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Polymer membranes have been used extensively for Angstrom-scale separation of solutes and molecules. However, the pore size of most polymer membranes has been considered an intrinsic membrane property that cannot be adjusted in operation by applied stimuli. In this work, we show that the pore size of an electrically conductive polyamide membrane can be modulated by an applied voltage in the presence of electrolyte via a mechanism called electrically induced osmotic swelling. Under applied voltage, the highly charged polyamide layer concentrates counter ions in the polymer network via Donnan equilibrium and creates a sizeable osmotic pressure to enlarge the free volume and the effective pore size. The relation between membrane potential and pore size can be quantitatively described using the extended Flory-Rehner theory with Donnan equilibrium. The ability to regulate pore size via applied voltage enables operando modulation of precise molecular separation in-situ. This study demonstrates the amazing capability of electro-regulation of membrane pore size at the Angstrom scale and unveils an important but previously overlooked mechanism of membrane-water-solute interactions.

Developing engineered surfaces with scaling resistance is an effective means to inhibit surface-mediated mineral scaling in various industries including desalination. However, contrasting results have been reported on the relationship between scaling potential and surface hydrophilicity. In this study, we combine a theoretical analysis with experimental investigation to clarify the effect of surface wetting property on heterogeneous gypsum (CaSO

Enhancing the water permeance while maintaining the solute rejection of a nanofiltration (NF) membrane can potentially result in significant cost-reduction for NF-a membrane process that excels in several unique environmental applications of growing interests. In this work, we demonstrate for the first time that intercalation of surfactant self-assemblies in the polyelectrolyte multilayer (PEM) can lead to significant performance enhancement of salt-rejecting dense NF membranes fabricated using layer-by-layer assembly of polyelectrolytes. Specifically, the intercalation of sodium dodecyl sulfate (SDS) bilayers in a PEM comprising poly(diallyldimethylammonium chloride) (PDADMAC) and poly (sodium 4-styrenesulfonate) (PSS) resulted in a decrease in PEM thickness, increase in pore size, and a smoother and more hydrophilic surface. The water permeance of the resulting PEM NF membrane increased by 100% without compromising the rejection of Na2SO4. Experiments with a quartz crystal microbalance also provide direct evidence that the intercalation of the surfactants substantially reduces the subsequent adsorption of the polyelectrolytes of a similar charge. Based on its mechanism of performance enhancement, surfactant intercalation may become a universally applicable and highly cost-effective approach for dramatically enhancing the performance of PEM NF membranes.

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.

The development of membranes capable of precise solute-solute separation is still in its burgeoning stage without a standardized protocol for evaluating selectivity. Three types of membrane processes with different driving forces, including pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration, have been applied in this study to characterize solute-solute selectivity of a commercial nanofiltration membrane. Our results demonstrated that selectivity values measured using different methods, or even different conditions with the same method, are generally not comparable. The cross-method incomparability is true for both apparent selectivity, defined as the ratio between concentration-normalized fluxes, and the more intrinsic selectivity, defined as the ratio between the permeabilities of solutes through the active separation layer. The difference in selectivity measured using different methods possibly stems from the fundamental differences in the driving force of ion transport, the effect of water transport, and the interaction between cations and anions. We further demonstrated the difference in selectivity measured using feed solutions containing single-salt species and that containing mixed salts. A consistent protocol with standardized testing conditions to facilitate fair performance comparison between studies is proposed.

The safe and economical management of shale gas flowback and produced water (FPW) with the goal of zero-liquid discharge (ZLD) is of paramount significance to the sustainable development of the energy industry. This challenge is still widely impending, due to severe limitations related to the complexity of FPW streams and high costs associated with their treatment. A long-term feasible solution is represented by solar-driven interfacial evaporation (SIE), a low-cost and environmentally friendly desalination technology. Technical and economic analyses show that even in the Sichuan Basin, where solar intensity is low and the volume of FPW is large, 4000 m2 of solar still would be sufficient to accomplish ZLD with a capital cost lower than $ 1 m−3, significantly cheaper than traditional membrane-based and thermal-based technologies (with costs above $ 15 m−3). Beneficial products, including condensate water and crystalline salts, may also be effectively recovered in this process, although design improvements are needed in this area. This study also discusses possible solutions to address the passage of volatile organic compounds into the effluent and to achieve the smart recovery of strategic resources, such as lithium and rare earth elements. The major current challenges of solar-driven evaporation for the beneficial management of FPW are scaling and fouling, cost-effective latent heat recovery, the scalability of the systems, and efficient water production and salts harvesting. These issues require ad hoc research efforts, analyses, and pilot testing.

Membrane distillation (MD) has potential to become a competitive technology for managing hypersaline brine but not until the critical challenge of mineral scaling is addressed. The state-of-the-art approach for mitigating mineral scaling in MD involves the use of superhydrophobic membranes that are difficult to fabricate and are commercially unavailable. This study explores a novel operational strategy, namely, negative pressure direct contact membrane distillation (NP-DCMD) that can minimize mineral scaling with commercially available hydrophobic membranes and at the same time enhance the water vapor flux substantially. By applying a negative gauge pressure on the feed stream, NP-DCMD achieved prolonged resistance to CaSO4 scaling and a dramatic vapor flux enhancement up to 62%. The exceptional scaling resistance is attributable to the formation of a concave liquid–gas under a negative pressure that changes the position of the water–air interface to hinder interfacial nucleation and crystal growth. The substantial flux enhancement is caused by the reduced molecular diffusion resistance within the pores and the enhanced heat transfer kinetics across the boundary layer in NP-DCMD. Achieving substantial performance improvement in both the scaling resistance and vapor flux with commercial membranes, NP-DCMD is a significant innovation with vast potential for practical adoption due to its simplicity and effectiveness.

Recovering nitrogen from source-separated urine is an important part of the sustainable nitrogen management. A novel bipolar membrane electrodialysis with membrane contactor (BMED-MC) process is demonstrated here for efficient recovery of ammonia from synthetic source-separated urine (∼3772 mg N L-1). In a BMED-MC process, electrically driven water dissociation in a bipolar membrane simultaneously increases the pH of the urine stream and produces an acid stream for ammonia stripping. With the increased pH of urine, ammonia transports across the gas-permeable membrane in the membrane contactor and is recovered by the acid stream as ammonium sulfate that can be directly used as fertilizer. Our results obtained using batch experiments demonstrate that the BMED-MC process can achieve 90% recovery. The average ammonia flux and the specific energy consumption can be regulated by varying the current density. At a current density of 20 mA cm-2, the energy required to achieve a 67.5% ammonia recovery in a 7 h batch mode is 92.8 MJ kg-1 N for a bench-scale system with one membrane stack and can approach 25.8 MJ kg-1 N for large-scale systems with multiple membrane stacks, with an average ammonia flux of 2.2 mol m-2 h-1. Modeling results show that a continuous BMED-MC process can achieve a 90% ammonia recovery with a lower energy consumption (i.e., 12.5 MJ kg-1 N). BMED-MC shows significant potential for ammonia recovery from source-separated urine as it is relatively energy-efficient and requires no external acid solution.

Although membrane distillation (MD) has been identified as a promising technology to treat hypersaline wastewaters, its practical applications face two prominent challenges: membrane wetting and fouling. Herein, we report a facile and scalable approach for fabricating a Janus MD membrane comprising a dense polyvinyl alcohol (PVA) surface layer and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate. By testing the Janus membrane in direct contact MD experiments using feeds containing a sodium dodecyl sulfate (SDS) surfactant or/and mineral oil, we demonstrated that the dense Janus membrane can simultaneously resist wetting and fouling. This method represents the simplest approach to date for fabricating MD membranes with simultaneous wetting and fouling resistance. Importantly, we also unveil the mechanism of wetting resistance by measuring the breakthrough pressure and surfactant permeation (through the PVA layer) and found that wetting resistance imparted by a dense hydrophilic layer is attributable to capillary pressure. This new insight will potentially change the paradigm of fabricating wetting-resistant membranes and enable robust applications of MD and other membrane contactor processes facing challenges of pore wetting or/and membrane fouling.

Mineral scaling constrains membrane distillation (MD) and limits its application in treating hypersaline wastewater. Addressing this challenge requires enhanced fundamental understanding of the scaling phenomenon. However, MD scaling with different types of scalants may have distinctive mechanisms and consequences which have not been systematically investigated in the literature. In this work, we compared gypsum and silica scaling in MD and demonstrated that gypsum scaling caused earlier water flux decline and induced membrane wetting that was not observed in silica scaling. Microscopic imaging and elemental mapping revealed contrasting scale morphology and distribution for gypsum and silica, respectively. Notably, while gypsum crystals grew both on the membrane surface and deep in the membrane matrix, silica only formed on the membrane surface in the form of a relatively thin film composed of connected submicrometer silica particles. We attribute the intrusion of gypsum into membrane pores to the crystallization pressure as a result of rapid, oriented crystal growth, which leads to pore deformation and the subsequent membrane wetting. In contrast, the silica scale layer was formed via polymerization of silicic acid and gelation of silica particles, which were less intrusive and had a milder effect on membrane pore structure. This hypothesis was supported by the result of tensile testing, which showed that the MD membrane was significantly weakened by gypsum scaling. The fact that different scaling mechanisms could yield different consequences on membrane performance provides valuable insights for the future development of cost-effective strategies for scaling control.

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.

Selective removal or enrichment of targeted solutes including micropollutants, valuable elements, and mineral scalants from complex aqueous matrices is both challenging and pivotal to the success of water purification and resource recovery from unconventional water resources. Membrane separation with precision at the subnanometer or even subangstrom scale is of paramount importance to address those challenges via enabling "fit-for-purpose" water and wastewater treatment. So far, researchers have attempted to develop novel membrane materials with precise and tailored selectivity by tuning membrane structure and chemistry. In this critical review, we first present the environmental challenges and opportunities that necessitate improved solute-solute selectivity in membrane separation. We then discuss the mechanisms and desired membrane properties required for better membrane selectivity. On the basis of the most recent progress reported in the literature, we examine the key principles of material design and fabrication, which create membranes with enhanced and more targeted selectivity. We highlight the important roles of surface engineering, nanotechnology, and molecular-level design in improving membrane selectivity. Finally, we discuss the challenges and prospects of highly selective NF membranes for practical environmental applications, identifying knowledge gaps that will guide future research to promote environmental sustainability through more precise and tunable membrane separation.

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.

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.

Desalination has become an essential toolset to combat the worsening water stress resulting from population and industrial growth and exacerbated by climate change. Various technologies have been developed to desalinate feedwater with a wide spectrum of salinity. While energy consumption is an important consideration in many desalination studies, it is challenging to make (intuitive) sense of energy efficiency due to the different mechanisms of various desalination processes and the very different separations achieved. This perspective aims to provide an intuitive, thermodynamics-based interpretation of energy efficiency by illustrating how energy consumption breaks down into minimum energy of separation and the irreversible energy dissipation. The energy efficiencies of different desalination processes are summarized and rationalized based on their working mechanisms. Notably, a new concept called the

We report in this study a scalable and controllable approach for fabricating robust and high-performance superhydrophobic membranes for membrane distillation (MD). This novel approach combines electro-co-spinning/spraying (ES2) with chemical vapor welding and enables the formation of robust superhydrophobic (r-SH) membranes that are mechanically strong, highly porous, and robustly superhydrophobic. Compared with superhydrophobic membranes obtained using surface deposition of fluorinated nanoparticles, the r-SH membranes have more robust wetting properties and higher vapor permeability in MD. MD scaling experiments with sodium chloride and gypsum show that the r-SH membrane is highly effective in mitigating mineral scaling. Finally, we also discuss the mechanism of scaling resistance enabled by superhydrophobic membranes with a highlight on the roles of the surface-bound air layer in reducing the crystal-membrane contact area, nucleation propensity, and ion-membrane contact time.

Capacitive deionization (CDI) is an emerging technology capable of selective removal of ions from water. While many studies have reported chemically tailored electrodes for selective ion removal, the selective removal of divalent cations (i.e., hardness) over monovalent cations can simply be achieved using membrane CDI (MCDI) equipped with ion exchange membranes (IEMs). In this study, we use both experimental and modeling approaches to systematically investigate the selective removal of Ca2+ over Na+. Specifically, the impacts of current density, hydraulic retention time, and feed composition on the selectivity of Ca2+ over Na+ were investigated. The results from our study suggest a universal correlation between the ratio of molar fluxes and the ratio of spacer channel ion concentrations, regardless of operating conditions and feed composition. Our analysis also reveals inherent and universal trade-off relationships between selectivity and the Ca2+ removal rate and between selectivity and the degree of Ca2+ removal. This fundamental understanding of the mechanism of selective ion removal in MCDI can also be applied to flow-electrode CDI processes that employ IEMs.

Capacitive deionization (CDI) as a class of electrochemical desalination has attracted fast-growing research interest in recent years. A significant part of this growing interest is arguably attributable to the premise that CDI is energy efficient and has the potential to outcompete other conventional desalination technologies. In this review, systematic evaluation of literature data reveals that while the absolute energy consumption of CDI is in general low, most existing CDI systems achieve limited energy efficiency from a thermodynamic perspective. We also analyze the causes for the relatively low energy efficiency and discuss factors that may lead to enhanced energy efficiency for CDI.

Optimizing system design and operation of reverse osmosis (RO) systems requires an in-depth comprehension of the intrinsic tradeoff between RO mass transfer kinetics and energetics. In this study, we demonstrate that this critical trade-off can be quantified using the relationship between the average water flux and the specific energy consumption (SEC). We derive analytical expressions to quantify the average water flux and SEC for single stage, two stage, and closed circuit RO processes. These analytical expressions are useful for system design and operation optimization as they facilitate direct comparison of the kinetic and energetic efficiencies between RO processes with different operation conditions and system configurations. Finally, we compare the kinetics and energetics of the three system configurations using these analytical expressions and discuss their relative advantages and disadvantages in RO desalination.

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.

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.

Reverse osmosis (RO), currently the most energy efficient desalination process, is inherently more energy intensive compared to conventional fresh water treatment technologies, as it is constrained by the thermodynamics of separation of saline solutions. Therefore, pushing the energy consumption towards the thermodynamic limit of separation would lead to significant long-term savings in energy cost. In this work, we quantitatively demonstrate the potential of energy reduction for RO desalination using staged operations with both multi-stage direct pass and closed-circuit configurations. We relate the minimum specific energy of desalination (i.e., the minimum energy required to generate a unit volume of permeate water) to the number of stages in each configuration, and elucidate the fundamental difference between the two configurations. Our analysis suggests that although it is theoretically impossible to reach the thermodynamic minimum energy of separation with closed-circuit RO, this configuration is robust and much more practical to implement than the multi-stage direct pass RO.

Forward osmosis (FO) has been extensively investigated in the past decade. Despite significant advancements in our understanding of the FO process, questions and challenges remain regarding the energy efficiency and current state of the technology. Here, we critically review several key aspects of the FO process, focusing on energy efficiency, membrane properties, draw solutes, fouling reversibility, and effective applications of this emerging technology. We analyze the energy efficiency of the process, disprove the common misguided notion that FO is a low energy process, and highlight the potential use of low-cost energy sources. We address the key necessary membrane properties for FO, stressing the importance of the structural parameter, reverse solute flux selectivity, and the constraints imposed by the permeability–selectivity tradeoff. We then dispel the notion that draw solution regeneration can use negligible energy, highlighting the beneficial qualities of small inorganic and thermolytic salts as draw solutes. We further discuss the fouling propensity of FO, emphasizing the fouling reversibility of FO compared to reverse osmosis (RO) and the prospects of FO in treating high fouling potential feed waters. Lastly, we discuss applications where FO outperforms other desalination technologies and emphasize that the FO process is not intended to replace RO, but rather is to be used to process feed waters that cannot be treated by RO.

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.

We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal relative flow rate that maximizes the overall energy efficiency of the PRO-MD system for given working temperatures and concentration. In the case of unlimited mass and heat transfer kinetics, the energy efficiency of the system can be analytically determined based on thermodynamics. Our assessment indicates that the hybrid PRO-MD system can theoretically achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and a working solution of 1.0 M NaCl. When mass and heat transfer kinetics are limited, conditions that more closely represent actual operations, the practical energy efficiency will be lower than the theoretically achievable efficiency. In such practical operations, utilizing a higher working concentration will yield greater energy efficiency. Overall, our study demonstrates the theoretical viability of the PRO-MD system and identifies the key factors for performance optimization.

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.

Theoretical and experimental approaches were employed to study the effect of aggregation on the affinity between nanoparticles (NPs) and a flat surface that is quantified by the attachment efficiency. Computer simulations were used to generate virtual aggregates formed via either diffusion limited cluster aggregation or reaction limited cluster aggregation. The colloidal interactions between the simulated aggregates and a flat surface were evaluated based on the surface element integration approach. It was found that the strength of colloidal interaction for the aggregated NPs was on the same order of magnitude as those for the primary particles and was significantly weaker than that for an equivalent sphere defined by the gyration radius of the aggregate. The results from the deposition experiments using quartz crystal microbalance suggested that the attachment efficiencies (unfavorable deposition) for aggregated NPs were higher at the initial stage but later became similar to that of the primary NPs when equilibrium deposition was reached. The high initial affinity was postulated to be attributable to secondary minimum deposition. These results suggest that it is the size of the primary particles, not that of the aggregates, that determines the strength of the colloidal interaction between the aggregate and an environmental surface.

An extension of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was developed to model the potential energy of interaction between a soft particle (particle coated with either uncharged polymer or polyelectrolyte) and a rigid flat surface prior to the particle-surface contact. An uncharged polymer is predicted to reduce the barrier of interaction energy curve by shifting the contact frontier away from the particle core and as a result increases the affinity between the coated particle and the uncoated surface. For particles coated with polyelectrolyte, the particle–surface interaction is primarily regulated by the thickness and charge density of the polyelectrolyte layer. For either type of coating, increasing ionic strength is predicted to reduce the barrier of interaction potential and thus increase the affinity between the particle and the surface. A more comprehensive model was also established for a polyelectrolyte-coated particle of charged core surface, in which the importance of segment density of the coating layer was demonstrated. Limitations of these models were discussed especially for particles of very low segment density to which the post-contact interaction would become important.

Nanomaterials such as fullerene C60, carbon nanotubes (CNTs), and other fullerenes show unique electrical, chemical, mechanical, and thermal properties that are not well understood in the context of the environmental behavior of this class of carbon-based materials. In this study, aqueous suspensions of three fullerenes nanoparticles, C60, single-wall (SW) and multi-wall (MW) CNTs were prepared by sonication and tested for reactive oxygen species (ROS) production and inactivation of Vibrio fischeri, a gram-negative rod-shaped bacterium, under ultraviolet (UV)-A irradiation. We show that ROS production and microbial inactivation increases as colloidal aggregates of C60, SWCNT, and MWCNT are fractionated to enrich with smaller aggregates by progressive membrane filtration. As the quantity and influence of these more reactive fractions of the suspension may increase with time and/or as the result of fractionation processes in the laboratory or the environment, experiments evaluating photo-reactivity and toxicity endpoints must take into account the evolution and heterogeneity of nanoparticle aggregates in water.

Multi-wall carbon nanotube (MWCNT)/aromatic polyamide (PA) nanocomposite membranes were synthesized by a polymer grafting process. Surface morphology, roughness, and mechanical strength of the resultant nanocomposite membranes were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and micro-strain analysis, respectively. SEM and AFM images showed that MWCNTs were well dispersed in the PA matrix. Measurements of mechanical properties of this composite showed increasing membrane strength with increasing MWCNT content with monotonic increases in Young's modulus, toughness, and tensile strength. The addition of MWCNTs also improved the rejection of both salt and organic matter relative to the 10% PA membrane base case. The nanocomposite membrane synthesized with 15 mg/g MWCNT in a 10% PA casting solution rejected NaCl and humic acid by factors of 3.17 and 1.67 respectively relative to the PA membrane without MWCNTs, while membrane permeability decreased by 6.5%.

We present a hybrid osmotic heat engine (OHE) system that uses draw solutions with an organic solvent for enhanced thermal separation efficiency. The hybrid OHE system produces sustainable energy by combining pressure-retarded osmosis (PRO) as a power generation stage and membrane distillation (MD) utilizing low-grade heat as a separation stage. While previous OHE systems employed aqueous electrolyte draw solutions, using methanol as a solvent is advantageous because methanol is highly volatile and has a lower heat capacity and enthalpy of vaporization than water. Hence, the thermal separation efficiency of a draw solution with methanol would be higher than that of an aqueous draw solution. In this study, we evaluated the performance of LiCl-methanol as a potential draw solution for a PRO-MD hybrid OHE system. The membrane transport properties as well as performance with LiCl-methanol draw solution were evaluated using thin-film composite (TFC) PRO membranes and compared to the results obtained with a LiCl-water draw solution. Experimental PRO methanol flux and maximum projected power density of 47.1 L m(-2) h(-1) and 72.1 W m(-2), respectively, were achieved with a 3 M LiCl-methanol draw solution. The overall efficiency of the hybrid OHE system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages under conditions with and without heat recovery. The modeling results demonstrate higher OHE energy efficiency with the LiCl-methanol draw solution compared to that with the LiCl-water draw solution under practical operating conditions (i.e., heat recovery90%). We discuss the implications of the results for converting low-grade heat to power.

We investigate the performance of pressure retarded osmosis (PRO) at the module scale, accounting for the detrimental effects of reverse salt flux, internal concentration polarization, and external concentration polarization. Our analysis offers insights on optimization of three critical operation and design parameters--applied hydraulic pressure, initial feed flow rate fraction, and membrane area--to maximize the specific energy and power density extractable in the system. For co- and counter-current flow modules, we determine that appropriate selection of the membrane area is critical to obtain a high specific energy. Furthermore, we find that the optimal operating conditions in a realistic module can be reasonably approximated using established optima for an ideal system (i.e., an applied hydraulic pressure equal to approximately half the osmotic pressure difference and an initial feed flow rate fraction that provides equal amounts of feed and draw solutions). For a system in counter-current operation with a river water (0.015 M NaCl) and seawater (0.6 M NaCl) solution pairing, the maximum specific energy obtainable using performance properties of commercially available membranes was determined to be 0.147 kWh per m(3) of total mixed solution, which is 57% of the Gibbs free energy of mixing. Operating to obtain a high specific energy, however, results in very low power densities (less than 2 W/m(2)), indicating that the trade-off between power density and specific energy is an inherent challenge to full-scale PRO systems. Finally, we quantify additional losses and energetic costs in the PRO system, which further reduce the net specific energy and indicate serious challenges in extracting net energy in PRO with river water and seawater solution pairings.

The transport of uncoated silver nanoparticles (AgNPs) in a porous medium composed of silica glass beads modified with a partial coverage of iron oxide (hematite) was studied and compared to that in a porous medium composed of unmodified glass beads (GB). At a pH lower than the point of zero charge (PZC) of hematite, the affinity of AgNPs for a hematite-coated glass bead (FeO-GB) surface was significantly higher than that for an uncoated surface. There was a linear correlation between the average nanoparticle affinity for media composed of mixtures of FeO-GB and GB collectors and the relative composition of those media as quantified by the attachment efficiency over a range of mixing mass ratios of the two types of collectors, so that the average AgNPs affinity for these media is readily predicted from the mass (or surface) weighted average of affinities for each of the surface types. X-ray photoelectron spectroscopy (XPS) was used to quantify the composition of the collector surface as a basis for predicting the affinity between the nanoparticles for a heterogeneous collector surface. A correlation was also observed between the local abundances of AgNPs and FeO on the collector surface.