Functionalized Carbon Nanotube Nanocomposite Membranes for Water Desalination: Experimental Study. Introduction

Transcription

1 Functionalized Carbon Nanotube Nanocomposite Membranes for Water Desalination: Experimental Study Wai-Fong Chan 1, Hang-Yan Chen 2, Eva Marand 1, and J. Karl Johnson 2 1 Virginia Polytechnic Institute and State University, VA, USA 2 University of Pittsburgh, PA, USA Introduction The increasing demand for fresh water has become a serious issue in today s world. Desalination of seawater using reverse osmosis (RO) is one way to meet this demand.[1] Thin film composite membranes based on aromatic polyamides are predominantly used as RO materials. However, these materials are far from optimum. [2] Faster filtration rates and higher resistance to chemical and biological attack are required for the next generation of RO membranes. Carbon nanotubes are promising materials for use in membranes because they have shown remarkably high water transport rate.[3-10] Molecular simulations and experimental studies have demonstrated that the transport of fluids through CNTs is orders of magnitude faster than through other nanoporous materials due to the unprecedented smoothness and regularity of the CNT pores.[2], [5], [7], [10], [11], [12], [13] The transport of water in CNTs has been shown to give flow rates that are faster than predicted by classical hydrodynamics.[14], [15] Our group has focused on using functionalized single-walled CNTs (SWNTs) with zwitterion groups which act as molecular gatekeepers at the entrance of CNTs to enhance the blockage of salt ions. Moreover, zwitterion groups should prevent biofouling of the membrane since zwitterion-treated surfaces have shown good resistance to cell adhesion.[16], [17], [18] We have synthesized zwitterionic SWNTs (Z-SWNTs) using the functional group with the following structure: -COO- (CH 2 ) 3 -N + (CH 3 ) 2 -(CH 2 ) 2 COO -,[19]. We then fabricated Z-SWNTs/polyamide nanocomposite membranes and examined the performance of the membrane in terms of permeation flux and ion rejection ratio. Note that the synthesis method we have developed can be used for large-scale manufacture of CNT membranes, in contrast to previous methods in synthesizing CNT membranes,[20], [21] which require CNTs to be grown on a substrate in a CVD reactor, and therefore, are non-scalable. Materials and Methods Materials The following chemicals were used as received from Sigma-Aldrich: 1,3,5- Benzenetricarbonyl trichloride (trimesoyl chloride, TMC), 1,3-phenylenediamine (mphenylenediamine, MPD) and sodium dodecylbenzenesulfonate (SDBS). All chemical were of analytical grade. Polyethersulfone ultrafiltration membrane (PES) was provided by Trisep Corporation (Goleta, CA). CNT Functionalization Carboxylate functionalized CNTs of outer diameter 15 Å and length 1 μm were purchased from Nano Lab Inc. (Waltham, MA).[22] The COOH-functionalized CNTs were

2 produced by chemical vapor deposition (CVD). Concentration of -COOH groups in the CNTs was approximately 2-7 wt% (as determined by titration). The functionalized CNTs were reacted with thionyl chloride (SOCl 2 ) at 65 C for 36 hours and the -COOH groups were replaced by COCl groups. The acylated CNTs were then esterificated using 3-dimethylamino-1-propanol, (CH 3 ) 2 -N-C 3 H 6 -OH. This was followed by a ring-opening reaction of the lactone, in which β- propiolactone was opened to form an acid group and attached to the tertiary amine on the functional group.[19], [23] The resulting zwitterionic group had a positive charge at the tertiary amine group and negative charge at the carboxylated group. Membrane Fabrication Briefly, the fabrication process was divided into three steps, as shown schematically in Figure 1. First, the PES support was pretreated by soaking in 0.5 wt% SDBS solution to open the pores and to increase the hydrophilicity. The support was sandwiched between two round Poly(tetrafluoroethylene) (PTFE) frames. Afterwards, a predetermined amount of functionalized CNTs was poured on the support. In order to get aligned CNTs dispersing on the support, we used high-vacuum filtration to remove the solvent while retaining the CNTs.[4], [8] The third step in the membrane fabrication process was interfacial polymerization, forming a polyamide, PA, film on top of the aligned CNTs. In this step, the fabrication side of support with aligned CNTs was wetted with an aqueous diamine solution containing 2 wt% MPD and 0.2 wt% of SDBS at ambient temperature for 2 min. The membrane was then immediately placed on a glass plate at which excess MPD solution was removed by squeezing the membrane using a glass roller. The dried membrane was then wetted by n-hexane solution containing 0.5% (w/v) TMC solutions for 90 seconds. The resulting nanocomposite membrane, Z-CNT/PA was subsequently heat-cured at 68 C for 5 min. After oven curing, the fresh nanocomposite membrane was washed thoroughly with DI water, submersed in fresh DI water and stored in a laboratory refrigerator at 4 C. Membrane Characterization The surfaces and cross-section of the membranes were characterized by field emission scanning electron microscopy (FESEM, LEO 1550). A small piece of fabric-free membrane sample was frozen in liquid nitrogen and fractured cryogenically. Pressure-driven experiments were carried out on a laboratory-scale cross-flow membrane test unit as shown schematically in figure 2, capable of pressures from 25 to 1000 psi. This test unit is comprised of a stainless steel membrane cell, high pressure pump (Hydra-cell pump, Warner Engineering), back-pressure regulator (US Paraplate), bypass valve (Swagelock), feed water reservoir (Nalgene), operated in closed loop mode with retentate being circulated into the feed water reservoir. The concentration analysis of the cations present in the permeant was measured by an atomic adsorption spectrophotometry (AAS; Perkin Elmer 5100, Wellesley, MA). Feed solutions were kept between 25 to 30 C. Membrane Permeation tests The membrane cell was pressurized to the designated hydraulic pressure by adjusting the speed of the pump and the flow rate of the retentate. For each testing pressure the permeation flux was allowed to equilibrate for 30 min before any permeant collection. A known amount of permeant was collected in a glass vial within a given period of time. The density of water was taken to be g/cm 2 at ambient temperature, the volumetric flow rate was calculated from

3 V Q =, where V is the permeate volume (liter), t is the permeation time (hour) and A is the t A effective membrane area (m 2 ). The flow rate, Q, was recorded in the units of liter per square meter per hour (LMH). The atomic absorption spectrophotometer (AAS) was calibrated using standard solutions, which contained 5, 10, 15 and 20 ppm of the specific cations. Thus, the concentrations of cations in the feed, C, and the permeant, C, were measured and the salt rejection ratio (in percent) was calculated from C p R (%) = C f f Results/Discussion Membrane Microstructure The surface morphologies of the PES support, PA and Z-CNTs/PA nanocomposite membranes were studied using FESEM and images are shown in Figure 3, parts A, B, and C, respectively. The cross-sectional view of the Z-CNTs/PA membrane, also taken by FESEM, is shown in Figure 3-D. The neat PES support (A) has a relatively smooth and porous surface with pore sizes ranging approximately from 6 to 20 nm. After interfacial polymerization, a thin PA skin layer with ridge-valley shape was formed on the top of the PES substrate (B) and acted as a barrier layer in separating salt ions from water. In the Z-CNTs/PA nanocomposite membrane image (C), all the nanotubes are covered by interfacially polymerized PA. Due to the random packing of Z-CNTs, the surface roughness of the nanocomposite membrane is greatly increased relative to plain PA. The cross-section of the Z-CNTs/PA membrane (D) shows that nanotubes are embedded in PA with semi-aligned orientation (examples indicated by arrows). RO Performance Two nanocomposite membranes were fabricated. One with 0 wt% and the other with 26 wt% of zwitterion functionalized CNTs embedded in the plain PA matrix. The weight percentage reflects the percentage of CNTs in the selective PA layer. These membranes were tested for water and ion flux by using a pressure drop of 2.41 Mpa (350 psi) with a feed solution containing 2000 ppm of designated cations. Na +, Mg 2+ and K + were used in the testing and their forms in the salt were NaCl, MgSO 4 and KCl respectively. The water flux and ion rejection ratio are shown in Table 1. For all three cations, the membrane experienced almost three-fold increase in water flux after adding Z-SWNTs. For Na +, water flux increased from 12.0 LMH (liter per square meter per hour) to 32.8 LMH, while the rejection ratio had a slight increase from 97.0 % to 97.6 %. Unlike Na + ion, Z-SWNTs/PA membrane had lower rejection towards Mg 2+ and K + ions. After adding Z-SWNTs, the rejection rate of the membrane for Mg 2+ decreased from 99.2 % to 97.6 %, and from 93.7 % to 92.5 % for K +. Water Flux The dramatic increase in water flux is mainly due to the transport through Z-SWNTs. The smooth inner surface of CNTs enhanced the water transport while the zwitterion groups rejected salt ions at the end of tubes and maintained high rejection rate. On the other hand, if the increase of flux was caused by any defects introduced from the addition of CNTs, a drastic decrease in salt rejection would be expected. This clearly was not the case. The fact that water transport p (1)

4 occurred through the carbon nanotubes is also consistent with the molecular simulation results reported by Johnson et al.,[24] which show that zwitterion functionalized CNTs reduced the conductance of ions while allowing an acceptable conductance of water molecules. With two zwitterions at the tube end, simulation showed complete rejection of ions (zero conductance) and a transport of about 100 water molecules per tube per nanosecond. It is this phenomenon, which improves the water transport without trading-off ion rejection. Ion Rejection The rejection data in Table 1 clearly shows that the polyamide coating rejects ions based upon their molecular size. Notice that Mg 2+ has the highest hydrated radius (4.38 Å) and its rejection rate in plain PA is as high as 99.2%. As the hydrated radii of ions decrease, a reduction in their rejection ratio is observed. Na +, whose hydrated radius is 3.58 Å, and K +, with a hydrated radius of 3.31 Å, have rejection ratio of 97.0 % and 93.7 % respectively. In Z-SWNTs/PA membrane, water and ions would first pass through a very thin layer of PA coating covered on top of all the nanotubes, at which size exclusion dominates and rejects ions as in the plain PA. When water and ion molecules reach the tube end, Z-SWNTs offer fast water transport through the inside of the tubes while the ions are retained in the PA phase. However, the drop of rejection rate for Mg 2+ after adding Z-SWNTs reveals the highest separation limitation for our nanocomposite membrane. When the Z-SWNTs are introduced into the membrane during the fabrication, some of them dry in the air and form a bundle-type structure (as shown in Figure 2- D). This reduces the dispersity of CNTs and thus decreases the quality of PA coating. These nonuniform coatings occur at different micro-areas in the membrane and therefore cause the rejection ratio to be limited to 97.6 % on the average. Similar rejection ratio has been observed for both Na + and Mg 2+ ions during the testing of Z-SWNTs/PA. On the other hand, it is also possible that nanoscaled voids are formed around the wall of Z-SWNTs due to the incompatibility between organic polyamide and inorganic carbon nanotube wall. These nanovoids showed minimal effect during the RO for Na + ion, but became observable in rejecting smaller molecules like K +, giving more passage for the smaller ions and resulting in a small drop in rejection ratio. Salt Concentration Effects NaCl salt was used in these experiments. The feed concentration was varied between 50 to 2000 ppm to test for ion concentration effects on the rejection ratio. Plain PA membrane (no Z-SWNTs) and the nanocomposite membrane with 20 wt% Z-SWNTs were fabricated and tested. The pressure drop applied was 3.65 Mpa (530 psi) for each test in these experiments. The results, plotted in Figure 4, show an increase in the salt rejection ratio with ion concentration for both the plain PA membrane and the nanocomposite membrane with 20 wt% Z-SWNTs. The increase in rejection ratio with increasing ion concentration is opposite to what has observed by Fornasiero et al.,[20] who found that the ion rejection ratio decreased with increasing ion concentration in the feed, and dropped to zero when the ion concentration was equal to 10 mm (about 750 ppm) for KCl. According to Donnan equilibrium theory, the cause of rejection drop in their experiment was due to the electrostatic screening effects which most of the charged nanofiltration (NF) membrane experienced. Fornasiero et al. utilized negatively-charged CNTs in fabricating NF membrane and rejected ions by counteracting the negative ions in the feed solution. This type of exclusion mechanism can be easily disrupted with increasing ionic strength/concentration in the feed solution. In contrast, Ji et al.[25] show that zwitterion-

5 functionalized membrane is stable in salt rejection regardless the change of feed concentration. Note that our plain PA membrane, which is uncharged inside the pores, exhibits an increase in rejection rate while increasing the concentration of feed solution. This same trend has been observed for other PA-type membranes.[26], [27] We therefore attribute the increase in rejection ratio with increasing feed concentration, observed from the Z-SWNTs/PA membrane in Figure 4, to both the PA component and zwitterion functional groups on the CNTs. It also agrees with the previous cation experiments, which suggest that the exclusion mechanism of the nanocomposite membrane is dominated by the size of ions as opposed to charge repulsion. Conclusions Zwitterion functionalized CNTs/Polyamide membrane was fabricated to improve RO performance in separating common salt ions from water. The water permeation flux increased threefold after embedding functionalized CNTs into the polymer matrix, which was attributed to the atomically-smooth inner surface of CNTs. The separation of the PA skin layer dominated the ion rejection mechanism by size exclusion even when the carbon nanotubes were introduced into the polyamide coating. However, due to the random packing of nanotubes, the quality of the polyamide coating decreased and resulted in an upper limit of the ion rejection ratio, which was around 97.6% regardless of the molecular size of the ion. We also demonstrated that charge exclusion has negligible effect for separating ions in our nanocomposite membrane because no electrostatic screening effect was observed when the ionic strength of the feed solution increased. All results show that zwitterions offer steric hindrance at the tip of SWNTs in separating salt ions, rather than electrostatic repulsion.

6 Figure 1. Cross-section schematics of the fabrication procedure for Z-SWNTs nanocomposite membrane. (A) PES ultrafiltration membrane, composed of a thin PES layer cast on a nonwoven polyester web, soaked in surfactant solution to cleanse the pores and to increase hydrophilicity. The membrane was then sandwiched by two round frames made out of PTFE. (B) Zwitterion functionalized SWNTs, deposited onto the pretreated PES support, through vacuum filtration. (C) Interfacial polymerization of polyamide carried out between functionalized CNTs at which MPD presented in the aqueous solution crosslinks with TMC in non-aqueous solution. (D) Photograph of the top of Z-SWNTs nanocomposite membrane that is exposed to the feed.

7 Figure 2. Schematic diagram of apparatus in RO membrane system and the cross-section of the membrane test cell. A B C D

8 Figure 3. Field emission scanning microscopy images of the PES support, plain PA and Z- SWNTs/PA membranes. The surface morphologies of (A) PES support, (B) PA and (C) Z- SWNTs/PA. (D) shows the cross-sectional view of a Z-SWNTs/PA membrane. Figure 4. Salt rejection as a function of NaCl feed concentration in plain PA (black curve with open circle) and a nanocomposite membrane with 20wt% Z-SWNTs (orange curve with solid circle). Feed pressure was 3.65 Mpa (530 psi). Table 1. Water flux and ion rejection ratio for different cations. Pressure was 2.41 Mpa (350 Psi); Feed concentration was 2000 ppm. Hydrated Flux (LMH) Rejection (%) Cation Radii[28] 26wt% 26wt% (Å) Plain PA Plain PA Z-SWNTs/PA Z-SWNTs/PA Na + (NaCl) ± ± ± ± 0.6 Mg 2+ (MgSO 4 ) ± ± ± ± 0.8 K + (KCl) ± ± ± ± 0.1

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