Water Transport Mechanisms Research Articles

Influence of Gas Humidity and Flow Rate on the Water Movement Direction through an Anion Exchange Membrane Fuel Cell

Environmental concerns lead to a continually growing demand for clean energy generation. Thus, alternative energy sources or green fuels attract attention. The anion exchange membrane fuel cell (AEMFC) emerged as an excellent alternative due to its air CO2-free emissions, low-cost electrocatalyst used at the cathode, and industrial-scale membrane production development [1-3].Many efforts have been made to have AEMFC performances like those already shown by the proton exchange membrane fuel cell (PEMFC). Excellent results have been obtained for the lasted technology regarding power densities and operational cell life [4]. It should be recognized that due to the nature of the electrode reactions in AEMFC, Eqs. 1-2, the performance enhancement of the overall cell reaction, Eq. 3, depends not only on the selected catalysts and ionomers/membranes but also on the cell's water management. The complexity of water management relies on the simultaneous production and consumption of water in anode and cathode, respectively, where the production is two-fold higher than the consumption. This effect might lead to anode flooding during the hydrogen oxidation reaction (HRR), Eq. 1. In contrast, during oxygen reduction reaction (OER), Eq. 2, the risk of membrane drying is latent as long as a water imbalance arises between both anodic and cathodic sides. It has been confirmed quantitatively that water direction movement could go from anode to cathode in a hydrophilic membrane (diffusion transport mode); however, using a membrane with hydrophobic properties promotes the water to travel from cathode to anode (electro-osmotic drag, EOD mode). In this context, it is convenient to identify the water direction trend under variable selected operating conditions. In particular, achieving the anode-to-cathode water direction would improve the AEMFC performance by avoiding flooding on the anode by cathode hydration through water diffusion through the membrane. Therefore, this work tested the influence of symmetric and asymmetric flow rates and relative humidities within an AEMFC using a hydrophilic membrane. According to both modes of transport, the AEMFC performance was studied using polarization curves.The experimental procedure involved trapping water out of each half-cell during a chronoamperometry in steady-state conditions. Also, the water incoming was quantified independently for each selected operating condition in order to make a mass balance (Figure 1a). The variable operating conditions were the reactant flow rate, low (125 cm3 min-1) and high (250 cm3 min-1), and the reactant humidity level.In Figure 1b, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min-1 for anode and cathode) were used in the AEMFC, suggesting membrane dehydration. On the contrary, the water diffusion dominance appears under high (250 cm3 min-1) symmetric and asymmetric flow rates between anode and cathode; under these conditions, the membrane dehydration might be suppressed by anode flooding alleviation. Once the flow conditions are given, no changes in water transport mode were detected for changes in the humidity level (Figure 1c-d). The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm-2 at 60°C, Figure 1e) than those obtained by diffusion (62.8 mW cm-2 at 60°C, Figure 1f). The most suitable operating condition is under water-diffusion at non-saturated streams (below 65%), where a decrease in performance is experienced as the humidifier temperature increases from 60°C to 80°C (supersaturated flow), with peak power densities of 62.8 and 3.7 mW cm-2, respectively.

Water Movement Direction through an Anion Exchange Membrane Fuel Cell Exerted By Gas Humidity and Flow Rate: A Simulation Study

The growing demand for clean energy generation has been urged due to the threat of climate change by greenhouse emissions (1 kg of CO2 per 1 kW h of generated electricity). In 2021, the electricity generation rose to 1530 TW h, 62% coming from fossil fuels [1]. To lower this significant fossil fuels usage, the development and use of alternative energy sources and green fuels have reached a formidable level of interest. Renewable solar and wind energy sources are currently available; however, those appear to be limited by natural fluctuations in energy production relegated to day- and night-time wheatear or geographical regions [2]. Electrochemical energy storage and conversion technologies can be employed to minimize production variations as an alternative to this limitation. The anion-exchange membrane fuel cell (AEMFC) is an excellent alternative for energy generation due to its air CO2-free emissions, low-cost electrocatalyst used at the cathode, and industrial-scale membrane production [3]. The design of a fuel cell involves several tightly interrelated factors, such as electrode and membrane materials, reactor geometry, and characterization of the reaction environment in terms of the momentum, mass and heat transport, electrode kinetics, and current distribution, to find the best operational conditions to reach a suitable energy efficiency [4]. In either case, the present work deals with designing a hybrid flow-field single-fuel cell. The monopolar anode plate is a multiple-serpentine flow field that benefits the water drainage. On the other hand, the cathode plate is an interdigitated leaf-type flow field that might ensure an efficient gas reactant distribution over the entire electrode geometric area.A two-phase flow CFD simulation is performed to analyze the influence of the flow field patterns on the mass transport processes in the electrodes. Moreover, the water movement direction through the membrane as a function of the operational conditions is presented. The commercial code COMSOL Multiphysics was used to solve the transport equations through the finite element method.In the experimental field, the water movement direction between the anodic and cathodic sides shows the capability to favor the electro-osmotic drag (EOD) or the diffusion water transport mechanism under different gas flow rates and humidities, as shown in Figure 1a. Accordingly, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min-1 for anode and cathode) were used in the AEMFC. On the contrary, the water diffusion dominance appears under asymmetric flow rates between anode and cathode (125 and 250 cm3 min-1, respectively). The different tested relative humidities do not influence the water direction once the flow rates are given. The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm-2, in Figure 1b) than those obtained by diffusion (62.8 mW cm-2, in Figure 1c). The most suitable operating condition is for water diffusion dominance at non-saturated streams, where a decrease in performance is experienced as the flow becomes saturated (at higher humidifier temperature). A close agreement between simulation and experimental results was obtained regarding current-potential curves and water movement through the membrane dictated by electro-osmotic drag and water diffusion.

Water transport in calcium silicate hydrate nanoscale pores by molecular dynamics: Unsaturated steady-state diffusion and suction

Water transfer in calcium silicate hydrate (CSH) nanoscale pores is an important factor affecting the durability of cement-based materials. Using molecular dynamics, the transport behaviour of nanoscale water in unsaturated CSH gel pores can be simulated to explore the transport law and mechanism of unsaturated water in the gel pores. In this paper, two kinds of transport models were established: an unsaturated steady-state diffusion model and a suction model. The effects of pore size and saturation level on the steady-state density distribution, dipole orientation, dipole moment and diffusion coefficient of water in each region were studied. Then, the influence of temperature, pore diameter, initial saturation level and tortuosity on the water absorption rate in the suction process was analysed, and the CSH molecular formula for the saturation state was calculated. The results showed that the distribution of water in the CSH gel pore was mainly related to the saturation level and wall position, and the percolation effect was triggered when the saturation of the gel pore reached 53 %. Water molecules located in the interlayer zone, adsorption zone and free zone exhibit the laws of the free zone > adsorption zone > interlayer zone due to the differences in geometric constraints and bonding. With the increase in the saturation level of water in the gel pore, the overall diffusion coefficient of water presents a change law of first rising and then falling. A higher temperature will magnify the effect on the promotion of transport. The pore size of the gel pore is positively correlated with the diffusion capacity of water, while higher initial saturation level and tortuosity will both inhibit the absorption rate of the gel pore. The CSH formula in the saturated stable state is (CaO)1.65SiO2(H2O)2.1. For cement-based materials with a high proportion of large gel pores (above 2 nm), limiting the saturation level in the gel pores below the percolation threshold (preferably below 45 %) as far as possible can effectively inhibit the water transport capacity. The simulation results are in good agreement with the data in the literature.

Numerical upscaling of nonlinear flow in low-permeability sandstone matrix system

The matrix system's apparent permeability model based on nano- and micro-scale mechanism modeling is limited in its role in low-permeability reservoir numerical simulation due to the lack of suitable up-scaling methods. This paper developed a numerical simulation method to evaluate the microporous structure's apparent permeability contribution to total permeability of the heterogeneous representative elementary volume (REV)-scale matrix under macroscopic pressure gradients. First, a local apparent liquid permeability (LALP) model considering the heterogeneous microporous structure and multiple water-transport mechanisms was established to describe the nonlinear seepage behavior of fluid in microscopic computational domain. Then, a multiresolution-image-based REV-scale domain reconstruction method was proposed, in which the microporous structural characteristics were quantified by fractal heterogeneous-porous-media model, and the REV-scale matrix was subsequently stochastic reconstructed based on the porosity distribution characteristics of microporous structures. Finally, the microscopic LALP model and macroscopic continuum model were fully coupled to numerical simulate the nonlinear seepage progress in REV-scale matrix domain. The comparisons between simulation results and experimental results demonstrated that the developed numerical simulation method can effectively predict the nonlinear flow velocity curve of REV-scale sandstone matrix. The influences of internal factors (microporous structural characteristics and seepage fluid properties) and external factors (temperature and pressure) on absolute permeability and apparent permeability of REV-scale matrix were investigated. The results demonstrated that (1) microporous structural characteristics determines the approximate level of intrinsic reservoir permeability, especially the average maximum pore size of discrete domain; (2) with microporous structure's permeability distribution heterogeneity increasing, the macroscopic intrinsic permeability decreases and the differences between apparent permeability and intrinsic permeability increase.

Numerical Simulation and Analysis of Water and Suspended Sediment Transport in Hangzhou Bay, China

Hangzhou Bay is a large, high-turbidity shallow bay located on the southern side of the Changjiang Estuary, China. The process and dynamic mechanisms of water and sediment transport in the bay are not yet clear. An improved three-dimensional sediment numerical model that combined various dynamic factors was established to simulate and analyze these mechanisms. The residual current cannot properly represent the net water and sediment transport, and the residual unit width water flux (RUWF) and residual unit width sediment flux (RUSF) were used to explain the water and sediment transport. The results of numerical simulations indicate that in summer, the surface RUWF from the Changjiang Estuary near Nanhui Cape flows westward along the coast, in which the major part flows southward to the Zhenhai area, and the small part flows further westward along the north coast and then turns to the south coast and eastward, forming the water transport pattern of north-landward and south-seaward, which is stronger in the spring tide than in the neap tide. The bottom RUWF near Zhenhai flows northward to Nanhui Cape in the neap tide, which is larger in the neap tide than in the spring tide. In the middle and western parts of the bay, the RUWF has the same pattern as the surface water transport and is stronger in the spring tide than in the neap tide. The pattern of RUSF is roughly similar to the water flux transport. During the spring tide, the water and sediment transport fluxes near Nanhui Cape are from the Changjiang Estuary into Hangzhou Bay, but from Hangzhou Bay into the Changjiang Estuary during the neap tide. In the winter, the distributions of RUWF, RUSF, and suspended sediment concentration (SSC) are similar to those in the summer. In addition, the distance of surface water transport westward along the north coast is shorter than that in the summer, the magnitude of the bottom RUWF is smaller than that in the summer due to the weaker salinity gradient, and the bottom RUSF near Nanhui Cape is weaker than that in the summer during the neap tide. The net transect water flux (NTWF) and the net transect sediment flux (NTSF) near Nanhui Cape are from the Changjiang Estuary into Hangzhou Bay during the spring tide; during the neap tide, the NTWF is still from the Changjiang Estuary into Hangzhou Bay, but the NTSF is from Hangzhou Bay into the Changjiang Estuary because the SSC is much higher in the bottom layer than in the surface layer. The dynamic reason for the temporal and spatial variation in RUWF and RUSF is that the barotropic pressure gradient force is larger than the baroclinic pressure gradient force during the spring tide and is the opposite during the neap tide.

Xylem-Inspired Hydrous Manganese Dioxide/Aluminum Oxide/Polyethersulfone Mixed Matrix Membrane for Oily Wastewater Treatment.

Ultrafiltration membrane has been widely used for oily wastewater treatment application attributed to its cost-efficiency, ease of operation, and high separation performance. To achieve high membrane flux, the pores of the membrane need to be wetted, which can be attained by using hydrophilic membrane. Nevertheless, conventional hydrophilic membrane suffered from inhomogeneous dispersion of nanofillers, causing a bottleneck in the membrane flux performance. This called for the need to enhance the dispersion of nanofillers within the polymeric matrix. In this work, in-house-fabricated hydrous manganese dioxide–aluminum oxide (HMO-Al2O3) was added into polyethersulfone (PES) dope solution to enhance the membrane flux through a xylem-inspired water transport mechanism on capillary action aided by cohesion force. Binary fillers HMO-Al2O3 loading was optimized at 0.5:0.5 in achieving 169 nm membrane mean pore size. Membrane morphology confirmed the formation of macro-void in membrane structure, and this was probably caused by the hydrophilic nanofiller interfacial stress released in PES matrix during the phase inversion process. The superhydrophilic properties of PES 3 in achieving 0° water contact angle was supported by the energy-dispersive X-ray analysis, where it achieved high O element, Mn element, and Al elements of 39.68%, 0.94%, and 5.35%, respectively, indicating that the nanofillers were more homogeneously dispersed in PES matrix. The superhydrophilic property of PES 3 was further supported by high pure water flux at 245.95 L/m2.h.bar, which was 3428.70% higher than the pristine PES membrane, 197.1% higher than PES 1 incorporated with HMO nanofiller, and 854.00% higher than PES 5 incorporated with Al2O3 nanofillers. Moreover, the excellent membrane separation performance of PES 3 was achieved without compromising the oil rejection capability (98.27% rejection) with 12 g/L (12,000 ppm) oily wastewater.

Two-phase Flow Dynamics at the Interface Between GDL and Gas Distributor Channel Using a Pore-Network Model

For improved operating conditions of a polymer electrolyte membrane (PEM) fuel cell, a sophisticated water management is crucial. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. Pore-scale modeling of diffusion layers and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the cathode layers, a particular challenge is the combination and interaction of the multi-phase flow in the porous material of the gas diffusion layer (GDL) with the free flow in the gas distributor channels. The formation, growth and detachment of water droplets on the hydrophobic, porous surface of the GDL have a major influence on the mass, momentum and energy exchange between the layers. A dynamic pore-network model is used to describe the flow through the porous GDL on the pore-scale. To capture the droplet occurrence and its influence on the flow, this dynamic two-phase pore-network model is extended to capture droplet formation and growth at the surface of the GDL as well as droplet detachment due to the gas flow in the gas distributor channels. In this article, the developed model is applied to single- and multi-tube systems to investigate the general drop behavior. These rather simple test-cases are compared to experimental and numerical data available in the literature. Finally, the model is applied to a GDL unit cell to analyze the interaction between two-phase flow through the GDL and drop formation at the interface between GDL and gas distributor channel.

Transport behavior of water and ions through positively charged nanopores

• Investigating the application of charge-modified nanopores in water-ion separation. • Studying the effect of positively charged nanopore on water permeability. • Unveiling the complex interactions between charge-modified nanopore and hydration ions. Two-dimensional nanoporous membranes are promising materials for seawater desalination. However, the effect of nanopore charge properties on desalination performance, especially on water permeability, has not yet been systematically investigated. In this work, molecular dynamics simulations were carried out to elucidate the impact of nanopore charge modification (variated from 0 to 1.0e) on water permeability and ions conduction. The highest water permeability was observed when the modification was approximately 0.25e. The nanopore charge conditions significantly impacted water dynamic behaviors, leading to the transition of water transport processes from continuum state to non-continuum state with the increase of nanopore charge. In addition, we found that the positively charged nanopores not only effectively impede cation permeation, but also strongly impact the diffusion behavior of anions. This work unveils in-depth insights into the mechanism of saline water transport through the charge-modified nanopores and highlights the importance of the nanopore charge states for desalination. Meanwhile, it provides useful guidelines for the rational design of optimal nanoporous membranes in the future.

Evaluation of test procedures for durability and permeability assessment of concretes containing calcined clay

The durability and permeability of concrete could be investigated by different test methods that evaluate various transport mechanisms of water and ions in cement-based materials. This research has investigated the suitability of conventional permeability and durability assessment test methods for evaluating the properties of concrete mixtures containing calcined clays and limestone calcined clay cements (LC3). The water absorption, capillary water sorptivity, water penetration, surface electrical resistivity, rapid chloride ions migration coefficient, chloride ions’ penetration profile, and diffusion coefficient were examined in concrete mixtures. A rapid approximation method has also been proposed to estimate the kaolinite content of clays based on the chemical composition data. According to the results, water absorption test based on BS 1881-122 could not accurately specify the permeability properties of binary and LC3 blends. Further, the initial rate of capillary water absorption had an appropriate correlation with the results of water penetration and electrical resistivity test methods.

Water movement through an anion exchange membrane fuel cell (AEMFC): Influence of gas humidity and flow rate

The water movement between the anodic and cathodic sides of a fuel cell (AEMFCs) motivated the analysis of the effect exerted by the operational conditions on the water direction using a commercial anion exchange membrane (IONOMR®). This work shows the capability to favor the electro-osmotic drag (EOD) or the diffusion water transport mechanisms under different gas flow rates and humidities. Accordingly, the results demonstrated that the EOD predominance was promoted when low symmetric flow rates (125 cm3 min−1 for anode and cathode) were used in the AEMFC, suggesting membrane dehydration. On the contrary, the water diffusion dominance appears under asymmetric flow rates between anode and cathode (125 and 250 cm3 min−1, respectively); under these conditions, the membrane dehydration might be suppressed by anode flooding alleviation. The AEMFC performance was affected by the water transport mechanism, giving lower peak power densities for EOD-dominated systems (35.2 mW cm−2 at 60 °C) than those obtained by diffusion (62.8 mW cm−2 at 60 °C). The most suitable operating condition is under water-diffusion at non-saturated streams (below 65%), where a decrease in performance is experienced as the humidifier temperature increases from 60 °C to 80 °C (supersaturated flow), giving peak power densities of 62.8 and 3.7 mW cm−2, respectively.

Characteristics of Water Transport of Membrane Electrolyte over Selected Temperature for Proton Exchange Membrane Fuel Cell.

The water contents at both the anode and cathode of PEMFCs depend on the water-transport mechanism at the membrane. The humidity at the outside layers of the membrane determines the diffusion of water through it. The operating temperatures and pressures regulate the humidity conditions in the system. Because these parameters are nonlinear, the water-transport mechanism is analyzed via the difference in the water concentration on each side of the membrane. In this work, an experimental configuration is designed to investigate the diffusion mechanism of water through the membrane. A flat membrane module is tested in an isothermal test chamber to test the influence of temperature on the water-absorption and -transport characteristics of Nafion 117 and Nafion 211 membranes. A parametric study is conducted to test the water-transport mechanism at an operating pressure of 1 bar; temperatures of 30 °C, 50 °C, 70 °C and 90 °C; and a relative humidity ranging from 10% to 100%. The results indicate that the water content of Nafion 211 is higher than that of Nafion 117. The water content and diffusion coefficient are proportional to the operating temperature. In addition, the diffusion coefficient reaches its peak at conditions of 1 bar, 100% humidity, and 90 °C for both membrane types.

Experimental evaluation of the dynamic water-blocking effect in coalbed methane reservoir

Field observations indicate that almost 59% of coalbed methane (CBM) wells in the southern Qinshui Basin showed insufficient daily gas production (∼600 m3/t); this is caused by the water-blocking effect (WBE) attributed to the invasion of fracturing fluids or the accumulation of formation water. WBE can cause serious damages to the low-permeability reservoir but has not been paid sufficient attention to in CBM studies. To enhance understanding of the dynamic WBE in the multiphase flow process, we conducted core flooding experiments on two typical Chinese bituminous coal and anthracite coal samples. During the flooding experiments, real-time nuclear magnetic resonance (NMR) measurements of the change of water signal in the core were used to calculate the gas-water relative permeability and to evaluate the dynamic WBE. The results highlight that a more pronounced WBE occurred in anthracite coal compared with bituminous coal. The gas breakthrough model can calculate the gas breakthrough time, which is a useful parameter for evaluating water blocking. The water-blocking threshold pressure gradient (TPG) was determined based on experimental data. The results reveal that the water-blocking TPGs for both samples follow a positive power-law relationship with the normalized water saturation. The water-blocking TPG is approximately an order of magnitude higher than that of the conventional gas TPG. A high water-blocking TPG value reflects stronger gas/water interference during multiphase flow, exerting a negligible impact on the gas migration through the coalbed, which should be prioritized in CBM production. The implications of this study are important for a better understanding of the interaction mechanism of gas and water transport in coal reservoirs during the gas production process.

2D Model Porous Transport Layers for Interfacial Characterization in Polymer Electrolyte Water Electrolysis

Polymer electrolyte water electrolysis (PEWE) is a key technology in the realization of a future hydrogen economy. The thorough understanding and optimization of each of its components in a synergistic matter is essential in the advancement of this technology. In this study, we aim at understanding the role of the porous transport layer (PTL) interface in contact with the catalyst layer (CL) [1] and its impact on PEWE performance losses.We addressed the PTL/CL interface by creating a matrix of well-defined two-dimensional PTL materials [2]. These 2D-PTLs are fabricated by precise laser drillings of varying sizes on thin Ti-sheets, allowing us to accurately control geometrical properties such as interfacial contact area, pore size, triple-phase boundaries (TPB), and surface roughness. This systematic approach, combined with in-depth electrochemical analysis, provides insights into how the interface parameters affect the catalyst layer utilization at different current densities leading to severe differences in mass transport losses. Furthermore, by combining operando X-ray tomographic microscopy [3] and electrochemical characterization under selective conditions, we were able to get insights into the water transport mechanism inside the CL as a function of the different interfacial regions at the PTL/CL interface.The findings related to this work will potentially contribute to a future more rational design of PTL and CL structures.[1] T. Schuler, T.J. Schmidt, F.N. Büchi, J. Electrochem. Soc. 166 (2019) F555–F565.[2] Z. Kang, J. Mo, G. Yang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr, M.M. Mench, F.-Y. Zhang, Energy Environ. Sci. 10 (2017) 166–175.[3] S. De Angelis, T. Schuler, M.A. Charalambous, F. Marone, T.J. Schmidt, F.N. Büchi, J. Mater. Chem. A 9 (2021) 22102–22113. Figure 1

Modeling of Two Phase Flow in a Hydrophobic Porous Medium Interacting with a Hydrophilic Structure

Fluid flow through layered materials with different wetting behavior is observed in a wide range of applications in biological, environmental and technical systems. Therefore, it is necessary to understand the occuring transport mechanisms of the fluids at the interface between the layered constituents. Of special interest is the water transport in polymer electrolyte membrane fuel cells. Here, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. This is crucial to choose optimal operating conditions and improve the overall cell performance. Pore-scale modeling of gas diffusion layers (GDLs) and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the hydrophobic porous GDL and the hydrophilic gas distributor, a particular challenge is the combination and interaction of the different material structures and wetting properties at the interface and its influence on the flow. In this paper, a modeling approach is presented which captures the influence of a hydrophilic domain on the flow in a hydrophobic porous domain at the interface between the two domains. A pore-network model is used as the basis of the developed concept which is extended to allow the modeling of mixed-wet interactions at the interface. The functionality of the model is demonstrated using basic example configurations with one and several interface pores and it is applied to a realistic GDL representation in contact with a channel-land structured gas distributor.

Flow Reduction in Pore Networks of Packed Silica Nanoparticles: Insights from Mesoscopic Fluid Models.

A modified many-body dissipative particle dynamics (mDPD) model is rigorously calibrated to achieve realistic fluid-fluid/solid interphase properties and applied for mesoscale flow simulations to elucidate the transport mechanisms of heptane liquid and water, respectively, through pore networks formed by packed silica nanoparticles with a uniform diameter of 30 nm. Two million CPU core hours were used to complete the simulation studies. Results show reduction of permeability by 54-64% in heptane flow and by 88-91% in water flow, respectively, compared to the Kozeny-Carman equation. In these nanopores, a large portion of the fluids are in the near-wall regions and thus not mobile due to the confinement effect, resulting in reduced hydraulic conductivity. Moreover, intense oscillations in the calculated flow velocities also indicate the confinement effect that contests the external driven force to flow. The generic form of Darcy's law is considered valid for flow through homogeneous nanopore networks, while permeability depends collectively on pore size and surface wettability. This fluid-permeability dependency is unique to flow in nanopores. In addition, potential dependence of permeability on pore connectivity is observed when the porosity remains the same in different core specimens.

Molecular insights into the structure-property relationships of 3D printed polyamide reverse-osmosis membrane for desalination

3D-printing is an emerging method for manufacturing polyamide (PA) reserve osmosis (RO) membranes for water treatment and desalination, which can precisely control membrane structural properties, such as thickness, roughness, and resolution. However, the synthesis-structure (i.e., degree of cross-linking (DC), m-phenylenediamine/trimesoyl chloride (MPD/TMC) ratio, and membrane thickness) to property (permeability and water-salt selectivity) relationships for these membranes has not been well understood. At the same time, a microscopic understanding of the physical mechanism of water and salt transport is needed to guide the design of high-performance 3D-printed membranes and improve the printing efficiency. Thus, the atomic-scale transport features and energetics of water and salt ions are studied at high pressure for the 3D-printed PA RO membranes with the different DCs and MPD/TMC ratios through non-equilibrium molecular dynamics (NEMD) simulations. Factoring in membrane structure properties, rejection ratio of salt ions and pressure-dependent water flux, 3D-printed PA membranes having an MPD/TMC ratio of 3.0:2.0 and a DC between 80%∼90% attains ideal performance: high water flux, high rejection of salt ions, and excellent structural integrity. Mechanistically, water permeability for highly cross-linked PA RO membranes depends on the temporary on-and-off channels that allow water molecules to jump from one cavity to another at high pressure. In addition, higher pressures cause rapid compaction of PA membranes’ free volume and membrane thickness. Membrane failure at high pressure is determined by the DC and MPD/TMC ratios-dependent compressive yield strength. In short, these findings provide physical insights for optimizing existing PA membranes and designing next-generation desalination membranes at the molecular level.

Desalination Potential of Aquaporin-Inspired Functionalization of Carbon Nanotubes: Bridging Between Simulation and Experiment.

Outstanding water/ion selectivity of aquaporins paves the way for bioinspired desalination membranes. Since the amino acid asparagine (Asn) plays a critical role in the fast water conduction of aquaporins through hydrogen bonding interactions, we adapted this feature by functionalizing carbon nanotubes (CNTs) with Asn. We also studied a nonpolar amino acid and carboxylate functional groups for comparison. Computation of the ideal performance of individual CNTs at atomistic scale is a powerful tool for probing the effect of tip-functionalized CNTs on water and ion transport mechanism. Molecular simulation study suggests that steric effects required for ion rejection compromise fast water conductivity; however, an Asn functional group having polarity and hydrogen bonding capability can be used to balance this trade-off to some extent. To test our hypothesis, we incorporated functionalized CNTs (f-CNTs) into the in situ polymerized selective polyamide (PA) layer of thin film nanocomposite membranes and compared their experimental RO desalination performance. The f-CNTs were found to change the separation environment through modification of cross-linking density, thickness, and hydrophilicity of the PA layer. Asn functionalization led to more cross-linked and thinner PA layer while hydrophilicity is improved compared to other functional groups. Accordingly, water permeance is increased by 25% relative to neat PA with a salt rejection above 98%. Starting from the nanomaterial itself and benefiting from molecular simulation, it is possible to design superior membranes suited for practical applications.

Impacts of Vegetation Changes on Land Evapotranspiration in China During 1982–2015

Evapotranspiration (ET) bridges the hydrological and energy cycle through vegetation transpiration (T), soil evaporation (ES), and canopy interception evaporation (EI). Transpiration to evapotranspiration ratio (T/ET) quantifies the water use efficiency of terrestrial ecosystems explaining the mechanism of vegetation water transport and water–carbon interactions. This study employed GIMMS LAI3g data to improve the CLM4.5 land surface scheme of RegCM4.6. We designed two simulation experiments, each with control (CTL) and sensitivity (SEN), simulating the interannual variability of vegetation on ET and T/ET in China from 1982 to 2015. Studies show China has experienced a greening trend, especially in mid-south China and South China. Leaf area index (LAI) increased significantly (0.002 m2m−2yr−1). ∆LAI (SEN input LAI data minus CTL input LAI data) and ∆T/ET (T/ET data output by SEN minus T/ET data output by CTL) have shown significant positive correlations. The impacts of LAI on T/ET are more prominent during spring and winter than in autumn and summer. Compared with T/ET and LAI (R = 0.70), the correlation between ET and LAI is moderate (R < 0.5), indicating that vegetation has a higher impact on T/ET than ET. The impact of vegetation anomalies (positive and negative LAI anomalies) on T/ET and ET is spatially different, mainly due to dominant factors affecting ET and T/ET changes. In spring, summer, and autumn, &T (transpiration changes corresponding to vegetation anomalies) is the leading factor affecting both ET and T/ET regionally, and &T has a stronger influence on T/ET than ET, especially in summer. Vegetation anomalies have a stronger influence on T/ET than ET; and the influence of positive vegetation anomalies on ET and T/ET is greater than that of negative vegetation anomalies, especially in spring and autumn. This study reveals the mechanisms behind vegetation processes and their influences on the water and heat fluxes at the land–atmosphere interface and provides a strong scientific basis for studying the water cycle under climate warming.

Nonlocal interactions between vegetation induce spatial patterning

Vegetation pattern provides useful signals for vegetation protection and can be identified as an early warning of desertification. In some arid or semi-arid regions, vegetation absorbs water through nonlocal interaction of roots. In this study, we present a vegetation model with nonlocal interaction which is characterized by an integral term with a kernel function. Mathematical analysis provides the conditions for the generation of stationary pattern. Numerical simulations exhibit different spatial distributions of vegetation. Densities of vegetation and water show an inverse relationship at same spatial locations due to water transport mechanism. The results reveal that the interaction intensity and the shape of the kernel function can cause the transition of vegetation pattern. Specifically, the vegetation biomass increases as the interaction intensity decreases or as the nonlocal interaction distance increases. We demonstrate that the nonlocal interactions between roots of vegetation is a key mechanism for the formation of vegetation pattern, which provides a theoretical basis for the preservation and restoration of vegetation.

3D quantitative characterization and flow simulation of granite residual soil based on CT scanning

<p indent="0mm">This paper aims to reveal the water transport mechanism in soils with macropores. Based on the combination of the X-ray CT scanning and image processing technologies, a three-dimensional (3D) equivalent pore network model (PNM) with macropores (&gt;0.15 mm) is constructed, and the size of the optimal representative volume unit (REV) is determined to be 300×300×300. Then, the quantitative characterization of the topological structure of the macropore space is carried out followed by the PNM single-phase flow and the 3D macropore water-gas two-phase flow simulation. The results show that there are more pores with smaller pore diameters, and more than 80% of the pores are concentrated within <sc>0.15–1.00 mm.</sc> The pore topological space structure is quite different, and the coordination number is mainly less than 20, but some can reach more than 60. The average coordination number of pores is between 3–7, and the average connected pore radius is greater than <sc>2.00 mm.</sc> The single-phase flow simulation find that the complexity of the pore structure and the resolution of X-ray CT scanning resulted in a certain difference between the simulated and measured permeability, but the difference of the main seepage direction is only 3.81%. The two-phase flow simulation find that the front edge of flow is almost linear at the beginning of seepage, then it tend to be fingering, and finally, it become irregular. The residual gas in the pores is mainly distributed in blocks at the edges of pores, dead ends, and sudden changes of pore throats. With the increasing seepage time, the saturation of the water phase shows an upward trend, while the saturation of the gas phase shows a downward trend, then gradually tend to be stable, and finally, the saturation of the water phase and the residual gas phase maintain at 94.92% and 5.08%, respectively. This study can better reveal the meso-seepage mechanism of reconstructed soil, and provide a certain reference for further understanding the law of pore flow in soil.