Environ. Sci. Technol. 2005, 39, 7684-7691
High Flux Filtration Medium Based on Nanofibrous Substrate with Hydrophilic Nanocomposite Coating
and the pore size was in the range of 2.7-0.17 µm. Electrospun nonwoven nanofibrous mats thus have a larger specific surface area and smaller pore size than commercial nonwoven textiles, making them excellent candidates for use in gas filtration (5), enzyme supports (6), and biomedical applications (7, 8).
XUEFEN WANG, XUMING CHEN, KYUNGHWAN YOON, DUFEI FANG, BENJAMIN S. HSIAO,* AND BENJAMIN CHU* Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400
With the continuous decline of available freshwater supplies, recycling of municipal, industrial, and commercial wastewaters has gained more interest in recent environmental policy (9). By considering forthcoming new environmental regulations, nanofiltration (NF) and ultrafiltration (UF) have become even more important for applications in water treatment such as oil/water emulsions. NF and UF membranes are effective in the removal of oily microemulsions, but they often suffer from low flux due to limited permeability and surface fouling (10, 11). To improve the fouling problems, extensive efforts have been attempted. For example, several new nonporous thin film composite (TFC) membranes and surface-modified interfacial composite membranes for water filtration have been reported recently (12). They exhibited significantly improved antifouling properties compared to conventional UF and reverse osmosis (RO) membranes. Mayes and co-workers (13-16) demonstrated new water filtration membranes by employing amphiphilic, self-organizing comb copolymers, consisting of a hydrophobic backbone and hydrophilic side chains as surface segregating additives or coatings to create high surface porosity and low fouling performance. Freeman and coworkers synthesized microphase-separating block copolymers comprised of low surface energy fluorinated blocks and hydrophilic blocks as nonporous water purification membranes (17, 18).
A novel high flux filtration medium, consisting of a threetier composite structure, i.e., a nonporous hydrophilic nanocomposite coating top layer, an electrospun nanofibrous substrate midlayer, and a conventional nonwoven microfibrous support, was demonstrated for oil/water emulsion separations for the first time. The nanofibrous substrate was prepared by electrospinning of poly(vinyl alcohol) (PVA) followed by chemical cross-linking with glutaraldehyde (GA) in acetone. The resulting cross-linked PVA substrates showed excellent water resistance and good mechanical properties. The top coating was based on a nanocomposite layer containing hydrophilic polyetherb-polyamide copolymer or a cross-linked PVA hydrogel incorporated with surface-oxidized multiwalled carbon nanotubes (MWNTs). Scanning electron microscopy (SEM) examinations indicated that the nanocomposite layer was nonporous within the instrumental resolution and MWNTs were well dispersed in the polymer matrix. Oil/ water emulsion tests showed that this unique type of filtration media exhibited a high flux rate (up to 330 L/m2‚h at the feed pressure of 100 psi) and an excellent total organic solute rejection rate (99.8%) without appreciable fouling. The increase in the concentration of surface-oxidized MWNT in the coating layer generally improves the flux rate, which can be attributed to the generation of more effective hydrophilic nanochannels for water passage in the composite membranes.
Introduction Electrospinning is a process that can produce continuous polymer fibers with nanoscaled diameters (1-3). For fibers electrospun from polymer solutions, the presence of residual solvent in the electrospun fibers would induce bonding of intersecting fibers, creating a strong cohesive interconnected porous structure. The nonwoven nanofibers can assemble into a membranelike web that exhibits good tensile strength and is extremely lightweight. For example, Kim and coworkers have evaluated the physical properties of nonwoven mats composed of nylon-6 nanofibers (4). Depending on the fiber diameter, the Brunauer-Emmett-Teller (BET) surface areas of electrospun mats could be estimated to be between 9 and 51 m2/g, while the porosity varied from 25% to 80% * Address correspondence to either author. Phone: (631) 6327793 (B.S.H.); (631) 632-7928 (B.C.). Fax: (631) 632-6518. E-mail:
[email protected] (B.S.H.);
[email protected] (B.C.). 7684
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 19, 2005
In this study, we demonstrate a different type of TFC membrane by using the lightweight and high porosity electrospun nanofibrous substrate to support a thin hydrophilic nonporous polymer nanocomposite coating layer. The thin nonporous hydrophilic coating layer can minimize the fouling problem by preventing the solute adsorption on the surface and the solute access to the porous structure below it. The water flux rate of the composite membrane is mainly dependent on the selection of the coating layer (hydrophilicity) and its thickness (e.g., as thin as permissible). To increase the water permeability (the flux rate), pore channels with appropriate sizes have been introduced into the coating layer. This approach was inspired by some recent studies using organic-inorganic polymer composite membranes for applications in molecular separations. For example, Zoppi and co-workers (19) incorporated ZrO2 into the Pebax copolymer coating by using a sol-gel method to prepare hybrid membranes for water filtration, although there was no marked improvement in the water permeability of the hybrid membrane. Merkel and co-workers reported that the incorporation of nonporous, nanoscale, fumed silica particles in high-free-volume glassy polymers simultaneously enhanced both gas permeability and organic vapor/permanent gas selectivity (20, 21). They suggested that the nanoscale particles (99.8%) even when the MWNT content was as high as 8% (the values of rejection did not vary significantly). When the MWNT content was 12%, the water flux was found to increase significantly. The corresponding rejection decreased to 98.3%, which is still relatively high. This may be because the coating layer prepared with high MWNT content had some microscopic defects or cracks due to strong filler interactions, resulting in a less perfect covering of the surface and slightly higher permeation of oil and surfactant. At higher MWNT content (16%), the top coating layer became too brittle to be useful for water filtration. Table 2 lists results of the flux rate and the total organic rejection for a series of membranes based on cross-linked PVA nanofibrous substrates coated with a pure lightly crosslinked PVA hydrogel or PVA hydrogel/MWNT nanocomposite layer. Results similar to those in Table 1 were obtained for membranes with the PVA hydrogel/MWNT coating; i.e., the incorporation of the MWNT in the coating layer increased the water flux. It is interesting to see that when the content of MWNT was the same and the rejection rate was similar, the membrane with the PVA/MWNT coating exhibited a higher flux rate than that with the Pebax/MWNT coating, even though the PVA coating layer (∼1.8 µm) was thicker than the Pebax coating layer (∼0.8 µm). For example, a very high water flux rate (330 L/m2‚h) accompanied by high rejection rate (99.8%) was achieved by the membrane with PVA hydrogel/MWNT coating having 10 wt % MWNT. This may be because hydrophilic macromolecules tend to swell considerably during ultrafiltration and the top layer is expanded to provide additional space for the water transport. To test this hypothesis, swelling tests were carried out for two base-coating materials. Freestanding films of pure Pebax 1074 and of pure lightly cross-linked PVA hydrogel were immersed in distilled water for 48 h. The water uptake per gram of the Pebax 1074 film was 0.51 g, while the water uptake per gram of the PVA hydrogel film was 1.63 g. The swelling results suggested that the hydrophilicity of the Pebax 1074 coating was less than that of the PVA coating layer, and thus water could be more accessible in the PVA hydrogel. It should be noted that the degree of swelling in PVA hydrogel could be controlled by varying the PVA cross-linking density, which had not been optimized in this study. The rejection data for PVA hydrogel/MWNT samples were similar to those for Pebax/MWNT samples (Tables 1 and 2). The water permeabilities for both kinds of composite membranes were enhanced by the incorporation of MWNTs into the nonporous coating layer, and the rejection values for both kinds of composite membranes were essentially unaffected by the presence of up to 8 wt % MWNT in Pebax and 10 wt % MWNT in the PVA matrix. The effects of MWNT can be explained as follows. As mentioned in the Experimental Section, the surface of MWNT had a graphite layered structure with a very low surface tension, when compared with that of the hydrophilic
coating materials (Pebax 1074 and PVA). To improve the compatibility between MWNT and these hydrophilic polymers, oxidation treatment was performed on MWNTs to generate carboxylic acid (-COOH), carbonyl (-CdO), and hydroxy (-OH) functional groups on the surface. The density of the acidic groups was relatively high (up to 1.8 mmol/g). Thus, the surface of MWNT could possess nanophase domains: hydrophobic aromatic regions and hydrophilic acidic regions. When oxidized MWNTs were incorporated into the polymer matrix, the amphiphilic MWNT surface would disrupt the polymer chain packing in the interface and could introduce nanoscaled cavities to affect the transport property of the coating layer. For instance, these functional groups could interact with PVA chains through chemical bonding (via the cross-linking agent GA in the solution) or hydrogen bonding between the acidic group on the surface of oxidized MWNTs and the hydroxyl groups on the PVA chains. The cavities formed between hydrophilic PVA chains and hydrophobic aromatic regions on the surface of oxidized MWNT could provide additional pathways for water permeation. Therefore, although the composite coating layers were macroscopically nonporous, as had been confirmed by SEM, microscopically effective nanochannels were produced through the incorporation of surface-oxidized MWNTs into the polymer matrix. As a result, the values of water permeability in composite membranes increased systematically with increasing MWNT concentration. The (external) channel size could, in principle, be regulated by the degree of surface oxidation in MWNT and thereby be used to manipulate the permeability and selectivity of the nanocomposite layer. However, the oxidation treatment of the MWNT had not yet been optimized to achieve an optimal membrane performance. It should be noted that the adhesion between surface-oxidized MWNT and the polymer matrix was very good (Figure 7C). Thus, the incorporation of MWNT offered two unique advantages: (1) an improvement in the mechanical strength of the coating layer and (2) an increase in the water permeability of the coating layer. There is another mechanism that may also enhance the penetration of water in the nanocomposite layer containing MWNT. That is water may possibly permeate through the nanochannels within the carbon nanotubes. Thus, with the increase of MWNT concentration, the number of internal nanochannels increases, leading to a higher water flux. In reality, we envision that the enhancement of filtration flux in the nanocomposite layer may be due to both external (surface) and internal channels of MWNT. In summary, novel high flux composite membranes for water filtration were developed based on a cross-linked electrospun PVA substrate coated with a nonporous hydrophilic polymer/MWNT nanocomposite layer. The electrospun nanofibrous substrates provided good tensile strength, and an extremely lightweight and interconnected porous structure with a large specific surface area, making them excellent candidates as filtration supporting scaffolds. Filtration results using oil/water emulsions suggested that the incorporation of surface-oxidized MWNT could (1) modify the packing of hydrophilic chains in the interface, thereby producing effective external nanochannels for water permeation, and (2) additionally provide internal nanochannels for water permeation from the nanoscale opening of carbon nanotubes. The values of water permeability for composite membranes with Pebax 1074 or PVA hydrogel nanocomposite coatings increased substantially with increasing MWNT content, while the filtration rejection efficiency for both membranes was essentially unaffected by the presence of MWNT (up to 8 wt % MWNT in Pebax and 10 wt % MWNT in PVA). The PVA hydrogel/MWNT coating layer displayed more accessible free volume for water transport than the Pebax 1074/MWNT coating layer. The composite membrane VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7689
with PVA hydrogel/MWNT (10 wt %) coating exhibited excellent organic solute rejection (99.8%) and very high water flux (up to 330 L/m2‚h) with no detectable fouling over a 24 h operating period. It is imperative to point out that the above nanofibrous membranes with nanocomposite coating have not yet been optimized, and the chosen material system (i.e., PVA for both nanofibrous scaffold and coating layer) may not be the most suitable one for long-term operation (e.g., over 1 month). For example, one concern about the PVA nanocomposite coating is that MWNT may be essentially washed out of the cross-linked matrix over time. Although we do not have any data to address this concern, we believe that the demonstrated system paves a new way to fabricate very high flux filtration media while a retaining good rejection rate. For long-term operational stability, other proven filtration materials such as PVDF and polysulfone must be used to generate more stable electrospun scaffolds. In addition, the chemical composition and the thickness of the nanocomposite coating layer should be further optimized to achieve better filtration performance. These will be the subjects of our future study.
Acknowledgments Financial support of this work was provided by the Office of Naval Research (N000140310932). The authors acknowledge the helpful assistance of Dr. Rich Solinaro from the Pall Corporation for the construction of the cross-flow filtration cell.
(15) (16)
(17)
(18) (19)
(20) (21)
(22) (23)
Literature Cited (1) Reneker, D. H.; Chun, I. Nanometer diameter fibers of polymer, produced by electrospinning. Nanotechnology 1996, 7, 216223. (2) Huang, Z.; Zhang, Y.; Kotaki M.; Ramakrishna, S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos. Sci. Technol. 2003, 63, 2223-2253. (3) Frenot, A.; Chronakis, I. S. Polymer nanofibers assembled by electrospinning. Curr. Opin. Colloid Interface Sci. 2003, 8, 6475. (4) Ryu, Y. J.; Kim, H. Y.; Lee, K. H.; Park, H. C.; Lee, D. R. Transport properties of electrospun nylon 6 non-woven mats. Eur. Polym. J. 2003, 39, 1883-1889. (5) Gibson, P.; Schreuder-Gibson, H.; Rivin, D. Transport properties of porous membranes based on electrospun nanofibers. Colloids Surf., A: Physicochem. Eng. Aspects 2001, 187-188, 469-481. (6) Jia, H.; Zhu, G.; Vugrinovich, B.; Kataphinan, W.; Reneker, D. H.; Wang, P. Enzyme-carrying polymeric nanofibers prepared via electrospinning for use as unique biocatalysts. Biotechnol. Prog. 2002, 18, 1027-1032. (7) Zong, X.; Ran, S.; Kim, K.; Fang, D.; Hsiao, B. S.; Chu, B. Structure and morphology changes during in vitro degradation of electrospun poly(glycolide-co-lactide) nanofiber membrane. Biomacromolecules 2003, 4, 416-423. (8) Reneker, D. H.; Hou, H. Electrospinning. Encyclopedia of Biomaterials and Biomedical Engineering, Wnek, G. E., Bowlin, G. L., Eds.; Marcel Dekker: New York, 2004; Vol. 1, pp 543-550. (9) Van der Bruggen, B.; Vandecasteele, C.; Van Gestal, T.; Doyen, W.; Leysen, R. A review of pressure-driven membrane processes in wastewater treatment and drinking water production. Environ. Prog. 2003, 22, 46-56. (10) Marshall, A. D.; Munro, P. A.; Tragardh, G. The effect of protein fouling in microfiltration and ultrafiltration on permeate flux, protein retention and selectivity: A literature review. Desalination 1993, 91, 65-108. (11) Faibish R. S.; Cohen, Y. Fouling and rejection behavior of ceramics and polymer-modified ceramics membranes for ultrafiltration of oil-in-water emulsions and microemulsions. Colloids Surf., A 2001, 191, 27-40. (12) Freeman, B. D.; Pinnau, I. Gas and liquid separations using membranes: An overview. ACS Symp. Ser. 2004, 876, 1-23. (13) Akthakul, A.; Solinaro, R. F.; Mayes, A. M. Antifouling polymer membranes with subnanometer size selectivity. Macromolecules 2004, 37, 7663-7668. (14) Hester, J. F.; Banerjee, P.; Won, Y.-Y.; Akthakul, A.; Acar, M. H.; Mayes, A. M. ATRP of amphiphilic graft copolymers based on 7690
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 19, 2005
(24) (25) (26)
(27) (28) (29) (30)
(31)
(32) (33)
(34) (35)
PVDF and their use as membrane additives. Macromolecules 2002, 35, 7652-7661. Hester, J. F.; Banerjee, P.; Mayes, A. M. Preparation of proteinresistant surfaces on poly(vinylidene fluoride) membranes via surface segregation. Macromolecules 1999, 32, 1643-1650. Hester, J. F.; Mayes, A. M. Design and performance of foulresistant poly(vinylidene fluoride) membranes prepared in a single-step by surface segregation. J. Membr. Sci. 2002, 202, 119-135. Arnold, M. E.; Nagai, K.; Spontak, R. J.; Freeman, B. D.; Leroux, D.; Betts, D. E.; DeSimone, J. M.; DiGiano, F. A.; Stebbins, C. K.; Linton, R. W. Microphase-separated block copolymer comprising low surface energy fluorinated blocks and hydrophilic blocks: synthesis and characterization. Macromolecules 2002, 35, 3697-3707. DiGiano, F. A.; Roudman, A.; Arnold, M.; Freeman, B. Novel block copolymers as nanofiltration materials. Environ. Eng. Sci. 2002, 19, 497-511. Zoppi, R. A.; Soares, C. G. A. Hybrids of poly(ethylene oxideb-amide-6) and ZrO2 sol-gel: preparation, characterization, and application in processes of membranes separation. Adv. Polym. Technol. 2002, 21, 2-16. Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A. J. Ultrapermeable, reverse-selective nanocomposite membranes. Science 2002, 296, 519-522. Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A. J. Sorption, transport, and structural evidence for enhanced free volume in poly(4-methyl-2-pentyne)/fumed silica nanocomposite membranes. Chem. Mater. 2003, 15, 109123. Dai, W. S.; Barbari, T. A. Hydrogel membranes with mesh size asymmetry based on the gradient cross-linking of poly(vinyl alcohol). J. Membr. Sci. 1999, 156, 67-79. Ding, B.; Kim, H. Lee, S.; Shao, C.; Lee, D.; Park, S.; Kwag, G.; Choi, K. Preparation and characterization of a nanoscale poly(vinyl alcohol) fiber aggregate produced by an electrospinning method. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 12611268. Pinnau, I.; Freeman, B. D. Polyether-polyamide block copolymers: versatile materials for membrane separations. Polym. Mater. Sci. Eng. 2002, 86, 108. Nunes, S. P.; Sforca, M. L.; Peinemann, K.-V. Dense hydrophilic composite membranes for ultrafiltration. J. Membr. Sci. 1995, 106, 49-56. Sforca, M. L.; Nunes, S. P.; Peinemann, K.-V. Composite nanofiltration membranes prepared by in situ polycondensation of amines in a poly(ethylene oxide-b-amide) layer. J. Membr. Sci. 1997, 135, 179-186. Bondar, V. I.; Freeman, B. D.; Pinnau, I. Gas transport properties of poly(ether-b-amide) segmented block copolymers. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 2463-2475. Bondar, V. I.; Freeman, B. D., Pinnau, I. Gas transport properties of poly(ether-b-amide) segmented block copolymers. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 2051-2062. Blume, I.; Pinnau, I. Polyamide-polyether block copolymer composite separation membrane, method of preparation and use. U.S. Patent 4,963,165, 1990. Hu, H.; Bhowmik, P.; Zhao, B.; Hamon, M. A.; Itkis, M. E.; Haddon, R. C. Determination of the acidic sites of purified singlewalled carbon nanotubes by acid-base titration. Chem. Phys. Lett. 2001, 345, 25-28. Yao, L.; Haas, T. W.; Guiseppi-Elie, A.; Bowlin, G. L.; Simpson, D. G.; Wnek, G. E. Electrospinning and stabilization of fully hydrolyzed poly(vinyl alcohol) fibers. Chem. Mater. 2003, 15, 1860-1864. Zong, X.; Kim, K.; Fang, D.; Ran, S.; Hsiao, B. S.; Chu, B. Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer 2002, 43, 4403-4412. To test the reliability of the UV method, we determined the organic concentration of the permeate by measuring the chemical oxygen demand (COD) (using titration method: KMnO4-H2C2O4 oxidation-reduction titration) of the permeate (p) and feed (f) solution to calculate the organic rejection (1 CODp/CODf). The results from the titration method were in good agreement with those from the UV method, which demonstrated that the UV method was reasonable to determine the organic concentration. Chu, B. Laser Light Scattering, Basic Principles and Practice, 2nd ed.; Academic Press: New York, 1991; p 14. Tomihata, K.; Ikada, Y. Crosslinking of hyaluronic acid with glutaraldehyde. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 3553-3559.
(36) Kesting, R. E. Synthetic Polymeric Membranes, 1st ed.; McGrawHill: New York, 1971. (37) Akthakul, A.; McDonald, W. F.; Mayes, A. M. Noncircular pores on the surface of asymmetric polymer membranes: evidence of pore formation via spinodal demixing. J. Membr. Sci. 2002, 208, 147-155. (38) Nakao, S. Determination of pore size and pore size distribution: 3. Filtration membranes. J. Membr. Sci. 1994, 96, 131165.
(39) Andrews, R.; Jacques, D.; Minot, M.; Rantell, T. Fabrication of carbon multi-wall nanotube/polymer composites by shear mixing. Macromol. Mater. Eng. 2002, 287, 395-403.
Received for review March 16, 2005. Revised manuscript received July 26, 2005. Accepted July 27, 2005. ES050512J
VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
7691
