Water Flux and Ion Rejection by Imogolite Nanotubes Incorporated Polymeric Membranes Operated in Thermally Driven Desalination.

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1 Water Flux and Ion Rejection by Imogolite Nanotubes Incorporated Polymeric Membranes Operated in Thermally Driven Desalination Ming Li, Ph.D Candidate, Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY Phone: (307) , Jonathan A. Brant, Ph.D, Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY Phone: (307) , Introduction Membrane processes are widely used in water treatment for applications ranging from particulate separation to desalination. Persistent challenges for these processes include relatively high specific energy consumption, trade-offs between ion selectivity and water flux, and improving the chemical and physical robustness of membrane materials. Efforts aimed at overcoming many of these, as well as other, challenges have focused on the integration of engineered nanomaterials, such as zeolites, silica, silver, and carbon nanotubes into polymeric and ceramic matrices to create nanocomposite membranes [An W., et al. (2014)]. In a previous study thinfilm composite (TFC) membranes, which consist of a micro-porous support layer and a dense active layer, were developed to achieve to increase achievable specific flux values while maintaining a high selectivity [Wang X., et al. (2005)]. Nanotubes are an attractive type of nanofiller for making composite membranes because of their ability to enhance water transport across the dense active layer [Majumder, M. et al. (2005)]. The researchers [Hummer, G., Rasaiah, J. C. and Noworyta, J. P., (2001)] found that water transport is enhanced within the interior of carbon nanotubes (pore throat), counter to intuition, due to the lack of friction between the water molecules and the hydrophobic interior of the nanotube. Unfortunately the applicability of carbon nanotubes is limited by their poor dispersion in polymers. Overcoming some of the limitations associated with carbon nanotubes is source of motivation for exploring the feasibility of using other nanotubular structures in composite membranes. Imogolite Nanotubes Imogolite nanotubes [(OH)3Al2O3SixGe(1-x)OH] are naturally hydrophilic materials [Cradwich, P. D., et al. (1972)], which is expected to improve their ability to be dispersed in polymeric matrices relative to that for carbon nanotubes. Effective incorporation of nanotubes into polymeric matrices depends on the abilities to uniformly disperse the nanotubes through the matrix. Due to their high aspect ratio, imogolite nanotubes are highly anisotropic. Thus, the nanotubes will tend to agglomerate instead of forming separated individual tubes because of van der Waals force. As a result, in order to take full advantages of the structure of imogolite nanotubes, it is important to develop a method to disperse into polymeric gel. Imogolite can be well dispersed in water at a low acidic condition because its outer hydrophilic aluminum hydroxide (Al-OH) surface can adsorb protons. Multiple nanotube alignment methods have been investigated. One type of alignment route that can be applied is to infiltrate a polymer solution into an imogolite nanotube array. However, this 1

2 method usually leads to inhomogeneous imogolite nanotube condensation, membrane defects and non-vertically oriented imogolite nanotubes during the capillary condensation induced by solvent evaporation [Kim, S., et al. (2014)]. Another candidate approach is to infiltrate polymer monomers into the pre-aligned nanotube array, and then in-situ polymerization [Huang, H., et al. (2005)]. It is possible to polymerize polymeric monomers with pre-aligned imogolite nanotubes to synthesize vertically aligned imogolite nanocomposite membranes. However, this approach opens another question about how to synthesize pre-aligned imogolite nanotubes. Pervaporation Pervaporation is a thermally-driven membrane process in which water transport occurs through solution-diffusion and the water evaporates from the permeate side of the membrane. In general, the feed solution is at an elevated temperature, relative to the condensing stream, in order to create a vapor pressure gradient across the membrane. Because the driving force is a vapor pressure gradient, rather than a hydraulic pressure gradient, pervaporation processes can treat waters having salt concentrations in excess of those appropriate for reverse osmosis (RO). Development of membrane materials whose characteristics are tailored to the unique operating conditions in pervaporation is key to unlocking the potential of this process in desalination applications. Hydrophilic polymers such as poly (vinyl alcohol) (PVA) and polyamide (PA) are widely applied in pervaporation. However, hydrophilic membranes have severe swelling problems. Imogolite nanotubes can be not only homogeneously dispersed in polar polymers but also have the potential to crosslink polymeric chains, which decreases membrane swelling. Membrane Material Selection Optimum utilization of a membrane system often directly correlated with membrane material selection. The proper selection of polymer is mainly impacted by the need of membrane design and engineering, different system modules and working conditions (Table 1). The control of polymer chemistry and reinforce filler can improve the performance and stability of the membrane system. Objectives The objectives of this project were as follows: 1. Evaluate the physical and surface chemical properties of nanocomposite polymeric films containing imogolite nanotube. 2. Characterize and quantify the influence of nanotubes dispersion and alignment on the mass transport of water through nanocomposite membranes. 3. Investigate the influence of modifying the inner surfaces of imogolite nanotubes to membrane processes. 2

3 Table 1 Advantages and Disadvantages of Polymers being Considered for Formation of the Imogolite Nanotube Composite Membranes Membrane Material Cellulose Acetate (CA) Polysulfone (PSF) Poly (vinyl alcohol) (PVA) Polyvinylidene Fluoride (PVDF) Thin-Film Composite (TFC) Ethylene Propylene Diene Monomer (EPDM) Polyurethance (PU) Nitrile (NBR) Hydrogenated NBR Silicone Advantage Disadvantage Notes Low price, less fouling Exceptional temperature and ph resistance Low price; high resistance to hydrophobic foulants; operating temperature up to 90 C and mild to relatively strong acidic and basic conditions High resistance to hydrocarbons and oxidization High flux and rejection, good temperature and ph resistance High weathering, aging and ozone resistance; high resistance to polar materials (because EPDM is non-polar); operating temperature from -60 C to 170 C Good performance in presence of hydrocarbon oils and aromatic solvent; Operating temperature - 45 C to 70 C Good solvent and oil resistance (Nitrile is very polar); operating temperature -40 C to 120 C Saturated polymer; improved resistance to heat, ozone and chemicals; operating temperature -30 C to 150 C High resistance to heat, ozone, electrical current and strong bases; operating temperature - 65 C to 315 C Low tolerance to extreme temperature and ph; microorganism corrosion Low tolerance to oil, grease, fat and polar solvents Swelling in water; low tolerance to hydrophilic organic mater No good and consistent separation characteristics Low tolerance to oxidizing environment Low flame resistance, high fouling to aliphatic or aromatic hydrocarbon oils Damaged by concentrated acids, ketones, esters, etc. Low resistance to polar solar solvent; susceptible to heat and ozone (Nitrile is unsaturated polymer) Very expensive Degrade in solvents, oils and concentrated acids hydrophilic makes it less prone to fouling Practically the only membrane used in food and dairy application; hydrophobic; a modified hydrophilic PSF can be used for oil emulsions High degree of crosslinkage can reduce membrane swelling; hydrophilic nanofillers disperse well Hydrophobic; Most commercial TFC membranes use PSF as support layer; two or three thin-layer design; Typically used in some pulp, paper, food processing Co-polymer of acrylonitrile and butadiene; typically used in oily or greasy wastes. Its carbon-silicone-oxygen bonds are different from the carbon-carbon bonds in normal polymers Neoprene Fluorelastomers (FKM) High resistance to ozone, acids, fats, greases and aliphatic oils; temperature -40 C to 120 C Resistance to most chemicals, oils and heat; operating temperature from -40 C to 200 C Degradation by strong acids, ketones and hydrocarbon containing aromatic and nitrogen Very expensive Chlorinated butadiene polymer; generally used in petrochemical, meat processing or oily/greasy wastes Carbon-fluorine bonds and high insaturation; used in some chemical applications 3

4 Experimental Approach Synthesis of Imogolite Nanotubes Imogolite nanotubes are synthesized using different precursors and conditions. Using germanium precursor the imogolite nanotubes can get larger diameters (expected 3-4 nm) compared to silica precursor (expected 1-2 nm). And the aluminogermanate nanotubes synthesized in the autoclave are expected to be much longer (expected 1000 nm in length) than by traditional slow hydrolysis method (200 nm in length). Table 2 Synthesis of Imogolite Nanotube Precursors Slow Hydrolysis Autoclave Tetraethyl orthosilicate and aluminum perchlorate Germanium ethoxide and aluminum perchlorate Short nanotube with small diameter Short nanotube with large diameter Fail to form nanotube Long nanotube with large diameter Preparation of Imogolite/PVA/PAN Membrane The prepared imogolite solution was mixed with 5% PVA solution with a certain ratio under vigorous stir. Then the imogolite/pva solution was sonicated for 15 min to get good imogolite dispersion. Crosslinking agent glutaraldehyde (GA) and catalyst HCl was added into imogolite/pva solution under vigorous stirred at 40 C for 1 hour with a volume ratio as imogolite/pva : 25% GA : 1N HCl = 100 ml : 1 ml : 0.1 ml. Then the mixed solution was coated onto polyacrylonitrile (PAN) ultrafiltration membrane (GE Osmonics Ultrafilic UF) using a spin coater and dry in room temperature for 24 hours. The nanocomposite membrane was coated three times to get a defect free active layer. As control, the pure PVA/PAN membrane was made by coating 5% crosslinked PVA onto PAN substrate three times and dry for 24 hours. Dead-ended Filtration Cell Water flux and salt rejection measurements for the different membranes were done using a deadend filtration cell (Sterlitech, Kent, WA). Tests were done using a 10,000 mg/l sodium chloride solution (ph ~ 7) in order to simulate brackish water desalination conditions. All tests were done at a feed pressure of 200 psi and a stirring speed of 300 rpm. A commercially available RO membrane from DOW FILMTEC (XLE membrane) was also evaluated to do a comparative analysis with the PVA composite membranes. Result and Discussion Synthesis of Imogolite Nanotubes The as synthesized imogolite nanotube have 1000 nm in length and 1 nm in diameter. Most nanotubes agglomerate together to form bundles instead of individual nanotubes (Figure 1). 4

5 (a) (b) Figure 1. AFM images of imogolite nanotubes synthesized by traditional slow hydrolysis method. (a) single imogolite nanotube; (b) agglomeration of imogolite nanotubes. The presence of imogolite nanotubes in the PVA film was confirmed by Hyperspectral Microscopy. Using this approach, the surface area of the uppermost portion of the composite film was determined to be approximately 15% imogolite. The spectrum peaks at 450 nm represent imgolite (green lines in Figure 3). These results indicate that the imogolite nanotubes were dispersed in the PVA thin-film; however, the uniformity of this dispersion and the aggregation state of the nanotubes remains to be determined. Notably, the contact angle with water for the PVA film remained relatively unchanged following the integration of the nanotubes ( water = 55.6 for the pure PVA film; ( water = 55.0 for the PVA/imogolite film). Therefore, for at least the PVA to imogolite concentration ration reported here the hydrophilicity of the PVA film was not affected by the presence of the imogolite. Future tests will investigate other concentration rations (imogolite:pva) to determine how/if the imogolite can induce greater hydrophilicity in the composite film. Figure 2. The dispersion of imogolite/pva film (volume ratio of imogolite solution to 5% PVA solution was 1:7) Influence of Imogolite Nanotubes on Water Flux Water flux tests were done using a 10,000 mg/l sodium chloride solution. The constant feed pressure of 200 psi was used in all of the tests. The addition of imogolite nanotubes into the PVA film resulted in an increase (133%) in water flux from 0.30 m 3 m -2 day -1 to 0.40 m 3 m -2 day -1 for the pure PVA and the PVA/imogolite composite films, respectively. In addition, the PVA/imogolite 5

6 composite had a comparable (slightly higher) water flux than did a commercially available RO membrane (DOW FilmTec XLE) (Figure 3). Interestingly, the PVA/imogolite composite membrane was characterized by no flux decline, while both the pure PVA and RO membranes were when treating the brine solution. Because these tests were done in a dead-end test configuration the flux decline is attributed to concentration polarization. The lack of flux loss with the composite membrane remains to be resolved. Because the imogolite nanotubes did not alter the hydrophilicity of the PVA film the improved water flux must be a function of alterations in the physical structure of the PVA film, which resulted in enhanced water transport. Future tests will focus on resolving the specific mechanisms by which the nanotubes affected the water transport. Additional tests are also required to quantify how the imogolite nanotubes affected salt rejection by the membrane. Regardless, these initial results are promising Imogolite/PVA DOW Brackish RO Pure 5% PVA Permeate Flux, m 3 m -2 day Time (min) Figure 3. Permeate flux of dead-ended RO module using10,000 mg/l NaCl solution as feed solution. ( ) Imogolite incorporated 5% PVA membrane on PAN support; ( ) DOW FilmTec XLE RO membrane; ( ) Pure 5% PVA membrane on PAN support. Conclusions Based on the preliminary data gathered thus far we can draw the following preliminary conclusions: Imogolite nanotubes can be dispersed into polymeric matrices to form non-porous thinfilms suitable for desalination applications without modification of their external surface chemistry (surfactant grafting etc.). Incorporation of imogolite nanotubes into PVA increases the water flux for the resulting composite film when compared to the pure PVA polymer. Water fluxes for the composite 6

7 films are comparable to, or greater than, those values for commercially available lowpressure/high rejection RO membranes. References An, W., et al. (2014), Natural zeolite clinoptilolite-phosphate composite Membranes for water desalination by pervaporation, Journal of Membrane Science, : p Cradwick, P.D., et al., Imogolite, a Hydrated Aluminum Silicate of Tubular Structure. Nature- Physical Science, (104): p. 187-&. Hummer, G., Rasaiah, J. C., & Noworyta, J. P. (2001). Water conduction through the hydrophobic channel of a carbon nanotube. Nature, 414(6860), Huang, H., et al. (2005), Aligned Carbon Nanotube Composite Films for Thermal Management, Advanced Materials, (13): p Kim, S., et al. (2014), Fabrication of flexible, aligned carbon nanotube/polymer composite membranes by in-situ polymerization, Journal of Membrane Science, : p Majumder, M., Chopra, N., Andrews, R., & Hinds, B. J. (2005). Nanoscale hydrodynamics: enhanced flow in carbon nanotubes. Nature, 438(7064), Wang, X., et al. (2005), High flux filtration medium based on nanofibrous substrate with hydrophilic nanocomposite coating, Environ Sci Technol, (19): p