PVDF hollow fiber composite membrane

Applied Surface Science 427 (2018) 288–297

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Recycling of phenol from aqueous solutions by pervaporation with ZSM-5/PDMS/PVDF hollow fiber composite membrane Dan Li a , Jie Yao a,b,∗ , Hao Sun a , Bing Liu a , Sjack van Agtmaal c , Chunhui Feng c a b c

School of Environment, Harbin Institute of Technology, Harbin 150090, PR China National Engineering Center of Urban Water Resources, 202 Hehai Road, Harbin150090, PR China Evides Industriewater B.V. Schaardijk 150, 3063 NH Rotterdam, The Netherlands

a r t i c l e

i n f o

Article history: Received 27 April 2017 Received in revised form 18 August 2017 Accepted 24 August 2017 Keywords: PVDF hollow fiber membranes Modification Pervaporation Phenols

a b s t r a c t Zeolite (ZSM-5)/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) hollow fiber composite membrane was prepared by dynamic negative pressure. The influence of ZSM-5 silanization, coating time and concentration of ZSM-5 on the resulting pervaporation (PV) performance of composite membranes was investigated. The contact angle (CA) was used to measure surface hydrophobic property and it was found that the water contact angle of the membrane was increased significantly from 99◦ to 132◦ when the concentration of ZSM-5 increased from 0% to 50%. The morphology of the membrane was characterized by scanning electron microscope (SEM) and those SEM images illustrated that the thickness of the separating layer has obvious differences at varying coating times. Furthermore, the membranes were investigated in PV process to recycle phenol from aqueous solutions as feed mixtures. The impact of phenol concentration in feed, temperature and pressure of penetration side on the PV performance of membrane was studied systematically. When the ZSM-5 concentration was 40% and the coating time was 60 min, separation factor and phenol permeability were 4.56 and 5.78 g/(m2 h), respectively. ZSM-5/PDMS/PVDF membrane significantly improved the recovery efficiency of phenols. © 2017 Published by Elsevier B.V.

1. Introduction Phenol is presented in the wastewater of various industries, such as refineries, coking operation, coal processing, pharmaceuticals, plastics and wood products [1,2]. Phenol is difficult to be biodegraded due to its fatal toxicity to most microorganisms [3]. Therefore, the removal of phenols constitutes a key step before the biological treatment process. Low energy consumption Pervaporation (PV) has been extensively studied as a key technology of wastewater treatment and is regarded as the most promising candidate for selectivity separation of volatile organic compounds (VOCs) from their aqueous solutions [4–6]. In PV, separation of the membrane depends on solutiondiffusion coefficients of the feed liquids in the dense homogeneous asymmetric membrane. The determinant factor of PV is to find out the suitable membrane materials for the specific system. Several membrane materials such as chitosan [7], cellulose acetate [8], polysulfone [9], polyvinyl alcohol [10] and polyvinylidene fluoride

∗ Corresponding author at: School of Environment, Harbin Institute of Technology, Harbin 150090, PR China. E-mail address: [email protected] (J. Yao). http://dx.doi.org/10.1016/j.apsusc.2017.08.202 0169-4332/© 2017 Published by Elsevier B.V.

(PVDF) [11] are used in the removal of organic matter from aqueous solutions. PVDF, a semi-crystalline polymer exhibiting high thermal stability and excellent aging resistance [12,13], has been widely used in applications [14–16]. PVDF shows good membrane forming properties to prepare flat sheet or hollow fiber membrane. Hollow fiber membrane has a larger membrane area for a fixed module volume than flat-sheet membrane does [17]. However, the hydrophobicity of PVDF is limited. Therefore, the fabrication of a more hydrophobic PVDF membrane is essential for membrane PV development [18]. Physical and chemical methods are commonly used to strengthen hydrophobicity of membranes. The hydrophobicity of membranes depends on both the low surface energy and the surface roughness. PDMS has an affinity for organic matter, which is stable in both acidic and alkaline solution [19]. PVDF was selected as the support membrane material for its good chemical stability, while PDMS was chosen as coating layer for its low surface energy. M. L. Yeow et al. have reported that the fabrication of divinyl-PDMS/PVDF composite hollow fiber membranes for the purpose of selectively separating the xylene from nitrogen is clearly demonstrated with recovery greater than 95% [20]. As an inorganic hydrophobic material, Zeolite has regular crystal structure and uniform pore structure (4 Å–8 Å). The excellent adsorption selectivity

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of zeolite for organic matter results from its unique structural channels [21–24]. Although several studies have observed that organic or ceramic composite membrane exhibited considerable potential for removal of volatile organic compounds from aqueous solutions [6,25,26], reports regarding the separation of phenol from aqueous solutions through ZSM-5/PDMS/PVDF hollow fiber composite membrane have not been published as far as we know. The agglomerate phenomena of zeolite particles cause defects on composite membrane surface [27]. So far, immersion and coating method are widely used methods for the hydrophobic modification of membranes. However, it is extremely difficult for the modification of hollow fiber membranes. Existing method would block the membrane channel and thus decrease the permeability of membranes in the hydrophobization process. In this work, zeolite surface was first grafted by noctyltriethoxysilane (OTES) and then mixed with PDMS solution immediately in order to disperse ZSM-5 zeolite in the PDMS matrix. A layer of PDMS solution filled with the dispersed ZSM-5 zeolite was coated on PVDF hollow fiber membranes through dynamic negative pressure. The characterization of the modified PVDF membrane was assessed by SEM-EDS and CA, and the influence of zeolite concentration, coating time, the content of phenol in feed, temperature and pressure of the penetration side on membrane PV performance was studied systematically. 2. Experimental 2.1. Materials The commercial PVDF hollow fiber membrane module (average pore size 0.16 ␮m, hollow fiber inner diameter 0.8 mm, external diameter 1.16 mm, porosity 85%, membrane area 553 cm2 ) were purchased from Institute of Biological & Chemical Engineering, Tianjin Polytechnic University. Polydimethylsiloxane (PDMS) with molecular weight 20000, n-octyltriethoxysilane (OTES) and dibutyltindilaurate (DBTDL) were supplied by Heowns Biochem Technologies Co. Ltd., China. Sodium hydroxide, n-heptane, analytical reagent (National Medicine Group Chemical Reagent Co. Ltd.) were used. ZSM-5 (average particle size 1 um), which is used as filler was supplied by Tianjin Nankai Catalyst Co. Ltd. China.

289

Fig. 1. The mechanism of surface modification of hollow fiber membranes.

Fig. 2. PV process setup. 1. Feed tank, 2. Constant temperature water bath, 3. Thermometer, 4. Hollow fiber membrane module, 5. Peristaltic pump, 6. Liquid nitrogen trap, 7. Condensate collecting bottles, 8. Buffer tank, 9. Vacuum pump, 10. Vacuum gauge.

ing solution through the tube side of hollow fiber module via a peristaltic pump, the shell side of the hollow fiber membrane module was evacuated through a vacuum pump (0.04 MPa). The flow direction of the coating solution was reversed every 5 min. At the surface of the PVDF membrane appeared a composite layer after the cross-link had lasted for 48 h. This unique membrane has two hydrophobic layers, including ZSM-5/PDMS and PVDF, which could make it highly selective toward organic molecules (Fig. 1).

2.2. Fabrication of the composite membranes

2.3. Characterization

Residual organic solvents and other impurities in PVDF membranes must be removed prior to the chemical modification. Thus, PVDF membrane was prepared in situ by circulating sodium hydroxide solution (NaOH, 60 ◦ C, concentration 2 mol/L) into the tube-side of the membrane module via a peristaltic pump for 15 min. Hollow fiber membrane were rinsed in deionized water to adjust effluent water pH being neutral. Finally, the membranes were dried at 30 ◦ C. ZSM-5 zeolite were added to OTES/n-heptane solution through stirring, ultrasonic oscillation, then a certain amount of PDMS was added. The n-heptane, ZSM-5, OTES and PDMS were aggregated in the mass ratio of 30:70:2.5:0.5 [6,28], with continuous stirring at room temperature, to acquire the ZSM-5 zeolite silanization solution. PDMS cross linking solution was prepared by dissolving PDMS, a cross-linking agent (TEOS) and a catalyst (DBTDL) with a mass ratio of 10:1:0.1 in n-heptane while stirring and subsequently, PDMS cross-linking solution immediately mixed with ZSM-5 zeolite solution, while stirring for 1 h, to acquire the ZSM-5/PDMS coating solution. Composite membranes were prepared by dynamic negative pressure coating method, i.e. by circulating the ZSM-5/PDMS coat-

The membrane morphology was characterized by Scanning Electron Microscopy (SEM), the content of C, O, F, Si elements on the membrane surface by Energy Dispersive X-ray spectroscopy (EDS) (Oxford INCA, Germany) combined with the SEM, and contact angle (CA) measurements using the goniometer (VAF-30) and deionized water as a liquid at room temperature. Fourier transform infrared (FTIR) technique was used to confirm the modification process. Spectra for original ZSM-5 and modified ZSM-5 were performed in the mid infrared range.

2.4. Pervaporation experiment The PV performance of the membrane for 1L phenol aqueous solution was tested in lab scale setup, shown in Fig. 2, in which a heated aqueous solution at 50 ◦ C was circulated continuously through the tube-side of the membrane module by a peristaltic pump with a flow rate of 10 L h−1 and with vacuum at the permeate side maintained in the 50 kPa. The permeate vapor was collected in a liquid nitrogen trap during the experiments. The phenol concentration was determined by 4-Aminoantipyrine spectrophotometry (HJ 503-2009).

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Fig. 3. SEM and EDS images of the membrane: (a) the cross section of non-modified ZSM-5/PVDF membrane; (b) the inner surface of non-modified ZSM-5/PVDF membrane; (c) non-modified ZSM-5/PVDF membrane EDS; (d) the cross section of modified ZSM-5/PVDF membrane; (e) the inner surface of modified ZSM-5/PVDF membrane; (f) modified ZSM-5/PVDF membrane EDS.

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Fig. 4. SEM images of the cross-sections of 10 min (a), 30 min (b), 50 min (c); and the surfaces of 10 min (d), 30 min (e), 50 min (f) membrane coating.

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The PV performance of a membrane is usually expressed in terms of the permeation flux J and separation factor ␣ as Formula (1) and (2) [29,30]. J=

W A · t

(1)

Where J is permeation flux (g m−2 h−1 ), W is permeation mass (g), A is the effective area of membrane (m2 ), t is the time of permeation (h). ␣=

YA /YB XA /XB

(2)

Where ␣ is separation factor, YA and XA are the mass fraction of the compound A in the permeation and feed, respectively. 3. Results and discussion 3.1. Morphology analysis of membrane The influence of native ZSM-5 and modified ZSM-5 on sections and internal surface microstructure morphology of ZSM-5 filled PDMS/PVDF hollow fiber membrane was studied by SEM and the surface element of the sample with EDS, as shown in Fig. 3. SEM images illustrated that the difference of thickness of separating layer was obvious in Fig. 3(a) and (d), with the concentration of ZSM-5 being 20%. The thickness of separating layer of ZSM-5 zeolites modified with silane coupling agent membrane is twice that of native ZSM-5 filled PDMS/PVDF membrane. The inner surface of native ZSM-5 filled PDMS/PVDF membrane is rough, therefore the zeolite particle clusters phenomenon is severe in Fig. 3(b), in which ZSM-5 zeolite particles are not dispersed evenly in the PDMS matrix. The main cause of non-uniformity is that there is very little interaction between the zeolite particles and PDMS and the zeolite filled PDMS suspension can be unstable. Therefore, in order to disperse uniform ZSM-5 zeolite in a PDMS matrix (Fig. 3(e)), the zeolite surface was first grafted with OTES and then mixed with the PDMS solution. As a result, an elevated dispersion was ensured by creating a stable covalent bond (-Si-O-Si-) between the hydrolyzed silane molecules and the hydroxyl groups (-OH) on the zeolite surface. PDMS chains could entangle with n-octyl chains to provide considerable interactions between the zeolite particles and PDMS matrix, which achieves a homogeneous dispersion [25]. The EDS results indicated that the silicon content of the separate layers is increased after modifying the ZSM-5 zeolite particle surface. To achieve uniform coating, the ZSM-5/PDMS/PVDF hollow fiber composite membrane was prepared by applying dynamic negative pressure instead of dipping or brushing. The concentration of ZSM-5 was fixed at 40%, the shell side of the hollow fiber membrane module was evacuated through vacuum pump (80 kPa). The SEM images of composite membrane at different coating times are shown in Fig. 4; from those cross-sectional images, it was found that the membranes consisted of an asymmetric structure with a thin top skin layer and a finger-like macro-voids region and that the thickness of separating layer increased with the duration of the coating. When coating time was being prolonged from 10 to 50 min, the corresponding separating layer thickness increased from 1.5 um to 5 um. 3.2. Structure analysis of membrane The surface functional groups of the original and grafted ZSM-5 zeolite were further studied via FTIR. The FTIR spectra of the original and grafted ZSM-5 zeolite are shown in Fig. 5. The peak at 1100, 800 cm−1 can be attributed to the asymmetric and symmetric stretching vibrations of Si-O-Si, respectively. The bending vibration of Si O Si was observed at 450 cm−1 . The result is in agreement

Fig. 5. FT-IR spectra of original and grafted ZSM-5 zeolite.

Fig. 6. Effect of ZSM-5 concentration on water contact angle of membrane. (a) Separation factor (b)Water flux.

with the available analytical result reported in research article [31]. A broad absorption band near 3400 cm−1 , was attributed to the OH stretching vibration. The peak at 2950 cm−1 and 2929 cm−1 were attributed to the asymmetric C H stretch from OTES [32]. These features were essentially identical for the ZSM-5 zeolite both before and after graft. Moreover, in the spectra of grafted ZSM-5 zeolite, it was apparent that the broad band at 3400 cm−1 decreased in intensity whereas the bands at 1100, 800, and 450 cm−1 (Si-O-Si) increased in intensity. This result illustrated some new Si O Si bonds forming in ZSM-5 zeolite, which may be attributed to the n-octylsilane groups grafted to the surface of the ZSM-5 particles. 3.3. Hydrophobicity of membrane In general, the hydrophobic property of membranes is determined by the comparison of contact angles [25,33]. Thus surface hydrophobicity of the unfilled and surface modified ZSM-5 filled PDMS/PVDF composite membrane was evaluated by water static contact angle measurement and the results are shown in Fig. 6. Increase in the water contact angle indicates that PVDF membrane surfaces were efficiently modified (Fig. 6), showing that the filling of ZSM-5 zeolite particles could clearly promote hydrophobicity of PDMS/PVDF composite membrane surfaces. The water contact angle rises straight with increasing ZSM-5 concentration, from 99◦ to 132◦ , when ZSM-5 concentration extended from 0% to 50%, enabling the surface of the composite membrane to be highly resistant to wetting. In addition, the contact angle of a

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293

Fig. 7. Effect of surface modification on PV performance.

Fig. 8. Effect of the ZSM-5 concentration on the PV performance.

ZSM-5/PDMS/PVDF membrane is larger than that of a PDMS/PVDF membrane, because the ZSM-5 particles distributed in PDMS matrix increase the composite membrane surface roughness, and thus demonstrating that the surface micro structures are the key to the preparation of hydrophobic surfaces. To further assess the impact of modified ZSM-5 on hydrophobic property, a comparison of PV performance between the PDMS/PVDF membranes filled with unmodified and surface modified ZSM-5 particles are shown in Fig. 7. In Fig. 7(a) it is displayed that the separation factor of non-modified ZSM-5 filled PDMS/PVDF membranes is practically constant at 1.5, which is nearly equivalent to that of the unfilled PDMS/PVDF membrane. The result can be attributed to the existence of non-selective defects in these unmodified ZSM-5 filled PDMS/PVDF membranes [34]. For modified ZSM-5 filled PDMS/PVDF membrane, the separation factor increased to 2.8 when the concentration of ZSM-5 increased from 0% to 40%. As shown in Fig. 7(b), the water permeate flux of both unmodified and OTES modified ZSM-5 filled PDMS/PVDF membranes decreased with increasing ZSM-5 loading. And at the same ZSM-5 loading, the water flux of OTES modified ZSM-5 filled PDMS/PVDF membrane was less than that of the unmodified membrane. These results agreed well with the desired purpose of ZSM-5 incorporation in PDMS/PVDF membranes—with the observation that OTES modified ZSM-5 zeolite was dispersed uniformly in PDMS matrix, and thus ZSM-5 create s a preferential pathway for phenol permeation due to the sorption selectivity [30]. 3.4. PV performance of membranes 3.4.1. Effect of ZSM-5 concentration Fig. 8 shows a comparison of PV performance (phenol flux, water flux and separation factor) among the composite membranes

Fig. 9. Effect of coating time on the PV performance.

with varying ZSM-5 concentration. It was observed that the phenol flux increased as the ZSM concentration increased up to 20%, and then decreased gradually. The flux of phenol slightly increased from 2.9 to 3.3 g/(m2 h) as the concentration of ZSM-5 increased from 0% to 20%. Water flux was reduced from 2.1 to 1.0 kg/(m2 h) when the concentration of ZSM-5 increased from 0% to 50%. Composite membranes with 50% ZSM-5 coating displayed one time increase in separation factor; since the membrane is hydrophobic, the membrane should preferentially permeate organic compounds rather than water and the permeation will be enriched in the organic compounds [27]. Hydrophobic ZSM-5 sustained a much better selectivity for organic molecules [35], which led to the sharp decrease of water flux of ZSM-5 filled PDMS/PVDF composite membranes as the zeolite loading increased. So the phenol flux slightly increased, while water flux decreased when the concentration of ZSM-5 increased from 0% to 50%. Wang Y et al. studied physically hydroxyl-functionalized MWCNTs incorporated into HTPB-based polyurethane membrane for separation of phenol/water mixture by pervaporation. The result showed that modified hydroxylfunctionalized MWCNTs incorporated polyurethane membranes obtained a separation factor of 2.8 at 80 ◦ C [36]. Zeolite with regular crystal structure would limit the free movement of PDMS molecular chain and inhibit the swelling effect and so filling the zeolite is one necessary condition for good separation. 3.4.2. Effect of coating time The effect of coating time on the PV performance was also studied and is given in Fig. 9. The water flux and phenol flux were evidently reduced with extended coating time, with a coating time from 10 min up to 60 min in the case of composite membrane

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Fig. 10. Effect of phenol content in feed on the PV performance.

Fig. 11. Effect of operational temperature on the PV performance.

concentration of organic matter on both sides of the membrane was calculated by Eq. (3) [42]. causing the phenol flux to decrease from 3.7 to 2.3 g/(m2 h). The corresponding water flux decreased from 1960 to 800 g/(m2 h) when the coating time was extended from 10 to 60 min. The phenol flux and water flux decreased while the separation factor increased with prolonged coating time. This observed phenomenon agrees with the literature [37]. Combined with the analysis of the SEM results, namely that separation layer thickness increased with the extension of coating time, while also making mass transfer resistance increase, it could be observed that the water flux and the phenol flux showed a decreasing trend, whereas the separation factor increased from 2.05 to 3.10 when the coating time was prolonged from 10 to 60 min. Generally, the flux through a membrane during PV is affected by the membrane thickness, diffusion of the molecule into the pores of the membrane, adsorption of the molecule on the membrane surface, and the fugacity of the molecule on the feed side [38].

3.4.3. Effect of feed concentration Fig. 10 shows that with the increase of phenol content in feed, phenol flux and water flux were both increased and when the concentration of phenol in feed increased from 1 to 5 g/L, the phenol flux increased from 2.5 g/(m2 h) to 12 g/(m2 h) accordingly indicating that increasing the phenol content in feed improves the mass transfer driving force, and furthermore increased phenol removal efficiency significantly. At the same time the water flux increased from 800 g/(m2 h) to 1300 g/(m2 h) when the concentration of phenol in feed increased from 1 to 5 g/L. The total flux increased while the separation factor decreased with increasing phenol concentration. This phenomenon agrees with the literatures [39,40]. As the phenol content in the feed increased, the swelling degree of the membrane increased due to the strong affinity of phenol for the membrane. This resulted in the diffusion of phenol and water occurring more easily. Therefore, the phenol flux and increase increased with increasing phenol content in feed. According to the solution-diffusion theory [41], the permselectivity of liquid mixtures through membranes by PV depends on both the differences in the solubility process and the diffusion process of the per-meant molecules in the composite membranes. However, the increase in the water diffusivity was much larger than that of phenol diffusivity in the diffusion separation process because the size of the water molecules is smaller than that of the phenol molecules. According to kinetic studies of mass transfer, organic compounds are separated from aqueous solution by PV process. The relation between the flux of volatile organic compounds and the



Ji = Ki  (Ci )L − (Ci )V



(3)

Where Ji is permeation flux of component i g/(m2 h), Ki is constant rate of component i mass transfer m/s, ␳ is the molar density of component i in feed mol/m3 , (Ci )L and (Ci )V are the mole fraction of the compound i in the feed and permeation, respectively. The increase of phenol concentration made the liquid phase concentration (Ci )L increase, however the other side of membrane V (Ci ) had changed slightly. Phenol flux increased linearly with the increase of phenol concentration in feed. The reduction of separation factor values was also noticed as a result of phenol concentration increase in feed. 3.4.4. Effect of feed-in temperature The influence of temperature on the flux and separation factor of composite membrane is presented in Fig. 11. The permeate fluxes depended strongly on the temperature difference between feed and the permeate side, meaning that increasing temperature contributed to an increasing value of phenol flux, water flux and separation factor. The phenol flux increased from 1.5 g/(m2 h) to 5.2 g/(m2 h) when the temperature of feed increased from 30 to 80 ◦ C and separation factor reached 4.2 at 80 ◦ C, about 70% higher than that at 30 ◦ C. Since increasing feed-in temperature was able to augment the saturated vapor pressure of phenol, which then produced a wide vapor pressure differential between feed and the permeate side of the PV membrane, and thus increasing the mass transfer driving force, the permeation flux increased. In addition, an increase in temperature lead to an increase in the kinetic energy of polymer chain, with the growth of free volume being beneficial to phenol and water molecules passing through the membrane. At the same time, the molecular thermal motion at high temperature is more violent than at low temperature, so the molecular diffusion free path greatly increased, thus enabling phenol and water fluxes to significant growth. It is commonly believed that the influence of operating temperature on flux in the pervaporation separation process is in accordance with Arrhenius Eq. (4) [43,44]. Ji = J0 exp

 −E  a

RT

(4)

Where Ji is permeation flux of component i g/(m2 h), J0 is permeability constant g/(m2 h), Ea is activation energy of component i J/mol, R is constant, 8.314 J/(mol K), T is absolute temperature K. Fig. 12 shows the Arrhenius equation fitting curve of permeate flux and temperature, in which activation energy Ea of the water and phenol are calculated through the relation of the straight line’s slope Ea /R. The activation energy was introduced as a sensitivity

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295

Fig. 12. Arrhenius equation fitting curve of permeate flux and temperature.

Fig. 13. Effect of operational pressure on the PV performance.

indicator of penetration flux change at different temperatures. Here activation energy of the phenol and water are equal to 22.33 kJ/mol and 17.28 kJ/mol, respectively. The growth of the phenol flux is greater than that of the water flux with an increase in temperature, so the separation factor has a significant growth. 3.4.5. Effect of operation pressure The pressure on the feed side of the membrane was fixed at 100 kPa, and the influence of pressure on flux and separation factor was investigated as the penetration side of membrane ranged from 5 to 30 kPa as given in Fig. 13. The phenol flux declined evidently with the extension of pressure on the penetration side of the membrane and phenol flux was equal to 5.8 g/(m2 h) and 4.2 g/(m2 h) when pressure was 5 kPa and 30 kPa, respectively, but water flux was reduced slightly. The reason is that increasing pressure on the penetration side of the membrane caused a decrease of the vapor pressure on both sides of the membrane and the PV driving force. Since the saturated vapor pressure of phenol is less than that of water, the increase of phenol impetus is far greater than that of water when the vacuum degrees of the penetration side increases. 3.4.6. Membrane stability For industrial applications of pervaporation, it is expected that the membrane performance can be kept stable for a long time duration. To investigate the stability of the pervaporation performance with modified ZSM-5 filled PDMS/PVDF composite membrane, a pervaporation experiment with diluted phenol solution (1 g/L) was performed for 20 h with intervals of 1 h. The results are shown in Fig. 14. As shown in Fig. 14a), It can be seen that the flux of phenol is maintained at about 6 g/(m2 h), while the separation fac-

Fig. 14. Effect of operational time on the PV performance.

tor decreased slightly. Fig. 14b) shows the change of the phenol concentration in the feed and permeation solutions during the prolonged operation. The concentration of phenol in the feed solution and in the permeation solutions changed slightly. By the test, flux and separation factor changed slightly during the 20 h which proved that the zeolite filled PDMS/PVDF composite membrane would keep the good performance long term. Table 1 shows the comparison between the separation performances of ZSM-5/PDMS/PVDF membrane and other membranes reported in the literature [36,37,45–48]. Table 1 contains the performance data reported in the literature for other membranes used to remove phenol from aqueous solutions at concentrations ranging from 0.5 to 3 wt%. Although it is difficult to make a direct comparison because the experimental conditions were not consistent, the permeation flux obtained with this ZSM-5/PDMS/PVDF membrane is remarkably high compared to those obtained with other membranes. Though the PDMS [47] and Polyurethane [48] membranes seem to have higher selectivity among all the membranes shown in Table 1, their permeate fluxes are insufficient. No long-term stability is reported in their work. So these membranes cannot meet the needs of industrial production. In conclusion, the ZSM-5 filled PDMS/PVDF membrane, developed in this work, is a suitable membrane for phenol separation, since it presents relatively good fluxes when compared with other inorganic solvents with good stability.

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Table 1 Pervaporation performance of phenol–water separation with different membranes. Membrane

Feed phenol conc. (wt%)

PDMSI MWNTs/PU PIM-1 HMDI-PTMG PDMS Polyurethane

3 0.5 1 1 2 1

ZSM-5/PDMS/PVDF

0.001

Temp. (◦ C)

60 80 70 60 70 80 70 80

Permeate flux (g m−2 h−1 ) Total

Phenol

147 175.56 210 550 370 620 400 1585

57 – 29 90 70 150 100 5.2

Separation factor

Reference

20 2.8 16 11.7 21 35 40 4.56

[45] [36] [46] [37] [47] [48] [48] This work

PDMSI: polydimethylsiloxaneimide; MWNTs/PU: Multi-walled carbon nanotubes were incorporated into HTPB-based polyurethane membrane; PIM-1: A polydioxane polymer with intrinsic microporosity; PDMS: polydimethylsiloxane.

4. Conclusions In summary, ZSM-5/PDMS/PVDF composite PV membranes were successfully fabricated by dynamic negative pressure and the water contact angle was increased significantly from 99◦ (ZSM-5 0%) to 132◦ (ZSM-5 50%). The modified hollow fiber membrane appeared more hydrophobic and gave less water permeation and a higher separation factor. SEM-EDS images analysis indicated that the ZSM-5/PDMS layer was attached on the surface as well as inside the porous structure of the PVDF membrane. ZSM-5 was dispersed into the PDMS solution, which improved the penetration property of composite membranes significantly. When the ZSM-5 concentration was 40% and the coating time was 60 min, separation factor and phenol permeability were 4.56 and 5.78 g/(m2 h), respectively, suggesting that the ZSM-5/PDMS/PVDF membrane can significantly improve the efficiency of phenol recovery. The performance of modified ZSM-5 filled PDMS/PVDF membrane could be kept stable for a long time duration.

Acknowledgment The present paper was supported by Science and Technology Plan Projects of Harbin (No. MJ20140055).

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