unsw.edu.au (G. Leslie).

Journal of Membrane Science 467 (2014) 292–302

light microscopy [8,14–18]. Although these techniques are able toproduce clear micrographs of FO membranes, the complex inter-action between the substrate structure and transport process isstill hard to resolve. Consequently, an analytical technique that canbe used in situ and in real time to identify changes that occuracross the membrane and a model that can relate such changes tothe functionality of the membrane is required.

Electrical impedance spectroscopy (EIS), as an non-invasive char-acterisation technique, has been used to study membrane structuresand fouling of different membranes filtration processes [19,20],including microfiltration [21], ultrafiltration [22], electrodialysis [23],reverse osmosis [24–29], and recently on forward osmosis [30].

EIS measurements are performed by injecting sequential alter-nating current i¼ i0 sin ðωtþθiÞ to the system (membrane) withthe voltage response v¼ v0 sin ðωtþθvÞ as well as the phasedifference between the current and voltage being measured. Fromthis information, the impedance, phase angle, conductance andcapacitance can be determined. Impedance is defined as a phasorwith magnitude

Z � v0i0ð cos θþ j sin θÞ ð1Þ

where θ¼θv�θi, j is the imaginary number defined by j2¼�1 Eq.(1) is readily transformed into an equivalent circuit having aconductance element in parallel with a capacitance element.Essentially, capacitance (C) is the capacity to store charge whileconductance (G) is the rate of transfer of charges. These para-meters can be represented as

C � � 1ω Zj j sin θ ð2Þ

G� 1jZjcosθ ð3Þ

Hence, writing the impedance as a function of capacitance andconductance gives

Zsystem ¼ 1Gþ jωC

ð4Þ

The membrane system of interest is non-homogenous, com-prised of various layers and salt solution with different dielectricand conductance properties, the conductance, gk, and capacitance,ck for different elements can be deduced at each frequency [20,28].This feature enables the properties of membrane layers to beassessed as well as monitored during filtration.

In the study by Gao et al. [30], three different type ofcommercial FO membranes were utilised under two different testconditions (static and dynamic). The conclusion of this study wasthat FO membranes exhibited lower global membrane conduc-tance (Gm) values when the structural parameter “S” governed bythe following equation was high.

S¼ tτε

ð5Þ

where t refers to thickness of membrane, τ is the tortuosity and εis the porosity respectively.

The more noteworthy aspect of the experimental data was thelarge variation of Gm between different membranes. This observa-tion was suggested to be effect of internal concentration polarisa-tion. With the orientation of the membranes, higher Gm

values were observed in active layer draw side (ALDS) orientation.In conclusion, the characterisation results successfully indicatedthat EIS is a promising handy tool to investigate ICP during FOprocesses.

To the best knowledge of the authors, the research done by Gaoet al. [30] is the first and only study relating EIS and FO processes.The major limitation of this work is however that only globalconductance and capacitance values were obtained while thedistinctive structure and the electrical properties of the FOmembranes were not addressed. Inspired by this and the absenceof more accurate and extensive information relating membranestructure to the complex transport phenomena during FO process,we herein present results of a study using EIS measurements tocharacterise FO membrane structure and functionality in situ andin real time in a 4 terminal chamber.

The rationale of this work is that a chamber equipped withsuitable electrodes for impedance spectroscopy measurements canmonitor the performance of FO membrane as well as the devel-opment of both external and internal concentration polarisationand reverse solute permeation during the filtration process. Themonitoring was made possible by analysing the changes in theelectrical properties over the frequency range of interest. This inturn allows a model to be fitted to the measured spectra, fromwhich it would be able, in principle, to reveal the existence of thenumber of elements or “layers”. Through this technique, this workhopes to be able to present EIS as a novel way of characterising theFO membrane structures and more importantly, the transportphenomenon within the membrane structure occurring in FOprocesses.

2. Materials and methods

2.1. Feed and draw solutions

The feed solution used was deionized water (18 MΩ cm)obtained from a Milli-Q ultrapure water purification system(Millipore, Australia) while the draw solution was composed ofpotassium chloride. The draw solution was prepared at concentra-tions ranging from 0.5 M to 1.5 M by dissolving analytical gradepotassium chloride salts (Ajax Finechem Pty Ltd, Sydney, Australia)in deionized water. Osmotic pressure of feed and draw solutionwas estimated using Van't Hoff equation [31]. A summary ofconcentration and osmotic pressure of feed and draw solutioncan be found in Table 1.

2.2. Membrane and characterisation

A commercial cellulose triacetate FO membrane withembedded support (CTA-ES) (Hydration Technologies Inc., Albany,OR) was used in this study. This membrane is a specially devel-oped FO membrane consisting of an ultrathin polyester screenmesh support with polyester fibres arranged orthogonally.

A Scanning Electron Microscopy (SEM, Hitachi 3400X) wasused to produce 2D images of cross-section and surface morphol-ogy of the membrane used. Prior to analysing the membranesamples by SEM, the samples were dried under vacuum at roomtemperature for 24 h and then sputter coated with a thin layerof gold.

Table 1Feed and draw solution used in this study.

Feed solution Draw solution Bulk osmotic pressure differential (bar)

Deionized water 0.5 M KCl 22.27Deionized water 1.0 M KCl 44.55Deionized water 1.5 M KCl 66.820.5 M KCl 1.0 M KCl 22.270.5 M KCl 1.5 M KCl 44.55

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302 293

2.3. Determination of water and salt permeability

Pure water flux (Jw) was evaluated in a laboratory-scale highpressure dead end cell with Milli-Q water at a series of appliedpressure ranging from 5 to 15 bar. The effective membrane areawas 12.5 cm2. The pure water permeability “A” of the membranewas determined by dividing the pure water flux by the appliedpressure (Jw/P).

Salt rejection R was determined by pressurising 50 mM KClsolution through the membranes at a feed pressure from 5 to15 bar with the potassium ion concentration in the permeatebeing measured using Inductively Couple Plasma – Optical Emis-sion Spectrophotometry. The salt rejection R is given by

R¼ 1�Cp

Cb

� �� 100% ð6Þ

where Cp and Cb are the potassium ion concentrations in thepermeate and the bulk feed, respectively.

2.4. Forward osmosis – electrical impedance spectroscopy setup

The FO crossflow unit used in this study was a 4 terminalchamber (INPHAZE, Australia) having symmetric channels on bothsides of the membrane and a set of four electrodes with one pair ofdisc electrodes located a large distance from the membrane beingused for injecting current and the other pair of electrodes locatedvery close to the membrane surface used for measuring voltageresponses (Fig. 1). The detail design of the chamber ensures thatthe voltage electrodes do not disturb the otherwise uniformcurrent distribution over the membrane. The unique configurationof this unit allows the impedance at the interface between currentinjecting electrodes and solutions to be eliminated [24,30].Furthermore, the location and large area of the current electrodeswhich ensures consistent distribution of current over the mem-brane sample, whereas conversely the voltage electrodes have avery small area and are located close to the membrane thusensuring more precise definition of the potential differencemeasured. The flat sheet FO membrane coupon, with an effectivemembrane area of approximately 5.7 cm2, was placed in betweenthe two voltage probes. Plastic mesh spacers and O-rings wereused in both channels to support the membrane and also toincrease the turbulence and reduce external concentration polar-isation. The chamber was placed in a Faraday cage to eliminateelectrical interferences during measurements and connected to ahigh resolution impedance spectrometer (INPHAZE, Sydney, Aus-tralia) by which the electrical properties of the membrane weredetermined over the range of frequencies (1–106 Hz).

All FO experimental runs were conducted in two possibleorientations of the membrane. The two different orientations were

active layer facing draw solution (ALDS) and active layer facingfeed solution (ALFS). This is otherwise commonly referred to aspressure retarded osmosis (PRO) and FO modes respectively. Themembrane coupons were soaked in Milli-Q water and kept at 4 1Cfor 24 h prior to use.

The feed and draw solutions were circulated in a co-currentconfiguration on both sides of the FO chamber by a dual headvariable speed peristaltic pump (Masterflexs L/S Cole Parmer,USA) at a constant flow rate of 10 mL/min. Pressures of the feedand permeate stream were monitored with pressure transducers(Universal CB1020, Labom, Germany). The experiments com-menced by using deionized water on both side of the membraneto allow for temperature equilibration as well as calibration of flowrate on both sides. After calibration, the draw solution wasswitched to the desired salt solution prepared earlier. The drawsolution reservoir was placed on an electronic balance (ML4002,Mettler Toledo, Switzerland) that was connected to a desktop PCto enable the cumulative weight of the permeate to be continu-ously measured. The water flux was determined by calculating therate at which the weight of draw solution increased at an intervalof 10 min. The conductivity of feed solution was also continuouslymonitored and recorded as a function of time using a conductivitymetre (WP-81, TPS, Australia). The pressure and the permeateproduction rate was logged using Labviews (National Instru-ments) software The total duration of the experiments was lessthan 4 h, which ensured a relatively constant draw concentration(variation less than 2%).

2.5. Equivalent subcircuit model fitting

A Maxwell–Wagner model was used to represent the electri-cally heterogeneous FO membrane system. In this approach thesystem was treated as a combination of sub circuits characterizingthe conductive and capacitive properties of electrically distinctlayers that exist in the system. The impedance of the system,specific to a total area of membrane, is presented as the generalform of the Maxwell–Wagner model

zMW ðωÞ ¼ 1gMW ðωÞþ jωcMW ðωÞ �∑zk ¼ ∑

NMW

k ¼ 1

1gkþ jωck

ð7Þ

where gMW and cMW represent the conductance and capacitance ateach angular frequency ω per, respectively. zk is the impedance ofthe kth element of NMW impedance elements in the system eachwith a conductance per unit area gk and capacitance per unit areack. Eq. (7) discloses that for just one layer, the Maxwell–Wagnerconductance and capacitance and also measurements of conduc-tance and capacitance are independent of frequency. However, the

Feed Draw

PT

PT

i- v- v+ i+

Spectrometer

Fig. 1. Schematic diagram of FO-EIS setup.

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302294

addition of one or more layers introduces dependency onfrequency.

The influence of a particular element on the measurementspredominates at a frequency given by the characteristic frequency

ωk �gkck

ð8Þ

The dielectric thickness of an element originating from a layeris determined by

dk ¼εkε0ck

ð9Þ

where εk is the dielectric constant of the element and ε0 is thepermittivity of free space (ε0¼8.85�10�12 F m�2).

Digital software (Conquartium Impedance Analyser) was usedto analyse the voltage measured across the membrane sample andthe corresponding to the AC current injected as well as the phasedifference between the current and voltage. The model fitting isbased upon a least squares error method that minimises theMaxwell–Wagner model functions.

Reduced�χ2 � χ2

N�NMWð10Þ

where N is the number of measurements in the spectra and NMW isthe number of Maxwell–Wagner elements; and

χ2 � ∑I

i ¼ 1

mi�ϕMW ωið Þσi

� �ð11Þ

In Eq. (11), mi and σi denote the mean and standard deviation ofith of N measurements at angular frequency ωi respectively whileϕMW represents one of the Maxwell–Wagner model functions.

The denominator of Eq. (10) is referred to as the degrees offreedom of the fitting. Initially, the system is programmed to onlyfit a single layer of Maxwell–Wagner element. However, it thenadds in additional elements, i.e. increasing NMW. The introductionof additional elements results in an expected decrease in χ2

(i.e. smaller least-squares-error) which is countered in theReduced�χ2 by a decrease in the degrees of freedom. TheReduced�χ2 approaches infinity as NMW approaches N. When thispoint is reached, there is insufficient data for the algorithm toresolve a model. The criteria required for successfully modelling ofcomplex elements is one that minimises Reduced�χ2 for both fitsto conductance and capacitance measurements.

The rationale of Maxwell–Wagner model fitting suggests thatfor the accurate determination of contribution by each layer,spectra of measurements spanning a large range of frequenciescovering characteristic frequency of each element is required asthe dependence of the measurement on frequency increases as thenumber of layers with differing characteristic frequency increases.

3. Results and discussion

3.1. Membrane intrinsic properties

The overall thickness of the CTA-ES FO membrane measuredusing a pair of Vernier callipers was found to be 90 μm. SEMimages suggested the membrane has a thickness of 50 μm (Fig. 2).This value varied depending on the position of the woven meshes.The meshes for CTA-ES had a fibre size of 33 mm and a fibre to fibredistance of 120–140 mm. These values are consistent with thevalues presented in previous studies [4,14,17].

The water permeability was measured to be 0.70 (70.03)L/m2 h bar with a corresponding apparent salt rejection of 72(79)%. These values are comparable with previous studies[8,12,14,32–35] while variations of salt rejection was related to

different salt concentration and applied pressures used. The saltrejection measured was the apparent salt rejection and dependson the conditions at which salt rejection was measured.

3.2. Flux behaviour of CTA-ES membranes at different membraneorientations

The flux through the CTA-ES membrane ranged from 2.5 to8.7 LMH depending on the osmotic pressure difference as well asmembrane orientations (Fig. 3). A proportional increase in fluxwould be expected with an increase in bulk osmotic pressuredifference between the draw and feed solutions. However, theincrease in flux as a function of bulk osmotic pressure differentialswas compromised by the onset of external and internal concen-tration poloarisation. An increase in flux from 2.5 (70.1) LMH to3.9 (70.2) was observed when changing the orientation fromALFS to ALDS, indicating the internal concentration polarisation,which cannot be minimised by cross-flow operation, has moresignificant adverse impact on flux compared to external concen-tration polarisation. In addition, approximately 50% of flux declinewas observed upon the addition of 0.5 M KCl to the feed solutiondue to the coupled effect of external concentration polarisationand internal concentration polarisation. These observations wereconsistent with previous studies on FO membranes [5,10,36,37].

3.3. Equivalent Maxwell–Wagner circuit model

3.3.1. Characterisation of Maxwell–Wagner elementsThe typical EIS spectra of capacitance and conductance as a

function of frequencies ranging from 1 to 106 Hz exhibited disper-sion throughout the spectra over this wide range of frequencies(Fig. 4a and b). This clearly suggests the presence of electricallydistinct layers with different time constants in the system [28].A Maxwell–Wagner model was employed to analyse the dispersionsobserved from the spectra both with pure water as feed, 0.5 M KCl asdraw and in the ALDS orientation (Fig. 4c and d).

The rationale behind the model fitting is illustrated in bothFig. 4c and Fig. 4d, which shows theoretical plots of Maxwell–Wagner models comprised of 1 to 7 elements overlaid onto thespectral measurements of conductance and capacitance. The plotof a Maxwell–Wagner model comprised of one element yieldselemental conductance and capacitance values that can be seen tobe constant in frequency, as predicted by Eq. (7). The addition of asecond element can be seen to introduce dispersions in theconductance and capacitance with frequency over a frequencyrange spanning the characteristic frequency for the second

AB

Fig. 2. SEM image of a cross section of CTA-ES membrane showing variations inmembrane thickness from 50 μm (A) to 90 μm (B).

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302 295

0

2

4

6

8

10

0 20 40 60 80

Jw (L

MH

)

D,B - F,B (Bar)

Dilutive ICP &

Dilutive ICP

Dilutive

Dilutive

0

2

4

6

8

10

0 20 40 60 80

Jw (L

MH

)

D,B - F,B (Bar)

Dilutive ECP &

Dilutive ECP

Fig. 3. Flux behaviour of CTA-ES membrane at different osmotic pressure and membrane orientations; schematic diagrams depicting forward osmosis membrane in ALFS(active layer feed side) (a) and ALDS (active layer draw side) (b) orientation; permeate flux measured at different osmotic pressure in ALFS (c) and ALDS (d) orientation. Thesolid lines refer to experiments with pure water as the feed and the dotted lines with a feed of 0.5 M KCl.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Con

duct

ance

(S m

-2)

Frequency (Hz)

Cap

acita

nce

(F m

-2)

Frequency (Hz)104

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

100 101 102 103 105 106104100 101 102 103 105 106

Element 1Elements 1,

Elements 1, 2, 3Elements 1, 2, 3, 4

Elements 1, 2, 3, 4, 5

Elements 1 to 6

Elements 1 to 7

Element 1Elements 1, 2

Elements 1, 2, 3

Elements 1, 2 , 3, 4Elements 1 to 5

Elements 1 to 6

Elements 1 to 7

Fig. 4. The measured (symbols) and Maxwell–Wagner model fitted (full lines) of the conductance (a) and capacitance (b) against frequency for data of Milli-Q as the feedsolution and 0.5 M KCl as the draw solution in ALDS orientation; and the contributions of the various “layers” or elements in the Maxwell–Wagner model (c) and (d).

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302296

element whilst retaining the constancy of the dispersions outsidethis range. The additions of the third, fourth, and subsequentelements can be seen to introduce additional dispersions only overranges spanning their respective characteristic frequencies.

Fig. 4c and d further illustrates that the Maxwell–Wagnerelement comprised of one-element fits the conductance andcapacitance data at the highest frequency but the addition of eachfurther element improves the fit to some additional 5 data points.So the goodness of the fit of the Maxwell–Wagner model to boththe conductance and capacitance data improves as more elementsare included and the Maxwell–Wagner capacitance and conduc-tance become more dependent on frequency. Hence, the extent ofthe goodness of the fits expands to frequency ranges of 105–106 Hzfor 2 elements, 104–106 Hz for 3 elements, 103–106 Hz for 4 ele-ments, and 102–106 Hz for 5 elements, 101–106 Hz for 6 elementsand 1–106 Hz for 7 elements. Thus, the capacitance and conduc-tance plots visually confirm the basis of accepting the 7 elementsmodel evaluated by the Conquartium Analyser software andjustified using Reduced�χ2 statistics.

The fit obtained with the 3-element model was a poor fit athigh frequencies (Fig. 5a and b), while the fit of the 5-elementmodel provide a good fit for the conductance spectrum (Reduced-χ2¼0.12) (Fig. 5c) but a only an improved fit the capacitancespectrum (Reduced-χ2¼2.92) (Fig. 5d). The fit of the 7-element

model yields an good fit to both the conductance spectrum(Reduced-χ2¼0.06) and the capacitance spectrum (Reduced-χ2¼0.61) (Fig. 5e and f).Table 3

Consequently the Maxwell–Wagner algorithm successfully iden-tified a total of 7 elements for the process using Milli-Q water as thefeed solution and 0.5 M KCl as the draw solution in ALDS orientation(Fig. 6 and Table 2). The sequence of the elements were determinedfrom the element of highest characteristic frequency to the lowestbut not necessarily in the physical order of any regions in themembrane system from which these elements originate. Each ele-ment is associated with a corresponding dielectric constant. Theelectrical properties at a particular frequency were used to facilitatethe characterisation of each element (Eq. 9). The assignment of theelement was based on dielectric constants values in the systembeginning with the dielectric constant of water (�79). From thewater in the bulk solution and into the membrane, the dielectricconstant decreases and would approach the value of the dielectricconstant value the material of the membrane [38] where thehydration level of the material is negligible.

Element 1: This element was attributed to the stagnant layer ofwater formed between the membrane and voltage probe near thesurface of the active layer of the membrane. Hence the εk valueused to obtain the thickness of this element was that of purewater at 79.

3 element fit

3 element fit

5 element fit

5 element fit

7 element fit

7 element fit

Fig. 5. Dispersions of conductance and capacitance with frequency of Maxwell–Wagner models fitted to an impedance spectrum illustrating the procedure for optimisingthe number of elements: (a) and (b) 3 elements; (c) and (d) 5 elements; (e) and (f) 7 elements.

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302 297

Element 2 and 3: The thickness values estimated for theseelements ranged from 33 to 61 μm and thus of comparabledimensions to the bulk membrane and too large to be assignedto the active layer. These elements were therefore attributed tolayers forming the porous support. The fairly high conductivity ofelement 2 compared to element 3 suggested that element 2 mightbe the layer that was closer to the inner side of the active layerwhile element 3 would be still within the porous support butcloser towards the Milli-Q feed side. The dielectric constant of thisregion will be governed by the porosity of this layer and thus thevolume fraction of water in the layer. The effective dielectricconstant is the given by:

ε¼ x79þð1�xÞεk ð12Þwhere x is the volume fraction of water and εk is the dielectricconstant of the polyester material (3.65).

Assuming a range of likely volume fractions of water (i.e.porosities) of between 0.4 and 0.6, the value of the dielectricconstant would be in the range 26–49. The total thickness of thesetwo layers estimated from this is in the range of 33–61 μm, closeto that estimated from the SEM images.

Element 4 and 5: The dielectric constant of cellulose triacetate is5.1. On hydration this might be slightly increased. Layer 4 had a

higher conductance than layer 5 and this suggests a slightly largerdielectric constant (and hence ion partitioning) than layer 5. Theelement with the higher conductance value, i.e. Element 4, wasassigned to that most immediately exposed to the draw solution.Assuming a plausible value of 6–8 for layer 4 and a value of 5.1 forthe tighter layer 5, the total thickness of these two layers wasestimated to be in the range 87–110 nm. This is a typical thick-nesses of the active layer of the cellulose triacetate membranes.

Elements 6 and 7: It was observed that these two elements hadlarge capacitance values in the lower frequency ranges (o102 Hz).The thicknesses deduced by using Eq. (9) would yield unrealisticvalues of atomic dimensions and therefore too small for theseelements to be assigned to a region in the system. Such observa-tions indicate that the origin of these elements were not realMaxwell–Wagner layers but instead were most likely due tophenomenological effect arising from modulations of ion concen-trations on either side of the membrane due to injection of ACcurrents during measurements [24,27,28,39–43].

At frequencies less than 102 Hz, diffusion polarisation of ions atthe membrane–solution interface, can be induced by the alternat-ing current (AC) used for impedance measurements. The occur-rence of this phenomenon is due to the alternating build-up anddepletion in time of ions at the membrane surface. With ACcurrents passing through the membrane, the transport of ionicsolutes undergoes an inversion during each half-cycle due to theeffect of polarity of the potential applied across the membrane(Fig. 7). Since the movements of the ions are limited by diffusion, ittakes time for the concentration profile at the membrane–solutioninterface to build up, or down. Hence, low frequencies, the timeavailable in each half cycle of the alternating current is longenough for the accumulation and depletion of ions to occur atthe membrane surface. This would lead to substantial changes inthe concentration profiles. In contrast, at high frequencies there is

g7 g6

c7 c6

g5 g4

c5 c4

g3 g2

c3 c2

g1

c1

Diffusionpolarisation

Active layerelements

Poroussupport layer

Stagnantwater

Bulk Membrane

Elements (k) 7 6 15 4 3 2

Fig. 6. Equivalent circuit for an in situ characterisation of forward osmosis processes with Milli-Q water as the feed solution and 0.5 M KCl as the draw solution in ALDSorientation.

Table 2Numerical values of conductance and capacitance for each Maxwell–Wagnerelement.

Elements,k

Conductancegk (S m�2)

Capacitanceck (F m�2)

Dielectricconstantεk

Thicknessesdk (m)

1 1.5 9.3�10�8 79 7.5�10�3

2 7.8 7.7�10�6 26–49 30�10�6–56�10�6

3 6.2 8.2�10�5 41 2.7�10�6–5.1�10�6

4 3.7 7.8�10�4 6–8 68�10�9–91�10�9

5 1.3 2.4�10�3 5.1 19�10�9

6 0.77 1.3�10�2 – N/A7 0.18 5.0�10�2 – N/A

Table 3Conductivity and thickness of each element obtained from Maxwell–Wagner model.

Elements, k Conductivity σk (s/m) Thicknesses dk (m)

ALFS ALDS ALFS ALDS

1 3.4E-03 1.1E-02 5.3E-03 7.5E-032 1.7E-03 7.1E-04 3.8E-03 9.1E-053 6.2E-05 2.7E-05 1.3E-05 4.3E-064 2.8E-07 2.2E-07 7.4E-08 5.8E-085 2.4E-08 2.4E-08 2.4E-08 1.9E-08

+ -

Cations

Anions

-

Anions

Cations

+

Fig. 7. Effect of polarity of applied voltage across the membrane on the transport ofions that leads to an AC concentration polarisation. During one half cycle theconcentration of cations on one side builds up whilst the anion concentrationbecomes depleted near the membrane surface as illustrated in the top diagram. Theopposite occurs on the other side of the membrane. The profiles are reversed in thenext half cycle of the applied AC signal (lower diagram).

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302298

insufficient time during half cycle for the movement of salt ions tocause any significant changes in the concentration profile [27].

3.3.2. Effect of filtration timeAn increase in conductance was observed over time, indicating

the accumulation of salt in the system (Fig. 8a). In this study whereno foulants were used, the dispersion of the capacitance withfrequency signals the existence of different elements within thesystem and/or associated with the diffusion polarisation effect.The contributions of the thickness of each element were repre-sented by step functions occurring at the characteristic frequenciesof the layers (Fig. 8b). The FO CTA-ES membrane consists of anactive layer, porous layer and an embedded woven mesh support.The top layer, which was the active layer of CTA, was approxi-mately 100 nm thick (Elements 4 and 5). Element 3 on the otherhand was �10�5 m in thickness, (Elements 2 and 3 in the ALDSorientation). Overall, when all these elements were added uptogether, the order of magnitude was in fact very consistent withthe thickness value found from SEM imaging (Fig. 1). Elements6 and 7, although modelled by Maxwell–Wagner elements, yieldedlarge capacitance values that suggested that these elements couldnot arise even from the thinnest possible layer of dielectricmaterial. They were therefore not represented in this plot.

A shift in characteristic frequency to the right direction of theaxis was observed, indicating the system was more conductive,which was due to more reverse solute diffusing from draw side tothe feed side as a function of time. As a result, the porous layerbecame more conductive and only one element could be detectedin the porous support layer. Conductivity is related to the ionicconcentrations via the equation below

σ ¼∑i

qi2CiDi

kTð13Þ

where qi is the ionic charge, Ci is the ionic concentration Di is theionic diffusion constant, k is the Boltzman constant and T is thetemperature [38]. The fact that conductivity is proportional to theconcentration indicates that the solution of higher concentrationhad a larger influence on the concentrations in these layers.Therefore, the lower the conductivity value, the more structureof the membrane can be detected. However, as the porous supportlayer became more conductive after 2 h's operation, less structurecould be resolved by EIS as well as frequency at which suchelements could be detected was beyond the range of frequencyused in this study.

3.3.3. Effect of membrane orientationChanging the membrane orientation to ALFS while maintaining

the same feed and draw solution (i.e. Milli-Q water as feed and0.5 M KCl as draw) revealed identical number of elementsalthough the distribution of elements showed slight differences(Fig. 9). The thicknesses were obtained from Maxwell–Wagnermodelling of the EIS data using reasonable values of the dielectricconstants (Table 2).

Though Fig. 9b showed that dielectric thickness of differentelements did not vary significantly by changing the membraneorientations there was a slight shift of elements towards the rightof the characteristic frequency axis which by the proportionality ofcharacteristic frequency and conductivity relation suggested thatthese elements possessed higher conductivity values. The sameconclusions can be drawn from the results shown in Fig. 9a wherethe difference in conductance can be clearly seen.

The distribution of elements 1, 2 and 3 in the ALFS orientationshowed some difference to ALDS orientation (Fig. 9b). In theALFS orientation only one element (d3) was detected in theporous support layer. By contrast, two elements within the porous

0.0

0.5

1.0

1.5

2.0

2.5

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06

Con

duct

ance

(S m

-2)

Frequency (Hz)

0 hr 2hr

1E-10

1E-08

1E-06

1E-04

1E-02

1E+01 1E+02 1E+03 1E+04 1E+05 1E+06 1E+07

Die

lect

ric

thic

knes

s (m

)

Characteristic frequency (Hz)

Stagnant water elements

Porous support elements

Active layer elements

Interfacial elements

d1

d3

d5

d1

d2

d3

d4

d5~ ~

d4

Fig. 8. Change in conductance as a function of frequency (a) and dielectricweighted thickness as a function of characteristic frequency (b) at 0 and 2 h ofthe process using Milli-Q water as the feed, 0.5 M KCl as the draw in ALDSorientation.

0.0

0.3

0.6

0.9

1.2

1.5

1.8

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06

Con

duct

ane

(S m

-2)

Frequency (Hz)

ALDS ALFS

1E-10

1E-08

1E-06

1E-04

1E-02

1E+01 1E+02 1E+03 1E+04 1E+05 1E+06 1E+07

Die

lect

ric

thic

knes

s (m

)

Characteristic frequency (Hz)

Stagnant water elements

Porous support elements

Active layer elements

Interfacial elements

d2

d3

d4d5

d1

d2

d3

d4d5

d1

~~

Fig. 9. Change in conductance as a function of frequency (a) and dielectricweighted thickness as a function of characteristic frequency (b) for the processusing Milli-Q water as feed, 0.5 M KCl as draw in both ALDS and ALFS orientation,respectively.

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302 299

support were detected in the ALDS orientation. In the ALFSorientation, the porous support was directly exposed to the saltsolution thus enabling salt ions to diffuse into the loosely packedporous support (i.e. internal concentration polarisation) therebycausing it to be highly conductive. When the conductivity of aregion was very high, less distinction between elements can bemade. As a result, the porous support layer was detected as amonolithic element. Another reason that limits the resolution ofthe extra subdivision of this region is that at the high concentra-tion of salt within support a much higher frequency range isrequired to characterise these elements. Element d2 of the ALFSorientation (Fig. 8b) is ascribed to a stagnant water layer becauseits characteristic frequency was only very slightly different fromd1. Such small differences do not allow d2 to be identified asanother element.

In comparison, in the ALDS orientation, the reverse diffusion ofsalt across the porous support layer could be detected. In this case,the porous support was facing the extremely low concentrationsolution side (i.e. Milli-Q water). Thus the difference in dielectricconstants of the elements was more distinct. Furthermore, the saltacross the porous layer was also diluted by the water thatpermeated through the membrane resulting in lower conductivity.The detection of two elements in the porous support layer wouldalso suggest the development of a concentration profile in theporous layer in the ALDS orientation.

3.3.4. Effect of draw solution concentrationIncreasing the draw solution concentration from 0.5 M to 1 M KCl

solution while keeping the membrane oriented in the ALDS resultedin an increase in conductance (Fig. 10a). From the perspective of FOoperation, an increase in draw solution concentration would increase

water flux but might also promote the degree of concentrationpolarisation. Presumably, the number of elements and its originobtained from impedance spectra should be the same as when 0.5 Mof draw solution was used except that the characteristic frequency atwhich each layer dominates should shift to the right due to higherconductivity in the case of 1 M KCl. While the origin of the elements(i.e. the characteristic frequency) that was detectable remainedthe same, only three Maxwell–Wagner elements were detected(Fig. 10b).

The step function in the dielectric thickness profile for the 1 Mdraw solution concentration can be seen to have indeed shifted tothe right of the characteristic frequency axis. However, notice thatelement 1 shifted a little to the left. Since this layer was a stagnantlayer of water, the reason for such an observation is that there is aslightly greater dilution by the stronger ability of the 1 M drawsolution to draw water across the membrane. This explanation fitswell into the finding that as draw solution concentration wasincreased, so did water flux, as might be expected. The thicknessdeduced for element 3 and 5 were close to one another in bothsituations and were of an order of magnitude that represents theporous layer and active surface layer respectively.

However, elements 2 and 4 were not detected for the 1 M drawsolution. For the 0.5 M draw solution these elements were detect-able and interpreted as sub-layers of the porous support layer andthe active layer most immediately exposed to draw solutionrespectively. The failure to detect both elements in the 1 M drawsolution was attributed to the dominance of conductive propertiesover capacitive properties as a consequence of exposure to the drawsolution of high concentration. This would lead to the regions beinghighly conductive which so that less structure could be resolved byEIS, particularly over the range of frequency used in this study.

0.0

1.0

2.0

3.0

4.0

5.0

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06

Con

duct

ane

(S m

-2)

Frequency (Hz)

0.5 M 1.0 M

1E-10

1E-08

1E-06

1E-04

1E-02

1E+01 1E+02 1E+03 1E+04 1E+05 1E+06 1E+07

Die

lect

ric

thic

knes

s (m

)

Characteristic frequency (Hz)

Stagnant water elements

Porous support elements

Active layer elements

Interfacial elements

d1

d3

d5

d1

d2

d3

d4

d5~ ~

Fig. 10. Change in conductance as a function of frequency (a) and dielectricweighted thickness as a function of characteristic frequency (b) for the processusing Milli-Q water as feed, 0.5 M and 1.0 M KCl as draw in ALDS orientation,respectively.

0.0

4.0

8.0

12.0

16.0

20.0

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06

Con

duct

ane

(S m

-2)

Frequency (Hz)

Feed 0.5M/Draw 1.5MDraw only 1.0M

1E-10

1E-08

1E-06

1E-04

1E-02

1E+03 1E+04 1E+05 1E+06 1E+07

Die

lect

ric

thic

knes

s (m

)

Characteristic frequency (Hz)

Active layer elements

Stagnant water elements

Porous support elements

d1

d3

d5

d1

d3

d5~~

Fig. 11. Change in conductance as a function of frequency (a) and dielectricweighted thickness as a function of characteristic frequency (b) for the processusing Milli-Q water as feed coupled to 1.0 M KCl as the draw and 0.5 M KCl as feedcoupled to 1.5 M KCl as draw in the ALDS orientation, respectively.

S.Y. Yeo et al. / Journal of Membrane Science 467 (2014) 292–302300

Specifically for the porous layer elements, the increase inconcentration of the draw solution resulted in more severe reversesolute permeation and consequently a significant increase inconductivity of these elements. The results from the EIS spectratherefore clearly demonstrated that although increasing drawsolution concentration increased the water flux, more severereverse solute diffusion also occurred.

3.3.5. Coupled effects of ICP and ECPComparisons were also made between a 0.5 M KCl feed coupled

to a 1.5 M KCl draw solution and a Milli-Q water feed coupled to a1.0 M KCl draw solution in the ALDS orientation (Fig. 11). Thecoupled effects of internal and external concentration polarisationresulted in a further increase in conductance and further shift incharacteristic frequency to the right on the frequency axis. Asdemonstrated previously, only three elements (i.e. the stagnantwater element, and bulk membranes) could be resolved using aMaxwell–Wagner model (Fig. 10b).

4. Conclusion

Maxwell–Wagner models were used to fit the data generatedfrom EIS measurements for the FO process using Milli-Q water as thefeed and 0.5 M KCl as the draw solutions in the ALDS orientation. Atotal of 7 electrically distinct elements which arise from themembrane as well as elements arising from diffusion polarisationwere successfully detected and quantified. The sum of dielectricweighted thickness of the bulk CTA-ES FO membrane derived fromthe model was consistent with the thickness of the membranederived from SEM images and measured using a pair of the Verniercallipers. The conductance measured by EIS increased as a function oftime and also with an increase of concentration of the draw solution.Both of these suggest a reverse solute diffusion. The onset of internalconcentration polarisation caused an increase in conductance in theporous support layers when positioning the membrane in the ALFSorientation or with coupled effects of internal and external concen-tration polarisation. However, the structure of the measured systemwas less distinctive. This was ascribed to the increase in conductivityof the measured system as well as the fact that the frequency atwhich such elements in principle could be detected was beyond therange of frequency used in this study. Nevertheless, it was possible torelate flux decline with changes in the electrical properties of thesystem during FO operation from the EIS spectra. The resultspresented suggest that electrical impedance spectra can be inter-preted to provide information on structure and thickness of FOmembranes. However, identification of coupled effects of internaland external concentration polarisation in situ remains elusive andrequires further improvement of signal to noise ratio at higherconcentrations and improvement in Maxwell–Wagner fitting algo-rithms. Such improvements would enable the application of EIStechniques for membrane characterisation and monitoring of filtra-tion performance in situ and in real time.

Acknowledgement

This research was supported under Australian Research Coun-cil's Discovery Projects funding scheme (DP130103766).

Nomenclature

A water permeability (LMH/bar)Ci molar concentration of ions (M)

C or c capacitance (F m�2)Di solute diffusivity of ions (m2/s)dk dielectric weighted thickness (m)G or g conductance (S m�2)GM global conductance (S m�2)i alternating currentv voltageJw water flux (LMH)k Boltzmann constantN number of elementsP pressure (bar)qi ionic chargeR salt rejection (%)S structural parameterT temperature (K)t thickness (m)x volume fraction of waterZ or z impedance

Greek symbols

ε porosity of membraneε0 permittivity of free space, 8.85�10�12 F m�2

εk dielectric constantϕ Maxwell–Wagner model functionπ osmotic pressure (bar)θ phase differenceσ conductivity (S/m)τ tortuosity of the membraneω angular frequency (Hz)

Subscripts

b bulk solutiond draw solutionf feed solutionk kth elementMW Maxwell–Wagnern Nth of elementsp permeatew water

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