Nanosized Linde Type A Zeolites Providing Water-Selective Transport Pathways Through Chlorine Tolerant Polymers in Molecular Sieve Nanocomposite (MoSI

Project: Research project

Project Details

Description

Project Narrative: Introduction: To sustain life during space flight, water is an absolute necessity. One of the challenges currently faced by NASA is providing clean water from the limited supplies available in the isolated environment of a space shuttle or station. Society also faces this challenge in isolated areas and developing countries. Water cannot feasibly be delivered from earth, as the American space shuttle fleet has been retired, so another alternative must be found. A viable option is recycling water from all available sources, namely urine and urine brine, through the use of membrane processes such as reverse osmosis (RO). The Water Recovery System currently in use can recover 70% of the water from wastewater, but this is still inadequate for the quantity of water required in space[1]. This recovery rate has the potential to be increased with the use of osmotic processes. However, commercially available reverse osmosis and forward osmosis (FO) membranes are very susceptible to degradation in the harsh conditions imposed by urine and urine brine. Low pH and chlorine both damage these membranes, and they are also not designed to retain small, neutral organic substances such as urea, generally found in urine[2]. It is readily apparent that an innovative membrane is needed that can endure these conditions and improve water recovery. Molecular sieves, crystalline structures with pores that distinguish between molecules of different sizes, are an acceptable solution, with fluxes and separation factors that exceed those of polymers[3]. Pure molecular sieve membrane synthesis often requires high temperatures and pressures, increasing the cost of production[4]. Thus, membranes which consist of molecular sieves dispersed within a polymer matrix are ideal, and may retain the desirable characteristics of the individual components while obviating their individual disadvantages. Linde type A (LTA) zeolites have already been successfully incorporated into polyamide thin film RO membranes[3]. Figure 1: Proposed membrane containing water-selective zeolites in a thin film barrier membrane. Hypothesis: Nanosized LTA zeolites can provide water-selective transport pathways through corrosion-tolerant polymers to create molecular sieve nanocomposite (MoSIN) membranes for RO. We present this concept schematically in Figure 1. Description of Proposed Research: In order to synthesize MoSIN membranes, I will perform research to investigate the fundamental transport properties of zeolite molecular sieves, the formation of polymeric thin films on support membranes, and the corrosion resistance and chlorine tolerance of selected materials. My first year in the PhD program will focus on understanding film formation and beginning investigation of chlorine tolerance. The second and third years will focus on the transport properties of zeolites and selection of robust materials for membrane synthesis. A main question of interest to our research group is how liquid transport occurs through molecular sieves. An understanding of this phenomenon will allow us to better direct the course of our research by determining if more attention should be given to the zeolite surfaces or the interface between the zeolite and polymer matrix. Liquid transport through molecular sieve particles has not been characterized due to difficulties with fabrication and measurement of transport properties of nanoscale materials. To isolate the intrinsic transport through the three potential pathways, we will use model systems (Figure 2) in which a specific number of large, single crystal zeolites are embedded into a thick, impermeable polymeric film (e.g. poly(ethylene terephthalate), high density polyethylene, polytetrafluoroethylene, and polyvinylidene chloride). Using pore-filled and pore-open zeolites with particle diameters that match the thickness of the polymer film will enable us to isolate the contribution of the pores of the zeolite and the zeolite interface to the overall liquid transport. The transport properties of other single-crystal zeolites and molecular sieves could easily be measured by this method. Second, I will focus on elucidating the fundamental parameters controlling the formation of chlorine-tolerant thin films. An understanding of the mechanism by which the film deposition occurs is essential in adequately investigating the controlling parameters. With the formulation of governing equations, our ability to determine the relevance of these parameters will be greatly Figure 2: Water transport pathways through the components of our nanocomposite membrane. Our model system utilizes an impermeable polymer and both pore-open and pore-closed zeolites to assist in elucidating the transport properties of the molecular sieves. enhanced. Formation of polymeric thin films on porous support materials is a complex phenomenon involving fundamental aspects of chemistry, physics, and materials science. In order to develop a precise understanding of the factors controlling the deposition of an impermeable thin polymeric film onto porous supports we will systematically vary properties of the casting solution (e.g., viscosity, miscibility) and properties of the support (e.g., porosity, hydrophilicity). We will also evaluate different methods to disperse nanoparticles within the casting solutions and thin films. Our main goal in understanding the deposition process is avoidance of defects in the thin film and intrusion of the film into the support membrane. While defects will compromise the integrity of the barrier membrane, intrusion will block the pores of the support and significantly reduce the ultimate water flux (Figure 3). Our proposed methods to avoid penetration of polymer into the support membrane during spray deposition are filling with an immiscible material, heating the support to cause immediate evaporation of casting solution, or coating the support. Materials used to coat the surface or fill the pores of the support membrane may be quite relevant in determining the ultimate surface morphology and transport properties of the thin film coating. We will develop an additional model system for a combination of the thin film casting solution and support membrane filler materials to systematically determine which properties of the solvent, filler material and support most influence these phenomena. We will characterize all synthesized membranes by permeation testing, scanning electron microscopy, atomic force microscopy, X-ray diffraction, and attenuated total-reflectance Fourier transform infrared spectroscopy (ATR-FTIR). I plan to enroll in two semesters of materials characterization courses to assist in my understanding of these characterization methods. Thirdly, attention will be given to the corrosion resistance of the materials used in membrane synthesis. Particularly in space technology, long-term robustness of our Figure 3: Linde Type A zeolite, showing the 4.2 diameter water selective pore. These zeolites, and potentially the polymer-zeolite interface, will provide the pathway for water through the nanocomposite membrane. Figure 4: Capillary forces and impingement of casting solution droplets result in filling of support layer pores with polymer during the film-formation process. We intend to find a method of preventing infiltration of polymer by filling the pores with another liquid, coating the support layer surface, or heating the support layer. membranes will be extremely important in application. As urine and urine brine have a unique composition including such corrosive substances as chlorine and sulfuric acid, testing the effects of these solutions of our particular polymers and zeolites will be necessary. We will produce a synthetic urine solution and urine brine that may be used for soaking zeolites and commercial films of the proposed polymers over periods of time varying from one week to six months. The viability of these polymers will be determined by permeation testing, tensile testing, scanning electron microscopy and ATR-FTIR at regular intervals. Based on previous research into chemical resistance, we predict little to no degradation of the proposed materials[5], [6], [7]. The polymeric barrier thin-film portion provides flexibility and stability, and reduces the cost of production over a zeolite-only membrane. Incorporating water-selective zeolites eliminates the need for a polymeric material that is simultaneously selective only for water and corrosion resistant. As the zeolites do not need to be specifically oriented in the film, synthesis of the final membranes should be simpler. Based on previous experiments which showed improved water flux with zeolite addition while maintaining salt rejection in polyamidezeolite nanocomposite membranes[3], we predict production of a membrane that will yield high flux of water and very low permeability of all other undesired substances. Based on the zeolite pore size of 4.2 [4] (Figure 4) and urea molecular diameter of 5.28 [8], we may reasonably predict that the zeolites will effectively reject urea, which is of main concern with urine-containing wastewaters. With correct polymer selection, it is quite feasible that we may find a corrosion resistant and water impermeable polymer base and zeolites may be incorporated into it. Overall, we expect the final membrane to be far more effective than pre-existing RO membranes. We additionally propose a forward osmosis system that would allow direct production of a potable beverage through a lower-energy process. This process is presented in Figure 5. As forward osmosis depends upon chemical potential, external pressures would not be required for filtration, allowing for a simpler, more cost-effective process. An edible draw solution will produce the fortified potable beverage, which may be directly ingested, or may be further purified for use in other applications. The removed draw solution may be recycled in the Figure 5: The proposed forward osmosis system for production of a fortified potable beverage directly from waste water feed. This design negates the need for applied pressure, reducing the energy requirements of the process. With addition of a unit process, purified water may also be recovered . case of purification. This process provides the added benefit of a nutritive product without the need for energy input. To increase my understanding of my research topics, I plan to enroll in elective courses on inorganic membranes and films, characteristics of nanostructured materials, membrane separations, and polymer synthesis, which constitute all available chemical engineering courses relevant to my research. I will be taking the maximum allowable 12 credits per semester to expedite the completion of my coursework, and gain as much relevant information as possible early in my graduate career to assist in completion of my research. Relevance to Space Technology and Broader Impacts: Due to the unique composite design of our proposed membranes, the potential applications are numerous. Our results would directly impact recycling by NASA on the international space station, but may also be applied to recycling of impaired waters (e.g. wastewater, sea-water, brackish water, and agricultural drainage), with potential for application in locations where such technologies are needed the most. With minimal alteration, this membrane design may also be used in processes such as biofuels separations and seawater desalination. In a growing society, water and energy are of primary concern; membranes that may help to provide solutions for both of these issues have the potential to be an extremely valuable asset. We will disseminate our results in the form of both publications and conference presentations. During the course of this research, I will be working with undergraduate students through the Fulton Undergraduate Research Initiative, which provides students with ten hours per week of lab work. Our lab has also accepted local high school students who are currently assisting in testing the corrosion resistance of the selected polymers and with tensile and permeation testing, and FTIR analysis. This is already providing students with hands-on experience in lab work and materials characterization that would otherwise be unavailable at the high-school level.
StatusFinished
Effective start/end date8/20/139/30/15

Funding

  • NASA: Goddard Space Flight Center: $208,618.00

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