Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran

Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran

Journal of Membrane Science 283 (2006) 65–73 Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4...

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Journal of Membrane Science 283 (2006) 65–73

Poly(vinyl alcohol)-iron oxide nanocomposite membranes for pervaporation dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran夽 Malladi Sairam a , Boya Vijaya Kumar Naidu a , Sanna Kotrappanavar Nataraj a , Bojja Sreedhar b , Tejraj M. Aminabhavi a,∗ a

Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580 003, India b Thermal Analysis Center, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 12 July 2005; received in revised form 6 June 2006; accepted 10 June 2006 Available online 16 June 2006

Abstract Poly(vinyl alcohol) (PVA)-based nanocomposite membranes were prepared by coprecipitation of different amounts of Fe(II) and Fe(III) taken in an alkaline medium and their pervaporation (PV) performances were investigated to dehydrate isopropanol, 1,4-dioxane and tetrahydrofuran (THF) from aqueous feeds containing 10–20 wt.% of water in isopropanol and 1,4-dioxane, 5–l5 wt.% of water in THF. The freestanding membranes were characterized by the dynamic mechanical thermal analyzer (DMTA), which showed a shift in glass transition temperature toward higher range along with an increase in storage modulus with increasing amount of iron oxide in the PVA matrix. Furthermore, thin layered membranes were cast on polyester fabric cloths as support layers to improve their PV separation performances for all the three mixtures over that of the pristine crosslinked PVA membrane. In particular, the composite membrane prepared by taking 4.5 wt.% of iron oxide showed an improved selectivity with a slight sacrifice in flux compared to membranes containing lower contents of iron oxide as well as the pristine crosslinked PVA membrane. Flux decreased with increasing content of iron in the PVA matrix, while selectivity increased systematically. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite membrane; Iron oxide; Poly(vinyl alcohol); Aqueous-organic mixtures; Pervaporation dehydration

1. Introduction The design and synthesis of new materials with nanodimensions has been an area of intense research in recent years, since such materials have gained a widespread interest in many areas of science and technology due to their remarkable changes in properties such as mechanical [1], thermal [2–5], electrical [6] and magnetic [7] as compared to virgin organic polymers. Particularly, the usage of inorganic–organic composite materials is becoming increasingly important due to their extraordinary properties, which arise from the synergism between the properties of the individual components as well as their interactions with the base matrix materials. Several ways of incorporating inorganic materials into organic polymers have been attempted in the earlier literature [8–10]. Among the many poly-

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This article is CEPS communication #84. Corresponding author. Tel.: 91 836 2215372; fax: +91 836 2771275. E-mail address: [email protected] (T.M. Aminabhavi).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.013

mers employed, poly(vinyl alcohol) (PVA) has been recently used to develop inorganic–organic nanocomposite hybrids [10]. PVA is known to be one of the most widely used membranes for the separation of water-organic mixtures in addition to its use as adhesives [11,12], coatings and paints [13] in view of its good film-forming nature, hydrophilicity, processibility, and good chemical resistivity [14,15]. In general, PVA membranes are known to exhibit high permselectivity and relatively low permeabilities. Their high water permselectivity is due to their good hydrophilicity and preferential attraction of water molecules, whereas their low permeability is attributed to their close molecular packing and high degree of crystallinity. However, the properties of PVA can be modified depending upon its polymerization conditions, drying, grinding or chemical modifications [14,15]. This has prompted researchers to employ PVA as a base polymer for preparing nanoparticles of metal oxides by in situ synthesis to form the nanocomposites [16–18]. Hydroxyl groups of PVA act as the chelating sites to interact with metal ions, leading to the formation of a closely packed three-dimensional network structure.

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The in situ synthesis of nanoparticles in a polymer matrix has the potential to control mean particle size as envisaged in earlier studies [8–10,18]. The homogeneous dispersion of particles in the polymer matrix would lead to the formation of nanocomposites with improved thermal, barrier and mechanical properties [16]. Such properties when coupled with the hydrophilic nature of PVA will lead to the formation of a membrane that can be used in pervaporation (PV) dehydration of aqueous-organic mixtures. Our previous efforts in the direction [19] of in situ polymerization of aniline in PVA has led to the development of polyaniline-based nanocomposite membranes with improved PV separation characteristics for water when compared to the pristine crosslinked PVA membrane. In principle, basically two approaches have been used to form such inorganic-polymer nanocomposites. The most obvious and common one is the incorporation of preformed inorganic particles into a polymer matrix by grinding and mixing, while the other method includes in situ synthesis of nanoparticles within the polymer matrix itself. In continuation of our earlier work [19], the latter method is used here to fabricate nanocomposite membranes of PVA containing iron oxide nanoparticles. The method adopted here is similar to the one used by Godovsky et al. [18], who synthesized the nanosized magnetite particles in situ within the PVA solution by precipitating Fe2+ ions or mixture of Fe2+ and Fe3+ ions with NaOH solution. Such nanoscale particles homogeneously dispersed in PVA matrix form the three-dimensional network structure due to the chelating ability of PVA [18]. Transparent freestanding membranes (40–50 ␮m thickness) were obtained by solution casting of PVA without any support, but these membranes did not yield good flux. Alternatively, in an effort to improve flux and selectivity, we was thought of casting much thinner membranes (10 ␮m) by casting PVA solution containing Fe nanoparticles on a polyester fabric. In this effort, we have prepared three nanocomposite membranes containing different amounts of iron oxide nanoparticles. The membranes prepared in this study were employed in PV dehydration of isopropanol, 1,4-dioxane and THF, whose separation from their aqueous mixtures is an important unit operation in process engineering areas. Among the many separation techniques, PV is today considered as a basic unit operation process with significant potential for the solution of various environmental issues and is an energy-intensive process. It is in this sense that PV has been used to separate azeotropic aqueousorganic mixtures chosen in this study. PV experimental data of the three nanocomposite membranes were compared with the pristine crosslinked PVA membrane. It is observed that the nanocomposite membranes were able to increase the selectivity to water for all the mixtures when compared to the control experiments performed on pristine crosslinked PVA membrane.

samples purchased from S.D. Fine Chemicals, Mumbai, India. Double distilled water produced in the laboratory itself was used throughout the work. The polyester fabric was of a commercial grade sample purchased from the local market. 2.2. Preparation of PVA-iron free-standing nanocomposite membranes Freestanding PVA-iron oxide nanocomposite films were prepared according to the reported procedure published before [18]. An aqueous solution containing FeSO4 ·7H2 O and FeCl3 ·6H2 O in 1:2 ratio was prepared and filtered through a Whatman #41 filter paper to remove the iron hydroxide formed. The above solution was added to 5 wt.% PVA aqueous solution and stirred well for uniform mixing. The resulting PVA-Fe2+ /Fe3+ solution was cast as a membrane on a clean glass plate and dried at ambient temperature. The dried PVA-Fe2+ /Fe3+ membranes were peeled off from the glass plate, immersed in a bath containing 4 M KOH solution and kept for 24 h until the generation of iron oxide. Membranes were removed from KOH bath, washed repeatedly by immersing in double distilled water until pH 7 was achieved and finally dried at ambient temperature. Totally, five freestanding nanocomposite membranes of PVA-Fe containing 3, 6, 12 and 24 wt.% of iron oxide were prepared and these were designated, respectively, as: PVA-Fe-3, PVA-Fe-6, PVA-Fe-12, PVAFe-18 and PVA-Fe-24. These films were used in DMTA analysis. 2.3. Preparation of PVA-iron nanocomposite membranes on a polyester fabric An aqueous solution containing FeSO4 ·7H2 O and FeCl3 ·6H2 O in 1:2 ratio was prepared and filtered through a Whatman #41 filter paper to remove iron hydroxide formed. The above solution was added to 5 wt.% PVA aqueous solution and stirred at ambient temperature. The resulting PVA-Fe2+ /Fe3+ solution was cast into film on a polyester fabric and dried at ambient temperature. The dried PVA-Fe2+ /Fe3+ membranes on polyester fabric were immersed in a bath containing 4 M KOH solution and kept for 24 h. Membranes were removed from the KOH bath, washed repeatedly by immersing in double distilled water until pH 7 was obtained and then again dried at the ambient temperature. During in situ coprecipitation, we have taken three different amounts (1.5, 3 and 4.5 wt.%) of Fe(II)/Fe(III) based on the weight of PVA to prepare three different nanocomposites on polyester fabrics (10 ␮m thickness layer) designated, respectively, as PVA-Fe(c)-1.5, PVA-Fe(c)-3.0 and PVA-Fe(c)-4.5, where (c) refers to composite membranes prepared on polyester fabrics. 2.4. Characterization of nanocomposites

2. Experimental 2.1. Materials Poly(vinyl alcohol) (MW: 125,000) with a degree of hydrolysis 87%, FeSO4 ·7H2 O, FeCl3 ·6H2 O, potassium hydroxide, isopropanol, 1,4-dioxane and THF were all of analytical grade

2.4.1. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical thermal analysis (DMTA) of the PVAiron oxide nanocomposites have been performed by using a Rheometric Scientific DMTA IV instrument operating at 1 Hz frequency. DMTA scans have been performed between 25 and 250 ◦ C at the heating rate of 5 ◦ C/min.

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2.4.2. Thermogravimetric analysis (TGA) TGA thermograms of the PVA-iron oxide nanocomposites and nanocomposites prepared on polyester fabrics were recorded on a Rheometric Scientific (STA-1500) equipment. TGA scans were done from 25 to 800 ◦ C at the heating rate of 20 ◦ C/min under an inert nitrogen atmosphere.

where FA is the mass% of water in the feed and PA is the mass% of water in permeate. A minimum of three independent readings was taken under the same experimental conditions with errors not exceeding >2–3%.

2.4.3. Sorption experiments Sorption experiments were performed gravimetrically at 30 ◦ C in water–isopropanol and water–1,4-dioxane feed mixtures containing 10, 15 and 20 wt.% water, whereas for waterTHF feed mixture, we used 5, 10 and 15 wt.% composition of water. Initial mass of the circularly cut (dia = 2.5 cm) crosslinked pristine PVA and PVA-Fe composite membranes was measured on a single-pan digital microbalance (model AE 240, Mettler, Switzerland) sensitive to ±0.01 mg, kept in different soaking mixtures for 48 h. At the end of 48 h, membranes were weighed after carefully blotting the surface-adhered excess liquid droplets to estimate accurately the amount of liquid mixture sorbed by the membrane at a particular time, t. The % sorption was calculated as   Ms − Md Sorption (%) = × 100 (1) Md

One of the most common methods to incorporate inorganic particles into a polymer is by grinding and mixing. An alternative method is the in situ synthesis of nanoparticles within the polymer matrix itself. In the present work, iron oxide nanoparticles were prepared in situ within the PVA matrix via coprecipitation of Fe(II) and Fe(III) in alkaline medium to obtain nanocomposite membranes that were used in PV separation experiments. During in situ coprecipitation, we have taken three different amounts (1.5, 3 and 4.5 wt.% of Fe(II)/Fe(III) based on the weight of PVA) to prepare three different nanocomposite membranes on polyester fabrics (10 ␮m thickness layer) designated, respectively, as PVA-Fe(c)-1.5, PVA-Fe(c)-3 and PVA-Fe(c)-4.5. The reaction between iron ions attached to hydroxyl groups of PVA and –OH groups of KOH leads to the formation of iron oxide nanoparticles (Fe2 O3 or Fe3 O4 ). The reaction is clear to the naked eye as the initially observed yellow colored film changed to brown color during alkali treatment due to the dominance of Fe2 O3 . However, the formation of Fe2 O3 or Fe3 O4 depends upon the molar ratio of Fe2+ to Fe3+ . Nanocomposite with intensive black color due to the formation of Fe3 O4 is favored when Fe2+ to Fe3+ ratio is 1:1, whereas brown color results due to the dominance of Fe2 O3 when Fe2+ to Fe3+ ratio was maintained at 1:2. The nanocomposite membranes thus obtained were brown in color.

where Ms is mass (g) of the swollen membrane and Md is mass (g) of the dry membrane. 2.4.4. Pervaporation experiments A detailed procedure of performing the PV experiments was reported in earlier literature [20,21]. PV experiments were carried out on a 100 mL batch level instrument operated at a vacuum level as low as 0.05 mm Hg in the permeate line. Membrane area was approximately 20 cm2 . Before starting of the PV experiment, test membrane was equilibrated for about 2 h with the feed mixture at 30 ± 1 ◦ C. A Mettler balance (model B 204-S, Greifensee, Switzerland, accuracy 10−4 g) was used for measurements of the weight and then to determine the flux, J (kg/m2 h) using the weight of liquids permeated, W (kg), effective membrane area, A (m2 ) and measurement time, t (h) as J=

W At

(2)

The analysis of feed and permeate samples was done using a Nucon Gas Chromatograph (model 5765) provided with a thermal conductivity detector (TCD) equipped with a DEGS or Tenax packed column of 1/8 in. i.d. having 2 m length. Oven temperature was maintained at 70 ◦ C (isothermal), while injector and detector temperatures were maintained at 150 ◦ C. Sample injection volume was 1 ␮L. Pure hydrogen was used as a carrier gas at a pressure of 0.75 kg/cm2 . The GC response was calibrated for the column and for known compositions of the respective water + organic mixtures. Calibration factors were fed into GC software to obtain the analysis for unknown samples. The pervaporation selectivity, α, was calculated as    PA 1 − FA α= (3) 1 − PA FA

3. Results and discussion

3.1. Thermogravimetric analysis (TGA) Thermal stability of the nanocomposites was evaluated by TGA. The amount of iron oxide present in nanocomposites was determined from the residual weight at 800 ◦ C. PVA and polyester fabric, used as a support, were decomposed completely (100%) below 800 ◦ C, whereas nanocomposites showed some residual weight at 800 ◦ C. However, the residual weight remained at 800 ◦ C is the amount of iron oxide present in nanocomposites, which increased with increasing amount of iron in nanocomposite membranes. 3.2. Dynamic mechanical thermal analysis (DMTA) DMTA results of PVA and PVA-iron oxide nanocomposites were studied in terms of loss tangent (tan δ) and storage modulus (E ) as a function of temperature (see Figs. 1 and 2). The peak of tan δ located at 72 ◦ C is the glass transition temperature (Tg ) of the pristine PVA, which was shifted gradually to higher temperature with increasing amount of iron oxide in the PVA matrix. The Tg of pristine PVA (72 ◦ C) shifted to 85 ◦ C for films containing 24% iron oxide. Similar observations were reported by DSC studies on PVA-based magnetic nanocomposites [16]. The shift in Tg was attributed to a physical-type of interaction between PVA and the fillers, which could reduce PVA chain mobility,

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membrane materials. The % sorption data of the nanocomposite membranes, viz., PVA-Fe(c)-1.5, PVA-Fe(c)-3 and PVAFe(c)-4.5 versus wt.% of water of the feed mixture are displayed in Fig. 3(a)–(c), respectively, for water + isopropanol, water + 1,4-dioxane and water + THF mixtures at 30 ◦ C. Since the crosslinked pristine PVA membrane has an uptake of >80% for 10 wt.% water containing feed mixture, its sorption curves were not displayed. Notice that nanocomposite membranes showed a systematic effect on their sorption characteristics, depending upon the extent of Fe particles present in the PVA matrix. For instance, lower the amount of Fe in the PVA matrix, higher will be the % sorption by the membranes

Fig. 1. tan δ curves of: (a) PVA, (b) PVA-Fe(c)-3%, (c) PVA-Fe(c)-6%, (d) PVAFe(c)-12%, (e) PVA-Fe(c)-18% and (f) PVA-Fe(c)-24% free-standing membranes.

thereby increasing the Tg . A sharp fall in E was observed in the glass transition region of the pristine crosslinked PVA and its nanocomposites. However, a shift in the onset of fall in E to higher temperature and a more gradual fall were observed with increasing amount of iron oxide particles in the nanocomposites. The storage modulus of nanocomposites above the Tg has increased by one order of magnitude as compared to pristine PVA. This type of increase in Tg and E values for nanocomposite membranes are due to a good adhesion between iron oxide nanoparticles and PVA matrix. 3.3. Sorption studies PV transport mechanism is well interpreted in terms of solution-diffusion model. Thus, preferential sorption characteristics of each membrane were explored in different feed compositions. Sorption, namely solubility of the membrane, is caused by the interaction of the penetrating species with the

Fig. 2. E curves of: (a) PVA, (b) PVA-Fe(c)-3%, (c) PVA-Fe(c)-6%, (d) PVAFe(c)-12%, (e) PVA-Fe(c)-18% and (f) PVA-Fe(c)-24% free-standing membranes.

Fig. 3. % Sorption vs. wt.% water in feed for: (a) water + isopropanol, (b) water + 1,4-dioxane and (c) water + THF mixtures. Symbols: () PVA-Fe(c)1.5, () PVA-Fe(c)-3 and () PVA-Fe(c)-4.5.

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Table 1 Pervaporation data of water + isopropanol mixture at 30 ◦ C J (kg/m2 h)

Water (wt.%) in

Fig. 4. % Degree of swelling vs. wt.% of water in feed for PVA-Fe(c)-3 membrane in different solvents. Symbols: () water + THF, () water + isopropanol and () water + 1,4-dioxane.

(seen from the plots presented in Fig. 3). The sorption characteristics of membranes, are therefore, influenced greatly by the amount of iron oxide in the PVA matrix as well as the amount of feed mixture water composition. Sorption also varied differently for the three different feed mixtures studied. For instance, the sorption tendencies of three nanocomposite membranes presented in Fig. 3(a)–(c) vary as per the sequence: water + TFIF > water + isopropanol > water + 1,4-dioxane under similar set of conditions of sorption experiments. A typical plot of % sorption versus wt.% water in feed mixtures of the present study displayed in Fig. 4 clearly demonstrates this effect for PVA-Fe(c)-3 membrane. Further, it can also be noticed that % sorption capacities of the membranes increased more than double for all the three feed mixtures at increasing concentrations of water in the feed mixtures. The amount of water sorbed by the membrane increased with increasing water content of the feed mixture. Hydrophilic groups of the membrane polymers are responsible for such preferential sorption tendencies towards water. 3.4. Membrane performance In the present study, nanocomposite membranes gave the optimum PV performance for PV dehydration of aqueousorganic feed mixtures in terms of selectivity with a small sacrifice in flux. As shown in Tables 1–3, their PV performances exceed those of the pristine crosslinked PVA membrane for all three feed mixtures of this study. Therefore, there is an improvement in selectivity due to the presence of Fe particles in PVA nanocomposite membranes when compared to the pristine PVA membrane. Thus, hydrophilicity of PVA alone is not essential for dehydrating organics, since it can result in low selectivity and poor mechanical strength in aqueous solution with a high flux. Therefore, we thought of using a novel approach of growing iron oxide particles into the PVA matrix, not only to increase the mechanical strengths of the membranes, but also to improve their selectivity to water at the cost of lower fluxes. Low water absorption capacity in addition to mechanical strength properties of the

α

Feed

Permeate

Pristine PVA 10 12.5 15 20

89.57 89.32 89.01 88.2

0.123 0.138 0.145 0.163

77 59 46 30

PVA-Fe(c)-1.5 10 12.5 15 20

93.11 92.55 92.21 91.36

0.095 0.096 0.097 0.101

122 87 67 42

PVA-Fe(c)-3 10 12.5 15 20

94.09 93.4 93.06 92.26

0.082 0.088 0.092 0.098

143 99 76 48

PVA-Fe(c)-4.5 10 12.5 15 20

98.12 97.56 97.16 96.51

0.079 0.084 0.087 0.093

470 280 194 111

derived nanocomposite membranes as a result of the presence of iron oxide particles will reduce the relaxation of PVA chains during PV, resulting in better selectivity with somewhat lower fluxes than pristine PVA membrane. In majority of literature studies, crosslinking of PVA membrane was done by glutaraldehyde to reduce membrane swelling and to impart strength properties. In the present study, PVA-Fe nanocomposite membranes could Table 2 Pervaporation results of water + 1,4-dioxane mixture at 30 ◦ C J (kg/m2 h)

Water (wt.%) in

α

Feed

Permeate

Pristine PVA 10 15 18.1 20

86.82 84.9 80.08 78.03

0.128 0.143 0.155 0.167

59 32 18 14

PVA-Fe(c)-1.5 10 15 18.1 20

90.09 87.26 84.39 81.25

0.098 0.108 0.119 0.124

82 39 24 17

PVA-Fe(c)-3 10 15 18.1 20

92.06 90.59 88.26 85.19

0.091 0.101 0.109 0.116

104 55 34 23

PVA-Fe(c)-4.5 10 15 18.1 20

94.12 92.71 89.97 87.01

0.084 0.091 0.099 0.112

144 72 41 27

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Table 3 Pervaporation results of water + THF mixture at 30 ◦ C Water (wt.%) in

J (kg/m2 h)

α

Feed

Permeate

Pristine PVA 5 6.7 10 15

91.72 91.29 90.57 89.76

0.210 0.214 0.217 0.222

210 146 86 50

PVA-Fe(c)-1.5 5 6.7 10 15

94.74 94.21 93.76 93.08

0.180 0.187 0.193 0.208

342 227 135 76

PVA-Fe(c)-3 5 6.7 10 15

95.68 95.54 94.89 94.16

0.139 0.145 0.155 0.173

421 298 167 91

PVA-Fe(c)-4.5 5 6.7 10 15

96.47 96.24 95.91 95.27

0.095 0.101 0.113 0.133

519 356 211 114

offer better mechanical strength and membrane performances without crosslinking by glutaraldehyde, but merely by incorporating iron oxide nanoparticles into the PVA matrix to form a three-dimensional network structure due to the coordination of Fe with the hydroxyl groups of PVA. Therefore, our protocol used to prepare the nanocomposite membranes giving sufficient mechanical strength, lower swelling and improved PV performances is novel. 3.4.1. Effect of filler concentration on nanocomposite membrane performance The effect of iron oxide content on selectivity and flux data are displayed in Fig. 5 for the PV separation of azeotropic feed mixtures. For all the mixtures, selectivity increased with increasing amount of iron oxide at the expense of flux. However, in case of water + isopropanol feed mixtures at the azeotropic composition, the decline in flux and increase in selectivity are somewhat gradual with increasing Fe content of the membranes. On the other hand, with water + 1,4-dioxane and water + THF mixtures (at the azeotropic composition), decline in flux and increase in selectivity were somewhat steep. This shows that selectivity and flux of the membranes vary depending upon the nature of the feed mixtures. The pristine crosslinked PVA membrane shows selectivity data of 18, 59 and 146 for water–1,4-dioxane, water–isopropanol and water–THF feed mixtures, respectively, at their respective azeotropic compositions of 18.1, 12.5 and 6.7 wt.% of water. However, with increasing iron oxide content of the PVA membranes, water selectivity values increased to 41, 280 and 356, respectively, for the same azeotropic compositions. Such an increase in selectivity is attributed to a reduction in membrane swelling with increasing amount of iron oxide in the membrane. Each iron ion will get involved in forming

Fig. 5. Water flux and selectivity vs. amount of iron oxide in the membrane at azeotropic composition for PVA-Fe(c)-4.5 membrane. Symbols: (䊉) flux and () selectivity.

the network structure with hydroxyl groups of PVA that will be responsible for a reduction in membrane swelling. As the amount of iron in the PVA matrix increases, there is the possibility of forming higher level three-dimensional network structures, which will further reduce membrane swelling, thereby increasing its selectivity to water. However, the flux data decreased systematically with increasing concentration of iron particles in the PVA nanocomposite membranes and these are much lower than those observed for pristine crosslinked PVA membrane for all the three feed mixtures studied. These results are in line with the sorption data discussed before. 3.4.2. Effect of feed composition on PV performance Selectivity data of the nanocomposite membranes are higher at lower concentrations of water in the feed, but flux values

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increased with increasing concentration of water. Results of selectivity and flux of the three feed mixtures versus wt.% of water in feed are displayed in Figs. 6 and 7, respectively. This type of permeation behavior may be explained as follows. As the contents of Fe, increase in the active layer of the membrane, larger number of –OH groups from PVA are involved in the chelation with Fe, thereby decreasing the overall hydrophilic nature of the composite membrane; thus, the

Fig. 7. Selectivity vs. wt.% of water in feed mixture at 30 ◦ C. Symbols have the same meanings as in Fig. 4.

membrane will become rigid to allow for the selective transport of water molecules through the membrane. As the separation mainly occurs in the dense amorphous region, hence as per solution/diffusion mechanism, the presence of iron oxide nanoparticles will result in the formation of more amorphous regions dispersed throughout the PVA matrix. In the meanwhile, as the extent of Fe in PVA matrix increases, the structure of PVA layer becomes tighter to attract organic compounds of the feed mixture, which will result in a lowering of the flux of the nanocomposite membranes as compared to pristine crosslinked PVA membrane. These data are also consistent with the sorption results of the membranes discussed before.

30 ◦ C.

Fig. 6. Water flux vs. mass% of water in feed mixture at Symbols: () pristine PVA, (䊉) PVA-Fe(c)-1.5, () PVA-Fe(c)-3 and () PVA-Fe(c)-4.5.

3.4.2.1. Water + isopropanol mixture. Dehydration of isopropanol, especially for 90% azeotropic composition arises from the demand in semiconductor industry, where isopropanol is

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being used as a cleaning agent for semiconductor chips. In case of water + isopropanol feed mixture, flux of the pristine PVA membrane increases from 0.123 to 0.163 kg/m2 h with increasing feed water composition from 10 to 20 wt.%. For the nanocomposite membranes also, flux increased with increasing amount of water in the feed. For instance, flux increased from 0.095 to 0.101, 0.082 to 0.098 and 0.079 to 0.093 kg/m2 h, respectively, for PVA-Fe(c)-1.5, PVA-Fe(c)-3 and PVA-Fe(c)4.5 nanocomposite membranes with increasing concentration of water in the feed from 10 to 20 wt.%. On the other hand, selectivity has shown the reverse trends. Pristine PVA membrane has a selectivity of 77 at 10 wt.% of water in the feed, which decreased to 30 at 20 wt.% of water, but nanocomposite membranes exhibited higher selectivity than pristine PVA membrane. For instance, PVA-Fe(c)-4.5 membrane has the highest selectivity of 470 for 10 wt.% of water in the feed, which decreased sharply to 111 at 20 wt.% of water. The decrease in selectivity and increase in flux with increasing concentration of water in the feed is due to increased swelling of the membrane at higher amount of water. When the membrane swells, free volume increases, thereby more of water molecules of the feed mixture will diffuse through the membrane thus, increasing the flux values with a subsequent decrease in selectivity. The maximum extractable concentration of water in the permeate was up to 98.12 wt.% for PVA-Fe(c)-4.5 membrane when compared to the lowest value of 89.6 wt.% for pristine PVA membrane.

feed mixtures. Compared to all the membranes, pristine PVA membrane has a higher flux of 0.222 kg/m2 h at 10 wt.% water in the feed, whereas PVA-Fe(c)-4.5 membrane has the least flux value of 0.095 at 5 wt.% of water. On the other hand, PVA-Fe(c)4.5 membrane has a highest selectivity of 519 for 5 wt.% of water in the feed, while a least selectivity of 50 was observed for the pristine crosslinked PVA membrane when tested for 15 wt.% water in the feed. Flux values increased with increasing amount of water in the feed, whereas selectivity values have declined. Compared to water + isopropanol and water + 1,4-dioxane mixtures, in case of water + THF, the PVA-Fe(c)-4.5 nanocomposite membrane could remove up to the maximum of 96.47 wt.% of water from feed side to the permeate side. Even at the azeotropic composition, this membrane could remove up to 96.24 wt.% of water, while a much lower amount (91.72 wt.%) of water was able to be removed from the pristine crosslinked PVA membrane for 5 wt.% of water in the feed. 3.5. Comparison of PV data with VLE results Fig. 8 displays the plots comparing vapor liquid equilibrium (VLE) data of the feed mixtures with those of PV results obtained

3.4.2.2. Water + 1,4-dioxane mixture. 1,4-Dioxane is an important organic liquid used in majority of organic synthesis as well as a media with water in electrochemical research. In case of water + 1,4-dioxane mixture, pristine PVA membrane has shown a flux value of 0.128 kg/m2 h for 10 wt.% water containing feed. This value is slightly higher than that observed for water + isopropanol feeds, which further increased to 0.167 kg/m2 h at higher concentration of water (i.e., 20 wt.%). The nanocomposite membranes exhibited somewhat lower flux values than pristine PVA membrane. For instance, PVA-Fe(c)4.5 membrane has the least flux value of 0.084 kg/m2 h for 10 wt.% water in the feed as compared to other nanocomposite membranes. For all the membranes, flux increased with increasing amount of water in the feed, but selectivity decreased. The PVA-Fe(c)-4.5 membrane exhibited highest selectivity of 144 for 10 wt.% of water in the feed, but its selectivity decreased rapidly to 27 at the higher concentration of water in the feed. Selectivity value of the nanocomposite membranes containing lower amount of Fe content (PVA-Fe(c)-1.5 and PVA-Fe(c)-3) have higher flux and lower selectivity values than PVA-Fe(c)4.5 nanocomposite membrane. However, the amount of water in permeate was more (94.12 wt.%) for PVA-Fe(c)-4.5 membrane as compared to rest of the membranes used in PV dehydration of 1,4-dioxane. Notice that pristine crosslinked PVA membrane could extract only up to 86.82 wt.% of water from the feed mixture. 3.4.2.3. Water + THF mixture. The observed flux and selectivity values of water + THF mixtures are higher than those observed for both water + isopropanol and water + 1,4-dioxane

Fig. 8. Comparison of vapor liquid equilibrium curve (䊉) with PV data () for water + organic mixtures at 30 ◦ C for PVA-Fe(c)-4.5 nanocomposite membrane.

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in this work. For the PVA-Fe(c)-4.5 nanocomposite membrane at 30 ◦ C, for all the three mixtures, the present PV results are quite above the VLE data, suggesting the effectiveness of PV process to separate the azeotropes of this study. Moreover, energy requirements of the PV process are much lower and the membrane acts as a third phase to achieve the separation by breaking the azeotrope effectively, while in distillation, a third component (entrainer, usually benzene) has to be added to affect the same separation; this will make the process environmentally hazardous. 4. Conclusions In an effort to improve the dimensional stability of PVA membrane and to enhance its PV performance, novel nanocomposites were prepared on a polyester fabric support by in situ synthesis of iron oxide particles in PVA matrix without crosslinking. However, only for testing purpose, the freestanding films of PVA were prepapred and characterized by DMTA, whereas both freestanding films and those prepared on a polyester fabric were subjected to TGA analysis. Membranes thus prepared were used to selectively separate water from isopropanol, 1,4-dioxane and THF feed mixtures by the PV technique. Sorption characteristics of the membranes were found to be greatly influenced by the amount of iron oxide present in the PVA membrane matrix as well as the wt.% of water in the feed mixtures in addition to the nature of the mixed feed media chosen for the study. It was noticed that membranes containing higher concentration of iron particles exhibited higher selectivity to water for all the feed mixtures chosen in this study. Flux values of the pristine crosslinked PVA membrane were higher than those of the nanocomposite membranes over the range of water concentrations studied for each mixture. This is attributed to higher swelling of the pristine crosslinked PVA membrane than the nanocomposite membranes, since the iron particles are able to engage the hydroxyl groups of PVA during the complexation reactions. It is further demonstrated that the membranes of this study are effective in separating water from the feed mixtures at their azeotropic compositions; however, if these separations were to be achieved by distillation, it would require more energy and the process is less ecofriendly. In all the cases, nanocomposite membranes exhibited superior membrane performances over the pristine crosslinked PVA membrane. Acknowledgements The authors appreciate the financial support from University Grants Commission (UGC), New Delhi, India (F141/2001/CPP-II) to establish Center of Excellence in Polymer Science (CEPS). Professor T.M. Aminabhavi and Dr. B.V.K. Naidu (RA) thank the Council of Scientific and Industrial Research (CSIR), Grant no. 80(0042)/02/EMR-II for a partial support of this study. This research is a collaborative effort between CEPS, Dharwad and IICT, Hyderabad under the MoU.

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