Enhanced Water Transport Research Articles

Elucidating the impact of polymer crosslinking and fixed carrier on enhanced water transport during desalination using pervaporation membranes

Crosslinking is a well-established technique to enhance chemical stability of polymer membranes but at the expense of lower flux. We previously demonstrated that this trade-off between chemical stability and separation performance can be overcome through the judicious selection of a crosslinking agent, 4-sulfophthalic acid (SPTA), which also functioned as a fixed carrier site. The mechanism underpinning this breakthrough remained unknown. Through molecular dynamic simulations and a series of complementary experiments, here we reveal that higher crosslinking degree can increase local water concentration close to the sulfonic acid groups, and enhance both long-distance water-sulfonic acid-interactions and water molecule mobility. These underpinned the significant increase in water flux of crosslinked pervaporation membranes during desalination. These results show that the dependence of water diffusivity on water concentration is reduced with higher crosslinking degree. More importantly, water diffusivity at the “dry membrane region” determined the overall water transport behaviour. These findings can potentially impact on the design of pervaporation membranes for production of potable water from brackish, sea and brine water.

Enhanced water transport in AEMs based on poly(styrene–ethylene–butylene–styrene) triblock copolymer for high fuel cell performance

Anion exchange membrane fuel cells (AEMFCs) have received a considerable amount of attention in the past decades as a lower cost alternative to proton exchange membrane fuel cells (PEMFCs).

Risk evaluation for the navigation environment of the LNG ship based on the cloud model

In order to enhance water transport safety of the LNG ship and improve the objectivity of risk assessment of LNG ships navigation environment, the cloud model which can realize the conversion between qualitative and quantitative evaluation indexes is used to evaluate the LNG ships navigation environment. Combined with the Delphi method, cloud weight and cloud model of indexes at all levels in the evaluation system are determined. At the same time, simulation is carried out by MATLAB software and cloud chart is obtained. And then we analyze the risk level of different factors in LNG ships navigation environment to get the evaluation results, which provides decision-making basis for navigation safety management. Overall, it is testified that the cloud model can reduce the randomness and fuzziness in the process of evaluating and the irrationality caused by the subjective assumption of weight distribution, which makes the results more reasonable and credible.

Along-the-Channel Impacts of Water Management and Carbon-Dioxide Contamination in Hydroxide-Exchange-Membrane Fuel Cells: A Modeling Study

Water management and carbon-dioxide contamination from ambient air remain key challenges for development and operation of high-performance hydroxide-exchange-membrane fuel cells (HEMFCs). In this work, a 2D computational model of an HEMFC cell, coupled with a 1D down-channel stepping algorithm, is used to explore water management and carbon-dioxide contamination issues along the channel. Variations in local current density along the channel due to changes in membrane hydration and a reduction in oxygen content of the cathode gas are quantified. Water transport from anode to cathode is critical for replenishing water consumed at the cathode, and the effects of varying flow rates and membrane transport properties on water management are explored. We then include carbon dioxide gas in the cathode and show that the formation of a pH gradient could explain the observed decrease in current density in HEMFCs exposed to CO2. Furthermore, the HEMFC acts as an electrochemical CO2 pump, causing an increasing concentration of CO2 gas in the anode stream that prevents self-purging of the membrane from carbonate to hydroxide form. These issues highlight the need for careful engineering of HEMFC systems and for advanced membrane development to enhance water transport and hydroxide selectivity.

Fabrication of a novel and green thin-film composite membrane containing nanovoids for water purification

Thin film nanocomposite (TFN) membranes, which incorporate nanomaterials in a crosslinked polyamide matrix, often show enhanced separation properties thanks to the additional pores or channels offered by these materials. In this study, we deliberately created nanovoids in the dense polyamide rejection layer by acid-etching copper nanoparticles (CuNPs) contained in a TFN membrane. Systematic membrane characterization confirmed the complete removal of CuNPs using 1% HNO3, which formed nanovoids of approximately 10 nm in size. The water flux of the etched membrane TFC-Cu50X was nearly quadrupled compared to that of the CuNPs loaded membrane TFC-Cu50. This significantly improved water flux can be ascribed to the enhanced water transport through these nano-sized voids. The nanovoids-enhanced approach provides new possibilities for synthesizing high performance membranes.

Impact of PEG additives and pore rim functionalization on water transport through sub-1nm carbon nanotube porins.

Carbon nanotubes represent one of the most interesting examples of a nanofluidic channel that combines extremely small diameters with atomically smooth walls and well-defined chemical functionalities at the pore entrance. In the past, sub-1 nm diameter carbon nanotube porins (CNTPs) embedded in a lipid membrane matrix demonstrated extremely high water permeabilities and strong ion selectivities. In this work, we explore additional factors that can influence transport in these channels. Specifically, we use stopped-flow transport measurements to focus on the effect of chemical modifications of the CNT rims and chaotropic polyethyleneglycol (PEG) additives on CNTP water permeability and Arrhenius activation energy barriers for water transport. We show that PEG, especially in its more chaotropic coiled configuration, enhances the water transport and reduces the associated activation energy. Removal of the static charges on the CNTP rim by converting -COOH groups to neutral methylamide groups also reduces the activation energy barriers and enhances water transport rates.

Enhancement of plant leaf transpiration with effective use of surface acoustic waves: effect of wave frequency.

Water transport in vascular plants provides remarkable opportunities for various engineering applications due to its highly efficient and powerless transportability. Several previous studies were conducted to regulate the biological responses of plants using noninvasive audible or ultrasound waves. However, the control mechanism of acoustic stimuli applied to plants has not been investigated yet. Thus, the practical application of these stimuli to real plants still exhibits technological limitations. This study experimentally investigated the effects of surface acoustic wave (SAW) frequency on plant transpiration to understand the acoustic-activated leaf transpiration and utilize the advantages of SAW. We captured consecutive images of the enhanced water transport in the test plant (Epipremnum aureum) by SAW at three different frequencies (10, 15, and 20 MHz). The dye solution at 15 MHz SAW presented the highest intensity value after 40 min of SAW stimulation. The excitation areas for 15 and 20 MHz SAWs were decreased to 42.3% and 22.6%, respectively, compared with that of 10 MHz SAW. The transpiration rates were directly measured to compare water transport enhancement quantitatively when different SAW frequencies were applied to the same plant leaves. The water transport in the leaves was maximized at 15 MHz SAW, regardless of excitation area.

Enhanced water transport and salt rejection through hydrophobic zeolite pores

The potential of improvements to reverse osmosis (RO) desalination by incorporating porous nanostructured materials such as zeolites into the selective layer in the membrane has spurred substantial research efforts over the past decade. However, because of the lack of methods to probe transport across these materials, it is still unclear which pore size or internal surface chemistry is optimal for maximizing permeability and salt rejection. We developed a platform to measure the transport of water and salt across a single layer of zeolite crystals, elucidating the effects of internal wettability on water and salt transport through the ≈5.5 Å pores of MFI zeolites. MFI zeolites with a more hydrophobic (i.e., less attractive) internal surface chemistry facilitated an approximately order of magnitude increase in water permeability compared to more hydrophilic MFI zeolites, while simultaneously fully rejecting both potassium and chlorine ions. However, our results also demonstrated approximately two orders of magnitude lower permeability compared to molecular simulations. This decreased performance suggests that additional transport resistances (such as surface barriers, pore collapse or blockages due to contamination) may be limiting the performance of experimental nanostructured membranes. Nevertheless, the inclusion of hydrophobic sub-nanometer pores into the active layer of RO membranes should improve both the water permeability and salt rejection of future RO membranes (Fasano et al 2016 Nat. Commun. 7 12762).

AQP4e-Based Orthogonal Arrays Regulate Rapid Cell Volume Changes in Astrocytes.

Water channel aquaporin 4 (AQP4) plays a key role in the regulation of water homeostasis in the brain. It is predominantly expressed in astrocytes at the blood-brain and blood-liquor interfaces. Although several AQP4 isoforms have been identified in the mammalian brain, two, AQP4a (M1) and AQP4c (M23), have been confirmed to cluster into plasma membrane supramolecular structures, termed orthogonal arrays of particles (OAPs) and to enhance water transport through the plasma membrane. However, the role of the newly described water-conductive mammalian isoform AQP4e is unknown. Here, the dynamics of AQP4e aggregation into OAPs and its role in the regulation of astrocyte water homeostasis have been studied. Using super-resolution structured illumination, atomic force, and confocal microscopies, the results revealed that, in female rat astrocytes, AQP4e isoform colocalizes with OAPs, affecting its structural dynamics. In hypoosmotic conditions, which elicit cell edema, OAP formation was considerably enhanced by overexpressed AQP4e. Moreover, the kinetics of the cell swelling and of the regulatory volume decrease was faster in astrocytes overexpressing AQP4e compared with untransfected controls. Furthermore, the increase in maximal cell volume elicited by hypoosmotic stimulation was significantly smaller in AQP4e-overexpressing astrocytes. For the first time, this study demonstrates an active role of AQP4e in the regulation of OAP structural dynamics and in water homeostasis.SIGNIFICANCE STATEMENT Water channel aquaporin 4 (AQP4) plays a key role in the regulation of water homeostasis in the brain. To date, only AQP4a and AQP4c isoforms have been confirmed to enhance water transport through plasmalemma and to cluster into orthogonal arrays of particles (OAPs). We here studied the dynamics, aggregation, and role in the regulation of astrocyte water homeostasis of the newly described water-conductive mammalian isoform AQP4e. Our main findings are as follows: brain edema mimicking hypoosmotic conditions stimulates the formation of new OAPs with larger diameters, due to the incorporation of additional cytoplasmic AQP4 channels and the redistribution of AQP4 channels of the existing OAPs; and AQP4e affects the dynamics of cell swelling and regulatory volume decrease in astrocytes exposed to hypoosmotic conditions.

Enhanced water permeability and tunable ion selectivity in subnanometer carbon nanotube porins.

Fast water transport through carbon nanotube pores has raised the possibility to use them in the next generation of water treatment technologies. We report that water permeability in 0.8-nanometer-diameter carbon nanotube porins (CNTPs), which confine water down to a single-file chain, exceeds that of biological water transporters and of wider CNT pores by an order of magnitude. Intermolecular hydrogen-bond rearrangement, required for entry into the nanotube, dominates the energy barrier and can be manipulated to enhance water transport rates. CNTPs block anion transport, even at salinities that exceed seawater levels, and their ion selectivity can be tuned to configure them into switchable ionic diodes. These properties make CNTPs a promising material for developing membrane separation technologies.

Hot channels engineer enhanced water transport

Designing the high-flux nanofluidic devices is still a challenge. In this work, we show by molecular dynamics simulations that the permeation of single-file water molecules through a carbon nanotube (CNT) can be significantly enhanced by means of heating up the CNT. Specifically, with the increase in channel temperature, the water flow exhibits a remarkable maximum behavior, corresponding to the decrease in water occupancy. The maximum flow is clearly caused by the channel vibration at high temperatures that leads to the breakdown of single-file water chain, suggesting a new mechanism for fast water conduction. Furthermore, with the increase in channel temperature, the water translocation time decreases monotonously and the flipping frequency of water dipole orientation increases as a whole. The distributions of occupancy, hydrogen bond number, dipole angle and axial density profiles also demonstrate unique behaviors and suggest the breakdown of single-file water chain. Our results provide a significant new method to breakdown the collective motion of single-file water chain and achieve the fast water transport, which is helpful for the design of high-flux nanofluidic devices.

Impact of Surface Ionization on Water Transport and Salt Leakage through Graphene Oxide Membranes

Nanopores in graphene-based membranes have shown great potential in enabling highly efficient water desalination. The effective engineering of these membranes necessitates a fundamental understanding of how the physical structure and chemical details of their nanopores govern their desalination performance. Using molecular dynamics simulations, we investigate how the ionization of the functional groups along the perimeter of the nanopores in single-layer graphene oxide membranes affects water transport and salt leakage. Moderate ionization enhances water transport and leads to higher salt leakage. With further ionization of the functionalization groups, water flux through the pore is reduced while the salt leakage shows little change. The significant role of functionalization groups’ ionization in modulating the water flux and salt leakage through the nanopores is attributed to the different organizations of water molecules and ions in and near the pore as the pore becomes charged due to ionization.

Root xylem plasticity to improve water use and yield in water-stressed soybean.

We tested the hypothesis that increasing the number of metaxylem vessels would enhance the efficiency of water uptake in soybean (Glycine max) and decrease the yield gap in water-limited environments. A panel of 41 soybean accessions was evaluated in greenhouse, rainout shelter, and rain-fed field environments. The metaxylem number influenced the internal capture of CO2 and improved stomatal conductance, enhancing water uptake/use in soybeans exposed to stress during the reproductive stage. We determined that other root anatomical features, such as cortex cell area and the percentage of stele that comprised cortical cells, also affected seed yield under similar growth parameters. Seed yield was also impacted by pod retention rates under drought stress (24-80 pods/plant). We surmise that effective biomass allocation, that is, the transport of available photosynthates to floral structures at late reproductive growth stages (R6-R7), enables yield protection under drought stress. A mesocosm study of contrasting lines for yield under drought stress and root anatomical features revealed that increases in metaxylem number as an adaptation to drought in the high-yielding lines improved root hydraulic conductivity, which reduced the metabolic cost of exploring water in deeper soil strata and enhanced water transport. This allowed the maintenance of shoot physiological processes under water-limited conditions.

Prism-patterned Nafion membrane for enhanced water transport in polymer electrolyte membrane fuel cell

Here, we report a simple and effective strategy to enhance the performance of the polymer electrolyte membrane fuel cell by imprinting prism-patterned arrays onto the Nafion membrane, which provides three combined effects directly related to the device performance. First, a locally thinned membrane via imprinted micro prism-structures lead to reduced membrane resistance, which is confirmed by electrochemical impedance spectroscopy. Second, increments of the geometrical surface area of the prism-patterned Nafion membrane compared to a flat membrane result in the increase in the electrochemical active surface area. Third, the vertically asymmetric geometry of prism structures in the cathode catalyst layer lead to enhanced water transport, which is confirmed by oxygen gain calculation. To explain the enhanced water transport, we propose a simple theoretical model on removal of water droplets existing in the asymmetric catalyst layer. These three combined effects achieved via incorporating prism patterned arrays into the Nafion membrane effectively enhance the performance of the polymer electrolyte membrane fuel cell.

Functionalization of flat sheet and hollow fiber microfiltration membranes for water applications.

Functionalized membranes containing nanoparticles provide a novel platform for organic pollutant degradation reactions and for selective removal of contaminants without the drawback of potential nanoparticle loss to the environment. These eco-friendly and sustainable technology approaches allow various water treatment applications through enhanced water transport through the membrane pores. This paper presents "green" techniques to create nanocomposite materials based on sponge-like membranes for water remediation applications involving chlorinated organic compounds. First, hydrophobic hollow fiber microfiltration membranes (HF) of polyvinylidene fluoride were hydrophilized using a water-based green chemistry process with polyvinylpyrrolidone and persulfate. HF and flat sheet membrane pores were then functionalized with poly(acrylic acid) and synthesized Fe/Pd nanoparticles. Surface modifications were determined by contact angle, surface free energy and infrared spectroscopy. The synthesized nanoparticles were characterized by electronic microscopy, X-ray spectrometry and image analysis. Nanoparticle sizes of 193 and 301 nm were obtained for each of the membranes. Depending on the concentration of the dopant (Pd) in the membrane, catalytic activity (established by trichloroethylene (TCE) reduction), was enhanced up to tenfold compared to other reported results. Chloride produced in reduction was close to the stoichiometric 3/1 (Cl-/TCE), indicating complete absence of reaction intermediates.

The influence of zeolite and powdered Bayburt stones on the water transport kinetics and mechanical properties of hydrated lime mortars

The purpose of this paper is an investigation of the possible role of zeolite and powdered Bayburt stones on the fresh and hardened properties of hydrated lime (CL90) mortars. Parameters studied in this paper form the main barriers to the use of hydrated lime in construction practice. Enhancement of these parameters is vital for mortar/substrate optimisation in masonry construction. The major concern of this paper therefore is the combination of wet mortar and brick substrate and most significantly the interaction between them at fresh and hardened states. The results show that transfer sorptivity and time to dewater hydrated lime mortars can be manipulated when zeolite and powdered Bayburt stones are used as replacement materials to the binder. Long setting time of CL90 mortars is decreased with the increasing replacement levels of both zeolite and powdered Bayburt stones. Experimental results also showed that the increasing replacement levels of zeolite and powdered Bayburt stones resulted in a dramatic increase in compressive strength and these results are also supported with the microstructural images. The ability to enhance water transport kinetics and mechanical properties of hydrated lime mortars with zeolite and powdered Bayburt stones should not be underestimated as this enables such materials to be used in construction practice more competently. These results have important practical consequences, not only in the initial adhesion of the mortar to the substrate but also in the strength of the set material and therefore the use of hydrated lime mortars may be encouraged if zeolite and Bayburt stones can improve the fresh and hardened state properties of these mortars.

Ion and water transport in charge-modified graphene nanopores**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB707601 and 2011CB707605), the National Natural Science Foundation of China (Grant No. 50925519), the Fundamental Research Funds for the Central Universities, Funding of Jiangsu Provincial Innovation Program for Graduate Education, China (Grant No. CXZZ13_0087), and the Scientific Research Foundation of Graduate

Porous graphene has a high mechanical strength and an atomic-layer thickness that makes it a promising material for material separation and biomolecule sensing. Electrostatic interactions between charges in aqueous solutions are a type of strong long-range interaction that may greatly influence fluid transport through nanopores. In this study, molecular dynamic simulations were conducted to investigate ion and water transport through 1.05-nm diameter monolayer graphene nanopores, with their edges charge-modified. Our results indicated that these nanopores are selective to counterions when they are charged. As the charge amount increases, the total ionic currents show an increase–decrease profile while the co-ion currents monotonically decrease. The co-ion rejection can reach 76.5% and 90.2% when the nanopores are negatively and positively charged, respectively. The Cl– ion current increases and reaches a plateau, and the Na+ current decreases as the charge amount increases in systems in which Na+ ions act as counterions. In addition, charge modification can enhance water transport through nanopores. This is mainly due to the ion selectivity of the nanopores. Notably, positive charges on the pore edges facilitate water transport much more strongly than negative charges.

Gas Diffusion Layer Materials and their Effect on Polymer Electrolyte Fuel Cell Performance – Ex Situ and In Situ Characterization

AbstractThe gas diffusion layer (GDL) has a vital role in the operation of a polymer electrolyte fuel cell (PEFC). Therefore, studying GDL characteristics and their effect on the cell performance is fundamental for the development of more efficient PEFCs.The work presented covers a selection of commercially available GDL types used in fuel cell development. It highlights some key GDL properties and their influence on PEFC performance. The results show that GDL materials have a direct effect on the ohmic and mass transport losses in the membrane electrode assembly (MEA). They also show that studying the effect of GDL properties on the performance is rather complex, due to the many interrelated properties.The study shows that GDL thickness has a significant effect on the mass transport properties of MEA, but has minimal effect on the ohmic losses. The bulk density of the substrate has a significant effect on the water transport properties and the maximum current density achieved. It is also found that woven and non‐woven GDLs achieve comparable performance at optimized operating conditions. Moreover, the felt fiber structure has higher ohmic resistance but achieves better performance than that of straight fiber carbon paper due to its enhanced water transport ability.

Water diffusion-transport in a synthetic dunite: Consequences for oceanic peridotite serpentinization

A series of San Carlos olivine aggregates, sintered at high pressure and high temperature, with two different porosities (around 1 and 10%) and grain sizes (1–5 μm and 0–38 μm) were reacted at 300 °C and 500 bars in the presence of pure water. The reaction progress was monitored magnetically and the composition and distribution of the reaction products were analyzed at the end of each experiment. Brucite formation mainly occurred at the aggregate surface as a result of both congruent olivine dissolution and aqueous Mg and Si buffering by the reaction products, i.e. brucite and lizardite. The measured reaction progress did not exceed 2.6% after 290 d, which strongly contrasts with previous studies performed on San Carlos olivine powders (i.e., isolated grains in aqueous solution). Hence, limited water transport through the intergranular region of the aggregate drastically decreased the olivine surface area accessible to water and thus slowed down the whole serpentinization process. When extrapolated to peridotite relevant olivine grain sizes, our experimental results indicate that the water diffusion rate will become so slow that the first layer of primary minerals exposed to water within a mesh structure must fully react before the next mineral layer starts reacting (“layer by layer” mechanism). This type of reaction-transport mechanism is obviously not consistent with the micro-scale serpentine distribution in the mesh of oceanic peridotite samples, therefore additional water transport pathways are required. Cracks formed under extensional thermal stresses are good candidates since, in comparison to grain boundary or reaction-induced fractures, they are wide enough to drastically enhance water transport in oceanic peridotites and therefore account for the observed textures. The ‘layer by layer’ mechanism inferred here can only set a lower time bound for serpentinization completion. Assuming a mesh size of 1 mm and an initial grain size of 100 μm and considering a temperature ranging from 100 to 300 °C with permanent water saturation, completion should take place within 100–1000 yr. Surprisingly, this duration represents only 1 to 10% of the estimated timescale of the natural serpentinization process, emphasizing the central role played by water availability in the natural reaction process.

Asymmetric bi-layer PFSA membranes as model systems for the study of water management in the PEMFC.

New bi-layer PFSA membranes made of Nafion® NRE212 and Aquivion™ E79-05s with different equivalent weights are prepared with the aim of managing water repartition in the PEMFC. The membrane water transport properties, i.e. back-diffusion and electroosmosis, as well as the electrochemical performances, are compared to those of state-of-art materials. The actual water content (the inner water concentration profile across the membrane thickness) is measured under operation in the fuel cell by in situ Raman microspectroscopy. The orientation of the equivalent weight gradient with respect to the water external gradient and to the proton flow direction affects the membrane water content, the water transport ability and, thus, the fuel cell performances. Higher power outputs, related to lower ohmic losses, are observed when the membrane is assembled with the lower equivalent weight layer (Aquivion™) at the anode side. This orientation, corresponding to enhanced water transport by back-flow while electroosmosis remains unaffected, results in the higher hydration of the membrane and of the anode active layer during operation. Also, polarization data suggest a different water repartition in the fuel cell along the on-plane direction. Even if the interest in multi-layer PFSA membranes as perspective electrolytes for PEMFCs is not definitively attested, these materials appear to be excellent model systems to establish relationships between the membrane transport properties, the water distribution in the fuel cell and the electrochemical performances. Thanks to the micrometric resolution, in situ Raman microspectroscopy proves to be a unique tool to measure the actual hydration of the membrane at the surface swept by the hydrated feed gases during operation, so that it can be used as a local probe of the water concentration evolution along the gas distribution channels according to changing working conditions.