Asymmetric Membranes for Gas Separations

Water scarcity is one of the greatest societal challenges facing humanity. Reverse osmosis (RO) desalination, a widely used membrane-based technology, has proven to be effective to augment water supply in water-stressed regions of our planet. However, progress in the design and development of RO membranes has been limited. To significantly enhance the performance of RO membranes, it is essential to acquire a deep understanding of the membrane separation and transport mechanisms. In this tutorial review, we cover the pivotal historical developments in RO technology, examine the chemical and physical properties of RO membrane materials, and critically review the models and mechanisms proposed for water transport in RO membranes. Based on recent experimental and computational findings, we conduct a thorough analysis of the key transport models-the solution-diffusion and pore-flow models-to assess their validity for accurately describing water transport in RO membranes. Our analysis involves examining the experimental evidence in favor of the solution-diffusion mechanism. Specifically, we explain whether the water content gradient within the membrane, cited as evidence for the key assumption in the solution-diffusion model, can drive a diffusive transport through RO membranes. Additionally, we review the recent molecular dynamics simulations which support the pore-flow mechanism for describing water transport in RO membranes. We conclude by providing future research directions aimed at addressing key knowledge gaps in water transport phenomena in RO membranes, with the goal of advancing the development of next-generation RO membranes.

Polymeric reverse osmosis (RO) and forward osmosis (FO) membranes have been predominantly used in membrane applications for water desalination. The membrane science has advanced in the past decades and various efforts have been employed to improve the membrane characteristics for enhanced water flux and impurities rejection capability. In this chapter, the progress of RO and FO membranes has been discussed in three sections: synthesis methods of RO and FO membranes, modification works done on the membranes and the formulation used for the synthesis of RO and FO membranes, with particular interest given to the incorporation of nanoparticles in the synthesis of thin film composite membrane.

An atomistic simulation study is reported to investigate the capability of dipeptide crystals as reverse osmosis (RO) membranes for water desalination. Eight dipeptides are considered, namely, Ala-Val (AV), Val-Ala (VA), Ala-Ile (AI), Ile-Ala (IA), Val-Ile (VI), Ile-Val (IV), Val-Val (VV), and Leu-Ser (LS). It is revealed that water flux is governed by both pore size and helicity. With a relatively larger pore size, AV, AI, VV, and LS exhibit a higher water flux than VA, IA, VI, and IV. Despite similar pore size in AI and VA, a higher flux is observed in AI because of a lower helicity. On the other hand, VI, LS, IV, and IA possess higher salt rejection (>90%, and 100% for VI) than the rest (<70%). The salt rejection is determined by the electric potential difference across the membrane, induced by the staggered arrangement of −NH3+ and −COO– groups in the dipeptides. This unique arrangement of charge groups is not observed in other types of RO membranes. A higher electric potential difference allows more ...

Chapter 9 - Polymer-based reverse osmosis membranes

Tailoring the internal void structure of polyamide films to achieve highly permeable reverse osmosis membranes for water desalination

The recognition of membrane separations as a vital technology platform for enhancing the efficiency of separation processes has been steadily increasing. Concurrently, 3D printing has emerged as an innovative approach to fabricating reverse osmosis membranes for water desalination and treatment purposes. This method provides a high degree of control over membrane chemistry and structural properties. In particular, when compared to traditional manufacturing techniques, 3D printing holds the potential to expedite customization, a feat that is typically achieved through conventional manufacturing methods but often involves numerous processes and significant costs. This review aims to present the current advancements in membrane manufacturing technology specifically tailored for water desalination purposes, with a particular focus on the development of 3D-printed membranes. A comprehensive analysis of recent progress in 3D-printed membranes is provided. However, conducting experimental work to investigate various influential factors while ensuring consistent results poses a significant challenge. To address this, we explore how membrane manufacturing processes and performance can be effectively pre-designed and guided through the use of molecular dynamics simulations. Finally, this review outlines the challenges faced and presents future perspectives to shed light on research directions for optimizing membrane manufacturing processes and achieving optimal membrane performance.

Membrane Filtration technique is being accepted worldwide as an environment friendly and energy efficient technique in Desalination Industry as compared to Thermal Desalination techniques. However, the performance of membranes which include permeate flux and rejection is affected by the membrane fouling. The properties of membrane and surface features such as porous structure, hydrophilicity/hydrophobicity charge, polymer characteristics, surface roughness determine the fouling potential of the membrane. The hydrophilic and smooth membrane surface is usually considered desirable in tackling membrane fouling issues. Therefore, many studies have focused on to enhance surface characteristics of membranes by surface coating with polymers and nanomaterials. Since, membrane coating is not done during fabrication of the most commercially available membranes, therefore, it is also important to determine the surface features of the commercially available membranes to investigate their membrane fouling potential. Thus, the objectives of this study were (1) to perform membrane surface characterization of commercial Reverse Osmosis (RO) and Nanofiltration (NF) membranes using techniques such as SEM, AFM, FTIR and XPS; (2) to measure hydrophilicity/hydrophobicity of commercial RO and NF membranes through water contact angle measurement using sessile drop method and (3) to measure the flux and percentage rejection of NF and RO membranes using Dead end filtration technique. Here, the characterization of membrane surface in terms of surface roughness, using SEM and AFM, showed that the commercial RO membrane had more ridge and valley structures and higher average surface roughness i.e. 71.24 nm as compared to NF membranes (6.63 nm). In addition, water contact angle measurements showed that the NF membrane was more hydrophilic as compared to RO membrane. The average contact angle found for RO membrane was 59.94°. On the other hand, it was observed that NF membrane is extremely hydrophilic in nature. Due to which, contact angle value was not obtained for most of the runs. The droplet could diffuse in less than 5 seconds. In addition, the dead-end filtration experiments showed that the RO membrane had much lower flux as compared to NF membrane. This can be associated with the pore structure of these membranes. Since, the NF membrane has porous structure, in oppose to RO membrane, the flux of the NF membrane is usually higher than the RO membranes. As the membrane surface roughness and hydrophobicity makes it more susceptible to the fouling leading to reduction in membrane flux and performance, it can be concluded from this study that there is a need for surface coating of RO membrane with suitable nanomaterials such as graphene oxide to improve its hydrophilicity and surface smoothness. This will eventually make the membrane more resistant to membrane fouling and will establish the use of membrane filtration technique in desalination industry in Qatar in the future. Microorganisms have been isolated from Gulf sea water, identified and differentiated and are being used to study the biofouling of RO and NF membranes, that would be coated to limit the fouling problems. Acknowledgement: This research was made possible by NPRP grant # [9-318-1-064] from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the author[s].

Reverse osmosis (RO) desalination is one of the main technologies for producing fresh water from seawater and other saline water sources. The membrane properties greatly affect the water productivity and energy costs in the reverse osmosis desalinatin processes. Recent years have seen significant research efforts devoted to developing high-performance RO membranes. This article reviews recent activities in the development of RO membranes with improved flux and salt rejection, chlorine tolerance, fouling resistance and thermal stability. In particular, this review mainly focuses on the modification of current polymeric membrane materials, and synthesis and separation performance of new polymer membranes, inorganic membranes and mixed matrix membranes.

Water desalination via reverse osmosis (RO) to produce fresh water represents an ideal solution to address water shortage. Membranes of large water permeability and high salt rejection are desired, and these properties are subject to the design of the membrane structure. The structural tunability of metal-organic frameworks (MOFs) therefore provides tremendous opportunities, but their potential has not yet been systematically explored. In this study, molecular dynamics simulations are conducted to investigate MOFs with a focus on a subclass of water stable Zirconium-based MOFs as RO membranes in water desalination. The results show that MOF membranes can indeed achieve a perfect salt rejection while allowing notably high permeability as compared to commercial polymeric membranes. Moreover, the structure-performance relationship is explored, and the critical role of channel homogeneity is identified. Overall, the outcomes of this study demonstrate the great promise of MOFs and provide guidelines on the selection and design of MOFs for effective and efficient water desalination.

Organically bridged polysilsesquioxane (PSQ)‐based reverse osmosis (RO) membranes have been studied for water desalination. In this work, RO membranes were prepared from tris[3‐(triethoxysilyl)propyl]amine (TTESPA), tris[3‐(diethoxymethylsilyl)propyl]amine (TDEMSPA), tris[(triethoxysilyl)methyl]amine (TTESMA) tris[(triethoxysilyl)propenyl]amine (TTESP2A), and tris[3‐(triethoxysilyl)prop‐2‐ynyl]amine (TTESP3A) by the sol–gel process and interfacial polymerization. It was found that the TTESPA‐derived membranes gave the best RO performance among those prepared. The membrane prepared by the interfacial polymerization of TTESPA showed water permeance and salt rejection of 7.3 × 10−13 m3/m2sPa and 95.6% for 2000 ppm NaCl aqueous solution, respectively, which were higher than those previously reported for a membrane similarly prepared from bis[(triethoxysilyl)propyl]amine (1.4 × 10−13 m3/m2sPa and 93.6%).

Fabrication of polymeric nanocomposite forward osmosis membranes for water desalination—A review

Reverse osmosis membranes for water desalination based on cellulose acetate extracted from Egyptian rice straw

Membranes are widely used in industrial separation processes, particularly for gas separation and desalination processes. To develop membrane materials with improved permeability, selectivity can achieve more energy-efficient membrane separations and reduce costs. Since composite membranes offer improved performance, the aim of this research is to develop polymer-based composite membranes with improved performance for gas separation and water desalination applications. First, in order to obtain a composite membranes with high chlorine tolerance, a carbonaceous poly(furfuryl alcohol) (PFA) composite membrane was synthesized at a low temperature carbonation by formation and post-treatment of a thin PFA layer on porous polymer substrates. The carbonaceous PFA membrane exhibits high selectivity and excellent chemical stability in seawater desalination. The low-temperature carbonization method developed in this study is promising for developing a wide range of other carbonaceous polymer composite membranes for water desalination. Next, in order to apply PFA to other applications, understanding the effects of polymerization conditions on the properties of the PFA composite membrane is required. The PFA membrane was fully characterized in terms of microstructure and separation properties. Suitable synthesis conditions for the preparation of PFA composite membranes with smooth surfaces and uniform structure were (1) FA/ H2SO4 molar ratios: 74-300, (2) polymerization temperatures: 80-100°C and (3) solvents: ethanol and acetone. The preparation conditions were also optimized. The PFA composite membrane prepared with a FA/ H2SO4 molar ratio of 250, a polymerization temperature of 80°C and with ethanol as the solvent exhibited the highest H2/N2 ideal selectivity (αH2/N2=24.9), and a H2 permeability of 206 Barrers. This work led to a better understanding of the effect of the preparation procedures on the membrane performance. In order to investigate the effects of the incorporation of molecular sieve nanoparticles on the membrane structure and membrane performance, silicalite-poly(furfuryl alcohol) (PFA) mixed matrix composite membranes were successfully synthesized based on the best synthesis condition obtained previously. The silicalite-PFA mixed matrix composite membrane with 20% w/w silicalite loading had a high ideal selectivity (αo2/N2= 3.5 and αco2/N2= 5.4) and a good permeability (Po2= 821.2, Pco2= 1263.7, PN2= 233.3 Barrers) at room temperature. This membrane can be a good candidate for oxygen enrichment applications. Finally, in order to investigate the effects of the incorporation of silicalite nanocrystals on the desalination property of polyamide membranes, silicalite nanocrystals were also incorporated into polyamide matrix to synthesize silicalite-polyamide mixed matrix membranes. With an increase in the loading of silicalite nanocrystals, the water flux of silicalite-polyamide mixed matrix composite membranes increased whereas the salt selectivity significantly decreased. The silicalite-polyamide mixed matrix composite membrane prepared from TMC-hexane with 0.5% (w/v) silicalite had water flux of 2.7×10-6 m3/m2·s and NaCl rejection of 50% at a feed pressure of 34.5 bar which 2000 ppm salt solution was used as the feed. The silicalite-polyamide mixed matrix composite membrane is promising for developing high water flux composite membranes for water desalination. In this research, composite membranes with improved permeability, selectivity and chemical resistance were successfully synthesized for desalination and gas separation. For desalination, carbonaceous PFA composite membranes with high chlorine tolerance and silicalite-PA mixed matrix composite membranes with high salt rejection and water flux were successfully obtained. For gas separation, an optimized composite membranes PFA synthesis condition was found and silicalite-PFA mixed matrix composite membranes with high O2/N2 separation were successfully synthesized.

The development of carbon-based reverse osmosis membranes for water desalination is hindered by challenges in achieving a high pore density and controlling the pore size. We use molecular dynamics simulations to demonstrate that graphene foam membranes with a high pore density provide the possibility to tune the pore size by applying mechanical strain. As the pore size is found to be effectively reduced by a structural transformation under strain, graphene foam membranes are able to combine perfect salt rejection with unprecedented water permeability.

Mechanical properties of water desalination and wastewater treatment membranes