Impact of support layer pore size on performance of thin filmcomposite membranes for forward osmosis

Liwei Huang, Jeffrey R. McCutcheon n

University of Connecticut, Department of Chemical and Biomolecular Engineering, Center for Environmental Sciences and Engineering,191 Auditorium Rd. Unit 3222, Storrs, CT 06269-3222

a r t i c l e i n f o

Article history:Received 29 August 2014Received in revised form7 January 2015Accepted 11 January 2015Available online 19 January 2015

Keywords:Forward osmosisPressure-retarded osmosisThin film compositeInterfacial polymerizationNylon 6,6

a b s t r a c t

Previous investigations of forward osmosis (FO) concluded that thin film composite (TFC) membranesshould be designed with hydrophilic supports to help mitigate internal concentration polarization andimprove water flux. A number of research groups and companies around the world have responded tothose findings by developing TFC membranes with hydrophilic supporting materials. However, there hasbeen few fundamental studies on how hydrophilic support structure affects selective layer formationand hence membrane performance. Here, a systematic investigation on the influence of support layerpore size on the osmotic performance of thin film composite membranes is conducted for the first time.Specifically, TFC membranes were made by interfacial polymerization to form a polyamide selectivelayer on top of a series of commercially available nylon 6,6 microfiltration membranes with similarphysical and chemical properties but different pore sizes. The interfacial polymerization process isaffected by the support pore dimensions and the resulting polyamide composite membranes exhibitedvarying film morphology, cross-linking degree, mechanical integrity, and permselectivity. Osmotic fluxtests show that the osmotic flux performances (water flux, salt flux and specific salt flux) are dependenton a permeability-selectivity trade-off which is in part impacted by the pore size of the support layer.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Forward osmosis (FO) is an emerging platform technology thatthat exploits the natural phenomenon of osmosis to address waterand energy scarcity [1–3]. When two solutions of differing concen-tration are placed on opposite sides of a semi-permeable mem-brane, an osmotic pressure differential is generated to drive thepermeation of water across the membrane from the dilute solution(the feed solution) to the concentrated solution (the draw solution).The immense promise of this technology has been demonstrated invarious applications such as concentrating high-value dissolvedsolids [4–6], seawater and brine desalination [7–9], and electricpower generation [10–12]. However, the further advancement andultimate commercialization of FO processes has been hinderedby the lack of an appropriately designed membrane. Until veryrecently, the only commercially available FO membrane has beenthe asymmetric cellulose triacetate (CTA) membrane from Hydra-tion Technology Innovations (HTI, Albany, OR). The hydrophilicnature of CTA favors osmotic transport, but its susceptibility tohydrolysis [13], its relatively low water permeance, and low saltrejection have limited its use to niche applications.

Another potential FO membrane platform is the thin filmcomposite (TFC) membrane. Commonly used in reverse osmosis(RO) and nanofiltration (NF), conventional TFC membranes arecomprised of an aromatic polyamide thin film formed in-situ ontop of an asymmetric polysulfone (PSu) mid-layer which is itselfcast by phase inversion over a polyester (PET) nonwoven backinglayer. The ultra-thin polyamide selective layer gives superiorpermeance and selectivity over conventional asymmetric inte-grated membranes, such as CTA [14]. Furthermore, TFC mem-branes are more flexible in their design as both the selective andsupport layers can be tailored for specific needs. These mem-branes, while accelerating the adoption of RO and NF, were nevertailored for use in any FO process and thus have performed poorlywhen tested under relevant conditions [15]. Poor performance hasbeen attributed to thick, dense and hydrophobic support layersthat, while necessary in RO and NF to provide mechanical integrityunder hydraulic pressure, create severe mass transfer resistancenear the interface of the selective thin film layer in FO. Thisphenomenon, widely described as internal concentration polariza-tion (ICP) [8,15,16], reduces effective osmotic driving force andresults in poor water flux performance.

To design TFC membranes specifically for FO processes, supportlayers must be redesigned to incorporate a combination ofcharacteristics. Support layers need to be thin, highly porous andhave a low tortuosity. The support layer must also be hydrophilic,

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Journal of Membrane Science

http://dx.doi.org/10.1016/j.memsci.2015.01.0250376-7388/& 2015 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ1 860 486 4601.E-mail address: jeff@engr.uconn.edu (J.R. McCutcheon).

Journal of Membrane Science 483 (2015) 25–33

allowing for complete wetting throughout the structure [17]. Thesupport layer must exhibit excellent chemical and thermal stabi-lity while retaining reasonable mechanical strength and be easy tofabricate at full scale. After all of this, the support layer mustadequately support the selective layer during both formation andoperation and that layer must match the permselectivity of aconventional RO TFC membrane.

Early efforts in developing high-flux TFC-FO membranes havebeen reported with flat-sheet and hollow-fiber platforms. Yip et al.invented one of the first TFC-FO flat sheet membranes with tailoredsupport layers consisting of both finger-like and sponge-like pores[18]. Bui et al. employed a new support fabrication technique knownas electrospinning to develop nanofiber based TFC membranes withintrinsically low resistance [19]. Wang et al. designed novel hollowfiber TFC membranes with high water flux [20]. All these studies,however, focus on support structures but not support chemistry. Inlarge part, conventional support materials, such as hydrophobic PSuand polyethersulfone are still used.

Other groups have addressed this issue by considering morehydrophilic polymers as support materials. For instance, somehave considered sulfonated polysulfone [21] and sulfonated poly(ether ketone) blends for use in TFC membrane substrates [22].These studies still require a hydrophobic polymer to be blended toavoid severe swelling or even dissolution in water, ultimatelymaking the support somewhat hydrophobic. A chemical modifica-tion method has been reported by Arena describing the modifica-tion of the PSu support layers of commercial TFC RO membranesusing polydopamine, a novel bio-inspired hydrophilic polymer[23]. These efforts resulted in dramatically improved water fluxcompared to the native membranes, but complete modification ofthe support is difficult with the polyamide layer already formed.More recently, intrinsically hydrophilic polymers have been con-sidered as alternatives to modified hydrophobic materials as newTFC substrates. FO membranes supported by polyacrylonitrile(PAN) [24], cellulose triacetate [25] and cellulose acetate propio-nate [26] phase-inversion films as well as PAN/cellulose acetateblended nanofibers [27] have been previously reported. None ofthese efforts considered support materials fabricated on an indus-trial scale. Our previous work considered a commercially-availablehydrophilic nylon 6,6 microfiltration (MF) membrane from 3M™made on a continuous line as a support for a TFC membrane [28].We found that we could make a high performance TFC with thisunconventional support, but we only considered a single mem-brane (a 0.1 μm pore size membrane). There are, in fact, severalother membrane pore sizes that are available as well. By usingthese membranes, we can control the material and vary the poresize of the support, giving us a tool for understanding how poresize impacts polymide formation on a hydrophilic support.

There are only a few studies on the role of support membraneproperties in formation of polyamide composite membranes inrecent years. Singh et al. found that two PSu supports with differentpore dimensions result in TFC membranes with different saltrejections [29]. Gosh et al. proposed a conceptual model describingthe role of support membrane pore structure and chemistry (i.e.pore size, porosity, and hydrophilicity) on performance of polyamidebased composite membranes [30]. Tian et al. adopted this model toexplain the permselectivity difference of two TFC membranessupported by two nanofibers with different fiber diameters [31].Yet most of these studies are based on hydrophobic polymersubstrates. It is uncertain that the conclusions drawn from thesestudies hold for intrinsically hydrophilic supports. In addition, thesupports used in these studies are either lab-scale hand-cast films orelectrospun nanofiber mats. It is difficult to keep consistency whencasting in a lab and a commercial platform is far more consistent.

In this study, we explored the use of a series of commerciallyavailable nylon 6,6 MF membranes as supports for FO TFC

membranes. We found that support layer pore size played animportant role in polyamide properties and osmotic flux perfor-mance. These performance differences were not caused by thesupport layer itself, but rather differences in polyamide selectivelayer formation and mechanical support during operation.

2. Experimental

2.1. Materials

Four types of multi-zoned nylon 6,6 MF membranes with differentpore sizes (STL01, BLA010, BLA020 and BLA045) were provided by 3MPurification Inc. (Meriden, CT) as the support membranes for the TFCmembranes. The details of membrane structure (i.e. pore size andporosity) and other characteristics will be discussed in Section 3.1. M-phenylenediamine (MPD) and trimesoyl chloride (TMC) were pur-chased from Sigma-Aldrich. Hexane, the solvent for TMC, was pur-chased from Fisher Scientific. Deionized water (DI) obtained from aMilli-Q ultrapure water purification system (Millipore, Billerica, MA)was used as the solvent for diamine monomers. Sodium chloride waspurchased from Fisher Scientific. A commercial asymmetric cellulosetriacetate (HTI-CTA) FO membrane (Hydration Technology InnovationsInc., Albany, OR) was provided for comparison.

2.2. Interfacial polymerization of TFC membrane

Nylon 6,6 MF support membranes were first taped onto a glassplate with the side supporting the polyamide film facing up. Theplates were then immersed into a 1.0% (w/v) aqueous MPD solutionfor 120 s. Excess MPD solution was removed from the supportmembrane surface using a rubber roller. The support membraneswere then dipped into a solution of 0.15% (w/v) TMC in hexane for60 s to form an ultrathin polyamide film. The resulting compositemembranes were air dried for 120 s and subsequently cured in anair-circulation oven at 80 1C for 5 min for attaining the desiredstability of the formed structure [32]. The TFC polyamide mem-branes were thoroughly washed and stored in deionized water at4 1C before testing.

2.3. Support layer characterization

The thickness of the support membranes was measured using adigital micrometer at 5 different locations for each membranesample. A CAM 101 series contact angle goniometer (KSV Com-pany, Linthicum Heights, MD) was used to measure the contactangle of the support surface. The values were taken as an averageof at least five points with a volume of 1071 μL.

The support surface porosity was analyzed as a ratio of total porearea vs. total image area using Image-J software based on top-surfacescanning electron microscopy (SEM) images) using a cold cathodefield emission scanning electron microscope JSM-6335F (JEOL Com-pany, USA). This technique, though widely adopted in previous studiesto quantify surface pore size and porosity of phase-inversion films[30,33], is challenging for our support membranes as there is nodistinct surface interface. Images therefore are thresholded to distin-guish the surface from the bulk. Results might slightly differ if otherswere to analyze the same image since the threshold value is userbiased. We do, however, keep the image contrast and threshold valueconstant for all SEMs to ensure consistency between samples. Beforeimaging, samples were kept overnight in a desiccator and thensputter coated with a thin layer of platinum to obtain better contrastand to avoid charge accumulation.

The average porosity (ε) of the substrate was determined by agravimetric method which measures the weight of DI water as thewetting agent contained in membrane pores. The following equation

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–3326

was used to calculate the porosity of the membrane.

ε¼ ðWwet�WdryÞ=ρwater

� �V

ð1Þ

where Wdry is the weight of dry membrane, Wwet is the total weightof membrane after wetting with water, ρwater is the water density,and V is the total volume of the sample that can be obtained bymeasuring the dimensions. We assume that the hydrophilic structurefully wets.

2.4. Selective layer characterization

Surface morphology and cross-sectional structure of the TFCmembranes were also imaged with SEM. Samples were preparedfor cross-sectional imaging using a freeze fracture techniqueinvolving liquid nitrogen. A razor blade was submerged into liquidnitrogen with the sample simultaneously and then used to quicklycut the sample into half once removed from the liquid nitrogen.The thickness of the selective layer was measured at 3 differentlocations for each membrane and then averaged.

The cross-linking degree of formed polyamide film can beexamined based on the element ratio of O/N [31,34]. The chemicalformula of fully cross-linked polyamide is [C15H10O3N3] in whichthe O/N ratio is 1; while the O/N ratio is 2 for the fully linearstructure of [C15H10O4N2]. The relative fractions of cross-linkedportion, m, and linear portion, n, were calculated based on thefollowing relations:

mþn¼ 1 ð2Þ

ON¼ 3mþ4n3mþ2n

ð3Þ

The O/N ratio can be experimentally determined by performingelement composition analysis using X-ray photoelectron spectro-scopy (XPS) (Kratos, AXIS 165, UK) with a monochomated Al K-αsource. A normal sample position of 0 degree to detector directionwas applied. A charge neutralization system was used to obtainhigh-resolution spectra for the insulating polymers by reducingthe surface charge. Pass energy of 160 eV and 20 eV were used forsurvey and high-resolution spectroscopy. No chemical degradationof the surface membrane was found due to the exposure to X-rays.All spectroscopy were calibrated to hydrocarbon C 1 s peak at284.6 eV. Sensitivity factors of C 1s, N 1s and O 1s from themanufacture were used for quantitative calculation.

2.5. Determination of transport properties in cross-flow reverseosmosis

A bench-scale cross-flow RO testing unit was used to evaluatethe pure water permeance, A, observed salt rejection, %R, andsolute permeability, B, of the TFC membranes and the HTI controlmembrane (HTI-CTA) at 2071 1C using a method described else-where [28]. The system was operated at 150 and 250 psi with afixed cross-flow velocity of 0.26 m/s (Re �1200) using DI or a2000 ppm NaCl feed solution to determine A and %R, respectively.The determined A and B were used to derive structural parameterin FO tests.

2.6. Evaluation of osmotic water flux and reverse salt flux

Osmotic water flux and reverse salt flux of polyamide TFCmembranes were evaluated using a custom lab-scale cross-flowforward osmosis system described in details elsewhere [23]. A1.5 M sodium chloride solution was used as the draw while DIwater was the feed at a temperature of 2071 1C. Osmotic fluxtests were carried out with the membrane oriented in both PRO

mode (the membrane active layer faces the draw solution) and FOmode (the membrane active layer faces the feed solution). Thecross-flow velocities were kept at 0.18 m/s for both the feed anddraw sides. Conductivity of the feed was measured to estimate thereverse salt flux through the membrane.

The osmotic water flux, Jw, was calculated by dividing thevolumetric flux by the membrane area. By measuring the con-ductivity of the feed solutions at certain times during the tests, thereverse salt flux, Js, was calculated by dividing the NaCl mass flowrate by the membrane area. The specific salt flux [35,36], Js/Jw, wasdetermined as the ratio of the reverse salt flux and the water flux.The structural parameter (S) was determined by using the follow-ing equation [18] where the membrane is orientated in FO mode:

S¼ DJw

� �ln

BþAπD;b

Bþ JwþAπF;m

� �ð4Þ

In this equation, D is the diffusion coefficient of the draw solute,Jw is the measured water flux, B is the solute permeability, A is thepure water permeance, πD,b is the bulk osmotic pressure of thedraw, and πF,m is the osmotic pressure at the membrane surface onthe feed side (0 atm for DI feed).

The tortuosity of the support can be estimated using thefollowing Equation that defines structural parameter [15].

S¼ tτε

ð5Þ

where t, τ, and ε are the thickness, tortuosity and porosity of thesupport, respectively.

3. Results and discussion

3.1. Support layer characterization

All MF substrates share the similar surface morphology(Fig. 1(a), (c), (e), and (f)) and cross-sectional structure(describedelsewhere [28]). Their other characteristics are listed in Table 1.Except the STL01, the other MF membranes have an asymmetricstructure consisting of three regions: (1) a large-pore region at theupstream side of the membrane; (2) a nonwoven scrim used as amechanical support to facilitate manufacturing; and (3) a small-pore region on the downstream side of the membrane. The poresizes of the upstream and downstream sides of STL01 are thesame. For interfacial polymerization, the membrane was orientedto have the smallest pores supporting the selective layer to avoiddefects. These four supports are labeled based on their pore size(Support-0.025, Support-0.1, Suppport-0.2, and Support-0.45). Theoverall porosity of the support membranes ranges from approxi-mately 55% to 70%, gradually increasing with the pore size. Thesurface porosity, on the other hand, ranges from 46.8% to 54.7%,lower than the overall porosity but substantially higher thanconventional RO support materials [30]. Note that a higher surfaceporosity at the support-selective layer interface might help toimprove the osmotic water flux because the selective layer is notshadowed by the support. The contact angle of the membranesurface is measured to be approximately 41–421 for all foursupports, indicating their intrinsic hydrophilicity.

3.2. Selective layer characterization

3.2.1. Scanning electron micrographsThe top surface SEM images for polyamide selective layers built

on different supports are shown in Fig. 1. Defect free films withtypical ridge-and-valley structure were obtained for all TFC mem-branes. Smoother surfaces are seen in TFC membranes based on

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33 27

Fig. 1. Top surface SEM images of different supports and corresponding TFC membranes (Magnification 5000� ): (a) Support-0.025; (b) TFC-0.025; (c) Support-0.1;(d) TFC-0.1; (e) Support-0.2; (f) TFC-0.2; (g) Support-0.45; (h) TFC-0.45.

Table 1Properties of nylon 6,6 MF supports with different pore sizes.

Support-0.025 (STL01) Support-0.1 (BLA010)a Support-0.2 (BLA020) Support-0.45 (BLA045)

Average pore size (μm) Top zone 0.025 0.1 0.2 0.45Bottom zone 0.025 0.45 0.65 0.8

Contact angle (degree)b 41.971.7 40.573.1 41.074.6 40.774.7Thickness (μm) 14272 18174 17672 18174Surface porosity (%)b 46.870.9 51.171.9 53.271.5 54.773.1Porosity (%) 54.270.5 57.770.1 66.870.1 70.571.0Structural parameter (μm)c 22107170 19407240 12207380 14007160Estimated tortuosityd 8.4 6.5 4.6 5.5

a Support-0.1 data is from [28].b Measured on the small pores zone (i.e. the surface for interfacial polymerization).c Structural parameter is determined by fitting the experimental results into Eq. (4).d Tortuosity is estimated by dividing structural parameter by the thickness and porosity.

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–3328

larger pore size support (i.e. TFC-0.45) than those with smallerpores (i.e.TFC-0.025).

The cross-sectional SEM images for polyamide TFC membranesare shown in Fig. 2. The average thickness of the polyamide selectivelayers was measured to be approximately 0.10 μm for all foursupports, indicating that the thickness of the selective layer isindependent of the support pore size. Different results have beenobserved by Singh et al. that the polyamide coated on smaller poresize (70 nm) PSu support was thicker than that coated on bigger poresize (150 nm). This difference could be attributed to the hydropho-bicity of the PSu supports. Singh believes the smaller pores of thehydrophobic PSu support might be resistant to penetration of MPDaqueous solution, limiting polyamide formation to the surface. Largerpores favor the penetration of MPD into the pores, leading toformation of polyamide film inside the pore. This is not the casefor our membranes, though, since MPD aqueous solution easily wetout the entire nylon 6,6 support structure. In addition, the selectivelayer is better supported by the smaller pores than by the largerpores. The selective layer spanning over the large voids seen onlarger pore size support (i.e. TFC-0.45) is susceptible to failure at highhydraulic pressure.

3.2.2. Cross-linking degreeThe relative atomic concentrations and composition ratios of

the polyamide selective layer are presented in Table 2. All fourTFC membranes exhibited a cross-linking degree between 0 and 1,implying the presence of a partially cross-linked and partially linearpolyamide structure. With increasing the support pore size from0.025 μm to 0.45 μm, the resulting polyamide cross-linking degreedecreased from 0.69 to 0.37. In other words, under the same interfacialpolymerization condition, lower selectivity membranes results fromlarger pores size supports. This finding indicates that the supportstructure not only performs as a mechanical anchor for the selectivelayer, but directly impacts the interfacial polymerization processes andstrongly affects the properties of the formed selective layer.

3.3. Polyamide formation mechanism

The difference in cross-linking degree can be explained by aconceptual model to illustrate the polyamide formation mechanismproposed by Ghosh [30], which is shown schematically in Fig. 3.

According to this model, after the excess MPD solution was removed,the aqueous phase meniscus was concave in hydrophilic pores(the better wetting forces the meniscus to drop below the supportinterface). In addition, MPD is likely to diffuse out of the pore slowlydue to the favorable hydrogen bonding interactions between MPDand polar functional groups in the nylon 6,6 membrane pore walls[30]. This “drag” on the MPD slows the diffusion rate and allows thepolyamide formation to occur deeper inside the pore.

Given the fact that four supports exhibited a small deviation ofsurface porosity (i.e. between 46.8% and 54.7%), it is expected that asmaller pore size support has more pores and than the larger pores.In addition, the degree of concavity on the surface of support withsmaller pore size is also higher. As a result, the support with smallerpores has greater surface area at the polymerization interface. Thishigher surface area provides more sites for MPD partitioning intothe TMC organic phase and results in a higher local MPD/TMC ratioand cross-linking degree.

3.4. Reverse osmosis tests

Table 3 summarizes the performances of TFC membranes inreverse osmosis. The rejection of the polyamide selective layersslightly decreased with increasing the pore size of the support thenshowed dramatic change for a support of 0.45 μm pore size. Purewater permeance A, on the other hand, gradually increased withincreasing the pore size. One cause for the perm-selectivity variationis the cross-linking degree difference. TFC membrane based onsmaller pores support exhibits a higher cross-linking degree, whichis responsible for its higher selectivity. TFC membranes with a lower

Fig. 2. Cross-sectional SEM images of polyamide-nylon 6,6 composite membranes built on different supports: (a) TFC-0.025; (b) TFC-0.1; (c)TFC-0.2; and (d) TFC-0.45.

Table 2Relative atomic concentrations determined by X-ray photoelectron spectroscopy(XPS) and composition ratios of aromatic polyamide TFC Membranes.

C (%) O (%) N (%) O/N m

Fully cross-linked 75 12.5 12.5 1 1Fully linear 71.4 19.1 9.5 2 0TFC-0.025 73.4 14.7 11.9 1.23 0.69TFC-0.1 72.8 15.5 11.7 1.33 0.58TFC-0.2 69.9 18.1 12.0 1.50 0.40TFC-0.45 73.1 16.3 10.6 1.53 0.37

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33 29

cross-linking degree give a higher water permeation rate due to morecarboxylic acid group present in the linear portion of polyamide.

Also note that the TFC-0.45 exhibited less than 50% NaClrejection at 150 psi and failed at 250 psi. It also exhibited highersolute permeability than the TFC-0.2. The difference in cross-linking degree might not explain this dramatic difference sincethe cross-linking degree of TFC-0.45 is only slightly lower thanTFC-0.2. We are confident that the large poor size does notadequately support the polyamide layer under hydraulic pressure.

3.5. Effect of support pore size on osmotic flux

3.5.1. Structural parameter and tortuosity determinationThe osmotic water flux of the four TFC membranes is presented

in Fig. 4. The structural parameter for the TFC and HTI membranescan then be derived by fitting the water flux data obtained in FOas well as A and B values obtained in RO (Table 3) into Eq. (2).Generally, the support with larger pores has lower S. TFC-0.45 did

not follow this trend because the A and B values measured duringRO are impacted by the damage to the polyamide layer. Theartificially high A and B values impact the model substantially,producing strange results. This is one of the challenges of using ROto characterize membranes for use in FO. RO places the polyamidelayer under stresses that are not present in FO, meaning that themembrane properties may not be the same between the twoprocesses. This has been observed by Tiraferri et al. in their recentstudy [37]. They developed a new methodology for simultaneousdetermination of A, B and S only by means of a FO experiment andfound that those values were different than those obtained by thestandard approach using both RO and FO. While this method isreliant on fitting data to a model that may not be entirely accurate,it is among the first studies to consider an alternative approach tousing RO to characterize A and B.

Fig. 3. Schematic diagram of polyamide thin film formation onto hydrophilic supports with different pore sizes. Figure modified from [30].

Table 3Summary of salt rejection (%R), pure water permeability coefficient (A) and solutepermeability (B) of HTI and TFC membranes built on different MF supports.Experimental conditions: 2000 ppm NaCl feed solution, cross-flow velocity of0.26 m/s, and temperature of 20 1C.

%R A (LMH/bar) B (LMH)a

150 psi 250 psi

HTI 85.4 94.8 0.599 0.942TFC-0.025 97.7 97.5 0.673 0.122TFC-0.1b 95.8 96.5 0.917 0.300TFC-0.2 92.9 92.9 1.548 0.697TFC-0.45 47.5 Fail 1.930 18.14

a Determined at a hydraulic pressure of 150 psi.b TFC-0.1 data is from [28].

Fig. 4. Osmotic water flux of HTI and TFC membranes built on different nylon 6,6MF supports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water asthe feed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1 data is from [28].

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–3330

Based on our porosity measurements, the lower S values calculatedfor larger pore size substrates were attributed to tortuosity differences.Tortuosity of the supports is estimated by dividing structural para-meter by the thickness and porosity of the support membranes. Thecalculated tortuosities for our membranes seem to be far greater thana typical tortuosity value of a phase-inversion film (i.e. 1oτo2).They are consistent, however, with other studies characterizingtortuosity of phase-inversion supports or membranes using the samemethod [38,39]. One possible cause for the seemingly abnormaltortuosity value is that the model for S calculation does not considerECP on both the support side and selective layer side, which wouldover-estimate S and hence τ. In addition, dead-end pores might bepresent that contribute to porosity but not to mass transport. Theinterface between the selective layer and the support layer is also notincorporated into the structural parameter in any way. The selectivelayer is partially screened by the substrate and acts as anotherresistance. This effect however, is not taken into account in modelingof bulk structural parameters. We do notice in Table 1 that thetortuosity decreases (generally) with increasing surface porosity,suggesting that less screening of the selective layer by the supportlayer reduces mass transport resistance.

Regardless of the absolute value of τ, we still can observe atrend that smaller pore support membranes with lower porositygenerally have higher tortuosity than the ones with large pores.This finding is consistent with previous investigations on otherporous media. Mualem and Dagan [40] suggested a pore-levelrepresentation of tortuosity of soil media that is related inverselyto a power function of the pore radius, r, taking the form τ¼r–b,where b is an empirical parameter. Armatas [41] developed acomputational model to study flow and transport process of smallmolecules in porous media and also found that the modeledtortuosity is related inversely to the pore size of the channel,where the parameter decreased significantly as the size of poresincreased. This means that pore channels with small pore sizeexhibit tighter restrictions through tortuous pathways.

3.5.2. Water flux performance analysisFurther analyzing the water flux data shown in Fig. 4, we found

that in both PRO and FOmode the water flux first gradually increasedwith increasing support pore size until pore size reaches to 0.2 μm,and then dramatically dropped when pore size is increased to0.45 μm. In PRO mode, TFC-0.2 achieved approximately two-foldhigher water flux than the other three TFCs as well as the HTI. TheTFC-0.45 exhibited the worst performance among the four. Fluxes inthe FO mode were lower than the PRO, as is typical for many FOmembrane tests, because of the severe ICP that impacts osmoticdriving force. TFC-0.2 still performed the best among the four TFCsbut its performance only matches the HTI in this orientation.

TFC-0.2 outperforms the other three membranes for tworeasons. The TFC-0.2 exhibited relatively high water permeancein RO tests, reducing resistance to water transport in osmotic tests.Second, its open and highly porous structure with lower tortuositycreates less resistance to water transport and solute diffusion, asindicated by its lower structural parameter. TFC-0.025 and TFC-0.1,while more selective than the TFC-0.2, suffer from their lowerwater permeance and higher structural parameter. This stronglysuggests a “permeance-selectivity” type tradeoff for FO mem-branes. The performance of the TFC-0.45, on the other hand, isin a different category since the polyamide layer is insufficientlysupported to know exactly what its permselective properties arein FO. The lower flux in the PRO mode can be attributed to therelatively high salt flux among 4 TFC membranes (approximately20 times higher than TFC-0.025 and TFC-0.1, and 3 times higherthan TFC-0.2), which causes a lower effective osmotic pressure andsevere ICP in the support layer.

3.5.3. Reverse salt fluxThe reverse salt flux performances of our TFC and HTI mem-

branes are shown in Fig. 5. With increasing pore size of thesupport, the salt flux increased in both PRO and FO modes, whichcorresponds to the trend of decreasing cross-linking degree.Compared to HTI, our TFC membranes showed lower or equalreverse salt flux in both orientations. Among those, the TFC-0.025and TFC-0.1 achieved approximately 15 times lower salt flux thanthe HTI membrane largely due to their superior selectivity com-pared to integrated asymmetric membranes. The TFC-0.2, with1 to 2 fold higher water flux, still exhibited lower salt flux thanHTI. Interestingly enough, TFC-0.45, our least selective TFC, stillshowed approximately equal salt flux to HTI, even though itpossesses an order of magnitude higher B when tested in RO.Again this is probably due to the poor stability of the PA layerunder hydraulic pressure. These results suggest that in RO, theselectivity is determined by combined effects of selective layerproperties and mechanical support provided by the substrate. InFO, selectivity is primarily determined by the intrinsic selectiveproperties of the polyamide layer, or cross-linking degree.

3.5.4. Specific salt fluxSpecific salt flux represents the amount of draw solute loss per liter

of water produced [35,36]. Lower specific salt flux is desirable as itindicates a higher “efficiency”, meaning that the membrane passesmore water per unit of salt lost to reverse salt flux. As shown in Fig.6,

Fig. 5. Reverse salt flux of HTI and TFC membranes built on different nylon 6,6 MFsupports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water as thefeed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1data is from [28].

Fig. 6. Specific salt flux of HTI and TFC membranes built on different nylon 6,6 MFsupports. Experimental conditions: 1.5 M NaCl as the draw solution; DI water as thefeed; cross-flow velocity of 0.18 m/s; 20 1C. TFC-0.1 data is from [28].

L. Huang, J.R. McCutcheon / Journal of Membrane Science 483 (2015) 25–33 31

TFCs based on smaller pore size substrates (i.e. TFC-0.025 and TFC-0.1)generally have lower specific salt flux than those with larger pores (i.e.TFC-0.2 and TFC-0.45). Compared to the HTI membrane, our TFCmembranes exhibited 10–30 times lower specific salt flux in PROmode and 3–9 times lower in FO mode. The exception is the TFC-0.45,which exhibited a matched or slightly higher specific salt flux than theHTI membrane.

4. Conclusions

During the course of this work, we identified that the supportlayer pore size does have a significant impact on TFC membraneperformance in both RO and FO. Using a unique series of MFmembranes available from 3M, we were able to focus on pore sizeas a singular independent variable impacting water and salt fluxperformance. Generally, we found that smaller pore supportsyielded denser and more selective polyamide layers with betterpressure tolerance. However, the osmotic flux performance waslimited by a more tortuous structure. We found that support poresizes of around 0.2 μm favors both high water flux and low salt flux,but even this membrane did not exhibit the lowest specific salt fluxof our group (0.1 μm).

An important finding of this study is that pore size is not afeature of the structural parameter, yet it can impact the fluxperformance dramatically in FO and PRO. Membrane fabricatorsmust keep this in mind when designing new membranes. It is notonly the thickness, tortuosity and porosity that must be adjustedto minimize structural parameter. The pore size and its impact onboth structure parameter and selective layer formation must beunderstood if we are to design high performance membranes forFO or PRO.

Acknowledgements

The authors gratefully acknowledge funding from the NationalScience Foundation (CBET #1067564), the Environmental Protec-tion Agency STAR Program (R834872), and the 3M™ Non tenuredFaculty Award. Furthermore, we thank 3M for providing micro-filtration membranes and for assisting with some of the SEMimaging. Hydration Technologies Innovations provided their for-ward osmosis membrane for this work. We would also like toacknowledge Dr. Heng Zhang and graduate student XiaoqiangJiang in University of Connecticut for their help in XPS character-ization and analysis.

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