Nanofibrous Microfiltration Membrane Based on Cellulose

Publication Date (Web): December 5, 2011. Copyright © 2011 ..... Advanced Healthcare Materials 2014 3 (10), 1546-1550 ... Surface chemistry, morpholo...
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Nanofibrous Microfiltration Membrane Based on Cellulose Nanowhiskers Hongyang Ma, Christian Burger, Benjamin S. Hsiao,* and Benjamin Chu* Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States S Supporting Information *

ABSTRACT: A multilayered nanofibrous microfiltration (MF) membrane system with high flux, low pressure drop, and high retention capability against both bacteria and bacteriophages (a virus model) was developed by impregnating ultrafine cellulose nanowhiskers (diameter about 5 nm) into an electrospun polyacrylonitrile (PAN) nanofibrous scaffold (fiber diameter about 150 nm) supported by a poly(ethylene terephthalate) (PET) nonwoven substrate (fiber diameter about 20 μm). The cellulose nanowhiskers were anchored on the PAN nanofiber surface, forming a cross-linked nanostructured mesh with very high surface-to-volume ratio and a negatively charged surface. The mean pore size and pore size distribution of this MF system could be adjusted by the loading of cellulose nanowhiskers, where the resulting membrane not only possessed good mechanical properties but also high surface charge density confirmed by the conductivity titration and zeta potential measurements. The results indicated that a test cellulose nanowhisker-based MF membrane exhibited 16 times higher adsorption capacity against a positively charged dye over a commercial nitrocellulose-based MF membrane. This experimental membrane also showed full retention capability against bacteria, for example, E. coli and B. diminuta (log reduction value (LRV) larger than 6) and decent retention against bacteriophage MS2 (LRV larger than 2).

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nanofibrils.14 Moreover, the surface of these ultrafine cellulose nanofibers is covered with the negatively charged carboxylate groups as well as the aldehyde groups, generated by the oxidation of C6-hydroxyl groups, enabling them to adsorb virus or toxic metal ions.10,19 High flux and low-pressure drop MF membranes based on electrospun scaffolds have also been recently demonstrated.20−22 In these studies, when membranes were challenged with 0.2 μm particles and proteins (e.g., BSA) as models for bacteria, two to five times higher water permeability and similar particle retention (compared with commercial membranes such as Millipore GS0.22) ratio were obtained. However, the mean pore size of these tested electrospun MF membranes seemed to be too large to retain smaller bio-organisms (e.g., viruses). To enhance the filtration capability against virus but without sacrificing the permeability of electrospun MF membranes, we have considered varying pathways to increase their electrostatic charges to promote the adsorption capability. In the present study, ultrafine cellulose nanowhiskers were infused into an electrospun polyacrylonitrile (PAN) nanofibrous scaffold supported by a mechanically strong polyethylene terephthalate (PET) nonwoven substrate. The impregnated cellulose nanowhiskers possessed very high negative surface charge density and thus provided high adsorption capacity to remove positively charged species, such as crystal violet (CV) dye (as a model in this study). We

INTRODUCTION Shortage and contamination of global drinking water requires the development of highly efficient water purification techniques such as membrane filtration.1,2 However, for membrane technology, significant challenges in terms of limited available materials, cost effectiveness and environmental concerns have to be overcome in order to meet this goal.3,4 In the present study, we demonstrate the use of ultrafine cellulose nanowhiskers as an effective means to modify the effectiveness of nanofiber-based microfiltration (MF) membranes for drinking water purification. These ultrafine cellulose nanofibers (diameter 5−10 nm), derived from biomass, have received a great deal of interest due to their good chemical, mechanical, thermal, and benign environmental properties.5,6 They have been used in various applications such as nanocomposites,7 nanopapers,8 gas/water barrier layers,9,10 nanotemplates,11 and tissue engineering.12 Conventional fabrication of cellulose nanofibers involves the acid/alkali treatment, followed by the grinder/fluidizer defibrillation process, where the fiber length (several micrometers) to diameter (few tens to few hundred nanometers) ratios are usually small and nonuniform.7,13 Typically, highly corrosive chemicals have to be used in the initial step of the chemical treatment. Recently, an aqueous oxidation system based on TEMPO/NaBr/NaClO (TEMPO is 2,2,6,6-tetramethylpiperidinooxy) has been developed and employed to fabricate various types of ultrafine cellulose/chitin nanofibers and nanowhiskers.10,14−18 The nanofibers prepared by this method possess very fine and uniform diameters (e.g., 5−10 nm) when compared with those conventionally prepared © 2011 American Chemical Society

Received: October 11, 2011 Revised: December 1, 2011 Published: December 5, 2011 180

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tension of 15.9 dyn/cm was used to wet the membrane for measurements.26 Tensile Stretching. All MF membranes (including the PAN/PET referenced membrane, ultrafine cellulose nanowhiskers infused PAN/ PET membrane, and GS0.22), cut into a dog bone shape with dimensions of 30 mm × 10 mm × 0.03 mm, were uniaxially stretched at room temperature using a modified Instron 4442 tensile apparatus under symmetric deformation. PET layer was removed before stretching. The initial length between the Instron clamps was 10 mm, and the stretching rate was maintained at 5 mm/min. Adsorption Capacity against Positively Charged Molecules. The adsorption capacity of CV in the nanofibrous MF membrane was measured batch-wise using the following procedure. We immersed 0.05 g of cellulose nanowhisker-based membrane (without PET layer) and of commercial GS0.22 membrane, respectively, in 20 mL of CV aqueous solution (10 mg/L, pH 7.0) on a shaking bed for a predetermined time (10−180 min) at 20 °C. The amount of the CV adsorbed onto the membrane was calculated from the concentration change of the CV solution before and after the adsorption, as determined by optical absorption at 590 nm. The CV adsorption capacity as a function of time was continuously monitored. After 2 h, when the concentration of CV reached equilibration and the adsorption of CV onto the cellulose nanowhisker membrane was saturated, the adsorption capacity was defined as the maximum value measured under this condition. The maximum adsorption capacity of the nanofibrous membrane (without PET layer) and that of the commercial GS0.22 membrane were determined by using Langmuir adsorption isotherms with the following procedures. Membranes (0.03 g) were immersed into 10 mL of CV solution at different concentrations, ranging from 5 mg/L to 1000 mg/L, and shaken for 2 h at 20 °C. An analysis of the relationship between the adsorption capacity of membranes and the CV concentration was performed using the Langmuir adsorption equation.27

have tested the nanofibrous membranes containing cellulose nanowhiskers against two bacteria (i.e., B. diminuta and E. coli) and one virus model (i.e., bacteriophage MS2). The demonstrated membranes were proven to be effective media that can simultaneously remove bacteria and viruses, whereas high flux and low pressure drop were also obtained. They are very suitable for applications in low-energy drinking water purification from lakes, rivers, and ponds.23,24

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EXPERIMENTAL SECTION

Materials. Ultrafine cellulose nanowhiskers were prepared from microcrystalline cellulose (Aldrich) first by the oxidation method using TEMPO/NaBr/NaClO aqueous solution at pH 11, followed by the mechanical treatment with a homogenizer (Cole Parmer, VCX-400), as described in our previous study.10 A cellulose nanowhisker suspension in water with a concentration of 0.4 wt % was obtained from the above procedure for this study. Electrospun PAN (Mw ≈ 150 kDa, Aldrich) nanofibrous scaffolds were fabricated by continuous electrospinning of a PAN nanofibrous scaffold (width: 45 cm; thickness: about 45 μm) onto a PET nonwoven substrate (AWA-161, Japan) using the equipment developed at Stony Brook.22 The commercial membrane GS0.22 (with negatively charged surface) based on nitrocellulose was purchased from Millipore. A polybead microsphere particle suspension (with the average diameter about 0.20 μm in 2.6 wt % concentration) was purchased from Polysciences and was diluted in deionized (DI) water to 100 ppm, which was used as the model feed suspension to simulate contaminant water containing waterborne bacteria. CV (ACS reagent, > 90.0% anhydrous basis) was obtained from Aldrich. Two types of bacteria, B. diminuta and E. coli, were purchased from American Type Culture Collection (ATCC), and one virus model, bacteriophage MS2, was prepared by incubation using the procedures previously demonstrated.25 Preparation of Cellulose Nanowhisker-Based Nanofibrous MF Membranes. A composite nanofibrous membrane (electrospun PAN on PET nonwoven) in the form of a disk sample with 25 mm diameter was latched in a dead-end filtration cell (Millipore, model 8050), where 0.1 g of cellulose nanowhisker aqueous suspension (0.4 wt % concentration, pH 7.0) was gradually infused into the membrane disk by gravity. The membrane disk was subsequently heated to 100 °C for 10 min to cross-link the cellulose nanowhisker network. The amount of cellulose nanowhisker loading in the membrane was kept at 20 mg/cm3. Transmission Electron Microscopy. A transmission electron microscope (TEM, FEI BioTwinG2), operating at an accelerating voltage of 120 kV was equipped with an AMT digital camera, photographic film capability, and goniometer/stage tilt capability. The samples were prepared by solution coating of a TEM grid (Ted Pella) with 0.01 wt % aqueous cellulose nanowhisker suspension, followed by staining with 2.0 wt % aqueous uranyl acetate solution. Scanning Electron Microscopy. A scanning electron microscope (SEM, LEO 1550) equipped with a Schottky field emission gun (10 kV) and a Robinson backscatter detector was used for the SEM micrographs in both cross-sectional and top views. The crosssectioned samples were prepared by freeze-fracturing of the waterwetted membrane in a liquid nitrogen bath. The samples were sputtered with gold in an argon atmosphere. Microfiltration Performance by Particle Retention. The particle filtration using the cellulose nanowhisker-based MF membrane was performed using a total of 10 mL of polybead suspension under a flow rate of 1.0 mL/min at 19.3 kPa, where the particle concentrations of permeate and feed solutions were measured with a total organic carbon analyzer (TOC-5000, Shimadzu).22 The filtration experiments were reproduced three times, and the results were taken as the averaged values. Porometry. The bubble point, mean pore size, and pore size distribution of the investigated MF membranes were measured using a capillary flow porometer (FPA-1500A, Porous Materials, Ithaca, NY, USA). A wetting fluid Galwick (Porous Materials) with a surface

1/qe = 1/qm + kd/qm × (1/ce)

(1)

where qm is the maximum adsorption at monolayer coverage (mmol·g−1) and kd is the Langmuir adsorption equilibrium constant (L/mg), reflecting the energy of adsorption. Water Contact Angle Measurement. Water contact angles of electrospun PAN membrane, cellulose nanowhisker-based nanofibrous membrane, and commercial GS0.22 membrane were measured with CAM200 optical contact angle meter, KSV Instrument. We used 10 μL of Milli-Q water as the probe liquid for the measurements. At least three tests were taken for each sample, and the averaged data were collected. Surface Charge Density Evaluation. The surface charge density of cellulose nanowhisker-based MF membranes was evaluated by the conductivity titration method.17 In this measurement, 0.18 g of the nanofibrous MF membrane (without PET layer) was suspended in 70 mL of water, and the suspension was acidified with concentrated HCl (36.5%) to pH 2.0. A sodium hydroxide standard aqueous solution (0.02 mol/L) was used to titrate the solution to pH 10.4 while monitoring the conductivity during the titration process. The conductivity and pH curves showed the presence of strong acid (i.e., excess HCl) and weak acid that corresponds to the content of carboxyl groups on the surface of the membrane. The surface charge density was then calculated by the content of the carboxyl groups (equal molar to the negative charges) normalized by the weight of the membrane. Zeta Potential Measurement. The zeta potential of nanofibrous MF membranes was determined by a SurPASS electrokinetic analyzer (Anton Paar Company) based on the streaming potential and streaming current measurement.28 In addition to the analyzer, this instrument includes a measuring cell, electrodes, and a data control system. For each measurement, a rectangular sample (dimensions: 10 mm × 20 mm × 0.03 mm) was carefully affixed onto the sample holder with double-sided adhesive tape. The sample holder was inserted into the adjustable gap cell (AGC), where the gap spacing was adjusted in the range of 50−150 μm. Two measuring heads with Ag/ 181

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Figure 1. (a) TEM image of cellulose nanowhiskers fabricated by the TEMPO/NaBr/NaClO oxidation method. (The inset shows the electron diffraction pattern.) (b) SEM images of cross-sectional views of PAN electrospun nanofibrous scaffold and (c) cellulose nanowhisker-modified PAN electrospun nanofibrous scaffold. Schematic representations of electrospun nanofibrous scaffolds: (d) scaffold before nanowhisker infusion; (e) infused nanowhiskers forming loose cross-linked mesh; and (f) nanowhiskers collapsed onto the scaffold, forming bundles. AgC1 electrodes were attached to the AGC. A maximum pressure of 300 mbar was chosen, under which a linear relationship between the pressure and the flow rate had been established. In this measurement, 1 mM KCl solution prepared in DI water was used as the background electrolyte. Each sample was first rinsed at the maximum pressure (300 mbar) for 180 s, and the streaming current was subsequently measured at the same pressure (300 mbar) for 20 s. The streaming current measurement was performed in both flow directions after the samples had been rinsed sufficiently with the electrolyte solution. The zeta potential as a function of the pH value (the equipment had the autotitration capability) was estimated by using the Helmholtz− Smoluchowski equation29

ζ = (dI /dP) × [η /(ε × ε0)] × (L /A)

LRV = log(C f /C p)

(3)

where Cf and Cp are the bacterial concentrations of the feed and permeate, respectively. The agar plates were incubated at 37 °C for 24 h, and the number of bacterial colonies was counted.30,31 Bacteriophage Test. MS2 bacteriophage (diameter ∼27 nm) with an isoelectric point (pI) = 3.9 was used as a model virus particle to evaluate the viral adsorption capacity of cellulose nanowhiskerbased MF membranes at 20 °C.10 In this test, MS2 phage with an initial concentration of ∼106 pfu/mL was suspended in phosphate buffer solution (pH 7.2). The suspension (10 mL) was filtered through the membrane (effective area: 4.0 cm2) at a constant flow rate of 192 L/m2 h, where the pressure drop was measured. For the pfu measurements, E. coli (ATCC 15597-B1) was used as a viral host bacterium, and trypticase soy agar plates (Teknova) were used to cultivate 10−100 pfu/plate at various dilution ratios of the permeate. All experiments were repeated at least three times.

(2)

where dI/dP is the slope of streaming current versus differential pressure, η is the electrolyte viscosity, ε0 is the permittivity, ε is the dielectric coefficient of electrolyte, L is the length of the streaming channel, and A is the cross-section of the streaming channel. Bacterial Test. Spiked B. diminuta or E. coli bacteria were stirred in dechlorinated water at room temperature. Different concentrations of bacteria, ranging from 106 to 107 colony forming units (cfu)/mL, were prepared for this study. The bacterial challenge test was performed with 50 mL of B. diminuta (or E. coli) suspension (106∼107 cfu/mL) in a dead-end filtration stirred cell (Millipore 8050-5122). Before the test, the filtration cell and all other glass equipment were autoclaved (15 min at 120 °C). The retention test was carried out at room temperature (22 °C) under a stirring condition (stirring rate = 300 rpm). Both values of permeation flux and pressure drop were monitored during the test. The retention was expressed as the log reduction value (LRV) defined as

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RESULTS AND DISCUSSION Morphology of Cellulose Nanowhiskers and Cellulose Nanowhisker-Based Nanofibrous Membrane. Figure 1a shows that the cellulose nanowhiskers produced by the TEMPO oxidation method had diameters in the range of 5− 10 nm and lengths in the range of 200−400 nm. The electron diffraction pattern of the cellulose nanowhiskers probed by TEM showed a structure of cellulose I type crystal.32 The contents of carboxylate and aldehyde groups on the surface of cellulose nanowhiskers were 1.0 and 0.3 mmol/g cellulose, respectively, according to the titration experiments.17,33 The 182

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nanofibrous MF membrane investigated in this study was based on a nonwoven electrospun PAN nanofibrous scaffold (Figure 1b) deposited onto a PET nonwoven microfiber substrate, where cellulose nanowhiskers were impregnated into the PAN nanofibrous scaffold (Figure 1c). In Figure 1c, cellulose nanowhiskers were collapsed onto the surface of PAN nanofibers, forming an entangled, partially bundled, and partially cross-linked mesh anchored on the PAN nanofibrous scaffold. Compared with the undecorated nanofibrous scaffold, this leads to a substantial increase in the effective surface-to-volume ratio in the nanofibrous membrane as well as to an improvement in the mechanical properties of the electrospun scaffolds as confirmed by the tensile strength tests to be discussed below. Figure 1d−f illustrates the formation of the nanowhisker mesh on the surface and the junction points of the electrospun scaffold. Figure 1e illustrates that the limiting case of complete nanowhisker dispersion, forming a cross-linked 3-D network, loosely fills the space within the electrospun scaffold. Figure 1f shows the opposite limiting case of infused nanowhiskers completely collapsed onto the electrospun scaffold as a result of their rigid rod-like nature, forming bundles. Experimental SEM images of the cellulose nanowhisker-based MF membrane (Figure 1c) show features of both of these two limiting cases; that is, some cellulose nanowhiskers collapsed into bundles, but the rest still formed a space-filling cross-linked network with substantially increased surface-to-volume ratio relevant for particle adsorption. The cellulose nanowhisker meshes were very stable during the filtration process because of hydrogen bonding among the nanowhiskers as well as chemical cross-linking induced by the thermal treatment.10 Microfiltration Performance of Cellulose Nanowhisker-Based Nanofibrous MF Membranes. The filtration performance of the cellulose nanowhisker-based nanofibrous MF membranes was evaluated in terms of pore size parameters, water flux, and retention ratio of 0.2 μm particles in suspension. The results are summarized in Table 1.

Figure 2. Pore size distribution of membrane consisting of electrospun PAN nanofibrous scaffolds and cellulose nanowhisker-based nanofibrous membrane.

distribution of membranes consisting of electrospun PAN nanofibrous scaffolds and cellulose nanowhisker-based nanofibrous membranes. In the electrospun PAN nanofibrous membrane, the mean pore size was 0.38 μm, whereas a broad distribution of pore size was observed. After impregnation with ultrafine cellulose nanowhiskers, the mean pore size of the membrane decreased to 0.22 μm, and the distribution became narrower. A commercial membrane, GS0.22, prepared from mixtures of nitrocellulose and acetyl cellulose ester (also with negatively charged surface) was compared under the same experimental conditions. From the SEM image of GS0.22 (as seen in Figure S1 in the Supporting Information), a tortuous structure with some dead-end pores was observed. The porosity of GS0.22 measured by the weighing method was only 52%, whereas that of PAN nanofibrous scaffolds and cellulose nanowhisker-based nanofibrous membrane was >80%. The water flux of GS0.22 was found to be only half that of the cellulose nanowhiskerbased nanofibrous MF membrane, even though it had a similar pore size. Moreover, the retention ratio for 0.2 μm particles by GS0.22 was slightly lower than that of the MF membrane based solely on the electrospun PAN nanofibrous scaffold and much lower than that of the cellulose nanowhisker-based nanofibrous MF membrane. Mechanical Properties of Cellulose NanowhiskerBased Nanofibrous MF Membrane. It is of interest to note that the mechanical properties of the cellulose nanowhisker-based nanofibrous membrane were improved significantly when compared with those of the membrane consisting of electrospun PAN nanofibrous scaffold without modification, as shown in Figure 3.

Table 1. Pore Sizes, Water Flux, and Particle Retention Ratio of the PAN Electrospun Membrane, Cellulose Nanowhisker-Based Nanofibrous Membrane, and Commercial GS0.22 MF Membrane

samples PAN electrospun membrane cellulose nanowhiskerbased nanofibrous MF membrane GS0.22 MF membrane

maximum pore size (μm)

mean pore size (μm)

water flux (Lm−2 h−1 kPa−1)

0.2 μm particle retention (%)

0.70

0.38

83 ± 2

13.8

0.41

0.22

59 ± 2

97.7

0.48

0.23

25 ± 2

12.2

As expected, the pore sizes (both maximum pore size and mean pore size) of the nanofibrous scaffold decreased as a result of the introduction of cellulose nanowhiskers. Consequently, the pure water flux decreased by ∼28% when compared with that of the nanofibrous scaffold before cellulose nanowhisker impregnation. However, it was impressive to note that the 0.2 μm particle retention ratio increased from 13.8% to 97.7%, implying a very high retention capability for most waterborne bacteria, including the rather small B. diminuta (dimension: 0.3 μm diameter × 0.9 μm length), with test results to be discussed later. Figure 2 illustrates the pore size

Figure 3. Mechanical properties of membranes consisting of electrospun PAN nanofibrous scaffolds and cellulose nanowhiskerbased nanofibrous membrane.

The membrane consisting of electrospun PAN nanofibrous scaffolds showed a yield point (5.7 MPa at 2.8%), whereas no 183

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reached at 0.5 h by the nanowhisker-based nanofibrous MF membrane, whereas it was reached at about 1.5 h by the GS0.22 membrane. This difference could be explained by the higher hydrophilicity of the cellulose nanowhisker surface than that of the commercial membrane surface, as it was seen that the water contact angle of the cellulose nanowhisker surface was 16.9° (the water contact angle of the electrospun PAN nanofibrous scaffold surface was 50.6°) and that of GS0.22 was 56.3°. The hydrophilic surface of the cellulose nanowhisker-based MF membrane probably reduced the dramatic decrease in permeation flux due to the fouling of tested particles by hydrophobic interaction as seen in GS0.22.34 It was interesting to note that the adsorption capacity (using 10 mg/L CV aqueous solution) of the cellulose nanowhisker-based nanofibrous MF membrane was about two times higher than that of GS0.22 (Figure 4a). Encouraged by this result, adsorption isotherms of the membranes were further investigated by using different concentrations of CV solutions, with the maximum adsorption capacity of these membranes being obtained from the Langmuir adsorption equation. The results are shown in Figure 4b. The Langmuir isotherm model assumes a monolayer adsorption onto the surface with a finite number of identical sites, with all sites being energetically equivalent and no interactions between the adsorbed molecules. In Figure 4b, all points showed a linear relationship with the deviation R2 > 99.3%, which confirmed that the adsorption process of both membranes obeyed the first-order dynamics.27 It was found that the maximum adsorption capacity of the cellulose nanowhisker-based nanofibrous MF membrane for CV was 16 times higher than that of GS0.22, supporting the hypothesis that the former had a very high surface-to-volume ratio. Considering the equimolar interactions between the carboxylate groups on the cellulose nanowhisker surface and the amino groups in CV, the surface charge density of the cellulose nanowhisker could be estimated based on the maximum adsorption capacity of the membrane. Meanwhile, a conductivity titration experiment was also carried out to determine the effective amount of carboxylate groups on the surface of the cellulose nanowhisker-based nanofibrous MF membrane. The surface charge density of the membrane calculated based on the maximum adsorption capacity approach was 0.17 mmol/(g membrane), which matched well with the 0.22 mmol/(g membrane) determined by the titration experiment. The zeta potentials of the cellulose nanowhisker-based nanofibrous MF membrane ranged from −70.8 to −80.4 mV when the pH value changed from 5.4 to 8.9. At pH 7.0, the zeta potential (−75.2 mV) was very negative,35 which provides further strong evidence of the high adsorption capacity of the cellulose nanowhisker-based nanofibrous MF membrane for positively charged species. Evaluation of Bacteria and Bacteriophage Removal Capability. For practical applications, two bacteria, E. coli and B. diminuta, as well as a bacteriophage, MS2, were used to challenge the cellulose nanowhisker-based nanofibrous MF membranes. The results are listed in Table 2. The MF membrane based on the unmodified electrospun PAN nanofibrous scaffold showed a fully retentive capability (i.e., LRV = 6) against E. coli due to its relatively large size (0.5 μm diameter × 2.0 μm length), whereas its LRV against B. diminuta (0.3 μm diameter × 0.9 μm length) was only ∼4. However, the retention for B. diminuta could also be increased to 6 (fully

yield point could be observed during the tensile stretching of the cellulose nanowhisker-based nanofibrous membrane. The Young’s modulus and the ultimate tensile strength for the cellulose nanowhisker-based nanofibrous membrane were 375 ± 15 and 14.3 ± 0.4 MPa, respectively, which were about two times higher than those (i.e., 226 ± 20 and 8.5 ± 0.3 MPa) of the unmodified membrane containing only electrospun nanofibrous scaffolds. This is strong evidence of the stabilization of the electrospun scaffold by the formation of cross-linked cellulose nanowhisker mesh, in agreement with the SEM images (Figure 1). The elongation-to-break ratio of the cellulose nanowhisker-based nanofibrous membrane (∼23.0 ± 3.0%) did not change significantly from the unmodified membrane because the scaffold base was still the PAN nanofibers. The mechanical properties of GS0.22 were also determined under the same conditions, as shown in Figure S2 (Supporting Information). The ultimate tensile strength of GS0.22 was 5.6 ± 0.3 MPa, and the elongation-to-break ratio was 9.4 ± 1.0%; both are lower than those of nanofibrous membranes. Adsorption Evaluation Using the Positively Charged Crystal Violet Dye. The CV dye, having a molecular diameter of ∼1.6 nm, was used as a representative of positively charged molecules to determine the static adsorption capacity and the adsorption isotherm of the cellulose nanowhisker-based nanofibrous MF membrane. The optical absorbance of CV at 590 nm was monitored by UV−vis spectroscopy to determine the adsorption capacity. Figure 4 a illustrates the adsorption

Figure 4. (a) Adsorption capacity of cellulose nanowhisker-based nanofibrous MF membrane and GS0.22 against time, and (b) respective Langmuir adsorption isotherms for both membranes.

capacity of 10 mg/L CV by the cellulose nanowhisker-based nanofibrous MF membrane as well as the commercial GS0.22 as a function of time. In Figure 4a, it was found that the adsorption of CV by the cellulose nanowhisker-based nanofibrous MF membrane was much quicker than that by GS0.22. This feature would be beneficial for dynamic adsorption in practical applications. With the 10 mg/L CV aqueous solution, equilibrium adsorption was 184

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Table 2. Permeate Flux, Pressure Drop, and LRV against Ecoli, B. diminuta, and MS2 of Electrospun PAN Nanofibrous Membrane, Cellulose Nanowhisker-Based Nanofibrous MF Membrane and GS0.22 samples

permeate flux (L/ m2h)

pressure drop (kPa)

E. coli (LRV)

B. diminuta (LRV)

MS2 (LRV)

192

1.1

6

4

0

192

3.0

6

6

2

192

7.6

6

PAN electrospun nanoscaffolds cellulose nanowhisker nanofibrous membrane GS0.22

ASSOCIATED CONTENT

S Supporting Information *

SEM image and the mechanical properties of the commercial membrane GS0.22. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Phone: (631) 632-7793; E-mail: [email protected]; Fax: (631) 632-6518 (B.S.H.). Phone: (631) 632-7928; E-mail: [email protected] (B.C.).

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ACKNOWLEDGMENTS Financial support of this work was provided by the National Science Foundation (DMR-1019370).

retentive) after impregnating cellulose nanowhiskers into the electrospun nanofibrous scaffold. The negatively charged cellulose nanowhisker-based nanofibrous membrane with relatively high charge density could also provide an LRV of 2 against MS2 (pI = 3.9), a bacteriophage under neutral conditions (e.g., pH 7.2), whereas the negatively charged commercial filter GS0.22 had only an LRV of 1 against MS2. This behavior may be explained by the high affinity between cellulose and proteins (bacteriophages),36 where the high surface charge density of cellulose nanowhiskers may also play a role. It should be noted that the pressure drop of the cellulose nanowhisker-based nanofibrous membrane was much lower than that of GS0.22 under the same flow rate, which is consistent with the MF results (Table 1). The higher porosity of the cellulose nanowhisker-based nanofibrous membrane (>80%) than that of GS0.22 (52%) definitely results in a faster velocity of fluid traveling through the pores of the membrane.

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CONCLUSIONS

A cellulose nanowhisker-based nanofibrous MF membrane was prepared by impregnating ultrafine cellulose nanowhiskers into the PAN electrospun nanofibrous scaffold, resulting in a crosslinked nanofibrous mesh with very high surface-to-volume ratio. The resulting membrane possessed a mean pore size of 0.22 μm, a narrow pore size distribution, and a high retention ratio (LRV = 6) against bacteria: E. coli and B. diminuta (through screening filtration). The impregnated cellulose nanowhiskers had very high surface (negative) charge density, as confirmed by the adsorption capacity of positively charged dye (e.g., CV) and a zeta potential of −75.2 mV. It should be noted that the negatively charged cellulose nanowhisker-based nanofibrous MF membrane also exhibited an LRV of 2 against bacteriophage MS2 as compared with an LRV of 1 for the commercial membrane GS0.22 at a similar high flux but much lower pressure drop probably due to the distribution of carboxylate groups on the surface of cellulose nanowhiskers and the affinity between cellulose and proteins. It should be noted that the use of negatively charged cellulose nanowhiskers is not the most effective way to remove viruses through adsorption. Recently, we have used a positively charged polymer, that is, polyethylenimine (PEI), to modify the cellulose nanowhisker surface and render the network into a positively charged one.17 The resulting membrane could be expected to have an LRV of 4 against MS2. The details will be presented in a future publication. 185

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Biomacromolecules

Article

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