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Vapor permeation of aqueous ethanol mixtures through agarose membranes
Journal of Membrane Science 459 (2014) 114–121
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Vapor permeation of aqueous ethanol mixtures through agarose membranes Yukiko Fujita, Masakazu Yoshikawa n Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 12 October 2013 Received in revised form 22 January 2014 Accepted 24 January 2014 Available online 15 February 2014
Vapor permeation membranes were obtained from a “green” polymer, agarose. Agarose and crosslinked agarose membranes preferentially transported water vapor from aqueous ethanol mixtures by vapor permeation. Diffusion experiments revealed that the permselectivity toward water vapor was expressed by both solubility selectivity and diffusivity selectivity. Differential calorimetric melting endotherms of the membranes were studied to clarify the state of water in the membrane. The results revealed that there were a couple of states of water in the membrane, such as bound and free (bulk) water. The present study suggests that the existence of bound water in a membrane is requisite to selectively transport water vapor from aqueous ethanol mixture by vapor permeation. In addition, preliminary study revealed that the membranes were applicable to forward osmosis. & 2014 Elsevier B.V. All rights reserved.
Keywords: Agarose Dehydration Forward osmosis Permselectivity Vapor permeation
1. Introduction Membrane separation is expected to solve the critical global issues we have faced: (1) environmental problems, such as water pollution, air pollution, and so forth, (2) energy/resource problems, such as water resources, fuel cell, concentration difference power generation, biomass production, etc., and (3) health problems, such as artificial organs, production and separation of optically pure medicines, and so forth. From above, membranes are expected to contribute to the globe as “Green Membranes” like green chemistry, which is spread as sustainable chemistry. “Green Membranes” are indispensable to support safe, secure, and sustainable society. There can be found various polymeric materials, such as synthetic polymers, derivatives of natural polymer, and natural polymers, which are utilized as candidate materials for membranes [1]. It is an interesting and an important subject to obtain membranes from natural polymers, in other words, environmentally benign “green” polymers or derivatives of them. To this end, the authors' research group has been studying molecularly imprinted membranes from cellulose acetate [2–4] or proteins from Geobacillus thermodenitrificans DSM465 [5,6], molecularly imprinted nanofiber membranes from cellulose acetate [7], pervaporation membranes from agarose [8–10], agarose/hydrophilic polymer [11], agarose/ derivative of natural polymer [12] or soybean polysaccharide [13], and vapor permeation membranes from gelatin/polyamideimide
n
Corresponding author. Tel.: þ 81 75 724 7816; fax: þ81 75 724 7800. E-mail address: [email protected] (M. Yoshikawa).
http://dx.doi.org/10.1016/j.memsci.2014.01.052 0376-7388 & 2014 Elsevier B.V. All rights reserved.
[14] or G. thermodinitrificans DSM465/polyamideimide [15]. Chiral separation was studied adopting egg shell membranes [16]. DNA, which was considered as a polyanion with a huge molecular weight, was adopted as membrane materials and chiral separation [17–19] and selective transport of oxygen with DNA complex membranes [20] were studied. Contrary to this, polycation of natural polymer or derivative of that was also expected to give membranes for separation. Quaterinized chitosan/lipid complex membranes showed chiral separation ability [21] and selective separation of bioethanol by vapor permeation [22]. Among three of critical global issues, in the present study, the authors' research group focused their attentions on the energy/ resource problems. Agarose, which is a linear polymer of alternating D-galactose and 3,6-anhydro-L-galactose from red algae, is one of the most abundant natural polymers, but it has not been used as a membrane material excepting materials for gel filtration, gel electrophoresis, and matrices for immobilizing biologically active molecules, and so forth [23]. From this, the possibility of application of agarose membrane to pervaporation of MeOH/MTBE [8,11,12] and that of aqueous organic mixtures [9,10] was investigated. Considering the application of agarose membrane to dehydration of fermentation products, vapor permeation is thought to be more practical than pervaporation; since, a given membrane is in contact with fermentation broth in pervaporation. This leads to fouling, which is a cause of reduction in flux. Contrary to this, vapor permeation, in which vapor of the permeants is contacting the membrane, eliminates the effect of not only fouling but also concentration polarization in liquid phase separation. Previous studies revealed that agarose membrane preferentially
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transported water from aqueous organic mixtures by pervaporation [9,10]. From nature of agarose and results of previous experiments [9,10], agarose is one of the promising natural polymers to provide water permeable membrane by vapor permeation. In the present study, vapor permeation of aqueous ethanol with agarose and crosslinked agarose membranes was studied. The expression mechanism of permselectivity toward water vapor was studied in terms of solubility and diffusivity selectivities, and state of water in the membrane was also investigated. In addition, possibility of application of these agarose membranes to forward osmosis was preliminarily studied.
2. Experimental 2.1. Materials Agarose (Standard Low-mr) was purchased from Bio-Rad Laboratories and used without further purification. Sodium azide, acetone, glutaraldehyde (GA), hydrochloric acid, ethanol were used without further purification. Deionized water was employed throughout the experiments.
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an experimental error. Three membrane samples were adopted for each determination. 2.5. Vapor permeation Vapor permeation was carried out at a reflux temperature of each aqueous ethanol mixture, under atmospheric pressure [ca. 0.101 MPa (ca. 1.0 atm)]. The apparatus was connected to a vacuum line and permeate was condensed and trapped by liquid nitrogen in the same manner in the pervaporation experiments [14,15]. The effective membrane area was 17.3 cm2 and the downstream pressure was maintained at 26.7–106.6 Pa (0.20–0.80 mmHg). Separation analysis was carried out on a Shimadzu GC-7APT gas chromatography instrument with a 3.0-m-long column packed with polyethylene glycol 6000 (Shimalite TPA). The separation factor, αH2 O=EtOH , is defined as
αH2 O=EtOH ¼ ðY H2 O =Y EtOH Þ=ðX H2 O =X EtOH Þ
ð2Þ
Yis are the weight fractions in permeate and Xis are in vapor in feed. 2.6. Measurement of diffusion coefficient
2.2. Membrane preparation Non-crosslinked agarose membrane (Aga) was prepared as reported previously [8]. The thickness of the membrane was around 25 μm. Crosslinked agarose membrane was prepared as follows: the agarose gel, which was prepared as above, was placed in 200 cm3 of acetone/water mixture (80/20, vol./vol.) containing 0.25 mol dm 3 of GA and 0.01 mol dm 3 of HCl. The crosslinking reaction was allowed to proceed at ambient temperature for 1 day (Aga–GA-1) or for 7 days (Aga–GA-7). After the reaction, the crosslinked agarose membranes were washed with deionized water and then dried as non-crosslinked agarose membrane (Aga) was dried [8]; in other words, the crosslinked membrane was sandwiched between two porous Teflon sheets and both the sides facing outwards were covered with a few sheets of filter papers. The crosslinked membrane, thus covered, was allowed to evaporate water at ambient temperature under the compression of the constant pressure of 104 g cm 2. The thickness of the crosslinked membranes thus obtained, such as Aga–GA-1 and Aga–GA-7, was determined to be ca. 25 μm. 2.3. IR spectroscopy IR spectra measurements were performed on membranes using Perkin-Elmer Spectrum GX: 64 scans at a resolution of 4 cm 1. 2.4. Water content The water content was measured as follows: after the membrane was dried in vacuo at 50 1C for over 24 h to the constant weight, the dried membrane was immersed in deionized water at 40 1C for 1 week. After the swelling equilibrium was attained, the membrane sample was weighed immediately after blotting free the surface water. The water content Wt (g-H2O/g-membrane) is defined by W t ¼ ðW s –W d Þ=W d
ð1Þ
where Ws and Wd denote the weights of water swollen and dry membrane, respectively. The weight of dry membrane, Wd, was determined after drying the water swollen membrane under vacuum at 50 1C for 24 h. The difference in weight of the membrane before water content measurement and that after measurement was within
The permeation of H2O or ethanol vapor through the membrane in the present study was measured at 92 1C, which corresponds to the reflux temperature for a weight fraction of H2O in liquid feed of ca. 0.90 [24]. From the time–transport curve for each vapor, the apparent diffusion coefficient was determined by applying the time-lag method [1,25–27]. 2.7. Differential scanning calorimetry (DSC) of water in the membrane The differential scanning calorimeter used in the present study was a Shimadzu DSC-60. A membrane sample, which was prepared in the same way as described for the measurement of the water content, was hermetically sealed to prevent evaporation. The sample was cooled down to 80 1C by using liquid nitrogen and then heated at a scanning rate of 10 1C min 1 up to 30 1C. Nitrogen at the flow rate of 300 cm3 min 1 was used throughout all DSC measurements. The bulk (freezing) water content was determined from the area of melting endotherm concerned with the amount of freezing water. The bound (non-freezing) water content was determined from the difference between the freezing water and total water content. The total water content was varied in order to study the influence of the total water content on the states of water in the membrane. The adjustment of total water content was performed by allowing the membrane to evaporate at ambient atmosphere for a suitable time.
3. Results and discussion 3.1. Preparation of membranes Fig. 1 shows the scheme for the introduction of crosslinking into agarose membrane by glutaraldehyde (GA). Hydroxyl groups in agarose were involved in the crosslinking reaction between agarose and GA. In other words, the introduction of crosslinking by GA leads to the increase in amount of methylene linkages and simultaneous decrease in that of hydroxyl group in the membrane. These changes would be reflected in IR spectra of the pristine and crosslinked agarose membranes. The IR spectra for these membranes are shown
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Fig. 1. Scheme of crosslinking reaction of agarose (Aga) with glutaraldehyde (GA).
in Fig. 2. Adopting the absorption band at the wave number of 2902 cm 1, which was assigned to the stretching band of C–H, and that of 3301 cm 1, assigned to O–H stretching band, the absorbance ratios of C–H stretching band to O–H stretching band, ACH/AOH, for the three types of membrane were determined. As shown in Fig. 2, the ACH/AOH ratios were increased in the following order: AgaoAga– GA-1oAga–GA-7. This order revealed that the crosslinking was introduced by the reaction with GA and the degree of crosslinking was increased with the increase in the crosslinking time, though the degree of crosslinking could not be determined quantitatively. The introduction of crosslinking was also confirmed by the water content, Wt. Water contents for these membranes were determined as follows: Wt for Aga, 10.8 70.1 g-H2O/g-membrane; for Aga–GA-1, 7.0 70.2 g-H2O/g-membrane; and for Aga–GA-7, 4.17 0.3 g-H2O/g-membrane. The water content decreased with the introduction of crosslinking into the membrane. Such phenomena were often observed after the introduction of crosslinking into hydrophilic polymeric materials [28–31]. 3.2. Vapor permeation of aqueous ethanol mixtures Fig. 2. FT-IR spectra of Aga (a), Aga–GA-1 (b), and Aga–GA-7 (c).
The results of vapor permeation of aqueous ethanol vapor mixtures through the present membranes are shown in Fig. 3. The membrane performances for the present membranes are plotted against weight fraction of water in feed solution. As expected, the three types of membrane preferentially transported water vapor from aqueous ethanol vapor mixtures. The introduction of crosslinking into the membrane led to the decrease in flux as often observed [32–34]. Against expectation, the permselectivity toward water was slightly enhanced by the introduction of crosslinking.
In the present study, Aga–GA-1, which was treated with GA for 1 day, showed the maximal permselectivity toward water vapor. 3.3. Mechanism for the expression of permselectivity toward water vapor There are a couple of methods to study the factors governing permselectivity toward water vapor. One is as follows: diffusivity
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selectivity (SDðH2 O=EtOHÞ ), which is defined in Eq. (3), is determined from the separation factor (αH2 O=EtOH ) and solubility selectivity (SSðH2 O=EtOHÞ ), which is defined in Eq. (4), obeying the solution– diffusion theory [1,35]. In other words, SDðH2 O=EtOHÞ is determined by Eq. (5) using αH2 O=EtOH and SSðH2 O=EtOHÞ . In this case, SSðH2 O=EtOHÞ is determined by conducting sorption experiments. SDðH2 O=EtOHÞ ¼ DH2 O =DEtOH
ð3Þ
where DH2 O and DEtOH are the diffusion coefficients of H2O and EtOH, respectively. SSðH2 O=EtOHÞ ¼ SH2 O =SEtOH
ð4Þ
where SH2 O and SEtOH are the solubility coefficients of H2O and EtOH, respectively. SDðH2 O=EtOHÞ ¼ aH2 O=EtOH =SSðH2 O=EtOHÞ
ð5Þ
The other method is a reverse way, that is, SSðH2 O=EtOHÞ is determined from αðH2 O=EtOHÞ and SDðH2 O=EtOHÞ as given in Eq. (6), where SDðH2 O=EtOHÞ is determined by the time–transport curve [25–27,35]. SSðH2 O=EtOHÞ ¼ aH2 O=EtOH =SDðH2 O=EtOHÞ
ð6Þ
As often suggested in pervaporation [36], the distribution of permeant in a given membrane for vapor permeation and that for a sorption experiment might be considerably different. In the distribution for vapor permeation, one surface of the membrane, which is in contact with the feed vapor, is swollen like pervaporation. The state of the membrane in contact with the feed vapor may be considered to resemble that of the membrane for the sorption experiment. The other
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surface of the membrane, which is in contact with vacuum in vapor permeation, maintains its dry state and contributes to the expression of permselectivity like pervaporation. The state of the membrane on the downstream side is considerably different from that of the upstream side. In other words, the state of the membrane surface in contact with vacuum, which plays an important role for permselectivity, is different from that of the sorption experiment even though the sorption experiment is carried out in a vapor phase. From this, the actual solubility selectivity should be higher than the estimated value using sorption data. That is, values of solubility selectivity obtained by sorption experiment can be considered to be minimum values. In contrast, the diffusivity selectivity calculated in Eq. (5) can be considered to be maximal values; in other words, the actual diffusivity selectivity should be lower than that estimated by using αðH2 O=EtOHÞ and SSðH2 O=EtOHÞ . From above, the governing factors elucidated by the latter method might have a greater accuracy than that by the former method. Consequently, diffusivity selectivity was determined so that the factors governing the permselectivity toward water vapor could be elucidated. Typical time–transport curves obtained experimentally of H2O vapor and ethanol vapor through the Aga–GA-1 membrane at 92 1C are shown in Fig. 4. In the present study, vapor permeation experiments were carried out at each reflux temperature, which was not equal to 92 1C, which was the reflux temperature of the aqueous ethanol solution with the weight fraction of water being 0.90 [24]. But diffusion coefficients were measured at 92 1C in the present study. Diffusion coefficients of H2O and ethanol in the present membranes were determined by the time-lag method [25–27,35] from the permeation data of pure vapor at a prescribed vapor pressure. Strictly speaking, the obtained diffusion coefficients in the present study are apparent diffusion coefficients since neither sorption isotherms of H2O
Fig. 3. Effect of feed composition on the vapor permeation of H2O/EtOH mixtures through various membranes. (Downstream pressure, 26.7–106.6 Pa (0.20–0.80 mmHg); operating temperature, each reflux temperature.)
Fig. 4. Time–transport curves of H2O vapor (a) and EtOH vapor (b) through Aga–GA-1 membrane at 92 1C.
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In these two Eqs. (7) and (8), D0 is the D value as the permeant concentration approaches 0; γ and β are the coefficients characteristic of the membrane/permeant interaction and C denotes the permeant concentration in the membrane. The latter equation, Eq. (8), is used to simulate the concentration dependence of both H2O and ethanol diffusion in the present study. D0 and β values for the present membranes are summarized in Table 3 together with the diffusivity selectivities (SDðH2 O=EtOHÞ ). The D0 values for H2O and EtOH were decreased with the introduction of crosslinking into the membrane as expected. The β values for H2O were decreased with the introduction of crosslinking. This is due to the suppression of swelling by the introduction of crosslinking. Contrary to this, the β values for EtOH were slightly increased with the introduction of crosslinking. The increase in amount of methylene linkages and simultaneous decrease in that of hydroxyl group in the membrane led to enhance the affinity between membrane and EtOH and, as a result, to a slight increase in β values. In the case that the diffusivity selectivities summarized in Table 3 were adopted, the solubility selectivities were calculated by using Eq. (6) in the present study. The determined solubility selectivities for the three types of membrane are shown in Fig. 6. Strictly speaking, the solubility selectivities for the membranes at the weight fraction of water in feed solution of 0.9 are thought to be correct values. Others
and EtOH in the membranes nor state of the membranes were known [37,38]. It is necessary to know the diffusivity selectivity of the active layer, which is thought to govern permselectivity and to be in the dry state. To accomplish this, vapor with low activity should be in contact with the membrane. In the present study, each vapor, of which activity was below 0.03 at 92 1C, was in contact with the membrane. This enabled the diffusion coefficient to be determined for a given vapor in the active layer of the membrane in vapor permeation. The resulting determined diffusion coefficients for H2O and ethanol in the present membranes are summarized in Tables 1 and 2, respectively. As an example, the diffusion coefficients for H2O and ethanol for Aga–GA-1 membrane are plotted as a function of activity of vapor in Fig. 5. The diffusion coefficient often depends on the local concentration of permeant itself [39–46]. In some cases, the concentration dependence of the diffusion coefficient has been reported to be linear by D ¼ D0 ð1 þ γ CÞ
ð7Þ
And in other cases, an exponential form was observed, which can be represented by D ¼ D0 expðβ CÞ
ð8Þ
Table 1 Diffusion coefficients of H2O through the membranes. p (Pa)
800 1200 1600 2000
p (mmHg)
6 9 12 15
a ( ¼p/p0)
Aga
0.011 0.016 0.021 0.026
Aga–GA-1
Aga–GA-7
θ/s
D (cm2 s 1)
θ/s
D (cm2 s 1)
θ/s
D (cm2 s 1)
166 132 107 85
6.28 10 9 7.89 10 9 9.74 10 9 1.23 10 8
230 190 163 139
4.53 10 9 5.48 10 9 6.39 10 9 7.49 10 9
256 220 191 171
4.07 10 9 4.73 10 9 5.45 10 9 6.09 10 9
p0 ¼ 75.6 kPa (567.1 mmHg); membrane thickness, ca. 25 μm. Table 2 Diffusion coefficients of EtOH through the membranes. p (Pa)
2400 3200 4000 4800
p (mmHg)
18 24 30 36
a ( ¼p/p0)
0.014 0.019 0.023 0.028
Aga
Aga–GA-1 2
1
θ/s
D/cm s
1195 1083 979 877
8.72 10 10 9.62 10 10 1.06 10 9 1.19 10 9
Aga–GA-7 2
1
θ/s
D/cm s
1333 1185 1073 968
7.81 10 10 8.79 10 10 9.71 10 10 1.08 10 9
θ/s
D/cm2 s 1
1442 1274 1136 1030
7.22 10 10 8.18 10 10 9.17 10 10 1.01 10 9
p0 ¼ 170 kPa (1276.7 mmHg); membrane thickness, ca. 25 μm.
Fig. 5. Concentration dependence of diffusion coefficients of water (a) and ethanol vapor (b) through Aga–GA-1 membrane at 92 1C. (The saturation vapor pressure p0 of water at 92 1C is 75.6 kPa (567.1 mmHg); that of ethanol at 92 1C is 170 kPa (1276.7 mmHg).)
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Table 3 Determined diffusion coefficients, β values, and diffusivity selectivities. Membrane
Aga Aga–GA-1 Aga–Ga-7 a
H2O
SDðH2 O=EtOHÞ a
EtOH
109D0/cm2 s 1
β
1010D0/cm2 s 1
β
3.86 3.17 3.05
41.4 33.3 27.0
6.35 5.68 5.16
22.3 23.0 24.4
6.08 5.58 5.91
SDðH2 O=EtOHÞ ¼DðH2 O=EtOHÞ .
Fig. 7. DSC heating curves of dry Aga–GA-1 membrane (a), Aga–GA-1/H2O (b)–(h), and pure H2O (i).
Fig. 6. Effect of feed composition on solubility selectivity, SSðH2 O=EtOHÞ , for the present membranes. (The horizontal broken line shows a solubility selectivity of unity.)
from 0.7 to 0.1 are reference values. The solubility selectivity toward water for the membranes showed a preference for water over ethanol. The lowest solubility selectivity toward water vapor for Aga membrane was determined at the weight fraction of water in feed solution of 0.90. The determined solubility selectivity toward water vapor was calculated to be 1.05 using αH2 O=EtOH and SDðH2 O=EtOHÞ , which was still over unity. The results obtained in the present study revealed that the permselectivity toward water in vapor permeation was expressed by both diffusivity selectivity and solubility selectivity. In Fig. 6, solubility selectivity had inclination to decrease with the increase in weight fraction of water in feed solution. This is due to the swelling of membrane by water vapor. 3.4. State of water in the membrane The anomalous properties of water in plant and animal cells and tissues, synthetic polymers, and especially in the synthetic polymeric membranes have been extensively studied. The attempts to correlate these anomalous properties with the state of water in such substances have met with varying degrees of success [47,48]. In reverse osmosis, it was reported that the state of water, which was preferentially transported through a given reverse osmosis membrane, was different from that of bulk water [49–55]. In other words, melting point depression of water in the reverse osmosis membrane was observed. In pervaporation, similar phenomena were observed in water permeable membranes [9,10,13,56,57] and alcohol permselective polydimethylsiloxane membrane [58–60]. Bound water in agarose had been observed by 1H NMR [61] and thermal analyses [62]. In the present study, the states of water in three types of membrane were studied and the relationship between the state and their membrane performance was investigated. Such an approach had not been carried out in vapor permeation as far as we have known.
Fig. 8. Relationship between enthalpic heat for H2O in Aga–GA-1 membrane and total H2O content.
Melting endotherms of water in Aga–GA-1 membrane, of dry Aga– GA-1 membrane, and of bulk water are shown in Fig. 7 as an example. The area of endothermic curve at 0 1C decreased with decrease in water content in the membrane (Fig. 7(d)–(h)). In Fig. 7(b) and (c), endotherms of water were hardly observed around its melting point of 0 1C, even though water was still present in Aga–GA-1 membrane. No other new endotherm was observed in the temperature range 80 1C to 30 1C. The membrane in Fig. 7(b) contained 6.50 10 5 g of water. The endotherm of 1.00 10 4 g of bulk water is shown in Fig. 7(i). If most of the water in Aga–GA-1 membrane could be assigned as bulk water, the endotherms in Fig. 7(c) and (b) should be observed. Fig. 8 shows the relationship between enthalpic heat and water content for Aga–GA-1 membrane. The observed points in Fig. 8 fall on a straight line with a slope equal to the heat of melting for water of 333.4 J g 1 (79.8 kcal g 1) [63]. Extrapolation of ΔH ¼0 intercepts the water content axis at a point that is the total bound water content of the sample. The bound water content, Wb, was determined to be 0.45 g-H2O/g-membrane for Aga–GA-1 membrane. For other membranes it was determined to be 0.19 g-H2O/gmembrane for Aga and 0.26 g-H2O/g-membrane for Aga–GA-7. It is an interesting and indispensable task to elucidate the relationship between nature of water in the membrane and
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Fig. 9. Relationship between separation factor (αðH2 O=EtOHÞ ) and total water content (a), amount of bound water (b), and fractional bound water (c) in the membrane.
permselectivity. This will lead to future molecular engineering to obtain suitable vapor permeation membrane materials for the dehydration of bioethanol. To this end, permselectivities toward water are plotted against total water content in the membrane (Fig. 9(a)), amount of bound water in the membrane (Fig. 9(b)), and fractional bound water in the membrane (FBW) (Fig. 9(c)). The fractional bound water in the membrane, FBW, is defined by FBW ¼ W b =W t
ð9Þ
From Fig. 9, as reported previously [13,57], permselectivity toward water gave a good correlation with fractional bound water, FBW. The bound water in water permeable vapor permeation membrane would not dissolve ethanol like bound water in reverse osmosis membranes rejected salts [52]. In the present study, membranes with fractional bound water of unity were not obtained. Vapor permeation membranes, of which water consists of just bound water, gave higher permselectivity toward water as observed in pervaporation membranes from quaterinized poly[3(N0 , N0 -dimethyl)aminopropylacrylamide-co-acrylonitrile] membranes [57].
4. Preliminary study on forward osmosis Nowadays, renewable and clean energy is required in order to realize a safe, secure, and sustainable society. Naturally occurring energy, such as tidal energy, geothermal one, wind one, solar one, and so forth are potential energy sources for our future. Among them, the mixing of fresh water with seawater is one of the plausible ones. Osmotic salination converter [64] and generation of electric power by mixing of fresh and seawater [65] cannot be attained without membranes. Membranes play an important role in these challenges: a water permeable membrane plays a leading character in the former challenge and both cation and anion exchange membranes a leading role in the latter one. The present membranes showed permselectivity toward water. This stimulated us to survey the potential of forward osmosis [66–68] for the membranes. 25.0 wt% aqueous NaCl solution was adopted as draw solution and pure water as feed. Water flows were determined to be 6.61 L m 2 h 1 for Aga membrane, 5.37 L m 2 h 1 for Aga–GA-1, and 5.27 L m 2 h 1 for Aga–GA-7.
5. Conclusions Vapor permeation membranes were obtained from a “green” polymer, agarose. Separation of aqueous ethanol mixtures through the agarose and crosslinked agarose membranes was studied by vapor permeation. These membranes preferentially transported water vapor. Diffusion experiments revealed that the permselectivity
toward water vapor was expressed by both solubility selectivity and diffusivity selectivity. Differential calorimetric melting endotherms of the membranes were studied to clarify the state of water in the membrane. The results revealed that there were a couple of states of water in the membrane, such as bound and free (bulk) water. The present study suggests that the existence of bound water in a membrane is requisite to preferentially transport water vapor from aqueous ethanol solution by vapor permeation. In addition, preliminary study revealed that these membranes were applicable to forward osmosis. References [1] M. Mulder, Basic Principles of Membrane Technology, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1996. [2] J. Izumi, M. Yoshikawa, T. Kitao, Enantioselective permeation of racemic glutamic acid through alternative molecularly imprinted cellulose acetate membranes, Membrane 22 (1997) 149–154. [3] M. Yoshikawa, T. Ooi, J. Izumi, Alternative molecularly imprinted membranes from a derivative of natural polymer, cellulose acetate, J. Appl. Polym. Sci. 72 (1999) 493–499. [4] M. Yoshikawa, J. Izumi, M.D. Guiver, G.P. Robertson, Recognition and selective transport of nucleic acid components through molecularly imprinted polymeric membranes, Macromol. Mater. Eng. 286 (2001) 52–59. [5] M. Yoshikawa, K. Kawamura, A. Ejima, T. Aoki, S. Sakurai, K. Hayashi, K. Watanabe, Green polymers from Geobacillus thermodenitrificans DSM-465 – candidates for molecularly imprinted materials, Macromol. Biosci. 6 (2006) 210–215. [6] M. Yoshikawa, K. Kawamura, K. Watanabe, Evaluation of the recognition ability of molecularly imprinted proteins by surface plasmon resonance (SPR) spectroscopy, Membrane 32 (2007) 42–44. [7] Y. Sueyoshi, C. Fukushima, M. Yoshikawa, Molecularly imprinted nanofiber membranes from cellulose acetate aimed for chiral separation, J. Membr. Sci. 357 (2010) 90–97. [8] M. Yoshikawa, T. Yoshioka, J. Fujime, A. Murakami, Pervaporation separation of MeOH/MTBE through agarose membranes, J. Membr. Sci. 178 (2000) 75–78. [9] K. Masaki, M. Yoshikawa, Pervaporation of aqueous organic mixtures through agarose membranes, Membrane 26 (2001) 155–157. [10] M. Yoshikawa, K. Masaki, M. Ishikawa, Pervaporation separation of aqueous organic mixtures through agarose membrane, J. Membr. Sci. 205 (2002) 293–300. [11] M. Yoshikawa, T. Yoshioka, J. Fujime, A. Murakami, Pervaporation separation of MeOH/MTBE with hydrophilic polymer/agarose blended membranes, Membrane 26 (2001) 259–264. [12] M. Yoshikawa, T. Yoshioka, J. Fujime, A. Murakami, Pervaporation of methanol/ methyl tert-butyl ether mixtures through agarose/hydroxyethylcellulose blended membranes, J. Appl. Polym. Sci. 86 (2002) 3408–3411. [13] M. Yoshikawa, K. Nakai, D. Komatsubara, A. Nakao, State of water in soybean polysaccharide membranes and their pervaporation performance, Membrane 33 (2008) 118–123. [14] M. Yoshikawa, A. Higuchi, M. Ishikawa, M.D Guiver, G.P. Robertson, Vapor permeation of aqueous 2-propanol solutions through gelatin/Torlons poly (amide/imide) blended membranes, J. Membr. Sci. 243 (2004) 89–95. [15] M. Yoshikawa, K. Kawamura, A. Ejima, T. Aoki, K. Watanabe, M.D Guiver, G.P. Robertson, Thermostable natural proteon polymers from Geobacillus thermodenitrificans DSM465 as membrane materials from vapor permeation, Membrane 29 (2004) 384–387. [16] Y. Kondo, M. Yoshikawa, Egg shell membranes for chiral separation, Membrane 26 (2001) 228–230. [17] M. Yoshikawa, M. Maruhashi, Y. Iwamoto, N. Ogata, Chiral separation or racemic amino acids through DNA–polydiallyldimethylammonium polyion complex membranes, Macromol. Symp. 249–250 (2007) 557–561.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Vapor permeation of aqueous ethanol mixtures through agarose membranes Yukiko Fujita, Masakazu Yoshikawa n Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 12 October 2013 Received in revised form 22 January 2014 Accepted 24 January 2014 Available online 15 February 2014
Vapor permeation membranes were obtained from a “green” polymer, agarose. Agarose and crosslinked agarose membranes preferentially transported water vapor from aqueous ethanol mixtures by vapor permeation. Diffusion experiments revealed that the permselectivity toward water vapor was expressed by both solubility selectivity and diffusivity selectivity. Differential calorimetric melting endotherms of the membranes were studied to clarify the state of water in the membrane. The results revealed that there were a couple of states of water in the membrane, such as bound and free (bulk) water. The present study suggests that the existence of bound water in a membrane is requisite to selectively transport water vapor from aqueous ethanol mixture by vapor permeation. In addition, preliminary study revealed that the membranes were applicable to forward osmosis. & 2014 Elsevier B.V. All rights reserved.
Keywords: Agarose Dehydration Forward osmosis Permselectivity Vapor permeation
1. Introduction Membrane separation is expected to solve the critical global issues we have faced: (1) environmental problems, such as water pollution, air pollution, and so forth, (2) energy/resource problems, such as water resources, fuel cell, concentration difference power generation, biomass production, etc., and (3) health problems, such as artificial organs, production and separation of optically pure medicines, and so forth. From above, membranes are expected to contribute to the globe as “Green Membranes” like green chemistry, which is spread as sustainable chemistry. “Green Membranes” are indispensable to support safe, secure, and sustainable society. There can be found various polymeric materials, such as synthetic polymers, derivatives of natural polymer, and natural polymers, which are utilized as candidate materials for membranes [1]. It is an interesting and an important subject to obtain membranes from natural polymers, in other words, environmentally benign “green” polymers or derivatives of them. To this end, the authors' research group has been studying molecularly imprinted membranes from cellulose acetate [2–4] or proteins from Geobacillus thermodenitrificans DSM465 [5,6], molecularly imprinted nanofiber membranes from cellulose acetate [7], pervaporation membranes from agarose [8–10], agarose/hydrophilic polymer [11], agarose/ derivative of natural polymer [12] or soybean polysaccharide [13], and vapor permeation membranes from gelatin/polyamideimide
n
Corresponding author. Tel.: þ 81 75 724 7816; fax: þ81 75 724 7800. E-mail address: [email protected] (M. Yoshikawa).
http://dx.doi.org/10.1016/j.memsci.2014.01.052 0376-7388 & 2014 Elsevier B.V. All rights reserved.
[14] or G. thermodinitrificans DSM465/polyamideimide [15]. Chiral separation was studied adopting egg shell membranes [16]. DNA, which was considered as a polyanion with a huge molecular weight, was adopted as membrane materials and chiral separation [17–19] and selective transport of oxygen with DNA complex membranes [20] were studied. Contrary to this, polycation of natural polymer or derivative of that was also expected to give membranes for separation. Quaterinized chitosan/lipid complex membranes showed chiral separation ability [21] and selective separation of bioethanol by vapor permeation [22]. Among three of critical global issues, in the present study, the authors' research group focused their attentions on the energy/ resource problems. Agarose, which is a linear polymer of alternating D-galactose and 3,6-anhydro-L-galactose from red algae, is one of the most abundant natural polymers, but it has not been used as a membrane material excepting materials for gel filtration, gel electrophoresis, and matrices for immobilizing biologically active molecules, and so forth [23]. From this, the possibility of application of agarose membrane to pervaporation of MeOH/MTBE [8,11,12] and that of aqueous organic mixtures [9,10] was investigated. Considering the application of agarose membrane to dehydration of fermentation products, vapor permeation is thought to be more practical than pervaporation; since, a given membrane is in contact with fermentation broth in pervaporation. This leads to fouling, which is a cause of reduction in flux. Contrary to this, vapor permeation, in which vapor of the permeants is contacting the membrane, eliminates the effect of not only fouling but also concentration polarization in liquid phase separation. Previous studies revealed that agarose membrane preferentially
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transported water from aqueous organic mixtures by pervaporation [9,10]. From nature of agarose and results of previous experiments [9,10], agarose is one of the promising natural polymers to provide water permeable membrane by vapor permeation. In the present study, vapor permeation of aqueous ethanol with agarose and crosslinked agarose membranes was studied. The expression mechanism of permselectivity toward water vapor was studied in terms of solubility and diffusivity selectivities, and state of water in the membrane was also investigated. In addition, possibility of application of these agarose membranes to forward osmosis was preliminarily studied.
2. Experimental 2.1. Materials Agarose (Standard Low-mr) was purchased from Bio-Rad Laboratories and used without further purification. Sodium azide, acetone, glutaraldehyde (GA), hydrochloric acid, ethanol were used without further purification. Deionized water was employed throughout the experiments.
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an experimental error. Three membrane samples were adopted for each determination. 2.5. Vapor permeation Vapor permeation was carried out at a reflux temperature of each aqueous ethanol mixture, under atmospheric pressure [ca. 0.101 MPa (ca. 1.0 atm)]. The apparatus was connected to a vacuum line and permeate was condensed and trapped by liquid nitrogen in the same manner in the pervaporation experiments [14,15]. The effective membrane area was 17.3 cm2 and the downstream pressure was maintained at 26.7–106.6 Pa (0.20–0.80 mmHg). Separation analysis was carried out on a Shimadzu GC-7APT gas chromatography instrument with a 3.0-m-long column packed with polyethylene glycol 6000 (Shimalite TPA). The separation factor, αH2 O=EtOH , is defined as
αH2 O=EtOH ¼ ðY H2 O =Y EtOH Þ=ðX H2 O =X EtOH Þ
ð2Þ
Yis are the weight fractions in permeate and Xis are in vapor in feed. 2.6. Measurement of diffusion coefficient
2.2. Membrane preparation Non-crosslinked agarose membrane (Aga) was prepared as reported previously [8]. The thickness of the membrane was around 25 μm. Crosslinked agarose membrane was prepared as follows: the agarose gel, which was prepared as above, was placed in 200 cm3 of acetone/water mixture (80/20, vol./vol.) containing 0.25 mol dm 3 of GA and 0.01 mol dm 3 of HCl. The crosslinking reaction was allowed to proceed at ambient temperature for 1 day (Aga–GA-1) or for 7 days (Aga–GA-7). After the reaction, the crosslinked agarose membranes were washed with deionized water and then dried as non-crosslinked agarose membrane (Aga) was dried [8]; in other words, the crosslinked membrane was sandwiched between two porous Teflon sheets and both the sides facing outwards were covered with a few sheets of filter papers. The crosslinked membrane, thus covered, was allowed to evaporate water at ambient temperature under the compression of the constant pressure of 104 g cm 2. The thickness of the crosslinked membranes thus obtained, such as Aga–GA-1 and Aga–GA-7, was determined to be ca. 25 μm. 2.3. IR spectroscopy IR spectra measurements were performed on membranes using Perkin-Elmer Spectrum GX: 64 scans at a resolution of 4 cm 1. 2.4. Water content The water content was measured as follows: after the membrane was dried in vacuo at 50 1C for over 24 h to the constant weight, the dried membrane was immersed in deionized water at 40 1C for 1 week. After the swelling equilibrium was attained, the membrane sample was weighed immediately after blotting free the surface water. The water content Wt (g-H2O/g-membrane) is defined by W t ¼ ðW s –W d Þ=W d
ð1Þ
where Ws and Wd denote the weights of water swollen and dry membrane, respectively. The weight of dry membrane, Wd, was determined after drying the water swollen membrane under vacuum at 50 1C for 24 h. The difference in weight of the membrane before water content measurement and that after measurement was within
The permeation of H2O or ethanol vapor through the membrane in the present study was measured at 92 1C, which corresponds to the reflux temperature for a weight fraction of H2O in liquid feed of ca. 0.90 [24]. From the time–transport curve for each vapor, the apparent diffusion coefficient was determined by applying the time-lag method [1,25–27]. 2.7. Differential scanning calorimetry (DSC) of water in the membrane The differential scanning calorimeter used in the present study was a Shimadzu DSC-60. A membrane sample, which was prepared in the same way as described for the measurement of the water content, was hermetically sealed to prevent evaporation. The sample was cooled down to 80 1C by using liquid nitrogen and then heated at a scanning rate of 10 1C min 1 up to 30 1C. Nitrogen at the flow rate of 300 cm3 min 1 was used throughout all DSC measurements. The bulk (freezing) water content was determined from the area of melting endotherm concerned with the amount of freezing water. The bound (non-freezing) water content was determined from the difference between the freezing water and total water content. The total water content was varied in order to study the influence of the total water content on the states of water in the membrane. The adjustment of total water content was performed by allowing the membrane to evaporate at ambient atmosphere for a suitable time.
3. Results and discussion 3.1. Preparation of membranes Fig. 1 shows the scheme for the introduction of crosslinking into agarose membrane by glutaraldehyde (GA). Hydroxyl groups in agarose were involved in the crosslinking reaction between agarose and GA. In other words, the introduction of crosslinking by GA leads to the increase in amount of methylene linkages and simultaneous decrease in that of hydroxyl group in the membrane. These changes would be reflected in IR spectra of the pristine and crosslinked agarose membranes. The IR spectra for these membranes are shown
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Fig. 1. Scheme of crosslinking reaction of agarose (Aga) with glutaraldehyde (GA).
in Fig. 2. Adopting the absorption band at the wave number of 2902 cm 1, which was assigned to the stretching band of C–H, and that of 3301 cm 1, assigned to O–H stretching band, the absorbance ratios of C–H stretching band to O–H stretching band, ACH/AOH, for the three types of membrane were determined. As shown in Fig. 2, the ACH/AOH ratios were increased in the following order: AgaoAga– GA-1oAga–GA-7. This order revealed that the crosslinking was introduced by the reaction with GA and the degree of crosslinking was increased with the increase in the crosslinking time, though the degree of crosslinking could not be determined quantitatively. The introduction of crosslinking was also confirmed by the water content, Wt. Water contents for these membranes were determined as follows: Wt for Aga, 10.8 70.1 g-H2O/g-membrane; for Aga–GA-1, 7.0 70.2 g-H2O/g-membrane; and for Aga–GA-7, 4.17 0.3 g-H2O/g-membrane. The water content decreased with the introduction of crosslinking into the membrane. Such phenomena were often observed after the introduction of crosslinking into hydrophilic polymeric materials [28–31]. 3.2. Vapor permeation of aqueous ethanol mixtures Fig. 2. FT-IR spectra of Aga (a), Aga–GA-1 (b), and Aga–GA-7 (c).
The results of vapor permeation of aqueous ethanol vapor mixtures through the present membranes are shown in Fig. 3. The membrane performances for the present membranes are plotted against weight fraction of water in feed solution. As expected, the three types of membrane preferentially transported water vapor from aqueous ethanol vapor mixtures. The introduction of crosslinking into the membrane led to the decrease in flux as often observed [32–34]. Against expectation, the permselectivity toward water was slightly enhanced by the introduction of crosslinking.
In the present study, Aga–GA-1, which was treated with GA for 1 day, showed the maximal permselectivity toward water vapor. 3.3. Mechanism for the expression of permselectivity toward water vapor There are a couple of methods to study the factors governing permselectivity toward water vapor. One is as follows: diffusivity
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selectivity (SDðH2 O=EtOHÞ ), which is defined in Eq. (3), is determined from the separation factor (αH2 O=EtOH ) and solubility selectivity (SSðH2 O=EtOHÞ ), which is defined in Eq. (4), obeying the solution– diffusion theory [1,35]. In other words, SDðH2 O=EtOHÞ is determined by Eq. (5) using αH2 O=EtOH and SSðH2 O=EtOHÞ . In this case, SSðH2 O=EtOHÞ is determined by conducting sorption experiments. SDðH2 O=EtOHÞ ¼ DH2 O =DEtOH
ð3Þ
where DH2 O and DEtOH are the diffusion coefficients of H2O and EtOH, respectively. SSðH2 O=EtOHÞ ¼ SH2 O =SEtOH
ð4Þ
where SH2 O and SEtOH are the solubility coefficients of H2O and EtOH, respectively. SDðH2 O=EtOHÞ ¼ aH2 O=EtOH =SSðH2 O=EtOHÞ
ð5Þ
The other method is a reverse way, that is, SSðH2 O=EtOHÞ is determined from αðH2 O=EtOHÞ and SDðH2 O=EtOHÞ as given in Eq. (6), where SDðH2 O=EtOHÞ is determined by the time–transport curve [25–27,35]. SSðH2 O=EtOHÞ ¼ aH2 O=EtOH =SDðH2 O=EtOHÞ
ð6Þ
As often suggested in pervaporation [36], the distribution of permeant in a given membrane for vapor permeation and that for a sorption experiment might be considerably different. In the distribution for vapor permeation, one surface of the membrane, which is in contact with the feed vapor, is swollen like pervaporation. The state of the membrane in contact with the feed vapor may be considered to resemble that of the membrane for the sorption experiment. The other
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surface of the membrane, which is in contact with vacuum in vapor permeation, maintains its dry state and contributes to the expression of permselectivity like pervaporation. The state of the membrane on the downstream side is considerably different from that of the upstream side. In other words, the state of the membrane surface in contact with vacuum, which plays an important role for permselectivity, is different from that of the sorption experiment even though the sorption experiment is carried out in a vapor phase. From this, the actual solubility selectivity should be higher than the estimated value using sorption data. That is, values of solubility selectivity obtained by sorption experiment can be considered to be minimum values. In contrast, the diffusivity selectivity calculated in Eq. (5) can be considered to be maximal values; in other words, the actual diffusivity selectivity should be lower than that estimated by using αðH2 O=EtOHÞ and SSðH2 O=EtOHÞ . From above, the governing factors elucidated by the latter method might have a greater accuracy than that by the former method. Consequently, diffusivity selectivity was determined so that the factors governing the permselectivity toward water vapor could be elucidated. Typical time–transport curves obtained experimentally of H2O vapor and ethanol vapor through the Aga–GA-1 membrane at 92 1C are shown in Fig. 4. In the present study, vapor permeation experiments were carried out at each reflux temperature, which was not equal to 92 1C, which was the reflux temperature of the aqueous ethanol solution with the weight fraction of water being 0.90 [24]. But diffusion coefficients were measured at 92 1C in the present study. Diffusion coefficients of H2O and ethanol in the present membranes were determined by the time-lag method [25–27,35] from the permeation data of pure vapor at a prescribed vapor pressure. Strictly speaking, the obtained diffusion coefficients in the present study are apparent diffusion coefficients since neither sorption isotherms of H2O
Fig. 3. Effect of feed composition on the vapor permeation of H2O/EtOH mixtures through various membranes. (Downstream pressure, 26.7–106.6 Pa (0.20–0.80 mmHg); operating temperature, each reflux temperature.)
Fig. 4. Time–transport curves of H2O vapor (a) and EtOH vapor (b) through Aga–GA-1 membrane at 92 1C.
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In these two Eqs. (7) and (8), D0 is the D value as the permeant concentration approaches 0; γ and β are the coefficients characteristic of the membrane/permeant interaction and C denotes the permeant concentration in the membrane. The latter equation, Eq. (8), is used to simulate the concentration dependence of both H2O and ethanol diffusion in the present study. D0 and β values for the present membranes are summarized in Table 3 together with the diffusivity selectivities (SDðH2 O=EtOHÞ ). The D0 values for H2O and EtOH were decreased with the introduction of crosslinking into the membrane as expected. The β values for H2O were decreased with the introduction of crosslinking. This is due to the suppression of swelling by the introduction of crosslinking. Contrary to this, the β values for EtOH were slightly increased with the introduction of crosslinking. The increase in amount of methylene linkages and simultaneous decrease in that of hydroxyl group in the membrane led to enhance the affinity between membrane and EtOH and, as a result, to a slight increase in β values. In the case that the diffusivity selectivities summarized in Table 3 were adopted, the solubility selectivities were calculated by using Eq. (6) in the present study. The determined solubility selectivities for the three types of membrane are shown in Fig. 6. Strictly speaking, the solubility selectivities for the membranes at the weight fraction of water in feed solution of 0.9 are thought to be correct values. Others
and EtOH in the membranes nor state of the membranes were known [37,38]. It is necessary to know the diffusivity selectivity of the active layer, which is thought to govern permselectivity and to be in the dry state. To accomplish this, vapor with low activity should be in contact with the membrane. In the present study, each vapor, of which activity was below 0.03 at 92 1C, was in contact with the membrane. This enabled the diffusion coefficient to be determined for a given vapor in the active layer of the membrane in vapor permeation. The resulting determined diffusion coefficients for H2O and ethanol in the present membranes are summarized in Tables 1 and 2, respectively. As an example, the diffusion coefficients for H2O and ethanol for Aga–GA-1 membrane are plotted as a function of activity of vapor in Fig. 5. The diffusion coefficient often depends on the local concentration of permeant itself [39–46]. In some cases, the concentration dependence of the diffusion coefficient has been reported to be linear by D ¼ D0 ð1 þ γ CÞ
ð7Þ
And in other cases, an exponential form was observed, which can be represented by D ¼ D0 expðβ CÞ
ð8Þ
Table 1 Diffusion coefficients of H2O through the membranes. p (Pa)
800 1200 1600 2000
p (mmHg)
6 9 12 15
a ( ¼p/p0)
Aga
0.011 0.016 0.021 0.026
Aga–GA-1
Aga–GA-7
θ/s
D (cm2 s 1)
θ/s
D (cm2 s 1)
θ/s
D (cm2 s 1)
166 132 107 85
6.28 10 9 7.89 10 9 9.74 10 9 1.23 10 8
230 190 163 139
4.53 10 9 5.48 10 9 6.39 10 9 7.49 10 9
256 220 191 171
4.07 10 9 4.73 10 9 5.45 10 9 6.09 10 9
p0 ¼ 75.6 kPa (567.1 mmHg); membrane thickness, ca. 25 μm. Table 2 Diffusion coefficients of EtOH through the membranes. p (Pa)
2400 3200 4000 4800
p (mmHg)
18 24 30 36
a ( ¼p/p0)
0.014 0.019 0.023 0.028
Aga
Aga–GA-1 2
1
θ/s
D/cm s
1195 1083 979 877
8.72 10 10 9.62 10 10 1.06 10 9 1.19 10 9
Aga–GA-7 2
1
θ/s
D/cm s
1333 1185 1073 968
7.81 10 10 8.79 10 10 9.71 10 10 1.08 10 9
θ/s
D/cm2 s 1
1442 1274 1136 1030
7.22 10 10 8.18 10 10 9.17 10 10 1.01 10 9
p0 ¼ 170 kPa (1276.7 mmHg); membrane thickness, ca. 25 μm.
Fig. 5. Concentration dependence of diffusion coefficients of water (a) and ethanol vapor (b) through Aga–GA-1 membrane at 92 1C. (The saturation vapor pressure p0 of water at 92 1C is 75.6 kPa (567.1 mmHg); that of ethanol at 92 1C is 170 kPa (1276.7 mmHg).)
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119
Table 3 Determined diffusion coefficients, β values, and diffusivity selectivities. Membrane
Aga Aga–GA-1 Aga–Ga-7 a
H2O
SDðH2 O=EtOHÞ a
EtOH
109D0/cm2 s 1
β
1010D0/cm2 s 1
β
3.86 3.17 3.05
41.4 33.3 27.0
6.35 5.68 5.16
22.3 23.0 24.4
6.08 5.58 5.91
SDðH2 O=EtOHÞ ¼DðH2 O=EtOHÞ .
Fig. 7. DSC heating curves of dry Aga–GA-1 membrane (a), Aga–GA-1/H2O (b)–(h), and pure H2O (i).
Fig. 6. Effect of feed composition on solubility selectivity, SSðH2 O=EtOHÞ , for the present membranes. (The horizontal broken line shows a solubility selectivity of unity.)
from 0.7 to 0.1 are reference values. The solubility selectivity toward water for the membranes showed a preference for water over ethanol. The lowest solubility selectivity toward water vapor for Aga membrane was determined at the weight fraction of water in feed solution of 0.90. The determined solubility selectivity toward water vapor was calculated to be 1.05 using αH2 O=EtOH and SDðH2 O=EtOHÞ , which was still over unity. The results obtained in the present study revealed that the permselectivity toward water in vapor permeation was expressed by both diffusivity selectivity and solubility selectivity. In Fig. 6, solubility selectivity had inclination to decrease with the increase in weight fraction of water in feed solution. This is due to the swelling of membrane by water vapor. 3.4. State of water in the membrane The anomalous properties of water in plant and animal cells and tissues, synthetic polymers, and especially in the synthetic polymeric membranes have been extensively studied. The attempts to correlate these anomalous properties with the state of water in such substances have met with varying degrees of success [47,48]. In reverse osmosis, it was reported that the state of water, which was preferentially transported through a given reverse osmosis membrane, was different from that of bulk water [49–55]. In other words, melting point depression of water in the reverse osmosis membrane was observed. In pervaporation, similar phenomena were observed in water permeable membranes [9,10,13,56,57] and alcohol permselective polydimethylsiloxane membrane [58–60]. Bound water in agarose had been observed by 1H NMR [61] and thermal analyses [62]. In the present study, the states of water in three types of membrane were studied and the relationship between the state and their membrane performance was investigated. Such an approach had not been carried out in vapor permeation as far as we have known.
Fig. 8. Relationship between enthalpic heat for H2O in Aga–GA-1 membrane and total H2O content.
Melting endotherms of water in Aga–GA-1 membrane, of dry Aga– GA-1 membrane, and of bulk water are shown in Fig. 7 as an example. The area of endothermic curve at 0 1C decreased with decrease in water content in the membrane (Fig. 7(d)–(h)). In Fig. 7(b) and (c), endotherms of water were hardly observed around its melting point of 0 1C, even though water was still present in Aga–GA-1 membrane. No other new endotherm was observed in the temperature range 80 1C to 30 1C. The membrane in Fig. 7(b) contained 6.50 10 5 g of water. The endotherm of 1.00 10 4 g of bulk water is shown in Fig. 7(i). If most of the water in Aga–GA-1 membrane could be assigned as bulk water, the endotherms in Fig. 7(c) and (b) should be observed. Fig. 8 shows the relationship between enthalpic heat and water content for Aga–GA-1 membrane. The observed points in Fig. 8 fall on a straight line with a slope equal to the heat of melting for water of 333.4 J g 1 (79.8 kcal g 1) [63]. Extrapolation of ΔH ¼0 intercepts the water content axis at a point that is the total bound water content of the sample. The bound water content, Wb, was determined to be 0.45 g-H2O/g-membrane for Aga–GA-1 membrane. For other membranes it was determined to be 0.19 g-H2O/gmembrane for Aga and 0.26 g-H2O/g-membrane for Aga–GA-7. It is an interesting and indispensable task to elucidate the relationship between nature of water in the membrane and
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Fig. 9. Relationship between separation factor (αðH2 O=EtOHÞ ) and total water content (a), amount of bound water (b), and fractional bound water (c) in the membrane.
permselectivity. This will lead to future molecular engineering to obtain suitable vapor permeation membrane materials for the dehydration of bioethanol. To this end, permselectivities toward water are plotted against total water content in the membrane (Fig. 9(a)), amount of bound water in the membrane (Fig. 9(b)), and fractional bound water in the membrane (FBW) (Fig. 9(c)). The fractional bound water in the membrane, FBW, is defined by FBW ¼ W b =W t
ð9Þ
From Fig. 9, as reported previously [13,57], permselectivity toward water gave a good correlation with fractional bound water, FBW. The bound water in water permeable vapor permeation membrane would not dissolve ethanol like bound water in reverse osmosis membranes rejected salts [52]. In the present study, membranes with fractional bound water of unity were not obtained. Vapor permeation membranes, of which water consists of just bound water, gave higher permselectivity toward water as observed in pervaporation membranes from quaterinized poly[3(N0 , N0 -dimethyl)aminopropylacrylamide-co-acrylonitrile] membranes [57].
4. Preliminary study on forward osmosis Nowadays, renewable and clean energy is required in order to realize a safe, secure, and sustainable society. Naturally occurring energy, such as tidal energy, geothermal one, wind one, solar one, and so forth are potential energy sources for our future. Among them, the mixing of fresh water with seawater is one of the plausible ones. Osmotic salination converter [64] and generation of electric power by mixing of fresh and seawater [65] cannot be attained without membranes. Membranes play an important role in these challenges: a water permeable membrane plays a leading character in the former challenge and both cation and anion exchange membranes a leading role in the latter one. The present membranes showed permselectivity toward water. This stimulated us to survey the potential of forward osmosis [66–68] for the membranes. 25.0 wt% aqueous NaCl solution was adopted as draw solution and pure water as feed. Water flows were determined to be 6.61 L m 2 h 1 for Aga membrane, 5.37 L m 2 h 1 for Aga–GA-1, and 5.27 L m 2 h 1 for Aga–GA-7.
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