Article pubs.acs.org/IECR
Preparation and Performance of PET-Braid-Reinforced Poly(vinylidene fluoride)/Graphene Hollow-Fiber Membranes Junqiang Hao,†,‡ Changfa Xiao,*,†,‡ Tai Zhang,† Jian Zhao,† Zuwei Fan,† and Li Chen† †
School of Textiles and ‡State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China S Supporting Information *
ABSTRACT: Poly(ethylene terephthalate)- (PET-) braid-reinforced poly(vinylidene fluoride)/graphene (PBR−PVDF/GE) hollow-fiber membranes were prepared by a dry−wet spinning process; they consisted of an outer-coated PVDF/GE separation layer and inner PET hollow tubular braids. GE was employed to increase the hydrophobicity of the membranes. The influences of the GE concentration in the casting solution on the structure and performance of the PBR−PVDF/GE hollow-fiber membranes were investigated in terms of the pore size distribution, water contact angle (CA), water entry pressure (WEP), and differential scanning calorimetry (DSC), and the morphologies of the membranes were observed by field-emission scanning electron microscopy (FESEM). The results indicated that the as-prepared membranes completely repelled water during an 8-h continuous oil/water separation process and were hydrophobic and had a high WEP. The PBR−PVDF/GE hollow-fiber membrane with 0.5 wt % GE exhibited the optimal performance and was used for reusability testing in oil/water separation. The separation efficiency of the membrane did not deteriorate with increasing number of cycles, showing an outstanding durability and reusability.
1. INTRODUCTION Oily wastewater is generated in many industries, such as the petrochemical, food, textile, leather, steel, and metal finishing industries,1 as well as in oil product transportation. If the oily wastewaters are discharged directly, they will do serious harm to local aquatic environments. Conventional treatments for oil/ water separation, including adsorption, oil skimming, centrifugation, air flotation, coagulation, de-emulsification, and flocculation,1−4 have been extensively used. However, these methods have some drawbacks, including low efficiencies, high costs, corrosion, and even secondary pollution. Compared with these methods, membrane separation technology has many advantages in the treatment of oily wastewaters, such as no additives, no secondary pollution, and low costs.5 In principle, both hydrophilic-oleophobic membranes and hydrophobic-oleophilic membranes could be used for oil/water separations.6 Generally, the pollutants in oily wastewater mainly arise from the oil phase. Hydrophilic membranes preferentially promote water permeation rather than oil permeation, resulting in a much higher water flux and good resistance to fouling. For these reasons, hydrophilic membranes have attracted great research attention.7 However, the oil contents in most oil/water mixtures to be treated are relatively low, so hydrophilic membranes for oil/water separations must afford huge water permeate fluxes, thus requiring large membrane areas and high energy consumption. In terms of workload, oil/water mixtures should be treated with hydrophobic membranes for duty reduction. Kong and Li8 prepared flat poly(vinylidene fluoride) (PVDF) membranes with different pore sizes and porosities and used them for oil removal from a dilute oil/water mixture. This proved to be feasible for oil as a permeable material in oil/ water mixtures based on oil affinity. Gorouhi et al.9 applied hydrophobic polypropylene (PP) membranes for the treatment © 2016 American Chemical Society
of oily wastewater and found that increased temperature, pressure, and flow rate improved both the permeate flux and the water content in the permeate. Ahmad et al.10 prepared polysulfone (PSf) mixed-matrix membranes with functionalized SiO2 nanoparticles to improve the PSf hydrophobicity. The permeate flux of the modified membrane (PSf-5) was 17.32 L/ m2·h, whereas that of the unmodified membrane (PSf-0) was 1.08 L/m2·h for the oil/water separation. However, the hydrophobic membrane as reported had some common drawbacks including consistently reduced hydrophobicity and low mechanical strength.11 At present, ultrafiltration (UF) and microfiltration (MF) hollow-fiber membranes that consist of a skin layer and a support layer are popularly used to treat various manufacturing industry wastewaters. However, hollow-fiber membranes are liable to be damaged or broken during the high-pressure cleaning process or by a disturbance in the aerated airflow during the water treatment process. It is definite that the excellent mechanical properties of hollow-fiber membranes are desired.12 Xiao and co-workers13,14 demonstrated that the method of coating a separation layer on a high-strength hollow tubular braid is effective in improving the mechanical properties of hollow-fiber membranes. Kolon Industries Inc.15 developed a braid-reinforced (BR) hollow-fiber membrane that comprised a reinforced material of tubular braids and a resinous thin film coated on the surface of the reinforced material. Wang et al.16 prepared poly(ethylene terephthalate) (PET) fiber tubular braids/PVDF composite hollow-fiber membrane by a PVDF Received: Revised: Accepted: Published: 2174
November 22, 2015 February 2, 2016 February 3, 2016 February 3, 2016 DOI: 10.1021/acs.iecr.5b04428 Ind. Eng. Chem. Res. 2016, 55, 2174−2182
Article
Industrial & Engineering Chemistry Research separation layer coating on the PET fiber tubular braids by a dry-jet spinning method. The membranes as-prepared showed good properties, including high flux, high strength, and high rejection. PVDF membranes have a wide range of applications, including membrane distillation, gas separation, oil/water separation, biomedical materials, and electrochemicals,17−19 because PVDF has excellent physical and chemical properties. PVDF membranes exhibit good hydrophobic performance for oil/water separation; however, their hydrophobicity decreases dramatically the in continuous use.17 Teoh and Chung20 prepared PVDF/polytetrafluoroethylene (PTFE) hollow-fiber membranes for membrane distillation using the phase inversion method, yielding a resultant water contact angle (CA) of 103°. Efome et al.21 prepared flat nanocomposite PVDF/SiO2 membranes by the phase inversion technique, which showed that the addition of superhydrophobic SiO2 nanoparticles enhanced the hydrophobicity of the nanocomposite membranes. Hence, the blending method is a simple and effective method for improving the hydrophobicity of PVDF-based membranes. As a single-atom-thick sheet composed of sp2-hybridized carbon atoms, graphene (GE) is highly hydrophobic.22 GE hybrid materials have been employed for oil adsorption and oil/ water separation, including PVDF/GE porous materials,23 magnetic polymer-based GE foam,24 and graphene-based sponges.25 Zha et al.23 prepared superhydrophobic PVDF/GE porous materials by a simple method of diffusion and freezedrying; the resulting porous materials were superhydrophobic and oleophilic. Liu et al.24 prepared magnetic polymer-based GE foam (MPG) through the synergistic effects of the deposition of Fe3O4 nanoparticles on GE sheets and the selfassembly of GE on polyurethane (PU) sponges; the resulting MPG exhibited superhydrophobicity, superoleophilicity, and good reusability. Nguyen et al.25 prepared GE-based sponges by a dip-coating method, obtaining sponges with high adsorption capacities to a broad range of oils with high oil/water selectivities, good recyclability, low weight, and an excellent oil adsorption capacity approaching 165 g/g. However, none of these previously developed materials can be used for continuous oil/water separation. In this work, we prepared a PET-braid-reinforced PVDF (PBR−PVDF)/GE hollow-fiber membrane with stable hydrophobicity and mechanical strength that can be used for continuous oil/water separation by a facile method of concentric-circle coating. GE was blended into PVDF/GE solutions, which were coated twice on the PET tubular braids. The structure and performance of the PBR−PVDF/GE hollowfiber membranes were investigated in terms of membrane morphology, pore size, and water entry pressure (WEP). Finally, the continuous oil/water separation performance and reusability of the membranes were tested using a laboratoryscale continuous setup.
purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co., Ltd. (Tianjin, China). Congo red was purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Hydrophobic SiO2 particles (40 nm) were purchased from Guangzhou GBS High-Tech & Industry Co., Ltd. (Guangzhou, China). PET tubular braids were supplied by Tianjin Boanxin Co., Ltd. (Tianjin, China). Kerosene was obtained from Tianjin Kailida Chemical Co., Ltd (Tianjin, China). All chemicals were used directly without further purification. 2.2. Membrane Fabrication. The PBR−PVDF/GE hollow-fiber membranes were prepared by the dry-wet spinning process (Figure 1) according to the following steps. To
Figure 1. Schematic of the spinning setup.
enhance the interfacial bonding between the surface coating layer and the braids, the PET fiber tubular braids were washed with 3 wt % NaOH aqueous solution for 2 h at 90 °C;16,26 then, the product was washed with distilled water to remove residual NaOH and dried to a constant weight. After this pretreatment, the hollow tubular braids were coated with the PVDF/GE solutions at 70 °C and then guided through a water coagulation bath at 30 °C. The primary PBR−PVDF/GE hollow-fiber membranes formed were kept in water for at least 8 h and stored in ethanol for at least 12 h to remove the residual solvents and DOP additive. Then, this process was repeated again to attain PBR−PVDF/GE hollow-fiber membranes. The air-gap distance and take-up speed were set at 20 cm and 120 cm/min, respectively. The PVDF/GE solutions were prepared by blending PVDF, DMAc, GE, and hydrophobic additives under vigorous stirring in a flask at 70 °C for 5 h (Table 1). Ultimately, five membranes with different GE concentrations in the casting solution (0, 0.1, 0.3, 0.5, and 0.7 wt %) were obtained, labeled as membranes M0, M1, M2, M3, and M4, respectively. 2.3. Membrane Characterization. 2.3.1. Shear Viscosity of Casting Solutions. The shear viscosities of the PVDF/GE solutions were measured with a rotational rheometer (Bohlin CVO, Malvern Instruments, Malvern, U.K.) under different shear rates at ambient condition. 2.3.2. Morphology. The morphologies of the membranes were observed by field-emission scanning electron microscopy (FESEM; S4800, Hitachi, Japan). Each membrane sample was immersed in ethyl alcohol, t-butanol/ethyl alcohol (50/50 vol %), and t-butanol for 12 h before observation. After being freeze-dried, the samples were cut off with a razor blade. In
2. EXPERIMENTAL SECTION 2.1. Materials. PVDF resin (product no. 6010) was purchased from Solvay Advanced Polymers (Parachute, CO). N,N-Dimethylacetimide (DMAc, 98%) was obtained from Samsung Fine Chemical Co., Ltd. (Ulsan, Korea). GE (KNGG5, thickness < 5 nm, flake size = 0.1−5 μm) was purchased from Xiamen Knano Graphene Technology Co., Ltd. (Xiamen, China). Dioctyl phthalate (DOP), t-butanol, ethylene glycol, nbutyl alcohol, methyl alcohol, ethyl alcohol, and NaOH were 2175
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membrane modules were immersed in kerosene oil for 10 min before the measurement. Then, the pressure imposed on the membranes was increased slowly until the first water droplet dripped (initial pressure = 0 MPa, step size = 0.02 MPa). Each pressure was continuously tested for 30 min, and five measurements were carried out to obtain the average value. 2.3.5. Differential Scanning Calorimetry (DSC). The crystalline properties were determined by DSC (DSC200F3, NETZSCH Corporation, Selb, Germany) under N2. The separation layers of the membrane samples were heated from room temperature to 250 °C at a rate of 10 °C/min. The degrees of crystallization were calculated according to the equation
Table 1. Recipes and Spinning Parameters of PBR−PVDF/ GE Membranes with Different Casting Solutions membrane
GE (wt %)
M0 M1 M2 M3 M4
0.0 0.1 0.3 0.5 0.7
DMAc (wt %) 73 72.9 72.7 72.5 72.3 parameter
PVDF (wt %)
DOP (wt %)
SiO2 (wt %)
13 13 13 13 13
10 10 10 10 10
4 4 4 4 4
casting solution temperature (°C) water coagulation temperature (°C) air gap (cm) take-up speed (cm/min)
value 70 30 20 120
Xc =
addition, the separation layers of the membranes were broken after the membranes had been frozen in liquid nitrogen for 10− 15 s, for clear observation of the cross-sectional structures of the separation layers. 2.3.3. Pore Size, Pore Size Distribution, and Porosity. The pore sizes and pore size distribution of each sample were determined using a capillary flow porometer (Porous Materials Inc., Ithaca, NY), and the values were calculated from the pressure of the gas flow. The gravimetric method was used to assess membrane porosity, which was calculated from the weight of liquid immersed in the membrane pores. n-Butyl alcohol was used as the wetting liquid. The porosity (ε) was calculated as W1 − W2 ε (%) = × 100% (π /4)(D2 − d 2)lρ (1)
ΔH × 100% ΔHf
(2)
where ΔH is the enthalpy of fusion for the separation layers of the membrane (J/g) and ΔHf is the enthalpy of fusion for a 100% crystalline sample of PVDF (104.7 J/g).27 2.3.6. Continuous Oil/Water Separation Tests. In the continuous oil/water separation tests, membrane M0 could not afford total oil/water separation, as a water droplet could be forced to permeate from the outside to the inside of the membrane at a pressure of −0.1 MPa. Consequently, only the other membranes (i.e., membranes M1−M4) were subjected to further study. Negative-pressure dead-end filtration experiments with the PBR−PVDF/GE hollow-fiber membranes were conducted using a laboratory-scale continuous setup, as shown in Figure 3. In this study, the work pressure was set at −0.1 MPa
where D is the outer diameter (cm), d is the inner diameter (cm), and l is the length of the sample membrane (cm); ρ is the density of n-butyl alcohol (ρ = 0.81 g/mL); W1 is the weight of wet membrane (g); and W2 is the weight of dry membrane (g). 2.3.4. Contact Angles of Different Liquids and Water Entry Pressure. Contact angles of different liquids were measured using a contact angle goniometer (DSA-100, Krüss GmbH, Hamburg, Germany). The contact angle values of water, ethylene glycol, n-butyl alcohol, methyl alcohol, and ethyl alcohol were obtained. For each membrane sample, five measurements were carried out to obtain the average value for each liquid. The WEP of each membrane sample was measured using a laboratory-scale microfiltration setup, as shown in Figure 2. The
Figure 3. Schematic of the setup of the PBR−PVDF/GE hollow-fiber membranes for continuous oil/water separation.
according to the WEPs of the PBR−PVDF/GE hollow-fiber membranes. Kerosene was used as the test liquid to evaluate the continuous oil/water separation performance of the PBR− PVDF/GE hollow-fiber membranes in the negative-pressure dead-end filtration experiments. The volume ratio of the kerosene/water mixture was 1:1. Each of the negative-pressure dead-end filtration experiments was conducted in two steps. First, the PBR−PVDF/GE hollowfiber membrane was packed into the membrane module and then dipped into the kerosene/water mixture, placed so that it was at the oil/water interface. Second, the PBR−PVDF/GE hollow-fiber membrane module was connected to a vacuum system for the continuous removal of kerosene from the water surface (Figure 3). To determine whether water was present in the permeate, the water was dyed with Congo red. The change
Figure 2. Schematic of the setup for microfiltration experiments. 2176
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Industrial & Engineering Chemistry Research in kerosene flux was recorded at different time intervals (from 0 to 8 h in steps of 30 min). The kerosene flux was calculated as
J=
V At
(3)
where V is the oil flow volume of permeation (L), A is the effective area of the membrane (m2), and t is the filtration time (h). The relative flux of each membrane was calculated as
m=
J J0
(4)
where J0 is the flux of the membrane 0.5 h into the separation process (L/m2·h) and J is the flux of the membrane during the separation process (L/m2·h). 2.3.7. Kerosene Removal Percentage and Membrane Reusability. The kerosene removal percentage was determined according to the continuous oil/water separation test method described in section 2.3.6. The volume ratio of the kerosene/ water mixture was 1:1 (100 mL/100 mL). The kerosene removal percentage was calculated as η=
Vt × 100% V0
Figure 4. Effect of GE concentration on the viscosity of the casting solutions.
casting solutions. Even so, the value of the viscosity was still higher than that of the casting solution without GE. Therefore, the take-up speed was controlled at the level of 120 cm/min. A fibrous crisscross structure of PET tubular braids (Figure 5a) and an irregular stack structure of GE (Figure 5b) were
(5)
where η is the kerosene removal percentage (%), Vt is the volume of kerosene collected (L), and V0 is the initial volume of kerosene (L). To evaluate the membrane reusability, the fluxes of the PBR−PVDF/GE hollow-fiber membranes for kerosene/water separation were tested for 12 h after five cycles. After each filtration of the oil/water mixture, the membrane was rinsed and cleaned using ethyl alcohol for 15 min to remove the adsorbed kerosene on the surface and internal pore walls of the PBR−PVDF/GE hollow-fiber membranes and then dried at room temperature for the next filtration. The flux recovery rate was calculated according to the equation r=
Fi × 100% F1
Figure 5. Morphologies of (a) washed PET tubular braids and (b) the GE used in this study.
clearly observed. The mean pore sizes of the as-prepared membranes are listed in Table 2. Surface pores can be clearly observed on the outer surfaces of all five membranes from the enlarged FESEM micrographs in Figure 6 (left column). For membranes M0−M3, the amount of membrane surface pores decreased, but the mean pore size of the membranes increased with increasing amount of GE in the casting solution (Table 2). This change can be attributed to the incorporation30 of added SiO2 and GE. Compared to those on membrane M3, the amount and size of the surface pores on membrane M4 seemed lower and uneven because of the poor dual dispersion, regarding the high viscosity of the casting solution for membrane M4 (Figure 4). As shown in the middle column of Figure 6, a microporous coating layer was clearly observed at the outer surface of the tubular braids after they had been coated with the PVDF/GE casting solutions. Interfacial bonding occurred between the separation layer and the braid as a result of the infiltration of the casting solutions. The outer diameters of the membranes and the thicknesses of the separation layers are listed in Table 2. In terms of the maximum GE concentration (0.7 wt %), the thickness of the separation layer fluctuated greatly, showing a small mean pore size, which were attributed to the low fluidity and high viscosity of the casting solution at high GE concentration (Figure 4). As shown in Figure S1 (Supporting Information) and the right column of
(6)
where Fi is the initial kerosene flux of membrane M3 in cycle i of kerosene/water separation, i is the cycle number (i = 2, 3, 4, 5), and F1 is the initial kerosene flux of membrane M3 in the first cycle.
3. RESULTS AND DISCUSSION 3.1. Effects of GE Amount on Membrane Performance. The effect of the amount of GE on the viscosity of the casting solution is shown in Figure 4. Clearly, the viscosities of the coating solutions changed with increasing shear rate and exhibited the characteristics of a pseudoplastic fluid.28 The casting solution for membrane M0 without GE showed a low viscosity that exhibited no significant change with increasing shear rate. With increasing amount of GE in the casting solutions, the viscosity first increased and then decreased. In particular, the viscosity of the casting solution for membrane M4 changed obviously, which can be explained by the fact that GE is a rigid and flaky two-dimensional carbon material with a very high Young’s modulus (1.1 × 103 GPa).29 When the shear rate was very low, the presence of rigid GE enhanced the flow resistance of the casting solutions. When the shear rate exceeded a certain value, the flow resistance effect of GE gradually weakened, resulting in a decreased viscosity of the 2177
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Industrial & Engineering Chemistry Research Table 2. Characterization of Membranes M0−M4 membrane M0 M1 M2 M3 M4
outer diameter (mm)
separation layer thickness (μm)
mean pore size (μm)
bubble pore size (μm)
± ± ± ± ±
77.74 ± 24.85 101.59 ± 23.83 92.53 ± 18.47 92.46 ± 18.78 87.28 ± 27.05
0.09 0.08 0.14 0.16 0.10
0.97 0.16 0.18 0.19 0.15
2.24 2.19 2.14 2.24 2.15
0.11 0.06 0.16 0.09 0.08
porosity (%) 38.53 39.41 46.14 47.26 45.53
± ± ± ± ±
1.43 1.18 1.62 1.42 1.78
GE to the casting solution, the pore sizes of membranes M1− M3 were in the range from 0.07 to 0.18 μm, narrower than those of membrane M0. The pore size range with the largest percentage in membranes M1−M3 showed an increasing trend with increasing amount of GE. About 85% of the membrane M1 pore size was concentrated in the range from 0.08 to 0.09 μm, nearly 68% of the membrane M2 pore size concentrated in the range from 0.11 to 0.12 μm, and more than 85% of the membrane M3 pore size was in the range from 0.16 to 0.18 μm. The changes in the membrane pore sizes of membranes M1− M3 might be due to the incorporation of SiO2 and GE. However, the pore size distribution of membrane M4 was narrower than that of membrane M3, and nearly 80% of the membrane M4 pores were concentrated in the range from 0.1 to 0.11 μm (Figure S2). This might be attributed to poor dualdispersion, given the high viscosity of the membrane M4 casting solution (Figure 4). As shown in Table 2, the mean pore size of membrane M3 was the largest. The mean pore sizes of the membrane corresponded to the pore size distributions of the membranes, as shown in Figure 7 and Figure S2. Zisman31 introduced an empirical method for treating contact angle data for different liquids on the same material. If these contact angle data are plotted in terms of the liquid surface tension versus the cosine of the contact angle and extrapolated to cos θ = 1, a critical wetting surface tension (γc) value can be obtained for the highest-surface-tension liquid that will completely wet the solid, with a contact angle of 0°. This approach can be used to evaluate the hydrophobicity of membranes. As shown in Table 3 and Figure S3, it was found that the critical wetting surface tensions of the as-prepared membranes were approximately 21 mN·m−1, far lower than the liquid surface tension of water, indicating that the membranes were hydrophobic. Also, the water CAs of the membranes were all greater than 90° (Figure 8a). GE played a positive role in the improvement of the water CAs of the membranes for membranes M1 and M2. As a result of GE addition, the water CA values of membranes M1 and M2 were slightly higher than that of membrane M0. However, the surface wettabilities of membranes M1−M4 did not reach a superhydrophobic effect because of the smooth membrane surfaces from the phase inversion method.32 Generally, smooth membrane surfaces allow limited air entrapment within the surface microstructure and are thus associated with relatively high surface tensions that decrease the water CA. As shown in Figure 8b, the WEPs of membranes M1−M4 definitely exceeded that of membrane M0, which is attributed to the hydrophobic effect of GE. To the best of our knowledge, the WEP is influenced by factors such as the maximum membrane pore size, water CA, surface roughness, and surface chemical properties.34 The surface roughness and water CA values of membranes M1−M4 were adjacent to those of membrane M0, so these factors can be ruled out for the difference in WEPs in this study. As mentioned in the above
Figure 6. FESEM images of membranes (a) M0, (b) M1, (c) M2, (d) M3, and (e) M4: (1) Partial enlargement of the outer surface, (2) cross section, and (3) partial enlargement of the cross section.
Figure 6, the membranes exhibited a spongy-like pore structure with only a few fingerlike pores close to the outer surface. The interfaces of two coats within the separation layer are marked by red rectangles (Figure S1 and the right column of Figure 6). This could be caused by dissolution by DMAc during the second coating process and enhanced the combination between the inner and outer layers. The inner layers had looser pore structures than the outer layers of the as-prepared membranes. This was attributed to the dense skin layer after the first coating, which prohibited the casting solution from infiltrating inside the layer and affected the solvent exchange during membrane formation. In this case, the mean size of inner layers became smaller than that of the outer layers. GE irregularly embedded itself in the separation layers (red ellipses in Figure S1 and the right column of Figure 6) except for membrane M0. The pore sizes and pore size distributions of the membranes are shown in Figure 7 and Figure S2. Membrane M0 had a wider pore size distribution than membranes M1−M4. About 70% of the membrane M0 pores were smaller than 0.1 μm, and the other membrane pores were in the range of 0.1−1 μm. A membrane with such microscale pores cannot be used in a continuous oil/water separation process. After the addition of 2178
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Figure 7. Pore size distributions of membranes (a) M0, (b) M1, (c) M2, and (d) M3.
Table 3. Contact Angles of Different Liquids and Critical Wetting Surface Tensions (γc) for the Membranes contact angle (deg) water
ethylene glycol
n-butyl alcohol
methyl alcohol
ethyl alcohol
γc (mN·m−1)
M0 M1 M2 M3 M4
93.83 97.91 98.56 95.31 93.96
60.55 57.74 60.59 63.34 58.19
14.21 15.09 14.86 18.68 16.58
15.56 17.62 19.60 18.52 18.38
17.75 16.76 17.28 18.86 15.44
21.03 21.52 21.46 20.59 21.31
γla (mN·m−1)
72.75
46.50
24.60
22.60
22.30
−
membrane
a
Liquid surface tension at 20 °C.33
permeating the membrane pores. Kohonen37 found that the roughness of capillary walls could dramatically increase the wettability of the capillaries. Compared to those of membrane M3, the membrane pore channels of membrane M4 became rougher with increased GE, because more GE was distributed in the membrane pore channels. As a result, the WEP of membrane M4 was lower than that of membrane M3. Therefore, the appropriate range of GE usage is from 0.3 to
discussion, the added GE incorporated with SiO2 influenced the pore sizes of the as-prepared membranes. It was found that the bubble pore sizes of membranes M1−M4 were much smaller than that of membrane M0 (Table 2); the bubble pore size is associated with the WEP of a membrane, as described by the Young−Laplace equation.35 Moreover, the surface tension of GE (46.70 mN·m−1)36 is lower than that of water, showing a hydrophobic effect, which promoted the resistance to water 2179
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Figure 8. (a) Water CAs and (b) WEPs of the as-prepared membranes.
nucleation agent, resulting in an increased degree of crystallization. This is consistent with the results reported by Cui et al.40 The melting temperature (Tm) decreased with increasing GE content, which is attributed to the high coefficient of thermal conductivity of GE.41 In addition, the degree of crystallization influences the diffusion rate and the flux of the membrane with regard to the transport of liquids because diffusion takes place primarily in the amorphous regions.42 This fact can be used to explain the observation that the kerosene flux of membrane M4 was lower than those of membranes M2 and M3, as depicted in Figure 10a. 3.2. Continuous Oil/Water Separation Performances of the Membranes. In the collected permeating fluid, no dyed water droplets were observed by the naked eye, showing outstanding performance in the continuous oil/water separation tests. As shown in Figure 10a, membrane M1 exhibited a high flux in the initial phase, and then the flux decreased rapidly, finally reaching a relatively steady value. In comparison, the initial flux and the final flux of membrane M2 were higher than those of membrane M1, because of the larger mean pore size and porosity of membrane M2. The flux of membrane M3 showed a similar trend. However, membrane M4 exhibited a lower flux than membrane M3, which was caused by the small mean pore size of membrane M4. In the continuous oil/water separation, there were no particles plugging the membrane pores for the filtrate of pure kerosene. Therefore, the decrease in flux with work time might be caused by a change in membrane pore structure under high work pressure as described by Li et al.43 Figure 10b shows the relative fluxes of the as-prepared membranes. The relative fluxes of membranes M1−M4 decreased rapidly at the beginning of the oil/water separation process and finally reached 0.25, 0.26, 0.32, and 0.34, respectively. The ultimate relative flux value after 8 h of oil/water separation increased with increasing GE contents. The GE sheets were randomly dispersed in the separation layers of the membranes, resulting in a stable membrane pore structure, because of the rigid nature of the GE sheets for supporting the separation layers. 3.3. Kerosene Removal Percentages and Reusability Characteristics of the Membranes. Figure S4 shows the kerosene removal percentages of membranes M1−M4. The kerosene removal percentages of membranes M1−M4 increased rapidly at the beginning of the oil/water separation process and reached to 90.67%, 98.67%, 99.37%, and 98.67%, respectively. The results indicated better performances than were reported by Ebrahimi et al.,44 who obtained an oil removal
0.5 wt %. In this case, a high WEP could guarantee oil/water separation against water at a high working pressure. DSC curves and data on the separation layers of the asprepared membranes are displayed in Figure 9 and listed in
Figure 9. DSC curves for the separation layers of membranes M0− M4.
Table 4. DSC Data for the Separation Layers of Membranes M0−M4 membrane
Tm (°C)
ΔH (J/g)
degree of crystallization (%)
M0 M1 M2 M3 M4
157.54 156.75 156.69 156.39 156.24
36.12 36.93 38.72 40.34 42.97
34.50 35.27 36.98 38.53 41.04
Table 4. The degrees of crystallization were nearly 34−42%, which were lower than those of membranes prepared by thermally induced phase separation.38 In this study, pure water was used as the coagulation bath, and the demixing process occurred rapidly, which resulted in insufficient time to induce crystallization.39 The degree of crystallization increased with increasing GE content. GE played the role of a hetero2180
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Figure 10. (a) Kerosene fluxes of membranes M1−M4 from oil/water separation and (b) relative fluxes of as-prepared membranes M1−M4.
membranes were hydrophobic and had high WEPs. The membranes were used for the continuous separation of kerosene/water mixtures in a laboratory-scale continuous setup. PBR−PVDF/GE hollow-fiber membranes with 0.5 wt % GE were found to provide an optimal performance in oil/ water separation. The separation efficiency of the membranes did not deteriorate with increasing numbers of cycles, showing an outstanding durability and reusability performance.
percentage of 84% in the treatment of oily wastewater using ultrafiltration processes. According to the oil/water separation results, membrane M3 was used for an evaluation of reusability. Figure 11 shows the
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04428. FESEM cross-sectional images of partial enlargements of the membranes (Figure S1); pore sizes and pore size distributions of membranes M0−M4 (Figure S2); Zisman plots for membranes M0−M4 (Figure S3); and kerosene removal percentages of the membranes (Figure S4) (PDF) Video showing the continuous separation process (AVI)
■
Figure 11. Reusability of membrane M3 in the treatment of a kerosene/water mixture.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
reusability of membrane M3 for five cycles of kerosene/water mixture treatment. There was a similar trend for each cycle, namely, a high flux in the initial phase, followed by a rapid decrease and then a gradual approach to a relatively steady flux. The initial flux of membrane M3 decreased slightly with increasing number of cycles, but the flux recovery rates remained above 90%, so the final flux of membrane M3 exhibited no significant change. These results indicate the outstanding reusability performance of membrane M3.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21274109) and the Science and Technology Plans of Tianjin (14JCZDJC37300).
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4. CONCLUSIONS PBR−PVDF/GE hollow-fiber membranes were prepared by a concentric-circles coating method, by coating PVDF/GE solutions onto PET tubular braids. After the addition of GE to the casting solutions, the pore size distribution of the asprepared membranes exhibited a narrow range, and the mean pore size of membrane M3 was the largest. GE played a positive role in the improvement of the water CAs and WEPs of the membranes. The results indicated that the as-prepared membranes repelled water completely during 8 h of a continuous oil/water separation process and that the
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