Menachem Elimelech

Brackish water desalination is imperative for meeting water demands in arid regions far from the seashore. Reverse osmosis, the leading desalination technology, removes nearly all calcium and magnesium ions, which are essential in drinking and irrigation water. Multistep process schemes combining reverse osmosis with ion-selective membrane processes can maintain or reintroduce these minerals without external chemical addition. Previous efforts emphasized membrane processes that retain multivalent ions, focusing primarily on nanofiltration and monovalent-selective electrodialysis. The potential of processes where monovalent ions are retained and divalent preferentially transported through the membrane has not been studied systematically. Here, we explored applying divalent-selective electrodialysis to transfer calcium and magnesium from the influent into the brackish water reverse osmosis permeate …

Pressure-driven membrane desalination (PMD), such as reverse osmosis or nanofiltration, is an energy-efficient technology that addresses water shortages by using saline waters to augment freshwater supplies. This Primer describes several key methodological aspects of PMD, including membrane fabrication, characterization and performance evaluation; system modelling; process configurations; and applications. Thin-film composite polyamide membranes represent the state of the art in reverse osmosis and nanofiltration membranes and are the focus of the membrane development discussion. First, thin-film composite polyamide membrane fabrication using interfacial polymerization and alternative methods is discussed, followed by an exploration of techniques for characterizing the morphological, structural and interfacial properties. Experimental procedures and model frameworks for evaluating membrane …

Membranes consist of pores and the walls of these pores are often charged. In contact with an aqueous solution, the pores fill with water and ions migrate from solution into the pores until chemical equilibrium is reached. The distribution of ions between outside and pore solution is governed by a balance of chemical potential, and the resulting model is called a Donnan theory, or Donnan equation. Including a partitioning coefficient that does not depend on salt concentration results in an extended Donnan equation `of the first kind'. Recently, an electrostatic model was proposed for ions in a pore based on the arrangement of ions around strands of polymer charge, including also ion activity coefficients in solution. That framework leads to an extended Donnan equation `of the second kind', which has extra factors depending on ion concentrations in the pores and salt concentration in solution. In the present work, we set up another Donnan model of the second kind by evaluating the Coulombic interactions of ions in a cylindrical pore, including the interaction of ions with the charged pore walls and between the ions. We assume that counterions are near the pore wall while coions distribute over the center region. Starting from a complete analysis, we arrive at an elegant expression for the chemical potential of ions in such a pore. This expression depends on coion concentration, pore size, and other geometrical factors, but there is no additional dependence on counterion concentration and charge density. This model predicts the Coulombic contribution to the chemical potential in the pore to be small, much smaller than predicted by the electrostatic …

Reverse osmosis (RO) stands as the state-of-the-art desalination technology, owing to its high energy efficiency and low cost. Further improvements in the performance of this technology require a fundamental understanding of transport mechanisms in RO membranes. For decades, the solution-diffusion model has been the prevalent approach to describe water transport mechanism in RO membranes. In this model, water first partitions into the membrane and then diffuses down a concentration gradient of water within the RO membrane. However, recent experimental and theoretical findings pose serious challenges to the fundamental assumptions of this theory. In this perspective, we discuss seven critical flaws in the solution-diffusion model and explain why the model fails to describe water transport in RO membranes. Instead, through our careful analyses, we determine that the pore-flow model, in which a …

During the final revision of this manuscript, a modification to Figure 1 B resulted in an error of a factor of 10 in this figure. Note that the corresponding number quoted in the text (percentage cost of water treatment of< 0.046%) remains correct. However, the numbers represented in the published figure are off by a factor of 10 (and therefore also are inconsistent with the number quoted in the text). The discussion and conclusions remain unchanged, as 0.46% and 0.046% are both still insignificant cost contributions; the correction just further reinforces the conclusions and is consistent with the original numbers quoted in the text. The corrected version of Figure 1 B appears below.

Thin-film composite (TFC) membranes, the backbone of modern reverse osmosis and nanofiltration, combine the high separation performance of a thin selective layer with the robust mechanical support. Previous studies have shown that heat released during interfacial polymerization (IP) can have a significant impact on the physical and chemical structure of the selective layer. In this study, we develop a multilayer transient heat conduction model to analyze how the thermal properties of the materials used in TFC fabrication impact interfacial temperature, focusing on support-free (SFIP), conventional (CIP), and interlayer-modulated IP (IMIP). Using a combination of analytic solutions and computational models, we demonstrate that the thermal effusivities of fluid and material layers can have a significant effect on the temporal evolution of interfacial temperature during IP. In CIP, we show that the presence of a …

In this perspective, we critically assess the competitiveness of salinity gradient energy (SGE) as a renewable energy source. While scientifically intriguing and gaining research attention, SGE encounters formidable challenges in competing with established renewable technologies, such as solar and wind energy. Even the most advanced SGE technology, pressure retarded osmosis, remains far from cost competitive, primarily because of its low energy density and conversion efficiency. These constraints appear to be fundamental, unlikely to be resolved solely through material or process advancements. While integrating SGE in applications such as energy storage or desalination has been actively explored, it still fails to present compelling value propositions in the context of their specific applications. Therefore, we strongly encourage the research community to critically examine the practical impacts of research …

Thin-film composite reverse osmosis membranes have remained the gold standard technology for desalination and water purification for nearly half a century. Polyamide films offer excellent water permeability and salt rejection but also suffer from poor chlorine resistance, high fouling propensity, and low boron rejection. We addressed these issues by molecularly designing a polyester thin-film composite reverse osmosis membrane using co-solvent–assisted interfacial polymerization to react 3,5-dihydroxy-4-methylbenzoic acid with trimesoyl chloride. This polyester membrane exhibits substantial water permeability, high rejection for sodium chloride and boron, and complete resistance toward chlorine. The ultrasmooth, low-energy surface of the membrane also prevents fouling and mineral scaling compared with polyamide membranes. These membranes could increasingly challenge polyamide membranes by …

Membrane distillation (MD), an emerging thermal desalination technology for more than five decades, has not been widely commercialized largely due to the use of hydrophobic membranes. Here we introduce fabric distillation (FD), a novel membraneless thermal desalination technology, as a transformative alternative to MD. FD shares similar working principles with MD but distinguishes itself by employing hydrophilic fabrics instead of hydrophobic membranes for vapour–water separation. We outline the requirements of a desirable fabric for FD and show that common hydrophilic cotton and linen are applicable. Subsequently, we propose feasible FD configurations with experimental demonstrations. Through rigorous module-scale analysis, we reveal that FD outperforms MD in thermal desalination performance. Additionally, we elucidate the interaction of the membrane or fabric with ubiquitous substances in …

Metal–organic framework (MOF) membranes have emerged as promising candidates for efficient water purification. However, challenges related to limited spatio-temporal control over metal–ligand interactions and inherent film fragility hinder the scale-up and widespread adoption of MOF-based membranes. Here we report a nanoreactor-confined crystallization strategy that enables rapid and roll-to-roll fabrication of high-performance ultra-thin (∼25 nm) MOF hybrid membranes (0.33 m × 35 m) at mild conditions. This strategy leverages metal-chelated polydopamine nanoparticles as reactors to grow membranes with hierarchical polymer–MOF interconnected structures that promote remarkable stability against chlorine and varying pH levels, precise solute–solute selectivity and high water permeance. The robustness of the resulting membranes facilitates their assembly into spiral-wound membrane modules …

We extend the classical Flory-Rehner theory for the expansion and compression of porous materials such as cross-linked polymer networks and membranes. The theory includes volume exclusion, affinity with the solvent, and finite stretching of the polymer chains. We also modify this equilibrium theory, which applies to equal expansion of a material in all directions, to the situation where a material can only expand in a single direction, as is the case when a thin polymer layer is tightly bound to a support structure. We further extend this equilibrium model to the case where a pressure is applied across a thin polymer layer, such as a membrane, and liquid flows across the membrane. The theory describes how in the direction of liquid flow the membrane is increasingly compacted and becomes less porous, with compaction intensifying at higher applied pressures. We present results of example calculations for a thick …

The rational design of high-performance desalination membranes is critical to enable sustainable water treatment applications. However, conventional polymeric membranes suffer from insufficient stability especially under harsh chemical conditions. Here we show a novel robust ceramic-based UiO-66 metal–organic framework nanoporous membrane molecularly engineered with missing linkers, enabling more challenging chemically harsh desalination applications. The membranes show competitive desalination performance, which is higher than most state-of-the-art asymmetric and thin-film composite polymeric osmotic membranes. Experimental and molecular simulation results indicate that introducing missing-linker defects substantially increases water flux, allowing faster transport of water clusters with a lower energy barrier via enlarging the pore size of metal–organic framework nanochannels. Notably, the …

Reverse osmosis and nanofiltration are membrane-based methods that remove solutes from solvent, for instance they remove salts from water (desalination). In these methods, an applied pressure is the driving force for solvent to pass the membrane, while most of the solutes are blocked. Very important in the theory of mass transport is the concentration polarization layer (CP layer), which develops on the upstream side of the membrane. Because of the CP layer, the solvent flux through the membrane is reduced while leakage of solutes through the membrane increases, and both these effects must be minimized. So it is very important to understand and describe the nature of the CP layer accurately, especially to find a good estimate of the CP layer mass transfer coefficient, . This is also important for the accurate characterization of membranes in a test cell geometry. We theoretically analyze the structure of the CP layer using three levels of mathematical models. First, we present a modification of an equation for by Sherwood et al. (1965) and show that it works very well in a zero dimensional model. Second, we evaluate a one-dimensional model that is more accurate, which can incorporate any equation for the flow of solvent and solutes through the membrane, and which also makes use of the new modified Sherwood equation. Finally, we fully resolve the complete channel in a two-dimensional geometry, to validate the lower-order models and to illustrate the structure of the CP layer. The overall conclusion is that for typical test cell conditions, the modified Sherwood equation can be used to characterize the CP layer, also when solvent flux …

The COVID-19 pandemic sparked public health concerns about the transmission of airborne viruses. Current methods mainly capture pathogens without inactivation, leading to potential secondary pollution. Herein, we evaluated the inactivation performance of a model viral species (MS2) in simulated bioaerosol by an electromagnetically enhanced air filtration system under a 300 kHz electromagnetic induction field. A nonwoven fabric filter was coated with a 2D catalyst, MXene (Ti3C2Tx), at a coating density of 4.56 mg·cm–2 to absorb electromagnetic irradiation and produce local heating and electromagnetic field for microbial inactivation. The results showed that the MXene-coated air filter significantly enhanced the viral removal efficiency by achieving a log removal of 3.4 ± 0.15 under an electromagnetic power density of 369 W·cm–2. By contrast, the pristine filter without catalyst coating only garnered a log …

Desalination is increasingly essential to ensure water access as climate change and population growth stress fresh water supplies. Already in use in water-stressed regions around the world, desalination generates fresh water from salty sources, but forms a concentrated brine that requires disposal. There is a growing push for the adoption of zero/minimal liquid discharge (ZLD/MLD) technologies that recover additional water from this brine while reducing the liquid volumes requiring disposal. This analysis evaluates the cost, energy, and sustainability impacts of 7 overarching treatment trains with 75 different configurations. ZLD/MLD water recoveries are found to range from 32.6-98.6%, but with steep energy and cost tradeoffs that underscore the crucial role of ion-specific separations, heat integration, and clean energy sources. Ultimately, this analysis explores key tradeoffs between costs, energy, and water recovery, highlighting the increasingly tight connections at the central to the energy-water nexus and desalination.

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor–catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving …

While electrodialysis (ED) demonstrates lower energy consumption than reverse osmosis (RO) in the desalination of low salinity waters, RO continues to be the predominant technology for brackish water desalination. In this study, we probe this skewed market share and project the potential for future disruption by ED through systematic assessment of the levelized cost of water (LCOW). Using rigorous process- and economic-models, we minimize the LCOW of RO and ED systems, highlighting important tradeoffs between capital and operating expenditure for each technology. With optimized current state-of-the-art systems, we find that ED is more economical than RO for feed salinities ≤ 3 g L−1, albeit to a minor extent. Considering that RO is a highly mature technology, we focus on predicting the future potential of ED by evaluating plausible avenues for capital and operating cost reduction. Specifically, we find that …

Coagulation is one of the most common treatment processes for the removal of contaminants from water, representing ‘the first line of defence’ for drinking water safety. However, the macroscopic and descriptive theories of trivalent metal-based coagulation have limited the optimization of its performance in the removal of natural organic matter species, which are major precursors of hazardous disinfection by-products. In this study, we have extended the coagulation theory to the functional group level, highlighting the fundamental importance of η-H2O and η-OH groups on aluminium precipitates, and finding that the selectivity for natural organic matter during coagulation is determined by their functional groups. Drawing upon the fundamental characteristics of coagulants and organic substances, this study has elucidated their behaviour during the coagulation process and offers valuable theoretical insights to guide …

Conventional phototrophic cultivation for microalgae production suffers from low and unstable biomass productivity due to limited and unreliable light transmission outdoors. Alternatively, the use of a renewable lignocellulose-derived carbon source, cellulosic hydrolysate, offers a cost-effective and sustainable pathway to cultivate microalgae heterotrophically with high algal growth rate and terminal density. In this study, we evaluate the feasibility of cellulosic hydrolysate-mediated heterotrophic cultivation (Cel-HC) for microalgae production by performing economic and environmental comparisons with phototrophic cultivation through techno-economic analysis and life cycle assessment. We estimate a minimum selling price (MSP) of 4722 USD/t for producing high-purity microalgae through Cel-HC considering annual biomass productivity of 300 t (dry weight), which is competitive with the conventional phototrophic …

Membrane filtration has been widely adopted in various water treatment applications, but its use in selective solute separation for resource extraction and recovery is an emerging research area. When a membrane process is applied for solute–solute separation to extract solutes as the product, the performance metrics and process optimization strategies should differ from a membrane process for water production because the separation goals are fundamentally different. Here we used lithium (Li) magnesium (Mg) separation as a representative solute–solute separation to illustrate the deficiency of existing performance evaluation framework developed for water–solute separation using nanofiltration (NF). We performed coupon- and module-scale analyses of mass transfer to elucidate how membrane properties and operating conditions affect the performance of Li/Mg separation in NF. Notably, we identified an …

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate …

Bipolar membranes (BPMs), a special class of ion exchange membranes with the unique ability to electrochemically induce either water dissociation or recombination, are of growing interest for environmental applications including eliminating chemical dosage for pH adjustment, resource recovery, valorization of brines, and carbon capture. However, ion transport within BPMs, and particularly at its junction, has remained poorly understood. This work aims to theoretically and experimentally investigate ion transport in BPMs under both reverse and forward bias operation modes, taking into account the production or recombination of H+ and OH–, as well as the transport of salt ions (e.g., Na+, Cl–) inside the membrane. We adopt a model based on the Nernst–Planck theory, that requires only three input parameters─membrane thickness, its charge density, and pK of proton adsorption─to predict the concentration …

We set up a theory for the expansion and compression of porous materials such as cross-linked polymer networks. The theory includes volume exclusion, affinity with the solvent, and finite stretching of the polymer chains. We extend this equilibrium theory to the case that a pressure is applied across a thin layer of such a material, for instance a membrane, and liquid flows across this layer. The theory describes how in the direction of liquid flow the membrane is increasingly compacted (becomes less porous), and the more so at higher applied pressures. For thin membranes used in reverse osmosis, a method to desalinate water, we calculate that for commercially available membranes the porosity decreases only very little across the membrane, in line with experimental observations that for pure solvents up to high pressures water flux is proportional with pressure.

In the original version of this article, an incorrect version of Fig. 4 was displayed. The original Fig. 4 was inconsistent with the source data uploaded. The source data were correct. The scale of the y-axes in Fig. 4d and the inset graphs of 4a and c have been updated. The data points have been replotted for the Local Mg2+ line in Fig. 4b and the “Local” line in Fig. 4e. Additionally, two errors were found in the “Supplementary Codes” of the original Supplementary Information. The first error was in line 120, where “Pressure= 4” should be corrected to “Pressure= 6”. The second error was in line 124, where “A= 18” should be corrected to “A= 18.8”. Three additional functions (namely,‘PCF’,‘licl’and ‘mgcl2’) are needed to generate the source data for Fig. 4. These functions were called in the function “SDEM” and were not included in the submission or uploaded to GitHub. They have now been added in the corrected …

Low-salt rejection reverse-osmosis (LSRRO) is a novel technology that has been proposed as a possible desalination solution capable of treating hypersaline brines that traditional seawater reverseosmosis (SWRO) cannot. Additionally, LSRRO can operate under similar hydraulic pressures of SWRO, thereby using less energy than currently employed by thermal desalination processes, such as mechanical vapor compression (MVC). This study presents unique experimental evaluation of LSRRO, building on previously published process modeling. Five feed streams, from simple sodium chloride solutions (70 g/L-TDS) to oil and gas produced water from the Permian Basin (133 g/L-TDS), were used to assess the performance of a 3-stage LSRRO pilot system, using water recovery and stream compositions as testing parameters. The results of this study confirm the capability of LSRRO to achieve recoveries in …

Boron removal from aqueous solutions has long persisted as a technological challenge, accounting for a disproportionately large fraction of the chemical and energy usage in seawater desalination and other industrial processes like lithium recovery. Here, we introduce a novel electrosorption-based boron removal technology with the capability to overcome the limitations of current state-of-the-art methods. Specifically, we incorporate a bipolar membrane (BPM) between a pair of porous carbon electrodes, demonstrating a synergized BPM–electrosorption process for the first time. The ion transport and charge transfer mechanisms of the BPM–electrosorption system are thoroughly investigated, confirming that water dissociation in the BPM is highly coupled with electrosorption of anions at the anode. We then demonstrate effective boron removal by the BPM–electrosorption system and verify that the mechanism for …

Achieving controllable fine-tuning of defects in catalysts at the atomic level has become a zealous pursuit in catalysis-related fields. However, the generation of defects is quite random, and their flexible manipulation lacks theoretical basis. Herein, we present a facile and highly controllable thermal tuning strategy that enables fine control of nanodefects via subtle manipulation of atomic/lattice arrangements in electrocatalysts. Such thermal tuning endows common carbon materials with record high efficiency in electrocatalytic degradation of pollutants. Systematic characterization and calculations demonstrate that an optimal thermal tuning can bring about enhanced electrocatalytic efficiency by manipulating the N-centered annulation–volatilization reactions and C-based sp3/sp2 configuration alteration. Benefiting from this tuning strategy, the optimized electrocatalytic anodic membrane successfully achieves >99 …

Clarifying the reaction pathways at the solid–water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn3+ and MnOX) and Fenton and Fenton-like catalytic oxidation (e.g., Fe2+ and FeOCl catalyzing H2O2, Co2+ and Co3O4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the …

Nanofiltration—a technology that selectively extracts critical materials from streams—can help secure resources while minimizing wasteful and unsustainable extraction practices. However, next-generation nanofiltration membranes must be designed with a fit-for-purpose framework in mind to fully harness its capabilities in resource conservation and minimize trade-offs.

Ceramic membranes are a promising alternative to polymeric membranes for selective separations, given their ability to operate under harsh chemical conditions. However, current fabrication technologies fail to construct ceramic membranes suitable for selective molecular separations. Herein, we demonstrate a molecular-level design of ceramic thin-film composite membranes with tunable subnanometer pores for precise molecular sieving. Through burning off the distributed carbonaceous species of varied dimensions within hybrid aluminum oxide films, we created membranes with tunable molecular sieving. Specifically, the membranes created with methanol showed exceptional selectivity toward monovalent and divalent salts. We attribute this observed selectivity to the dehydration of the large divalent ions within the subnanometer pores. As a comparison, smaller monovalent ions can rapidly permeate with an …

Water scarcity is one of the greatest societal challenges facing humanity. Reverse osmosis (RO) desalination, a widely used membrane-based technology, has proven to be effective to augment water supply in water-stressed regions of our planet. However, progress in the design and development of RO membranes has been limited. To significantly enhance the performance of RO membranes, it is essential to acquire a deep understanding of the membrane separation and transport mechanisms. In this tutorial review, we cover the pivotal historical developments in RO technology, examine the chemical and physical properties of RO membrane materials, and critically review the models and mechanisms proposed for water transport in RO membranes. Based on recent experimental and computational findings, we conduct a thorough analysis of the key transport models—the solution–diffusion and pore-flow models …

Salt permeability of polyamide reverse osmosis (RO) membranes has been shown to increase with increasing feed salt concentration. The dependence of salt permeability on salt concentration has been attributed to the variation of salt partitioning with feed salt concentration. However, studies using various analytical techniques revealed that the salt (total ion) partitioning coefficient decreases with increasing salt concentration, in marked contrast to the observed increase in salt permeability. Herein, we thoroughly investigate the dependence of total ion and co-ion partitioning coefficients on salt concentration and solution pH. The salt partitioning is measured using a quartz crystal microbalance (QCM), while the co-ion partitioning is calculated from the measured salt partitioning using a modified Donnan theory. Our results demonstrate that the co-ion and total ion partitioning behave entirely differently with increasing …

Separation of specific ions from water could enable recovery and reuse of essential metals and nutrients, but established membrane technologies lack the high-precision selectivity needed to facilitate a circular resource economy. In this work, we investigate whether the cation/cation selectivity of a composite cation-exchange membrane (CEM), or a thin polymer selective layer on top of a CEM, may be limited by the mass transfer resistance of the underlying CEM. In our analysis, we utilize a layer-by-layer technique to modify CEMs with a thin polymer selective layer (∼50 nm) that has previously shown high selectivity toward copper over similarly sized metals. While these composite membranes have a CuCl2/MgCl2 selectivity up to 33 times larger than unmodified CEMs in diffusion dialysis, our estimates suggest that eliminating resistance from the underlying CEM could further increase selectivity twofold. In contrast …

The successful implementation of thin-film composite membranes (TFCM) for challenging solute-solute separations in the pharmaceutical industry requires a fine control over the microstructure (size, distribution, and connectivity of the free-volume elements) and thickness of the selective layer. For example, desalinating antibiotic streams requires highly interconnected free-volume elements of the right size to block antibiotics but allow the passage of salt ions and water. Here, we introduce stevioside, a plant-derived contorted glycoside, as a promising aqueous phase monomer for optimizing the microstructure of TFCM made via interfacial polymerization. The low diffusion rate and moderate reactivity of stevioside, together with its nonplanar and distorted conformation, produced thin selective layers with an ideal microporosity for antibiotic desalination. For example, an optimized 18-nm membrane exhibited an …

The transport of salt and water through “leaky” membranes is of fundamental importance in many disciplines of science and engineering. One important application is in low-salt-rejection reverse osmosis (LSRRO), an emerging membrane-based brine concentration technology with the potential to reduce the energy consumption and cost of brine management. For further development and optimization of the LSRRO technology, models that accurately describe salt transport through LSRRO membranes are critically needed. In this work, we use the solution-friction model to describe salt transport in LSRRO and demonstrate that the model can be simplified to the classic Spiegler-Kedem-Katchalsky model, where the phenomenological parameters of salt permeability and reflection coefficient are expressed in terms of friction and partitioning coefficients. We obtained membranes with different salt permeabilities by …

The surface of polyamide reverse osmosis (RO) membranes which regulates interface-dominated phenomena, such as partitioning and fouling, is the one facing the feed during operation. However, the opposite surface of the polyamide selective layer, the one facing the permeate and in contact with the polysulfone porous support, is commonly analyzed in quartz crystal microbalance (QCM) measurements due to limitations of state-of-the-art transfer methodologies. Such measurements on the back surface cannot be generalized because the polyamide layer is chemically and morphologically asymmetric. Herein, we introduce a simple method to coat QCM sensors with polyamide active layers in the correct orientation (i.e., exposing their front surface) and show that interface-dominated phenomena differ significantly between orientations. We start by describing a transfer protocol to coat any surface with a polyamide …

The unavoidable and detrimental formation of silica scale in engineered processes necessitates the urgent development of effective, economic, and sustainable strategies for dissolved silica removal from water. Herein, we demonstrate a rapid, chemical-free, and selective silica removal method using electrosorption. Specifically, we confirm the feasibility of exploiting local pH dynamics at the electrodes in flow-through electrosorption, achieved through a counterintuitive cell configuration design, to induce ionization and concomitant electrosorption of dissolved silica. In addition, to improve the feasibility of silica electrosorption under high-salinity solutions, we developed a silica-selective anode by functionalizing porous activated carbon cloths with aluminum hydroxide nanoparticles (Al(OH)3–p–ACC). The modification markedly enhances silica sorption capacity (2.8 vs 1.1 mgsilica ganode–1) and reduces the specific …

Climate change is directly impacting energy consumption, water availability, and agricultural production. Among the global efforts to address the root causes of carbon emissions, numerous emerging technologies have been proposed to accelerate sustainable development for achieving carbon neutrality. While science-based discovery for emerging technologies, such as the development of novel materials, may help enhance sustainable development, analyzing the system design and economic viability is imperative for assessing the feasibility of the technology for upscaling and successful commercialization. Herein, we demonstrate the crucial importance of process modeling and techno-economic analysis by evaluating three emerging technologies at the water-energy nexus: direct seawater electrolysis, salinity gradient energy harvesting, and membrane-based thermal desalination. We show that the synergistic …

Efforts toward developing membranes for aqueous separations beyond desalination have intensified, in attempts to achieve zero liquid discharge and a circular economy. Treatment of unconventional wastewaters and brines as well as recovery of valuable species require separation of solutes and ions. Recently, tethered electrolyte active-layer membranes (TEAMs) with dense ionizable brush polymers grafted from cellulose ultrafiltration supports were introduced as a robust, highly controllable membrane platform for these aqueous separations. In this study, we investigate crosslinking of single-block TEAMs to increase the effective charge density and coverage of pores by the active layer, and to possibly tap into size-based exclusion mechanisms. We also determine if crosslinking multiblock TEAMs comprising block copolymers of both negative and positive charge can better align blocks, thereby improving ion …

The release of wastewaters containing relatively low levels of nitrate (NO3−) results in sufficient contamination to induce harmful algal blooms and to elevate drinking water NO3− concentrations to potentially hazardous levels. In particular, the facile triggering of algal blooms by ultra-low concentrations of NO3− necessitates the development of efficient methods for NO3− destruction. However, promising electrochemical methods suffer from weak mass transport under low reactant concentrations, resulting in long treatment times (on the order of hours) for complete NO3− destruction. In this study, we present flow-through electrofiltration via an electrified membrane incorporating nonprecious metal single-atom catalysts for NO3− reduction activity enhancement and selectivity modification, achieving near-complete removal of ultra-low concentration NO3− (10 mg-N L−1) with a residence time of only a few seconds (10 s …

Some of graphene's exciting properties that inspired countless studies over the last two decades are exclusive to defect-free, few-layer graphene (FLG, ≤4 layers) produced by mechanical exfoliation. Despite graphite exfoliation having been thoroughly studied, the influence of the graphite source on the successful exfoliation of FLG remains unexplored. In this work, we examine the physicochemical properties and exfoliation performance of various graphite types (e.g., natural crystalline flake, natural vein, and synthetic) from around the globe (e.g., Alaska, Tanzania, China, etc.). We first established a normalization method for the facile comparison of FLG (Φ) yields between separately exfoliated graphites. Using this approach, we find that the relative yield of FLG ranges from 0% to 22% across graphite types, with regular natural crystalline flakes (NCF-R) performing the best overall (ΦNCF-R = 16% ± 5%). FLG yield …

Electrified processes are a versatile way of removing a wide range of contaminants from water, especially those that are difficult to treat using conventional methods. Electrified processes do not need treatment chemicals and use renewable energy more efficiently. In this Review, we present the fundamental principles of several electrified water treatment processes, discuss the crucial role of electrode materials in the interfacial processes that drive contaminant transport and transformation, and comprehensively review the state of knowledge in electrode material development. Further, we analyse the advantages and limitations of current and emerging electrode materials and discuss strategies for developing advanced electrode materials. Finally, we outline a path towards next-generation water and wastewater treatment systems based on electrified processes.

Effective removal of hexavalent chromium (Cr(VI)) from water is challenging due to the need for a highly selective process. Efficient chromium removal may potentially be achieved through the use of redox-assisted flow-through electrosorption. In this study, graphitized nanodiamonds (NDs) were annealed under various conditions and the ND with the lowest internal resistance was applied to an activated carbon cloth electrode. The ND-modified electrode was then used as the cathode in a flow-through electrode cell with pristine carbon cloth as the anode. Effective chromium removal was found through a dual pathway mechanism, whereby Cr(VI) is directly electrosorbed at the anode while Cr(VI) is reduced to Cr(III) at the cathode, and subsequently precipitated as Cr(OH)3 under the locally high cathodic pH conditions. The effects of flow rate and charging/discharging voltage on Cr(VI) removal were further …

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH …

Modern technology relies on an undisrupted supply of metals, yet many metals have limited geological deposits. Recovering metals from wastewater and brine could augment metal stocks, but there is little guidance on which metals to prioritize for recovery or on the techno-economic viability of extraction processes. Here we critically assess the potential for recovering metals from wastewater and brine. We first look at which metals are critical for recovery on the basis of their supply risks and the impacts of those supply restrictions. We then assess the feasibility of recovering these metals from various water sources by estimating the required operational costs to match market prices. Next we discuss the limitations of established separation technologies that may inhibit the practicality and scalability of metal recovery from water. We conclude by highlighting materials and processes that could serve as more sustainable …

Electrochemistry can provide an efficient and sustainable way to treat environmental waters polluted by chlorinated organic compounds. However, the electrochemical valorization of 1,2-dichloroethane (DCA) is currently challenged by the lack of a catalyst that can selectively convert DCA in aqueous solutions into ethylene. Here we report a catalyst comprising cobalt phthalocyanine molecules assembled on multiwalled carbon nanotubes that can electrochemically decompose aqueous DCA with high current and energy efficiencies. Ethylene is produced at high rates with unprecedented ~100% Faradaic efficiency across wide electrode potential and reactant concentration ranges. Kinetic studies and density functional theory calculations reveal that the rate-determining step is the first C–Cl bond breaking, which does not involve protons—a key mechanistic feature that enables cobalt phthalocyanine/carbon …

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water–energy …

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 …

Reverse osmosis (RO) is one of the most successful membrane technologies for desalination and contaminant removal from water. RO is applied globally, and can be used for both small- and large-scale applications. To characterize membrane performance, standard testing uses membrane coupons and a NaCl solution in a labscale setup under controlled conditions. Ideally, experiments are done for a range of applied hydrostatic pressures and salt concentrations, with water flux and salt rejection measured in each experiment. This full dataset can then be checked for internal consistency, and all these data must then be described by a comprehensive theoretical framework, i.e., we need an appropriate set of equations to parametrize these data. Parameters derived from this procedure, such as water and salt permeability, can then be compared to those obtained in other studies, for other membranes, salts, or …

Reverse osmosis (RO) is a method to desalinate water, where water containing salts is pushed through a membrane while salt ions are rejected by the membrane. Very important in the theory of mass transport in RO is the concentration polarization (CP) layer, which develops on the upstream side of the membrane because of a combination of salt convection and diffusion. Because of the CP-effect, the salt concentration at the membrane surface is higher than in the channel, and this increases the osmotic pressure there, and thus transmembrane water flux is reduced (the osmotic pressure acts against water flux), while salt leakage through the membrane increases. So it is very important to understand and describe the CP-layer accurately. We analyze a one-dimensional geometry, which is of relevance for a typical lab-scale RO setup using small membrane coupons where the solution on the feed side of the membrane is stirred. For this geometry, the standard film layer approach is often used that assumes a stagnant film layer of a defined thickness, which however does not exist in reality. We set up a model without that assumption but including refreshment of solution because of the flow of water along the membrane due to stirring. We show that the `exponential law' for the CP-layer that is predicted by the the film model, also applies for this more accurate model. We further improve the model by including the activity coefficient of salt ions, as described by the Bjerrum theory that is based on ion-ion Coulombic interactions. We show how including this activity correction leads to a reduction of the diffusional driving force at high concentration, and …

Water reuse is rapidly becoming an integral feature of resilient water systems, where municipal wastewater undergoes advanced treatment, typically involving a sequence of ultrafiltration (UF), reverse osmosis (RO), and an advanced oxidation process (AOP). When RO is used, a concentrated waste stream is produced that is elevated in not only total dissolved solids but also metals, nutrients, and micropollutants that have passed through conventional wastewater treatment. Management of this RO concentrate─dubbed municipal wastewater reuse concentrate (MWRC)─will be critical to address, especially as water reuse practices become more widespread. Building on existing brine management practices, this review explores MWRC management options by identifying infrastructural needs and opportunities for multi-beneficial disposal. To safeguard environmental systems from the potential hazards of MWRC …

Polyelectrolyte multilayer membranes (PEMs) produced by the sequential, layer-by-layer deposition of polyelectrolytes on porous supports have been shown to significantly reject ions in dilute saline solutions. However, polyelectrolyte thin films are susceptible to swelling or detachment from the substrate in higher salinities and extreme pH conditions, such that their performance is highly dependent on feed water composition. In this study, we introduce tethered electrolyte active-layer membranes (TEAMs), whereby charged block copolymers are covalently grafted-from a porous support, in aim of reducing the stimuli response of layered polyelectrolyte membranes under variable solution conditions. Cellulose support layers were modified using surface-initiated atom transfer radical polymerization of neutral precursor polymers, which were then converted into charged blocks after polymerization. An ultrathin layer of a …

Reverse osmosis (RO) and electrodialysis (ED) are the two most important membrane technologies for water desalination and treatment. Their desalination and transport mechanisms are very different, but on a closer look also have many similarities. In this tutorial review, we describe state-of-the-art theory for both processes, focusing on simple examples that are helpful for the non-specialist and for classroom teaching. We describe relevant theory for ion and water transport and the coupling with theory for chemical and mechanical equilibrium on membrane/solution interfaces. For RO of neutral solutes, we explain the solution-friction (SF) model which is closely related to the classical sieving or pore flow model. The SF model includes advection, diffusion, and solute partitioning, and leads to simple relationships for the coupled fluxes of water and solutes (and thus for solute retention as well), also when a diffusion …

Gypsum scaling via crystallization is a major obstacle limiting the applications of membrane-based technologies and heat exchangers in engineered systems. Herein, we perform the first comparative investigation on the impacts of natural organic matter (Suwannee River humic acid, SRHA) and colloidal particles on the gypsum crystallization process in terms of induction time and crystal morphology. Results show that the presence of SRHA significantly increases the induction time of gypsum crystallization. Specifically, at a solution saturation index of 4.92, the induction time increases 6.5-fold in the presence of 6 mg/L SRHA, compared to the case without SRHA. SRHA also alters the morphology of the formed calcium sulfate crystals, resulting in a polygon-like shape, differing from the characteristic needle-like shape of gypsum in the absence of additives. These changes in crystal morphology are attributed to the …

Formation of mineral scale on a material surface has profound impact on a wide range of natural processes as well as industrial applications. However, how specific material surface characteristics affect the mineral-surface interactions and subsequent mineral scale formation is not well understood. Here we report the superior resistance of hexagonal boron nitride (hBN) to mineral scale formation compared to not only common metal and polymer surfaces but also the highly scaling-resistant graphene, making hBN possibly the most scaling resistant material reported to date. Experimental and simulation results reveal that this ultrahigh scaling-resistance is attributed to the combination of hBN’s atomically-smooth surface, in-plane atomic energy corrugation due to the polar boron-nitrogen bond, and the close match between its interatomic spacing and the size of water molecules. The latter two properties lead to strong …

Microporous organic nanotubes (MONs) hold considerable promise for designing molecular-sieving membranes because of their high microporosity, customizable chemical functionalities, and favorable polymer affinity. Herein, we report the use of MONs derived from covalent organic frameworks to engineer 15-nm-thick microporous membranes via interfacial polymerization (IP). The incorporation of a highly porous and interpenetrated MON layer on the membrane before the IP reaction leads to the formation of polyamide membranes with Turing structure, enhanced microporosity, and reduced thickness. The MON-modified membranes achieve a remarkable water permeability of 41.7 L m−2 h−1 bar−1 and high retention of boron (78.0%) and phosphorus (96.8%) at alkaline conditions (pH 10), surpassing those of reported nanofiltration membranes. Molecular simulations reveal that introducing the MONs not only …

Since the advent of thin-film composite polyamide membranes brought forth a breakthrough in desalination and water purification membranes nearly half a century ago, recent years have only witnessed marginal improvements in the water-salt selectivity of these membranes. The slow progression is partly attributable to limited understanding of membrane synthesis–structure–performance relationships. A centralized archive of reverse osmosis membrane (RO) characterization data may lead to a shared understanding of features that maximize RO performance and unify research efforts. The Open Membrane Database (OMD), which can be found at www.openmembranedatabase.org, is a growing database of over 600 water purification and desalination membranes that are sourced from peer-reviewed journals, patents, and commercial product data. Here, we outline the detailed functionality of the database, the …

Low-salt-rejection reverse osmosis (LSRRO) is a novel reverse osmosis (RO)-based technology that can highly concentrate brines using moderate operating pressures. In this study, we investigate the performance of LSRRO membrane modules and systems using module-scale analysis. Specifically, we correlate the observed salt rejection of an LSRRO module with the water and salt permeabilities of the RO membrane. We then elaborate the impact of membrane properties and operating conditions on the performance of a 2-stage LSRRO, providing design guidelines for LSRRO systems. We further compare the performance of 2-stage and 3-stage LSRRO systems, showing that an LSRRO system with more stages is not always favored due to a larger energy consumption. The performance of a 3-stage LSRRO in treating different feed solutions for minimal/zero liquid discharge (MLD/ZLD) applications is then …

State-of-the-art polymeric membranes are unable to perform the high-precision ion separations needed for technologies essential to a circular economy and clean energy future. Coordinative interactions are a mechanism to increase sorption of a target species into a membrane, but the effects of these interactions on membrane permeability and selectivity are poorly understood. We use a multilayered polymer membrane to assess how ion-membrane binding energies affect membrane permeability of similarly sized cations: Cu2+, Ni2+, Zn2+, Co2+, and Mg2+. We report that metals with higher binding energy to iminodiacetate groups of the polymer more selectively permeate through the membrane in multisalt solutions than single-salt solutions. In contrast, weaker binding species are precluded from diffusing into the polymer membrane, which leads to passage proportional to binding energy and independent of …

The concentration of disinfection byproducts in tap water varies considerably across China and is statistically related to bladder cancer incidence rates. Anthropogenic factors are shown to have a notable influence on water quality. Countries and regions experiencing rapid socioeconomic development should consider adopting solutions to increase the safety of drinking water.

Inorganic scaling caused by precipitation of sparingly soluble salts at supersaturation is a common but critical issue, limiting the efficiency of membrane-based desalination and brine management technologies as well as other engineered systems. A wide range of minerals including calcium carbonate, calcium sulfate, and silica precipitate during membrane-based desalination, limiting water recovery and reducing process efficiency. The economic impact of scaling on desalination processes requires understanding of its sources, causes, effects, and control methods. In this Critical Review, we first describe nucleation mechanisms and crystal growth theories, which are fundamental to understanding inorganic scale formation during membrane desalination. We, then, discuss the key mechanisms and factors that govern membrane scaling, including membrane properties, such as surface roughness, charge, and …

The lack of electron donors in oxygen-rich aquatic environments limits the ability of natural denitrification to remove excess nitrate, leading to eutrophication of aquatic ecosystems. Herein, we demonstrate that electron-rich substances in river or lake sediments could participate in long-distance electron rebalancing to reduce nitrate in the overlying water. A microstructure containing Dechloromonas and consisting of an inner layer of green rust and an outer layer of lepidocrocite forms in the sediment-water system through synergetic evolution and self-assembly. The microstructure enables long-distance electron transfer from the sediment to dilute nitrate in the overlying water. Specifically, the inner green rust adsorbs nitrate and reduces the kinetic barrier for denitrification via an Fe(II)/Fe(III) redox mediator. Our study reveals the mechanism of spontaneous electron transfer between distant and dilute electron donors …

The development of novel materials with ion-selective nanochannels has introduced a new technology for harvesting salinity gradient (blue) energy, namely nanopore power generators (NPGs). In this study, we perform a comprehensive analysis of the practical performance of NPG in both coupon-size and module-scale operations. We show that although NPG membrane coupons can theoretically generate ultrahigh power density under ideal conditions, the resulting power density in practical operations at a coupon scale can hardly reach 10 W·m−2 due to concentration polarization effects. For module-scale NPG operation, we estimate both the power density and specific extractable energy (i.e., extractable energy normalized by the total volume of the working solutions), and elucidate the impact of operating conditions on these two metrics based on the interplay between concentration polarization and extent of …

Despite decades of dominance in separation technology, progress in the design and development of high-performance polymer-based membranes has been incremental. Recent advances in materials science and chemical synthesis provide opportunities for molecular-level design of next-generation membrane materials. Such designs necessitate a fundamental understanding of transport and separation mechanisms at the molecular scale. Molecular simulations are important tools that could lead to the development of fundamental structure–property–performance relationships for advancing membrane design. In this Perspective, we assess the application and capability of molecular simulations to understand the mechanisms of ion and water transport across polymeric membranes. Additionally, we discuss the reliability of molecular models in mimicking the structure and chemistry of nanochannels and transport …

Removal of organic micropollutants from water through advanced oxidation processes (AOPs) is hampered by the excessive input of energy and/or chemicals as well as the large amounts of residuals resulting from incomplete mineralization. Herein, we report a new water purification paradigm, the direct oxidative transfer process (DOTP), which enables complete, highly efficient decontamination at very low dosage of oxidants. DOTP differs fundamentally from AOPs and adsorption in its pollutant removal behavior and mechanisms. In DOTP, the nanocatalyst can interact with persulfate to activate the pollutants by lowering their reductive potential energy, which triggers a non-decomposing oxidative transfer of pollutants from the bulk solution to the nanocatalyst surface. By leveraging the activation, stabilization, and accumulation functions of the heterogeneous catalyst, the DOTP can occur spontaneously on the …

The quality of drinking-water supplies is of fundamental importance to public health and sustainable development. Here, we provide a spatial assessment of the tap-water quality across mainland China. We examine natural and anthropogenic origins of low quality as well as its association with public health risks. By quantifying key indicators, including total organic carbon, ionic conductivity and disinfection by-products (DBPs), we find that precipitation is a crucial factor driving the change of organic matter content and ionic conductivity of tap-water, especially for arid and semi-arid regions. Although the concentration of DBPs is closely related to the organic matter content, the occurrence of highly toxic DBPs is more subject to anthropogenic factors such as economic development and pollution emission. We show that nanofiltration is an effective point-of-use treatment to reduce the adverse effects of DBPs. The …

There is a need for membranes and processes that can selectively separate target ions from other similar ionic species. Recent studies have shown that electrified processes for ion removal such as membrane capacitive deionization (MCDI) and electrodialysis (ED) are selective towards specific ionic species, but selectivities are generally limited. Here, we demonstrate that an ion-selective polymer coating can significantly enhance ion selectivities for MCDI processes. We focused on the preferential removal of Ca2+ over Na+ and used the conductive and sulfonated polymer poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) as a model selective ion-exchange coating. We first measured the permeability of Ca2+ and Na+ in freestanding PEDOT:PSS membranes of varying crosslink density and found that the permeability of Ca2+ was six times greater than that for Na+ in optimized membranes …

In our recent published paper 1, we presented an entirely new design of a Janus electrocatalytic membrane and demonstrated its efficient water decontamination performance. The Janus electrocatalytic membrane demonstrated in situ singlet oxygen (1O2) formation inside the membrane porous structure, enabling enhanced removal of contaminants from water in a single-pass electrofiltration at very low energy consumption and without the addition of chemical precursors. The enhanced water decontamination performance was ascribed to the electrocatalytic membrane design. The Janus membrane integrates both the anodic and cathodic reactions within the membrane porous structure during flow-through filtration. This unique membrane structure induces spatial confinement within the membrane inner pores and enhances convective mass transport of reactants. In contrast to traditional electrofiltration designs …

Novel observations in nanofluidic systems, such as ultrafast water transport, have directed recent research efforts toward developing new materials for water purification. Characterizing the transport properties of these materials is often restricted to exceptionally small areas and low pressures to avoid damage or defect formation. Consequently, observable flow rates are often limited to a few nL h–1, even for materials with intrinsically high permeability. In this work, we apply laser interferometry to indirectly measure extremely low flow rates from permeable materials. We first demonstrate the principles of interferometry for measuring the rate of a moving mirror. We then validate the technique for nanoscale liquid level displacements by measuring the rate of water evaporation from a cylindrical vessel before (0.40 nm s–1) and after (undetectable) sealing the vessel. When applying interferometry to membrane …

The superior catalytic property of single-atom catalysts (SACs) renders them highly desirable in the energy and environmental fields. However, using SACs for water decontamination is hindered by their limited spatial distribution and density on engineered surfaces and low stability in complex aqueous environments. Herein, we present copper SACs (Cu1) anchored on a thiol-doped reactive membrane for water purification. We demonstrate that the fabricated Cu1 features a Cu–S2 coordination─one copper atom is bridged by two thiolate sulfur atoms, resulting in high-density Cu-SACs on the membrane (2.1 ± 0.3 Cu atoms per nm2). The Cu-SACs activate peroxide to generate hydroxyl radicals, exhibiting fast kinetics, which are 40-fold higher than those of nanoparticulate Cu catalysts. The Cu1-functionalized membrane oxidatively removes organic pollutants from feedwater in the presence of peroxide, achieving …

Securing decarbonized economies for energy and commodities will require abundant and widely available green H2. Ubiquitous wastewaters and nontraditional water sources could potentially feed water electrolyzers to produce this green hydrogen without competing with drinking water sources. Herein, we show that the energy and costs of treating nontraditional water sources such as municipal wastewater, industrial and resource extraction wastewater, and seawater are negligible with respect to those for water electrolysis. We also illustrate that the potential hydrogen energy that could be mined from these sources is vast. Based on these findings, we evaluate the implications of small-scale, distributed water electrolysis using disperse nontraditional water sources. Techno-economic analysis and life cycle analysis reveal that the significant contribution of H2 transportation to costs and CO2 emissions results in an …

Ion–surface interactions can alter the properties of nanopores and dictate nanofluidic transport in engineered and biological systems central to the water–energy nexus. The ion adsorption process, known as “charge regulation”, is ion-specific and is dependent on the extent of confinement when the electric double layers (EDLs) between two charged surfaces overlap. A fundamental understanding of the mechanisms behind charge regulation remains lacking. Herein, we study the thermodynamics of charge regulation reactions in 20 nm SiO2 channels via conductance measurements at various concentrations and temperatures. The effective activation energies (Ea) for ion conductance at low concentrations (strong EDL overlap) are ∼2-fold higher than at high concentrations (no EDL overlap) for the electrolytes studied here: LiCl, NaCl, KCl, and CsCl. We find that Ea values measured at high concentrations result …

Designing single-species selective membranes for high-precision separations requires a fundamental understanding of the molecular interactions governing solute transport. Here, we comprehensively assess molecular-level features that influence the separation of 18 different anions by nanoporous cellulose acetate membranes. Our analysis identifies the limitations of bulk solvation characteristics to explain ion transport, highlighted by the poor correlation between hydration energy and the measured permselectivity (R2 = 0.37). Entropy-enthalpy compensation, spanning 40 kilojoules per mole, leads to a free-energy barrier (∆G‡) variation of only ~8 kilojoules per mole across all anions. We apply machine learning to elucidate descriptors for energetic barriers from a set of 126 collected features. Notably, electrostatic features account for 75% of the overall features used to describe ∆G‡, despite the relatively …

Capacitive deionization (CDI) is an emerging membraneless water desalination technology based on storing ions in charged electrodes by electrosorption. Due to unique selectivity mechanisms, CDI has been investigated towards ion-selective separations such as water softening, nutrient recovery, and production of irrigation water. Especially promising is the use of activated microporous carbon electrodes due to their low cost and wide availability at commercial scales. We show here, both theoretically and experimentally, that sulfonated activated carbon electrodes enable the first demonstration of perfect divalent cation selectivity in CDI, where we define “perfect” as significant removal of the divalent cation with zero removal of the competing monovalent cation. For example, for a feedwater of 15 mM NaCl and 3 mM CaCl2, and charging from 0.4 V to 1.2 V, we show our cell can remove 127 μmol per gram carbon …

Reverse osmosis (RO) is the most important membrane technology for the desalination of water. Measured water and salt fluxes are traditionally analyzed in the context of the solution-diffusion (SD) model which leads to a water permeability, A, and a salt permeability, B. However, this parametrization of the salt flux is not correct for water desalination by RO membranes, because these membranes show markedly different retentions for different feed salt concentrations, a classical observation in the literature, and this effect is not captured by the SD model. Thus, the traditional salt permeability B is not an intrinsic property of these membranes. We present a new analysis for desalination of a 1: 1 salt, which follows from a transport theory that is based on the assumption that coions are strongly excluded from the membrane, and we demonstrate that it accurately describes a large dataset of salt retention by an RO …

Industrially synthesized ammonia (NH3) is essential to support the world population because approximately 85% of global NH3 production is used as agricultural fertilizer. 1 Recently, NH3 has also been considered as a promising carbonfree H2 carrier for energy storage and transportation. 2, 3 Ammonia’s relatively high boiling point of− 33.4 C (at atmospheric pressure) enables facile liquefaction for compatibility with existing liquid fuel infrastructure. 4 Further, NH3 has a higher volumetric energy density (12.92− 14.4 MJ L− 1) than compressed H2 (4.5 MJ L− 1) and liquefied H2 (8.49 MJ L− 1). 4 Ammonia may be used directly as a fuel in vehicles powered by spark-ignition engines or fuel cells, 5 and ammonia-fueled vehicles are being developed by companies such as Hydrofuel, Hyundai, and Mitsui OSK Lines. 6 Consequently, global NH3 production capacity is expected to expand to 289.83 million tons in 2030 (1 …

Conventional water disinfection methods such as chlorination typically involve the generation of harmful disinfection byproducts and intensive chemical consumption. Emerging electroporation disinfection techniques using nanowire-enhanced local electric fields inactivate microbes by damaging their outer structures without byproduct formation or chemical dosing. However, this physical-based method suffers from a limited inactivation efficiency under high water flux due to an insufficient contact time. Herein, we integrate electrochlorination with nanowire-enhanced electroporation to achieve a synergistic flow-through process for efficient water disinfection targeting bacteria and viruses. Electroporation at the cathode induces sub-lethal damages on the microbial outer structures. Subsequently, electrogenerated active chlorine at the anode aggravates these electroporation-induced injuries to the level of lethal …

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel …

The robustness of carbon nanomaterials and their potential for ultrahigh permeability has drawn substantial interest for separation processes. However, graphene oxide membranes (GOms) have demonstrated limited viability due to instabilities in their microstructure that lead to failure under cross-flow conditions and applied hydraulic pressure. Here we present a highly stable and ultrapermeable zeolitic imidazolate framework-8 (ZIF-8)-nanocrystal-hybridized GOm that is prepared by ice templating and subsequent in situ crystallization of ZIF-8 at the nanosheet edges. The selective growth of ZIF-8 in the microporous defects enlarges the interlayer spacings while also imparting mechanical integrity to the laminate framework, thus producing a stable microstructure capable of maintaining a water permeability of 60 l m−2 h−1 bar−1 (30-fold higher than GOm) for 180 h. Furthermore, the mitigation of microporous …

Despite recent advancements in photocatalysis enabled by materials science innovations, the application of photocatalysts in water treatment is still hampered due to low overall efficiency. Herein, we present a TiO2 photocatalytic process with significantly enhanced efficiency by the introduction of micro–nano bubbles (MNBs). Notably, the removal rate of a model organic contaminant (methylene blue, MB) in an air MNB-assisted photocatalytic degradation (PCD) process was 41–141% higher than that obtained in conventional macrobubble (MaB)-assisted PCD under identical conditions. Experimental observations and supporting mechanistic modeling suggest that the enhanced photocatalytic degradation is attributed to the combined effects of increased dissolution of oxygen, improved colloidal stability and dispersion of the TiO2 nanocatalysts, and interfacial photoelectric effects of TiO2/MNB suspensions. The …

The hypersensitivity of state-of-the-art polyamide-based membranes to chlorine is a major source of premature membrane failure and module replacement in water desalination plants. This problem can currently only be solved by implementing pre- and post-treatment processes involving additional chemical use and energy input, thus increasing environmental, capital, and operational costs. Herein, we report a chlorine-, acid-, and base-resistant desalination membrane comprising a cross-linked epoxide-based polymer-selective layer with permanent positive charges. These novel membranes exhibit high mono- and divalent salt rejection (81% NaCl, 87% CaCl2, 89% MgCl2) and a water permeance of ∼2 L m–2 h–1 bar–1, i.e., desalination performance comparable to that of commercially available nanofiltration membranes. Unlike conventional polyamide-based membranes, this new generation of epoxide-based …

Dissolved silica is a major concern for a variety of industrial processes owing to its tendency to form complex scales that severely deteriorate system performance. In this work, we present a pretreatment technology using a Joule-heated sponge to rapidly remove silica from saline waters through adsorption, thereby effectively mitigating silica scaling in subsequent membrane desalination processes. The adsorbent sponge is fabricated by functionalizing two-dimensional layered double hydroxide (LDH) nanosheets on a porous, conductive stainless-steel sponge. With the application of an external voltage of 4 V, the Joule-heated sponge achieves 85% silica removal and 95% sponge regeneration within 15 min, which is much more efficient than its counterpart without Joule-heating (360 min for silica adsorption and 90 min for sponge regeneration). Material characterization and reaction kinetics analysis reveal that …

The preparation of thin films of nanostructured functional materials is a critical step in a diverse array of applications ranging from photonics to separation science. New thin‐film fabrication methods are sought to harness the emerging potential of self‐assembled nanostructured materials as next‐generation membranes. Here, the authors show that nanometer‐scale control over the thickness of self‐assembled mesophases can be enacted by directional photopolymerization in the presence of highly photo‐attenuating molecular species. Metrology reveals average film growth rates below ten nanometers per second, indicating that high‐resolution fabrication is possible with this approach. The trends in experimental data are reproduced well in numerical simulations of mean‐field frontal photopolymerization modeled in a highly photo‐attenuating and photo‐bleaching medium. These simulation results connect the …

Electrodialysis (ED) is an electro-driven desalination technology that relies on the selective transport of ions through ion exchange membranes. Though several approaches have been developed to model and evaluate the performance of ED, mechanistic ion-transport models, which rigorously solve the fundamental Nernst-Planck equation, remain some of the most reliable and utilized. However, complexity of the involved transport phenomena prevents analytical solutions of such models, and numerical solutions can be prohibitively intensive. Here, we use an equivalent circuit analogue to derive a simple correlation equation that predicts the energetic performance of ED for brackish water desalination. Specifically, our correlation equation predicts the specific energy consumption of ED for a given productivity, set of desalination parameters (i.e., feed salinity, salt removal, water recovery), and system properties. The …

Membrane separation has enjoyed tremendous advances in relevant material and engineering sciences, making it the fastest growing technology in water treatment. Although membranes as a broad-spectrum physical barrier have great advantages over conventional treatment processes in a myriad of applications, the need for higher selectivity and specificity in membrane separation is rising as we move to target contaminants at trace concentrations and to recover valuable chemicals from wastewater with low energy consumption. In this review, we discuss the drivers, fundamental science, and potential enabling materials for high selectivity membranes, as well as their applications in different water treatment processes. Membrane materials and processes that show promise to achieve high selectivity for water, ions, and small molecules—as well as the mechanisms involved—are highlighted. We further identify …

Diffusion cannot be a major water transport mechanism in osmotic membranes because of the lack of true water concentration gradient within the membrane. Due to the semipermeable property of osmotic membranes, water concentration in the membrane is virtually constant because of the absence of salts. The recently confirmed porous structure of the skin layer of osmotic membranes cannot support the basis to exclude bulk water flow in the membrane as assumed in the classic solution-diffusion model. Herein we demonstrate that the concentration difference of water at the membrane-solution interface manifests itself as a negative hydraulic pressure in the membrane. Hence, the only possible driving force for water movement in osmotic membranes is hydraulic pressure gradient. Osmotically driven membrane processes are characterized with negative pressure within the membrane below the water vapor …

This study demonstrates the proof-of-concept of biogas sparging to control membrane fouling during sludge centrate pre-concentration by forward osmosis (FO). Sludge centrate sparging by biogas reduced membrane fouling (measured by water flux decline) and filtration time by two and eight times, respectively, compared to FO operation without biogas sparging at the same water recovery of 60%. In addition, the water flux was almost fully recovered by physical flushing when biogas sparging was applied. Biogas sparging also resulted in a significant improvement in the enrichment of organic, ammonia, and phosphate to close to the theoretical value based on mass balance calculation. In other words, organic matter and nutrients were retained in the bulk solution for subsequent recovery. Fouling mitigation and nutrient enrichment improvement by biogas sparging could be attributed to carbonate buffering to …

Minimum and zero liquid discharge (MLD/ZLD) are emerging brine management strategies that attract heightened attention. Although conventional reverse osmosis (RO) can improve the energy efficiency of MLD/ZLD processes, its application is limited by the maximum hydraulic pressure (ΔPmax) that can be applied in current membrane modules. To overcome such limitation, novel RO-based technologies, including osmotically assisted RO (OARO) and low-salt-rejection RO (LSRRO), have been proposed. Herein, we utilize process modeling to systematically compare the energy consumption of OARO and LSRRO for MLD/ZLD applications. Our modeling results show that the specific energy consumption (SEC) of LSRRO is lower (by up to ∼30%) than that of OARO for concentrating moderately saline feed waters (<∼35,000 mg/L TDS) to meet MLD/ZLD goals, whereas the SEC of OARO is lower (by up to ∼40 …

Synthesizing nanopores which mimic the functionality of ion-selective biological channels has been a challenging yet promising approach to advance technologies for precise ion–ion separations. Inspired by the facilitated fluoride (F–) permeation in the biological fluoride channel, we designed a highly fluoride-selective TiO2 film using the atomic layer deposition (ALD) technique. The subnanometer voids within the fabricated TiO2 film (4 Å < d < 12 Å, with two distinct peaks at 5.5 and 6.5 Å), created by the hindered diffusion of ALD precursors (d = 7 Å), resulted in more than eight times faster permeation of sodium fluoride compared to other sodium halides. We show that the specific Ti–F interactions compensate for the energy penalty of F– dehydration during the partitioning of F– ions into the pore and allow for an intrapore accumulation of F– ions. Concomitantly, the accumulation of F– ions on the pore walls also …

Understanding the salt–water separation mechanisms of reverse osmosis (RO) membranes is critical for the further development and optimization of RO technology. The solution-diffusion (SD) model is widely used to describe water and salt transport in RO, but it does not describe the intricate transport mechanisms of water molecules and ions through the membrane. In this study, we develop an ion transport model for RO, referred to as the solution-friction model, by rigorously considering the mechanisms of partitioning and the interactions among water, salt ions, and the membrane. Ion transport through the membrane is described by the extended Nernst–Planck equation, with the consideration of frictions between the species (i.e., ion, water, and membrane matrix). Water flow through the membrane is governed by the hydraulic pressure gradient and the friction between the water and membrane matrix as well as …

Engineered nanoporous materials have been extensively employed in the environmental field to take advantage of increased surface area and tunable size exclusion. Beyond those benefits, recent studies have uncovered that the confinement of traditional environmental processes within several nanometer pores exerts unique nanoconfinement effects, such as enhanced adsorption capacity, reaction kinetics, and ion selectivity, compared to their analogous processes without spatial confinement. In this review, we provide a systematic discussion covering the current understanding of nanoconfinement effects reported across diverse fields using similar materials and structures as those being explored in environmental technologies. We further abstract the underlying fundamental physical and chemical principles including molecular orientation and rearrangement, reactive center creation, noncovalent binding, and …

Hazardous gases threaten human health and the environment due to their toxicity and flammability. Managing such risks requires reliable gas detection techniques. However, accurate detection of target environmental gases remains challenging. Recent advances in materials science have enabled the combination of molecular separation membranes with chemical sensors to achieve high selectivity in the detection of target gas molecules. Herein, we discuss the underlying gas separation mechanisms for different types of membrane overlayers, including graphene oxide, metal-organic frameworks, and metal oxides, as well as their integration with sensing materials. Furthermore, we discuss the relationship between sensing performance and surface chemistry, catalytic properties, and membrane nanostructure. We conclude by highlighting future directions for selective detection of environmental gas species.

Synthetic polymer membranes are enabling components in key technologies at the water–energy nexus, including desalination and energy conversion, because of their high water/salt selectivity or ionic conductivity. However, many applications at the water–energy nexus require ion selectivity, or separation of specific ionic species from other similar species. Here, the ion selectivity of conventional polymeric membrane materials is assessed and recent progress in enhancing selective transport via tailored free volume elements and ion–membrane interactions is described. In view of the limitations of polymeric membranes, three material classes—porous crystalline materials, 2D materials, and discrete biomimetic channels—are highlighted as possible candidates for ion‐selective membranes owing to their molecular‐level control over physical and chemical properties. Lastly, research directions and critical …

Manganese(III/IV) oxide minerals are known to spontaneously degrade organic pollutants in nature. However, the kinetics are too slow to be useful for engineered water treatment processes. Herein, we demonstrate that nanoscale Mn3O4 particles under nanoscale spatial confinement (down to 3–5 nm) can significantly accelerate the kinetics of pollutant degradation, nearly 3 orders of magnitude faster compared to the same reaction in the unconfined bulk phase. We first employed an anodized aluminum oxide scaffold with uniform channel dimensions for experimental and computational studies. We found that the observed kinetic enhancement resulted from the increased surface area of catalysts exposed to the reaction, as well as the increased local proton concentration at the Mn3O4 surface and subsequent acceleration of acid-catalyzed reactions even at neutral pH in bulk. We further demonstrate that a reactive …

Electrified membranes (EMs) have the potential to address inherent limitations of conventional membrane technologies. Recent studies have demonstrated that EMs exhibit enhanced functions beyond separation. Electrification could enhance the performance and sustainability of membrane technologies and stimulate new applications in water and wastewater treatment. Herein, we first describe EM materials, synthesis methods, electrofiltration modules, and operating modes. Next, we highlight applications of EMs in water decontamination, purification, and disinfection. Additionally, we discuss state-of-the-art electrification methods for controlling membrane organic fouling, biofouling, and inorganic scaling. We also evaluate the energy consumption of EMs for water treatment and fouling control. We conclude by discussing the challenges for improving the stability and practicality of EMs and by proposing pathways …

Heterogeneous advanced oxidation processes (AOPs) allow for the destruction of aqueous organic pollutants via oxidation by hydroxyl radicals (•OH). However, practical treatment scenarios suffer from the low availability of short-lived •OH in aqueous bulk, due to both mass transfer limitations and quenching by water constituents, such as natural organic matter (NOM). Herein, we overcome these challenges by loading iron oxychloride catalysts within the pores of a ceramic ultrafiltration membrane, resulting in an internal heterogeneous Fenton reaction that can degrade organics in complex water matrices with pH up to 6.2. With •OH confined inside the nanopores (∼ 20 nm), this membrane reactor completely removed various organic pollutants with water fluxes of up to 100 L m–2 h–1 (equivalent to a retention time of 10 s). This membrane, with a pore size that excludes NOM (>300 kDa), selectively exposed smaller …

The transport of hydrated ions across nanochannels is central to biological systems and membrane-based applications, yet little is known about their hydrated structure during transport due to the absence of in situ characterization techniques. Herein, we report experimentally resolved ion dehydration during transmembrane transport using modified in situ liquid ToF-SIMS in combination with MD simulations for a mechanistic reasoning. Notably, complete dehydration was not necessary for transport to occur across membranes with sub-nanometer pores. Partial shedding of water molecules from ion solvation shells, observed as a decrease in the average hydration number, allowed the alkali-metal ions studied here (lithium, sodium, and potassium) to permeate membranes with pores smaller than their solvated size. We find that ions generally cannot hold more than two water molecules during this sterically limited …

High-pressure reverse osmosis (HPRO) has been recently proposed for the energy-efficient desalination of hypersaline brines. To advance this technology, substantial innovation is needed, including the capacity to test membranes at ultra-high hydraulic pressures. Here, we report on the development of an apparatus designed and constructed specifically for coupon-scale membrane research in HPRO. The capabilities of the closed-loop crossflow system were demonstrated through desalination experiments at hydraulic pressures up to 150 bar and feed concentrations up to 2.0 M (117 g L−1) NaCl. Several safety-related design considerations are highlighted in addition to the design of a custom-built membrane test cell and high-pressure seal to ensure leak-free, safe operation. Corrosion control protocols were also developed to enable long-term system compatibility with corrosive hypersaline feed solutions. Lastly …