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Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Achieving such a precise separation using membranes requires Angstrom scale pores with a high level of pore size uniformity. Herein, we demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization (SARIP). The dynamic, self-assembled network of surfactants facilitates faster and more homogeneous diffusion of amine monomers across the water/hexane interface during interfacial polymerization, thereby forming a polyamide active layer with more uniform sub-nanometre pores compared to those formed via conventional interfacial polymerization. The polyamide membrane formed by SARIP exhibits highly size-dependent sieving of solutes, yielding a step-wise transition from low rejection to near-perfect rejection over a solute size range smaller than half Angstrom. SARIP represents an approach for the scalable fabrication of ultra-selective membranes with uniform nanopores for precise separation of ions and small solutes., Separating molecules or ions with sub-Angstrom scale precision is important but technically challenging. Here, the authors demonstrate that precise solute-solute separation can be achieved using polyamide membranes formed via surfactant-assembly regulated interfacial polymerization.

Membrane distillation (MD) is a thermal desalination process with the capability of harnessing low-grade waste heat to treat hypersaline brine. For this reason, MD has been actively explored as a p...

Various nanofillers have been adopted to enhance the thermal conductivity of polymer nanocomposites. While it is widely believed that the contact thermal resistance between adjacent nanofillers can play an important role in limiting thermal conductivity enhancement of composite materials, lack of direct experimental data poses a significant challenge to perceiving the effects of these contacts. This study reports on direct measurements of thermal transport through contacts between silver nanowires (AgNWs) with a poly(vinylpyrrolidone) (PVP) interlayer. The results indicate that a PVP layer as thin as 4 nm can increase the total thermal resistance of the contact by up to an order of magnitude, when compared to bare AgNWs, even with a larger contact area. On the other hand, the thermal boundary resistance for PVP/silver interfaces could be significantly lower than that between polymer-carbon nanotubes (CNTs). Analyses based on these understandings further show why AgNWs could be more effective nanofillers than CNTs.

In recent years, the development of nanopore-based membranes has revitalized the prospect of harvesting salinity gradient (blue) energy. In this study, we systematically analyze the energetic performance of nanopore-based power generation (NPG) at various process scales, beginning with a single nanopore, followed by a multipore membrane coupon, and ending with a full-scale system. We confirm the high power densities attainable by a single nanopore and demonstrate that, at the coupon scale and above, concentration polarization severely hinders the power density of NPG, revealing the common, yet significant, error in linearly extrapolating single-pore performance to multipore membranes. Through our consideration of concentration polarization, we also importantly show that the development of materials with exceptional nanopore properties provides limited enhancement of practical process performance. For a full-scale NPG membrane module, we find an inherent tradeoff between power density and thermodynamic energy efficiency, whereby achieving a high power density sacrifices the energy efficiency. Furthermore, we derive a simple expression for the theoretical maximum energy efficiency of NPG, showing it is solely related to the membrane selectivity (i.e., S2/2). Through this relation, it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e., S = 1), reinforcing our optimistic full-scale simulations which result in a (practical) maximum energy efficiency of 42%. Finally, we assess the net extractable energy of a full-scale NPG system which mixes river water and seawater by including the energy losses from pretreatment and pumping, revealing that the NPG process-both in its current state of development and in the case of highly optimistic performance with minimized external energy losses-is not viable for power generation.

Electrochemical capacitive deionization (CDI) is a promising technology for distributed and energy-efficient water desalination. The development of high-performance, low-cost capacitive electrodes is critical for enhancing CDI performance and scaling up its applications. Here, we report a novel design of highly compact, free-standing nanoporous carbon film electrodes for high-performance CDI. Such porous electrodes are fabricated by slip-roll compressing polypyrrole (PPy)-derived activated microporous carbon particles (PPy-AMC) with a small amount of polymer binder (5 wt%). The unique PPy-AMCs are synthesized from a rigid polymer precursor using a scalable carbonization-activation process, which is adopted in the manufacturing of commercial activated carbons. With small and uniform particle size, large specific surface area and short diffusion length, the PPy-AMC-based compact carbon electrodes offer very high salt adsorption capacity and enable very fast desalination, which significantly outperforms the state-of-the-art porous carbon-based CDI electrodes. This work provides an important strategy to design and fabricate compact yet porous carbon electrodes for efficient water desalinization and can be potentially extended to other applications such as energy storage and conversion.

Failure of membrane distillation (MD) due to pore wetting by amphiphilic molecules has recently received growing interests because it is a critical challenge to overcome for MD to be applicable for treating unconventional feed water. Recent MD studies using feed solutions containing surfactants have elucidated fundamental mechanism of wetting and generated practical solutions for wetting mitigation. However, what remains unclear is the impact of surfactant species on pore wetting kinetics. Based on a recently developed kinetic model for surfactant-induced pore wetting in MD, we hypothesize that the surface excess concentration of a surfactant is the most important surfactant property in affecting the pore wetting kinetics. In this study, we performed controlled MD wetting experiments using seven different types of surfactants and measured their respective breakthrough time as a quantitative metric for wetting kinetics. Our experiments reveal a good linear correlation between the surface excess concentration and breakthrough time for most but one tested surfactant. When surface excess concentration and diffusion coefficient are both considered, the model-simulated breakthrough time matches the experimentally measurement remarkably well for all tested surfactants.

Conventional capacitive deionization (CDI) with carbon electrodes is a desalination process based on the formation of electrical double layer. Intercalation capacitive deionization (ICDI), a category of CDI based on intercalation materials as electrodes, achieves desalination by inserting ions into the crystal lattice of the electrode when a voltage is applied. It has been proven numerically that a thermodynamically reversible CDI cycle always consumes electrical work that equals the Gibbs free energy of the separation. We conducted a thermodynamic analysis of a four-stage reversible cycle for both symmetric and asymmetric ICDI using Frumkin isotherm to describe the electrode-solution chemical equilibrium. We provided both analytical and numerical proof showing the electrical work to complete a four-stage ICDI cycle is exactly identical to the Gibbs free energy of separation. Our thermodynamic analysis also shows ICDI is typically more energy efficient than CDI if constant voltage charging and discharge are performed to complete the same separation.

Appendix CMembrane pore wetting is a unique and important technical challenge for membrane distillation (MD). While the general principle of pore wetting is well known, the detailed mechanism of pore wetting induced by surfactants that can actively adsorb onto membrane pore surface has not been theoretically elucidated. In this study, we developed a theoretical model, based on surfactant transport in a partially wetted membrane pore under the pseudo-steady state assumption, to quantify the kinetics of pore wetting. The theoretical model predicts several key dependences of wetting kinetics on operating parameters and solution properties, which are highly consistent with results from MD experiments using feed solution containing sodium dodecyl sulfate. It was found that kinetics of pore wetting is strongly dependent on vapor flux, surfactant concentration, but relatively independent of the transmembrane hydraulic pressure. The critical surfactant concentration below which pore wetting does not occur was also predicted by the wetting model. Finally, the impact of surfactant species on wetting kinetics was also discussed.

Enhancing the membrane water permeability without undermining selectivity has strong potential to reduce the cost of loose nanofiltration (LNF) which attracts growing interest in water and wastewat...

Pore wetting is a unique and important technical challenge that can lead to process failure in membrane distillation (MD) using hydrophobic membranes. While it is well known that both low-surface-tension and water miscible liquids, such as alcohols, and amphiphilic molecules, such as surfactants, are effective wetting agents, the detailed mechanisms for these agents to induce pore wetting remain unclear. In particular, the role of surface adsorption in surfactant-induced wetting remains to be elucidated. This study provides fundamental insights to understanding the mechanism of pore wetting induced by these two different wetting agents. Using a recently developed wetting monitoring technique based on single-frequency transmembrane impedance, we experimentally probe the kinetics of wetting frontier propagation. We demonstrate that ethanol-induced wetting is instantaneous whereas surfactant-induced wetting is dynamic with its kinetic rate dependent on several critical factors. We also develop a theoretical model, based on the assumption of quasi-equilibrium adsorption, to successfully explain the important features experimentally observed in surfactant-induced wetting. Specifically, it was found that the kinetics of surfactant-induced wetting strongly depends on the vapor flux and the bulk concentration of surfactants in the feed solution, but surprisingly not on the transmembrane hydraulic pressure difference. The results from our study also suggest that while the presence of surfactants promotes wetting, adsorption of surfactants onto pore surface actually deters pore wetting instead of promoting it by rendering the surface hydrophilic.

Nanofiltration (NF) membranes with ultrahigh permeance and high rejection are highly beneficial for efficient desalination and wastewater treatment. Improving water permeance while maintaining the high rejection of state-of-the-art thin film composite (TFC) NF membranes remains a great challenge. Herein, we report the fabrication of a TFC NF membrane with a crumpled polyamide (PA) layer via interfacial polymerization on a single-walled carbon nanotubes/polyether sulfone composite support loaded with nanoparticles as a sacrificial templating material, using metal-organic framework nanoparticles (ZIF-8) as an example. The nanoparticles, which can be removed by water dissolution after interfacial polymerization, facilitate the formation of a rough PA active layer with crumpled nanostructure. The NF membrane obtained thereby exhibits high permeance up to 53.5 l m−2h−1 bar−1 with a rejection above 95% for Na2SO4, yielding an overall desalination performance superior to state-of-the-art NF membranes reported so far. Our work provides a simple avenue to fabricate advanced PA NF membranes with outstanding performance., Nanofiltration membranes are important for water desalination technologies, but designing membranes that achieve both high permeance and high salt rejection remains challenging. Here, the authors use sacrificial nanoparticles in the membrane fabrication process, leading to crumpled structures with ultrahigh permeance.

It is a widely accepted principle that a thermodynamically reversible desalination process should consume the Gibbs free energy of separation. This principle has been shown in reverse osmosis and has important practical implications in reducing its energy consumption. Capacitive deionization (CDI) with carbon electrodes, a desalination process based on electrical double layer (EDL) formation, should also follow such a principle when it operates in a thermodynamically reversible way. Inspired by a previous thermodynamic analysis on a three-stage reversible CDI process using the Gouy-Chapman-Stern model, we conducted a thermodynamic analysis of a four-stage reversible CDI cycle using the modified Donnan model. This analysis better reflects the cyclic nature of practical CDI operations and account for the significant EDL overlap in nanosized micropores of realistic CDI electrodes. Our analysis of CDI cycles with different separations and final discharge voltages shows that the electrical work to complete a four-stage cycles is numerically exactly identical to the Gibbs free energy of separation, as long as the cycle is operated in a thermodynamically reversible manner.

Significant progress has been made over recent years in capacitive deionization (CDI) to develop novel system configurations, predictive theoretical models, and high-performance electrode materials. To bring CDI to large scale practical applications, it is important to quantitatively understand the intrinsic tradeoff between kinetic and energetic efficiencies, or the relationship between energy consumption and the mass transfer rate. In this study, we employed both experimental and modeling approaches to systematically investigate the tradeoff between kinetic and energetic efficiencies in membrane CDI (MCDI). Specifically, we assessed the relationship between the average salt adsorption rate and specific energy consumptions from MCDI experiments with different applied current densities but a constant effluent salinity. We investigated the impacts of feed salinity, diluted water salinity, diluted water volume per charging cycle, and electrode materials on the kinetics-energetics tradeoff. We also demonstrate how this tradeoff can be employed to optimize the design and operation of CDI systems and compare the performance of different electrode materials and CDI systems.

Fouling on submerged surfaces is a major limiting factor for membranes, heat exchangers, and marine vessels as it induces mass and heat transfer resistances that increase operating costs and lead to system failures. While the role of surface roughness on fouling has been extensively studied, the specific effect of surface roughness on fouling is debated in literature. In this study, we employed force spectroscopy based on atomic force microscopy with two model colloidal probes to elucidate the role of surface roughness on foulant-surface interactions. Specifically, we quantified the strength and characteristic lengths of the interactions between the colloidal probes and hydrophilic and hydrophobic surfaces with and without surface texture. We found that hydrophilic surfaces are generally less prone to foulant adhesion than hydrophobic surfaces and that increasing roughness of a hydrophilic surface mitigates foulant adhesion. In comparison, we found that increased roughness of a hydrophobic surface increases the attractive foulant-surface interaction, and thus, its fouling propensity. Based on the results from this study, the implications for developing surfaces with fouling resistance are also examined.

In this study, we developed composite membranes with a hydrophobic substrate and a hydrophilic top surface using electrospinning to mitigate oil fouling in membrane distillation (MD). The electrospinning approach can be universally applied to any hydrophobic membrane substrate and to ensure the non-wetting condition of the substrate due to the electrospun fibrous structure. Using this approach, polytetrafluoroethylene (PTFE) hydrophobic substrate was coated with two different hydrophilic fibrous networks, including a cellulose acetate (CA) fibrous network and a nanocomposite fibrous network comprising CA and silica nanoparticles (SiNPs). We characterized the pristine and the modified membranes using contact angle measurements and tensiometer-based oil probe force spectroscopy, and tested the anti-fouling performance of the different membranes in MD experiments using a saline crude-oil emulsion as the feed solution. While both coatings offered significant improvement in oil fouling resistance compared to the substrate PTFE membrane, the nanocomposite CA-SiNPs fibrous coating outperformed the CA coating in terms of hydrophilicity, oil adhesion resistance, and anti-oil-fouling performance in MD experiments.

The water-energy nexus has motivated the quest for new membrane-based technologies that target potable water and energy production. To this end, membrane distillation (MD), forward osmosis (FO) and pressure retarded osmosis (PRO) provide alternative means for the sustainable production of freshwater and electricity from feed water with high fouling potential such as wastewater. MD is a thermally driven process that can utilize low grade (latent) heat sources, while FO and PRO harness osmotic gradients as the driving force. High rejection of contaminants, compact modular design and low fouling propensity make these membrane technologies suitable for treating different types of wastewater. However, the application of feed solutions with high loads of organic matter and bacteria prompts the development of microbial fouling (biofouling), which significantly reduces system performance. Therefore, mitigating biofouling by minimizing bacterial attachment and enhancing the biofilm cleaning efficiency is imperative. We stress that in-depth exploration of the impacts imposed by biofilm in MD, FO and PRO systems is essential before developing new approaches for biofouling mitigation. This comprehensive review compiles the driving forces of these non-pressurized membrane systems, while focusing on the current knowledge regarding the various impacts of biofouling. Moreover, we highlight current and future research directions that focus on the development of new approaches to minimize MD, FO and PRO biofouling.

Pore wetting is an important failure mechanism unique to membrane distillation (MD). The existing approach of wetting detection based on distillate conductivity works only when a membrane has failed in the presence of fully wicked-through pores. In this study, we develop a novel and simple method, based on measurement of cross-membrane impedance, for monitoring the dynamics of membrane pore wetting and enabling early detection of imminent wetting-based membrane failure. Using Triton X-100 to induce pore wetting in direct contact MD experiments, we demonstrated the rapid response of single-frequency impedance to partial pore wetting long before any change in distillate conductivity was observed. We also conducted an MD experiment using alternating feed solutions with and without surfactants to elucidate the mechanism of surfactant-induced pore wetting. Our experimental observations suggest that surfactant-induced pore wetting occurred via progressive movement of the water–air interface and that adsorption ...

Membrane distillation (MD) is a promising desalination technology that is capable of utilizing low-grade thermal energy to treat highly saline feed water. Conventional hydrophobic MD membranes limit the application of MD to desalination of relatively clean water without amphiphilic contaminants, as those amphiphilic constituents promote wetting of MD membrane and failure of the MD process. Here, we report a facile approach to fabricate superamphiphobic MD membranes with anti-surfactant-wetting property based on coaxial electrospinning. Silica nanoparticles were used as the sheath solution to create electrospun fibers with nanoscale roughness on individual fibers. After surface fluorination, the fabricated membrane exhibited superamphiphobicity as reflected by very high sessile drop contact angles with both water and oil. Robust MD performance in the presence of surfactants was observed with the superamphiphobic membrane, but not with commercial hydrophobic membranes or amphiphobic membrane without local nanoscale roughness.

Membrane distillation (MD) has been identified as a promising technology to desalinate the hypersaline wastewaters from fracking and other industries. However, conventional hydrophobic MD membranes are highly susceptible to fouling and/or wetting by the hydrophobic and/or amphiphilic constituents in these wastewaters of complex compositions. This study systematically investigates the impact of the surface wetting properties on the membrane wetting and/or fouling behaviors in MD. Specifically, we compare the wetting and fouling resistance of three types of membranes of different wetting properties, including hydrophobic and omniphobic membranes as well as composite membranes with a hydrophobic substrate and a superhydrophilic top surface. We challenged the MD membranes with hypersaline feed solutions that contained a relatively high concentration of crude oil with and without added synthetic surfactants, Triton X-100. We found that the composite membranes with superhydrophilic top surface were robustly resistant to oil fouling in the absence of Triton X-100, but were subject to pore wetting in the presence of Triton X-100. On the other hand, the omniphobic membranes were easily fouled by oil-in-water emulsion without Triton X-100, but successfully sustained stable MD performance with Triton X-100 stabilized oil-in-water emulsion as the feed solution. In contrast, the conventional hydrophobic membranes failed readily regardless whether Triton X-100 was present, although via different mechanisms. These findings are corroborated by contact angle measures as well as oil-probe force spectroscopy. This study provides a holistic picture regarding how a hydrophobic membrane fails in MD and how we can leverage membranes with special wettability to prevent membrane failure in MD operations.

Recent progress in developing anti-fouling membranes suggests that integrating low-surface-energy functional groups into a hydrophilic matrix can generate a robust fouling resistant surface that not only mitigates the attachment, but also facilitates the detachment, of hydrophobic foulants. Excellent anti-fouling performance with such surfaces with heterogeneous surface energy has been reported in pressurized membrane processes. However, the impact of low-surface-energy functional groups on the membrane fouling resistance in membrane distillation (MD) has not been investigated and is the goal of the current study. In this study, we compared the fouling behaviors between a reference hydrophobic polyvinylidene fluoride (PVDF) membrane and three composite membranes with chitosan based hydrogel surface but different amount of perfluoroalkyl functional groups. We first fabricated and characterized the three composite membranes, then challenged them in MD experiments with a crude-oil-emulsion as the feed solutions, and finally conducted force spectroscopy to probe the interaction between an oil droplet and the three composite membranes as well as the reference PVDF membrane. The results from MD fouling tests and force spectroscopy suggest that incorporating perfluoroalkyl functional groups into the hydrogel matrix did significantly improve the anti-fouling performance and reduce oil adhesion onto the membrane surface as long as they were not excessive on the surface.

We review recent advances, limitations, and prospects of solar-thermal desalination for sustainable, low-cost water production., Solar-thermal desalination (STD) is a potentially low-cost, sustainable approach for providing high-quality fresh water in the absence of water and energy infrastructures. Despite recent efforts to advance STD by improving heat-absorbing materials and system designs, the best strategies for maximizing STD performance remain uncertain. To address this problem, we identify three major steps in distillation-based STD: (i) light-to-heat energy conversion, (ii) thermal vapor generation, and (iii) conversion of vapor to water via condensation. Using specific water productivity as a quantitative metric for energy efficiency, we show that efficient recovery of the latent heat of condensation is critical for STD performance enhancement, because solar vapor generation has already been pushed toward its performance limit. We also demonstrate that STD cannot compete with photovoltaic reverse osmosis desalination in energy efficiency. We conclude by emphasizing the importance of factors other than energy efficiency, including cost, ease of maintenance, and applicability to hypersaline waters.

This review introduces the development history of the widely used NF model, i.e., the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE), from the emergence of its predecessors to its current form. We present the details of DSPM-DE with assumptions and equations that account for each mechanism. Model inputs and outputs, as well as the key parameters and the experimental procedures for determining these parameters are also explained. Furthermore, the DSPM-DE is applied to investigate NF performance under various conditions. Specifically, the DSPM-DE is employed to provide a mechanistic interpretation of the well-known selectivity-permeability tradeoff that is more typically applied to describe the behavior of non-porous membrane. Based on the simulations using the DSPM-DE, we also discuss strategies on enhancing NF performance via tuning membrane properties. NF membranes with thin active layer and small nanopores that are strongly hydrophilic and charged are beneficial to achieving a high perm-selectivity. Analysis is also performed on the separation of divalent and monovalent cations using DSPM-DE. Finally, we discuss the uncertainties of model parameters and their impacts on the model prediction. We also discuss the validity of fundamental model assumptions and the prospects on potential improvements of NF modeling to enhance the utility in predicting NF performance. Machine learning-based models, once trained with a large set of data, are likely more powerful than DSPE-DE or any physical NF model in future application of performance prediction.

Wetting induced by salt-scaling and surfactants is the Achilles heel of membrane distillation, especially for concentration of high salinity wastewater. Herein, we rationally developed a membrane with robust wetting resistance by integrating superhydrophobic-omniphobic surface and anti-deformable pores into one system. The membrane was first developed by electrospinning, which was then modified with surface roughness, and followed by coating of polydimethylsiloxane to weld the intersecting fibers and fluoroalkylsilane to lower the membrane surface energy. The product exhibits excellent wetting resistance when concentrating the high salinity NaCl solution from 20 to 38 wt% (saturation condition), the simulated reverse osmosis concentrated water, the gypsum and the low-surface-tension high salinity wastewater. Moreover, the mechanism of membrane wetting resistance was also systematically discussed based on the experiment and computer simulation. It reveals that the superhydrophobic-omniphobic surface could stabilize the surface-bound air layer chemically, thus reducing the contact of crystals and surfactant with the membrane surface. Simultaneously, the anti-deformable pore also helps it overcome the asymmetrical hydraulic disturbance on enlarging membrane pore size, thus stabilizes the surface-bound air layer structurally. The presented development will provide a platform to understand and achieve wetting and scaling inhibition MD membrane for high salinity wastewater treatment.

We herein report a novel approach based on electrochemical impedance spectroscopy (EIS) for in-situ monitoring of the adsorption kinetics in preparing polyelectrolyte multilayer nanofiltration membranes using layer-by-layer (LbL) deposition. Unlike existing methods for monitoring adsorption kinetics, this new approach is non-destructive and applicable to various substrates (as it does not use the substrate as a sensor). The model nanofiltration membrane used in this study is prepared by alternate depositions of Poly(diallyldimethylammonium chloride) and Poly(sodium 4-styrenesulfonate) on a polyacrylonitrile ultrafiltration membrane as the support. In each deposition step, the EIS measurements yield two important parameters, including the interfacial layer solution resistance and the film resistance, for probing the extent of polyelectrolyte deposition and membrane performance. The extent of polyelectrolyte deposition as probed by the EIS measurements is well corroborated by independent measurements of the membrane surface potential and the nanofiltration performance including water permeability and Na2SO4 rejection. This EIS-based approach enables the optimization of membrane fabrication using LbL deposition by conveniently identifying the minimum deposition time required to attain surface saturation.

The efforts of developing anti-oil-fouling membranes have so far been focusing on tailoring membrane surface wetting properties, whereas the impacts of surface charge are often eclipsed. In this study, we investigated the impacts of surface charge and wetting property on oil fouling kinetics in membrane distillation (MD). Two composite membranes with in-air hydrophilic and underwater oleophobic surfaces, one of positive charge and the other of negative charge, were fabricated by modifying a hydrophobic polyvinylidene fluoride (PVDF) membrane with hydrophilic polyelectrolytes with different charges. The modified composite membranes were compared with the reference PVDF membrane for their contact angles, adhesion force curves, and fouling kinetics in MD processes. It was found that the negatively charged composite membrane performed the best in mitigating fouling by the negatively charged oil emulsion, followed by the positively charged composite membrane, with the pristine PVDF membrane being the most susceptible to oil fouling in MD experiments. The results from underwater oil CA measurements and oil probe force spectroscopy corroborated the results in the fouling experiments. The impact of surface charges on oil-membrane interaction and associated mechanism were also discussed.

The recent development of high performance thin film composite membrane has been a major technological thrust in the research of forward osmosis (FO). While most of the recently developed FO membranes are of flat sheet (FS) geometry, hollow fiber (HF) FO membranes have attracted significant attention in recent years due to their promising prospect in full-scale applications. Existing studies on HF FO membrane fabrication and characterization exclusively apply the mass transfer equations developed for FS membranes. Whether or not these mass transfer equations for FS membranes are applicable for membranes with HF geometry remains theoretically unclear. In this paper, accurate analytical equations are derived to describe mass transfer of water and solute across an HF membrane. These equations take into account the curvature effect of the HF membranes and thus have very different mathematical forms from those for FS membranes. A systematic comparison of the mass transfer equations between HF and FS membranes was also conducted using both simulated and experimentally measured flux data. The results from such a comparison suggest that the mass transfer equations for FS membranes are in general applicable for an HF geometry, which provides the theoretical basis for the application of the well-established FS mass transfer equations in characterizing HF membranes.

Although significant technological advances have been made in recent years on pressure retarded osmosis (PRO), its practical viability remains unclear as few studies have been conducted at an integrated system level to quantify the potential of net energy output. In this study, we develop a framework to assess the net energy output of a PRO system by first quantifying the gross energy output via solving the mass transfer equations for a full-scale PRO module, and then incorporating the major energy losses from pretreatment, flow circulation, and inefficient energy recovery. We also propose a novel concept called net membrane power density that is strongly relevant to the capital cost of a PRO system. Finally, we describe an approach, based on the quantifiable specific net energy and net membrane power density, for assessing the economic viability of a PRO system. Albeit using seawater/river water PRO as the context for illustrating our approach, the assessment framework developed is universally applicable to PRO systems with any solution pairing. The results from this study clearly show the impacts of various parameters on the practical performance of a PRO system, thereby providing important guidance to the improvement of its design and operation.

Interfacial materials with special wettability have become a burgeoning research area in materials science in the past decade. The unique surface properties of materials and interfaces generated by biomimetic approaches can be leveraged to develop effective solutions to challenging environmental problems. This critical review presents the concept, mechanisms, and fabrication techniques of interfacial materials with special wettability, and assesses the environmental applications of these materials for oil-water separation, membrane-based water purification and desalination, biofouling control, high performance vapor condensation, and atmospheric water collection. We also highlight the most promising properties of interfacial materials with special wettability that enable innovative environmental applications and discuss the practical challenges for large-scale implementation of these novel materials.

Bipolar membrane electrodialysis (BMED) is a promising, cost-effective technology for treating highly saline wastewater toward zero-liquid-discharge. Instead of producing low-value minerals of sodium chloride and sulfate, BMED can produce a valuable concentrated acid and base for industrial reuse. In this study, we first develop a numerical model to describe the mass transfer and energy consumption of a BMED process. We then present a systematic framework based on performance tradeoff curves for evaluating the performance of a BMED process that captures the inherent tradeoff between energy efficiency and the kinetic rate of the process. Such a performance tradeoff provides the technical basis for comparison between different BMED operations and systems and for technoeconomic analysis. Using such a framework and Na2SO4 as a model feed solution, we evaluate the impacts of initial feed concentration, target concentration of NaOH, and the ratio between the initial volumes of feed and seed base solutions on the BMED performance. Finally, we also demonstrate that a novel 3D junction bipolar membrane can enhance the performance of the BMED process.

Membrane capacitive deionization (MCDI) can be typically operated with constant voltage (CV) or constant current (CC) mode in the charging stage. While a series of previous studies have compared both charging modes to identify the better operating mode, neither their performance evaluation protocols were consistent, nor did their conclusions unanimously converge. This study presents a new framework to evaluate and compare MCDI performance, considering the kinetic efficiency, the energetic efficiency, and the intrinsic trade-off between the two. A key prerequisite for making rational comparison of performance between MCDI operations is that the operations being compared should all result in the same target adsorption. With this key prerequisite and the new evaluation framework based on the trade-off curve between kinetic and energetic efficiencies, our experimental assessment and theoretical analysis suggest that whether CC or CV charging is more efficient is strongly dependent on the target adsorption and, to a less extent, on the kinetic rate of charging. However, the advantage in energy or kinetic efficiency of one charging mode over that of the other is relatively small in all cases. Our study also reveals that, for a given MCDI system, there exist regimes of target adsorptions and kinetic rates that can only be achieved by either CC or CV charging, or even regimes that can be achieved by neither charging mode. In summary, this study revises our current understanding regarding the comparison of the two typical charging modes in MCDI, and introduces a new framework for comparing the performance of different MCDI and CDI operations.

A novel Janus membrane integrating an omniphobic substrate and an in-air hydrophilic, underwater superoleophobic skin layer was developed to enable membrane distillation (MD) to desalinate hypersaline brine with both hydrophobic foulants and amphiphilic wetting agents. Engineered to overcome the limitations of existing MD membranes, the Janus membrane has been shown to exhibit novel wetting properties unobserved in any existing membrane, including hydrophobic membranes, omniphobic membranes, and hydrophobic membranes with a hydrophilic surface coating. Being simultaneously resistant to both membrane fouling and wetting, a Janus membrane can sustain stable MD performance even with challenging feed waters and can thus potentially transform MD to be a viable technology for desalinating hypersaline wastewater with complex compositions using low-grade-thermal energy.

In this study, we fabricated a composite membrane for membrane distillation (MD) by modifying a commercial hydrophobic polyvinylidene fluoride (PVDF) membrane with a nanocomposite coating comprising silica nanoparticles, chitosan hydrogel and fluoro-polymer. The composite membrane exhibits asymmetric wettability, with the modified surface being in-air hydrophilic and underwater oleophobic, and the unmodified surface remaining hydrophobic. By comparing the performance of the composite membrane and the pristine PVDF membrane in direct contact MD experiments using a saline emulsion with 1000 ppm crude oil (in water), we showed that the fabricated composite membrane was significantly more resistant to oil fouling compared to the pristine hydrophobic PVDF membrane. Force spectroscopy was conducted for the interaction between an oil droplet and the membrane surface using a force tensiometer. The difference between the composite membrane and the pristine PVDF membrane in their interaction with an oil droplet served to explain the difference in the fouling propensities between these two membranes observed in MD experiments. The results from this study suggest that underwater oleophobic coating can effectively mitigate oil fouling in MD operations, and that the fabricated composite membrane with asymmetric wettability can enable MD to desalinate hypersaline wastewater with high concentrations of hydrophobic contaminants.