Water Permselectivity Research Articles

Under-oil superhydrophilic TiO2/poly(sodium vinylphosphonate) nanocomposite for the separation of water from oil

Under-water superoleophobic filters lose their water separation capability if they are contaminated with oil. In contrast, an oil contaminant in a filter that is both superoleophobic underwater and superhydrophilic underoil can be readily displaced with water. Even after contamination by oil, such a filter can self-clean in water and re-create and maintain an integral water film in the pores to enable sustained water separation. Unfortunately, there have been only a few reported examples of under-oil superhydrophilic materials. We report herein a novel under-oil superhydrophilic material consisting of TiO2 nanoparticles that are coated by an excess of a sodium salt of poly(vinylphosphonic acid) (PVPA.nNa). This nanocomposite can be applied as a coating onto various substrates such as glass plate, glass wool, cotton ball, and stainless-steel metal mesh for permanent self-cleaning and hexadecane collection. More interestingly, a coated cotton ball could be affixed to the bottom end of a standing tube and allows the selective permeation of water from a stirred water/hexadecane mixture. Unfortunately, a coated metal mesh with larger pores and thus much higher water flux could not achieve the separation most likely because the weakness of the water film in the mesh pores induces a negative value of Laplace pressure. We achieve this separation for the coated metal mesh by attaching a thin electrospun superhydrophobic mat onto its lower side to thus yield a Janus filter. Our assumption is that the unidirectional upward water-pulling force of the Janus filter in this case helps to stabilize the water film in the metal mesh. Due to the facile synthesis of TiO2-PVPA·nNa and the demonstrated applications of the TiO2-PVPA·nNa coating, this study should inspire further research in high-flux filters for water separation from oil.

Selective Permeation of Water through Angstrom-Channel Graphene Membranes for Bioethanol Concentration.

Graphene-based laminate membranes have been theoretically predicted to selectively transport ethanol from ethanol-water solution while blocking water. Here, robust angstrom-channel graphene membranes (ACGMs) fabricated by intercalating carbon sheets derived from chitosan into thermally reduced graphene oxide (GO) sheets are reported. ACGMs with robust and continuous slit-shaped pores (an average pore size of 3.9 Å) are investigated for the dehydration of ethanol. Surprisingly, only water permeates through ACGMs in the presence of aqueous ethanol solution. For the water-ethanol mixture containing 90wt% ethanol, water can selectively permeate through ACGMs with a water flux of 63.8±3.2kg m-2 h-1 at 20°C and 389.1±19.4kg m-2 h-1 at 60°C, which are over two orders of magnitude higher than those of conventional pervaporation membranes. This means that ACGMs can effectively operate at room temperature. Moreover, the ethanol can be fast concentrated to high purity (up to 99.9wt%). Therefore, ACGMs are very promising for production of bioethanol with high efficiency, thus improving its process sustainability.

Selective Vapor Permeation Behavior of Crosslinked PAMPS Membranes.

The effect of crosslinking on vapor permeation behavior of polyelectrolyte membranes was studied. Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) membranes were crosslinked by using crosslinkers with different lengths between the reactive ends. Crosslinked membranes with a longer crosslinking length showed lower water vapor permeability due to the lower sorption coefficient. It was also shown that the permeation behavior of PAMPS membranes was more affected by sorption than diffusion. For chemical protection applications, the ratio of water over chemical warfare agent permeability (i.e., selectivity) was measured. Due to the high water solubility of polyelectrolytes, crosslinked PAMPS allowed for the selective permeation of water over harmful chemical vapor, showing a selectivity of 20. The addition of electrospun nylon nanofibers in the membranes significantly improved the selectivity to 80, since the embedded nanofibers effectively reduced both diffusion and sorption coefficients of chemical warfare agents.

Screen printing directed synthesis of covalent organic framework membranes with water sieving property

Screen printing is introduced to direct the synthesis of crack-free and thickness-tunable TpPa(OH)2 covalent organic framework membranes. A smooth precursor layer is firstly screen printed and then fully crystallised into the TpPa(OH)2 membrane. Molecular-scale pores endow the membrane with a fast water-sieving property, which is promising in water desalination.

Artificial water channels enable fast and selective water permeation through water-wire networks.

Artificial water channels are synthetic molecules that aim to mimic the structural and functional features of biological water channels (aquaporins). Here we report on a cluster-forming organic nanoarchitecture, peptide-appended hybrid[4]arene (PAH[4]), as a new class of artificial water channels. Fluorescence experiments and simulations demonstrated that PAH[4]s can form, through lateral diffusion, clusters in lipid membranes that provide synergistic membrane-spanning paths for a rapid and selective water permeation through water-wire networks. Quantitative transport studies revealed that PAH[4]s can transport >109 water molecules per second per molecule, which is comparable to aquaporin water channels. The performance of these channels exceeds the upper bound limit of current desalination membranes by a factor of ~104, as illustrated by the water/NaCl permeability-selectivity trade-off curve. PAH[4]'s unique properties of a high water/solute permselectivity via cooperative water-wire formation could usher in an alternative design paradigm for permeable membrane materials in separations, energy production and barrier applications.

Organotemplate-free synthesis of ZSM-5 membrane for pervaporation dehydration of isopropanol

ZSM-5 membrane was prepared on tubular macroporous alpha-alumina support using a different synthesis route. The effects of organic template agent and Si/Al ratio of the synthesis gel on morphology, structure, and separation performance of the ZSM-5 membrane used for dehydration of isopropanol were investigated. High water perm-selectivity ZSM-5 membrane with a thickness of about 3.0 um and a low Si/Al ratio of 10.1 was successfully prepared from organotemplate-free synthesis gel with a molar composition of SiO2 : 0.050Al2O3 : 0.21Na2O : NaF : 51.6H2O at 175 degree Celcius for 24 h. The ZSM-5 membrane exhibited high pervaporation performance with a flux of 3.92 kg/(m2.h) and corresponding separation factor of higher than 10,000 for dehydration of 90 wt.% isopropanol/water mixture at 75 degree Celcius.

Unidirectional liquid transportation and selective permeation for oil/water separation on a gradient nanowire structured surface

Liquid transport and permeation on solid surfaces with gradient wettability had attracted tremendous research attention to solve some important problems of interface science and technology. On account of the existing problem of film pore blocking during the oily water separation process, however, it still remains a challenge to achieve rapid, continuous, high flux liquid mixture separation, e.g., oil/water separation, on a micro-structured surface. Here we demonstrate a strategy to achieve unidirectional motion and selective permeation of underwater oil droplet and water on the Cu(OH)2 nanowires structured copper foil/mesh surface with a density and length gradient. The gradient Cu(OH)2 nanowires fabricated by anodic oxidation method with anodization time gradient, shows wettability gradient from hydrophilic/oleophobic to superhydrophilic/superoleophobic and adhesion force gradient to underwater oil from high to low. As a result, the underwater oil droplet can only move unidirectionally from the superhydrophilic/superoleophobic scope to the hydrophilic/oleophobic area on the nanowires structured copper foil/mesh surface, meanwhile, water can only permeate on the superhydrophilic area, but not permeate on the hydrophobic area of the nanowires structured copper mesh. Therefore, a new way was presented to provide more contact area of oil/water mixture, owing to the unidirectional motion of underwater oil droplets and selective permeation of water, for rapid, continuous, high flux oil/water mixture separation with different interfacial tension in engineering field, which would also be promising to develop smart interface materials for microfluidic devices.

Ultrapermeable graphene oxide membranes with tunable interlayer distancesviavein-like supramolecular dendrimers

Graphene oxide membranes with tunable interlayer distances by using vein-like supramolecular dendrimers as crosslinkers for ultrafast and selective water permeation.

Molecular Dynamics Simulation Study of Polyamide Membrane Structures and RO/FO Water Permeation Properties.

Polyamide (PA) membranes possess properties that allow for selective water permeation and salt rejection, and these are widely used for reverse osmotic (RO) desalination of sea water to produce drinking water. In order to design high-performance RO membranes with high levels of water permeability and salt rejection, an understanding of microscopic PA membrane structures is indispensable, and this includes water transport and ion rejection mechanisms on a molecular scale. In this study, two types of virtual PA membranes with different structures and densities were constructed on a computer, and water molecular transport properties through PA membranes were examined on a molecular level via direct reverse/forward osmosis (RO/FO) filtration molecular dynamics (MD) simulations. A quasi-non-equilibrium MD simulation technique that uses applied (RO mode) or osmotic (FO mode) pressure differences of several MPa was conducted to estimate water permeability through PA membranes. A simple NVT (Number, Volume, and Temperature constant ensemble)-RO MD simulation method was presented and verified. The simulations of RO and FO water permeability for a dense PA membrane model without a support layer agreed with the experimental value in the RO mode. This PA membrane completely rejected Na+ and Cl− ions during a simulation time of several nano-seconds. The naturally dense PA structure showed excellent ion rejection. The effect that the void size of PA structure exerted on water permeability was also examined.

Novel Membrane Material Based on Polybutadiene and Polydimethylsiloxane for Gas Separation and Hydrophobic Pervaporation

A method is proposed for the synthesis of new membrane materials based on polydimethylsiloxane (PDMS) and polybutadiene (PB). It has been shown that all components of the mixture completely enter into the hydrosilylation reaction and form a chemically crosslinked composite material. It has been found that in the region of low PB concentrations, the composite has a less crosslinked and dense structure. The gas transport properties of the synthesized materials have been experimentally investigated. The proposed materials have higher selectivity for organic components than the industrially used membrane polymer PDMS, which is determined by their high sorption selectivity. A membrane containing 17 wt % PB has enhanced fluxes of organic components and an increased separation factor for all alcohols (ethanol, n-propanol, and n-butanol) examined relative to those of the PDMS membrane in the pervaporative separation of water–alcohol solutions. At the same time, the ethanol–water permselectivity of such a membrane is greater than 1, a value that has been first achieved by modifying PDMS with polymers. Materials of this type have a great potential for the creation of membranes with high permeability and selectivity in the recovery of volatile organic compounds from aqueous media.

Zwitterion-Functionalized Graphene Oxide Incorporated Polyamide Membranes with Improved Antifouling Properties.

In this study, zwitterionic polymer poly(sulfobetaine methacrylate) (PSBMA) functionalized graphene oxide (GO) nanocomposites (GO-PSBMA) were synthesized and incorporated into the active layer of a polyamide membrane to improve its water perm-selectivity and fouling-resistant properties. GO-PSBMA nanocomposite contained covalently tethered PSBMA brushes on GO sheets, which were grown by activators regenerated by the electron transfer-atom transfer radical polymerization technique via the "graft-from" strategy. The grafting of zwitterionic PSBMA partially neutralized the surface charge of GO and increased its dispersibility in organic solvent. The incorporation of the GO-PSBMA-1h nanocomposite in the active layer of the polyamide membrane significantly improved surface hydrophilicity of the membrane and reduced its charge density. A near twofold increase in water permeation flux, with the nonsignificant change in MgSO4 and NaCl rejection, was achieved after the incorporation of 0.3 wt % of GO-PSBMA-1h in the membrane casting solution. With an improved water affinity, the fabricated nanocomposite membrane exhibited a near 80% reduction in bacterial ( Escherichia coli) attachment in comparison to the control membrane, even after 48 h of culture. In a crossflow filtration test, the nanocomposite membrane exhibited less of a reduction in the flux associated with bovine serum albumin fouling and salt ion scaling. The results demonstrated that incorporating zwitterionic polymer-decorated GO in the polyamide skin layer is a promising method to fabricate thin film nanocomposite membranes with improved water flux and fouling resistance.

High-Performance Thin-Film Composite Membrane with an Ultrathin Spray-Coated Carbon Nanotube Interlayer

We fabricated a thin-film composite (TFC) forward osmosis (FO) membrane with an ultrathin spray-coated carbon nanotube (CNT) interlayer. The impact of the CNT interlayer on the polyamide (PA) layer structural properties and transport behavior in FO were investigated. Results indicate that the CNT interlayer provides an interface which enables the formation of a highly permeable and selective PA layer with a large effective surface area for water transport, while inhibiting the formation of a flower-like PA structure inside the substrate pores. The TFC-FO membrane with the CNT interlayer exhibited a much greater water flux than previously reported for FO membranes, while maintaining comparable salt rejection. Specifically, a membrane perm-selectivity or ratio of water (A) to salt permeability coefficients (B) (A/B value) of 39 bar–1 was achieved for the TFC-PA-CNT membrane. Implications of the results for the fabrication of high-performance TFC-FO membranes are further discussed.

Incorporating Graphene Oxide into Alginate Polymer with a Cationic Intermediate To Strengthen Membrane Dehydration Performance.

Two-dimensional graphene oxide (GO) in hybrid membranes provides fast water transfer across its surface due to the abundant oxygenated functional groups to afford water sorption and the hydrophobic basal plane to create fast transporting pathways. To establish more compatible and efficient interactions for GO and sodium alginate (SA) polymer chains, cations sourced from lignin are employed to decorate GO (labeled as cation-functionalized GO (CG)) nanosheets via cation-π and π-π interactions, providing more interactive sites to confer synergetic benefits with polymer matrix. Cations from CG are also functional to partially interlock SA chains and intensify water diffusion. And with the aid of two-dimensional pathways of CG, fast selective water permeation can be realized through hybrid membranes with CG fillers. In dehydrating aqueous ethanol solution, the hybrid membrane exhibits considerable performance compared with bare SA polymer membrane (long-term stable permeation flux larger than 2500 g m-2 h-1 and water content larger than 99.7 wt %, with feed water content of 10 wt % under 70 °C). The effects of CG content in SA membrane were investigated, and the transport mechanism was correspondingly studied through varying operation conditions and membrane materials. In addition, such a membrane possesses long-term stability and almost unchanged high dehydration capability.

Functionally graded membranes from nanoporous covalent organic frameworks for highly selective water permeation

Functionally graded membranes with finely tuned COF gradients were fabricated for highly selective water permeation.

Enhanced hydrophilicity and water-permeating of functionalized graphene-oxide nanopores: Molecular dynamics simulations

In this work, molecular dynamics simulations were employed to study the permeation of ethanol-water mixtures through single-layer graphene oxide (GO) nanopores functionalized with COOH (carboxyl) and COO- (ionized carboxyl) groups. GO-COOH nanopore shows regular behavior with competitive permeation between the two species in the mixture. However, GO-COO- pore exhibits selective permeation of water, suggesting COO- functionalized GO sheets could provide the initial barrier to block ethanol permeation and enhance dehydration separation in GO-based membranes. Our simulation presents the underlying mechanism of the selective water transport is not determined by the pore-size sieving, but has been ascribed as the strong molecular affinity between water and GO-COO- surface. This interfacial interaction can induce the preferential interfacial adsorption and pore occupation for water, simultaneously impede the transport path of ethanol into the nanopore, and consequently give rise to the selective penetration of water in the mixture. The simulation results provide the direct theoretical evidence that ionization of carboxyl groups on GO sheets can be applied to improve the dehydration permeation of alcohol-water mixtures across GO-based separation membranes.

High efficient water/ethanol separation by a mixed matrix membrane incorporating MOF filler with high water adsorption capacity

In order to promote the selective water permeation through porous filler, hydrophilic MOF-801 crystals with superior water adsorption ability were incorporated into chitosan (CS) matrix to fabricate MOF-801/CS mixed matrix membranes (MMMs) for pervaporation dehydration of ethanol. Both the experimental and molecular simulation results confirm the selective adsorption of water molecules in MOF-801 crystals, while the free volume and the lowest energy sorption sites analyses demonstrate the subdued diffusion of ethanol molecules through MOF-801. As a result, the porous structure in MOF-801 provides additional transport pathways for water molecules, and meanwhile makes the transport pathways of ethanol molecules more tortuous, thus achieving simultaneously enhanced flux and separation factor. The optimized membrane with MOF-801 loading of 4.8wt% exhibits the total flux of 1937g/m2h and separation factor of 2156.

Potential of Pervaporation and Vapor Separation with Water Selective Membranes for an Optimized Production of Biofuels—A Review

The development of processes based on the integration of new technologies is of growing interest to industrial catalysis. Recently, significant efforts have been focused on the design of catalytic membrane reactors to improve process performance. In particular, the use of membranes, that allow a selective permeation of water from the reaction mixture, positively affects the reaction evolution by improving conversion for all reactions thermodynamically or kinetically limited by the presence of water. In this paper, how pervaporation (PV) and vapor permeation (VP) technologies can improve the catalytic performance of reactions of industrial interest is considered. Specifically, technological approaches proposed in the literature are discussed with the aim of highlighting advantages and problems encountered in order to address research towards the optimization of membrane reactor configurations for liquid biofuel production in large scale.

High‐Efficiency Water‐Transport Channels using the Synergistic Effect of a Hydrophilic Polymer and Graphene Oxide Laminates

Graphene oxide (GO) laminates possess unprecedented fast water‐transport channels. However, how to fully utilize these unique channels in order to maximize the separation properties of GO laminates remains a challenge. Here, a bio‐inspired membrane that couples an ultrathin surface water‐capturing polymeric layer (<10 nm) and GO laminates is designed. The proposed synergistic effect of highly enhanced water sorption from the polymeric layer and molecular channels from the GO laminates realizes fast and selective water transport through the integrated membrane. The prepared membrane exhibits highly selective water permeation with an excellent water flux of over 10 000 g m−2 h−1, which exceeds the performance upper bound of state‐of‐the‐art membranes for butanol dehydration. This bio‐inspired strategy demonstrated here opens the door to explore fast and selective channels derived from 2D or 3D materials for highly efficient molecular separation.

Preparation and Characterization of Water Perm-selectivity ZSM-5 Zeolite Membrane Using Fluoride Route

Water perm-selectivity ZSM-5 zeolite membranes were prepared from fluoride media without organic template on the low cost macroporous α-Al2O3 tubular support by secondary growth. A varying temperature hot dip-coating method with different size of crystal was used to form a flat and continuous perfect seed layer. The effects of concentration of the small seed solution on the surface of seed layer, the morphology and pervaporation property of the ZSM-5 membrane were investigated. The results show that when the concentration of the small seed solution is 0.2wt%, the prepared ZSM-5 zeolite membrane displays high water perm-selectivity properties in the pervaporation dehydration of 10wt% H2O/IPA and 10wt% H2O/Ac OH mixtures at 75℃, which fluxes are 3.64 kg/(m2·h) and 0.61 kg/(m2·h), separation factor up to 3204 and 1321, respectively.

Rapid Preparation of Acid-Stable and High Dehydration Performance Mordenite Membranes

Highly intergrown mordenite membranes were rapidly prepared on seeded aluminum supports by microwave-assisted synthesis. Both the thicknesses of the zeolite layer and the sizes of the zeolite crystals of the mordenite membranes were highly dependent on the synthesis time. With a SiO2:0.08Al2O3:0.2Na2O:0.1NaF:35H2O precursor synthesis gel, a compact and 0.75 μm thick mordenite crystal layer was successfully prepared on the aluminum support at 170 °C for 3 h. The as-synthesized mordenite membrane exhibited excellent water perm-selectivity and long-term stability for dehydration of alcohol/water, acetic acid/water, and acetic acid/ethanol/water/ethyl ester mixtures by pervaporation. For dehydration of 90 wt % acetic acid/water solution, the corresponding flux and separation factor (water over acetic acid) of the membrane remained at 0.44 kg·m–2·h–1 and 2300, respectively, at 75 °C even after 59 days of immersion.