Highly permeable cellulose acetate nanofibrous composite membranesby freeze-extraction

Faizal Soyekwo, Qiu Gen Zhang n, Chao Deng, Yi Gong, Ai Mei Zhu, Qing Lin Liu n

Department of Chemical & Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China

a r t i c l e i n f o

Article history:Received 1 June 2013Received in revised form3 December 2013Accepted 6 December 2013Available online 15 December 2013

Keywords:Composite membraneUltrafiltrationCellulose acetateNanofiberFreeze-extraction

a b s t r a c t

Ultrafiltration is widely used in waste water treatment and has become more crucial with increasingconcerns in the living environment. Here we demonstrate a facile method to prepare 10 nm-diametercellulose acetate (CA) nanofiber dispersions from very dilute CA solutions via freeze-extraction, andfurther fabricate nanofibrous composite membranes for ultrafiltration. The nanofibrous compositemembranes are fabricated by directly filtering the dispersions on a CA microfiltration membrane(support), on which an ultrathin free-standing nanofibrous layer is formed. This layer, acting as aseparation layer, has a uniform porous structure and ultrahigh porosity of up to 71%. The as-preparedmembranes present ultrahigh water permeability and high efficient separation performance forultrafiltration. The membrane with a 458 nm-thick nanofibrous layer has ferritin rejection of 90.7%and water flux of 3540 l m2 h1 bar1 that is almost 10 times greater than that of most commercialmembranes. These newly developed CA nanofibrous composite membranes have a great potentialapplication in various separation processes.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ultrafiltration is important and widely used in water treatment,food industry and life science, and has become more crucial withincreasing concerns in the living environment. However, mostcommercial membranes have a low porous separation layer and alarge flow resistance leading to a small permeation flux [13].Very recently, one-dimensional nanomaterials (e.g. nanofibers,nanowires and nanotubes) have attracted increasing interest andcontributed significantly to fabrication of nanofibrous compositemembranes [47]. Due to their peculiar properties, they impartunique structural characteristics to the membranes, such as highpermeability, high porosity, high surface area and interconnectedpores [4]. Currently many one-dimensional inorganic materials areused to produce high efficient nanofibrous composite membranes,including carbon nanotubes [79], inorganic nanowires [10],carbonaceous nanofibers [6], metal oxide nanofibers [11], andmetal hydroxide nanofibers [7,12]. The resulting membranes showa great potential in ultrafiltration. However, most ultrafiltrationmembranes are prepared from polymers because of their low costand high processability. Thus, it is highly imperative to develop afacile approach for the fabrication of high efficient ultrafiltration

membranes from polymer nanofibers for various separationprocesses.

Various techniques have been applied to prepare polymernanofibrous membranes. The electrospinning technique is themost versatile one and has been used to produce nanofibrousmembranes with pore sizes usually on the sub-micrometer scale[4,1315]. The diameters of electrospun nanofibers are generally inthe range of micrometer or sub-micrometer. However, ultrafiltra-tion membranes fabricated from ultrafine electrospun nanofibersare now a great challenge [16]. The drawback of electrospunnanofibers lies in their poor diameter control and the difficultyin fabricating ultrathin membrane with a controlled thickness [5].Alternatives have been explored to produce polymer nanofibers,such as molecular self-assembly, lithography, template synthesis,and interfacial polymerization [1723]. However, it is difficult tofabricate ultrafiltration membranes by these methods due to theirintricate operations and specific outcome of nanofiber. Therefore,the development of facile and versatile pathways of synthesis ofpolymer nanofibers to fabricate nanofibrous composite mem-branes for ultrafiltration is an important and interesting task.

In this work, we demonstrate a facile and green approach tofabricate CA nanofibrous composite membranes for ultrafiltration.Briefly, CA nanofiber dispersions were first prepared via thefreeze-extraction method and then filtrated on a CA microfiltrationmembrane to form a free-standing nanofibrous layer. Fig. 1adepicts the as-prepared nanofibrous composite membranes withan ultrathin CA nanoporous layer (separation layer) covered on a

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Journal of Membrane Science

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.memsci.2013.12.014

n Corresponding authors.E-mail addresses: qgzhang@xmu.edu.cn (Q.G. Zhang),

qlliu@xmu.edu.cn (Q.L. Liu).

Journal of Membrane Science 454 (2014) 339345

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CA macroporous membrane (support, Fig. 1b) with a cut-off of0.20 m. CA, the acetate ester of cellulose, is one of the earliestmaterials used and still finds utility in membrane separationprocesses due to its outstanding formability, reasonable resistanceto degradation by chlorine and low fouling. Membranes madefrom CA are nowadays used in seawater desalination, waterpurification, wastewater treatment, concentration of fruit juices,and life science [24]. The as-fabricated nanoporous membraneswith a tiny membrane resistance show ultrahigh water flux andgood ferritin rejection during ultrafiltration.

2. Experimental

2.1. Materials

CA, with average molecular weight of 30,000 g mol1 andapproximately 54.556.0% acetic acid bonded, was purchasedfrom Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). CAmembranes, with a cut-off of 0.2 m and 25 mm in diameter, werepurchased from Toyo Roshi Kaisha Ltd. (Japan). Dimethylsulfoxide(DMSO), N-methyl-2-pyrrolidone (NMP), N, N-dimethylacetamide(DMAc), and glacial acetic acid (GAA), of analytical grade, werepurchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,China). 5 nm gold nanoparticle solution and ferritin (from equinespleen) were obtained from British Biocell International andSigma-Aldrich, respectively.

2.2. Preparation of CA nanofiber dispersions

CA solutions were initially prepared by dissolving CA in each ofthe solvents (DMAc, DMSO, NMP and GAA) with vigorous stirringat 60 1C for 6 h to obtain homogenous transparent solutions of0.1 mg ml1 CA. 10 ml of the CA solutions was then directly frozenin liquid nitrogen contained in a 250 ml beaker to form a thickwhite solid. 90 ml of ethanol (at 40 1C) was then added in thebeaker and the contents were then transferred to a refrigeratormaintained at about 40 1C for 24 h. The resulting freeze-extracted solution containing 0.01 mg ml1 CA was the expectedCA nanofiber dispersion.

2.3. Fabrication of CA nanofibrous composite membranes

A given volume of the as-prepared CA nanofiber dispersionswas filtered through a CA microfiltration membrane placed on aglass filter holder (Millipore Co., USA) at a suction vacuumpressure of 80 kPa. The filtration area was 2.27 cm2. Duringfiltration, the CA nanofibers were freely deposited on the CAmembrane to form a free-standing nanoporous layer, resulting inthe formation of the CA nanofibrous composite membrane. Thethickness of the nanofibrous layer could be easily controlled byregulating the volume of the dispersion filtered.

2.4. Filtration experiments

Filtration was performed using the glass filter holder men-tioned above at the suction vacuum pressure of 80 kPa. Pure waterflux (J, l m2 h1 bar1) of the CA nanofibrous composite mem-branes is measured by filtering 100 ml of water across themembrane, and calculated by

J V=Atp 1where V is the volume of the water filtered (l), A is the effectivemembrane filtration area (m2), t is the filtration time (h), and p isthe suction pressure across the membrane (bar).

Ferritin and 5 nm gold nanoparticle solutions were used toevaluate the separation performance of the CA nanofibrous com-posite membranes. 20 ml of 20 mg ml1 ferritin solution wasfiltered on the fabricated membranes using the glass filter holderat the suction vacuum pressure of 80 kPa. Filtration of 5 nm goldnanoparticles was also conducted in a similar way using anaqueous solution (20 ml) that had been diluted 20 times fromthe original solution. The feed and the permeate solutions werecharacterized using an ultraviolet and visible (UVvis) spectro-photometer (Model UV-1800 series, Shimadzu, Japan). The rejec-tion (R, %) is calculated by

R 1Cp=Cf 100% 2where Cf and Cp are the concentration of the ferritin or 5 goldnanoparticles in the feed and the permeate, respectively.

2.5. Characterization

Transmission electron microscopy (TEM) (JEM-2100F, 200 kV,JEOL, Japan) was used to observe the CA nanofiber dispersions.A dispersion was initially diluted 10 times using deionized waterand then dropped on a copper grid coated with a carbon film anddried in the air to obtain the sample for TEM observation.Morphology of the CA nanofibrous composite membranes wascharacterized using scanning electron microscopy (SEM, LEO1530,Germany). Before the observation, all the samples were goldplated to prevent charging and improve the membranes' electroconductivity, and the samples for cross-sectional observation wereprepared by freeze-fracturing in liquid nitrogen.

3. Results and discussion

3.1. Preparation and characterization of CA nanofiber dispersions

Freeze-extraction used to prepare CA nanofiber dispersions is afeasible technique that has also been used to prepare microporousmembranes and highly porous polymer scaffolds. In this method, ahigh concentration (usually above 5 wt%) polymer solution isfrozen, and then immersed in a non-solvent bath at a temperaturelower than the freezing point of the polymer solvent, resulting inthe formation of porous structures with porosity of up to 80% [2527].

Fig. 1. (a) Schematic structure of CA nanofibrous composite membranes. (b) Top-viewSEM image of the CA microfiltration filter with a cut-off of 0.2 m.

F. Soyekwo et al. / Journal of Membrane Science 454 (2014) 339345340

Although this technique has been used to produce polymers withfunctional aggregation of polymer chains from dilute polymer solu-tion, however, there are no reports on their morphology and structure[2830].

In this work, a very dilute solution (0.1 mg ml1 CA) wasprepared by dissolving CA in a good solvent, and rapidly frozenin liquid nitrogen and then immersed into an ethanol bath at40 1C to fulfill the extraction process. Four solvents of DMSO(18.5 1C), NMP (24.4 1C), DMAc (20 1C) and GAA (16.0 1C) withmelting points above 40 1C were used as solvents of CA. Theresulting colorless and transparent solutions were the desired CA

nanofiber dispersion. Fig. 2 shows the TEM images of CA nanofi-bers. It is noted that the CA nanofibers are gradually formed fromnanosheets to nanofibers in the order of the solvent of DMSO,NMP, DMAc and GAA. This may be due to the difference in thesolubility parameter of the solvent with CA. Of those solvents,DMSO has a similar value (26.7 MPa1/2) to CA (25.9 MPa1/2), GAA(21.4 MPa1/2) has the biggest difference, and NMP and DMAc havethe solubility parameters of 22.9 and 22.7 MPa1/2, respectively[31]. Clearly, very thin and fine quasi one-dimensional nanofiberscan be observed in the dispersion by the use of GAA (Fig. 2d).The nanofibers with a diameter of about 10 nm are mainly

Fig. 2. TEM images of the CA nanofiber dispersions using different solvents: (a) DMSO, (b) NMP, (c) DMAc and (d) GAA. The scale bar is 0.5 m.

Table 1Properties and separation performances of the CA nanofibrous composite membranes.

Solvent Volume of dispersions (ml) The nanofibrous layer Water flux (l m2 h1 bar1) Rejection (%)

Thickness (nm) Porosity (%) 5 nm gold Ferritin

DMSO 5 333 37177138 50.270.3 83.471.410 792 63.0 24997101 53.270.5 92.470.9

NMP 5 500 51277165 33.270.3 39.370.810 1083 71.0 49567153 43.770.4 56.670.6

DMAc 5 250 33047122 51.870.5 83.170.710 708 60.9 1919787 71.870.4 82.371.5

GAA 5 458 35407176 56.070.4 90.771.310 833 54.8 20847134 65.770.6 98.170.9

F. Soyekwo et al. / Journal of Membrane Science 454 (2014) 339345 341

agglomerated to form a nanofiber network that would easilydeposit to form an ultrathin free-standing nanoporous layer onthe macroporous supports.

3.2. Formation and properties of CA nanofibrous compositemembranes

CA nanofibrous composite membranes were prepared bydirectly filtering the as-formed CA nanofiber dispersions over theCA microfiltration membrane. Filtration is a feasible way tofabricate ultrathin nanoporous membranes from nanofiber disper-sions, and has its own advantages such as the homogeneity of themembrane, maximal mechanical integrity and interconnected pores

throughout the membrane, and controlled thickness by adjustingthe nanofiber concentration and the volume of the dispersionfiltered [4,9]. A series of CA nanofibrous composite membraneswas fabricated in this work, as listed in Table 1.

Their morphological structure is unique for different CA sol-vents used. Fig. 3 shows SEM images of the membranes preparedfrom 5 ml of nanofiber dispersions. The membranes are stablesince the nanofibers strongly adhere to the support. The top-viewsurface morphologies reveal an orderly distribution of the nano-fibers over the support to form an extensive network of inter-connected pores made by fine netlike nanofibers. The nanofibersare also agglomerated to form a network of compactly arrangednanofibers. This results in the formation of a highly porous structure

Fig. 3. Top-view (left) and cross-sectional (right) SEM images of the CA nanofibrous composite membranes prepared from 5 ml of nanofiber dispersions using differentsolvents: (a) DMSO, (b) NMP, (c) DMAc and (d) GAA. The scale bars of the top-view and the cross-section are 0.5 and 1.0 m, respectively.

F. Soyekwo et al. / Journal of Membrane Science 454 (2014) 339345342

with a narrow pore size distribution. The membranes with the mostuniformly porous structure were obtained using DMAc and GAAas a solvent, the biggest and the smallest pore structures wereproduced by the use of NMP and GAA, respectively. The sameporous structure was observed for the membranes prepared from10 ml of the nanofiber dispersions (Fig. 4).

The cross-sectional images clearly show the formation of afree-standing nanofibrous layer of CA nanofibers with a typicalasymmetrical pore structure on the macroporous support(Figs. 3 and 4). The nanofibrous layer consists of a stack of denselypacked netlike CA nanofibers acting as an active barrier which isresponsible for the separation capacity of the as-formed compositemembranes. The layer thickness is in the range 275500 nm using5 ml of the nanofiber dispersions, and less than 1 m using 10 mlof nanofiber dispersions, as listed in Table 1. The membraneproduced has the thinnest and the thickest nanofibrous layersusing DMAc and NMP as a solvent, respectively.

The free-standing nanofibrous layer is highly porous from SEMobservation. The porosity was estimated by the increase () inthe thickness and the weight (m) of the free-standing layerprepared from 10 ml of nanofiber dispersions over that prepared

from 5 ml of nanofiber dispersions using the equation

Porosity % 1m=CAAf ilm

! 100 3

where is the density of cellulose acetate and A is the effectivearea of the membrane. The estimated porosity is presented inTable 1. The free-standing nanoporous layer produced using NMPas a solvent has the highest porosity of 71.0% that is at least 7 timesgreater than the skin layer of the most commercial polymermembranes [32,33]. Meanwhile, the layer formed using GAA as asolvent has the lowest porosity of 54.8%. It is expected that the as-formed composite membranes with an ultrathin highly porousnanoporous layer should have ultrahigh water flux duringpressure-driven filtration.

3.3. Separation performance of the CA nanofibrous compositemembranes

To evaluate the pore size of the as-prepared CA nanofibrouscomposite membranes, experiments were performed by filtration

Fig. 4. Cross-sectional SEM images of the CA nanofibrous composite membranes prepared from 10 ml of nanofiber dispersions using different solvents: (a) DMSO, (b) NMP,(c) DMAc and (d) GAA. The scale bar is 1.0 m.

0.0

0.2

0.4

0.6

0.8

1.0Ferritin solution

Permeate

Abs

orba

nce

Wavelength (nm)

Feed

200 250 300 350 400 300 400 500 600 700 800

0.00

0.05

0.10

0.15

0.20

Permeate

Feed

Abs

orba

nce

Wavelength (nm)

5 nm gold nanoparticle solution

Fig. 5. UVvis absorption spectra of the feed and the permeate of ferritin (a) and 5 nm gold nanoparticle (b) solutions separated by the CA nanofibrous composite membraneprepared from 5 ml of the nanofiber dispersion using GAA as a solvent.

F. Soyekwo et al. / Journal of Membrane Science 454 (2014) 339345 343

of ferritin and 5 nm gold nanoparticle solutions in the dead-endmode. As listed in Table 1, with the exception of using NMP as asolvent, the membranes prepared from 5 ml of nanofiber disper-sions have good ferritin rejection and 5 nm gold nanoparticlerejection of about 50%. As noted previously, the membraneprepared using NMP as a solvent has the biggest pores and thehighest porosity, and that using GAA has the smallest pores andthe lowest porosity. Thus, the latter should have the smallest cut-off during membrane filtration. As shown in Fig. 5, the membraneusing GAA as a solvent has ferritin rejection of 90.7% and 5 nmgold nanoparticle rejection of 56.0%.

The thickness of the separation layer has a significant impacton the membrane properties. Compared to the membranes pre-pared from 5 ml of nanofiber dispersions, the membranes pre-pared from 10 ml of nanofiber dispersions have less water fluxes,and higher rejection for ferritin and 5 nm gold nanoparticles(Table 1). Of those, the membrane prepared using GAA has aperfect ferritin rejection of 98.1%. The ferritin rejection of the708 nm-thick membrane by use of DMAc is slightly less than thatof the 250 nm-thick membrane. This is because the latter withfewer nanofibers deposited has a rougher membrane surface thatwould adsorb more ferritin molecules, leading to a higher ferritinrejection than the former.

Pure water flux, an important parameter, is measured to evaluatethe separation performance of the CA nanofibrous composite mem-branes. All the as-prepared membranes show ultrahigh water fluxes(Table 1). The membranes prepared from 5ml of nanofiber disper-sions have water fluxes of above 3304 l m2 h1 bar1. Of those,the membrane with a 500 nm-thick nanofibrous layer (NMP used asa solvent) has the highest value of 5127 l m2 h1 bar1. An increasein the membrane thickness leads to a decrease in water permeation.When compared with typical commercial flat sheet ultrafiltrationmembranes [34,35], the as-prepared CA nanofibrous compositemembranes exhibited much higher water fluxes. The membranewith a 458 nm-thick nanofibrous layer (GAA used as a solvent) has awater flux of 3540 l m2 h1 bar1 that is almost 10 times greaterthan that of most commercial membranes.

On the other hand, the relationship between the water flux (J)and the applied ultrafiltration pressure (p) for composite mem-branes can be described by the CarmanKozeny equation [31],

J pRbaseRskin

4

where is the water viscosity (Pa s), and Rbase and Rskin are theresistance of the support and the skin layer, respectively. Here, theCA membrane with a cut-off of 0.20 m has water flux of10,331 l m2 h1 bar1, and therefore has a resistance (Rbase) of3.481010 m1. The calculated resistance (Rskin) of the compositemembrane with a 500 nm-thick layer (NMP used as a solvent) is3.531010 m1, which is similar to that of the support. Thisreveals that the as-formed nanofibrous layer with the tiny resis-tance allows ultrafast permeation of water through the resultantcomposite membrane during the pressure-driven filtration.

4. Conclusions

In summary, a series of CA nanofiber dispersions was producedfrom very dilute CA solutions by the freeze-extraction method, andfurther used to fabricate nanofibrous composite membranes. Thediameter of the CA nanofibers is about 10 nm from TEM observa-tion. The as-prepared CA nanofibrous composite membranes havean ultrathin free-standing layer of CA nanofibers covered on a CAmicrofiltration membrane. This layer has an ultrahigh porosityof up to 71% that is at least 7 times greater than the skin layer ofmost commercial polymer membranes. These resulting composite

membranes show a great potential for a wide application inmembrane filtration. They have ultrahigh water flux and showgood ferritin rejection. GAA was the best solvent for CA to preparethe highly efficient nanofibrous composite membranes. The result-ing membrane with a 458 nm-thick nanofibrous layer has ferritinrejection of 90.7% and water flux of 3540 l m2 h1 bar1 that isalmost 10 times greater than that of most commercial membranes.This newly proposed facile approach will contribute tremendouslyto the development of highly efficient ultrafiltration membranesand nanoporous materials in various separation processes.

Acknowledgments

The research was supported by National Nature Science Founda-tion of China Grant nos. 21076170, 21107089 and 21376194, theresearch fund for the Doctoral Program of Higher Education(20120121120013) and the Fundamental Research Funds for theCentral Universities (No. 2012121029).

Nomenclature

A effective filtration area (m2)Cf concentration in feed (mg ml1)Cp concentration in permeate (mg ml1)m increase in the weight (kg)p suction pressure (bar)p pressure difference (Pa)R rejection (%)Rbase resistance of the support (m1)Rskin resistance of the skin layer (m1)t filtration time (h)V water volume (l)

Greek letter

water viscosity (Pa s) density of cellulose acetate (g cm3) increase in the thickness (nm)

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Highly permeable cellulose acetate nanofibrous composite membranes by freeze-extractionIntroductionExperimentalMaterialsPreparation of CA nanofiber dispersionsFabrication of CA nanofibrous composite membranesFiltration experimentsCharacterization

Results and discussionPreparation and characterization of CA nanofiber dispersionsFormation and properties of CA nanofibrous composite membranesSeparation performance of the CA nanofibrous composite membranes

ConclusionsAcknowledgmentsReferences