Water Permeation Rate Research Articles

Water and ion sorption, diffusion, and transport in graphene oxide membranes revisited

Graphene oxide (GO) membranes show promising separation performance, including high water permeation rate and rejection of ions or molecules larger than the interlayer distance of adjacent GO sheets. However, the fundamental water and small ion transport properties of GO membranes are still debated due to the heterogeneity of the physical and chemical structures of GO sheets. Here, we report the sorption, diffusion, and permeation properties of water and ions in GO membranes. The water permeability coefficient of GO membranes is in the range 3.0 × 10−7–1.0 × 10−6cm2/s and strongly depends on the flake size, the oxidation level, and the drying process. This value is not as high as reported in the literature and is similar to that of polymeric materials used for desalination, such as polyamide. Unexpectedly, a high water partition coefficient (0.93) was observed in GO membranes compared to those of other hydrophilic polymers. This contributes to the water permeability based on a solution-diffusion mechanism. Consequently, the water diffusivity of GO membranes is lower than that of hydrophilic polymers with similar water permeability due to the high tortuosity caused by its lamellar structure. In this study, we showed that GO membranes exhibited unique ion selective sorption and permeation behaviors compared to conventional polymeric membrane materials. The physical and chemical modifications of GO membranes strongly affect the transport properties of water and ions, indicating the possibility of the further improvement in the separation performances of GO-based membrane materials for liquid separation.

Dipeptides Embedded in a Lipid Bilayer Membrane as Synthetic Water Channels.

Water channels are essential to life sciences and many biological processes. We report a molecular simulation study on dipeptides embedded in a lipid (dipalmitoylphosphatidylcholine) membrane as synthetic water channels. Five dipeptides are examined including FF, FL, LF, and LL (with hydrophilic channels) and AV (with hydrophobic channel). It is found that AV is unstable in the lipid membrane due to incompatible interaction between the hydrophilic external surface of AV and the hydrophobic lipid tails; whereas FF, FL, LF, and LL with hydrophobic external surface exhibit good stability. In the four hydrophilic channels FF, FL, LF, and LL, water chains are formed; the number of chains ranges from multiple, two to one depending on channel diameter; moreover, water undergoes single-file diffusion and the mobility is enhanced with increasing channel diameter. The permeation rate of water in the FF channel is 9.20/ns, about three times that in aquaporin; however, the rate in FL, LF, and LL is much slower. Intriguingly, the rate can be tuned by a lateral stress/strain on the lipid membrane. The simulation study provides fundamental understanding on the stability of dipeptide channels embedded in a lipid membrane, quantitatively characterizing water structure, dynamics, and permeation in the channels. These microscopic insights are useful for the development of new water channels.

Amphoteric Ion-Exchange Membranes with Significantly Improved Vanadium Barrier Properties for All-Vanadium Redox Flow Batteries.

All-vanadium redox flow batteries (VRBs) have attracted considerable interest as promising energy-storage devices that can allow the efficient utilization of renewable energy sources. The membrane, which separates the porous electrodes in a redox flow cell, is one of the key components in VRBs. High rates of crossover of vanadium ions and water through the membrane impair the efficiency and capacity of a VRB. Thus, membranes with low permeation rate of vanadium species and water are required, also characterized by low resistance and stability in the VRB environment. Here, we present a new design concept for amphoteric ion-exchange membranes, based on radiation-induced grafting of vinylpyridine into an ethylene tetrafluoroethylene base film and a two-step functionalization to introduce cationic and anionic exchange sites, respectively. During long-term cycling, redox flow cells containing these membranes showed higher efficiency, less pronounced electrolyte imbalance, and significantly reduced capacity decay compared to the cells with the benchmark material Nafion 117.

Reverse osmosis desalination modules containing corrugated membranes – Computational study

Computational fluid dynamics simulation was carried out for three-dimensional desalination modules containing triangular and square ribs attached to the membrane surface. The solution-diffusion membrane transport model was applied along the surface of the corrugated membrane. The membrane flux model which couples the water permeation rate with local salt concentration impose a selective removal of components in the feed channel. The local water flux, salt concentration, wall shear stress, and Sherwood number were monitored over the surface of membranes to determine effects of eddy promoter corrugations on mitigation of concentration polarization and the total water flux. Simulations are conducted using a laminar model for Reynolds number of 100 and k-ω SST turbulence model for Reynolds number of 400 and 1000. Mathematical model and numerical methods employed are validated by comparing predictions against measurements reported earlier. Predicted results agree quantitatively and qualitatively with previous experimental results for membranes with semi-circular cross-sectioned ribs. The results show that corrugated membranes especially triangular chevrons enhance membrane performance profoundly at all flow rates. Water permeation rate is increased, concentration polarization is alleviated, and the potential fouling in the module is reduced by introducing corrugated membranes.

Molecular dynamics simulations of water flow enhancement in carbon nanochannels

Fluidic flow in carbon nanochannels or nanopores has shown great application prospects in nanofiltration. Therefore, enhancing water flux while maintaining their filtration property is crucial to further extend their applications. In this work, inspired by the hourglass-shaped aquaporin water channel, we proposed a method to optimize water flux in carbon nanochannels using conical carbon nanochannels. Adopting nonequilibrium molecular dynamics simulations, water flow in a series of conical carbon nanochannels was simulated. The results showed that the conical channel with apex angle of 19.2° (38.9°) had the optimum water flux when water flowed from the base (tip) side to the tip (base) side of the conical channel, and the water flux was nearly twice as the recently developed MoS2 desalination membrane. Then, the physical mechanism for the conical channel optimizing water permeation was revealed through detailed analyses of potential of mean forces, average number of hydrogen bonds and pressure distributions of the simulation systems. Finally, the microscopic water structures in these channels were also analyzed to further rationalize the optimizing mechanism. This work indicates that other than decreasing the membrane thickness, regulating the channel configuration is a more effective method to enhance water permeation rate in channels with limited channel sizes.

Carbon-based building blocks for alcohol dehydration membranes with disorder-enhanced water permeability

Graphene oxide (GO) thin films have demonstrated outstanding water permeability and excellent selectivity towards organic molecules and inorganic salts, unlocking a new exciting direction in the development of nanofiltration, desalination and pervaporation membranes. However, there are still high concerns about their stability at basic pH and under cross-flow conditions. The stabilization of graphene oxide can be achieved by thermal or chemical reduction; but stacked layers of reduced GO tend to form ordered and compact graphite-like structures, thus preventing their application as molecular separation membranes. In this work, a humic acid-like biopolymer (HAL), extracted from organic compost with a yield of ∼20%, was used to fabricate composite GO-HAL membranes. The HAL brings a high degree of disorder to the membrane structure, with the benefit of an increased water permeation rate. Upon thermal stabilization, the membrane with a biopolymer loading of 30% presented an ideal water/ethanol selectivity of 45 and a water permeance that is 33% higher than the pristine graphene oxide membrane. The enhanced water permeability along with the good water/ethanol selectivity makes the GO-HAL membranes promising devices for alcohol dehydration technologies.

Preparation and characterisation of polyethersulfone/hydrous ferric oxide mixed matrix membranes with improved hydrophilicity for treatment of oily waste water

The rapid growth in oil and gas industry has led to the large production of oily wastewater. The massive amount of oily wastewater derived from the industry has raised concerns in community especially its adverse impact to the environment. Membrane technology has been in the spotlight in recent advancement to treat the oily wastewater. The major obstacle regarding the membrane technology is fouling due to surfactant adsorption and/or oil droplets plugging the pore, which would lead to a severe decline of the flux and rejection rate. HFO nanoparticles are incorporated into the PES membrane matrix with the aim to improve the hydrophilicity, water permeability as well as the antifouling properties of the membrane. HFO is abundant and easily obtained making it the perfect candidate in developing economical and energy saving membrane operation. Hydrous ferric dioxide (HFO) nanoparticles were synthesised via chemical precipitation method and incorporated in polyethersulfone (PES) to fabricate nanocomposite mixed matrix membranes (MMMs) for ultrafiltration (UF). The resulting membranes were characterised by SEM, FTIR, contact angle goniometer, before further subjected to water permeation test. It was found that contact angle of membrane decreased remarkably with an increase in HMO nanoparticle loading (state the value/ percentage decrement). The pore size at the skin layer however decreased as observed by SEM. As for the UF experiments, pure water permeation rate increased remarkably with increasing nanoparticle loading.

A sustainable application of recycled tyre crumbs as insulator in lightweight cellular concrete

End-of-life waste tyres are known to have negative economic and environmental impacts due to the difficulty associated with their disposal and recycling. The use of recycled tyre crumb (RTC) as a component of construction materials has emerged as a potentially sustainable solution to this environmental issue. There exists extensive research covering the use of RTC as a filler in conventional concrete. However, there are very limited studies focused on insulation properties of RTC in lightweight cellular concrete (LCC) as a sustainable application that can add value to the final product. This study investigates the effects of different RTC contents on compressive strength, porosity, thermal conductivity, sound insulation and water permeability of LCC as an insulator. A rather homogeneous distribution of tyre crumbs within LCC structure was achieved. It has been found that LCC samples containing RTC are suitable as insulators because the sound and thermal insulation are improved as rubber content is increased compared to the sample with similar density but without RTC. The insulation properties of this product also come with the same total porosity and a significant reduction in the rate of water permeability compared to the sample without RTC. 3D restructuring of micro-CT scan images of this composite revealed the effects of tyre crumb on the total and local porosity of this composite. Despite the fact that addition of tyre crumb reduced the strength, surface treatment of crumb rubber with sodium hydroxide solution exhibited significant improvement in the compressive strength of LCC as the result of better physical bonding with hydrated cement proven by scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) analysis.

Progress in Understanding Degradation Mechanisms and Improving Stability in Organic Photovoltaics.

Understanding the degradation mechanisms of organic photovoltaics is particularly important, as they tend to degrade faster than their inorganic counterparts, such as silicon and cadmium telluride. An overview is provided here of the main degradation mechanisms that researchers have identified so far that cause extrinsic degradation from oxygen and water, intrinsic degradation in the dark, and photo-induced burn-in. In addition, it provides methods for researchers to identify these mechanisms in new materials and device structures to screen them more quickly for promising long-term performance. These general strategies will likely be helpful in other photovoltaic technologies that suffer from insufficient stability, such as perovskite solar cells. Finally, the most promising lifetime results are highlighted and recommendations to improve long-term performance are made. To prevent degradation from oxygen and water for sufficiently long time periods, OPVs will likely need to be encapsulated by barrier materials with lower permeation rates of oxygen and water than typical flexible substrate materials. To improve stability at operating temperatures, materials will likely require glass transition temperatures above 100 °C. Methods to prevent photo-induced burn-in are least understood, but recent research indicates that using pure materials with dense and ordered film morphologies can reduce the burn-in effect.

Numerical analysis of hollow fiber membranes for desalination

Water desalination by reverse osmosis hollow fiber membrane has been widely used to produce fresh water. This work numerically characterizes flux performance of the membrane, concentration polarization and potential fouling sites in the reverse osmosis desalination module containing hollow fiber membranes arranged in an inline and staggered configuration. Steady k-ω SST turbulence model is utilized to study membrane performance. An accurate membrane flux model, the solution-diffusion model, is employed. Hollow fiber membrane surface is treated as a functional boundary where the rate of water permeation is coupled with local concentration along the membrane surface. The rate of water permeation increases and concentration polarization decreases as the feed flow rate is increased. Hollow fiber membranes in the staggered geometry perform better than those in the inline geometry. It is proven by the present study that desalination modules containing hollow fiber membranes should be designed and optimized by careful consideration of their configurations. It is demonstrated here that flows in the hollow fiber bank becomes strongly time dependent at high flow rates and that transient effects could profoundly influence hollow fiber membrane flux performance and characterization of concentration polarization.

Fabrication and improved performance of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) for packaging by addition of high molecular weight natural rubber

ABSTRACTThe packaging industry is searching for alternative materials to attain environmental sustainability. Poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate (PHBV) is a semicrystalline polymer that meets this sustainability goal since it is bioderived and biodegradable. However, its brittle nature and relatively high water permeation and transmission rates make it unsuitable for packaging applications. In addition, PHBV has poor mechanical, thermal, and rheological properties above 160 °C, limiting its use in cast sheets and thermo‐formed packaging applications. To improve these properties, new blends of PHBV with high molecular weight natural rubber at 5, 10, 15, and 25% by weight were fabricated, and physico‐chemical properties of the blends were characterized. The rubber in the blends aided in the following: increased thermal stability since the complex viscosities of the blends were improved by one log over pure PHBV at 170 °C, created more uniform melting peaks attesting to improved homogeneity, decreased water permeation to a level similar to that of traditional thermoplastics; increased the elongation at break, and stabilized the Young's modulus. Therefore, these blends can potentially be used in‐place of traditional, petroleum‐based thermoplastics in cast sheets and thermoforms. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43937.

Effect of titania nanotubes on the flux and separation performance of polyethersulfone membranes

The improvement of membrane performances in terms of separation and permeation is a constant research problem. In this study, polyethersulfone (PES) hollow fiber nanocomposite membranes were fabricated with titania nanotubes (TNT) synthesized via hydrothermal method used as inorganic filler to improve separation and membrane permeation. The membranes were prepared using dry/wet phase inversion process. The concentration of PES was fixed at 18% and TNT was added at concentration of 0.1% wt. The membranes were characterized in terms of pure water permeation rate (PWP), proteins (BSA) rejection, porosity and water contact angle. The results showed that addition of TNT improves the water flux and rejection rate as well as increasing hydrophilicity of the membrane. The flux of membrane is improved more than 20% while the rejection has been improved from 79% to 96%. The addition of TNT into the membrane matrix has shown that photocatalytic property can be introduced to a normal membrane.

Comparison of polyelectrolyte coated PVDF membranes in thermopervaporation with porous hydrophobic membranes in membrane distillation using plate-and-frame modules

In thermopervaporation the same economically favorable driving force as in membrane distillation, i.e., a temperature difference between feed and permeate for the transport, is used but with non-porous thin-film composite membranes. Membrane pores cannot be wetted and long-term operational stability can be achieved with the appropriate coating layer, but normally with a decrease of the flux compared to membrane distillation with porous hydrophobic membranes.Porous asymmetric PVDF membranes were made to achieve low permeation resistance and pores which could be overcoated with polyelectrolyte polymers. This coating prohibits pore wetting and strongly reduces adsorption of organic substances.Those membranes showed a high permeation rate for water due to a structure of phase-separated hydrophilic and hydrophobic three-dimensional domains. The permeation rates of these composite membranes for water is between 6 and 12l/(hm2) at a feed temperature of 60°C and permeate at a temperature of 40°C of a 2% saline solution feed depending on the operational parameters. This is only a slight reduction of 10–15% in permeation rate compared to membrane distillation with porous hydrophobic membranes.In whey dewatering experiment this membrane showed a constant performance over 4days in intermittent operation mode and stability in cleaning with strong alkaline solution.

Sorption of polar organic solvents and water by graphite oxide: Thermodynamic approach

Sorption of polar organic solvents CH3OH, C4H8O (THF), CH3CN, C3H7NO (DMF), C2H6OS (DMSO), C5H9NO (NMP) and water was quantitatively evaluated for Hummers (H-GO) and Brodie (B-GO) graphite oxides at T = 298K and at melting temperature (Tm) of the solvents. H-GO showed stronger sorption compared to B-GO for all studied solvents and the increase of sorption upon lowering temperature was observed for both H-GO and B-GO. Thermodynamic equations allowed to explain earlier reported “maximums” of swelling/sorption in the binary systems H-GO – solvent at Tm. The specific relation between the values of enthalpies of sorption and melting leads to the change of sign in enthalpies of sorption at Tm and causes maximal swelling/sorption. The same thermodynamic explanation was given for the “maximum” on the swelling vs. pressure dependence in B-GO and H-GO – H2O systems earlier reported at pressure of phase transition “liquid water-ice VI”. Notably higher sorption of H2O was observed for H-GO compared to H-GO membrane (H-GOm) at high relative humidity (RH), RH > 0.75. Experimental sorption isotherm of H-GOm was used to simulate permeation rates of water through H-GOm and to estimate effective diffusion coefficient of water through the membrane.

Multiscale Simulation Method for Flow and Mass-Transfer Characteristics in a Reverse Osmosis Membrane Module

This paper presents a numerical simulation method for calculating flow and mass-transfer characteristics in an entire membrane sheet module comprised of feed and permeate channels. The channels, including the spacers, were coarse-grained as the porous media for simulation of the entire membrane sheet. This is because the previous unit cell concept, which directly calculates the flow and solute concentration fields around the spacers, cannot be extended to the meter-sized computational domain owing to computational load limitations. We first carried out the unit cell simulation of the spacer-filled channel to obtain the flow and mass-transfer characteristics. The obtained flow and mass-transfer characteristics were then used for modeling porous media. The flow and solute mass fraction fields were then calculated for a membrane sheet, and the effect of the spacer arrangement on the membrane sheet performance was investigated. From the relationship between the pressure drop and water permeation rate, signifi...

Water Permeability across Symmetric and Asymmetric Droplet Interface Bilayers: Interaction of Cholesterol Sulfate with DPhPC.

Cellular membranes employ a variety of strategies for controlling the flow of small molecules into the cytoplasmic space, including incorporation of sterols for modulation of permeability and maintenance of lipid asymmetry to provide both sides of the membrane with differing biophysical properties. The specific case of cholesterol asymmetry, especially, is known to have profound effects in neurological cellular systems. Synthetic membrane models that can readily determine valuable physical parameters, such as water transport rates, for sterol-containing membranes of defined lipid composition remain in demand. We report the use of the droplet interface bilayer (DIB), composed of adherent aqueous droplets surrounded by a lipid monolayer and immersed in a hydrophobic medium, for measurement of water permeability across the membrane, with rapid visualization and ease of experimental setup. We studied droplet bilayer membranes composed of the prototypical synthetic membrane lipid (i.e., the archaeal lipid DPhPC) as well as of symmetric and asymmetric DIBs formed by DPhPC and sodium cholesterol sulfate (S-Chol). The presence of S-Chol in DPhPC in symmetric DIB reduced the passive water permeability rate (P(f)) at all concentrations and increased the activation energy (E(a)) to 17-18 kcal/mol. When only one side of the DIB contains S-Chol (asymmetric DIB), an E(a) of 14-15 kcal/mol was obtained, a value intermediate that of pure lipid and symmetrical DIB containing lipid and S-Chol. Our data are consistent with a capability for regulation of water transport by one leaflet independent of the other. The engineering of our various systems is believed to have implications for garnering detailed knowledge regarding the transport of small moieties across bilayers in a wide variety of lipid systems.

Preparation and characterization of graphene oxide/TiO2 blended PES nanofiltration membrane with improved antifouling and separation performance

Polyethersulfone (PES)-based mixed matrix nanofiltration membrane was developed by blending with partially reduced graphene oxide (rGO)/TiO2 nanocomposite. The effect of rGO/TiO2 contents on the morphology and performance of the prepared membranes was investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM) and water contact angle analysis. The blended membranes displayed improved water permeability and fouling resistance compared to the bare PES. When the content of rGO/TiO2 was 0.15wt.%, the water flux reached a maximum value (45.0kg/m2h) nearly twice of that of the bare PES membrane (23.1kg/m2h). Fouling resistance of the membranes studied by bovine serum albumin (BSA) solution filtration showed that 0.1wt.% rGO/TiO2 membrane had the best antifouling property. Nanofiltration performance of the prepared membranes was evaluated by rejection of three organic dyes with different molecular weights (i.e. C. I. Reactive Green 19, C. I. Direct Yellow 12 and C. I. Reactive Blue 21). The rGO/TiO2/PES membranes showed better dye removal performance than the bare PES. Compared with TiO2/PES and GO/PES membranes, rGO/TiO2/PES membranes presented the best water permeation rate, antifouling ability and dye rejection.

Effects of operating conditions and membrane structures on the performance of hollow fiber forward osmosis membranes in pressure assisted osmosis

Recently, forward osmosis (FO) has received much attention as an advanced water treatment technology. Although the FO process has begun to spread widely worldwide, it still has some problems – such as the low water permeation rate compared with those required for some applications – that must be solved for its commercial application. To achieve the higher water flux and lower reverse salt flux, pressure assisted osmosis (PAO), in which pressure is applied to a feed solution (FS), has recently been proposed. In this work, experiments were carried out to investigate membrane structures and operating conditions in the PAO process by using three types of cellulose triacetate (CTA) — hollow fiber (HF) membranes. The HF membranes are preferable in the FO process because a high packing density and a large specific surface area can be obtained in the HF module. In addition, the HF membrane module has four ports and does not require a spacer between the membranes. The effects of the applied pressure, the draw solution concentration, and the structure parameter of the HF membranes on the PAO performance were investigated. The water flux in the PAO process was theoretically analyzed. The calculated results satisfactorily agreed with experimental data.

Design and fabrication of zwitter-wettable nanostructured films.

Manipulating surface properties using chemistry and roughness has led to the development of advanced multifunctional surfaces. Here, in a nanostructured polymer film consisting of a hydrophilic reservoir of chitosan/carboxymethyl cellulose capped with various hydrophobic layers, we demonstrate the role of a third design factor, water permeation rate. We use this additional design criterion to produce antifogging coatings that readily absorb water vapor while simultaneously exhibiting hydrophobic character to liquid water. These zwitter-wettable films, produced via aqueous layer-by-layer assembly, consist of a nanoscale thin hydrophobic capping layer (chitosan/Nafion) that enables water vapor to diffuse rapidly into the underlying hydrophilic reservoir rather than nucleating drops of liquid water on the surface. We characterize these novel films using a quartz crystal microbalance with dissipation monitoring (QCM-D) and via depth-profiling X-ray photoelectron spectroscopy (XPS) in addition to extensive testing for fogging/antifogging performance.

Surface modification and performance enhancement of nano-hybrid f-MWCNT/PVP90/PES hemodialysis membranes

The use of nanoparticles for surface modification and performance enhancement of membranes is another latest trend in membrane technology. In this work acid functionalized multi wall carbon nanotubes (f-MWCNT) was initially mixed with polyvinylpyrrolidone (PVP) in dimethylformamide (DMF) to form nano-composites (NCs) and then blended with polyethersulfone (PES). The PES/PVP–f-MWCNT nano-hybrid hemodialysis membrane was formed via the phase inversion process. The membranes were characterized and their performances were then evaluated in terms of pure water permeation rates (PWP), urea, creatinine and lysozyme rejection. The results revealed that, compared to the pristine PES membrane, the PES nano-hybrid hemodialysis membranes were more hydrophilic; possess high PWP rate up to 72.20L·m-2·h-1, exhibited 58.82% reduced protein absorption, and better uremic waste clearance of 56.30%, 55.08% and 27.90% of urea, creatinine and lysozyme respectively. Thus the addition of NCs in the membranes indeed modified the surface and enhanced the performance of the PES membranes.