sodium alginate two-active-layer composite membranes

sodium alginate two-active-layer composite membranes

Accepted Manuscript Title: Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes Author: Chengy...

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Accepted Manuscript Title: Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes Author: Chengyun Gao Minhua Zhang Jianwu Ding Fusheng Pan Zhongyi Jiang Yifan Li Jing Zhao PII: DOI: Reference:

S0144-8617(13)00841-2 http://dx.doi.org/doi:10.1016/j.carbpol.2013.08.057 CARP 8047

To appear in: Received date: Revised date: Accepted date:

5-4-2013 19-8-2013 22-8-2013

Please cite this article as: Gao, C., Zhang, M., Ding, J., Pan, F., Jiang, Z., Li, Y., & Zhao, J., Pervaporation dehydration of ethanol by hyaluronic acid/sodium alginate two-active-layer composite membranes, Carbohydrate Polymers (2013), http://dx.doi.org/10.1016/j.carbpol.2013.08.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights (for review)

► Two-active-layer

composite

membranes

were

prepared

by

sequential

spin-coating. ► Sorption and diffusion behavior in capping layer and inner layer were studied. ► Positioning of HA layer and NaAlg layer in membrane affected separation factor.

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► HA/Alg/PAN membrane exhibited higher pervaporation performance.

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*Manuscript

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Pervaporation dehydration of ethanol by hyaluronic

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acid/sodium alginate two-active-layer composite membranes

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Chengyun Gaoa,b, Minhua Zhangb, Jianwu Dinga,b, Fusheng Pana, Zhongyi Jianga,

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Yifan Lia, Jing Zhaoa

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a

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Chemical Engineering and Technology, Tianjin University, 92# Road Weijin, District

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Nankai, Tianjin 300072, China

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Key Laboratory for Green Chemical Technology of Ministry of Education, School of

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b

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Petrochemical Technology, Tianjin University, Tianjin 300072, China

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Key Laboratory for Green Technology of Ministry of Education, R & D Center for

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To whom correspondence should be addressed. Address: School of Chemical

Engineering & Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin, 300072, P. R. China. Tel: 86-22-2350 0086. Fax: 86-22-2350 0086. E-mail: [email protected]

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Abstract:

The composite membranes with two-active-layer (a capping layer and an inner

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layer) were prepared by sequential spin-coatings of hyaluronic acid (HA) and sodium

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alginate (NaAlg) on the polyacrylonitrile (PAN) support layer. The SEM showed a

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mutilayer structure and a distinct interface between the HA layer and the NaAlg layer.

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The coating sequence of two-active-layer had an obvious influence on the

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pervaporation dehydration performance of membranes. When the operation

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temperature was 80oC and water concentration in feed was 10 wt.%, the permeate

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fluxes of HA/Alg/PAN membrane and Alg/HA/PAN membrane were similar, whereas

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the separation factor were 1130 and 527, respectively. It was found that the capping

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layer with higher hydrophilicity and water retention capacity, and the inner layer with

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higher permselectivity could increase the separation performance of the composite

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membranes. Meanwhile, effects of operation temperature and water concentration in

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feed on pervaporation performance as well as membrane properties were studied.

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Keywords: Hyaluronic acid; Sodium alginate; Two-active-layer composite membrane;

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Pervaporation; Ethanol dehydration

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Introduction Pervaporation, as a promising membrane-based separation technology for the

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dehydration of solvents, has undergone a rapid development in recent years.

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Solution-diffusion mechanism is proposed and widely accepted (Binning et al., 1961;

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Lonsdale, 1982) to elucidate the mass transfer of pervaporation process: selective

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sorption of feed species on the membrane surface, selective diffusion through the

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membrane and desorption into a vapor phase on the permeate side. Among these three

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steps, selective sorption and diffusion in membranes are often the rate-governing steps.

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At present, the composite membrane, which comprises a separation layer (active layer)

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and a support layer is the most commonly used configuration. Ideally, the separation

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layer of membranes should possess the surface with high sorption capacity and

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selectivity, and the bulk with low diffusion resistance and high diffusion selectivity.

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Materials for the separation layer of the one-active-layer pervaporation membranes

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have to bear these properties all-in-one, which limits the selection scope of materials.

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It is well known that the composite membranes assign the separation performance and

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the mechanical strength demands into the separation layer and the support layer

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individually. The performance can be highly improved by choosing the appropriate

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materials for the separation layer and the support layer. It is thus envisaged that if the

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similar strategy is adopted, the separation layer is further divided into the capping

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layer and the inner layer (two-active-layer) to intensify the adsorption and diffusion

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individually, as shown in Fig.1, the selection scope of the materials for separation

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layer may be dramatically expanded and the separation performance may be improved

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in a much broader range, since the surface and bulk properties could be varied by the

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rational tuning of the capping layer and the inner layer.

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Fig. 1. Schematic structure of the two-active-layer composite membrane.

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Rational screening of hydrophilic materials for capping layer and inner layer is of

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great importance for the separation performance of two-active-layer membranes. Till

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now, several kinds of polymer-based materials have been employed in the dehydration

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process of ethanol-water aqueous solution by pervaporation. Huang et al. (1999)

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prepared two-ply membranes by successive castings of NaAlg and CS solutions for

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the pervaporation dehydration of 95 wt.% ethanol-water mixture. When CS was

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selected as capping layer facing to the feed solution, the separation factors was 147,

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while that of the membranes with NaAlg facing to the feed solution was 1062. These

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results suggested that coating sequence had an obvious effect on membrane separation

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performance. In our previous study (Ding et al., 2012), composite membranes with

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two-active-layer, HA and CS, were constructed through alternative self-assembly and

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spin-coating. It was found that the separation factor of the HA/CS/PAN membrane

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(704) with CS as inner layer was remarkably higher than that of CS/HA/PAN

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membrane (104) with HA as inner layer. A probable explanation was HA layer had

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higher water sorption selectivity and CS layer had higher diffusion selectivity. In the

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above studies, the capping layer and the inner layer are fabricated from cationic

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polyelectrolyte and anionic polyelectrolyte, individually. Herein, we want to explore

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the separation performance of the two-active-layer membranes where the capping

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layer and the inner layer are both fabricated from anionic polyelectrolytes.

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As one common hydrophilic carbohydrate polymer, sodium alginate (NaAlg),

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which is extracted from marine brown algae, has exhibited excellent performance as a

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pervaporation membrane material (Bhat & Aminabhavi, 2006, 2007; Kalyani et al.,

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2006; Kanti et al., 2004; Kumar Naidu et al., 2005; Kurkuri et al., 2002a; Kurkuri et

88

al., 2002b; Magalad et al., 2010; Uragami & Saito, 1989). The carboxylic and

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hydroxyl groups play a crucial role in preferential water sorption and diffusion

90

through the membranes (Yeom & Lee, 1998) .Since alginate has the excellent

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permselectivity toward water, it may be the suitable material to offer higher separation

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performance (Huang et al., 1999; Shi et al., 1998). As another common hydrophilic

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carbohydrate polymer, hyaluronic acid (HA) (Kim et al., 2004; Ma et al., 2010;

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Tomihata & Ikada, 1997) has demonstrated high water sorption selectivity in ethanol

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aqueous solution and high water retention capacity (500mL/g) due to a large number

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of hydroxyl groups and carboxyl groups. Therefore, HA may have potential to

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intensify the sorption/dissolution of water.

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In this study, two-active-layer composite membranes were fabricated by sequential

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spin-coatings of HA and NaAlg on the PAN support layer. Effect of coating sequences

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and operation conditions on structure and performance variation of the

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two-active-layer membranes for dehydration of 90 wt.% ethanol-water mixture were

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explored.

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2. Experiments

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2.1 Materials The flat-sheet ultrafiltration membrane, which had a molecular weight cut-off of

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100,000 and was made up of PAN porous layer and polyethylene terephthalate

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nonwoven fabric substrate, was obtained from Shanghai MegaVision Membrane

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Engineering & Technology Co. Ltd. (Shanghai, China). NaAlg (viscosity of 1 wt.%

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aqueous solution at 20oC: 0.02Pa•s ) was purchased by Tianjin Guangfu Ltd. (Tianjin,

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China). HA (MW 1,200,000 Da, sodium salt) was obtained by Qufu Haixin Industrial

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Co. Ltd. (Qufu, China). Glutaraldehyde (GA; 25 wt.%), hydrochloric acid (HCl),

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calcium chloride and absolute ethanol were purchased from Tianjin Kewei Ltd.

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(Tianjin, China). Deionized water was used in all experiments.

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2.2 Membrane preparation

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The flat-sheet PAN membrane was immersed into deionized water for two days to

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remove glycerol on the membrane surface, and air-dried at room temperature. The

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NaAlg solution was prepared by dissolving NaAlg into deionized water with stirring

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for two hours, and then kept aside for one hour to remove the bubble in the solution.

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HA solution was prepared by dissolving hyaluronic acid into deionized water with

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stirring for two hours, and then added GA solution to initiate cross-linking and

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adjusted pH to about 4.5 with 0.01M HCl. After half an hour, stirring was stopped and

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the sample was kept aside for degassing

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Membranes were prepared by two-step spin-coating. NaAlg solution was firstly

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coated on PAN support layer by spin-coater and dried at room temperature. Then the

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NaAlg membrane was cross-linked by immersing into the CaCl2 aqueous solution for

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ten minutes and then rinsed with deionized water and air-dried. After that, the second

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coating was carried on. HA solution was spin-coated on the membrane surface and

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air-dried. The resultant membrane was denoted as HA/Alg(X)/PAN, where X (unit:

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mol/L) meant Ca2+ concentrations of solutions that were used to cross-link NaAlg

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membrane. Similarly, the Alg(X)/HA/PAN membrane was prepared by sequential

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spin-coatings of first hyaluronic acid and then sodium alginate.

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2.3 Characterization

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FT-IR spectra for each layer of the composite membrane were acquired by using a

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Nicolet-560 Fourier transform infrared spectrometer with scan range of 4000-600

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cm-1.The static water contact angles of the membrane surfaces were measured by

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contact angle goniometer (JC2000C, Powereach Co., Shanghai, China) for the

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investigation of the hydrophilic/hydrophobic property. The contact angle of each

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membrane sample was measured ten times at different locations. The cross-section

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morphology of membranes was observed by a Nanosem 430 field emission scanning

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electron microscope (FESEM). Samples were freeze-fractured in liquid nitrogen and

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then were sputtered with gold prior to scanning.

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2.4 Swelling and sorption measurement

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The swelling degree of homogenous HA and NaAlg membranes was measured by

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immersing membranes into deionized water and 90 wt.% ethanol-water mixture for 24

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hours. Then the membranes were carefully wiped off the surface liquid with tissue

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146 147

paper. The degree of swelling was calculated as follows: Degree of swelling 

Ms  Md 100% Md

(1)

The adsorbed liquid in membranes was desorbed by an apparatus with a vacuum

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pump and condensed in a liquid nitrogen trap, which was analyzed by gas

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chromatography. All the experiments were repeated three times.

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Sorption selectivity (αs) was calculated by Eq. (2):

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s 

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Where WM and EM were the weight fractions of water and ethanol in the membrane,

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respectively; Wf and Ef were the weight fractions of water and ethanol in the feed

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solution, respectively.

Eq. (3):

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  D

 s

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According to solution–diffusion theory, diffusion selectivity (αD) was calculated by

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(2)

(3)

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WM / EM Ef / Ef

cr

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where α is the separation factor of single-active-layer HA/PAN or CS/PAN membranes.

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2.5 Pervaporation experiment

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Pervaporation experiments were carried out for dehydration of ethanol/water

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mixtures. The pervaporation apparatus were described in our previous study (Peng et

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al., 2005). The effective surface area was 25.6 cm2. The concentrations of the feed

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solution and the permeate were measured by gas chromatography (USA Agilent 4890).

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The permeate flux ( J ) and the separation factor (  ) was calculated as follows

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J

Q At

(4)

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

Wp / Ep Wf / Ef

(5)

Where Q is the mass of the permeate collected in time t, and A is the effective

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membrane area. Where Wp and Ep were the weight fractions of water and ethanol in

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the permeate solution, respectively; Wf and Ef were the weight fractions of water and

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ethanol in the feed solution, respectively.

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Generally, permeate flux and separation factor are affected strongly by operation

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conditions (such as temperature and water concentration in the feed), which change

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vapor pressure difference between two sides of membrane (driving force), as well as

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membrane intrinsic properties. In order to investigate the intrinsic properties of

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membranes with variation of feed concentration and temperature, effect of the driving

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force should be removed. Baker (Baker et al., 2010) presented a method in term of

179

permeability ( Pi ), permeance ( Pi / l ) and selectivity (  ), which normalized the

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driving force. They were used to evaluate variations of membrane properties with

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operation conditions.

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The component flux can be written as

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Ji 

Pi L L sat ( i 0 xi 0 pi 0  pil ) l

(6)

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Where for component i , J i is the partial permeate flux, Pi is the permeability, l

185

is the thickness of the active layer in the composite membranes,  i0L is the activity

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coefficient in the feed solution, xiL0 is the mole fraction in the feed solution, pisat is 0

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the saturated vapor pressure of pure component i , pil is the partial pressure in the

188

permeate vapor. It is implicit in the equation (6) that the concentration polarization 9

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189

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effect is negligible. Therefore the permeance ( Pi / l ) was calculated as follows: Pi Ji  L L sat l ( i 0 xi 0 pi 0  pil )

(7)

Since the vacuum pump is used in the permeate side and the vapor pressure is

192

approximately vacuum, pil can be considered as zero. The activity coefficient and

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the pure component vapor pressure can be calculated by computer simulation

194

programs (ChemCad, HYSYS, ProSim or Aspen Plus).

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The selectivity (  ) is defined as the ratio of the permeability or the permeance of two components:

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 ij 

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3. Results and Discussion

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3.1 Membrane characterization

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3.1.1 FT-IR

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Pi Pi / l  Pj Pj / l

(8)

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(d)

(c)

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(b)

(a)

3380cm

4000

201

3500

1570cm-1 1376cm-1

-1

1436cm-1 -1 1615cm-1 - 1050cm (-COO ) (-C-O-C)

(-OH)

3000

2500

2000

1500

1000

Wave numbers (cm-1)

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Fig. 2. FTIR spectra of (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;

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and (d) HA/Alg(0.5)/PAN membranes

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Fig.2 showed FT-IR spectra of Alg(0.5)/PAN, HA/PAN, Alg(0.5)/HA/PAN and

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HA/Alg(0.5)/PAN membranes, respectively. The spectra of Alg(0.5)/PAN, HA/PAN,

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Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes showed the same peak at

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3380cm-1 which was ascribed to –OH, 1050cm-1 which was ascribed to C-O-C(Ma et

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al., 2010), 1615cm-1 and 1436cm-1 which were ascribed to asymmetric COO-

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stretching vibration and symmetric COO- stretching vibration(Papageorgiou et al.,

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2010). In addition, The bending vibration of the N-H(1570cm-1) and –CH3(1376cm-1)

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were presented in the spectra of HA/PAN and HA/Alg(0.5)/PAN, respectively. It

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implied that HA was successfully deposited on HA/PAN and HA/Alg(0.5)/PAN.

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Meanwhile, the absorption peak of Alg(0.5)/HA/PAN was similar to that of

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Alg(0.5)/PAN which was also indicated that NaAlg was successfully deposited on

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Alg(0.5)/HA/PAN and Alg(0.5)/PAN.

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3.1.2 SEM

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Fig. 3 showed SEM cross-section images of Alg(0.5)/PAN, HA/PAN,

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Alg(0.5)/HA/PAN and HA/Alg(0.5)/PAN membranes, respectively. It was indicated

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that the active layer with a thickness about 500nm was uniformly and tightly

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deposited on the porous PAN support layer. The cross-section of Alg(0.5)/HA/PAN

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and HA/Alg(0.5)/PAN membranes in Fig. 3 (c-d) clearly showed the two-active-layer

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structure and a distinct interface between those two layers. The thickness of capping

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layers of HA/Alg/PAN and Alg/HA/PAN membranes was about 150 nm, while the

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thickness of inner layers was about 400 nm.

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Fig. 3. SEM images of cross-section: (a) Alg(0.5)/PAN; (b) HA/PAN; (c) Alg(0.5)/HA/PAN;

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and (d) HA/Alg(0.5)/PAN. Scale: 2 μm.

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3.1.3 Water contact angle

ed

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Table 1. Water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN, HA/Alg(X)/PAN

231

and HA/PAN membranes.

Membrane

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Contact angle (°)

Membrane

Contact angle (°)

52 ± 3

Alg(0)/PAN

51 ± 3

Alg(0.02)/HA/PAN 34 ± 2

Alg(0.02)/PAN

33 ± 2

Alg(0.1)/HA/PAN

24 ± 1

Alg(0.1)/PAN

25 ± 2

Alg(0.5)/HA/PAN

23 ± 1

Alg(0.5)/PAN

23 ± 1

HA/Alg(0)/PAN

37 ± 2

HA/PAN

36 ± 2

HA/Alg(0.02)/PAN 34 ± 2

--

--

HA/Alg(0.1)/PAN

33 ± 2

--

--

HA/Alg(0.5)/PAN

33 ± 2

--

--

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Alg(0)/HA/PAN

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Table 1 showed water contact angles of Alg(X)/HA/PAN, Alg(X)/PAN,

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HA/Alg(X)/PAN and HA/PAN membranes. It was observed that with the increase of

235

Ca2+ concentrations (X) in the aqueous solutions that used to crosslink NaAlg layer,

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the contact angle of Alg(X)/HA/PAN and Alg(X)/PAN membranes became smaller,

237

indicating that hydrophilicity was enhanced by Ca2+ crosslinking. When X value was

238

the same, contact angles of Alg(X)/HA/PAN and Alg(X)/PAN membranes were

239

similar, which indicated that alginic acid layer was completely deposited. Similarly,

240

contact angles of HA/Alg(X)/PAN and HA/PAN membranes proved that HA was also

241

completely deposited on membranes.

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3.2 Pervaporation studies

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3.2.1 Swelling, sorption and diffusion characteristics

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Table 2(a). The swelling behavior of HA and NaAlg homogenous membranes

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in 90 wt.% ethanol-water mixture and water.

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Degree of swelling

90 wt.% ethanol-water

Water

HA

0.06

41

NaAlg

0.03

0.64

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The equilibrium sorption percentage of crosslinked HA and NaAlg homogenous

248

membranes, which were cross-linked by glutaraldehyde and 0.5M Ca2+ respectively,

249

were investigated and results were shown in Table 2(a). Both HA and Alg membranes

250

suffered much less swelling in 10 wt.% water-ethanol mixture than in water, which

251

meant that the membranes were stable for pervaporation under low water

252

concentration. Although Ca2+ cross-linked Alg layer showed higher hydrophilicity, HA 13

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swelled more evidently than Alg in both water and the feed solution. The swelling

254

degree of polymer in various solvents was determined not only by the

255

polymer-solvent affinity but also its spatial structure. The three dimentional

256

cross-linking of Alginate acid chains by Ca2+ can effectively suppress the swelling.

257

The extended chains of HA with lower crosslinking degree in the solvent lead to a

258

looser network, which can be accessed by more solvent molecules.

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Table 2(b). The sorption and diffusion behavior of HA and NaAlg homogenous

260

membranes in 10 wt.% water-ethanol mixture

ethanol

HA

5.41

0.59

NaAlg

1.42

1.58

s 83

an

water

Di / (10-12m2/s) water

ethanol

1.75

1.12

1.6

3.73

0.05

91.1

M

Ci / (10-2g/g)

us

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8.1

Sorption selectivity indicated the affinity of the membrane toward the components

262

to be separated. The sorption amount of water and ethanol and the sorption selectivity

263

of HA and NaAlg in the feed (10 wt.% water-ethanol mixture) were also shown in

264

Table 2(b). A big difference in the sorption amount with respect to water and ethanol

265

can be observed which indicated that both the HA and NaAlg had high affinity toward

266

water. And the HA (  s=83) exhibited much higher sorption selectivity than that of

267

NaAlg (  s=8.1).

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261

268

To discriminate the effects of solubility and diffusivity on permeability, Diffusion

269

selectivity was calculated by Eq.(3). The flux and separation factor of Alg/PAN

270

one-active-layer composite membranes were 925 g/m2 h and 738, while those of

271

HA/PAN membranes were 1705 g/m2 h and 130. It showed that diffusion selectivity

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of Alg (

=91.2) was higher than that of HA (

=1.6). It can be explained by

273

concentration-averaged diffusion coefficients ( Di ) of water and ethanol in HA and

274

NaAlg layer in the active layer of HA/PAN and Alg(0.5)/PAN composite membrane.

275

According to the Fick’s law, when the concentration profile is assumed linear along

276

the diffusion length in the active layer for simplicity,

277

Di 

278

the concentration-averaged diffusion coefficients Di can be calculated by Eq.(9) .

279

Diffusion coefficient of water molecules were higher both in HA and NaAlg, While

280

diffusion coefficients of ethanol molecules in crosslinked alginate were smaller than

281

that in HA. One reason was that the water molecules (dynamic diameter, 0.26nm)

282

were smaller than ethanol molecules (dynamic diameter, 0.52nm). Furthermore, large

283

free volume voids of HA in the feed, indicated by the swelling degree, promoted the

284

diffusion coefficient of ethanol. But the voids was too large to identify water and

285

molecules, thus, the diffusion coefficient of ethanol in HA was much higher than that

286

in alginate. In a word, HA exhibited much higher water sorption selectivity than that

287

of NaAlg, whereas NaAlg exhibited higher water diffusion selectivity than that of

288

HA.

289

3.2.2 Effect of the cross-linking of Alginate layer on the separation performance

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272

Jil Ci

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(9)

290

Ca2+ cross-linking was a common method to improve alginate membrane

291

performance. Fig. 4 showed the effect of Ca2+ concentration on pervaporation

292

performance

293

HA/Alg(X)/PAN membranes for dehydration of 90 wt.% ethanol-water mixture at

of

Alg(X)/PAN,

Alg(X)/Alg(X)/PAN,

Alg(X)/HA/PAN

and

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80oC. The permeate flux of those four membranes displayed the similar trend with the

295

increase of Ca2+ concentration. Membranes made of alginate which was not

296

cross-linked by Ca2+ exhibited lower flux and separation factor, compared with

297

cross-linked membranes. With the increase of Ca2+ concentration, both of the

298

hydrophilicity and the crosslinking degree were increased, which improved the

299

permselectivity of the membranes. Thus separation factors of these membranes were

300

all increased.

us

Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN

Separation factor

1000

800

800 600 400

M

600

200

400 0.05

0.10

2+

0.15

0.50

0

0.55

ed

2

Permeation flux (g/(m h))

1000

0.00

301

(b)1200

Alg/PAN Alg/Alg/PAN HA/Alg/PAN Alg/HA/PAN

1200

an

(a)

cr

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Ca concentration(mol/L)

0.00

0.05

0.10

0.15

0.50

0.55

2+

Ca concentration(mol/L)

Fig. 4 Effect of Ca2+ concentration on pervaporation performance of membranes, Alg/PAN,

303

Alg/Alg/PAN, Alg/HA/PAN and HA/Alg/PAN, for dehydration of 90 wt.%

304

ethanol-water mixture at 80℃.

3.2.3 Effect of coating sequences on the separation performance

Ac

305

ce pt

302

306

Separation performance of various composite membranes was shown in Fig. 4.

307

Comparison between the separation performance of HA/Alg/PAN and Alg/Alg/PAN

308

membranes showed that HA/Alg/PAN possessed a higher separation factor, while

309

their fluxes were similar. It showed that HA with high water sorption selectivity and

310

water retention capacity was more suitable as the capping layer than alginate. When it

311

was used as capping layer, more water was adsorbed by HA layer, which increased the 16

Page 17 of 29

312

driving force of water diffusion through the membranes. It can be concluded that

313

materials as capping layer should possess higher hydrophilicity and larger sorption

314

capacity to intensify solution process. Comparison

between

the

separation

performance

of

Alg/HA/PAN

and

ip t

315

Alg/Alg/PAN membranes showed that Alg/Alg/PAN after crosslinked possessed a

317

higher separation factor, while their fluxes were similar. It showed that alginate was

318

more suitable as the inner layer than HA. Crosslinked alginate had higher diffusion

319

selectivity than that of HA. When it was used as inner layer, the membranes possessed

320

high separation factor, which proved that materials as inner layer should possess

321

higher diffusion selectivity and small swelling degree.

M

an

us

cr

316

Separation performance of HA/Alg/PAN and Alg/HA/PAN two-active-layer

323

composite membranes was studied to verify the validity of the selection rules. It was

324

found that the separation factor of HA/Alg/PAN was higher remarkably than that of

325

Alg/HA/PAN membranes. HA/Alg(0.5)/PAN membrane possessed a separation factor

326

of 1130 and Alg(0.5)/HA/PAN membrane possessed a separation factor of 527, while

327

their permeate fluxes were 972 g/(m2 h) and 948 g/(m2 h) respectively. This further

328

proved that capping layer with higher hydrophilicity and water capacity, and inner

329

layer with higher permselectivity could increase the separation performance of the

330

composite

331

HA/Alg(0.5)/PAN membrane with Alg as the inner layer have higher separation

332

factor(1130) than that of HA/CS/PAN(704) which further verified that Alg have

333

higher diffusion selectivity than that of CS.

Ac

ce pt

ed

322

membranes.

Meantime,

compared

with

our

previous

study,

17

Page 18 of 29

334

3.2.4 Effect of operation temperature on the separation performance

335

1000

600

800 400

600 400

200 200 300

310

320

330

340

0 360

350

80

60

40

20

300

(d)

HA/Alg/PAN Alg/HA/PAN

600

1.0

0.5

300 200

310

320

330

340

M

300

350

7

0

360

Temperature (K)

ed

3

1 2.8

2.9

3.0

3.1

3.2

330

340

350

360

water ethanol Ep,w=36.98kJ/mol

5

4

Ep,e=18.71kJ/mol

2

320

Temperature (K)

6

lnJp

ce pt

4

310

7

Ep,w=43.63kJ/mol

5

300

(f)

water ethanol

6

lnJp

360

100

(e)

Ep,e=-0.71kJ/mol

3

3.3

2.8

2.9

3.0

3.1

3.2

3.3

1000/T,K Alg/HA/PAN

1000/T,K HA/Alg/PAN

Ac

340

350

an

Selectivity

400 1.5

337

339

340

HA/Alg/PAN Alg/HA/PAN

500

2.0

0.0

330

us

2.5

320

Temperature (K)

2

Permeance(ethanol) (g/(m h kPa))

(c)

338

310

Temperature (K)

336

ip t

2

1200

HA/Alg/PAN Alg/HA/PAN

100

cr

1400

800

0

(b)

1600

HA/Alg/PAN Alg/HA/PAN

Separation factor 2 Permeance(water) (g/(m h kPa))

, ,

1000

Permeation flux (g/(m h))

(a)

Fig. 5. Effect of operation temperature on pervaporation performance of HA/Alg/PAN and Alg/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.

341

(a) Effect of operation temperature on permeation flux and separation factor of HA/Alg/PAN and

342

Alg/HA/PAN membranes.(b) Effect of operation temperature on water permeance of HA/Alg/PAN

343

and Alg/HA/PAN membranes. (c) Effect of operation temperature on ethanol permeance of

344

HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of operation temperature on selectivity of 18

Page 19 of 29

345

HA/Alg/PAN and Alg/HA/PAN membranes. (e) Arrhenius plots of permeation flux for separation

346

of ethanol/water mixture by the HA/Alg/PAN membrane.(f) Arrhenius plots of permeation flux for

347

separation of ethanol/water mixture by the Alg/HA/PAN Membrane.

Operation conditions had a remarkable influence on flux and separation factor, and

349

its variation not only changed the driving force of species permeating through

350

membranes, but also the intrinsic properties of membranes. Fig. 5 showed effect of

351

operation temperature on pervaporation performance of HA/Alg(0.5)/PAN and

352

Alg(0.5)/HA/PAN membranes for dehydration of 90 wt.% ethanol-water mixture.

353

With increasing temperature, both flux and separation factor of two membranes went

354

up markedly (as shown in Fig.5 (a)). However, the separation factor of HA/Alg/PAN

355

was higher than that of Alg/HA/PAN at all the temperature point, while fluxes of these

356

two membranes were similar.

ed

M

an

us

cr

ip t

348

The flux was mainly affected by ethanol and water vapor pressure (driving force)

358

and membrane properties. With the increase of temperature, ethanol and water vapor

359

pressure increased and the permeating driving force was enhanced, which was

360

favorable to increase of flux. In order to study the effect of temperature on membrane

361

intrinsic properties, driving force normalized permeance and selectivity were

362

presented in Fig. 5(b-d). It could be found that water and ethanol permeances of

363

Alg/HA/PAN membrane varied in a wider range than those of HA/Alg/PAN,

364

indicating

365

temperature-depended. The dependence of permeation flux on temperature could be

366

expressed by Arrhenius equation (Eq.(10)) and the apparent activation energy could

Ac

ce pt

357

that

properties

of

Alg/HA/PAN

membranes

were

more

19

Page 20 of 29

367

be obtained by equation (10).

368

J p  Ap exp( 

369

Where Ap, Jp, Ep, R and T are the pre-exponential factor, permeation flux, apparent

370

activation energy, gas content and feed temperature, respectively.

)

(10)

ip t

RT

The Arrhenius plots of lnJp versus 1000/T for water and ethanol were shown in

cr

371

Ep

Fig. 5(e,f) . A linear relationship was observed, indicating the permeability followed

373

the Arrheninus law. With the temperature ranging from 303 to 353K, the apparent

374

activation energy for water in HA/Alg(0.5)/PAN membrane and Alg/HA/PAN

375

membrane were 43.63 kJ/mol and 36.98 kJ/mol, respectively. Simultaneously, the

376

apparent activation energy for ethanol in HA/Alg(0.5)/PAN membrane and

377

Alg/HA/PAN membrane were 18.71 kJ/mol and -0.71 kJ/mol, respectively. It could be

378

found that the apparent activation energy for water and ethanol in HA/Alg(0.5)/PAN

379

membrane were larger than that in Alg/HA/PAN membrane. Therefore, with the

380

increase of temperature, the increasing rate of water flux and ethanol flux in

381

HA/Alg(0.5)/PAN membrane were greater than that in Alg/HA/PAN membrane. So

382

the data of selectivity showed that the HA/Alg/PAN membrane was more

383

advantageous than that of Alg/HA/PAN membrane for dehydration of 90 wt.%

384

ethanol-water mixture.

385

3.2.5 Effect of water concentration in feed on the separation performance

Ac

ce pt

ed

M

an

us

372

386

20

Page 21 of 29

2500

1250 2000 1000 1500 750 1000

500

500

250 0

10

20

30

40

0

HA/Alg/PAN Alg/HA/PAN

50 45 40 35 30 25 20 15

0

(d)

20

30

40

HA/Alg/PAN Alg/HA/PAN

cr

HA/Alg/PAN Alg/HA/PAN

0.25

1000

0.20

0.15

0.10

us

800

Selectivity

600

400 0.05

an

2

Permeance(ethanol) (g/(m h kPa)

(c)

10

Water concentration in feed

Water concentration in feed

387

200

0.00

0

10

20

30

40

0

10

20

30

40

Water concentration in feed

Water concentration in feed

M

388

3000

1500

0

(b)

3500

HA/Alg/PAN Alg/HA/PAN

ip t

2

Permeation flux (g/(m h))

1750

, ,

Separation factor 2 Permeance(water) (g/(m h kPa))

(a) 2000

Fig. 6. Effect of water concentration in feed on pervaporation performance of HA/Alg/PAN and

390

Alg/HA/PAN membranes for dehydration of ethanol-water mixtures containing 1 wt.%, 5 wt.%,

391

10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% water at 80℃.

ed

389

(a) Effect of water concentration on permeation flux and separation factor of HA/Alg/PAN and

393

Alg/HA/PAN membranes.(b) Effect of water concentration on water permeance of HA/Alg/PAN

394

and Alg/HA/PAN membranes.(c) Effect of water concentration on ethanol permeance of

395

HA/Alg/PAN and Alg/HA/PAN membranes. (d) Effect of water concentration on selectivity of

396

HA/Alg/PAN and Alg/HA/PAN membranes.

Ac

ce pt

392

397

Fig. 6 showed effect of water concentration in feed on pervaporation performance

398

of HA/Alg(0.5)/PAN and Alg(0.5)/HA/PAN membranes for dehydration of

399

ethanol-water mixtures containing 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.% and 40

400

wt.% water at 80oC. With increasing water concentration in feed, fluxes of both

21

Page 22 of 29

membranes increased and separation factors decreased, which resulted from enhanced

402

swelling with increasing water concentration. Driving force normalized permeance

403

and selectivity were presented in Fig. 6(b-d). It was shown that permeances of water

404

and ethanol increased remarkably, suggesting that membrane became more relaxed

405

with the increase of water concentration and was favorable to feed species through

406

membrane.

cr

ip t

401

When water concentration was below 20 wt.%, water selectivity of HA/Alg/PAN

408

was higher than that of Alg/HA/PAN; When water concentration was equal to or

409

above 20 wt.%, reverse. It was probably because the swelling of HA capping layer

410

increased with the increase of water concentration, resulting in the decrease of

411

selectivity. So HA/Alg/PAN membranes were advantageous for dehydration of

412

ethanol/water solution with low water concentration, while Alg/HA/PAN membranes

413

were advantageous for high water concentration, because of better anti-swelling

414

property of Ca2+ cross-linked NaAlg layers.

415

4. Conclusion

ce pt

ed

M

an

us

407

The two-active-layer composite membranes, which comprise the capping layer

417

and the inner layer, were prepared by sequential coating of HA and NaAlg on the PAN

418

support layer and utilized for pervaporation dehydration of ethanol-water mixtures. It

419

was shown that membranes with different coating sequences had an obvious

420

difference in the dehydration performance. Under the operation condition of 80oC and

421

10 wt.% water concentration in feed, HA/Alg/PAN membrane possessed a separation

422

factor of 1130 and Alg/HA/PAN membrane possessed a separation factor of 527,

Ac

416

22

Page 23 of 29

while the permeate fluxes of both membranes were similar. It suggested that materials

424

with higher hydrophilicity and larger sorption capacity could be selected for

425

constructing the capping layer to intensify solution process, while materials with less

426

diffusion resistance and high diffusion selectivity could be used for constructing the

427

inner layer to enhance the diffusion process.

ip t

423

Acknowledgements

us

429

cr

428

The authors gratefully acknowledge the financial support from the National Science

431

Fund for Distinguished Young Scholars (21125627), the National basic research

432

program of China (2009CB623404), the Programme of Introducing Talents of

433

Discipline to Universities (No: B06006) and the Program for Changjiang Scholars and

434

Innovative Research Team in University (PCSIRT), Tianjin Natural Science

435

Foundation (10JCZDJC22600).

Ac

ce pt

ed

M

an

430

23

Page 24 of 29

Nomenclature A

effective membrane area (m2)

Ci

concentration of component

J

permeation flux (g/m2 h)

l

thickness of the active layer (m)

P

permeability

Pi / l

permeance

pisat 0

saturated vapor pressure of pure component i

pil

partial pressure of component i in the permeate side

Q

total mass of the permeate solution in time t (g)

t

time (h)

md

weight of the dry membrane (g)

ms

weight of the swollen membrane (g)

xiL0

mole fraction of component i in the feed solution

Wf , Ef

weight fractions of water and ethanol in the feed solution

Wp , Ep

weight fractions of water and ethanol in the permeate solution

WM

weight fractions of water and ethanol in the membrane



selectivity

s

sorption selectivity

D



 i0L

ce pt

ed

M

an

us

cr

ip t

(g/m3)

Ac



in the membrane

diffusion selectivity separation factor activity coefficient of component i in the feed solution

24

Page 25 of 29

References

2

1.Baker, R. W. et al. (2010). Permeability, permeance and selectivity: A preferred way

3

of reporting pervaporation performance data. Journal of Membrane Science,

4

348(1-2), 346-352.

ip t

1

2.Bhat, S. D., & Aminabhavi, T. M. (2006). Novel sodium alginate–Na+MMT hybrid

6

composite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane

7

and tetrahydrofuran. Separation and Purification Technology, 51(1), 85-94.

us

cr

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3.Bhat, S. D., & Aminabhavi, T. M. (2007). Pervaporation Separation Using Sodium

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Alginate and Its Modified Membranes—A Review. Separation & Purification

12

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4.Binning, R. et al. (1961). Separation of Liquid Mixtures by Permeation. Industrial & Engineering Chemistry, 53(1), 45-50.

ed

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an

8

5.Ding, J. et al. (2012). Enhancing the permselectivity of pervaporation membrane by

14

constructing the active layer through alternative self-assembly and spin-coating.

15

Journal of Membrane Science, 390-391, 218-225.

ce pt

13

6.Huang, R. Y. M.et al. (1999). Characteristics of sodium alginate membranes for the

17

pervaporation dehydration of ethanol–water and isopropanol–water mixtures.

18

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Ac

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7.Kalyani, S. et al. (2006). Blend membranes of sodium alginate and

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hydroxyethylcellulose for pervaporation-based enrichment of t-butyl alcohol.

21

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8.Kanti, P. et al. (2004). Dehydration of ethanol through blend membranes of chitosan

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and sodium alginate by pervaporation. Separation and Purification Technology,

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40(3), 259-266. 9.Kim, S. J. et al. (2004). Electrical behavior of polymer hydrogel composed of

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poly(vinyl alcohol)–hyaluronic acid in solution. Biosensors and Bioelectronics,

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Polymers, 61(1), 52-60.

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of

11.Kurkuri,

M.

sodium

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al.

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cr

10.Kumar Naidu, B. V. et al. (2005). Thermal, viscoelastic, solution and membrane

Synthesis

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an

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ip t

25

Carbohydrate

characterization

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polyacrylamide-grafted sodium alginate copolymeric membranes and their use in

33

pervaporation separation of water and tetrahydrofuran mixtures. Journal of Applied

34

Polymer Science, 86(2), 272-281.

ed

M

32

12.Kurkuri, M. D.et al. (2002b). Syntheses and characterization of blend membranes

36

of sodium alginate and poly(vinyl alcohol) for the pervaporation separation of

37

water + isopropanol mixtures. Journal of Applied Polymer Science, 86(14),

38

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40

Ac

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ce pt

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13.Lonsdale, H. K. (1982). The growth of membrane technology. Journal of Membrane Science, 10(2–3), 81-181.

41

14.Ma, J. et al. (2010). Facile fabrication of structurally stable hyaluronic acid-based

42

composite membranes inspired by bioadhesion. Journal of Membrane Science,

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364(1-2), 290-297.

44

15.Magalad, V. T. et al. (2010). Preyssler type heteropolyacid-incorporated highly

26

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water-selective sodium alginate-based inorganic–organic hybrid membranes for

46

pervaporation dehydration of ethanol. Chemical Engineering Journal, 159(1-3),

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75-83. 16.Papageorgiou, S. K. et al. (2010). Metal–carboxylate interactions in metal–alginate

49

complexes studied with FTIR spectroscopy. Carbohydrate Research, 345(4),

50

469-473.

cr

ip t

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17.Peng, F. et al. (2005). Significant increase of permeation flux and selectivity of

52

poly(vinyl alcohol) membranes by incorporation of crystalline flake graphite.

53

Journal of Membrane Science, 259(1-2), 65-73.

an

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18.Shi, Y. et al. (1998). Preparation and characterization of high-performance

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dehydrating pervaporation alginate membranes. Journal of Applied Polymer

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ed

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19.Tomihata, K., & Ikada, Y. (1997). Crosslinking of hyaluronic acid with

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glutaraldehyde. Journal of Polymer Science Part A: Polymer Chemistry, 35(16),

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20.Uragami, T., & Saito, M. (1989). Studies on Syntheses and Permeabilities of

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21.Yeom, C. K. et al. (1998). Characterization of sodium alginate membrane

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Polymer Science, 67(2), 209-219. 22. Moon G. Y. et al. (1999). Novel two-ply composite membranes of chitosan and

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28

Page 29 of 29